Beat Ernst, Gerald W. Hart, Pierre Sinay
Carbohydrates in Chemistry and Biology
Related Titles from WILEY-VCH A. E. Stiitz (ed.) Iminosugars as Glycosidase Inhibitors XIV, 397 pages with 152 figures, 24 in color, and 48 tables plus a comprehensive 123-table of glycosidase inhibitors 1999. ISBN 3-527-29544-5. Cloth
Y. Chapleur (ed.) Carbohydrate Mimics - Concepts and Methods XXVIII, 604 pages with 375 figures and 52 tables 1998. ISBN 3-527-29526-7. Cloth H. van Bekkum, H. Roper, A. G. Vorhaben (eds.) Carbohydrates and Organic Raw Materials I11 X, 315 pages with 156 figures and 68 tables 1996. ISBN 3-527-30079-1. Cloth
T. K. Lindhorst Essentials of Carbohydrate Chemistry and Biochemistry XIV, 323 pages with 244 figures and 11 tables 2000. ISBN 3-527-29543-7. Paperback
Beat Ernst, Gerald W. Hart, Pierre Sinay
Carbohydrates in Chemistry and Biology Part I
Chemistry of Saccharides
VOl. 1
Chemical Synthesis of Glycosides and Glycomimetics
@WILEY-VCH Weinheim - New York * Chichester Brisbane - Singapore * Toronto
Beat Emst, Gerald W. Hart, Pierre Sinay
Carbohydrates in Chemistry and Biology Part I
Chemistry of Saccharides
VOl. 2
Enzymatic Synthesis of Glycosides and Carbohydrate-Receptor Interaction
Weinheim - New York - Chichester Brisbane - Singapore - Toronto
Beat Ernst, Gerald W. Hart, Pierre Sinay
Carbohydrates in Chemistry and Biology Part I1
Biology of Saccharides
VOl. 3
Biosynthesis and Degradation of Glycoconjugates
Weinheim - New York * Chichester Brisbane - Singapore * Toronto
Beat Ernst, Gerald W. Hart, Pierre Sinay
Carbohydrates in Chemistry and Biology Part I1
Biology of Saccharides
VOl. 4
Lectins and Saccharide Biology
@WILEY-VCH Weinheim - New York * Chichester Brisbane * Singapore * Toronto
Prof. Dr. B. Ernst Institut fur Molekulare Pharmazie Universitat Basel Klingenbergstrasse SO 4051 Basel Switzerland
T~ -~~
Prof. Dr. G. W. Hart Dept. of Biological Chemistry Johns Hopkins University School of Medicine 725 N. Wolf St. Rm. 401 Hunterian Baltimore, MD 21205-2185 USA ___p
Prof. Dr. P. Sinay Dept. de Chimie, URA 1686 Ecole Normale SupCrieure 24 rue Lhomond 75231ParisCedex05 France
~~~p~~
~
~~~~
.
p
~
~
.
-1
~
~
~
This book was carefully produced. Nevertheless, editors, authors and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
!-.~p-p.-p-ppp
~p
p~
-~
Cover picture: 3,6-dideoxyhexose binding to antibody se155.4 Courtesy of David Bundle, University of Alberta
Library of Congress Card No.: applied for A catalogue record for this book is available from the British Library. Die Deutsche Bibliothek - CIP Cataloguing-in-Publication-Data A catalogue record for this publication is available from Die Deutsche Bibliothek ISBN 3-527-295 11-9
0 WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany). 2000 Printed on acid-free and chlorine-free paper. All rights reserved (including those of translation in other languages). No part of this book may be reproduced in any form - by photoprinting, microfilm, or any other means - nor transmitted or translated into machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Composition: Asco Typesetters, Hongkong. Printing: betz-druck gmbH, D-6429 1Darmstadt. Bookbinding: Wilhelm Osswald & Co., D-67433 Neustadt. Printed in the Federal Republic of Germany.
About the Editors
Beat Ernst studied chemistry at the ETH in Zurich, Switzerland, where he completed his PhD-thesis about novel tricyclic hydrocarbons under the guidance of Professors Oskar Jeger and Camille Ganter. After his post-doctoral research on tirandamycinic acid with Professor R. E. Ireland at caltech in Pasadena, he joined the Central Research Laboratories of Ciba-Geigy (now Novartis) in Basel where he headed the Carbohydrate Research Section. In 1998 he became Professor of Molecular Pharmacy at the University of Basel, Switzerland. His major research interests are Carbohydrate Chemistry and Glycobiologymainly the chemical and enzymatic syntheses of oligosaccharides, investigation of carbohydrate/lectin interactions and carbohydrate mimics. In 1991 he was awarded the Werner-Price of the Swiss Chemical Society, and in 1993 he became CIBA-fellow for his research contributions.
Gerald Warren Hart is Director and DeLamar Professor of Biological Chemistry at Johns Hopkins University in Baltimore, MD. He studied biology, and chemistry at Washburn University and completed his PhD-thesis in developmental biology at Kansas State University before he was appointed assistant professor of Biological Chemistry at Johns Hopkins University School of Medicine. From 1993 to 1997 he held a chair at the department of Biochemistry and Molecular Genetics at the University of Alabama before he returned to Johns Hopkins University. In 1989 he founded the journal Glycobiology which became the leading journal in the field. His research results and contributions to the field of glycobiology are honored with many prices and awards-President of the Society of Glycobiology, Member of the Council of the ASBMB, First International Glycoconjugate Organization Award, just to name a few.
VI
About the Editors Pierre Sinay studied chemistry at Ecole Nationale Superieure des Industries Chimiques, Nancy, France, and received his PhD from the University of Nancy. After his post-doctoral research at Harvard University he became Professor at the University of Orleans before he was appointed professor of organic chemistry at Pierre et Marie Curie University and Ecole Normale Superieure in Paris. His main research focuses on the chemical synthesis of oligosaccharides of biological relevance and on the development of new synthetic methods in the carbohydrate field. He is Editor-in-Chief of the journal Carbohydrate Letters and he has published numerous scientific papers and patents. In 1987 he was elected president of the organic chemistry division of the French Chemical Society. In 1978 he was awarded the Le Be1 Price of the French Chemical Society, in 1996 he was triple-honoured with the Desnuelles Price, the Bethellot Medal and with the election as corresponding member of the French Academy of Sciences.
Contents
Part I
.
Vol 1
Chemistry of Saccharides Chemical Synthesis of Glycosides and Glycomimetics List of Contributors ...................................................
LV
Abbreviations Used in Volumes 1 and 2 ...........................
LXIII
I
Chemical Synthesis of Glycosides ..................................
1
1
Introduction to Volumes 1 and 2 .......................................
3
2
Trichloroacetimidates .................................................. Richard R. Schmidt and Karl-Heinz Jung Introduction ............................................................ Methods ................................................................ 0-Glycosides ............... ......................................... Synthesis of Oligosaccharides ......................................... p-Glucosides, p.Galactosides, a.Rhanmosides. etc.................... Aminosugar Trichloroacetimidates.................................... p-Mannosides ......................................................... 2-Deoxyglycosides .................................................... Miscellaneous Compounds ........................................... Complex Oligosaccharides............................................ Inositol Glycosides.................................................... Glycosylation of Sphingosine Derivatives and Mimics .............. Glycosylation of Amino Acids ....................................... Polycyclic and Macrocyclic Glycosides ..............................
5
2.1 2.2 2.3 2.3.1
2.3.2 2.3.3 2.3.4 2.3.5
5 6 7 7 7 8 13 13 14 14 36 38 40 42
VIII
Contents
2.3.6 2.3.7 2.4 2.5 2.6 2.7
Glycosides of Phosphoric and Carboxylic Acids .................... Solid-Phase Synthesis................................................. S-Glycosides .......................................................... N- and P-Glycosides.................................................. C-Glycosides.......................................................... Conclusion and Outlook ............................................. References ............................................................
3
Iterative Assembly of Glycals and Glycal Derivatives: The Synthesis of Glycosylated Natural Products and Complex Oligosaccharides... Lawrence J . Williams. Robert M . Garbaccio. and Samuel J . Danishefsky Introduction .......................................................... Ciclamycin 0 .......................................................... Allosamidin ........................................................... KS-502 and Rebeccamycin ........................................... Extension to Thioethyl Donors ...................................... LewisY ................................................................. Globo H .............................................................. KH-1 .................................................................. Concluding Remarks ................................................. Acknowledgments .................................................... References ............................................................
3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9
4
4.1 4.2 4.2.1 4.2.2 4.2.3 4.3 4.3.1 4.3.2
4.4 4.4.1 4.4.2 4.4.3
44 45 49 51 51 53 53
61
61 64 66 69 74 76 82 86 90 90 90
Thioglycosides ........................................................ 93 Stefan Oscarson Introduction .......................................................... 93 Synthesis of Thioglycosides .......................................... 94 From Anomeric Acetates............................................. 94 From Glycosyl Halides ............................................... 95 Protecting Group Manipulations in Thioglycosides ................. 96 Glycosylations with Thioglycoside Donors .......................... 97 A Two-step Procedure: Transformation of Thioglycosides into 97 Other Types of Glycosyl Donors ..................................... 99 Direct Activation of Thioglycoside Donors ......................... 99 Heavy Metal Salt Promoters ......................................... Halonium, sulfonium and carbonium type promoters ............... 100 Single-ElectronActivation ........................................... 106 Other Types of Donors With an Anomeric Sulfur .................. 108 Applications of Thioglycosides....................................... 110 Block Syntheses, Orthogonal Glycosylations ........................ 110 Thioglycosides as Acceptors.......................................... 110 Thioglycosides as Both Donors and Acceptors ...................... 111 Intramolecular Glycosidations ....................................... 112 Solid Phase Synthesis ................................................. 113 Reference6 ............................................................ 113
Contents 5
5.1 5.2 5.3 5.3.1 5.3.2 5.3.3 5.3.4 5.3.5
5.3.6 5.4 5.4.1 5.4.2 5.4.3 5.4.4
6
6.1 6.2 6.3 6.4 6.5 6.6 6.7 7
7.1 7.2 7.3 7.4 7.4.1
IX
Glycosylation Methods: Use of Phosphites ........................... 117 Zhiyuan Zhang and Chi-Huey Wong Introduction ........................................................... 117 118 Preparation of Glycosyl Phosphites .................................. Glycosylation using Glycosyl Phosphites ............................ 119 119 Mechanism ............................................................ Low Temperature-Dependent Stereoselectivity ...................... 121 Glycosylation of Sialyl Phosphites ................................... 122 Glycosylation of C-2-Acylated Glycosyl Phosphites ................. 123 Glycosylation with C-2-O-Benzylated Glycosyl Phosphites ......... 124 Glycosylation using Glucosyl Phosphites with a Benzyl Group at C-2 .................................................................... 124 Glycosylation using Galactosyl and Fucosyl Phosphites with a 125 Benzyl Group at C-2.................................................. Glycosylation using other Glycosyl Phosphites with a Benzyl Group at C-2 .......................................................... 126 Glycosylation with 2-Deoxy Glycosyl Phosphites ................... 127 128 Other Applications of Glycosyl Phosphites .......................... Synthesis of CMP-NeuAc ..... .................................... 129 Synthesis of GDP-Fucose ............................................ 129 Formation of Glycosyl Phosphonate. ................................ 131 Transformation to other Types of Glycosyl Donor ..................131 131 Phosphate ............................................................. Phosphorimidate ...................................................... 131 132 References............................................................. Glycosylation Methods: Use of n-Pentenyl Glycosides ............... 135 Bert Fraser.Reid. G. Anilkumar. Mark R. Gilbert. Subodh Joshi. and Ralf Kraehmer Introduction ........................................................... 135 Fundamental Reactions .............................................. 135 Determination of Relative Reactivities ............................... 138 n-Pentenyl Orthoesters as Glycosyl Donors .......................... 141 144 n-Pentenyl Orthoesters as Latent C2 Esters .......................... 146 Protecting Groups .................................................... Solid-Phase Iterative Couple-Deprotect-Couple Strategy .......... 146 153 References .............................................................. Glycosylidene Diazirines .............................................. Andrea Vasella. Bruno Bernet. Martin Weber. and Wolfgang Wenger Introduction ........................................................... Synthesis of Glycosylidene Diazirines ................................ Stability of the Glycosylidene Diazirines ............................. Glycosidation by Glycosylidene Diazirines .......................... General Aspects .......................................................
155 155 155 158 158 158
X 7.4.2 7.4.3 7.5 7.6 7.7
8
8.1 8.2 8.2.1 8.2.2 8.2.3 8.2.4 8.2.5 8.2.6 8.2.7 8.2.8 8.2.9 8.3 8.3.1 8.3.2 8.3.3 8.3.4 8.3.5
Contents
Glycosidation of Strongly Acidic Hydroxy Compounds ............ 162 Glycosidation of Phenols ............................................. 162 163 Glycosidation of Fluorinated Alcohols .............................. Glycosylation of Weakly Acidic Hydroxy Compounds ............. 163 163 Glycosidation of Monovalent Alcohols .............................. Glycosidation of Diols and Triols ................................... 164 Synthesis of Spirocyclopropanes ..................................... 168 Addition to Aldehydes and Ketones ................................. 170 Exploratory Use of Diazirines: Formation of Glycosyl Phosphines, Stannanes. N.Sulfonylamines. Esters. Boranes. and Alanes. and of 1.l-Difluorides ........................................................ 171 Acknowledgments .................................................... 174 174 References ............................................................ Glycosylation Methods: Alkylations of Reducing Sugars ............ 177 Jun-ichi Tamura Introduction .......................................................... 177 Anomeric O-Alkylation .............................................. 177 Anomeric O-Alkylation of Ribofuranose with Primary Triflates: Effect of the Protecting Group at 0 - 5 of Ribofuranose ............. 178 Anomeric O-Alkylation of Mannofuranose with Primary Triflates: 179 The Crown Ether Effect .............................................. Anomeric O-Alkylation of Gluco- and Galactopyranoses with Primary Triflates: High P-Selectivity as a Result of the Reactive Anomeric p-Anion .................................................... 180 Anomeric O-Alkylation of Acyl-Protected Nucleophiles with Primary Triflates ...................................................... 181 Anomeric O-Alkylation of Mannopyranose with Primary Triflates: Possibility of Intramolecular Complexation of the Nucleophile .... 184 Anomeric O-Alkylation of KDO with Primary Triflates ............ 185 Anomeric O-Alkylation of Some Protected Aldoses with Primary 186 Triflate ................................................................ Anomeric O-Alkylation of Unprotected Aldoses with Primary 187 Triflate, Bromides, and Cyclic Sulfates .............................. Anomeric O-Alkylation with Secondary Triflates and Nonaflate ... 188 Glycosylation via the Locked Anomeric Configuration ............. 189 Synthesis of Methyl. Allyl. and Benzyl Glycosides via Stannylene Acetals ................................................................ 189 Epimerization at C-2 by the Locked Anomeric Configuration 189 Method ............................................................... The Locked Anomeric Configuration Method for Rhamnosyl Stannylene Acetal ..................................................... 190 The Locked Anomeric Configuration Method for Mannosyl Stannylene Acetal: Isomerization of Acetal [25. 261 ................. 190 The Locked Anomeric Configuration Method for Stannylene Acetal with the Glucose Configuration [25. 261 ..................... 191
Contents
XI
8.4
Conclusion ............................................................ References .............................................................
9
Other Methods of Glycosylation ...................................... 195 Luigi Panza and Luigi Lay 195 Introduction and Summary ........................................... Highlights ............................................................. 195 Enol Ethers ........................................................... 197 Endo-En01 Ethers ..................................................... 198 Exo-Enol Ethers ...................................................... 201 Endo-Glycals ......................................................... 202 Exo-Glycals .......................................................... 204 Vinyl Glycosides ...................................................... 206 1-Hydroxy Sugars ..................................................... 209 Acidic Activation ..................................................... 210 Acidic Activation With Additional Reagents ........................ 211 Dehydrative Glycosylation ........................................... 212 In the Presence of the Acceptor From the Beginning ................213 214 Mitsunobu Glycosylation ............................................. I-0-Silyl Glycosides .................................................. 215 Esters and Related Derivatives ....................................... 216 216 Esters .................................................................. Sugar Carbonates and Derivatives ................................... 221 Orthoesters and Oxazolines........................................... 223 Phosphorus and Sulfur Derivatives .................................. 229 References ............................................................. 233
9.1 9.1.1 9.1.2 9.1.3 9.1.4 9.1.5 9.2 9.2.1 9.2.2 9.2.3 9.2.4 9.3 9.3.1 9.3.2 9.3.3 9.3.4
10 10.1 10.2 10.3 10.4 10.4.1 10.4.2 10.5
10.5.1
192 193
Polymer-Supported Synthesis of Oligosaccharides.................... 239 Jiri J. Krepinsky and Stephen P . Douglas Introduction ........................................................... 239 General Reflections ................................................... 240 Polymer Supports ..................................................... 246 One-Phase Systems (Syntheses in Solution) .......................... 247 248 Polyethyleneglycol,. monomethylether (MPEG) ..................... 249 Linear Polystyrene .................................................... Linkers ................................................................ 250 Succinoyl Diester ..................................................... 250 Dioxyxylyl Diether (DOX) ........................................... 252 254 Chemistry Investigations ............................................. Two-Phase Systems (Syntheses on Solid Supports) ..................255 Controlled Pore Glass ................................................ 256 256 Cross-Linked Polystyrene ............................................. Polyethylene Grafts on Cross-Linked Polystyrene. ..................256 Linkers ................................................................ 259 259 Dialkyl- or Diaryl-Silyl ............................................... Thioglycoside Linkers ................................................ 259
XI1
10.6 10.7 10.8 10.9 11 11.1 11.1.1 11.1.2
11.1.3 11.2 11.2.1
11.2.2 11.3 11.3.1
11.3.2
Contents
Linkers Cleavable by Photolysis ..................................... Examples of Syntheses ............................................... Combinatorial Libraries .............................................. Capping ............................................................... Concluding Remarks ................................................. References ............................................................
260 260 261 262 262 262
Glycopeptide Synthesis in Solution and on the Solid Phase .......... 267 Horst Kunz and Michael Schultz Introduction .......................................................... 267 Which Protecting Groups are Suitable for Carbohydrates (Table 2)? ............................................................. 269 Which Glycosylation Methods are Useful for the Formation of Glycopeptides?........................................................ 271 Formation of Asparagine N-Glycosides ............................. 271 a-Fucosylation ........................................................ 271 272 Formation of the P-Lactosamine Linkage ........................... a-Sialylation .......................................................... 272 Glycopeptides Containing Particularly Sensitive Linkages .......... 272 Acid Sensitivity ....................................................... 273 273 Base Sensitivity ....................................................... Synthesis of Glycopeptides in Solution .............................. 274 0-Glycopeptides ...................................................... 274 Glycopeptides Carrying N-Acetylgalactosamine (Tn-Antigen)...... 274 Glycopeptides Carrying the T-Antigen (Gal-GalNAc) ............. 276 Glycopeptides Carrying the Sialyl T Antigen (NeuAca2,6[Galpl,31GalNAc) ....................................... 278 279 Glycopeptides Carrying 0-GlcNAc .................................. N-Glycopeptides ...................................................... 280 N-Glycopeptides Carrying Natural Saccharide Side-Chains ........ 280 N-Glycopeptides with Lewis-Type Saccharide Side-Chains ......... 285 Glycopeptide Synthesis on the Solid Phase .......................... 286 0-Glycopeptides ...................................................... 287 Glycopeptides Carrying N-Acetylgalactosamine (Tn-Antigen)...... 287 0-Glycopeptides Carrying the T Antigen (Gal-GalNAc) .......... 290 0-Glycopeptides Carrying the Sialyl Tn Antigen (NeuNAc~a2, 6. GalNAc) .............................................................. 291 0-Glycopeptides Carrying the 2, 3.Sialyl T Antigen ................. 293 0-Glycopeptides Carrying 0-GlcNAc Side-Chains .................294 295 0-Glycopeptides Carrying 0-Linked Fucose ........................ 0-Glycopeptides Carrying a Sialyl Lewis Antigen Structure ........ 296 N-Glycopeptides ...................................................... 297 The Construction of N-Glycopeptide Libraries on the Solid 298 Phase .................................................................. Sequential N-Glycopeptide Synthesis on the Solid Phase with Oligosaccharides from Natural Sources.............................. 299
Contents
XI11
11.4
Conclusion ............................................................ References .............................................................
12
Glycolipid Synthesis ................................................... 305 Hidehuru Ishida Introduction ........................................................... 305 Synthesis of Ganglio-Series Gangliosides ............................ 305 305 Retrosynthetic Analysis of Ganglioside G D l a ...................... Preparation of Sialylgalactose Donor as Building Block ............ 306 Construction of Oligosaccharide ..................................... 308 Transformation of Oligosaccharide into Glycolipid .................310 Synthesis of Polysialo Ganglio-Series Gangliosides.................. 311 Retrosynthetic Analysis of GQl b .................................... 313 313 Preparation of Building Block ........................................ Construction of Oligosaccharide ..................................... 314 315 Conclusion ............................................................ References ............................................................. 316
12.1 12.2 12.2.1 12.2.2 12.2.3 12.2.4 12.3 12.3.1 12.3.2 12.3.3 12.4 13
13.1 13.2 13.2.1
13.2.2 13.2.3 13.2.4 13.2.5
13.2.6 13.2.7 13.2.8 13.2.9 13.3 13.4
Stereoselective Synthesis of P-Mannosides............................ Vince Pozsguy Introduction ........................................................... Chemical Methods .................................................... Glycosylation with Mannosyl Donors ............................... Mannosylation using Insoluble Promoters ........................... The Sulfonate Approach .............................................. Intramolecular Mannosylation ....................................... Other Mannosyl Donor-Based Methods ............................. Epimerization of p-Glucopyranosides at C-2 ........................ The Oxidation-Reduction Approach ................................ Direct Inversion ....................................................... The 2-Ulosyl Donor Method ......................................... Anomeric O-alkylation ............................................... Alkylation of l-O-Metal Complexes ................................. The Stannylene Acetal Method ...................................... Miscellaneous Methods ............................................... Radical Inversion of the Anomeric Chirality of a-DMannopyranosides .................................................... Reductive Cleavage of Cyclic Orthoesters ........................... De nouo Syntheses .................................................... 2-Acetamido-2-deoxy-~-~-mannopyranosides ....................... Aryl p-D-mannopyranosides.......................................... l-Thio-P-D-mannopyranosides ....................................... P-D-Mannopyranosylamines .......................................... Enzymatic Synthesis .................................................. Conclusions ........................................................... References.............................................................
300 300
319 319 320 320 320 322 324 327 329 329 329 331 332 332 332 333 333 334 334 335 336 336 337 337 338 338
XIV 14
14.1 14.2 14.3 14.4 14.4.1 14.4.2 14.4.3 14.5 14.5.1 14.5.2 14.5.3
15
15.1 15.2 15.2.1 15.2.2 15.2.3 15.2.4 15.2.5 15.2.6 15.2.7 15.3 15.4 15.5 15.5.1 15.5.2 15.5.3 15.5.4 15.6 15.6.1 15.6.2 15.6.3 15.6.4 15.6.5
Contents
Special Problems in Glycosylation Reactions: Sialidations ........... 345 Mukoto Kiso. Hidehuru Ishida and Hirorni It0 Introduction .......................................................... 345 Sialidation by the Koenigs-Knorr Method .......................... 345 347 Sialidation Using an Auxiliary Group at C-3 ....................... Sialidation Using 2.Thioglycosides. Xanthates. or Phosphites of 349 Sialic Acids in Acetonitrile ........................................... Thioglycosides ........................................................ 349 Xanthates and Phosphites ............................................ 356 359 Reaction Mechanism ................................................. Further Solutions to the Problem .................................... 359 Combination of C-3 Auxiliary and Sterically less Hindered Sugar Acceptors ............................................................. 359 Combination of C-3 Auxiliary and Specific Activation of the 360 Anomeric Center C-2 ................................................. Thioglycoside of N ,N.Diacetylneuraminic Acid and Combination 363 with C-3 Auxiliary .................................................... References ............................................................ 364 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars ...... 367 Aluin Veyri2res Introduction .......................................................... 367 Electrophilic Additions to Glycals: Mechanistic Aspects and Applications to the Synthesis of 2-Deoxyglycosides . ................ 368 Protonation of Glycals ............................................... 369 370 Enzyme-Catalyzed Additions to Glycals............................. Halogenation of Glycals ............................................. 370 372 Bromo- and Iodoalkoxylation of Glycals ............................ 377 Epoxidation of Glycals ............................................... Addition of Sulfur Based Electrophiles to Glycals .................. 379 Addition of Selenium Based Electrophiles to Glycals ............... 382 The Cycloaddition Way to Glycosyl Transfer ....................... 384 385 Fluoroglycosylation of Glycals ....................................... 386 Glycosyl Donors with a C-2 Heteroatom ............................ 2-Bromo-2-deoxyglycosyl bromides .................................. 386 2-Deoxy-2-(thiophenyl)-glycosylfluorides ........................... 387 388 2,6.Anhydro-2-Thi o-Glycosyl Donors ............................... 1,2-Di-O-Acetyl-P-Hexopyranoses and N-Formylglucosamine Derivatives ............................................................ 392 2-Deoxyglycosyl Donors ............................................. 393 394 2-Deoxy-Hexopyranoses ............................................. Tert-Butyldimethylsilyl 2-Deoxyglycosides .......................... 394 1-0-Acyl- and Acetimidyl-2-Deoxy.Hexopyranoses ................. 394 395 2-Deoxyglycosyl Bromides and Fluorides ........................... S-(2-Deoxyglycosyl)phosphorodithioates............................ 396
Contents 15.6.6 15.6.7 15.6.8 5.7 5.7.1 5.7.2
6
16.1 16.2 16.2.1 16.2.2 16.3 16.3.1 16.3.2 16.3.3 16.3.4 16.4 16.4.1 16.4.2 16.4.3 16.5
17
17.1 17.2 17.3 17.4 17.5 17.5.1 17.5.2 17.6 17.7 17.8
XV
2-Deoxyglycosyl Phosphates. Phosphoramidites and Phosphites .... 397 2-Deoxy Thioglycosides .............................................. 398 399 2-Deoxyglycosyl Sulfoxides........................................... Other Approaches to 2-Deoxyglycosides............................. 400 Cyclization of Acyclic Sugars ........................................ 401 Use of Alkoxy-Substituted Anomeric Radicals ...................... 402 References............................................................. 403 Orthogonal Strategy in Oligosaccharide Synthesis ...................407 Osurnu Kun ie Introduction ........................................................... 407 Analysis of the Strategic Aspects of Oligosaccharide Synthesis ..... 408 General Aspects ....................................................... 408 The Pursuit of Efficiency in Oligosaccharide Synthesis ..............408 The Introduction of the Orthogonal Glycosylation Strategy ........ 410 Limitation of Current Concepts ...................................... 410 The Orthogonal Coupling Concept .................................. 412 413 What is Orthogonality Anyway? ..................................... Orthogonal Glycosylation and Solid-Phase Oligosaccharide 414 Synthesis .............................................................. The Orthogonal Glycosylation Strategy ............................. 414 Orthogonal Chain Elongation of Homo-Oligosaccharides: 414 Synthesis of Chito-Oligosaccharides [ 191 ............................. Orthogonal Coupling for Hetero-Oligomer Synthesis [22]...........418 Application to Polymer-Supported Synthesis [26] ...................420 421 Conclusions and Prospects ........................................... Acknowledgments .................................................... 424 References ............................................................. 424 Protecting Groups: Effects on Reactivity. Glycosylation Stereoselectivity. and Coupling Efficiency ............................ 427 Luke G. Green and Steven V. Ley Introduction ........................................................... 427 Glycosidic Mechanism ................................................ 428 Electronic and Torsional Effects ..................................... 430 Influence of Protecting Group on Donor Reactivity ................. 431 Stereoselectivity ....................................................... 436 Neighboring-Group Participation .................................... 436 437 Reactivity Control .................................................... influence of the Protecting Group on the Acceptor .................441 Steric Effects on Glycosylation ....................................... 443 444 Conclusions ........................................................... Acknowledgments .................................................... 445 References ............................................................. 446
XVI 18
18.1 18.2 18.2.1 18.2.2 18.3 18.4 19 19.1 19.2 19.2.1 19.2.2 19.2.3 19.2.4 19.2.5 19.2.6 19.3 19.3.1 19.3.2 19.3.3 19.3.4 19.3.5 19.3.6 19.4 19.4.1 19.4.2 19.4.3 19.5
Contents
Intramolecular Glycosidation Reactions .............................. 449 Jacob Madsen and Mikael Bols Introduction .......................................................... 449 Reactions in which the Tether Participates in the Reaction ......... 450 Tethering to the Glycosyl Donor .................................... 450 Carbon Tethers ....................................................... 450 Silicon Tethers ........................................................ 454 Tethering to the Leaving Group ..................................... 459 Reactions in which the Tether does not Participate in the Reaction 459 Conclusion ............................................................ 464 References ............................................................ 465 Classics In Total Synthesis of Oligosaccharides and Glycoconjugates....................................................... 467 Jean-Maurice Mallet and Pierre Sinay Introduction .......................................................... 467 Syntheses of Nod factors ............................................. 467 Introduction .......................................................... 467 The K . C . Nicolaou Synthesis (1992) [3]............................. 468 The J.-M. Beau Synthesis (1994) [ 121 ................................ 471 The T . Ogawa Synthesis (1994) [ 161 ................................. 475 The Y . Z . Hui Synthesis (1992) [ 181 ................................. 477 Conclusion ............................................................ 480 Synthesis of the Antithrombin-Binding Pentasaccharide Sequence in Heparin (1984) [ 19, 201 ............................................... 480 Introduction .......................................................... 480 An Overview of the Synthesis of the Protected Pentasaccharide 73 ..................................................................... 481 Synthesis of the Disaccharidic Bromide Donor 68 .................. 483 Synthesis of the Disaccharidic Acceptor 69 .......................... 484 Synthesis of the Protected Pentasaccharide 73 ....................... 484 Synthesis of the Active Site of Heparin .............................. 485 Total Synthesis of VIM-2 Ganglioside [31] .......................... 485 Introduction .......................................................... 485 The Total Synthesis of VIM-2-a General Strategy ................ 486 Preparation of the Key Protected Octasaccharide 87 ................ 487 Epilogue .............................................................. 490 References ............................................................ 491
I1
Synthesis of Oligosaccharide Mimics .................................
493
20
Synthesis of C-Oligosaccharides...................................... Troels Skrydstrup. Boris Vauzeilles. and Jean-Marie Beau Introduction .......................................................... The Anionic Approach ............................................... C5-Alkynyl Anions ...................................................
495
20.1 20.2 20.2.1
495 496 496
Contents
XVII
20.2.2 20.2.3 20.2.4 20.2.5 20.3 20.3.1 20.3.3 20.4 20.5
C1-Glycal Carbanions ................................................ 500 Anomeric Samarium Species ......................................... 502 C-Branched Carbanions .............................................. 506 C6-Phosphoranes ..................................................... 508 The Radical Approach ............................................... 511 Intermolecular Anomeric Radical Addition ......................... 511 513 Intramolecular Anomeric Radical Addition ......................... The Partial de Nuuo Approach ....................................... 518 The Cycloaddition and Rearrangement Approach ..................527 528 References .............................................................
21
531 Synthesis of Oligosaccharide Mimics: S-Analogs..................... Jon K. Fairweather and Hugues Driguez Introduction ........................................................... 531 General Synthesis ..................................................... 532 Preparation of Thioglycoses .......................................... 532 l-Thioglycoses ........................................................ 532 2-, 3-, 4-, 5.. or 6-Thioglycoses ....................................... 532 Selective S-Deprotection of Thioglycoses ............................ 533 Glycosylation Methods ............................................... 534 Establishment of 1,6-Thio Linkages .................................. 534 6-Thiodisaccharides ................................................... 534 6-Thiooligosaccharides ............................................... 538 538 Branched Thiocyclodextrins .......................................... Establishment of I , 4-Thio Linkages .................................. 541 1,4-Thiodisaccharides ................................................. 541 General Approaches .................................................. 541 SN~-Displacement on Triflates ....................................... 541 546 1,4-Thiooligosaccharides ............................................. Conventional Approaches ............................................ 546 548 Chemoenzymatic Approaches ........................................ 549 Michael Addition to Unsaturated Acceptors ........................ Solid-Support Synthesis........ ................................... 550 551 Establishment of 1, 3.Thio Linkages .................................. 1,3-Thiodisaccharides ................................................. 551 Conventional Methods ............................................... 551 Cyclic Sulfamidate and Aziridine .................................... 551 1,3-Thiooligosaccharides ............................................. 552 Establishment of 1,2.Thio Linkages .................................. 553 1,2-Thiodisaccharides................................................. 553 Conventional Methods ............................................... 554 Other Approaches .................................................... 555 Establishment of I , l-Thio Linkages .................................. 557 558 Establishment of Mixed Thio linkages ............................... Thiooligosaccharides and Proteins ................................... 558 The Conformation of Thiooligosaccharides in Solution ............. 558
21.1 21.2 21.2.1
21.3 21.3.1 21.3.2 21.3.3 21.4 21.4.1 21.4.2
21.5 21.5.1 21.5.2 21.6 21.6.1 21.7 21.8 21.9 21.9.1
XVIII
Contents
21.9.2
Enzyme-Substrate Interactions ...................................... a-Glucan- Active Enzymes ............................................
21.9.3 21.10
P-Glucan-Active Enzymes ............................................ Lectin-Ligand Interactions .......................................... Conclusion ............................................................ Acknowledgments .................................................... References ............................................................
22 22.1 22.2 22.2.1 22.2.2 22.3 22.4 22.4.1 22.4.2 22.4.3 22.4.4
Saccharide-Peptide Hybrids .......................................... Huns Peter Wessel Introduction .......................................................... Carbohydrate Amino Acids .......................................... Natural Carbohydrate Amino Acids ................................. Synthetic Carbohydrate Amino Acids ............................... Amide-Linked Carbohydrate Polymers .............................. Amide-Linked Carbohydrate Oligomers ............................. Solution Synthesis .................................................... Solid-Phase Synthesis................................................. Biological Activity .................................................... Conformational Properties ........................................... References ............................................................
565
Index ...................................................................
11
Part I
Chemistry of Saccharides
Vol. 2
Enzymatic Synthesis of Glycosides and CarbohydrateReceptor Interaction
111
Enzymatic Synthesis of Glycosides ..................................
23
On the Origin of Oligosaccharide Species-Glycosyltransferases in Action ................................................................ Dirk H . van den Eijnden Introduction ......................................................... Protein N-Glycosylation: Pre-assembly of Oligosaccharide-PPDolichol and en bloc Transfer ...................................... Trimming of the Polypeptide-Bound Oligosaccharide .............. Folding and Quality Control ........................................ Committed Steps in the Formation of Complex-Type Oligosaccharide Chains and Branching ............................. Topology of the Reaction Catalyzed by a Typical GlcNAcT ...... Elongation and Termination Reactions in the truns-Golgi ......... Activity with Branched Substrates ..................................
23.1 23.2 23.3 23.4 23.5 23.6 23.7 23.8
560 560 561 562 562 562 562
565 566 566 567 572 574 574 578 579 582 583
587 589 589 591 592 593 594 596 596 598
Contents
23.9 23.10 23.11 23.12 23.13 23.14 23.15 23.16 23.17 23.18 23.19 23.20 23.21 23.22 23.23 23.24 23.25 23.26 23.27 23.28 23.29 24 24.1 24.2 24.2.1
24.2.2
24.3 24.4
XIX
600 Branch Specificity .................................................... Essential Requirements for Activity with LacNAc ................. 601 Further Terminal Reactions in Complex-Type Oligosaccharide 602 Synthesis ............................................................. Specific Modifications of Polylactosaminoglycans .................. 603 The Invariable Core of N-linked Oligosaccharide Chains, and 606 Site- and Protein-Specific Processing ................................ Comparison of the Synthesis of Type 1 (Gal(P1-3)GlcNAcP-R) 607 and Type 2 (Gal(P1-4)GlcNAcP-R) Chains ........................ The LacdiN Ac Pathway of Complex-Type Oligosaccharide Synthesis ................... ....................................... 607 608 Protein O-Glycosylation ............................................. Glycosyltransferase Families ........................................ 608 610 Sialyltransferase Family ............................................. 611 a2-Fucosyltransferase Family ....................................... 612 a3/4-Fucosyltransferase Family ..................................... a3-Galactosyl/N-Acetylgalactosaminyltransferase(Histo-Blood Group ABO) Family ................................................ 613 P6-N- Acetylglucosaminyltransferase Family ........................ 613 Polypeptide N-Acetylgalactosaminyltransferase Family ............ 614 P4-N-AcetylgalactosaminyltransferaseFamily ...................... 615 P4-Galactosyltransferase Family .................................... 615 P3-Galactosyltransferase Family .................................... 617 P3-Glucuronyltransferase Family ................................... 617 Glycosyltransferases Standing Alone ................................ 617 618 Concluding Remarks ................................................ References............................................................ 618 Synthesis of Sugar Nucleotides ...................................... Reinhold Ohrlein Introduction .......................................................... Synthesis of Sugar Nucleotides ...................................... Chemical Synthesis .................................................. UDP-Activated Donors ............................................. CMP-Activated Sugars.............................................. GDP-Activated Donors ............................................. Comments ............................................................ Chemo-Enzymatic Synthesis ........................................ Uridine Diphosphate-Activated Donor Sugars ..................... CMP-Activated Sugars.............................................. GDP-activated sugars ............................................... Comments ............................................................ In situ Generation of Sugar Nucleotides ............................ Comments ............................................................ Outlook .............................................................. References............................................................
62.5 62.5 626 626 626 629 632 634 635 635 637 639 640 641 641 644 644
XX 25
25.1 25.2 25.2.1 25.2.2 25.2.3 25.2.4 25.2.5 25.2.6 25.2.7 25.3 25.3.1 25.3.2 25.3.3 25.3.4 25.4 25.5
26 26.1 26.2 26.3 26.4 26.4.1 26.4.2 26.4.3 26.4.4 26.4.5 26.5
27
27.1 27.2 27.2.1
Contents
Enzymatic Glycosylations with Glycosyltransferases ................ Ossi Renkonen Introduction ......................................................... In vitro Synthesis of the Core Region of 0.Glycans ................ Initialization of O-Glycan Biosynthesis ............................. Synthesis of Core 1 .................................................. Synthesis of Core 2 .................................................. Synthesis of Core 3 and Core 4 ..................................... In vitro Extension of Core 1 Glycans ............................... In vitro Extension of Core 2 Glycans ............................... Extension of Core 3 and Core 4 Glycans ........................... Enzymatic in vitro Synthesis of Polylactosamine Backbones ....... Enzymatic Synthesis of the Primary Chains of Blood Group i-Type ................................................................ Distal Branching of i-Type Polylactosamine Backbones ........... Central Branching of i-Type Polylactosamine Backbones.......... P4-Galactosylation in Polylactosamine Backbones ................. a3-Sialylation of N-Acetyllactosaminoglycansat the Terminal Gal a3-Fucosylation of Lactosamine Saccharides....................... References ........................................................... Recycling of Sugar Nucleotides in Enzymatic Glycosylation........ Kathryn M . Koeller and Chi-Huey Wong Introduction ......................................................... Glycosyltransferases of the Leloir Pathway and their Sugar Nucleotide Substrates ............................................... Design of Regeneration Systems .................................... Practical Regeneration Systems ..................................... UDP-Galactose ..................................................... Other UDP-Sugars .................................................. CMP-NeuAc ........................................................ GDP-Sugars ......................................................... Other Carbohydrate-Based Regeneration Systems ................. Conclusion ........................................................... References ........................................................... Enzymatic Glycosylations with Non-Natural Donors and Acceptors............................................................. Xiangping Qian. Keiko Sujino. and Monica M . Palcic Introduction ......................................................... Enzymatic Glycosylations ........................................... Galactosylations ..................................................... P 1.4-Galactosyltransferase .......................................... al.3-Galactosyltransferase ..........................................
647 647 648 648 648 649 650 650 651 651 651 652 653 654 656 656 657 659 663 663 663 665 666 666 669 671 676 680 682 683
685 685 686 686 686 688
Contents
27.2.2
27.2.3 27.2.4 27.3
28
28.1 28.2 28.3 28.3.1
28.3.2 28.4 28.4.1
28.4.2
29
29.1 29.2 29.3 29.4 29.5 29.6
XXI
Fucosylations ........................................................ Human Milk a173/4.Fucosyltransferase ............................. FucT I11 and VI ..................................................... FucT V ............................................................... Sialylations ........................................................... a2,3-Sialyltransferase and a2,6.Sialyltransferase .................... N.Acetylglucosaminylation .......................................... N-Acetylglucosaminyltransferase I, 11. and I11...................... N-Acetylglucosaminyltransferase V ................................. Summary ............................................................. Acknowledgments ................................................... References ............................................................
690 690 692 692 692 692 696 696 698 700 700 700
Solid-Phase Synthesis with Glycosyltransferases .................... Claudine AugC. Christine Le Narvor. and Andri Lubineau Introduction .......................................................... General Aspects ...................................................... Enzymatic Synthesis on Insoluble Supports ......................... Enzymatic Synthesis of Oligosaccharides ........................... Use of an Amino-Functionalized Water-Compatible Polyacrylamide Gel .................................................. Use of a Sepharose Matrix .......................................... Use of Controlled-Pore Glass ....................................... Enzymatic Synthesis of Glycopeptides .............................. Use of Controlled-Pore Glass ....................................... Use of Polyethylene Glycol Polyacrylamide (PEGA)............... Enzymatic Synthesis of Oligosaccharides and Glycoconjugates on Soluble Supports ..................................................... Enzymatic Synthesis of Oligosaccharides ........................... Use of Water-Soluble Amino-Substituted Poly(viny1 alcohol) ..... Use of Water-Soluble Glycopolymer Synthesized by Polymerization ....................................................... Enzymatic Synthesis of Glycolipids on Water-Soluble Polyacrylamide-Poly (N-acryloxysuccinimide) (PAN) .............. References ............................................................
705
Glycosidase-Catalysed Oligosaccharide Synthesis ................... David J. Vocadlo and Stephen G. Withers Introduction .......................................................... Background on Glycosidases ........................................ Basic Mechanisms ................................................... Synthesis by the ‘Thermodynamic’ Approach ...................... The Kinetic Approach ............................................... Recent Developments and New Directions ......................... References ............................................................
705 705 707 707 707 708 711 712 712 715 715 715 715 717 718 722 723 723 723 724 724 728 732 838
XXII 30
30.1 30.2 30.2.1 30.2.2 30.2.3 30.2.4 30.3 30.3.1 30.3.2 30.3.3 30.4 30.4.1 30.4.2 30.4.3
Contents
Production of Heterologous Oligosaccharides by Recombinant Bacteria (Recombinant Oligosaccharides) ........................... Roberto A . Geremia and Eric Samain Introduction ......................................................... Concept and Methodology of Heterologous (‘Recombinant’) Oligosaccharide Production in E. coli............................... Biosynthesis of Nod Factors ........................................ Expression Systems and Cloning Strategy .......................... High Cell-Density Cultivation ...................................... Purification of Recombinant Oligosaccharides ..................... Examples of Recombinant Oligosaccharides ....................... Production of Chitin Oligosaccharides in E . coli Expressing NodC ................................................................ Production of Nod Factor Precursors .............................. Production of Derivatives of N-Acetyllactosamine ................. Conclusions and Future Perspectives ............................... Production of Labeled Chitin Oligosaccharides to Study Their Interactions with Proteins ........................................... Improvement of Oligosaccharide Production, and Metabolic Engineering .......................................................... Production of More Complex Oligosaccharides .................... Acknowledgments ................................................... References ...........................................................
845 845 847 847 849 851 852 852 852 853 855 856 856 858 858 859 859
IV
Carbohydrate-Protein Interactions..................................
861
31
Protein-Carbohydrate Interaction: Fundamental Considerations ... Nikki F. Burkhalter. Sarah M . Dimick. and Eric J . Toone Introduction ......................................................... Association in Aqueous Solution ................................... Gas Phase Non Covalent Interactions .............................. Dipole-Dipole Interactions ......................................... Dipole-Induced Dipole ............................................. Dispersive Interactions .............................................. Specific Forces: Hydrogen Bonding and n - ( ~Bonding .............. The Effect of Water on Intermolecular Interactions ................ Coulombic Stabilization ............................................. Hydrogen Bonding .................................................. Dispersive Interactions .............................................. ‘Hydrophobic’ Interactions .......................................... The Evaluation of Protein-Carbohydrate Binding ................. Precipitin Assays ..................................................... Enzyme-Linked Lectin Assay (ELLA).............................. Isothermal Titration Microcalorimetry ............................. The Interpretation of Calorimetric Data ............................
863
31.1 31.2 31.2.1
31.2.2
3 1.2.3 31.3 31.3.1 31.3.2 31.3.3 31.4
863 864 864 864 866 867 868 869 870 871 872 872 876 877 878 878 882
Contents XXIII 31.4.1 3 1.4.2 31.4.3 31.5 31.6 31.6.1 31.6.2 31.6.3
32
32.1 32.2 32.3 32.4 32.5 32.6 32.7 32.8 32.9 32.9.1 32.9.2 32.9.3 32.9.4 32.10 32.10.1 32.10.2
Solvation/Desolvation ............................................... Solvation Entropy ................................................... Translational/Rotational Entropy ................................... Other Contributions to Thermodynamics of Association .......... Proton Transfer ...................................................... Salt Effects/Binding Site Reorganization ........................... van’t Hoff versus Calorimetric Enthalpies .......................... The Thermodynamics of Protein-Carbohydrate lnteraction ....... The Role of Multivalency in Protein-Carbohydrate Interaction ... Phenomenology ...................................................... The Energetic Consequence of Ligand Linkage .................... Enthalpic Contributions to AGi ..................................... Entropic Contributions to AGi ...................................... A Molecular Basis for the Cluster Glycoside Effect ................ Acknowledgments ................................................... References............................................................ Structural Analysis of Oligosaccharides: FAB.MS. ES-MS and MALDI-MS ......................................................... Anne Dell. Howard R. Morris. Richard Easton. Stuart Haslam. Maria Panico. Mark Sutton.Smitlz. Andrew J . Reason. and Kay-Hooi Khoo Introduction .......................................................... Fast Atom Bombardment-Mass Spectrometry (FAB-MS) ......... Matrix Assisted Laser Desorption Ionization-Time of Flight-Mass Spectrometry (MALDI-TOF-MS) .................................. Electrospray-Mass Spectrometry (ES-MS).......................... Appearance of Mass Spectra Obtained in FAB.MS. MALDI-MS and ES-MS Experiments ............................................ Assignment of Mass Values ......................................... Derivatisation ........................................................ Fragmentation Pathways ............................................ Protocols for MS Analysis........................................... Protocol l-Sample Loading for FAB-MS Analysis ............... Protocol 2-Sample Loading for NanoES-MS and MS-MS Analysis on the Q.TOF .............................................. Protocol 3-Sample Loading for LC-ES-MS and LC-ES-MS-MS on the Q.TOF ........................................................ Protocol 4-Sample Loading for MALDI-MS Analysis ........... Applications of FAB.MS. MALDI-MS and ES-MS in Glycobiology ......................................................... Case Study 1-Molecular Weight Profiling of Polysaccharides by MALDI-MS ......................................................... Case Study 2-Analysis of Glycoproteins by LC-ES-MS and FAB-MS .....................................................
882 883 884 885 885 885 886 887 901 901 905 906 907 910 911 911
915
915 915 917 918 919 921 921 922 924 924 925 925 925 926 926
XXIV 32.10.3 32.10.4 32.10.5 32.10.6 32.11 33 33.1 33.2 33.2.1 33.2.2 33.2.3 33.2.4 33.2.5
33.3 33.3.1 33.3.2 33.4 33.4.1 33.4.2 33.5 33.5.1 33.5.2 33.5.3 33.5.4 33.5.5 33.6 34
34.1 34.2 34.3 34.3.1
Contents Case Study 3-Characterization of a Novel N-Glycan by FAB-MS and FAB.MS.MS ......................................... Case Study 4-High Sensitivity Sequencing of a Novel Glycopeptide by Q-TOF ES-MS-MS and MALDI.MS ............ Case Study 5-FAB-MS Screening of Biological Samples for Glycan Content ...................................................... Case Study 6-MS Analysis of Mycobacterial Glycoconjugates . . Concluding Remarks ................................................ References ........................................................... Conformational Analysis in Solution by NMR ...................... S. W. Homans Introduction ......................................................... Solution Conformations of Oligosaccharides ....................... The NMR Technique ................................................ Conformational Parameters in Oligosaccharides ................... Conformational Restraints .......................................... I3C Isotopic Enrichment ............................................ Additional Conformational Restraints .............................. Exchangeable Protons ............................................... Heteronuclear Overhauser Effects................................... 13C- c Coupling-Constants ....................................... Dipolar Couplings ................................................... Experimental Restraints in Conformational Analysis .............. Restraining Protocol ................................................. Biharmonic Restraints ............................................... Time-Dependent Restraints ......................................... Dynamical Simulated Annealing .................................... Analysis of Oligosaccharide Dynamics ............................. Monte-Carlo Simulations ........................................... Molecular Dynamics Simulations ................................... A Case Study on Neu5Aca2-3GalP1-4Glc ......................... Resonance Assignments in Neu5Aca2-3Galfll-4Glc ............... ROE Connectivities ................................................. ‘Global Minimum’ Conformation of Neu5Aca2-3GalP1-4Glc .... Conformational Dynamics of Neu5Aca2-3Gal~1-4Glc............ Short-range vs Long-range Restraints .............................. Conclusions .......................................................... References ........................................................... Oligosaccharide Conformations by Diffraction Methods ............ Serge Pdrez. Catherine Gautier. and Anne Imberty Introduction ......................................................... General Analysis .................................................... Crystalline Conformations of Disaccharide Moieties .............. The Disaccharides ...................................................
930 933 935 942 944 945 947 947 947 947 948 949 949 950 950 952 953 954 955 955 955 957 957 958 959 959 959 960 960 961 962 963 966 966 969 969 970 973 973
Contents 34.3.2 34.4 34.5 34.6 34.7 34.8
35 35.1 35.2 35.3 35.4 35.5 35.6 35.7 35.8 36
36.1 36.2 36.3 36.4 36.5 36.6 36.7 36.8 36.9
37 37.1 37.1.1 37.1.2 37.2
XXV
The Analogs (S. C. N. ....). ......................................... Hydrogen Bonding in Crystalline Oligosaccharides ................ Packing Features ..................................................... Selected Examples ................................................... Crystalline Conformations of Oligosaccharides Complexed with Lectins................................................................ Concluding Remarks ................................................ References ............................................................
985 987 988 990 992 996 998
Transfer NOE Experiments for the Study of Carbohydrate-Protein Interactions ........................................................... 1003 Thomas Peters Introduction .......................................................... 1003 1004 The Transfer NOE Experiment ..................................... 1006 Measurement of trNOEs .......................................... Bioactive Conformations of Carbohydrate Ligands From trNOE Experiments .......................................................... 1008 Spin Diffusion may Generate Misleading Distance Constraints ... 1009 The Conformation of Sialyl Lewis' Bound to E-selectin ...........1011 Interaction of Bacterial Lipopolysaccharide Fragments with Monoclonal Antibodies .............................................. 1016 Conclusions and Future Directions ................................. 1019 References ............................................................ 1021 Carbohydrate-Protein Interactions: Use of the Laser Photo Chemically Induced Dynamic Nuclear Polarization(C1DNP)-NMR Technique ............................................................ 1025 Hans-Christian Siebert and Johannes F. G. Vliegenthart Introduction .......................................................... 1025 The CIDNP Method ................................................. 1026 CIDNP-related Molecular Modelling ............................... 1027 Applications .......................................................... 1027 Hevein-like Lectins .................................................. 1029 Galactoside-binding Lectins from Plant and Animal Origin ....... 1032 Sialidase from Clostridium Perfringens (Wild Type and Mutants) . 1037 CIDNP Analysis of Glycoproteins .................................. 1039 Conclusions .......................................................... 1040 Acknowledgments ................................................... 1041 1042 References ............................................................ Biacore ............................................................... 1045 Wolfgang Jager Introduction .......................................................... 1045 Real-time Analysis by Surface Plasmon Resonance ................1045 Information in a Sensorgram ........................................ 1047 Experimental Procedures ............................................ 1048
XXVI
Contents
37.2.1 31.2.2 37.2.3 37.2.4 37.2.5 37.3 37.3.1 37.3.2 37.3.3
Immobilization of Biomolecules at the Sensor Surface ............. 1048 Surface Regeneration ................................................ 1050 Interaction Analysis and Controls .................................. 1051 1052 Determination of Kinetic Rate Constants .......................... Affinity Determination .............................................. 1053 1054 Application Areas ................................................... Selectin Binding to a Glycoprotein Ligand ......................... 1054 Oligosaccharide Characterization ................................... 1055 In situ Modification of Immobilized Carbohydrates ................1056 References ........................................................... 1056
V
Carbohydrate-Carbohydrate Interactions ...........................
38
Carbohydrate-Carbohydrate Interactions ........................... 061 Dorothe Spillmann and Max M . Burger Introduction ......................................................... 1061 From Structural Components to Cell Recognition ................. 1063 Carbohydrate-Carbohydrate Interactions as Part of Structural Components ......................................................... 1063 Extracellular Matrix of Seaweeds-Agarose, Carrageenan and 1063 Alginate .............................................................. Cell Walls ............................................................ 1064 Mammalian Extracellular Matrix Components .................... 1066 Carbohydrate-Carbohydrate Interactions as Part of Recognition 1068 Keys? ................................................................. Carbohydrate Interactions in Invertebrates-The Marine Sponge Microciona prolifera as a Model System ............................ 1069 Carbohydrate Interactions in Vertebrates-Embryonal and Tumor 1071 Cells .................................................................. Repulsive Carbohydrate-Carbohydrate Interactions ............... 1072 Molecular Aspects of Carbohydrate Interactions ...................1074 1074 Polyvalence to Inforce Weak Interactions .......................... Arrangement of Motifs and the Possibility to Control Specificity . 1075 Molecular Basis of Carbohydrate-Carbohydrate Interactions ..... 1076 Experimental Approaches ........................................... 1078 General Considerations ............................................. 1078 Affinity Interactions ................................................. 1079 1079 Cell Binding Studies ................................................. Aggregation of de n o w Complexes ................................. 1081 Affinity Chromatography ........................................... 1082 Distribution between Compartments ............................... 1082 Microscopy .......................................................... 1083 Electron Microscopy ................................................ 1083 Atomic Force Microscopy .......................................... 1083 1084 Crystallography ......................................................
38.1 38.2 38.2.1
38.2.2
38.3 38.3.1 38.3.2 38.3.3 38.4 38.4.1 38.4.2
38.4.3 38.4.4
059
Contents XXVII
38.4.5 38.4.6 38.4.7 38.4.8
Mass Spectrometry .................................................. Nuclear Magnetic Resonance ....................................... Molecular Modelling ................................................ Tools ................................................................. Synthetic Oligosaccharides .......................................... Antibodies against Carbohydrate Motifs ........................... Cells .................................................................. References ............................................................
1085 1085 1086 1086 1086 1087 1088 1088
V1
Carbohydrate-Nucleic Acid Interactions ............................
1093
39
Carbohydrate-Nucleic Acid Interactions ............................ Heinz E . Moser Introduction .......................................................... Carbohydrates Binding to DNA .................................... Ene-Diyne Antibiotics and Antitumor Agents ...................... Esperamicins ......................................................... Calicheamicins ....................................................... Anthracyclins ........................................................ Pluramycins and Aureolic Acids .................................... Carbohydrates Binding to RNA .................................... Aminoglycosides ..................................................... References ............................................................
1095
39.1 39.2 39.2.1 39.2.2 39.2.3 39.3 39.3.1
Index ...................................................................
Part I1
Biology of Saccharides
Vol . 3
Biosynthesis and Degradation of Glycoconjugates Introduction to Volumes 3 and 4 ....................................... Abbreviations Used in Volumes 3 and 4 .............................
1095 1096 1096 1096 1100 1106 1111 1112 1113 1120 11
V LV
I
Biosynthesis of Glycoconjugates .......................................
1
1
Metabolism of Sugars and Sugar Nucleotides ......................... Hudson H . Freeze Introduction ............................................................ Basic Principles ........................................................ Transporters Deliver Monosaccharides to Cells ...................... Intracellular Sources of Sugars ........................................ Salvage .................................................................
3
1.1 1.2 1.3 1.4 1.4.1
3 3 4 5 5
XXVIII Contents 1.4.2
1.5 1.6 1.7 2
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10
3
3.1 3.2 3.3 3.4 3.5
Activation and Interconversion of Monosaccharides................. Glycogen ............................................................... Glucose ................................................................ Glucuronic acid ........................................................ Iduronic acid ........................................................... Xylose .................................................................. Mannose ............................................................... Fucose .................i ............................................... Galactose ............................................................. N-Acetylglucosamine................................................. N-Acetylgalactosamine............................................... Sialic acids ............................................................ Sugar Nucleotide Transporters ....................................... Control of Sugar Nucleotide Levels ................................. Possible Future Directions ........................................... References ............................................................
6 6 7 8 8 8 8 9 10 10 10 11 11 13 13 14
Nucleotide Sugar Transporters ....................................... Rita Gerardy-Schahn and Matthias Eckhardt Introduction .......................................................... General Considerations .............................................. The Requirement for Nucleotide Sugar Transporters and Their Mechanism of Function: A Comprehensive Overview of the Last 20 Years .............................................................. Molecular Cloning of Nucleotide Sugar Transporters .............. The Structure of Nucleotide Sugar Transporters .................... The Subcelluar Distribution of Nucleotide Sugar Transporters ..... Molecular Defects that Cause Inactive UDP-Galactose and CMP-Sialic Acid Transporters ....................................... Association Between Defects in Nucleotide Sugar Transporters and Diseases ............................................................... Involvement of Nucleotide Sugar Transporters in the Regulation of Glycosylation ......................................................... Future Perspectives ................................................... Acknowledgements ................................................... References ............................................................
19
Biosynthesis of Oligosaccharyl Dolichol .............................. Sharon S. Krag General Overview .................................................... Oligosaccharyl Dolichol .............................................. Key Enzymatic Steps in the Assembly Process ...................... Topology of the Assembly Process................................... Utilization of Oligosaccharyl Dolichol .............................. Acknowledgment ..................................................... References ............................................................
19 20
20 22 25 27 28 29 29 30 31 32 37 37 38 39 42 42 43 43
Contents XXIX 4
4.1 4.2 4.2.1 4.2.2 4.2.3 4.2.4 4.3 4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.4.5 4.4.6 4.4.7 4.4.8 4.5
5
5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8
6
6.1 6.2
Biochemistry and Molecular Biology of the N-Oligosaccharyltransferase Complex................................................... Roland Knauer and Ludwig Lehle Introduction ........................................................... Biochemistry of OST ................................................. Lipid-Saccharide Donor ............................................. Acceptor Specificity of OST .......................................... Catalytic Mechanism of OST ........................................ Regulation of OST Activity .......................................... Isolation of OST Complexes from Different Sources ................ Molecular Biology of OST ........................................... WBPl/OST48 ......................................................... SWPl/Ribophorin I1 ................................................. OSTl/Ribophorin I ................................................... OST3/OST6 ........................................................... OST5 .................................................................. OST4 .................................................................. OST2/DAD1 .......................................................... STT3 .................................................................. Structural Organization of the OST Complex ....................... Acknowledgments .................................................... References ............................................................. Processing Enzymes Involved in the Deglucosylation of N-Linked Oligosaccharides of Glycoproteins: Glucosidases I and 11 and Endomannosidase ..................................................... Robert G. Spiro Introduction ........................................................... Glucosidase I .......................................................... Glucosidase 11......................................................... Endo-a-mannosidase .................................................. Concerted Action of Deglucosylation Enzymes ...................... Mutants ............................................................... Role of Monoglucosylated N-Linked Oligosaccharides and Glucose Trimming Enzymes in Regulating Quality Control of Glycoproteins ......................................................... Effect of Glucosidase Inhibitors on Viral Proliferation .............. Acknowledgments .................................................... References............................................................. a-Mannosidases in Asparagine-linked Oligosaccharide Processing and Catabolism ........................................................ Kelley W. Moremen Overview .............................................................. Introduction ...........................................................
45 45 46 47 48 49 51 51 52 54 54 55 55 56 56 57 58 59 60 60
65 65 66 68 70 72 74
75 77 78 78
81 81 82
XXX 6.2.1 6.2.2 6.2.3 6.2.4 6.2.5 6.3 6.3.1 6.3.2
6.3.3
6.4
7 7.1 7.2 7.3 7.4 7.5
8 8.1 8.2
Contents
Roles of N - and O-Linked Glycans and Compartmentalization of Biosynthetic and Catabolic Reactions ............................... 82 82 Processing of Asn-Linked Oligosaccharides ......................... Early Trimming Events: importance for quality control glycoprotein degradation and anteriograde transport ..................... 85 Glycoprotein Catabolism: multiple routes for glycoprotein 87 breakdown ............................................................ Consequences of Genetic Defects in Oligosaccharide Biosynthesis and Catabolism ....................................................... 88 Mannosidases in Glycoprotein Processing and Catabolism ......... 89 89 Classification of Mannosidases ....................................... Class 1 Mannosidases: enzymes of the ER and Golgi ............... 93 ER mannosidase I subfamily ......................................... 93 95 Golgi mannosidase I sub-family ..................................... Fungal secreted mannosidases ....................................... 97 New genes with unknown functions ................................. 98 Class 2 Mannosidases: enzymes of the cytosol, ER, Golgi, and Lysosomes ............................................................ 98 99 Golgi mannosidase I1................................................. Lysosomal mannosidase ............................................. 101 Epididymal/sperm mannosidase .................................... 103 Heterogeneous cluster of mannosidase homologs among eukary a, 104 eubacteria, and archaea ............................................. Conclusions and Future Prospects .................................. 106 107 Acknowledgments ................................................... 107 References ........................................................... The Role of UDP-Glcyglycoprotein Glucosyltransferase as a Sensor of Glycoprotein Conformations ...................................... Armando J. Parodi Introduction ......................................................... General Properties ................................................... GT Recognizes Glycoprotein Conformations ...................... The Primary Structure of the UDP-Glcyglycoprotein Glucosyltransferase .................................................. The Role of Monoglucosylated Oligosaccharides in Glycoprotein Folding ............................................................... Acknowledgments ................................................... References ........................................................... Mannosyltransferases................................................ Peter Orlean Introduction ......................................................... Occurrence of Covalently-linked Mannose .........................
119 119 120 121 122 123 126 127 129 129 130
Contents XXXI
8.2.1 8.2.2 8.2.3 8.2.4 8.3 8.3.1 8.3.2 8.3.3 8.3.4 8.4 8.4.1
8.4.2
8.4.3 8.4.4 8.5 8.6
9
9.1 9.2 9.3 9.3.1 9.3.2 9.4 9.5 9.6
Eukaryotic Secretory Glycoproteins ................................ Glycophospholipids .................................................. Eubacterial and Archaeal Mannose-containing Molecules ......... C-linked Mannose ................................................... Biochemistry of Mannosyl Transfer ................................. Many Linkages, Two Donors ....................................... Donor Specificity .................................................... Acceptor Specificity.................................................. Structural Features of Man-T ....................................... Man-T Families and the Pathways They Participate in ............ Man-Ts of the ER [l-51 ............................................. Alglp ................................................................. Alg2p/Algl l p ........................................................ Dpmlp ............................................................... Alg3p ................................................................. Alg9p/PIG-Bp family ................................................ Pmtlp family ......................................................... Golgi Man-Ts and Fungal Mannan Synthesis ...................... Ochlp family ......................................................... ................................. Mnn9p family ........... MnnlOp/Mnnl l p family ............................................ Mnnlp family ........................................................ Ktrlp family ......................................................... “Missing” Eukaryotic Man-T ....................................... Eubacterial and Archaeal Man-T ................................... Coordinating Man Transfer with the Cell Cycle and Morphogenesis ....................................................... Concluding Remarks ................................................ Acknowledgments ................................................... References............................................................
130 130 130 130 131 131 131 132 132 133 134 134 134 135 135 135 136 136 137 137 138 138 138 138 139
Branching of N-Glycans: N-Acetylglucosaminyltransferases........ Hurry Schuchter Introduction .......................................................... Processing of N-Glycans within the Endomembrane Assembly Line .................................................................. General Properties of the N-Acetylglucosaminyltransferases....... Domain Structure .................................................... Targeting to the Golgi Apparatus ................................... UDP-GlcNAc:Manal-3R [GlcNAc to Manwl-31 p.1,2. N . Acetylglucosaminyltransferase I (GnT I, EC 2.4.1.101) ............ UDP-GlcNAc:Manal-6R [GlcNAc to Manal-61 p.1,2. N . Acetylglucosaminyltransferase 11 (GnT 11, E.C. 2.4.1.143). ........ The Role of GnT I and I1 in Mammalian Development ...........
145
139 140 140 140
145 146 148 148 150 150 152 153
XXXII 9.7 9.7.1 9.7.2 9.8 9.9 9.9.1 9.10 9.11
10 10.1 10.2 10.2.1 10.3 10.3.1 10.3.2 10.3.3 10.3.4 10.3.5 10.4 10.4.1 10.4.2 10.4.3 10.4.4 10.5 10.5.1 10.5.2 10.5.3 10.5.4
Contents UDP-G1cNAc:RI-Manal-6[GlcNAc~l-2Manul-3]Man~l-4R~ [GlcNAc to ManP 1-41 P- 1,4. N.Acetylglucosaminyltransferase 111 155 (GnT 111. E.C. 2.4.1.144). ........................................... 156 Overexpression of GnT 111 Activity ................................. 157 GnT 111 Activity and Cancer ........................................ UDP-GlcNAc:R1Manal-3R2 [GlcNAc to Manal-31 (3.1,4. N . Acetylglucosaminyltransferase IV (GnT IV, E.C. 2.4.1.145). ...... 157 UDP-GlcNAc:Rl Manal-6R2 [GlcNAc to Manal-61 P-1,6-NAcetylglucosaminyltransferase V (GnT V, E.C.2.4.1.155) ......... 158 159 GnT V Activity and Cancer ......................................... UDP-GlcNAc:Rl (R2)Manal-6R3 [GlcNAc to Manul-61 P-1, 4-Nl -Acetylglucosaminyltransferase VI (GnT VI) ............. 161 161 GnT VII and GnT VIII ............................................. 162 References ........................................................... The Galactosyltransferases .......................................... Nancy L . Shaper. Martin Charron. Neng- Wen Lo. Jane R. Scocca. and Joel H . Shuper Introduction ......................................................... Using the Databanks to Obtain Information on the Galactosyltransferases ............................................... Nomenclature ........................................................ The Dual Role of P4-Galactosyltransferase-I (P4GalT-I) in Oligosaccharide and Lactose Biosynthesis: The Early Days ....... p4GalT-I: Isolation and Characterization of cDNA Clones ....... The Murine NGalT-I Gene: Genomic Organization and Structure of the 5'-End ......................................................... P4GalT-I and Lactose Biosynthesis ................................. P4GalT-I and the Vertebrate P4GalT Gene Family ................ Evolution of the 04-Galactosyltransferase Gene Family ........... The Vertebrate P3Galactosyltransferase (P3GalT) Gene Family . . General Characteristics of the P3-Galactosyltransferase Gene Family Members .................................................... P3GalT-IV: UDP-ga1actose:GM 1 P3-galactosyltransferase (GM1 Synthase; GalT-3) ................................................... Other Vertebrate P-Galactosyltransferase Activities................ UDP-Ga1actose:Ceramide P-Galactosyltransferase (CGalT; EC 2.4.1.45). ............................................................. The Vertebrate u3-Galactosyltransferase Gene Family ............ u3-Galactosyltransferase (a3GalT: UDP-Gal:Galp4GlcNAca3Galactosyltransferase; EC 2.4.1.87) ................................. The Blood Group B a3-Galactosyltransferase (EC 2.4.1.37). ...... The Forssman Glycolipid Synthetase (EC 2.4.1.88). ............... Evolution of the u3GalT Gene Family .............................
175 175 177 177 178 181 181 182 182 184 185 186 187 187 187 188 188 190 191 191
Contents XXXIII
10.6
A UDP-Gal:Gal@3GalNAca4Galactosyltransferase Activity ..... 192 192 Acknowledgments ................................................... References............................................................ 192
11
Fucosyltransferases ................................................... 197 Ernest0 T. A . Marques. Jr. Introduction .......................................................... 197 198 General Characteristics .............................................. Nomenclature ........................................................ 198 199 Gene Structure ....................................................... Sequence Peptide Motifs ............................................. 199 Specificity ............................................................ 199 200 Protein Structure and Topology ..................................... Enzymatic Reaction Mechanism .................................... 201 203 Inhibitors ............................................................. 203 Specific Fucosyltransferases ......................................... GDP-Fucose: Fucal (Fucal,2Fuc)a2-fucosyltransferase ............ 204 GDP-Fucose: GalPl (Fucal,2Gal)a2-fucosyltransferase ............ 204 GDP-Fucose: Gal~l,4/3GlcNAc(Fucal,3/4GlcNAc)a3/ 4204 fucosyltransferases ................................................... 204 Blood group Lewis: FucT 111, V and VI ............................ Myeloid enzyme: FucT IV ........................................... 205 Leukocyte enzyme: FucT VII ....................................... 206 206 Neuronal enzyme: FucT IX ......................................... GDP-Fucose: Gal~l,3GlcNAc(Fuca1, 3GlcNAc) bacterial (Helicobacter pylori) a3-fucosyltransferase ......................... 207 GDP-Fucose: GlcNAc-N(Fuca1,6GlcNAc)a6fucosyltransferases . 207 GDP-Fucose: O-Ser(Fuca1-O-Ser)GlcNAc polypeptide 207 fucosyltransferases ................................................... Unconventional Types of Fucosylation: FucPl -P-Ser and cytoplasmic Fucal,2-GalP1,3-GlcNAc-Pro (Dictyostelium discoideum) 207 FucP 1-P-Ser .......................................................... 207 208 Fucal,2-GalP1,3-GlcNAc-Pro ...................................... Acknowledgments ................................................... 208 208 References............................................................
11.1 11.2 11.2.1 11.2.2 11.2.3 11.2.4 11.2.5 11.2.6 11.2.7 11.3 11.3.1 11.3.2 11.3.3
11.3.4 11.3.5 11.3.6 11.3.7
12
12.1 12.2 12.3 12.4 12.5 12.6 12.7
Sialyltransferases..................................................... Joseph T. Y. Lau and Sherry A . Wuensch Introduction .......................................................... General Features of Sialyltransferases .............................. Cloning and ldentification Strategies for Sialyltransferases ........ Sialyltransferase Classification and Nomenclature .................. The a2, 3-ST Family ................................................. The 1x2,6-ST Family ................................................. The 1x2.8.ST Family .................................................
213 213 213 215 216 216 217 218
XXXIV Contents 12.8
Regulation and Functionality of Sialyltransferases................. 219 221 References ...........................................................
13
Biochemistry of Sialic Acid Diversity................................ Roland Schauer Introduction ......................................................... Occurrence and Biosynthesis ........................................ General Biological Functions ....................................... N-Glycolylneuraminic Acid ......................................... 0-Acetylated Sialic Acids ........................................... 0-Methylated and 0-Sulfated Sialic Acids ......................... Acknowledgments ................................................... References ...........................................................
13.1 13.2 13.3 13.4 13.5 13.6
14 14.1 14.2 14.3 14.4 14.5 14.6 14.7
15
15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 15.9 15.10
Carbohydrate Sulfotransferases...................................... Steven D. Rosen. Annette Bistrup. and Stefan Hemmerich Introduction ......................................................... Basic Features of Sulfotransferase Reactions ....................... Tyrosine Sulfation ................................................... Diversity of Carbohydrate Sulfation ................................ Biochemical Demonstration of Carbohydrate Sulfotransferases ... Molecular Cloning of Carbohydrate Sulfotransferases ............. Primary Structures of Carbohydrate Sulfotransferases ............. Acknowledgments ................................................... References ........................................................... Novel Variant Pathways in Complex-type Oligosaccharide Synthesis ............................................................. Dirk H . van den Eijnden Introduction ......................................................... The lacNAc Pathway of Complex-type Oligosaccharide Synthesis Occurrence and Biology of 1acdiNAc-based Complex-type Oligosaccharides ..................................................... Biosynthesis of lacdiNAc Backbone Units .......................... The lacdiNAc Pathway of Complex-type Oligosaccharide Synthesis ............................................................. Other Shared Properties of 04-GalT and 04-GalNAcT ............ Cloning of a snail UDP-G1cNAc:GlcNAcP-R 04-Nacetylglucosaminyltransferase ....................................... The Chitobio Pathway of Complex-type Oligosaccharide Synthesis ............................................................. Competition Between Pathways ..................................... Concluding Remarks ................................................ References ...........................................................
227 227 227 229 231 234 238 239 239 245 245 245 246 246 249 250 252 256 256
261 261 261 262 263 264 266 266 267 267 269 269
Contents XXXV 16
16.1 16.2 16.3 16.4 16.4.1 16.4.2
16.5 16.6 16.7
17
17.1 17.2 17.2.1 17.2.2 17.2.3 17.2.4 17.2.5 17.3
Control of Mucin-Type 0-Glycosylation: 0-Glycan Occupancy is Directed by Substrate Specificities of Polypeptide GalNAcTransferases .......................................................... 273 Helle Hassan. Eric P. Bennett. Ulla Mandel. Michael A . Hollingsworth. and Henvik Cluusen Introduction .......................................................... 273 The Mammalian UDP-GalNAc: Polypeptide GalNAc-Transferase Gene Family ......................................................... 274 The GalNAc-Transferase Gene Family is Evolutionarily Old ..... 276 The Kinetic Properties of GalNAc-Transferase Isoforms are Different.............................................................. 278 Lessons from in vivo Analysis of GalNAc-transferase Substrate Specificities ........................................................... 219 Lessons from in vitro Analysis of the Acceptor Substrate Specificities of GalNAc-transferase Isoforms ....................... 280 Isoforms may have distinct acceptor substrate specificities ......... 281 Isoforms may have overlapping substrate specificities .............. 283 Isoforms may act in different order on substrates with multiple 283 acceptor sites ......................................................... Isoforms may require prior (GalNAc) glycosylation ............... 283 Expression of the GalNAc-Transferase Genes are Differentially Regulated ............................................................ 285 288 Predictive Value of in vitro 0-glycosylation? ....................... Conclusions and Future Perspectives................................ 288 References ............................................................ 289 Glycosyltransferase Inhibitors ........................................ Xianyping Qian and Monica M . Palcic Introduction .......................................................... Inhibitors of Glycosyltransferases................................... Inhibitors of Galactosyltransferases ................................. Inhibitors of pl, 4-galactosyltransferase ............................. Inhibitors of cr.l,3-galactosyltransferase............................. Inhibitors of Fucosyltransferases .................................... Inhibitors of 1x1,2-fucosyltransferases ............................... Inhibitors of a1,3-fucosyltransferases ............................... Inhibitors of Sialyltransferases ...................................... Inhibitors of a2, 6-sialyltransferase .................................. Inhibitors of a2, 3.sialyltransferase .................................. Inhibitors of N-Acetylglucosaminyltransferases .................... Inhibitors of Human Blood Group A and B Glycosyltransferases. Summary ............................................................. Acknowledgments ................................................... References............................................................
293 293 296 296 296 297 298 300 300 301 302 304 305 306 309 309 309
XXXVI Contents 18
18.1 18.2 18.3 18.4 18.5 18.6 18.7 18.8 18.9 18.10 18.11 18.12
19
19.1 19.2 19.3 19.4 19.5 19.6 19.7
20
20.1 20.2 20.3 20.3.1 20.3.2 20.3.3 20.3.4 20.3.5 20.4 20.4.1 20.4.2 20.4.3 20.5
Biosynthesis of the O-Glycan Chains of Mucins and Mucin Type Glycoproteins ........................................................ Inka Brockhausen Summary ............................................................. Introduction ......................................................... Structures of O-Glycans ............................................. Functions of Much Type O-Glycans ............................... Primary O-Glycosylation ............................................ Synthesis of O-Glycan Core 1....................................... Synthesis of O-Glycan Core 2 ....................................... Synthesis of O-Glycan Core 3 ....................................... Synthesis of O-Glycan Core 4 ....................................... Synthesis of O-Glycan Cores 5-8 ................................... Elongation and Branching Reactions ............................... Synthesis of Terminal Structures .................................... Acknowledgments ................................................... References ........................................................... Glycosyltransferases in Glycosphingolipid Biosynthesis ............. Subhash Basu. Kamal Das. and Manju Basu Introduction ......................................................... Fucosyltransferases in Glycolipid Biosynthesis ..................... Galactosyltransferases in Glycolipid Biosynthesis .................. N-Acetylgalactosaminyltransferases in Glycolipid Biosynthesis.... N-Acetylglucosaminyltransferases in Glycolipid Biosynthesis...... Sialyltransferases in Glycolipid Biosynthesis ....................... Glucuronyltransferases in Glycolipid Biosynthesis ................. Acknowledgments ................................................... References ........................................................... Biosynthesis of Glycogen ............................................ Peter J . Roach Summary ............................................................. Introduction ......................................................... Glycogenin and the Initiation of Glycogen Synthesis .............. History ............................................................... Properties ............................................................ Reaction Mechanism ................................................ Domain Structure ................................................... Function ............................................................. Glycogen Synthase and the Bulk Synthesis of Glycogen ........... Properties ............................................................ Structure of Glycogen Synthase ..................................... Branching Enzyme .................................................. Intermediates in the Biosynthesis of Glycogen .....................
313 313 313 314 314 315 315 317 319 319 319 320 321 324 324 329 329 329 332 334 336 337 340 342 342 349 349 350 351 351 351 352 352 354 354 354 355 356 357
Contents XXXVII 20.6
Conclusion ........................................................... Acknowledgments ................................................... References ............................................................
358 359 359
21
Biosynthesis of Hyaluronan .......................................... Paraskevi Heldin and Torvard C. Laurent Introduction .......................................................... Site of Biosynthesis .................................................. Biosynthetic Precursors .............................................. Hyaluronan Synthases ............................................... Microbial Enzymes .................................................. Vertebrate Synthases ................................................. Mechanism of Synthesis ............................................. Chain Elongation .................................................... Translocation ........................................................ Shedding ............................................................. Regulation of HA Synthesis ......................................... Concluding Remarks ................................................ Acknowledgments ................................................... References ............................................................
363
21.1 21.2 21.3 21.4 21.4.1 21.4.2 21.5 21.5.1 21.5.2 21.5.3 21.6 21.7
22
22.1 22.2 22.2.1 22.2.2 22.2.3 22.2.4 22.3 22.3.1 22.3.2 22.3.3
22.3.4
Biosynthesis of Chondroitin Sulfate and Dermatan Sulfate Proteoglycans ........................................................ Geetha Sugumaran and Barbara A4. Vertel Introduction .......................................................... Proteoglycan Structure .............................................. Proteoglycans and Their Core Proteins ............................. What Initiates GAG Chain Addition? .............................. The Linkage Region ................................................. .................................... CS and DS Chains ........... Biosynthesis of CS and DS Proteoglycans .......................... Biosynthesis of the Core Protein .................................... Origin of Sugar and Sulfate Precursors ............................. Addition of the Linkage Oligosaccharides .......................... Xylosylation .......................................................... Galactosylation ...................................................... Addition of GlcA and completion of the common tetrasaccharide linkage region ........................................................ Initiation of CS/DS chains by addition of the first GalNAc ....... Formation of the CS/DS Chains .................................... Addition of the repeating disaccharides ............................. Epimerization of GlcA to IdoA to form DS ........................ Sulfation of GalNAc................................................. Sulfation of uronic acid ..............................................
363 364 364 365 365 366 367 368 369 369 370 371 372 372
375 375 379 379 381 381 382 383 383 384 385 385 386 387 388 388 388 389 390 391
XXXVIII Contents 22.4
Concluding Remarks/Perspectives .................................. Acknowledgments ................................................... References ...........................................................
391 392 392
23
Biosynthesis of Heparin and Heparan Sulfate Proteoglycans ....... Lena Kjellin and U r Lindahl Introduction ......................................................... The Proteoglycans: Structure. Location and Functions ............ Biosynthesis of the Polysaccharide Backbone ...................... Outline of Polymer-Modification Reactions ........................ The N.Deacetylase/N.Sulfotransferases ............................. The C5-Epimerase ................................................... The 2-O-Sulfotransferase............................................ The 6-O-Sulfotransferases........................................... The 3 - 0 Sulfotransferases........................................... The Products. Heparin and Heparan Sulfate ....................... Interactions with Proteins ........................................... Regulation of HS Biosynthesis ...................................... References ...........................................................
395
23.1 23.2 23.3 23.4 23.4.1 23.4.2 23.4.3 23.4.4 23.4.5 23.5 23.6 23.7
24 24.1 24.2 24.2.1 24.2.2 24.2.3 24.2.4 24.3 24.3.1 24.3.2 24.3.3 24.3.4 24.4 24.5
25
25.1 25.2 25.2.1
395 396 396 397 399 399 399 400 400 400 401 402 403
Biosynthesis of Proteoglycans with Keratan Sulfates................ James L . Funderburgh Introduction: Keratan Sulfate Renaissance ......................... Keratan Sulfate Structure and Distribution ........................ Corneal KS .......................................................... Non-corneal KSI .................................................... KSII .................................................................. KSIII ................................................................. Keratan Sulfate Proteoglycans ...................................... SLRPs ................................................................ Aggrecan ............................................................. Cell-Associated KS .................................................. Brain ................................................................. Enzymatic Reactions of KS Biosynthesis ........................... Metabolic Control of KS Synthesis ................................. Acknowledgments ................................................... References ...........................................................
407
The Biosynthesis of GPI Anchors .................................... Yasu S. Morita, Alvaro Acosta.Serrano. and Paul T. Englund Introduction ......................................................... Structure of GPI Anchors ........................................... Glycan Core Modifications .........................................
417
407 407 408 409 409 410 410 410 411 411 412 412 413 414 414
417 417 417
Contents XXXIX 25.2.2 25.3 25.3.1 25.3.2 25.3.3 25.3.4 25.3.5 25.3.6 25.3.7 25.3.8 25.4 25.4.1 25.4.2 25.4.3 25.5 25.6
Variations in Anchor Lipid Structure ............................... GPl Precursor Synthesis ............................................. GlcNAc-PI Synthesis ................................................ GlcNAc-PI Deacetylation ........................................... Inositol Acylation .................................................... GPI Mannosylation .................................................. Transfer of EtN-P ................................................... Lipid Remodeling.................................................... Addition of Carbohydrate Side Chains ............................. Topology of Biosynthetic Pathways ................................. Attachment of the GPI Precursor to a Protein ..................... Basic Features ....................................................... Protein Machinery for GPI Addition ............................... Signal Sequence for GPI Addition. ................................. Evolution of GPI Biosynthesis ...................................... Future Studies ....................................................... Acknowledgments ................................................... References............................................................
419 419 420 421 421 422 423 423 424 424 425 425 426 426 426 427 427 427
26
435
26.1 26.2 26.3 26.3.1 26.3.2 26.3.3 26.3.4 26.3.5 26.3.6 26.4 26.5
Escherichiu coli Lipid A: A Potent Activator of Innate Immunity ... Teresa A . Garrett and Christian R . H . Raetz Introduction .......................................................... Structure of Lipopolysaccharide..................................... Lipid A Biosynthesis in E. coli ...................................... Acylation of UDP-GlcNAc ......................................... Disaccharide Formation ............................................. Phosphorylation by the Lipid A 4' Kinase .......................... Kdo Addition and the Late Acyltransferases ....................... Other Acyltransferases............................................... Transport of Lipid A and the Role of MsbA ....................... Lipid A Activation of Signal Transduction in Animal Cells ....... Summary ............................................................. Acknowledgments ................................................... References............................................................
435 435 437 439 440 440 441 442 442 444 447 447 447
11
Glycosidases ...........................................................
453
27
Lysosomal Degradation of Glycolipids............................... 455 Thomas Kolter and Konrad Sandhof Summary ............................................................. 455 Introduction .......................................................... 455 Mechanisms of Lysosonial Glycolipid Degradation ................ 456 Glycosidases ......................................................... 456
27.1 27.2 27.3 27.3.1
XL 27.3.2 27.3.3 27.3.4 27.3.5 27.3.6 27.4 27.4.1 27.4.2 27.4.3 27.4.4 27.4.5 27.4.6 27.4.7 27.5 27.5.1 27.5.2 27.6
28 28.1 28.2 28.3 28.4 28.4.1 28.4.2 28.4.3 28.5
28.5.1 28.5.2 28.5.3 28.6
29 29.1 29.2
Contents Topology of Endocytosis and Lysosomal Glycolipid Degradation Sphingolipid Activator Proteins ..................................... The GM2-activator and its role in lysosomal digestion ............ SAP-A to SAP-D .................................................... Lateral Pressure ...................................................... Lipid Composition .................................................. Membrane Curvature ............................................... Degradation of Selected Lipids ....... ........................... Ganglioside GM2 ................................................... Lactosylceramide .................................................... Glucosylceramide .................................................... Ceramide ............................................................. Sphingomyelin ....................................................... Sulfatide ............................................................. Galactosylceramide .................................................. Pathobiochemistry ................................................... Animal Models for Sphingolipidoses ............................... Therapy .............................................................. Future Directions .................................................... References ...........................................................
457 458 459 460 460 461 462 462 462 464 464 465 465 465 466 466 467 469 470 470
Lysosomal Degradation of Glycoproteins ........................... Nathan N . Aronson. Jr. Summary ............................................................. Introduction ......................................................... Roles of Lysosomes ................................................. Lysosomal Degradation of N-Linked Glycoproteins ............... General Features .................................................... Carbohydrate Digestion ............................................. Protein and Linkage Hydrolysis .................................... Formation of Thyroid Hormone via Lysosomal Degradation of Thyroglobulin ....................................................... Synthesis of Thyroid Hormone ..................................... Carbohydrate Degradation .......................................... Proteolysis ........................................................... Degradation of Free Polymannose-Type Oligosaccharides Derived from N-Linked Glycoproteins During Biosynthesis ................ References ...........................................................
473
Sialidases............................................................. Garry Taylor. Susan Crennell. Carl Thompson. and Marina Chuenkova Abstract .............................................................. Introduction .........................................................
473 473 474 475 475 476 476 477 477 478 479 479 481 485
485 485
Contents
XLI
29.3 29.4 29.5 29.6 29.7 29.8 29.9
Influenza Virus Neuraminidase ...................................... Paramyxovirus Hemagglutinin-Neuraminidase (HN) .............. Non-Viral Sialidases ................................................. Small Sialidases ...................................................... Large Sialidases ...................................................... T . cruzi Trans-Sialidase (TS) ........................................ Conclusion ........................................................... Acknowledgments ................................................... References ............................................................
486 487 487 490 491 491 493 494 494
30
Microbial Glycosidases............................................... Kenji Yamamoto. Su-Chen Li. and Yu-Teh L i Exo-Glycosidases .................................................... u-Glucosidase ........................................................ P-Glucosidase ........................................................ a-Galactosidase ...................................................... P-Galactosidase ...................................................... u.Mannosidase ....................................................... 0.Mannosidase ....................................................... P-N-Acetylhexosaminidase .......................................... u.N.Acetylga1actosaminidase ........................................ u-L-Fucosidase ....................................................... P.D.Fucosidase ....................................................... Sialidase .............................................................. KDNase .............................................................. u-L-Rhamnosidase ................................................... p-Xylosidase ......................................................... Endo.Glycosidases ...................................................
497
30.1 30.1.1 30.1.2 30.1.3 30.1.4 30.1.5 30.1.6 30.1.7 30.1.8 30.1.9 30.1.10 30.1.11 30.1.12 30.1.13 30.1.14 30.2 30.2.1 30.2.2 30.2.3 30.2.4 30.2.5 31
31.1 31.2 31.3 31.4 31.5 31.6 31.7 31.8
Endo.P.N.acetylg1ucosaminidase .................................... Peptide-N-glycanase F ............................................... Endo.u.N.acetylga1actosaminidase .................................. Endo-P-galactosidase ................................................ Endoglycoceramidase ................................................ References. ........................................................... Glycoprotein Processing Inhibitors ................................... Magid Osser and Alan D . Elbein Introduction ........................................................... Structural Classification ............................................. Distribution of Glycosidase Inhibitors in the Plant Kingdom ...... Isolation and Structural Determination ............................. Glycosidase Inhibitory Activity ..................................... Structure-Activity Relationships .................................... N-Linked Oligosaccharide Processing ............................... Inhibitors of N-Linked Oligosaccharide Processing.................
497 497 498 498 499 500 500 501 501 502 503 503 504 504 505 505 505 506 506 507 507 508 513 513 515 515 516 517 518 519 522
XLII 31.8.1 31.8.2 31.9
Contents Glucosidase Inhibitors ............................................... Mannosidase Inhibitors ............................................. Summary and Future Prospects ..................................... References ...........................................................
522 525 528 529
Index ...................................................................
11
Part I1
Biology of Saccharides
Vol. 4
Lectins and Saccharides Biology
Ill
Lectins ...............................................................
533
32
Plant Lectins ......................................................... Marilynn E. Etzler Summary ............................................................. Introduction ......................................................... Carbohydrate Specificity ............................................ Other Activities ...................................................... Structure ............................................................. Biological Roles ..................................................... Acknowledgments ................................................... References ...........................................................
535
32.1 32.2 32.3 32.4 32.5 32.6
33 33.1 33.2 33.3 33.4 33.5 33.6 33.7 33.8
34
34.1 34.2 34.3
Interactions of Oligosaccharides and Glycopeptides with Hepatic Carbohydrate Receptors ............................................. Yuan C. Lee and Reiko T. Lee Summary ............................................................. Introduction ......................................................... Molecular Characteristics of Hepatic Lectins ...................... Cellular Aspects of HL .............................................. Binding Specificity................................................... Photoaffinity Labeling ............................................... Subunit Organization on Rat Hepatocyte Surface ................. Applications ......................................................... References ........................................................... P-Type Lectins and Lysosomal Enzyme Trafficking ................ Patricia G. Marron-Teradu and Nancy M . Dahms Introduction ......................................................... Intracellular Trafficking of the MPRs .............................. Primary Structure and Biosynthesis of the MPRs ..................
535 535 536 539 540 543 546 547
549 549 550 551 552 553 557 558 559 560 563 563 564 566
Contents
XLIII
34.3.1 34.3.2 34.4 34.5 34.5.1 34.5.2 34.6
CI-MPR .............................................................. CD-MPR ............................................................. Lysosomal Enzyme Recognition by the MPRs ..................... Structural Determinants of Man-6-P Recognition .................. Expression of Mutant Forms of the MPRs ......................... Crystal Structure of the CD-MPR .................................. Conclusions .......................................................... Acknowledgments ................................................... References............................................................
35
The Siglec Family of I-Type Lectins ................................. 579 Paul R. Crocker and Soerye Kelm Introduction .......................................................... 579 The Immunoglobulin Superfamily and Carbohydrate Recognition 579 Siglecs as a Family of Sialic Acid Binding Proteins ................ 580 Biology of Siglecs .................................................... 581 Sialic Acids in Cellular Recognition ................................ 583 Mode of Carbohydrate Recognition by Siglecs ..................... 584 Importance of Multivalent Binding ................................. 588 Sialic Acid Recognition by the Immunoglobulin FoldEvolutionary Considerations ........................................ 588 Role of cis Interactions in Modulating Adhesion to Other Cells in trans ............................................................... 589 Sialoadhesin as a Macrophage Adhesion Molecule................. 590 Signalling Versus Adhesion Mediated by Siglecs ................... 591 Conclusions .......................................................... 592 Acknowledgments ................................................... 592 References............................................................ 592
35.1 35.2 35.3 35.4 35.5 35.6 35.7 35.8 35.9 35.10 35.11 35.12
36
36.1 36.2 36.2.1 36.2.2 36.3 36.3.1 36.3.2 36.3.3 36.3.4 36.3.5 36.3.6 36.3.7 36.4
C-Type Lectins and Collectins ....................................... Russell Wallis Summary ............................................................. Structure and Function of C-Type Animal Lectins ................. The Carbohydrate-Recognition Domain ............................ Ligand Binding ...................................................... Mannose-Binding Protein and Collectins ........................... Domain Organization ............................................... MBPs as Prototype Collectins ....................................... Ligand Binding by Serum MBP ..................................... MBP and Innate Immunity .......................................... Liver MBP ........................................................... Pulmonary Surfactant Proteins ...................................... Conglutinin and CL-43 .............................................. Conclusions ..........................................................
566 567 569 571 571 572 574 574 575
597 597 598 599 600 601 601 603 603 604 607 608 608 609
XLIV
Contents
Acknowledgments ................................................... References ...........................................................
609 609
Selectins .............................................................. Rodger P. McEver Introduction ......................................................... Structure of Selectins ................................................ Selectin Ligands ..................................................... Requirements for Selectins to Mediate Tethering and Rolling of Leukocytes under Hydrodynamic Flow ............................ Functions of Selectins and their Ligands in vivo ................... Conclusions .......................................................... References ...........................................................
613
Galectins ............................................................. Douglas N . W. Cooper and Samuel H . Barondes Introduction ......................................................... Galectin Structure ................................................... Novel Candidate Galectins .......................................... Unorthodox Subcellular Targeting .................................. Regulation of Galectin Expression .................................. Galectin Binding Specificity and Identified Ligands ................ Physiological Functions ............................................. Summary ............................................................. References ...........................................................
625 625 626 631 635 637 639 640 642 642
IV
Saccharide Biology ...................................................
649
39
Structures and Functions of Nuclear and Cytoplasmic Glycoproteins Robert S. Haltiwanger Introduction ......................................................... O-Linked N-Acetylglucosamine (0.GlcNAc) ....................... O-GlcNAc Appears to be a Regulatory Modification much like Phosphorylation ..................................................... Modulation of Protein Stability and Function by O-GlcNAc ..... Other Forms of Nuclear and Cytoplasmic Glycosylation .......... Unique Cytoplasmic Forms of Glycosylation ...................... Conventional Forms of Glycosylation in the Nucleus and Cytoplasm ........................................................... Nuclear and Cytoplasmic Lectins ................................... Conclusions .......................................................... Acknowledgments ................................................... References ...........................................................
65 1
37 37.1 37.2 37.3 37.4 37.5 37.6
38
38.1 38.2 38.3 38.4 38.5 38.6 38.7 38.8
39.1 39.2 39.2.1 39.2.2 39.3 39.3.1 39.3.2 39.3.3 39.4
613 613 614 619 621 621 622
651 652 653 655 658 658 660 661 662 662 662
Contents 40
40.1 40.2 40.3 40.3.1 40.3.2 40.3.3 40.4 40.5 40.6 40.7 40.7.1
40.7.2 40.7.3 40.7.4 40.7.5 40.8
41
41.1 41.2 41.3 41.3.1 41.3.2 41.3.3 41.3.4 41.3.5 41.4 41.4.1 41.4.2
XLV
Structure and Functions of Mucins .................................. 669 Joyce Taylor-Papadimitriou and Joy M . Burchell Classification of Mucins ............................................. 669 670 The Epithelial Mucins ............................................... Mucin Type O-Glycosylation Pathways ............................ 670 671 Initiation of O-Glycosylation ........................................ Chain Extension ..................................................... 671 Chain Termination ................................................... 671 Expression of Epithelial Mucins ..................................... 672 The Complex Gel-Forming Mucins: Processing and Function ..... 672 674 Epithelial Membrane Mucins ....................................... Studies Related to the MUCl Much ............................... 675 Changes in the Patterns of O-Glycosylation of MUCl in Breast 675 Cancer ................................................................ Differences in sites of glycosylation ................................. 675 Changes in the composition of O-glycans added to MUCl in 676 Breast Cancer ........................................................ Correlation in changes of Glycosyltransferase activities with changes in O-glycan structure in Breast Cancer .................... 676 Changes in Glycosylation of MUCl in other Cancers ............. 677 Effects of MUCl Expression on the Behavioral Properties of Cancer Cells.......................................................... 677 Effects on cell interactions and tumourogenicity .................... 677 678 MUCl Expression and Immune Responses ......................... Active Specific Immunotherapy Based on MUCl .................. 679 679 Animal models ....................................................... 680 Clinical studies ....................................................... Comments ............................................................ 681 References............................................................ 681 Biological Roles of Hyaluronan...................................... Bryan P . Toole Introduction .......................................................... Hyaluronan is a Biopolymer with Unusual Physical Properties .... Hyaluronan Binds to Several Types of Proteins (Hyaladherins) ... General Properties of Hyaladherins ................................. Structural Hyaluronan-Binding Proteins ............................ Hyaluronan Receptors ............................................... Intracellular Hyaluronan-Binding Proteins ......................... Inter-a-Trypsin Inhibitor ............................................ Hyaluronan-Dependent Pericellular Matrices Assemble Around Several Cell Types ................................................... Hyaluronan-Dependent Cellular “Coats” .......................... Assembly of Chondrocyte Pericellular Matrix ......................
685 685 685 687 687 688 688 689 689 690 690 691
XLVI 41.4.3 41.5 41.5.1 41.5.2 41.5.3 41.5.4 41.5.5 41.6
42 42.1 42.2 42.3 42.4 42.5 42.5.1 42.6 42.6.1 42.6.2 42.7 42.7.1 42.7.2 42.8 42.8.1 42.8.2 42.8.3 42.9 42.9.1 42.9.2 42.9.3 42.10
43 43.1 43.2
Contents
Tethering of Cell Surface Hyaluronan to Hyaluronan Synthase ... Hyaluronan Influences Cell Behavior During Morphogenesis and Tissue Remodeling .................................................. Migratory and Proliferating Cells are Surrounded by Hyaluronanenriched Matrices .................................................... Hydrated Pericellular Milieux Provide Cellular Pathways ......... Receptors Mediate Effects of Hyaluronan .......................... Hyaluronan-Cell Interactions in Limb Development ............... Hyaluronan-Cell Interactions in Other Physiological and Developmental Systems ............................................. Hyaluronan Plays a Crucial Role in Cancer ........................ References ...........................................................
691 693 693 693 693 694 695 696 696
Biological Roles of Heparan Sulfate Proteoglycans................. 701 Ofer Reizes. Pyong Woo Park. and Merton BernJeld Introduction ......................................................... 701 Heparan Sulfate Biosynthesis ....................................... 701 Functions of Heparan Sulfate ....................................... 702 703 Proteoglycans ........................................................ Intracellular Proteoglycans .......................................... 703 Serglycin and Heparin ............................................... 704 Cell Surface Heparan Sulfate Proteoglycans ........................ 705 Syndecans ............................................................ 705 Glypicans ............................................................ 706 Part-time Cell Surface Heparan Sulfate Proteoglycans ............. 707 Betaglycan ........................................................... 707 CD44 ................................................................. 708 Functions of Cell Surface Heparan Sulfate Proteoglycans ......... 708 Ligand Receptors .................................................... 708 709 Ligand Coreceptors .................................................. Shed Effectors ....................................................... 709 Extracellular Matrix Heparan Sulfate Proteoglycans and Their 710 Functions ............................................................ 710 Perlecan .............................................................. Agrin ................................................................. 712 Other Extracellular HSPGs ......................................... 713 713 Conclusions .......................................................... References ........................................................... 713 Biological Roles of Keratan Sulfate Proteoglycans.................. 717 Gary W. Conrad Introduction ......................................................... 717 718 Corneal Transparency ...............................................
Contents XLVII 43.3 43.4 43.5
Nerve Growth Cone Guidance ...................................... Cell Adhesion ........................................................ Other Possible Roles of KSPGs ..................................... Acknowledgment .................................................... References ............................................................
44
Developmental and Aging Changes of Chondroitin/Dermatan Sulfate Proteoglycans ........................................................ 729 J . Michael Sorrell. David A . Carrino. and Arnold I. Caplan Proteoglycans ........................................................ 729 Glycosaminoglycans ................................................. 729 Core Proteins ........................................................ 731 Hyalectans ........................................................... 731 Small Leucine-rich Proteoglycans ................................... 733 Chondrotin/Dennatan Sulfate Proteoglycans in Development and Aging ................................................................. 735 Core Proteins in Development. Aging. and Pathologies ............ 735 Chondroitin/Dermatan Sulfate Glycosaminoglycan Chains in Development. Aging. and Pathologies .............................. 736 Summary ............................................................. 740 References ............................................................ 740
44.1 44.2 44.3 44.3.1 44.3.2 44.4 44.4.1 44.4.2 44.5
45
45.1 45.2 45.3 45.4 45.5 45.6 45.7
46 46.1 46.2 46.3 46.4 46.5 46.6
719 721 722 723 723
Proteoglycans and Hyaluronan in Vascular Disease ................. Thomas N . Wight Introduction .......................................................... Proteoglycans and Hyaluronan ...................................... Versican (CSPGs) .................................................... Hyaluronan .......................................................... Decorin/Biglycan (DSPGs) .......................................... Perlecan/Syndecans (HSPGs)........................................ Summary ............................................................. Acknowledgments ................................................... References ............................................................
743
Functions of Glycosyl Phosphatidylinositols ......................... Nikola A . Baumann. Anant K. Menon. and David M . Rancour Introduction ........................................................... Parasite Coats: Extreme GPI-Anchoring ............................ Yeast GPIs and the Cell Wall ....................................... Paroxysmal Nocturnal Hemoglobinuria (PNH): Disease and Defects in GPI-Anchoring of Proteins .............................. GPIs in the Secretory and Endocytic Pathways ..................... Organization of GPI Proteins in the Plasma Membrane ...........
757
743 744 745 747 748 749 750 750 750
757 758 758 759 760 762
XLVIII
Contents
46.7 46.8 46.9 46.10 46.11
Association of GPI-Anchored Proteins with Caveolae ............. Detergent Insolubility and Signaling via GPI-Proteins ............. Membrane Release of GPI-Anchored Proteins ..................... GPIs as Second Messenger Signaling Molecules ................... Summary ............................................................. Acknowledgments ................................................... References ...........................................................
47
Glycosphingolipid Microdomains in Signal Transduction. Cancer. 771 and Development ..................................................... Sen-itiroh Hakomori and Kuzuko Handu Clustered GSLs as Functional Units ................................ 771 GSL Clusters. Associated with Signal Transducers. are Functional Units Separable from Caveolae ..................................... 772 Cell Adhesion Coupled with Signal Transduction Initiated by GSL Microdomain: Concept of Glycosignaling Domain (GSD) ........ 773 Role of GSLs in Control of Growth Factor and Hormone Receptors: Possible Relationship with GSL Microdomain ......... 774 Functional Role of Developmentally-Regulated and Tumor776 Associated GSLs ..................................................... References ........................................................... 778
47.1 47.2 47.3 47.4 47.5
48
48.1 48.2 48.3 48.3.1 48.3.2 48.4 48.5 48.6 48.7 48.8 48.9 48.10 48.10.1 48.10.2 48.10.3 48.10.4 48.11 48.12
The Primary Cell Walls of Higher Plants ........................... Jocelyn K. C. Rose, Mulcolm A . O’Neill, Peter Albersheim. and Alan Darvill Introduction (What is a Cell Wall?)................................. Purification of Cell Walls and Isolation of Wall Components ..... The Structural Components of the Primary Cell Wall ............. Cell Walls and the Diversity of Flowering Plants .................. The Structural Components of the Primary Wall .................. Biosynthesis of Wall Components .................................. Organization of the Plant Primary Cell Wall ....................... Cellulose-Xyloglucan Interactions .................................. Interactions Between Pectins and Other Cell Wall Components ... Glycoproteins in the Cell Wall ...................................... Heterogeneity in the Primary Cell Wall ............................ Function and Metabolism of Plant Primary Cell Walls ............ Mechanical Support ................................................. Regulation of Cell Expansion ....................................... Morphogenesis and Differentiation ................................. Plant Cell Wall Oligosaccharides in Defense and Cell Signalling .. Intercellular Transport and Storage ................................. Biotechnology and Future Directions in the Commercial Applications of Plant Primary Cell Walls ..........................
764 764 765 766 767 767 768
783 783 784 786 786 786 791 793 793 794 796 797 798 798 798 800 801 803 803
Contents
49
49.1 49.2 49.3 49.4 49.5 49.6
50
50.1 50.2 50.2.1 50.2.2 50.2.3 50.2.4 50.3 50.3.1 50.3.2 50.3.3
51
51.1 51.2 51.2.1 51.2.2 51.3 51.3.1 51.4
XLIX
Acknowledgment .................................................... References ............................................................
804 804
Glycolipids and Bacterial Pathogenesis .............................. ClifSord A . Lingwood Introduction .......................................................... Modulation of Glycolipid Receptor Function ...................... Stress Response and Glycolipid Receptors .......................... Subcellular Gb3 Trafficking ......................................... Model for Lipid Sorting Based on Chain Length ................... Glycosphingolipids and Signal Transduction ....................... Acknowledgments ................................................... References............................................................
809
Glycobiology of Viruses .............................................. Hildegard Geyer and Rudolf Geyer Summary ............................................................. General Aspects ...................................................... Functions of Viral Surface Glycoproteins ........................... Biosynthesis .......................................................... Function of Carbohydrate Substituents ............................. Oligosaccharide Diversity ........................................... Examples ............................................................. Friend Murine Leukemia Virus Complex ........................... Marburg Virus (MBGV) ............................................ Hepatitis B Virus (HBV) ............................................ Acknowledgments ................................................... References ............................................................
821
809 810 812 813 815 815 817 817
821 821 825 826 826 827 830 830 832 833 836 836
The Glycobiology of Influenza Viruses............................... 839 Stephen J. Stray and Gillian M . Air Introduction .......................................................... 839 Receptor Binding Proteins: Influenza A HAg and Influenza C HEF .................................................................. 840 Structure of Receptor Binding Domain and Mechanism of 843 Sialic Acid Recognition .............................................. HEF Esterase Domain and Mechanism of Cleavage ............... 844 844 Influenza NAm (types A and B) .................................... Mechanism of Sialic Acid Cleavage ................................. 845 Function of Viral Receptor Destroying Enzymes ................... 847 847 Acknowledgments ................................................... 848 References............................................................
L 52
52.1 52.2 52.3 52.4 52.5 52.6 52.7 52.7.1 52.7.2 52.7.3 52.7.4 52.7.5 52.8 52.9
53
53.1 53.2 53.3 53.3.1 53.3.2 53.3.3 53.4 53.4.1 53.4.2 53.4.3 53.4.4 53.4.5 53.4.6 53.4.7 53.5 53.6
Contents Glycobiology of Aids ................................................. Ten Feizi Abstract .............................................................. Introduction ......................................................... The Repertoire of N-Glycans on the Envelope Glycoprotein of HIV of Human Immunodeficiency Virus Produced in Different Cell Types............................................................ Evidence for the Occurrence of O-Glycans on the Envelope Glycoproteins of HIV-1 Produced in Certain Cell Lines ........... Oligosaccharides of gp 120 and gp 41 at N-Glycosylation Sites and Their Possible Influence on Viral Infectivity ................... gp 120 Glycosylation Can Influence Antigenicity and .. Immunogenicity ..................................................... Saccharides Recognized by Carbohydrate-binding Proteins and Antibodies as Potential Neutralization Epitopes on the Envelope Glycoprotein of HIV-1 .............................................. Lectins and Antibodies with Mannose-related Specificities ........ Antibodies to O-Glycan Sequences ................................. Antibodies to Blood Group A ...................................... Xeno-antibodies to Gala1-3Gal Sequence .......................... Potential Medical Relevance ........................................ Does Viral Oligosaccharide Display Influence Tissue Tropism? ... Concluding Remarks ................................................ Acknowledgment .................................................... References ...........................................................
851
Glycobiology of Protozoan and Helminthic Parasites ............... Richard D . Cummings. and A . Kwame Nyame Introduction ......................................................... General Classification of Parasites .................................. The Major Protozoan Parasites ..................................... Malaria .............................................................. . . ..................................................... Trypanosomiasis Leishmaniasis........................................................ Other Protozoan Parasites........................................... Entamoeba histolytica ............................................... Acanthamoeba ....................................................... Giardia lumblia ...................................................... Cryotosporidium parvum ............................................. Sarcocystis spp ....................................................... Toxoplusmu gondii., ................................................. Pneumocystis carinii ................................................. Helminthic Parasites ................................................. Carbohydrate-Binding Proteins in Parasitic Helminths ............
867
851 851 853 854 855 856 857 857 858 858 859 859 860 862 862 863
867 867 868 868 873 874 878 818 878 878 878 879 879 879 879 883
Contents 53.7 53.8
Unusual Glycans in Other Helminthic Parasites .................... Future Directions .................................................... Acknowledgments ................................................... References ............................................................
54
The Involvement of the Oligosaccharide Chains of Glycoproteins in Gamete Interactions at Fertilization ................................. Noritaka Hirohashi and William J . Lennarz Introduction .......................................................... Advantages of Marine Invertebrates as an Experimental System . . Induction of the Acrosome Reaction ................................ Studies in Sea Urchins ............................................... Studies in Starfish .................................................... Sperm-Egg Coat Binding ........................................... Studies in Mammals ................................................. Studies in Frog ....................................................... Studies in Ascidians .................................................. Studies in Sea Urchins ......... ................................... Carbohydrate as a Species-Specific Determinant ................... References ............................................................
54.1 54.2 54.3 54.3.1 54.3.2 54.4 54.4.1 54.4.2 54.4.3 54.4.4 54.5
55
55.1 55.2 55.3 55.4 55.4.1 55.4.2 55.5 55.6 55.6.1 55.6.2 55.6.3 55.7
56
56.1 56.2 56.3
LI 883 885 885 886
895 895 895 896 896 899 899 900 900 902 904 906 907
Glycosylation and Development ...................................... 909 Michde Aubery and Christian Derappe Summary ............................................................. 909 Introduction .......................................................... 911 Lectins as Tools to Analyze Changes in Cell-surface 911 Glycoconjugates During Development .............................. Cell-adhesion Molecules ............................................. 912 Neural Cell-adhesion Molecule ...................................... 912 The Adhesion Molecule L1 .......................................... 914 Glycosyltransferases ................................................. 914 Altered Expression of Endogenous Lectins During Development . 915 Galectins ............................................................. 915 Selectins .............................................................. 917 Other Endogenous Lectins .......................................... 917 Conclusion ........................................................... 918 References ............................................................ 918 Protein Glycosylation and Cancer ................................... James W. Dennis and Maria Granovskjj Introduction .......................................................... Protein Glycosylation Generates Molecular Diversity .............. Cancer Initiation and Progression ...................................
923 923 923 926
LII
Contents
56.4 56.5 56.6 56.7 56.8 56.9
Tumor Cell Proliferation ............................................ Cell Migration ....................................................... Sialylation and Metastasis........................................... Endogenous Lectins and Tumor Cell Adhesion .................... Carbohydrate Processing Inhibitors as Anti-Cancer Agents ....... Other Considerations ................................................ Acknowledgments ................................................... References ...........................................................
927 930 933 934 935 936 937 937
57
Lysosomal Storage Diseases ......................................... Nathan N . Aronson. Jr. Summary............................................................. Introduction ......................................................... Animal Models ...................................................... Mucopolysaccharidoses ............................................. Cathepsin K Deficiency and Pycnodysostosis ...................... Mouse Models for Tay-Sachs and Sandhoff Diseases .............. Impact of Lysosomal Diseases and Their Study .................... References ...........................................................
945
Genetic Diseases of Glycosylation ................................... Tomoya Akama and Michiko N . Fukuda Introduction ......................................................... CDGS ................................................................ CDGS Type I ........................................................ CDGS Type I1 ....................................................... HEMPAS ............................................................ References ...........................................................
959
57.1 57.2 57.3 57.4 57.5 57.6 57.7
58
58.1 58.2 58.2.1 58.2.2 58.3
59
59.1 59.2 59.3 59.3.1 59.3.2 59.3.3 59.3.4 59.3.5 59.3.6 59.4 59.5
Glycobiology of Helicobacter pylori and Gastric Disease ........... Karl-Anders Karlsson Introduction ......................................................... The Bacterial Surface and Molecular Mimicry ..................... Host Surfaces and H . pylori Recognition of Glycoconjugates: Unique Complexity.................................................. Sialic Acid ........................................................... Sulfatide ............................................................. Heparan Sulfate ..................................................... Fucose-Dependent Binding (H-1 and Lewis b) ..................... Gangliotetraosylceramide ........................................... Lactosylceramide .................................................... The Meaning of Multiple Binding Specificities ..................... Aspects for the Future ............................................... References ...........................................................
945 946 947 947 949 951 953 954
959 959 959 961 963 964 967 967 968 968 969 970 970 971 971 972 972 973 973
Contents
60 60.1 60.2 60.3 60.4 60.5 60.6 60.7 60.7.1 60.7.2 60.8 60.9 60.10 60.1 1 60.12 60.13 60.14 60.15
61
61.1 61.2 61.3 61.4 61.5
62 62.1 62.2 62.3 62.3.1
LIII
Immunoglobulin G Glycosylation and Galactosyltransferase Changes in Rheumatoid Arthritis .................................... 977 John S. Axford Introduction .......................................................... 977 977 Oligosaccharide Synthesis ........................................... Galactosyltransferase ................................................ 978 978 Immunoglobulin G .................................................. Rheumatoid Arthritis ................................................ 979 979 Quantification of IgG sugars in RAr ................................ RAr and Pregnancy .................................................. 980 980 Galactosylation of IgG .............................................. a3-Fucosylation of a1-Acid Glycoprotein .......................... 981 Agalactosyl-IgG and Rheumatoid Factor Binding ................. 982 Tissue-specific Galactosyltransferase Abnormalities in an Experimental Model of Rheumatoid Arthritis ...................... 983 Glycosylation Homeostasis within RAr Lymphocytes is Abnormal ............................................................ 985 Are the Rheumatoid Arthritis Associated Glycosylation Abnormalities Unique? .............................................. 986 Sugar Printing Rheumatic Disease is Possible ...................... 990 Rapid Profiling of IgG N-Glycans by Fluorophore-coupled Oligosaccharide Electrophoresis has the Potential of Differentiating 992 Rheumatic Diseases.................................................. In What Way could GTase Enzymatic Control be Abnormal? . . . . 992 Conclusion ........................................................... 993 References ............................................................ 994
Calnexin. Calreticulin and Glycoprotein Folding Within the Endoplasmic Reticulum .............................................. 997 Michael R. Leach and David B. Williams Structure and Properties of Calnexin and Calreticulin ............. 997 1000 Biological Functions ............................................... Mechanism of Action .............................................. 1002 Functional Relationship Between Calnexin and Calreticulin ..... 1005 Relationship with other ER Chaperones and Folding Catalysts . 1007 1008 References .......................................................... Glycobiology of The Nervous System .............................. Ronald L . Schnaar Introduction ........................................................ Nervous System Glycoconjugates-Overview .................... Nervous System Glycolipids ....................................... Galactosylceramides ...............................................
1013 1013 1013 1014 1014
LIV
Contents
62.3.2 62.4 62.4.1 62.4.2 62.5 62.6 62.6.1 62.6.2 62.7
Gangliosides and Related Anionic Glycosphingolipids ........... Nervous System Glycoproteins ..................................... Polysialic Acid ...................................................... The HNK-I Determinant .......................................... Nervous System Glycosaminoglycans ............................. Lectins in the Brain ................................................. Myelin-Associated Glycoprotein ................................... Other Nervous System Lectins ..................................... Concluding Remarks ............................................... References ..........................................................
1016 1019 1020 1020 1020 1021 1021 1022 1023 1023
63
Glycobiology of the Immune System ............................... Elizabeth F. Hounsell Infection and Pathogenesis ......................................... Control of the Immune Response .................................. Bacterial and Tumor Antigens. Mucins and Mucin-like Molecules ........................................................... Immunoglobulins and Pathology .................................. References ..........................................................
1029
63.1 63.2 63.3 63.4
64
64.1 64.2 64.2.1 64.2.2 64.2.3 64.3 64.3.1
64.3.2
Metabolic Engineering Glycosylation: Biotechnology’s Challenge to the Glycobiologist in the Next Millennium ...................... Thomas G. Warner Introduction ........................................................ Recent Developments in Carbohydrate Biosynthesis.............. Optimizing Sialylation of Recombinant Proteins by Metabolic Engineering Sialic Acid Biosynthesis............................... Optimizing Galactosylation of Recombinant Proteins by Metabolically Engineering Galactose Biosynthesis ................ Mannose Biosynthesis and Mannosylation of Recombinant Proteins ............................................................. Glycosylation Engineering Alternate Expression Hosts For Recombinant Protein Therapeutic Production .................... Engineering Glycosylation of Recombinant Proteins Expressed in Baculovirus-Insect Cells ......................................... Genes needed to supplement glycosylation of recombinant proteins in insect cells .............................................. Deleterious genes may need to be deleted or inhibited to enhance recombinant glycoprotein biosynthesis in insect cells.............. Engineering Glycosylation of Recombinant Proteins Expressed in Plants ............................................................. Genetic addition and supplementation needed to improve plant recombinant protein glycosylation ................................. Inhibition or deletion of plant glycosylation genes ................
1029 1033 1034 1036 1038
1043 1043 1044 1044 1049 1052 1053 1053 1054 1054 1056 1058 1059
Contents
64.4
Summary ........................................................... Acknowledgments ................................................. References ..........................................................
Index ...................................................................
LV
1059 1060 1060 11
List of Contributors (only main authors are listed)
Gillian Air Dept. of Biochemistry and Molecular Biology University of Oklahoma, Health Center BMSB 840, P. 0. Box 26901 Oklahoma City, OK 73190 USA Nathan N . Aronson Department of Biochemistry University of South Alabama MSB 2152 Mobile, AL 36688 USA Michele Aubery Ul80-INSERM U180 45 Rue Des Saints Peres 75006 Paris France Claudine Auge Laboratoire de Chimie Organique Multifonctionelle DR2 CNRS 462 Universite de Paris-Sud Inst. de Chimie Moleculaire (ICMO) 91405 Orsay Cedex France John S. Axford St. George’s Hospital Med. Ctr. Cranmer Terrace London SW17 ORE England
Samuel H. Barondes Langley Porter Psych. Inst. 401 Parnassus Avenue San Francisco, CA 94143-0984 USA Subhash Basu University of Notre Dame Stephan Chemistry Hall, Rm 443 Notre Dame, IN 46556 USA Jean-Marie Beau Univ. Paris-Sud Lab. de Synthese de Biomolecules URA CNRS 462, Inst. de Chimie 91405 Orsay France Merton Bernfield Harvard Medical School Enders Building, Room 961 300 Longwood Avenue Boston, MA 021 15 USA Mikael Bols Department of Organic Chemistry Aarhus University Langelandsgade 140 8000 Aarhus C Denmark
LVIII
List of Contributors
Inka Brockhausen Department of Medicine Queen’s University Etherington Hall, room 1021 Kingston, Ontario K7L 3N6 Canada Max Burger Friedrich Miescher Institut 4002 Basel Switzerland Arnold I. Caplan Case-Western Reserve University Cleveland, OH 44106 USA Henrik Clausen Department of Oral Diagnostics University of Copenhagen Norre Alle 20 DK-2200 Copenhagen N. Denmark Gary W. Conrad Kansas State University Ackert Hall Manhattan, KS 55606 USA Paul Crocker Department of Biochemistry The Welcome Trust Building University of Dundee Dundee, DDl 5HN Scotland Richard D. Cummings Department of Biochemistry and Molecular Genetics University of Oklahoma HSC 975 NE lofhSt., BRC 417 Oklahoma, OK 73104 USA Nancy Dahms Medical College of Wisconsin P. 0. Box 26509 8701 Watertown Plank Road Milwaukee, WI 53226-0509 USA Samuel J. Danishefsky Department of Chemistry Columbia University Havemeyer Hall
New York, NY 10027 and Laboratory for Bioorganic Chemistry The Sloan-Kettering Institute for Cancer Research 1275 York Avenue New York, NY 10021 USA Alan Darvill University of Georgia 220 Riverbend Road Athens, GA 30602 USA James W. Dennis Div. of Cancer and Cell Biology Mount Sinai Hosp., Res. Inst. 600 University Ave. Toronto, ON M5G 1x5 Canada Anne Dell Wolfson Laboratories Department of Biochemistry Imperial College of Science London SW7 2AY England Hugues Driguez CERMAV-CNRS B. P. 53 38041 Grenoble cedex France Dirk H. van den Eijnden Department of Medicinal Chemistry Vrije Universiteit Van der Boechorststraat 7 1081 BT Amsterdam The Netherlands Alan D. Elbein Dep. of Biochemistry and Molecular Biology University of Arkansas, Slot 516 4301 W. Markham Street Little Rock, AR 72205-7199 USA Paul Englund Johns Hopkins University School of Medicine 725 North Wolfe St. Baltimore, MD 21205 USA
List of Contributors Beat Ernst Institute of Molecular Pharmacy Pharmacenter Universitat Basel Klingenbergstrasse 50 4051 Basel Switzerland Mail:
[email protected] Marilynn E. Etzler University of California Davis, CA 95616 USA Ten Feizi Clinical Research Centre Watford Road Harrow, MDX HA1 3UJ England Bert Fraser-Reid Natural Products and Glycotechnology Research Institute Inc. 41 18 Swarthmore Road Durham, North Carolina 27707 USA Hudson H. Freeze The Burnham Institute 10901 N. Torrey Pines Road La Jolla, CA 92093 USA Michiko Fukuda Burnham Institute (formerly La Jolla Cancer Res. Fndm.) 10901 North Torrey Pines Road La Jolla, CA 92037 USA James L. Funderburgh Kansas State University Ackert Hall Manhattan, KS 66506 USA Robert M. Garbaccio Laboratory for Bioorganic Chemistry The Sloan-Kettering Institute for Cancer Research 1275 York Avenue New York, NY 10021 USA
Teresa Garrett Duke University Medical Center c/o Dr. C. Raetz P. 0. Box 3711 Durham, N C 27710 England Rita Gerardy-Schahn Institut fur Medizinische Mikrobiologie Medizinische Hochschule Hannover Carl-Neuberg-Strak 1 30625 Hannover Germany Roberto A. Geremia Centre de Recherches sur les M acromolkcules Vegttales Universite Joseph Fourier Cermav-CNRS, BP 53 38041 Grenoble Cedex 9 France Rudolph Geyer Biochemisches Institut Universitat Giessen Friedrichstr. 24 35392 Giessen Germany Gary R. Gray Department of Chemistry Institute of Technology University of Minnesota 207 Pleasant Street S. E. Minneapolis, MN 55455-0431 USA Sen-Itiroh Hakomori Pacific Northwest Research Foundation 720 Broadway Seattle, WA 98122 USA Robert S. Haltiwanger State University of New York Stony Brook, NY 11794-5215 USA Gerald W. Hart Department of Biological Chemistry John Hopkins University 401 Hunterian Building 725 N. Wolfe Street Baltimore, M D 21205-2185 USA
LIX
LX
List of Contributors
Steve W. Homans School of Biochemistry and Molecular Biology University of Yeeds LSZ 9ST United Kingdom
Horst Kunz Institut fur Organische Chemie Universitat Mainz J.-Joachim-Becher-Weg 18-20 55099 Mainz Germany
Hideharu Ishida Department of Applied Bio-organic Chemistry Gifu University Gifu 501-11 Japan
Joseph T. Y. Lau Roswell Park Memorial Institute 666 Elm Street Buffalo, NY 14263 USA
Wolfgang Jager Biacore AB Jechtinger Strasse 8 79 111 Freiburg Germany Osamu Kanie Glycoscience Laboratory Mitsubishi Kasei Institute of Life Sciences (MILS) Machida-shi Tokyo 194-8511 Japan Karl-Anders Karlsson Institute of Medical Biochemistry G~teborgUniversity Medicinaraegatan 9A P. 0. Box 440 405 30 Gateborg Sweden Makoto Kiso Department of Applied Bio-organic Chemistry Gifu University Gifu, 50 1-11 Japan Sharon S. Krag Department of Biochemistry Johns Hopkins University 615 North Wolfe Street Baltimore, MD 21205 USA J. J. Krepinsky Department of Medical Genetics and Microbiology University of Toronto Toronto, ON M5S 1A8 Canada
Torvard C. Laurent Department of Medical and Physiological Chemistry University of Uppsala Avd. for Med. Kemi, Box 575 751 23 Uppsala Sweden Yuan C. Lee Department of Biology Johns Hopkins University 3400 North Charles Street Baltimore, MD 21218 USA Ludwig Lehle Lehrstuhl fur Zellbiologie und Pflanzenphysiologie Universitat Regensburg 93040 Regensburg Germany William J. Lennarz Department of Biochemistry and Cell Biology SUNY Stony Brook Room 450, Life Sciences Bldg. Stony Brook, NY 11794-5215 USA Steven V. Ley Chemical Laboratory University of Cambridge Lensfield Road Cambridge CB2 1EW England Yu-Teh Li Department of Biochemistry, SL43 Tulane University School of Medicine 1430 Tulane Avenue New Orleans, LA 701 12 USA
List of Contributors Ulf Lindahl Medical and Physiological Chemistry Uppsala Universitet P. 0. Box 575 75 12 Uppsala Sweden
Kelly L. Moremen Complex Carbohydrate Research Center University of Georgia 220 River Bend Road Athens, GA 30602-7229 USA
Clifford A. Lingwood Department of Microbiology Hospital for Sick Children 555 University Avenue Toronto, OT M5G 1x8 Canada
Heinz E. Moser GeneSoft Inc Two Corporate Drive South San Francisco, CA 9080 USA
Jean-Maurice Mallet Department de Chimie, URA 1686 Ecole Normale Superieure 24 rue Lhormond 75231 Paris Cedex 05 France Ernesto Marques Department of Pharmacology and MoleculaI Science John Hopkins University School of Medicine 725 North Wolfe Street Baltimore, M D 21205-2185 USA Jamey Marth Howard Hughes Medical Institute University of California, San Diego 9500 Gillman Drive, 0625 La Jolla, CA 92093-0625 USA Rodger McEver Oklahoma Medical Research Ed. 825 NE 13'h Street Oklahoma City, OK 73 104 USA Anant K. Menon Department of Biochemistry University of Wisconsin-Madison 420 Henry Mall Madison, WI 53706-1569 USA
LXI
Reinhold Ohrlein Ciba Speciality Chemicals Inc. K-420.2.19 4002 Base1 Switzerland Peter Orlean 309 Roger A d a m Laboratory University of Illinois at Urbana 600 South Mathews Avenue Urbana, IL 61801 USA Stefan Oscarson Department of Organic Chemistry Stockholm University Arrhenius Laboratory Svante Arrhenius vag 12 10691 Stockholm Sweden Monica M. Palcic Department of Chemistry University of Alberta Edmonton, AB, T6G 2G2 Canada Luigi Panza Dipartimento di Chimica Organica e Industriale Via Venezian 2 1 20133 Milano Italy Armando J. Parodi Biquimicas-Fundaction Campomar University of Buenos Aires Antonio Machado 151 Buenos Aires, 1405 Argentina
LXII
List of Contributors
Serge Perez Centre de Recherches sur les MacromolCcules Vegetales UniversitC Joseph Fourier Cermav-CNRS, BP 53 38041 Grenoble Cedex 9 France
Konrad Sandhoff Institut fur Organische Chemie und Biochemie Universitat Bonn Gerhard-Domagk-Str. 1 53121 Bonn Germany
Thomas Peters Medizinische Universitat Zu Lubeck Institut fur Chemie Ratzeburger Allee 160 23538 Lubeck Germany
Harry Schachter Department of Biochemistry Hospital for Sick Children 555 University Avenue Toronto, ON M5G 1x8 Canada
Vince Pozsgay National Institute of Health 6 Center Dr. MSC 2720 Bldg. 6, Rm. 2A02 Bethesda, MD 20892-2720 USA
Roland Schauer Christian-Albrechts Universitat zu Kiel Olshausenstr. 40 24098 Kiel Germany
Christian Raetz Department of Biochemistry Duke University Medical Center P.O. Box 371 1 Durham, NC 27710 USA
Ronald L. Schnaar Department of Pharmacology and Molecular Science Johns Hopkins School of Medicine 725 North Wolfe Street Baltimore, MD 21205 USA
Ossi Renkonen University of Helsinki Institute of Biotechnology P. 0. Box 56 00014 University of Helsinki Finland
Richard R. Schmidt Fakultat fur Chemie Universitat Konstanz Postf. 5560 M 125 78457 Konstanz Germany
Peter J. Roach Department of Biochemistry Indiana University School of Medicine 635 Barnhill Dr. Indianapolis, IN 46202-5122 USA
Joel Shaper Oncology Center Johns Hopkins Hospital 600 North Wolfe Street Baltimore, MD 21287 USA
Steven D. Rosen Department of Anatomy University of California, San Francisco Box 0452 San Francisco, CA 94143 USA
Pierre Sinay Departement de Chimie Ecole Normale Superieure 24 rue Lhomond 75231 Paris Cedex 50 France
List of Contributors Dorothe Spillmann Department of Medical Biochemistry and Microbiology University of Uppsala The Biomedical Centre P. 0. Box 575 751 23 Uppsala Sweden Robert C. Spiro Department of Biological Chemistry Haward Medical School J o s h Research Laboratory 1 J o s h Place Boston, MA 02215 USA Jun-ichi Tamura Faculty of Education Tottori University Tottori 680-0945 Japan Garry Taylor Department of Biology and Biochemistry University of Bath Calverton Down Bath, BA2 7AY England Joyce Taylor-Papadimitriou Epithelial Cell Biology Laboratory Imperial Cancer Research Fund 44 Lincoln's Inn Field London, WC2A 3PX England Bryan P. Toole Department of Anatomy Tufts University HSC 136 Harrison Avenue Boston, MA 021 11 USA Eric J. Toone Department of Chemistry Duke University Durham, NC 27708-0346 USA Andrea T. Vasella Laboratorium fur Organische Chemie Eidgenossische Technische Hochschule Universitatsstrasse 19 8092 Zurich Switzerland
LXIII
Alain Veyrieres Ecole Nationale Superieure de Chimie de Rennes Campus du Beaulieu Avenue du General Leclerc 35700 Rennes France Barbara Vertel Department of Cell Biology and Anatomy Chicago Medical School 3333 Green Bay Road North Chicago, IL 60064-3095 USA Johannes F. G. Vliegenhart Bijvoet Center for Biomolecular Research Utrecht University P. 0. Box 80075 3508 TB Utrecht The Netherlands David J. Vocadlo Department of Chemistry University of British Columbia Vancouver B.C. V6T 1Zl Canada Russell Wallis Department of Biochemistry Glycobiology Institute University of Oxford South Parks Road Oxford, OX1 3QU England Thomas Warner 541 Wellington Drive San Carlos, CA 94070 USA Hans Peter Wessel F. Hoffmann-La Roche AG Pharma Research Discovery Chemistry PRPV-D Building 15/110 4070 Basel Switzerland Thomas N. Wight Department of Pathology SM-30 University of Washington School of Medicine Seattle, WA 98 195 USA
LXIV
List of Contributors
David B. Williams Department of Biochemistry University of Toronto Toronto, ON M5S 1A8 Canada Lawrence J. Williams Laboratory for Bioorganic Chemistry The Sloan-Kettering Institute for Cancer Research 1275 York Avenue New York, NY 10021 USA
Stephen G. Withers Department of Chemistry University of British Columbia 2036 Main Mall Vancouver, B.C. V6T 1Zl Canada Chi-Huey Wong Department of Chemistry Scripps Research Institute 10550 N. Torrey Pines Road La Jolla, CA 92037 USA
Abbreviations Used in Volumes 1 and 2
a1,3-GalT a2,3-SialT a2,6-SialT aGalF aGlcF aMuNeuAc apNPFuc apNPGal apNPGalNAc apNPGlc apNPGlcNAc apNPMan apNPNeuAc
A aa Ac AcHmb ACP AF AFM AgOTf AGP 019.
All AllNAc Aloc APC APP APS APS
a 1,3-GaIactosyltransferase a2,3-Sialyltransferase a2,6-sialyltransferase a-Galactosyl fluoride a-Glucosyl fluoride 4-Methylumbelliferone a-D-N-acetylneuraminic acid para-Nitrophenyl a-L-fucopyranoside para-Nitrophenyl a-D-galactopyranoside para-Nitrophenyl 2-acetamido-2-deoxy-a-~-galactopyranoside para-Nitrophenyl a-D-glucopyranoside para-Nitrophenyl 2-acetamido-2-deoxy-a-~-glucopyranoside para-Nitrophenyl a-D-mannopyranoside para-Nitrophenyl-a-D-N-acetyheuraminic acid Angstrom amino acid acetyl
N-2-acetoxy-4-methoxybenzyl acyl carrier protein aggregation factor atomic force microscopy silver trifluoromethanesulfonate (silver triflate) arabinogalactan protein asparagine-linked glycosylation ally1 N-Acetylallosamine Ally loxycarbony1 antigen-presenting cell acute phase protein adenosine phosphosulfate ammonium persulfate
LXVI
Abbreviations Used in Volumes I and 2
AR ARIS ASGP-R Asialo-GM1 Asn Asn GT ASTF-GP j3 1,4-GalT PManF PMuGlc PMuGlcNAc PMuXyl PoNPGal PoNPGlc PpClPMan PPhGal PPhGlc ppNP2dGlc PpNPGal PpNPGalNAc PpNPGlc PpNPGlcNAc PpNPMan PPNPXYl PpOMePhGlc PVGal 9-BBN BDA BHK bFGF BLAST BM BMP Bn Boc BOP bP Bu BU4NSSTr Bz Bzl CAA CAM CAN
acrosome reaction acrosome reaction-inducing substance asialoglycoprotein receptor p-D-Gal( 1-3)-b-~-GalNAc(1-4)-P-D-Gal( 1-4)-P-~-Glc-O-Cer asparagine asparagine glucosyltransferase (GalG1cNAcMan)zManGlcNAcz Asn P 1,4-Galactosyltransferase P-Mannosyl fluoride 4-Methylumbelliferyl P-o-glucopyranoside 4-Methylumbelliferyl 2-acetamido-2-deoxy-~-~-glucopyranoside 4-Methylumbelliferyl P-D-xylopyranoside ortho-Nitrophenyl P-D-galactopyranoside ortho-Nitrophenyl P-D-glucopyranoside para-Chlorophenyl P-D-mannopyranoside Phenyl P-D-galactopyranoside Phenyl P-D-glucopyranoside para-Nitrophenyl2-deoxy-P-~-arabino-hexopyranoside para-Nitrophenyl P-D-galactopyranoside para-Nitrophenyl 2-acetamido-2-deoxy-P-~-galactopyranoside para-Nitrophenyl P-D-glucopyranoside para-Nitrophenyl 2-acetamido-2-deoxy-~-~-glucopyranoside para-Nitrophenyl P-D-mannopyranoside para-Nitrophenyl P-D-xylopyranoside para-Methoxyphenyl P-D-glucopyranoside Vinyl 0-D-galactopyranoside 9-Borabicyclo[3.3.llnonane butane diacetal baby hamster kidney basic fibroblast growth factor basic alignment search tool basement membrane bis(monoacylg1ycero)phosphate benzyl tert-butoxycarbonyl benzotriazol-1-yloxy-tris(dimethy1amino)phosphoniumhexafluorophosphate base pair butyl Tetrabutylammoniumsaltoltriphenylmethanethiol benzoyl benzyl carbohydrate amino acids cell adhesion molecule Cer(1V)ammoniumnitrate
Ahhreviutions Used in Volumes 1 and 2 LXVII CAT CBMIT CBS CBS Cbz CCCP CD CDA CDGS CDMT cDNA CDTA Cer CHO CgOse4 CGTase CI CI8 CIAP ClAc ClNP CMLN CMP CMP CMP-NeuAc/ CMP-NeuSAc CMP-Sia CNBr CNS CoA Coll CPG CRD
cs
CSA CSA
csc CSPG CST CTL CTP Da DAF DAG DBA DBMP
chloramphenicol acetyltransferase 1,l-carbonylbis(3-methylimidazolium triflate Gal(S03)Pl- 1Cer Cerebroside sulfate benzyloxycarbonyl carbonyl cyanide m-chlorophenylhydrazone cation dependent cyclohexane-l,2-diacetal carbohydrate-deficient glycoprotein syndrome 2-chloro-4,6-dimethoxy-l,3,5-triazine complementary DNA diaminocyclohexane tetraacetic acid ceramide Chinese hamster ovary fi-~-Gal( 1-3)-o-~-GalNAc( 1-4)-fi-~-Gal( 1-4)-p-~-Glc cyclodextrin glucosyltransferase cation independent Cycloisomalto-octaose calf intestine alkaline phosphatase [ EC.3.1.3.11 chloroacetyl 2-Chloro-4-nitrophenyl calf mesenteric lymph node cytosine monophosphate cytidine 5'-monophosphate cytidine 5'-monophosphate N-acetylneuraminic acid Cytidine 5'-monophosphate sialic acid cyanogen bromide central nervous system coenzyme A sym-collidine controlled pore glass carbohydrate-recognition domain chondroitin sulfate Campher sulfonic acid chondroitin-4-sulfate chondroitin-6-sulfate chondroitin sulfate proteoglycan castanospermine cytotoxic T cell 5 '-Cytidinetriphosphate Dalton decay accelerating factor diacylglucosamine ((strain D x strain B) x strain A) mice 2,6-di-tert-butyl-4-methylpyridine
LXVIII Abbreviations Used in Volumes 1 and 2 DBU DCC DCPhth Dde DDP DDQ DEPC DF 2dGlc DIBALH DiBz DIG DIM DIPEA DMA DMAP DME DMF DMJ DMM DMPU DMSO DMTST DMTST 2,4-DNP DNJ DNJ Do1 Dol-P Dol-PP DP DPPA DRM DS DTBMP DTBP DTBPI DTT E-CAD ECM EDCD/EDCI EDTA EGF eIF EM
1,8-diazabicyclo[5.4.01undec-7-ene N ,N’-Dicyclohexylcarbodiimide 4,s-dichlorophthaloyl N- 1- (4,4-dimethy1-2,6-dioxocyc1ohexy1idene) ethy 1 dialkyl N,N-diethylphosphoramidites 2,3-Dichloro-S,6-dicyano-1,4-benzoquinone diethylphosphoryl cyanide deactivation factor 2-Deoxy-arabino-hexose Diisobutylaluminumhydride Dibenzoyl detergent-insoluble glycolipid complex 1,4-dideoxy-1,4-imino-~-mannitol diisopropylethylamine N ,N-dimethylacetamide 4-Dimethylaminopyridine 1,Zdimethoxyethane N ,N-dimethylformamide 1-deoxymannojirimycin dimethylmaleoyl N ,N’-dimethylhexahydropyrimidin-2-one dimethyl sulfoxide dimethyl(methy1thio)sulfonium tetrafluoroborate (dimethyl (methy1thio)sulfoniumtriflate) Dimethyl(methy1thio)sulfoniumtetrafluoromethansulfonate 2,4-Dinitrophenyl deoxynojirimycin 1,5-Dideoxy-l,S-irnino-D-glucitol (deoxynojirimycin) dolichol dolichyl phosphate dolichyl pyrophosphate degree of polymerization diphenylphosphoryl azide detergent-resistant membrane dennatan sulfate 2,6-di-tert-butyl-4-methylpyridine 2,6-di-tert-butylpyridine 2,6-di-tert-butylpyridiniumiodide dithiothreitol Epithelial Cadherin I extracellular matrix 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide ethylene diamine tetraacetic acid epidermal growth factor eukaryotic initiation factor electron microscopy
Abbreviations Used in Volumes I and 2 LXIX endo H EPGase ER ER ESI EST EtN EtN-P F FAK FCOE FGF Fmoc/FMOC FSP FT FTIR FTIR Fuc Fuc FucT GA GAG Gal Gal GalNAc GalT Gal- 1-P UT Gb3 Gb;l GC GC-MS GDI, GDP GDP GDP-Fuc GDP-Man GDPMP Gg3 GI GI1 Glc Glc GlcA GlcN GlcN
endo-0-N-acetylglucosaminidase H endopolygalacturonase endoplasmic reticulum endoplasmic reticulum electrospry ionization expressed sequence tag ethanolamine ethanolamine phosphate fusion protein focal adhesion kinase fluorophore-coupled oligosaccharide electrophoresis fibroblast growth factor 9-Fluorenylmethoxylcarbonyl(fluorenyl-9-methoxycarbonyl) fucose sulfate polysaccharide a-(1-3/4)-fucosyltransferase [ EC.2.4.1.651 Fourrier transform infra red Fourier transform infra-red (spectroscopy) fucose/fucosyl fucose fucosyltransferase Golgi apparatus glycosaminoglycan galactose galactose/galactosy1 N-acetylgalactosamine galactosyltransferase Gal- 1-P-uridyltransferase Gala+4Gal~1+4Glc~lA+lCer
GalNAcPl+3Gala1+4Gal~l+4Glc~l+lCer glucosyl cerdmide gas chromatography-mass spectrometrya-DNeuAc(2-3)-P-~-Gal(1-3)-p-~-GalNAc(1-4)-[a-DNeuAc(2-3)]-P-~-Gal(1-4)-p-~-Glc-o-CerGDP guanosine diphosphate guanosine 5'-diphosphate guanosine 5I-diphosphate fucose guanosine 5'-diphosphate mannose GDP-Man pyrophosphorylase
GalNAcP1+4Galpl+4Glcpl+lCer gastro-intestinal glucosidase I1 glucose/glucosyl glucose glucuronic acid 2-Amino-2-deoxy -glucopyr anose glucosamine
LXX
Abbreviations Used in Volumes I and 2
GkN3 Glc3NAc GlcNAc GlcNAc GlcNAc, GlcNAc-Oxazoline GlcNPht GlcNTeoc Glucal GLUT GM~ GMi
GM3 GM3 GMP GnT GP GPI GRIFIN GRP GSL GT GT GTA GTB H HA HAF HAg HAS HBP HBPyU hCG HEMPAS test HeP HEPES HEV HIV HIV
2-Azido-2-deoxy-~-glucopyranose 3-Acetamido-3-deoxy-~-glucopyranose N-acetylglucosamine 3,4,6-tri-O-acetyl-N-acetylglucosamine { p-~-GlcNAc( 1-4)) ( , - l ) - ~ - G l ~ N A ~ 2-Methyl-4,5-dihydro-(1,2-dideoxy-a-~-g~ucopyranoso) [2,1-d]-1,3-oxazole 2-Deoxy-2-phthalimido-~-glucopyranose 2-Deoxy-2-aminot~ethoxylcarbonyl-~-g~ucopyranose 1,5-Anhydr0-2-deoxy-~-arabino-hex1-enitol glucose transporter GalPl+3GalNAcf31 -+[NeuAcu2+3]Gal~l+4Glc~l+1Cer p-D-Gal(1-3)-p-~-GalNAc(1-4)-[u-~-NeuAc(2-3)1-p-~Gal( 1-4)-p-D-GlC-O-Cer P-D-GalNAc(1-4)-[u-~-NeuAc(2-3)]-p-~-Gal( 1-4)-p-~Glc-0-Cer NeuAca2+3Gal~l-+4G1c~l+ lCer a-~-NeuAc(2-3)-P-D-Gal(1-4)-p-~-Glc-O-Cer guanosine monophosphate N-acetylglucosaminyltransferase glycoprotein glycosyl phosphatidylinositol galectin-related interfiber protein glycine-rich protein glycosphingolipid p-(1-4)-galactosyltransferase [ EC.2.4.1.221 (glycosyltransferase) UDP-G1c:glycoprotein glucosyltransferase human blood group A glycosyltransferase human blood group B glycosyltransferase Fucul+2Galpl+3/4GlcNAcP hyaluronic acid, hyaluronate Hulicondriu aggregation factor hemagglutinin hyaluronate synthesizing enzyme hexosamine biosynthetic pathway 0-(benzotriazole-1-yl)-N,N,N,N-bis(tetramethy1ene)uronium hexafluorophosphate human chorionic gonadotropin hereditary erythroblastic multinuclearity with a positive acidified-serum-lysis (congenital dyserythropoietic anemia type 11) heparin N-(2-Hydroxyethylpiperazine)-N'-(2-ethanesulfon~c acid) high endothelial venules Human immunodeficiency virus human immunodeficiency virus
Abbreviations Used in Volumes 1 and 2 HL HMG HN HOAll HOAT HOBt HOMO HPLC HPLC HRGP HS HSD HSPG HUS HYCRON HYP ICAM IDCP IDCT IduA IGF Ig IgSF IIDQ IL IPG IU IV3NeuAc PGalNAcGbOse4 I V NeuAcII NeuAcCgOse4 K
kb kDa Kdn
Kdn/KDN Kdo Kdo K-4 Ki KIF Krn KS KSPG
LXXI
hepatic lectin high mobility group (proteins) hemagglutinin-neuraminidase Ally1 alcohol 1-hydroxy-7-azabenztriazole 1-hydroxybenzotriazole hydrate highest occupied molecular orbital high-performance liquid chromatography high performance liquid chromatography hydroxyproline-rich glycoprotein heparan sulfate highly sulfated domain heparan sulfate proteoglycan hemolytic uremic syndrome hydroxycrotyl-oligoethylene glycol-n-alkanoyl 4-hydroxyproline intercellular adhesion molecule iodonium dicollidine perchlorate iodonium dicollidine triflate iduronic acid insulin-like growth factor immunoglobulin immunoglobulin superfamily of proteins isobutoxyisobutyldihydroquinoline interleukin inositol phosphoglycan international unit p-D-GalNAc(1-3)-p-~-GalNAc(1-3)-u-~-Gal(1-4)-p-DGal(1-4)-~-Glc a-u-NeuAc(2-3)-P-~-Gal(1-3)-p-~-GalNAc(1-4)-[a-DNeuAc(2-3)]-P-~-Gal(1-4)-Dkinase enzyme activity coefficient kilobase (thousand base pairs) kiloDalton 2-keto-3-deoxy-nononicacid, formally 5-desamino-5hydroxyneuraminic acid 3-deoxy-~-y~ycero-D-gu~ucto-2-nonulosonic acid ketodeoxoctulosonic acid (3-deoxy-~-mannooctulosonic acid) 2-keto-3-deoxy-~-manno-octulosonic acid kinase enzyme activity coefficient, at equilibrium kinase enzyme activity coefficient, for inhibition kifunensine kinase enzyme activity coefficient, for structural modification(s) keratan sulfate keratan sulfate proteoglycan
LXXII Abbreviations Used in Volumes 1 and 2 LacAm LacCer LacNAc LacNAc LAD LAMP LBP Lc3 LDA Lea Lea Leb Lea, LeX LeX LeX Ley Leu Lev LNP LPS LRP LRR LUMO LYS mAb MAd MAF MAG MALDI-TOF MALDI-TOF Man ManNAc MAP MASP Man-T MBL MBP MBz MCD MDBK MDCK ME MEM MHC MK
lactosamine GalP 1+4GlcP1+ 1Cer N-acetyllactosamine N-acetyllactosamine leucocyte adhesion deficiency lysosomal associated membrane protein lipopolysaccharide-binding protein GlcNAcPl-.3Gal~1+4GlcP1+ 1Cer Lithium diisoprypropylamide GalP 1+3[Fucal+4]GlcNAcP1+3Gal~1+GlcP P-~-Gal(l-3)-[a-~-Fuc( 1-4)]-~-GlcNAc Fuca 1+2GalP 1+3[ Fucal-+4]GlcNAc~ 1+3GalP 1-4GlcP Lewis a, Lewis x determinants p-D-Gal(1-4)-[a-~-Fuc( 1-3)]-~-GlcNAc GalP 1+4[ Fucal+3]GlcNAcP 1+3GalP 1-+4GlcP Fuca 1-2GalP 1+4[Fucal+3]GlcNAcP 1+3GalP 1-4ClcP leucine levulinoyl lectin and nucleotide phosphohydrolase lipopolysaccharaide lipoprotein receptor-related protein leucine-rich repeat (amino acid/protein) lowest unoccupied molecular orbital lysine monoclonal antibody mucosal addressin Microciona prolifera aggrigation factor myelin-associated glycoprotein mass-analyzed laser desorption ionization-time of flight matrix-assisted-laser desorption ionization time of flight mass spectrometry mannose N-acetylmannosamine microtubule-associated protein MBP-associated serine protease mannosyltransferase mannose-binding lectin mannose-binding protein p-methoxybenzoyl macular corneal dystrophy Madin-Darby bovine kidney (cells) Madin-Darby canine kidney (cells) methoxyethyl methoxyethoxymethyl major histocompatibility complex myokinase
Abbreviations Used in Volumes 1 and 2 LXXIII mol.wt. MP MPEG MPR MPS M, MS MSD MTr Mu MUC NA NAc nAChR NAm NANA N-BOC NBS NCAM NCS N DP NDP NDST NEM 3NeuAcCgOse4 Neu5Ac NeuAc NGF NIH NIS NK nLc4 Nle NMK NMP NMR NOE NPG NS NTP Nu OD (2-OH)-Lea (2-OH)-Le" (2-OH)-sLea
molecular weight p-methoxyphenyl polyethylene glycol monomethyl ether mannose 6-phosphate receptor mucopolysaccharidosis apparent molecular weight mass spectrometry multiple sulfatase deficiency, Austin disease p-methoxytrityl 4-Methylumbelliferone mucin N-acetylated disaccharide unit N-acetyl nicotinic acetylcholine receptor neuraminidase N-acetylneuraminic acid N-tert-butoxycarbonyl N-bromosuccinimide neural cell adhesion molecule N-chlorosuccinimide nucleoside diphosphates nucleoside-diphosphate N-deacetylaselN-sulfotransferase N-ethylmaleimide p-~-Gal(1-3)-o-~-GalNAc( 1-4)-[a-~-NeuAc(2-3)]-P-DGal( 1-4)-D-Glc N-acetylneuraminic acid N-acetylneuraminic acid nerve growth factor National Institutes of Health N-iodosuccinimide natur a1 killer Gal~l+4GlcNAc~1+3Gal~1-t4Glc~1-+ 1Cer Norleucin nucleoside monophosphate kinase nucleoside monophosphate nuclear magnetic resonance nuclear Overhauser effect n-pentenyl glycoside N-sulfated disaccharide unit nucleoside triphosphate nucleophile optical density P-D-Gal(1-3)-[a-~-Fuc( 1-4)I-D-Glc p-D-Gal(1-4)-[a-~-Fuc(1-3)I-D-Glc a-NeuAc(2-3)-P-~-Gal(1-3)-[a-~-Fuc(1-4)]-~-Glc
LXXIV Abbreviations Used in Volumes I and 2 (2-OH)-sLe” OR ORF OSP OST OST P PA Pac PAMP PAPS PAPS PARP PBL pClP PCR PDGF PDI PEG PEGA PEP PEP PfP
pmu
PG PGM PGM Ph PhSeOTf PhSOTf Phth PI Piv PK PMB pNP-a-Man PNS PNZ POEPOP Ppase ppGaNTases PPK PRP PSA PSGL
a-NeuAc(2-3)-P-~-Gal(1-4)-[a-~-Fuc( 1-3)]-~-Glc dimethyl(methy1thio)sulfonium trifluoromethanesulfonate open reading frame 0-specific polysaccharides oligosaccharyltransferase N-oligosaccharyltransferase phosphate 2-Aminopyridine phenacyl pathogen-associated molecular pattern) 3 ‘-phosphoadenosine 5’-phosphosulfate phosphoadenosine phosphosulfate procyclic acidic repetitive protein peripheral blood leucocytes para-Chlorophenyl polymerase chain reaction platelet-derived growth factor protein disulfide isomerase poly(ethy1ene glycol) polyethylene glycol N,N-dimethylacrylamide copolymer phospho(eno1)pyruvate phosphoenolpyruvate pentafluorophenyl N,N,N’,N’-bis(tetramethylene)-O-pentafluorophenyluronium hexafluorophosphate phosphatidylglycerol phosphoglucomutase phosphoglucomutase phenyl Phenylseleniumtriflate Phenylsulfoniumtriflate phthaloyl phosphatidylinositol pivaloyl pyruvate kinase p-methoxybenzyl p-nitropheny1-a-D-mannoside peripheral nervous system p-nitrobenzyloxycarbonyl poly oxyethylene-polyoxypropylene pyrophosphatase UDP-Ga1NAc:polypeptide N-acetylgalactosaminyltransferases polyphosphate kinase proline-rich protein polysialic acid P-Selectin glycoprotein ligand
Abbreuiutions Used in Volumes I and 2 LXXV PSGL-1 PSGL- 1 PTG PYr 9.Y. RA RAr RACE rER RHAMM ROESY RT-PCR SA SAP SAT SBA SBu SDS SDS-PAGE Se SE Ser SEt SGC SGG SGGL SGLT sia sLea sLea, sLe' sLe' 6'-SLN SLRP SMC SP SPh SPH SSEA ST ST3 ST6 STF-GP SV
sw
P-selectin glycoprotein ligand-1 P-selectin glycoprotein ligand-1 peptidoglycan pyridine quantitative yield retinoic acid rheumatoid arthritis rapic amplification of cDNA ends rough endoplasmic reticulum receptor for hyaluronan-mediated motility rotating-frame nuclear Overhauser effect reverse transcription polymerase chain reaction sialyl saposin sialyltransferase soybean agglutinin Thiobutyl sodium dodecyl sulfonate sodium dodecylsulfate-polyacrylamide gel electrophoresis secretor enzyme 2-(trimethylsily1)ethyl serine Thioethyl sulfogalactosyl ceramide sulfogalactosyl glycerolipid sulfoglucuronyl glycolipid sodium-glucose transporter sialic acid a-~-NeuAc(2-3)-P-~-Gal( 1-3)-[ a-~-Fuc( 1-4)]-~-GlcNAc(sialyl Lewisa trisaccharide) sialylated Lewis a, sialylated Lewis x determinants a-~-NeuAc(2-3)-P-~-Gal( 1-4)-[a-~-Fuc( 1-3)]-~-GlcNAc(sialyl Lewis' trisaccharide) 6'4alyl-LacNAc small leucine-rich proteoglycan sialomucin complex spacer Thiophenyl saccharide-peptide hybrids stage specific embryonic antigen sulfurotransferase recombinant rat liver a-(2-3)-sialyltransferase [EC.2.4.99.6] rat liver a-(2-6)-sialyltransferase jEC.2.4.99.11 (NeuAcGalGlcNAcMan)2ManGlcNAqAsn synaptic vesicle-associated protein swainsonine
LXXVI Abbreviations Used in Volumes 1 and 2 TATU TBAF TBAI TBDMS TBDMSOTf TBDPS TBDPSCl TBTU Bu TCA Tcoc TCP TCR TDS TEMED Teoc TES TESOTf Tf20 TFA TFDMSCl TfOH TGF TGN THF Thr tlc TM TMD TMS TMSCN TMSN3 TMSOTf to1 Tos t-PA TriBz Troc Trt Ts UDP UDP UDP-Gal UDP-Gal
0-(7-azabenzotriazol-1-yl)-3-tetramethyluronium tetrafluoroborate Tetrabutylammoniumfluoride Tetabutylammoniumiodide tert-butyldimethylsilyl tert-butyldimethylsilyl trifluoromethanesulfonate (tert-butyldimethylsilyl triflate) tert-butyldiphenylsilyl tert-butyldiphenylsilylchloride 0-(benzotriazol- 1-yl)-N,N,N',N'-tetrauroniumtetrafluoroborate tert-butyl trichloroacetyl trichloroethyloxycarbonyl tetrachlorophthaloyl T cell receptor thexyldimethylsilyl tetramethylethylenediamine 2,2,2-trichloroethoxycarbonyl triethylsilyl triethylsilyl trifluoromethanesulfonate (triethylsilyl triflate) trifluoromethane sulfonic acid anhydride trifluoroacetic acid trifluoromethyldimethylsilylchloride trifluoromethanesulfonic acid (triflic acid) transforming growth factor trans Golgi network tetrahydrofuran threonine thin layer chromatography transmembrane trans-membrane domain trimethylsilyl trimethylsilyl cyanide trimethylsilyl azide trimethylsilyl trifluoromethanesulfonate (trimethylsilyl triflate) toluene p-toluenesulfonyl (tosyl) tissue plasminogen activator factor tribenzoyl
2,2,2-trichloroethoxycarbonyl triphenylmethyl (trityl) p-toluenesulfonyl (tosyl) uridine 5'-diphosphate uranyl diphosphate UDP-a-D-galactose uridine 5'-diphosphate-galactose
Abbreviations Used in Volumes I and 2 UDPGE UDP-Glc UDPGP UMD UMP UTP
uv vc
VE VL VSG
vsv
VT vWF WBP XYl XY12 Z ZP
LXXVII
uridine 5'-diphosphate-glucose epimerase [EC.5.1.3.2.] uridine 5'-diphosphate-glucose UDP-Glc pyrophosphorylase unmodified domain uranyl monophosphate uranyl triphosphate ultraviolet vitelline coat vitelline envelope vitelline layer variant surface glycoprotein vasicular stomatis virus verotoxin von Willebrand factor wheat germ agglutinin binding protein xylose p-D-xyl(1--4)-D-xyl benzyloxycarbonyl zona pellucida
Abbreviations Used in Volumes 3 and 4
A" aa ACP AGP 4 7
APC APP APS AR ARIS ASGP-R Asn Asn GT BHK bFGF BLAST BM BMP bP CAM CAT CCCP CD CDGS CDTA Cer CHO CI CMLN CMP
Angstrom amino acid acyl carrier protein arabinogalactan protein asparagine-linked glycosylation antigen-presenting cell acute phase protein adenosine phosphosulfate acrosome reaction acrosome reaction-inducing substance asialoglycoprotein receptor asparagine asparagine glucosyltransferase baby hamster kidney basic fibroblast growth factor basic alignment search tool basement membrane bis(monoacy1glycero)phosphate base pair cell adhesion molecule chloramphenicol acetyltransferase carbonyl cyanide m-chlorophenylhydrazone cation dependent carbohydrate-deficient glycoprotein syndrome diaminocyclohexane tetraacetic acid ceramide Chinese hamster ovary cation independent calf mesenteric lymph node cytosine monophosphate
LVIII
Abbreviations Uscd in Volumes 3 and 4
CNS CoA CRD CS CSPG CST CTL Da DAF DAG DBA DIG DIM DMJ DMSO DNJ Do1 Dol-P Dol-PP DP DRM DS ECM EDTA EGF eIF EM endo H EPGase ER EST EtN EtN-P F FAK FCOE FGF FSP FTIR Fuc FucT GA GAG Gal GalNAc GalT
central nervous system coenzyme A carbohydrate-recognition domain chondroitin sulfate chondroitin sulfate proteoglycan castanospermine cytotoxic T cell Dalton decay accelerating factor diacylglucosamine ((strain D x strain B) x strain A) mice detergent-insoluble glycolipid complex 1,4-dideoxy-l,4-imino-~-mannitol 1-deoxymannojirimycin dimethyl sulfoxide deoxynojirimycin dolichol dolichyl phosphate dolichyl pyrophosphate degree of polymerization detergent-resistant membrane dermatan sulfate extracellular matrix ethylene diamine tetraacetic acid epidermal growth factor eukaryotic initiation factor electron microscopy endo-p-N-acetylglucosaminidaseH endopol y galacturonase endoplasmic reticulum expressed sequence tag ethanolamine ethanolamine phosphate fusion protein focal adhesion kinase fluorophore-coupled oligosaccharide electrophoresis fibroblast growth factor fucose sulfate polysaccharide Fourier transform infra-red (spectroscopy) fucose fucosyltransferase Golgi apparatus glycosaminoglycan galactose N-acetylgalactosamine galactosyltransferase
Ahbreniutions Used in Volumes 3 and 4
GC GC-MS GDP GI GI1 Glc GlcA GlcN GlcNAc GLUT GnT GP GPI GRIFIN GRP GSL GT GTA GTB HA H Ag HAS HBP HEMPAS hCG HeP HEV HIV HL HMG HN HPLC HRGP HS HSD HSPG HUS ICAM lduA IGF Ig IgSF IL IPG
1u
LIX
glucosyl ceramide gas chromatography-mass spectrometry guanosine diphosphate gastro-intestinal glucosidase I1 glucose glucuronic acid glucosamine N-acetylglucosamine glucose transporter N-acetylglucosaminy ltransferase glycoprotein glycosyl phosphatidylinositol galectin-related interfiber protein glycine-rich protein glycosphingolipid UDP-G1c:glycoprotein glucosyltransferase human blood group A glycosyltransferase human blood group B glycosyltransferase hyaluronic acid, hyaluronate hemagglutinin hyaluronate synthesizing enzyme hexosamine biosynthetic pathway hereditary erythroblastic multinuclearity with a positive acidifiedserum-lysis test (congenital dyserythropoietic anemia type 11) human chorionic gonadotropin heparin high endothelial venules human immunodeficiency virus hepatic lectin high mobility group (proteins) hemagglutinin-neuraminidase high performance liquid chromatography hydroxyproline-rich glycoprotein heparan sulfate highly sulfated domain heparan sulfate proteoglycan hemolytic uremic syndrome intercellular adhesion molecule iduronic acid insulin-like growth factor immunoglobulin immunoglobulin superfamily of proteins interleukin inositol phosphoglycan international unit
LX
K kb kDa Kdn
Abbreviations Used in Volumes 3 and 4
kinase enzyme activity coefficient kilobase (thousand base pairs) kiloDalton 2-keto-3-deoxy-nononic acid, formally 5-desamino-5-hydroxyneuraminic acid 2-keto-3-deoxy-~-manno-octulosonic acid Kdo kinase enzyme activity coefficient, at equilibrium Keq kinase enzyme activity coefficient, for inhibition Ki kifunensine KIF kinase enzyme activity coefficient, for structural modification(s) Km keratan sulfate KS keratan sulfate proteoglycan KSPG lactosamine LacAm N-acetyllactosamine LacNAc leucocyte adhesion deficiency LAD lysosomal associated membrane protein LAMP lipopolysaccharide-binding protein LBP Lewis a, Lewis x determinants Lea, Le" leucine Leu lectin and nucleotide phosphohydrolase LNP lipopoly saccharaide LPS lipoprotein receptor-related protein LRP leucine-rich repeat (amino acid/protein) LRR lysine LYS monoclonal antibody mAb mucosal addressin MAd myelin-associated glycoprotein MAG MALDI-TOF matrix-assisted-laser desorption ionization time of flight inass spectrometry mannose Man microtubule-associated protein MAP MBP-associated serine protease MASP mannose-binding lectin MBL mannose-binding protein MBP mannosyltransferase Man-T macular corneal dystrophy MCD Madin-Darby bovine kidney (cells) MDBK Madin-Darby canine kidney (cells) MDCK major histocompatibility complex MHC molecular weight mol.wt. mannose 6-phosphate receptor MPR mucopolysaccharidosis MPS apparent molecular weight Mr multiple sulfatase deficiency, Austin disease MSD N-acetylated disaccharide unit NA N-acetyl NAc
Ahhreviutions Used in Volumes 3 and 4
nAChR NAm NANA NCAM NDP NDST NEM NGF NIH NK NMR NS ORF OST P PAMP PAPS PARP PRL PCR PDGF PDI PEP PG PGM PI pNP-a-Man PNS PRP PSA PSGL-1 PTG RA RAr RACE rER RHAMM RT-PCR SA SAP SAT SBA SDS-PAGE Se Ser SGC
nicotinic acetylcholine receptor neuraminidase N-acetylneuraminic acid neural cell adhesion molecule nucleoside-diphosphate N-deacetylaselN-sulfotransferase N-ethylmaleimide nerve growth factor National Institutes of Health natural killer nuclear magnetic resonance N-sulfated disaccharide unit open reading frame N-oligosaccharyltransferase phosphate pathogen-associated molecular pattern phosphoadenosine phosphosulfate procyclic acidic repetitive protein peripheral blood leucocytes polymerase chain reaction platelet-derived growth factor protein disulfide isomerase phosphoenolpyruvate phosphatidylglycerol phosphoglucomutase phosphatidylinositol p-nitrophenyl-a-D-mannoside peripheral nervous system proline-rich protein polysialic acid P-selectin glycoprotein ligand-1 peptidoglycan retinoic acid rheumatoid arthritis rapic amplification of cDNA ends rough endoplasmic reticulum receptor for hyaluronan-mediated motility reverse transcription polymerase chain reaction sialyl saposin sialyltransferase soybean agglutinin sodium dodecylsulfate-polyacrylamide gel electrophoresis secretor enzyme serine sulfogalactosyl ceramide
LXI
LXII
Abbreviations Used in Volumes 3 and 4
SGG SGGL SGLT sia sLea, sleX SLRP SMC ST
sv
SW TCR TGF TGN Thr tlc TM t-PA UDP UDP-Gal UMD UMP UTP UV
vc
VE VL VSG
vsv
VT vWF WBP XYl ZP
sulfogalactosyl glycerolipid sulfoglucuronyl glycolipid sodium-glucose transporter sialic acid sialylated Lewis a, sialylated Lewis x determinants small leucine-rich proteoglycan sialomucin complex sulfurotransferase synaptic vesicle-associated protein swainsonine T cell receptor transforming growth factor trans Golgi network threonine thin layer chromatography transmembrane tissue plasminogen activator factor uranyl diphosphate UDP-a-D-galactose unmodified domain uranyl monophosphate uranyl triphosphate ultraviolet vitelline coat vitelline envelope vitelline layer variant surface glycoprotein vasicular stomatis virus verotoxin von Willebrand factor wheat germ agglutinin binding protein xylose zona pellucida
Part I
Volume 1
I Chemical Synthesis of Glycosides
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
1 Introduction to Volumes 1 and 2
As no better man advances to take this matter in hand, I hereupon offer my own poor endeavours. I promise nothing complete; because any human thing supposed to be complete must for that very reason infallibly be faulty. (from Moby Dick, Herman Melville)
Our understanding of the relationship between carbohydrate structure and biological function is still far behind that of proteins and nucleic acids. Initially, carbohydrates were only recognized as structural and energy storage molecules (e.g. cellulose, chitin and glycogen). The development of novel tools to study carbohydrate function has permitted additional biological functions to be elucidated. Their roles in protein folding, cell signaling, fertilization, pathogen binding to host tissue, leukocyte trafficking and associated inflammatory responses, tumor cell metastasis, and regulation of hormone and enzyme activities are just a few selected examples. These findings led to an immense interest in the preparation of oligosaccharides and glycoconjugates as well as the understanding of their interaction with their natural receptors. Part 1 of Carbohydrates in Chemistry and Biology is designed to be a valuable collection of chemical and enzymatic methods for the synthesis of oligosaccharides. It also covers state-of-the-art knowledge of the interactions of carbohydrates with their natural receptors. Each of the chapters is written by a leading expert in the field and covers an impressive body of references. Volume 1 of the series begins with the most challenging aspect of synthetic carbohydrate chemistry, namely the stereospecific formation of glycosidic bonds. A number of chapters describe recently developed approaches and cover numerous significant advances in glycosidation techniques. Other chapters review the attachment of sugars through linkages not occurring in nature. Volume 1 concludes by summarizing one of the most exciting and rapidly evolving area in modern carbohydrate synthesis: the use of glycosyl transferases and glycosidases in the synthesis of oligosaccharides and their analogs.
4
1 Introduction to Volumes 1 and 2
Volume 2 of the series covers the interactions of carbohydrates with their natural receptors, i.e. proteins, nucleic acids, and other carbohydrates. Whereas carbohydrate-protein interactions are well documented, the investigation of carbohydratenucleic acid and carbohydrate-carbohydrate interactions is an emerging area. The Carbohydrates in Chemistry and Biology series provides a comprehensive and up-to-date compilation of information on the fields of glycochemistry (Volumes 1 and 2) and glycobiology (Volumes 3 and 4),available today. The volumes not only provide an expert overview of the entire field, but also serve as a valuable reference tool to both, established investigators and students in many disciplines. The editors are obliged to all authors for their excellent contributions. Special thanks goes to Dr. Anette Eckerle, Wiley-VCH, who managed the realization of this series. Beat Ernst, Ph.D. Professor of Molecular Pharmacy University of Base1 Pierre Sinay, Ph.D. Professor of Organic Chemistry Ecole Normale SupCrieure, Paris
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
2 Trichloroacetimidates Richard R. Schmidt and Karl-Heinz Jung
2.1 Introduction Glycoside bond formation generally requires activation of the sugar at the anomeric center. To this end, activation through anomeric oxygen-exchange reactions (the classical Koenigs-Knorr method and its modifications, sulfur based activations, etc.) or, alternatively, activation through retention of the anomeric oxygen (trichloroacetimidate method, phosphite and phosphate activation, etc.) have been proposed. The trichloroacetimidate method is well established [ 1-1 31 and although the first paper on this method was published as recently as 1980 [ 141 it has already become a classical method. This is essentially because only catalytic amounts of a promoter are required to provide very high glycosyl donor properties whereas the anomeric oxygen-exchange based methods generally require at least equimolar amounts of a promoter system, which is obviously often associated with disadvantages of various kinds. The basic principle of the trichloroacetimidate method [ 11 consists in a simple base-catalyzed addition of trichloroacetonitrile to the anomeric hydroxy group to generate the 0-glycosyl trichloroacetimidate (see Scheme 1). This reaction is generally high-yielding and, because of its reversibility, high anomeric control can often be achieved; in addition, competing reactions with non-anomeric hydroxy groups are not observed. In the second step, under mild acid/Lewis acid catalysis glycosylation of various nucleophiles (for instance, of alcohols, etc.) can be readily achieved; in general high yields and high anomeric control are obtained. The anomeric control is derived from the glycosyl donors’ anomeric configuration (inversion or retention), from anchimeric assistance, from the influence of the solvent, or from thermodynamic or kinetic effects [ 1- 131. Since 1980 (up to the end of 1998), application of the trichloroacetimidate method has been reported in more than 1100 papers and over 1200 different 0glycosyl trichloroacetimidates have been described in the literature. Obviously, the wealth of this chemistry cannot be compiled in a relatively short book chapter. A
6
*OH
2 Trichloroacetimidates
.
Base Catalysis
CI&
Reaction with CI&-CN not observed
- C - NH, .
:: t
Mild Aci? Catalysis
4 +
+OyNH
CCI,
HOR
Scheme 1.
quite comprehensive review [ 71 on the trichloroacetimidate method appeared in 1994, and this chapter is, therefore, essentially devoted to the literature since 1994. Of 686 references (1994-1998) on the use of the trichloroacetimidate method, because of space limitation, only ca 220 references are considered in this chapter. The typical examples selected illustrate the application of this method in different areas.
2.2 Methods The reported methods [ 141 for the formation and reaction of trichloroacetimidates are well established. Use of sodium hydride or cesium carbonate as a base for the reaction of 1-0-unprotected sugars with trichloroacetonitrile yields generally the thermodynamically favored 0-a-glycosyl trichloroacetimidates, whereas use of potassium carbonate yields 0-p-glycosyl trichloroacetimidates because of kinetic reaction control. Use of 1,8-diazabicyclo[5.4.01undec-7-ene (DBU) often yields a/p mixtures, mostly favoring the a-trichloroacetimidates. Recently, application of new base systems has been reported using 7-methyl-l,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) [ 151 or phase transfer catalysis (50% potassium hydroxide, dichloromethane, tetra-n-butylammonium hydrogen sulphate) which yield exclusively atrichloroacetimidates in very good yields (96%) [ 161. Besides the well established catalysts for glycosylation reactions with trichloroacetimidates, some new catalysts have been investigated. The use of silver trifluoromethanesulfonate in the reaction with phthalimido-protected glucosamine trichloroacetimidate gave higher yields and smaller amounts of a-glucoside than the use of boron trifluoride etherate; this procedure also avoids acyl migration in the acceptor molecule; but stoichiometric amounts of the promoter are required [ 17,
2.3 Q-Glycosides
7
IS]. Use of lithium perchlorate in dichloromethane requires a large excess of promoter [ 19, 20 1, but with lithium trifluoromethanesulfonate only 0.05 equivalents are required [21]. Both catalysts give high yields, but the a/P selectivity is low and has to be improved before general application is possible. The synthesis of other 0-glycosyl imidates, i.e. O-glucopyranosyl dichlorocyanoacetimidates, 2,3,5,6-tetrafluoropyridin-4-y1 glucopyranosides, etc., is under investigation [22, 231 thus variations in reactivity and selectivity are expected.
2.3 O-Glycosides 2.3.1 Synthesis of Oligosaccharides
P-Glucosides, P-Galactosides, a-Rhamnosides, etc. 1,2-trans-glycosides (P-glucosides, 0-galactosides, P-xylosides, a-mannosides, arhamnosides, etc.) are usually prepared with glycosyl donors containing participating neighboring groups (acetyl, benzoyl, pivaloyl, etc. ) at 2-OH. Some examples for Galp(1-3)Gal [24], GalP(1-3)GalN [25], and ~-Rhaa(l-2)Glc[26] are shown in Scheme 2. Instead of the methyl glycoside of azidogalactose, the corresponding glycosyl fluoride has also been used as acceptor [27]. In general glycosylation yields are high, irres,pective of the anomeric configuration of the trichloroacetimidate, and only one anomer is obtained. Occasionally, when glycosyl acceptors with more than one free OH group are used, high regioselectivity is obtained because of the different reactivity of the OH groups. It was found that trichloroacetimidate donors are especially appropriate for deactivated (fluorine containing) donors in contrast to other glycosyl donors (thioglycosides, etc.) [28]. A highly convergent synthetic method has been developed for the synthesis of cellooctaose and other cello-oligosaccharides via di- and tetrasaccharides which can be converted either into glycosyl donors by deallylation and transformation into the trichloroacetimidates, or into acceptors by deacetylation (Scheme 3) [29].The use of p-methoxybenzyl groups (PMB) instead of acetyl groups was less successful. The combination of acetyl, pivaloyl, and benzyl groups leads to the proper balance between stability and reactivity of trichloroacetimidates as donors and the acceptor substrates. High yields were achieved by using a specially developed high-vacuum system for glycosylations under anhydrous conditions. Trichloroacetimidates have also been used for the glycosylation of more complex carbohydrate acceptors (Scheme 4). Galp( 1-3)GlcN and other disaccharides were synthesized and converted into fluorescein-labeled acceptors which are used for sialyltransferase assays [301. Glucosyl diosgenine has been used as carbohydrate acceptor for further glycosylations with various trichloroacetimidates (Scheme 4) [31, 321. All glycosides were obtained in high yields and high stereoselectivity because of participating neighboring groups at 2-OH in the synthesis of dioscin, polyphyllin D, balanitin 7, and gracillin.
8
2 Trichloroacetimidates
Baw Bzo
OBI
+
BZO
Hr&
OCH3
75%
OBZ
BzO
OBz
HO
OBZ
GalP(1-3)Gal OBZ
Ph
OAc
OBz
BZO~ O ! & & O C H ,
Aug6 et al. 1241
O YCCI, NH
AcO
.
CH TMSOTf CI ,O°C
Ph
TMSOTf CICH,CH,CI,,
-1 8%
Ga@(1-3)GalN
AcO OAc
Bock et al. [251
CCI,
N,
CCI, Ad)
OANH
BFS. OEtz A
c O AcO
H
+
0 OAll
CH,CI,, -20°Cb
AcO OAll
OAc
L-Rhaa(1-2)Glc
95% Liu et al. "261
A c O AcO R q
+
6
HO BnO
0
PhtN
I
O YCCI, NH
R = OAC, H, F
R
I cot
TMSOTf CH2C12,00C CHzCIz,OoC
*
93%
93%
A
C
~ W AcO
~
Karnerling et al. [281
,OBn
O
-
~ PhtN
I
BZ& BnO BnO
0C8H17
Scheme 2.
Aminosugar Trichloroacetimidates The use of azidosugars and N-phthalimido-protected aminosugars is well established in carbohydrate chemistry. But many new protective groups have been proposed to enable deprotection under milder conditions or to apply orthogonal protection strategies (Scheme 5). For the synthesis of P-glycosides of aminosugars, protective groups with amide structure are mainly used; these are capable of neighboring group participation. But formation of stable oxazolines reduces the reactivity of the glycosyl donors, and phthaloyl groups are therefore preferred. They can be cleaved only under strongly basic conditions, for instance, with hydrazine. As an alternative, the use of the tetrachlorophthaloyl (TCP) group has been proposed [33];this can be more readily removed with ethylenediamineor with sodium borohydride followed by acid treatment (pH 5). The glycosylation reaction gives excellent yields and in acetonitrile as solvent exclusively the P anomers are obtained. Further applications of TCP-protected trichloroacetimidates are shown below (glycosyl amino acids, C-glycosides).
2.3 0-Glycosides
9
OAll
1. Se02IAcOH
bpiv
OPiv
HBnOo
~
O A OPiv
I
I
BnO OPiv
/
disaccharide-
OPiv
\
disaccharide-
BnO
OPiv
OPiv
OPiv
tetrasaccharideimidate
OPiv
tetrasaccharideacceptor CH&,
O°C
Nishimura, Nakatsubo 1291
OPlV ACO BnO
OPlV
BnO OPlV
OPlV
OPlV
OPlV
OPlV
Scheme 3.
Similar properties have been observed for 4,5-dichlorophthaloyl (DCPhth) protected trichloroacetimidates [ 341. Deprotection can be performed with ethylenediamine. The trichloroethoxycarbonyl (Teoc, Troc-both abbreviations are in use) group has already been applied successfully to aminosugars [35].Because it can be cleaved under reductive conditions (with zinc) it can be used for base-labile compounds and more importantly for demanding orthogonal protection strategies. For glycosylation reactions with N-Teoc-protected trichloroacetimidate-activated aminosugars very high yields and stereoselectivities are reported [36]. Application was also shown for the synthesis of some complex Lipid A analogs [37-391. Glycosylation of acceptors with Teoc-protected trichloroacetimidates, both containing long chain fatty acid residues, was performed in high yields and stereoselectivities. Further applications (glycolipids, glycopeptides, bacterial polysaccharides, solid-
10
2 Trichloroucetimidates
HO HO&
"Po AcHN
BF,.OEt,
+
CCI,
CH~CI, 76%
-
1. deprotection
HO
OH
2. fluorescein isothiocyanate
O(CH,),NHCbz NHAc
Slim et al. [301
HO
CCI,
AAcOc O ~ 6 ; * o I c c , , OAc
AcO
OH
OAC CCI,
L-Rhaa(l-2)GlcP(l-O)diosgenine (dioscin)
Xyl~(l-3)Glc~(1-2)Glc~(l-O)diosgenine (balanitin 7)
L-Araa(1-2)GlcP(1-0)diosgenine (polyphyllin D)
Glc~(1-3)Glc~(l-O)diosgenine
I
Rhaa(1-2)
(gracillin)
Hui et al. [31,321
Scheme 4.
phase synthesis) of Teoc-protected trichloroacetimidates are shown below. Similar results were obtained with p-nitrobenzyloxycarbonyl (PNZ) protected trichloroacetimidates [40]. Deprotection can be performed under reductive conditions either with hydrogen and palladium-on-carbon or with sodium dithionite under neutral or slightly alkaline conditions. If protective group manipulations are incompatible with basic or reductive conditions, use of the 2,5-dimethylpyrrole group is proposed; this can be treated with hydroxylamine hydrochloride to liberate the amino group [41, 421. Glycosylations with dimethylpyrrole containing trichloroacetimidates perform well, leading selectively to P-glycosides (1,2-trans-glycosides). Recently the dimethylmaleoyl (DMM) group has been proposed as an amino-protecting group which can be cleaved under mild aqueous basic and then acidic conditions (pH 5 ) [43]. Because of the anchimeric assistance and the electron-withdrawing character of the DMM group the corresponding trichloroacetimidates give stereoselectively P-glycosides in high yields. The dithiasuccinoyl (Dts) group [44] for amino group protection is discussed below (glycopeptides).
11
2.3 0-Glycosides BnO A AcO c o ~ O y C C l , +
TCPN
OH
n BnO O OBn q CH,CN, 91% r.t.
B
NH
81%
BnO 0
BnO
CI OBn
TCP = tetrachlorophthaloyl,
Castro-Palomino, Schmidt [33]
CI
0
CI
1. TMSOTf HO DCPhthN
73%
OMP
BnO DCPhthN
NH
0
OBn
AcNH
3. AcpO, pyridine 84%
wz:
DCPhth = 4,5-dichlorophthaloyl,
b
2.ethylenediamine
Ogawa et al. [34]
0 BnO
--,
0
Teoc = trichloroeihoxycarbonyl,
-k!
R = (S)-3-(tetradecanoyloxy)tetradecanoyl = (S)-3-(dodecanoyloxy)tetradecanoyl R" = (S)-3-(benzyloxy)tetradecanoyl
-OCH,CCI,
(Troc)
% % ;A
+
PNZ'
BnO
1. TMSOTf
NH
1. BF,.OEt, CHZCI,, -3OC
&
HO BnO
2. NaOMe, 79% quant.
OBn
OBn
'yNH
.
3. Na,S,O, quant. 4. Ac,O, pyr., 93% C
H NO,
OBn
AcNH
0
CCl,
PNZ = p-nitrobenzyloxycarbonyl, - - O
Kusumoto et al. [37-391
Quian, Hindsgaul [40]
~
1. TMSOTf
CCI,
--
OCH,
~~~&o,cc13 DMMN
6
HO BnO
AcO
NH
DMM = dimethylmaleoyl,
OBn
+
1. TMSOTf C 78% H,CI,
2.NaOH 3.HCI 4. Ac,O 97%
0 $CH3 CH,
Scheme 5.
OBn
CHZCI,, -25C 78% 2. NEt,,MeOH, H,Ob 3. NH20H, 8OC 4. Ac,O, pyr.
r. t.
.
AcO
BnO OCH.
Boons et al. [41]
AEoa:o+OBn AcNH
OBn
Schmidt et al. [43]
12
2 Trichloroacetimidates CONH,
CONH,
ACO
31% OAc 0
Welzel et al. 1461
TMSOTf CH,Cldhexane, 60%
r.?
Bnd
Scheme 6.
Finally, the synthesis of moenomycin-type analogs is described (Scheme 6). Use of the trichloroacetyl protecting group for aminosugar trichloroacetimidates as previously proposed [45] gave the desired disaccharide (31% yield) [46]. Because of silyl group migration under the glycosylation conditions, the tri-TBS-protected acceptor, the diglycosylated acceptor, and the oxazolidine of the donor were also formed as by-products. The trichloroacetyl group can be converted into the acetyl group either with zinc-acetic acid or with tributyltin hydride in the presence of AIBN. Better results were obtained with a 2-(trimethylsilyl)ethanesulfonyl-protected trichloroacetimidate [47].The desired P-glycoside was obtained stereospecificallyin 60% yield and further transformations are under investigation. Trichloroacetimidate derivatives for the preparation of aminosugar a-glycosides (1,2-cis-glycosides)are rare. The use of azidosugars is well established (examples are given below), but unfortunately they are not readily available. Azidoglucose, for example, is prepared either from glucose via glucal by azidonitration [48]or from glucosamine via diazo transfer with trifluoromethanesulfonyl azide [49]. The N-l(4,4-dimethyl-2,6-dioxocyclohexylidene)ethyl (Dde) group has been proposed as non-participating neighboring group for the synthesis of 1,2-cis-glycosides(Scheme 7) PI.
,OAc
Kellam et al. [501
ACO
DdeNH
~~
. .. .
0
Dde = N - l - ( 4 , 4 - d i m e t h y l - 2 , 6 ~ 1 x ~ c y c l ~ h e x ~ ~ ~ e ~ e ~ ~ ~ h y Scheme l a e n e ) e7.t 0
2.3 0-Glycosides
13
65%
CCl,
Furstner, Konetzki 1541
Scheme 8.
P-Mannosides
The synthesis of p-mannosides is crucial because p-mannoside formation is disfavored because of stereoelectronic and steric reasons. Besides the already known direct mannosylation reactions some indirect approaches which have been reported are based on the preparation of I)-glucosides and then inversion of the configuration at C-2 to yield the corresponding P-mannosides (Scheme 8). According to a previously introduced method [ 5 11, a p-glucoside, synthesized using a trichloroacetimidate as glycosyl donor, was converted into its 2-0-triflate [ 521. Intramolecular nucleophilic substitution afforded a Mano( 1-4)GlcNp( 1-4)GlcN trisaccharide occurring in the core-region of glycoproteins. In other examples 2-0-triflate intermediates were subjected to intramolecular nucleophilic substitution under inversion of the configuration by reaction with tetrabutylammonium nitrite [53] (see below, Scheme 26) or with tetrabutylammonium acetate [54]. The reaction has been significantly improved by means of ultrasound, but very good yields have been also obtained previously via a sulfonyl imidazylate [55]. 2-Deoxyglycosides
Direct glycosylation with 2-deoxyglycosyl donors is not employed because the corresponding glycosyl donors are quite labile and they lack stereocontrol by anchimerically assisting groups. A recent application of a previously developed method [56] has been reported; it uses a 2-deoxy-2-thiophenylglucosyltrichloroacetimidate which can be readily prepared from glucal (Scheme 9) [57]. After glycosylation the thiophenyl group can be removed by reduction methods. Because glucal is not readily available as starting material and the stereoselectivity of the glycosylation of carbohydrate acceptors is critically dependent on substituent X
14
2 Trichloroacetimidates
ROH, TMSOTf CH& -78% &I3
& '*OR TBDMSO
- -re$- - -b
Roush et al. [571
X = OTos, Br, H
BnO BnO
TBDMSO -&OR
PhS
a -unu- I
OCH.
1. TMSOTf
91% L 2. BuqSnH, AlBN 74%
-
BnO I
OCH.
Castro-Palomino,Schmidt I581
Scheme 9.
at the 2-position, a new method has been proposed. Glycosylation reaction with 2-O-thiobenzoyl-protected O-glucosyl trichloroacetimidates gave exclusively the 0-glucosides which could be converted into the 2-deoxy-P-glucosides by reduction with tributyltin hydride [58]. Starting from mannosyl trichloroacetimidates the 2-deoxy-a-glucosides were obtained in the same manner [ 5 8 ] . Miscellaneous Compounds
The synthesis of some rare glycofuranosyl donors has also been reported (Scheme 10). The trichloroacetimidates of D-glucofuranose [59], D-mannofuranose [59], and D-galactofuranose [59, 601 have been prepared and used for the synthesis of furanosyl glycosides. With the heptosyl trichloroacetimidate some neoglycoconjugates containing Hep( 1-4)Kdo and Hep(1-5)Kdo-4-phosphate-(2-6)GlcNAcresidues were synthesized which are part of the inner core region of bacterial lipopolysaccharides [61]. A 5-thio-~-fucosyltrichloroacetimidate was used for the synthesis of 5-S-~-Fuca( 1-2)Gal and other disaccharides which have inhibitory activity against a-L-fucosidases [621. Some 5-thiomannose-containing oligosaccharide mimetics were prepared with a 5-thiomannosyl trichloroacetimidate; their binding constants to concanavalin A were determined [63]. An example of intramolecular glycosylation has been reported with a Glca(12)Glc trichloroacetimidate [64, 651. Reaction with boron trifluoride-diethyl ether gave the cyclo-a(1-2)glucobiose (cyclokojibiose) with concomitant expulsion of the O-benzyl protecting group. 2-O-debenzylation previous to cyclization is not necessary because the oxygen is sufficiently nucleophilic and the reaction is entropically favored. A similar example has been reported with a fucose moiety [66]. Complex Oligosaccharides
Complex oligosaccharides are major parts of many biomolecules (glycolipids, glycoproteins, bacterial lipopolysaccharides, mucopolysaccharides, etc. ) and they have a variety of functions. Much interest has been focussed on Lewis antigen structures which are found in glycosphingolipids and on glycoproteins. Many syntheses have
2.3 0-Glycosides
15
AcO AAco & * I
O YCCI, NH
CCI,
LOB"
Plusquellec et al. 1591
Roy et al. 1601
Kosma et al. (611 HO O ,H
&
A d
OAc NH A d
OAc
+
Hashimoto et al. 1621
OAll
OH
BF3.OE1, CH2C12,-2OOC 86%
*
--b H : + HO
OH
,.OAc
Scheme 10.
been described for the Lewis' antigen (LeX)structure which has been found to be a tumor-associated antigen, and for the sialyl LewisX(s LeX)epitope which is involved in cell-adhesion and inflammation processes mediated by its binding properties to selectins. Several synthetic strategies with various building blocks and/or different protective group patterns have been employed, but in recent investigations two alternatives have been favored. A suitably protected azidolactose building block which is available from lactose via lactal, has been chosen (Scheme 11) because it already contains the desired Galp( 1-4)Glc:N linkage [67]. Because of the non-participating neighboring group at 2-OH, fucosylation proceeded exclusively to the a-glycoside and gave the LeXtrisaccharide repeating unit in very good yield (97%). Transformation into the trichloroacetimidate and coupling with the lactose moiety gave spacer-linked Le" which could be extended with the trisaccharide trichloroacetimidate into the spacerlinked dimeric LeXantigen. Following the alternative strategy (Scheme 12), an azidoglucose building block was chosen as starting material and glycosylated first with fucosyl trichloroacetimidate and after benzylidene ring opening with galactosyl trichloroacetimidate [68]. Both glycosylations were highly stereoselective and gave, because of suitable neighboring groups and reaction conditions, exclusively the anomers desired. The high yields were based on the use of the 'inverse procedure' (i.p.), i.e. addition of the trichloroacetimidate donor to the acceptor-catalyst mixture. The LeXtrisaccharide repeating unit was then transformed into the trichloroacetimidate donor and into the glycosyl acceptor and then coupled with trimethylsilyl triflate as catalyst in
16
2 Trichloroucetimidutes
NH
OBI
OBn
97%
Aco
OBZ
Windrnuller, Schmidt [671
Scheme 11.
~
h
T t HO
-
~
N3
~
~
~
TMSOTf, i.p. Et,O, r.t. 85%
TMSOTf, i.p. Et20, r.t. 71%
trisaccharide
hexasaccharide irnidate acceptor N3
imidate 081
dodecasaccharide imidate
n = 2 : dim.LeX n = 3 : trim.LeX n = 4 : tetram. LeX
Scheme 12.
Schmidt et al. 68
17
2.3 0-Glycosides
acetonitrile as solvent to yield stereospecifically the P-connected hexasaccharide by means of the 'nitrile effect'; this effect is particularly efficient with 2-azido sugars. The nonasaccharide and dodecasaccharide were prepared in the same way by coupling the hexasaccharide trichloroacetimidate with the tri- or hexasaccharide acceptor. Transformation into the trichloroacetimidates and reaction with the lactose acceptor yielded the octa-, undeca-, and tetradecasaccharides. After coupling with azidosphingosine and further transformations the dimeric, trimeric, and tetrameric Le" antigens were obtained. The synthesis strategy is highly convergent and allows variations in the saccharide structure and the lipid moiety. The ready a.vailability of glucosamine was reason to transfer this synthesis strategy to glucosamine building blocks which can be obtained directly from glucosamine. The N-tetrachlorophthaloyl (TCP) [69], N-trichloroethoxycarbonyl (Teoc) [70], and N-phthalimido (Pht) [711 protected glucosamine acceptors were glycosylated with the 0-galactosyl and 0-fucosyl trichloroacetimidates; both reactions proceeded with high stereoselectivity. The resulting Le" trisaccharide repeating unit was transformed to the corresponding trichloroacetimidates which were used for the glycosylation of the lactose acceptor to give, owing to neighboring group participation, the p( 1-3)-linked Le" pentasaccharide. The Lewisa (Lea) antigen is a blood group antigen; because of its structural similarity to the Re" antigen it has been synthesized in a similar manner (Scheme 13) [72]. The benzylidene-protected azidoglucose acceptor was first glycosylated with galactosyl trichloroacetimidate and, after benzylidene ring opening, with fucosyl trichloroacetimidate; both glycosylations could again be performed with high diastereoselectivity. Transformation into the trichloroacetimidate and reaction with the CCI,
CCI,
OTBS
TMSOTf Et20, r.t. 88%
N3
Ph T HO a O
T
D
BnO
TMSOTf, i.p. Et20, r.t.
AcO
*
OAc
78%
S
I
0KCC'3 NH
BF, OEt2 CH,C12, hexane
H
Schmidt et al. [;TO]
o
#
~
o
~
O
OBn B n
'
61%
FUCCU(~-4) HHO
O
aOTDS NTCP
Schmidt et al. [69]
Hio&
OTBS
I
Gal~(I-3)GlcNAc~(1-3)Gal~(1-4)Glc
N,
Toepfer, Schmidt [72]
Lea Toepfer, Schmidt [72]
Scheme 13.
18
2 Trichloroacetimidates
lactose acceptor in a non-polar solvent as catalyst with diethyl ether-boron trifluoride gave the Lea pentasaccharide. The p-linkage was stereospecifically formed under inversion of the configuration; typical conditions for this reaction type were used. The N-trichloroethoxycarbonyl (Teoc) protected glucosamine acceptor was proceeded in the same way [70]. The Lea trisaccharide was also prepared from the 3,4-di-O-unprotectedazidoglucose [72] and N-tetrachlorophthaloyl (TCP)-protected glucosamine [69] acceptors by attachment of the two sugar residues directly one after the other. The blood group antigen H and the LewisY (Ley) antigen, a stage-specific embryonal antigen (SSEA), have been synthesized using the same azidolactose building block (Scheme 14) [67]. By regioselective monofucosylation and by difucosylation of the 3,2'-di-O-unprotected azidolactose acceptor, the H-antigen trisaccharide and the Ley tetrasaccharide were synthesized in high yields. Transformation into the trichloroacetimidate and coupling with the lactose moiety furnished the spacer-
CH,Cl$hexane BF,.OEt, -10%
70%
!
o(cHd&~H, HO
OBZ
Ga@(1-4)GlcNAcp(1-3)Ga@(1-4)GlcP(1-0)Sp FuW( 1-2)
H-antigen
I
BFS.OEt2 CH,Cldhexane
H O + O - Y
OBZ
-1O°C
71%
F u c ~1(-2) F u c ~1(-3) Windmuller, Schmidt I671
Scheme 14.
2.3 0-Glycosides
19
-
trisaccharide imidate
BnO
--+ trisaccharide N3
TMSOTf. i.p. Et,O
TMSOTf, i.p. EtPO
85%
89%
acceptor BnO
TMSOTf CHSCN, -4OC 80%
I
COOCH,
FUctr(1-3)
dim. sLeX
1:
octa-
hexasaccharide imidate
Fuca(1-2) Galp(l-4)GlcNAcp(l-3) alp(l-4)Glcp(l-I)Cer
[
F U C ~-3) ('~
I
dim.LeY
BnO
Hummel, Schmidt [73]
Scheme 15.
linked H-antigen and Ley antigen, respectively. Another strategy has been applied to the synthesis of the dimeric Ley antigen (Scheme 15) [73]. According to the previously described synthesis of dimeric LeX (Scheme 12, Ref. [68].) the Le' trisaccharide has been prepared from the azidoglucose acceptor, the fucosyl and the galactosyl trichloroacetimidate, and the derived trisaccharide was used as trichloroacetimidate donor and as acceptor for the synthesis of the corresponding hexasaccharide. Coupling with the lactose moiety gave the octasaccharide which was subjected to a protective group change at the terminal galactose moiety. Regioselective protection of the 3-OH, 4-OH, and 6-OH groups and fucosylation at 2-OH gave the desired nonasaccharide. After coupling with azidosphingosine and further transformations the dimeric Ley antigen was obtained. For the synthesis of the sialyl Lewis' (sLe') antigen, the terminal galactose moiety of the octasaccharide was 6-0-benzyl-protected. Regioselective glycosylation of the 2,3,4-tri-O-unprotected acceptor with neuraminic acid phosphite as donor gave the desired 3-0-sialylated oligosaccharide which was coupled with azidosphingosine and transformed into the dimeric sLe' antigen by use of established methods. Further syntheses of the sLe' determinant follow the same strategy, but applying other glycosylation methods (Scheme 16). Fucosylation of a similar O-benzylideneprotected glucosamine acceptor performed well with a thioglycoside donor [74-761, but the galactosylation was critical because of the steric hindrance of the 4-OH group. The methylthio galactoside donor gave a 16% yield [74]; the trichloroaceti-
20
2 Trichloroacetimidates
HNAC
Ga@(1-4)GlcNAcP-X NeuAca(2-3) Fuca(1-3)
= NH2
Kretzschmar, et al. [74-761
Scheme 16.
midate was, therefore, again favored. Regioselective sialylation with the thioglycoside donor gave the sLeXdeterminant bearing an amino group or a spacer at the reducing end for attachment to peptides or for the generation of other conjugates. The synthesis was suitable for large-scale preparations 1761 and enabled the use of sialic acid derivatives with modifications at C-5 to synthesize sLeXanalogs and to investigate their binding properties to E- and P-selectins [75]. A similar strategy for the synthesis of the sLeXdeterminant was used by applying the glycal method (Scheme 17) [77]. The 3,4-di-O-unprotected glucal was regioselectively fucosylated ( a : = 7.5: 1) with fucosyl fluoride and then galactosylated with 0-galactosyl trichloroacetimidate to yield the Le" trisaccharide glycal. The 2,3,4-tri-O-unprotected galactose acceptor was then regioselectively sialylated with sialyl chloride as donor. Then application of the glycal method gave either the sLeX trisaccharide or-by extension with the tributyltin derivative of the lactal acceptor -the sLeXpentasaccharide glucal derivative which can be activated to provide a donor for the synthesis of conjugates. The synthesis is efficient and convergent, and it is worth mentioning that for the crucial galactosylation of the sterically hindered 4-OH group the trichloroacetimidate method has again been employed. The synthesis of LeXor sLeXanalogs or simplified structures, so-called mimetics, is of great interest for binding studies with selectins, thus high affinity ligands concomitantly with convenient chemical syntheses for novel anti-inflammatory drugs are evaluated. The synthesis of some only slightly modified Le" analogs containing Glc instead of GlcNAc [78], GalNAc instead of Gal [79],or 5-thiofucose instead of Fuc [801 has been similarly performed as described above using trichloroacetimidate donors. It has been found that sialylations are always crucial and expensive processes, therefore many syntheses for sLeXmimetics have been developed in which
2.3 0-Glycosides
21
OTDPS OBn
OTDPS
A
c
O
g
O
&
BZO
AgC104 SnCI, DTBP 59%
OBn
BF,~OEt, CH& -78OC 73%
Bzo
Danishefsky el al. 071
Gal~(1-4)GlcNAc~(l-3)Gal~(l-4)Glucal
Galp(1-4)GlcNAc
NeuAca(2-3) Fuca(1-3)
NeuAca(2-3) Fuca(1-3)
Scheme 17.
AcO
BnO
AcO
a
O
B
OAc
Ph q 'HO a O A l l 0 0 ~
TMSOTf CH& 87%
I
AgOTf Bu,SnCI, 90%
b
F u c ~1-4) (
n
I
AcO W o OAc & O A I I
3'-phospho-Lea-analog AcO
OBn OAc
Kiessling et al. [Sll
3'-C-carboxymethyl OBZ OBn
DMTST TMSOTf benzene, 6OC CH,CN, -1OOC 86% 67%
OBn
b
'~6
BnOoBn @OBn
I
Kiso el al.[831
NHAc
HO
Scheme 18.
Thoma el al. I841
22
2 Trichloroacetimidates
HO
OH
Wong et al. IS81
Scheme 19.
neuraminic acid is replaced by other acids (Scheme 18). A 3’-phospho-Lea analog [81] and a 3’-sulfo-Lex analog [82] were prepared from 3-0-unprotected 4,6-0benzylidene-glucose derivatives. As well established in the synthesis of LeXand sLeX (see above), for all glycosylations with galactose the trichloroacetimidates were successfully used as donors. For the synthesis of the 3’-C-carboxymethyl LeXanalog [831 the synthesis and use of the required galactose-derived trichloroacetimidate donor has been reported. A more rigidly fixed carboxylic group is contained in the sLeXmimetic with a six-membered ring fused to galactose bearing an equatorial carboxylic acid group [84]. The required branched galactose moiety has been prepared and used as trichloroacetimidate-activated donor. Further sLeXmimetics with other simplifications have been proposed (Scheme 19). The GlcNAc moiety has been replaced by cyclohexanediol which was used as acceptor and glycosylated first with the galactosyl trichloroacetimidate and then with the corresponding fucosyl donor [85]. Then a malonic acid derived group was introduced at the 2-OH of galactose. In an even more simplified structure the galactose moiety has been replaced by an aliphatic chain [86]. The acceptor which was prepared from glucal is first glycosylated with the fucosyl trichloroacetimidate and then converted into the malonic acid derivative. In a related mimetic, the malonic acid group is attached more rigidly at a benzene ring [87]. Glycosylation with the
2.3 0-Glycosides
A*
OAC
COOCH,
OBn
23
OBn
(GM, intermediate) pentasaccharide imidate
Gal~(I-3)GalNAc~(1-4)Gal~(l-4)Glc~(l-1)Cer NeuAca(2-3)
Schmidt et al. [891
Scheme 20.
fucosyl trichloroacetimidate resulted in a low yield only (35%), because of steric hindrance. A c:ompletelydifferent structure has been chosen with a terpenoid system [88]. Glycosylation of isosteriol with the mannosyl trichloroacetimidate gave a mimetic which selectively inhibits P-selectin, but not E- and L-selectins. Glycolipids other than the Lewis antigens have also been synthesized with trichloroacetimidate donors. For the synthesis of the ganglioside GM1 several strategies have been suggested [89, 901. Applying a convergent strategy (Scheme 20) [89] a 3,4-di-O-unprotected lactose acceptor was first sialylated with neuraminic acid phosphite as donor to give a GM3 intermediate, and then glycosylated with a Galp( 1-3)GalN trichloroacetimidate as donor; the latter can be easily prepared from azidoglucose and galactosyl trichloroacetimidate. Coupling with azidosphingosine by application of trichloroacetimidate activation of the pentasaccharide residue and then further transformations yielded GMI . Another strategy via a C A I intermediate performing sialylation at a later stage has also been reported [90]. The GM3 intermediate is also suitable for the synthesis of GM2 [91]Glycosylation of the GM3 intermediate with the Teoc-protected galactosamine trichloroacetimidate, which can be more readily synthesized from galactosamine than azidogalactose from galactal, and application of the established transformations, yielded GM2 which is required in a cancer immunization study with GM2-containing vaccines. A glycolipid of the neolucto series named 3-0-sulfo glucuronyl paragloboside is also of interest because of its immunological properties (Scheme 21) [92]. The syn-
24
2 Trichloroucetimidutes COOMe
k0.d-
H:p&
BzOO
OBn OSE
HO
8
BnO
gn&
OBn OSE
OBn
CCl,
OSE
94% TMSOTf
CH,CI,, r.1.
pentasaccharide imidate
N3
BF,.OEt, CHZCI,, O°C 72%
00~S-0-3GlcAP(1-3)GalP(I-4)GIcNAc~(1-3)Gal~(1-4)Glc~(l-l)Cer
3-0-sulfo glucuronyl paragloboside
Hasegawa et al. [921
Scheme 21.
thesis is based on glycosylation of a lactose acceptor with phthalimidoglucosyl trichloroacetimidate. Glycosylation with a disaccharide trichloroacetimidate, which was prepared from a glucuronic acid trichloroacetimidate and a galactose acceptor, and the usual transformations including introduction of the sulfo group yielded 3-0-sulfo glucuronyl paragloboside. A more straightforward synthesis of a related structure has been reported using a Galb( 1-4)GlcN disaccharide (lactosamine) which is contained in the neolucto structure as starting material (Scheme 22) [93]. Glycosylation of the lactose acceptor with the azidolactosyl trichloroacetimidate donor followed by sialylation and application of the standard azidosphingosine glycosylation procedure gave the neolacto ganglioside LM1. All the above mentioned examples contain P-glycosidic linkages which can be obtained by means of participating neighboring groups or-for azidosugars-by means of the nitrile effect. The synthesis of gangliosides of the globo series which contain also an a-galactose linkage is more difficult. From the azidogalactosyl trichloroacetimidate and a galactose acceptor the 2-0-benzyl-protected disaccharide was prepared (Scheme 23) [94, 951; this was converted into the trichloroacetimidate and used as donor for the glycosylation of the lactose acceptor. Using typical conditions favoring s,~-type reactions (non-participating neighboring group, strong Lewis acid, diethyl ether as solvent), the a-connected tetrasaccharide was obtained with high selectivity. Ensuing galactosylation, sialylation, and application of the
25
2.3 0-Glycosides TMSOTf CH2C12,-4OOC (SOYO) tetrasaccharide
b
I OBn
58%
OBn
OEt
pentasaccharideimidate
5
lF3.0Et2 CH2C12,O°C 43%
O h
NeuAca(2-3)Gal~(l-4)GlcNAc~(l-3)Gal~(1-4)Glc~(I-1)Cer Tietze et al. [931
LMl
Scheme 22.
Ph
Ph
Ph
TMSOTf
&:s
Ho BnO
OAll
HO
N,
OBn
BnO
OBn
OBn
1
CCI,
0
Y
NH
Y CCI,
Ph
Ph
TMSOTf, i.p EbO, -1OOC
Bds+E;ohloBn TMSOTf I
-'I
Ell0
AcOO
vNH I CCI,
TMSOTf CH,CN, -4OOC-I (a$= 3.5:l)
%En
AcO
62%
AcN
COOCH,
dAc
hexasaccharide imidate
N,
TMSOTf OBZ
Neu5Aca(2-3)Gal~(l-3)GalNac~(l-3)Gala(l-4)Gal~(1-4)Glc~(1-1)Cer Schmidt et al. 194,951
Scheme 23.
SGG
26
2 Trichloroacetirnidates
azidosphingosine glycosylation procedure yielded the sialylgalactosylgloboside (SGG), which bears the stage-specific embryonic antigen 4 (SSEA-4) structure. Another example of the globo series is the globo H (human breast cancer) antigen. It has been prepared by means of a similar strategy [96]. Although in glycopeptide synthesis, most strategies are based on the synthesis of the complete oligosaccharide moiety and then its attachment to the amino acid or peptide, glycosylations of glycosylated amino acid acceptors have also been reported (Scheme 24). Thus, glycosylation of the GalNa( I-0)Thr acceptor with
Ph
'%$
+
H
o
1. TMSOTf, CH2CI,, O°C 02%
b
2. Zn, &OH, AC20, THF 3. AqO. pyr. (80%)
TeocN
b
O YCCI, N H
core3
Fmoc[O X HC0,Bu' 3
FmocNH
Bock et al. [971 Ph
+
Ph
"q
Bock et al. 1441
1. TMSOTf, CH2C12,40°C 67% 2. Zn, 7OoC THF, AC20,HOAc)
AcNH
H
Ph
Ph
Ph
CCI,
Qiu, Koganti I981
w T s 0
'no
0
II
-15% 69%
AcN
OBn
Scheme 24.
OBn
OBn
OBn
2.3 O-Glycosides
27
N-trichloroethoxycarbonyl (Teoc)- [97] or the N-dithiasuccinyl (Dts)- [44] protected glucosamine trichloroacetimidates, respectively, afforded the core 3 building blocks. The P-glycosides were stereospecifically obtained by means of neighboring group participation. Use of the N-acetyl-protected galactosamine trichloroacetimidate gave stereoselectively the a-connected core 5 structure [98]. Thus, core 1, 2, 3 , 4, 5 , sialyl 5 , 6, and F1-a building blocks were synthesized [44, 97-99]. A sialylated glycosy1 serine building block was prepared with a disaccharide trichloroacetimidate donor and used in the solid-phase synthesis of the B-chain of human u2HS glycoprotein [loo, 1011. In the synthesis of branched complex type N-glycans the oligosaccharide part is usually prepared first; it is then attached to the amino acid, usually asparagine. For investigations of the function of glycoproteins the synthesis of libraries with variations of the antennary structures is required. A highly convergent chemoenzymatic approach has recently been reported (Scheme 25) [ 102-1041. The core trisaccharide Mar$( 1-4)GlcNP( 1-4)GlcN was synthesized via the P-glucoside and then inversion of the configuration by intramolecular nucleophilic substitution as described above (see Scheme 8:1, thus avoiding the crucial direct P-mannosylation. The trisaccharide acceptor is suitable for the attachment of various antennary structures with linear or branched trichloroacetimidate donors which were again synthesized by application of the trichloroacetimidate method. Several examples were prepared and used for investigations on further glycosylations with galactosidases and sialidases. Another versatile strategy enables the synthesis of diantennary and bisected diantennary oligosaccharides (Scheme 26) [ 531. The azidoglucose acceptor was glycosylated with the glucosyl and mannosyl trichloroacetimidates. The P-glucoside of the resulting trisaccharide was converted into the P-mannoside moiety by inversion of the configuration at C-2 via triflate formation and substitution with tetrabutylammonium nitrite. Further glycosylations with the mannosyl and the azidoglucosyl trichloroacetimidates gave the bisected diantennary pentasaccharide which was used as acceptor for extension with lactosamine trichloroacetimidate. After transformation of the nonasaccharide into the trichloroacetimidate the terminal glucosamine residue was attached at the reducing end. Thus, the synthesis is highly variable in the connection to the protein and in the antennary structures. Further syntheses applying direct 0-mannosylation have also been reported [ 105- 1071, and it was found, that the trichloroacetimidate method was superior to other glycosylation methods, especially for the synthesis of u-mannosides. The acidic mucopolysaccharides, which consist of disaccharide repeating units containing D-glucuronic or L-iduronic acid and glucosamine or galactosamine, respectively, are also members of the class of glycoproteins or proteoglycans. Because L-iduronic acid is not readily available it must be prepared. A new synthesis from Dglucose was recently described by formation of the exo-glucal, diastereoselective hydroboration, and oxidation of C-6 [ 108). Galactosamine is also a very expensive starting material and it has therefore been prepared from glucosamine by inversion of the configuration of the 4-OH group via the triflate, and used for the synthesis of dermatan sulfate fragments [ 1091. In another approach [ 1101 an L-idose acceptor was glycosylated stereoselectively (P: u = 2.5 : 1) with an azidogalactosyl trichloroacetimidate. The P-selectivity was achieved, without neighboring group participa-
28
2 Trichloroacetimidates
85% b
BnO
BnO
F
0
\
NPht
BF3 OEt2 CH2C12 84%
Ad)
Ad)
BF, OEt2 CH2CI2 -45% 76% Aco AcO
OAC
Aco Aco
GkNAc&l-6)
I MW(1-6)
I
GkNAcS(1-2)
1
Man~(1-4)GICNACP(1-4)GlcNA@N3
Unverzagt [102-1M]
I
GlcNAcP(l-P)Mana(l-3)
Scheme 25.
tion, in a non-polar solvent at low temperature by an SN2-type reaction. The resulting disaccharide was oxidized to the iduronic acid derivative and converted into the GalNp( 1 -4)IdoA trichloroacetimidate and into a corresponding acceptor which were then used for the next glycosylation step to provide a dermatan tetrasaccharide fragment; thus, because of neighboring group participation an a-glycosidic bond was formed stereospecifically. The tetra-, hexa-, and octasaccharide fragments of hyaluronic acid (Scheme 27) [ 1111 have been prepared by repeated glycosylation of a GlcNP( 1 -4)GlcA acceptor with a disaccharide trichloroacetimidate which was prepared from a trichloroacetyl
29
2.3 0-Glycosides
ACO AcO OBn
I
TMSOTf CH2C12,hexane 93%
Et20,-4OOC
68%
Bu~N'NO,
I
GlcNAc~(l-4)Man~(l-4)GlcNAc~(1-4)GlcNAc
I
Ga@(1-4)GlcNA1:b(1-2)Mana(1-3) Weiler, Schmidt 1531
Scheme 26.
(TCA)-protected glucosamine trichloroacetimidate and a glucuronic acid acceptor. All reactions gave, because of neighboring group participation, exclusively pglycosides in high yields. Some chondroitin fragments have been prepared starting from an azidoglucosyl trichloroacetimidate and a glucose acceptor (Scheme 28) [ 1 12, 1131. Despite the use of a non-participating neighboring group, the p-glycoside was obtained exclusively because in a non-polar solvent at low temperature with boron trifluoride as the catalyst an S,z-type reaction was favored. With trimethylsilyl triflate lower stereoselectivity (p :CI = 2 : 1) was observed. The disaccharide was oxidized to the glucuronic acid derivative and converted into the
30
2 Trichloroacetimidates
+
PhT-+
Ho* BzO
OMP
TCANH
OBz
O Y N H dCl,
q
89%
~
~
~
&
O
M
~
HO
TCANH
COOMe
-4 87%
Ph
TMSOTf
zqsy-;*oMe TCANH
Cm+ie
TCANH
COOMe
n=
hyaluronic acid
Blatter, Jaquinet
1,2,3
I1 111
Scheme 27.
GalNAc(1-4)GlcA trichloroacetimidate and the corresponding acceptor which were used again for glycosylation to yield a tetrasaccharide fragment. The di- and tetrasaccharide trichloroacetimidates were used for the glycosylation of a Galf3(13)Galf3(1-4)Xyl acceptor which represents a typical linkage of mucopolysaccharides to proteins. The resulting penta- and heptasaccharide trichloroacetimidates were used for the glycosylation of serine to yield chondroitin fragments with one or two repeating units; these compounds are of interest as acceptors for glycosyltransferases in enzymatic studies. Great biological interest has been shown in heparan sulfate or analogs of heparin, containing glucosamine and D-glucuronic or L-iduronic acid. A convergent synthesis has been reported (Scheme 29) [114], starting from an azidoglucose acceptor which was glycosylated with two iduronic acid trichloroacetimidates. The resulting two disaccharides were converted into the trichloroacetimidates and used consecutively for glycosylation of the suitably protected iduronic acid acceptor. Both aglycosidic bonds were formed stereospecifically with inversion of the configuration. Thus, a heparan trisulfate fragment has been prepared and used for binding studies to fibroblast growth factor-2. Syntheses of C-bridged heparan sulfate analogs [ 1151 and of heparin-like glycoconjugates which were used for thrombin binding studies [ 116, 1171, have also been reported. The synthesis of bacterial polysaccharides is of great importance for the development of antibacterial vaccines. A terminal a( 1-2)-linked rhamnose trisaccharide
2.3 0-Glycosides
31
YPh
Ph
Ph I
P' h
BF3.OEtz toluene. -5OOC
r
71%
1
5.0-70%
GalNAcP(1-4)GlcAP( n=1,2
Scheme 28.
HNFmoc
1-3)GalP(1-4)XylP(1 -0)Ser chondroitin
Ogawa et al. 1112,1131
32
2 Trichloroacetimidates
~
'
"
not reported
3
I
Lea
qn%
Lev0
OAc
w
HO
KCci3 84% TBDMSOTf NH
OAc
HO
I
OAll
toluene, -20°C
TBDMSOTf CHZCI,, -2OOC
IdoAa(l-4)GlcNa(l-4)ldoAa(i-4)GlcNa(l-4)IdoAal-OMe
6 0 , '
Sinay et al. [I141
Ao3Q
Scheme 29.
of the group B Streptococcus antigen has been synthesized with a 2-0-acetyl protected rhamnosyl trichloroacetimidate by repeated glycosylation and deprotection of, the 2-OH group [118]. A hexasaccharide fragment of the a(l-4)-linked 3-0methylmannose polysaccharide of Mycobacterium smegmatis has been convergently prepared, via a disaccharide building block [ 1191. From a 4-0-acetyl protected 3-0methylmannosyl trichloroacetimidate and a suitable acceptor a 3-0-methylmannose disaccharide was prepared which was converted into the trichloroacetimidate and used for repeated glycosylation and then deprotection of the 4'-OH group. All glycosidic bonds were stereospecifically formed owing to the participation of neighboring groups. Most bacterial polysaccharides are much more complex. Ninety 90 serotypes of Streptococcus pneumoniae are already known. The synthesis of a pyruvated tetrasaccharide repeating unit of Streptococcus pneumoniae type 27 has been described (Scheme 30) [ 1201. The disaccharide trichloroacetimidate donor was prepared from acetobromoglucose and the pyruvated glucosamine acceptor applying the KoenigsKnorr method. The a-linked disaccharide acceptor was prepared from a galactosyl trichloroacetimidate and a rhamnose acceptor. The use of a strong Lewis acid and solvent (diethylether) favoring an S,I -type reaction mechanism, gave the a-glycoside with high selectivity (a: = 9 : 1). Glycosylation of the disaccharide acceptor with the disaccharide trichloroacetimidate gave the tetrasaccharide repeating unit carrying a spacer which is suitable for the preparation of glycoconjugates. The syntheses of another tetrasaccharide repeating unit of this type [121] and four tetrasaccharide repeating units of Streptococcus pneumoniae type 6B [ 1221 have also been reported. The chemoenzymatic synthesis of a decasaccharide consisting of two repeating units of Streptococcus capsular polysaccharides type I11 group B (GBSP 111) (Scheme 31) [123] has been described. Glycosylation by the Koenigs-Knorr method of a 3,4-
a
2.3 0-Glycosides
61Yo
33
TMSOTf
6OoC
scheme 30.
di-O-unprotected glucosamine acceptor with acetobromogalactose gave regiospecifically the lactosamine disaccharide which was converted into an acceptor and into a trichloroacetimidate donor. Glycosylation of the disaccharide acceptor with acetobromoglucose, again under Koenigs-Knorr conditions, gave the trisaccharide building block which was transformed into the trichloroacetimidate donor. The disaccharide trichloroacetimidate was used for the glycosylation of a galactose acceptor; the resulting linear trisaccharide was glycosylated with a lactosyl trichloroacetimidate to give the pentasaccharide building block. Glycosylation with the trisaccharide trichloroacetimidate finally gave the desired octasaccharide. All pglycosidic bonds were formed stereospecifically, but the synthesis seems not to be straightforward because the tetrasaccharide repeating unit was not used twice as a building block. Finally, the neuraminic acid residues were enzymatically attached with a(2-3)sialyltransferase to yield the desired decasaccharide fragment. The synthesis of an octasaccharide mimic of two repeating units of the Streptococcus capsular polysaccharide type I11 group B (GBSP 111) with S-1-carboxyethyl groups replacing the siailic acids has also been reported [124]. A convergent synthesis has been reported for a hexasaccharide fragment of the Streptococcus group A cell-wall polysaccharide (Scheme 32) [ 1251. The 3 - 0 unprotected rhamnose acceptor was first glycosylated with the N-phthaloylprotected glucosamine trichloroacetimidate and then with a suitably protected
34
2 Trichloroucetimidutes
ACO
HS(CN12 tol. CHsNO2,
ACO
OSE
72%
.A;&oHg
OSE
Pht
NPhth
OAc
A
disaccharide acceptor
disaccharide imidate
J
-OAc
Ad)
Oh43
TMSOTf
Glc~(1-6)GlcNAc~(1-3)Gal~(l-4)Glc~(1 -6)GlcNAcP(1-3)GalPl-OMe
I
I
GaiP(1-4)
GalP(1-4)
1
a(2-3)sialyNransferase CMP-NedAc
Glc~(l-6)GlcNAc~(l-3)Gal~(l-4)Glc~(l-6)GlcNAc~(l-3)Gal~l-OMe
I
NeuAca(2-3)GalB(l-4)
I
NeuAca(2S)Ga@(1-4)
GBSP 111 Jennings et al. [I231
Scheme 31.
rhamnosyl trichloroacetimidate. The resulting trisaccharide was converted into the trichloroacetimidate and into an acceptor; coupling yielded the hexasaccharide with high stereoselectivity. A participating neighboring group is not necessary for the formation of the a-rhamnoside bond. An efficient synthesis has also been reported for the hexadecasaccharide fragment
35
2.3 0-Glycosides OAll
B
z
O
H
TESOTf CH2CI2, -75OC 58%
r
I '
Bz
OAc
trisaccharide imidate
Rhaa(l-2)Rhaa(1-3)-Rhaa(l-2)Rha GlcNAcp(1-3)
OBz
BZO
CH2CI2, TESOTf,-75OC 84%
TESOTf, CH2C12, r.t. 43%
GlcNAcp(1-3)
trisaccharide acceptor Pinto et al. [125]
Scheme 32.
of the Shigelln dysenteria type-1 0-specific polysaccharide (Scheme 33) [ 126, 1271. For the synthesis of the tetrasaccharide repeating unit by subsequent glycosylation of the rhamnose acceptor with azidoglucosyl chloride, methylthio galactoside, and rhamnosyl trichloroacetimidate, several glycosylation methods were applied. For the ensuing gl ycosylations with the tetrasaccharide as donor, the trichloroacetimidate method was favored. Repetitive glycosylation of the hexanoic acid acceptor gave the octa-, dodeca-, and hexadecasaccharides in high yields. Coupling with the linker to human serumalbumin (HSA) enabled the preparation of materials for vaccine studies. Further oligosaccharide fragments of the Rhizobium leguminosarum [ 128, 1291, CCI, CCI,
OANH N, SPh BnO
I
AgOTf DBMP CH&12, O°C
64%
OMBn I
I
MeOTf DBMP Et20, r.1. 58%
Brio
ClAcd
BF .OH
bez b
cH3c12,200c 8940
Rhaa(1-2)Gala(l-3)GlcNAca(l-3)Rhaa 1-O(CH,),COOMe n = 2-4
BnO
OAC
HO(CH,),COOMe
Pozsgay [126,1271
Scheme 33.
36
2 Trichloroacetimidates
BnO
OH
OBn
dBn B
n
En0 O q
68%1Sn(OTI)Z Et20/CH2CI,. r.t.
+
CCI,
Sn(OTI), Et20/CH2C12,r.t.
+
En0 B
n
O
g
0 OBn
I D-Gala(1-1)-D-rnyo-inositol
(Galactinol)
Mayer, Schmidt [ 1 1
Scheme 34.
Mycobacterium gornoae [ 1301, Mycobacterium arium [ 1311, and Vibrio cholerae [ 1321 polysaccharides have also been prepared with the help of trichloroacetimidates as glycosyl donors.
2.3.2 Inositol Glycosides The synthesis of galactinol gala( 1-l)-~-myo-inositol]which is involved in the biosynthesis of the raffinose family, has been reported (Scheme 34) [133]. The readily available L-myo-inositol derivative was glycosylated with the 0-benzyl protected galactosyl trichloroacetimidate. Because of reaction conditions favoring an S,I -type reaction mechanism, the a-glycoside was formed exclusively. In the same way D-Gala(1-1)-D-myo-inositol was obtained from the corresponding D-myoinositol derivative. The glycosylphosphatidyl inositols (GPIs) are an interesting class of compounds which serve as anchors for proteins and glycoproteins in membranes. They consist of many variants in the carbohydrate moiety and in the lipid part. The first total synthesis of a GPI anchor from yeast has been reported; it contains a ceramide residue with a phytosphingosine constituent (Scheme 35) [ 134, 1351. The synthesis has been performed in a convergent manner by glycosylation of 6-0-unprotected Dmyo-inositol acceptor with the azidoglucosyl trichloroacetimidate yielding stereospecifically the a-glycoside as a result of s,~-type reaction conditions. A similar
37
2.3 0-Glycosides
BBilOn 0
0
OAll
O-(-)-Mnt
"
G
TMSOTf
NH
o&.~~,
85% Et,O. r.t.
Pl
0
BnO
TMSOTf
00
;;oq I
uoo~OyO(-)-Mnt 0
cc13
TMSOTf
O YCCI, NH
HO
"YNH CCI,
HN
0
08 0- 6Mana(1-P)Mana(1 -P)Mana(l-4)GlcNa(1-6)lnositol-l-o,
H,Nlco':/-
I
0
Mana(l-2)
8
OH
Schmidt et al. 1134,1351
Scheme 35.
approach with a benzyl-protected azidoglucosyl trichloroacetimidate in dichloromethane gave much lower stereoselectivity [ 1361. The mannose tetrasaccharide building block was prepared by the consecutive glycosylation of the 6-0-unprotected mannose acceptor with three suitably protected O-mannosyl trichloroacetimidates. The two building blocks were coupled by glycosylation of the GlcNyx(1-1)inositol acceptor with the tetramannosyl trichloroacetimidate in very good yield. All a-
38
2 Trichloroacetimidates
mannoside bonds were stereospecifically formed. The synthesis was completed by attaching the phosphate and the ceramide phosphate moieties at the suitably deprotected positions. Other inositolglycans are of pharmaceutical interest because of their insulin-like effect. The synthesis of a 0-linked inositol glycoside using a linear strategy has been reported [ 1371. The azidoglucose acceptor was first glycosylated with mannosyl trichloroacetimidate. The resulting disaccharide was again converted into the trichloroacetimidate and used for the glycosylation of the inositol acceptor. The pseudotrisaccharide was stereoselectively (p :a = 3 : 1) obtained and then consecutively glycosylated with the three suitably protected mannosyl trichloroacetimidates. Introduction of the phosphate groups gave the desired inositolglycans; some related derivatives have also been synthesized.
2.3.3 Glycosylation of Sphingosine Derivatives and Mimics The azidosphingosine glycosylation method [ 138, 1391 is well established. It has been found that azidosphingosine is a better glycosyl acceptor than ceramide or other sphingosine derivatives. Some examples of the synthesis of glycosphingolipids with complex oligosaccharide trichloroacetimidates and azidosphingosine have already been shown (see above). Even when other glycosylation methods have been used for the synthesis of the oligosaccharide moiety, the trichloroacetimidate method has been preferentially chosen for the coupling to azidosphingosine (Scheme 36) [140]. A tetrasaccharide trichloroacetimidate of the ganglio series containing three neuraminic acid residues was used for the glycosylation of 3-0-protected azidosphingosine. Reduction of azide, attachment of the fatty acid, and deprotection
N.
TMSOTf CH,CI,. O°C 65%
I NeuAca(2-8)NeuAca(2-3)
Scheme 36.
OTBDPS
I
NeuAca(2-3)
GT'a
Hasegawa et al. [14W
2.3 0-Glycosidrs
6BZ
39
Higuchi et al. [154]
Scheme 37.
yielded the ganglioside GT1,. With the help of the azidosphingosine glycosylation method many other glycosphingolipids have been synthesized, ie. GD1, [ 1401, GD2 [141], GT3 [142], GT4 [142], GQlb [143, 1441, GQlba [145], sialylated globosides [ 1461, sulfated glucuronyl paraglobosides [ 1471, sulfated sLeX glycosphingolipids [ 1481, ganglioside lactams corresponding to G M I , GM2, GM3, and GM4 [ 1491, and some xylose analogs of glycosphingolipids [ 1501. Although the glycosylation of ceramides has been found to proceed with low yields [ 151, 1521, this method has been applied a few times, especially when special ceramide derivatives are required and their azido derivatives were not available (Scheme 37). ‘The synthesis of astrocerebroside A, which has been isolated from starfish and which is of interest because of its pharmacological activity, has been reported via glycosylation of its ceramide moiety with 0-glucosyl trichloroacetimidate [153]. A hematoside-type ganglioside called GAA-6 which has been also found in starfish, was prepared in the same way via glycosylation of the phytosphingosine derivative with the trisaccharide trichloroacetimidate [ 1541. To find potent selectin binding inhibitors, mimics not only of the carbohydrate part of glycosphingolipids, especially of LeXand sLeX(see above), but also mimics of various lipid analogs have been prepared. 2-Tetradecylhexadecanol has been proposed as lipid moiety, and was glycosylated with the sialylated lactosyl trichloroacetimidate to yield the GM3 analogous neoglycolipid [ 1551. With the same lipid moiety were also prepared the GM4 [155], LeX[156], 3’-C-carboxymethyl LeX [ 1571, and some sulfated [ 158- 1601 and phosphorylated [ 1601 neoglycolipids. Another lipid moiety containing a spacer of variable length was glycosylated with the required trichloroacetimidate to prepare sLeXneoglycolipids [ 1611. A double-chain bis-sulfone has been used for the glycosylation with the corresponding trichloroacetimidates in the synthesis of a globotriosyl neoglycolipid and some deoxy derivatives [162, 1631. Other lipid moieties have been used for various purposes (Scheme 38). The hexadecanediol derivative was glycosylated with the trisaccharide trichloroacetimidate [ 1641. The resulting glycoside was transformed into the 16-mercaptohexadecanyl glycoside and used for biosensor studies. Glycosylation of a trivalent acceptor with mannosyl trichloroacetimidate furnished a triantennary cluster mannoside; its
2 Trichloroucetimidutes
40
TMSOTf CH2C12,r.t. 71%
1 \OAc
AcO
?Oq
no
0
NH +
Y CCI,
TMSOTf CH2C12, -65°C b 74%
HO$
no
Lindhorst et al. 11651
+
m 2 w p e x a n e , r.t.
no
~
49-72%
-OH
*R
Schmidt et al. [1661
Scheme 38.
binding properties to lectins were investigated [ 1651. Long-chain diols were glycosylated with glucosyl trichloroacetimidate to synthesize bis-glycosides, furnishing new types of non-ionic surfactants of the 2/1-type (two sugar head groups, one lipophilic chain) [ 1661. Surfactants of the 2/2-type have also been prepared [ 1661. 2.3.4 Glycosylation of Amino Acids The well established method [167] for the preparation of a-glycosylated serine and threonine derivatives has also been applied to the attachment of complex oligosaccharides (Scheme 39). Glycosylation of the serine acceptor with the P-configured trisaccharide trichloroacetimidate gave the a-glycosylated product stereospecifically, thus furnishing a derivative of the Fla-antigen [ 1681. Other derivatives including the sialyl T (ST) tumor antigen [ 1691, have also been prepared. The use of N-acetyl-protected and 4,6-O-benzylidene-protectedgalactosamine trichloroacetimidates has been proposed for the synthesis of a-glycosides [170]. Because the cisbicyclic structure disfavors oxazoline formation, a-glycosides are formed exclusively even from the a-trichloroacetimidates. Thus the Thomsen-Friedenreich (TF) antigen has been prepared from the Galp( 1-3)GalNAc trichloroacetimidate and serine or threonine as acceptor [ 1711.
2.3 0-Glycosides
41
HNFmoc HO
GalP(1-4)GlcNP(l-G)GalNa(1-3)Ser
Danishefsky et al. [I681 NH
Ph HNFmoc HO . hCOOCH,COPh
BF3.0Etz,THF, -20%
Ad3
Koganti et al. [1711
68% kl,
HtjFmoc
rCm,
HO
m AGO $
R = H, CH3 R' = All, Pfp
R
TMSOTf, CH2CIz,r.t.
TeocN
HNAc
80-90% HoYNH CCI,
Saha, Schmidt 11721
k0% AcO
Ad3
TMSOTf, CHzCIz, 74%
b
I O YCCI, N H
'
O
H
% A +
DtsN
Fmoc-lle-Ser-Gly-lle-Gly- OCH,
A
Fmoc-lle-S~r-Gly-Ile-Gly
I
I
TMSOTf, CH2CI2, 78%
G Resin e +
A%$
0
I
O YCCI, N H
MeMal et a1.[1731
Fmoc-Ile-Ser-Gly-lle-Gly- OCH,
Scheme 39.
The preparation of Gala(1-3)Ser and its use for further syntheses has also been reported [98, 99, 170, 1711. The N-trichloroethoxy (Teoc)-protected glucosamine trichloroacetimidate has been employed for the synthesis of P-glycosidically linked amino acids [ 1721. Glycosylation of various protected serine and threonine esters gave P-glycosides, because of the presence of participating neighboring groups. The N-tetrachlorophthaloyl (TCP)-protected derivatives have been used similarly. After conversion into the N-acetyl derivatives, the building blocks are suitable for glycopeptide synthesis. A completely different approach for the synthesis of glycopeptides has been reported by glycosylation of the serine-containing peptides on the solid-phase [ 1731. A polyoxypropylene cross-linked polyethylene glycol resin (POEPOP, polyoxyethylene-polyoxypropylene) was chosen because it contains only ether bonds and no amide bonds, and several linkers were investigated. The use of an Ndithiasuccinyl (Dts)-protected glucosamine trichloroacetimidate as donor gave the P-glycosylated peptide whereas the use of the azidoglucosyl trichloroacetimidate
42
2 Trichloroacetimidates
gave, as expected, the a-glycosylated peptide. Galactosyl, mannosyl, and fucosyl trichloroacetimidates have been used for the synthesis of libraries of peptide acceptors with more than one serine residue. 2.3.5 Polycyclic and Macrocyclic Glycosides Glycosides of polycycles or macrocycles (anthracyclines, chalicheamicin, macrolactones, etc.) are of great interest because of their antibiotic and antitumor activity. The synthesis of calichearubicin A and B, which have the same carbohydrate moiety as calicheamicin, has recently been reported (Scheme 40) [174, 1751. The phenolic acceptor was stereospecifically glycosylated with rhamnosyl trichloroacetimidate in very good yield. The resulting a-glycoside was transformed into the trichloroacetimidate donor which was used for the glycosylation of the anthracycline acceptor with silver triflate as catalyst. The a-glycosidically linked calichearubicin A was stereoselectively ( a :P = 5 : 1) obtained. The P-glycosidically linked calichearubicin B was similarly prepared from the same trichloroacetimidate donor with boron trifluoride-diethyl ether as the catalyst and, despite the lack of a participating neighboring group, again the P-stereoselectivitywas high (P : a = 5 : 1, 35%). The syntheses of a modified carbohydrate moiety, a head-to-tail dimer of the calicheamicin oligosaccharide [ 1761, the 1,1'-disaccharide (trehalose) containing the carbohydrate moiety of everninomycin [ 1771, and the vancomycin disaccharide CH,
BF,.OEtz H " ; iTBDMGH3 ; ; . CH30
+
HO
OAc
OCH,
OCH,
CH2CI2 95%
OCH,
OCH,
TBDMSO
0%
\/ ?
H,CO
CU
OCH,
H:q Danishefsky et al. [174,1751
CH30
OH
OH
calichearubicinA
Scheme 40.
HO
a
43
2.3 0-Glycosides OTESMo
0
TMSOTf dioxaneflol., O'C 75% (a$=8:1)
OH
+
CCI,
HNWoOPMB b o T B D M s I
Nicolaou et al. [181,1821
OTBDMS
TMSOTf CH2CI , -78'C 91 % (i:a=5:1)
eleutherobin
OH
TMSOTf CH2CI2, -4O'C not reported F CCI,
C I A c O ) TBDMSO PhS
iPrC0
M &
HO
I
Me0
v
'Me
OH OH
HO
HO HO &
o
o
0
u iPrC0
iPrCO O L *k
OH OH
olivomycin A
Roush et al. [1831
Scheme 41.
[ 1781 have also been reported. The total synthesis of neocarcinostatin including the preparation of the N-methylfucosamine trichloroacetimidate and the glycosylation of the endiyne-type aglycone were recently described [ 179, 1801. The synthesis of eleutherobin, which has antitumor activity and cytotoxic properties similar to those of taxol, has also been reported (Scheme 41) [181, 1821. The carvone-derived acceptor was stereoselectively glycosylated with the arabinosyl trichloroacetimidate ( a :p = 8: 1). The resulting glycoside intermediate was treated as for sarcodictyins A and B to yield eleutherobin and eleuthosides A and B. Studies of the synthesis of olivomycin A have been reported [ 1831. Glycosylation of a glycal acceptor with a trichloroacetimidate and an acetate donor gave a trisaccharide which was transformed into the trichloroacetimidate and used for the glycosylation of the acyloin acceptor. The resulting P-glycoside is a model compound for the approach to the synthesis of olivomycin A.
44
2 Trichloroacetimidates
OH
80% (1:1)
I
BFa.OEt2 CH2C123 -80c
no
Jiang, Schmidt [184,1851
calonyctinA
Scheme 42.
The macrolactone calonyctin A, a plant growth regulator, has been synthesized (Scheme 42) [ 184, 18.51. The quinovose acceptor was glycosylated subsequently with the suitably protected quinovosyl, rhamnosyl, and quinovosyl trichloroacetimidates; all steps could be performed stereospecifically. Transformation of the tetrasaccharide into the trichloroacetimidate and glycosylation of the racemic hydroxy acid derivative gave two epimeric compounds. Separation and lactonization gave calonyctin A. A convergent strategy has been applied to the synthesis of Tricolorin A, a natural herbicide with cytotoxic activity against cultured P-388 and human breast cancer cells (Scheme 43) [186, 1871. The acceptor was prepared by glycosylation of the hydroxy acid derivative with the galactosyl and the glucosyl trichloroacetimidates. It was coupled with the disaccharide trichloroacetimidate which had been prepared before from the suitable rhamnose donor and acceptor. All steps proceeded with high stereoselectivity. The resulting tetrasaccharide was transformed into the macrolactone Tricolorin A. The synthesis of the disaccharide macrolactone lonitoside 1 has also been reported [ 1881. 2.3.6 Glycosides of Phosphoric and Carboxylic Acids
Trichloroacetimidates can be used for glycosylation of phosphoric and carboxylic acids without additional Lewis acid. Reaction of the fucosyl a-trichloroacetimidate
45
2.3 0-Glycosides TMSOTf CH,C12, r, t. 79%
AgOTf CH2C12,r. t. a4%
phT& PivO
0
A"Oo CCI,
YNH CCI,
CCI,
0
TMSOTf CH2C12,r.t. 75%
BnO
OBn
0 0
0
HO
Hd Larsson, Heathcock [186,1871
tricolorin A
Scheme 43.
with dibenzyl phosphate gave the b-fucosyl phosphate with stereospecific inversion of configuraticin (Scheme 44) [189]. Deprotection and coupling with GMP morpholidate yielded GMP fucose which was purified by preparative HPLC. Reaction of the galactosyl trichloroacetimidate with a uridine phosphonate derivative furnished a phosphonate analog of UDP-galactose [22] which might serve as glycosyl donor or as inhibitor for galactosyltransferases. Some nucleoside monophosphate sugars have also been prepared directly by reaction of 0-glycosyl trichloroacetimidates with nucleoside monophosphates [ 1901. Reaction of the glucosyl trichloroacetimidate with a ceramide- 1-phosphate derivative gave exclusively the P-glycoside, owing to the participating neighboring group [ 1911. Reaction of the benzyl-protected glucosyl P-trichloroacetimidate with a malonic acid derivative as acceptor gave, with inversion of configuration, the a-linked malonyl derivative which was designed to mimic the pyrophosphate moiety of UDP-glucose and to serve as inhibitor for glycosyltransferases [ 1921. 2.3.7 Solid-Phase Synthesis
Although solid-phase chemistry is well developed, progress has not yet been fully extended to oligosaccharide chemistry, because of the high demands on the polymer support and the lack of powerful analytical tools for monitoring reactions on solid phases. The synthesis of an a(1-2)-linked pentamannose moiety has recently been reported (Scheme 45) [193]; a Merrifield resin, a thio-linker, and a 2-0-acetyl protected mannosyl trichloroacetimidate were used. The glycosylation reaction was
46
2 Trichloroucetimidutes CCI,
0
Schmidt et al. [IS91 0 A*
OAc
AcO!&&oyCCI, OAC
0
0
II
+
NH
Schrnelzer 1221
HO AcO
OAc
Schmidt et al. [I911
CCI,
It II Wong et al. [I921
x
0
0
HO
OH
Scheme 44.
monitored by MALDI-TOF and, after oxidative cleavage from the solid phase, the oligosaccharide was obtained by preparative HPLC. The method described has been applied to other examples, i.e. the synthesis of the branched-core pentasaccharide moiety of complex type N-glycans [ 194, 1951. The same 2-0-acetyl-protected mannosyl trichloroacetimidate was used in a polymer-supported solution-phase synthesis of a tetramannose moiety with polyethylene glycol monomethyl ether (MPEG) as polymer-support [ 1961. The use of controlled-pore glass (CPG) as solid-phase support is under investigation in the synthesis of oligosaccharides [ 1971 and glyconucleotide conjugates [ 1981 with trichloroacetimidates as donors. The synthesis of polylactosamine oligosaccharides was performed with a Merrifield resin and a benzyl-type linker (Scheme 46) [199].The glycosylation was stereospecifically performed with a phthalimido-protected lactosamine trichloroacetimidate bearing a levulinoyl group as temporary protecting group which can be removed with hydrazinium acetate under mild conditions. After cleavage from the linker the octasaccharide was obtained in good yield (overall 42%).
47
2.3 Q-Glycosides r
1
bR
Rademann, Schmidt [I931
R = H, CH,
Scheme 45.
Heparan sulfate-like oligomers have been prepared by polyethylene glycol monomethyl ether (MPEG)-supported solution-phase synthesis (Scheme 47) [200]. Glycosylation of the succinyl linked acceptor (linkage not via the anomeric position) was performed with the disaccharide trichloroacetimidate up to five times, again using the levulinoyl group as temporary protecting group. The yield of the coupling step could be increased to 95% by optimizing the reaction temperature to 10 "C, and only the a-glycoside was obtained. Cleavage from the linker with lithium hydro-
,
Ph,hN
OAC
BnO
,
,
~ BnO
~
~
o
NH
A
C
C
~
NH,NH,.AcOH EtOH
,
OHn
1
TrtBF,
CH,CI,.
En0
OBn
OBn
r.t.
Ogawa et al. 11991 n = 1-3
Scheme 46.
48
2 Trichloroacetimidates
OMe
Dreef-Tromp et al. I2001
n = 2-5
Scheme 47.
peroxide and further transformations yielded the hexa-, octa-, deca-, and dodecasaccharide fragments of heparan sulfate. In a recently introduced method for the synthesis of glycopeptides the peptide was synthesized from monoglycosylated amino acid derivatives followed by glycosylation reactions on the same solid phase (Scheme 48) [201]. A polyethylene glycol N,N-dimethylacrylamide copolymer (PEGA) resin was used for the synthesis of the nonapeptide acceptor. When the resin-bound acceptor was thoroughly washed and dried before subsequent glycosylation with the benzoyl-protected trichloroacetimidate, the core 3 peptide derivative was stereospecifically obtained in good yield (62%). The synthesis of a-glycosides with trichloroacetimidates bearing no participating neighboring groups is much more difficult. The reactivity of the donor has to be well adapted. Nevertheless, glycosylation of the octapeptide acceptor with the benzoyl-protected azidogalactosyl trichloroacetimidate was stereospecific, but gave the core 5 peptide derivative in only 20% yield. Use of the corresponding less reactive 0-acetyl-protected trichloroacetimidate did not work. Further examples employed disaccharide trichloroacetimidates or performed two subsequent glycosylations to synthesize branched trisaccharides. Although the direct glycosylation of OH-unprotected serine-containing peptides did not work on this resin, the use of another resin, containing no amide bonds, gave good results (see above, Scheme 39) 11731. Further applications of polymer-supported glycosylations for the synthesis of
2.4 S-Glycosides
49
Ph ,002
I
Ac-Glu-Pro-Thr-Thr-Thr-Pro-lle-Thr-Thr7 Ac-Glu-Pro-Thr-Thr-Thr-Pro-lle-Thr-Thr-
I
‘Bu
l
‘Bu
l
‘Bu
I I
‘Bu ‘Bu
Ac H Resin
CH&
AcN H i
“I3 TMSOTf “I3
b Ac-Glu-Pro-Thr-Thr-Thr-Pro-Ile-Thr-Thr-NH2
-15OC
core 3
62%
*
TMSOTf CH2CI2, -1 5OC 20%
Ac-Pro-Thr-Thr-Thr-Pro-Ile-Thr-Thr-NH,
core 5
Bock et al. iZOl1
Scheme 48.
oligosaccharide libraries by a combinatorial approach have been reported; to this end, polymer-bound acceptors with several free hydroxy groups were glycosylated randomly with trichloroacetimidates [2021.
2.4 S-Glycosides Thio-linked analogs of oligosaccharides, i. e. Lewis’, sialyl Lewis’, etc., are of interest because of their improved stability to glycosidases. The synthesis of several examples by the trichloroacetimidate glycosylation method has been described (Scheme 49) [204, 2051. Glycosylation of the 3-thio galactose acceptor with a fucosyl trichloroacctimidate gave the a-disaccharide stereospecifically; this was converted into the 3,4-dithio glucose derivative via the triflate and then substitution with inversion of the configuration. Conversion into the disaccharide trichloroacetimidate and glycosylation of heptyl mercaptan gave the thio-linked disaccharide acceptor which was coupled with the neuraminic acid disaccharide to yield the sLeX analog containing only thio linkages and a glucose instead of a glucosamine moiety. For synthesis of the LeXanalog, the disaccharide intermediate was coupled with acetobromogalactose and transformed into the trisaccharide trichloroacetimidate. Glycosylation of the thio-linked acceptor, which was prepared from the lactosyl trichloroacetimidate and heptyl mercaptan, yielded the Le’ analogdus pentasaccharide. All p-glycosidic bonds were stereospecifically formed owing to the participation of neighboring groups. Similarly the thio-linked Lewisa [205]and ganglioside GM3 [206] epitopes have been prepared.
50
2 Trichloroucetimidutes
OH OH HO
LeXanabg
Eisele, Schmidt I2031
Scheme 49.
The benzyl-protected mannosyl a-trichloroacetimidate has been used for the glycosylation of 2-mercaptopyridine (Scheme 50) [207], but the stereoselectivity was low. The P-glycoside was separated and oxidized to the 2-pyridyl sulfone which was employed as glycosyl donor for the synthesis of C-glycosides. The corresponding azidogalactose and N-acetylgalactosamine derivatives have also been prepared [208]. An S-glycosylated amino acid derivative has been synthesized by glycosylation of a D-cysteine derivative with the L-rhamnosyl trichloroacetimidate [209]. The Sglycosyl amino acid was used for the synthesis of an 5'-glycopeptide derivative of a luteinizing hormone-releasing hormone (LH-RH) antagonist. The S-glucosylated L-
2.6 C-Glycosides
51
(pa= 5:4) Beau et al. 12071
CCI,
c
HS
50%
c
Kessler et al. [2091
Scheme 50.
cysteine has been prepared applying the same method [210]. Some glycosylations of thiophenol [21 I ] and thiobenzyl alcohol [212]derivatives with trichloroacetimidates have also been reported.
2.5 N- and P-Glycosides Trichloroacetimidates are also suitable glycosyl donors for the synthesis of Nglycosides. For instance, reaction of N-phthaloyl-protected glucosamine trichloroacetimidate with trimethylsilyl azide gave, owing to neighboring group participation, only the p-glycosyl azide (Scheme 51) [213]. In the synthesis of Nglycopeptides the glucosamine azide is a suitable building block at the reducing end. After reduction of the azide to the amino group it can be linked to aspartate via amide bond formation. Another example of the synthesis of N-glycosides is the reaction of the ribopyranosyl trichloroacetimidate with 2-(3-pyrazolyl)pyridine [2141. The resulting nucleotide was employed for investigations on non-helical supramolecular nanosystems. A P-glycoside has been prepared with the hemiacetal-type trichloroacetimidate and trimethyl phosphite [2151. The resulting diastereomeric phosphonates can be considered as P-analogs of uronic acids, and their glycosides are of interest in investigations with glycosidases.
2.6 C-Glycosides Application of trichloroacetimidates to the synthesis of aromatic C-glycosides, i. e. vitexin, isovitexin, isoembigenin, etc. which are of interest because ot their physiological properties, is well established [216, 2171. A furan derivative was recently
52
2 Trichloroucetimidutes
0
&lo phq *'
NPht
CCI,
KNH
TMSN3 TMSOTf, CH2CI,, -78'C) 700,"
lto et al. k131
NPht
Eschenrnoser et al. [2141 CCI,
BnLv OPWH,),
P(OCH& TMSOTf, CH3CN, 4'C 43%
BnO
w *
OAll
+
Bgo%oAl,
OAll
OPWH,),
1:l
Vasella et al. [2151
Scheme 51.
glycosylated with glucosyl trichloroacetimidate to yield the P-glycoside stereospecifically; this served as intermediate in the synthesis of visnagine analogs (Scheme 52) [218]. The synthesis of bis(C-glycosyl) compounds [219] and of glycosides with glucosamine [220] has also been reported. Occasionally the 0-glycoside intermediates could be isolated and subjected to Fries-type rearrangement to furnish the final C-glycosides. _OBn
+
O n% 'h
O Y N H
TMSOTt CH2C12,-300C,
&CH,
0
'
OH
g
EilO
77% OBn
iCI, BnO 0 n
Schmidt et al. [2181
TMSCN
Aa&o$cl, Ad)
OAc
CH2C12, TMSOTfr.t. qu
CN
Shiozaki et al. [222,223]
References
53
A known method [221] for the preparation of C-glycosyl cyanides from trichloroacetimidates has been applied to the reaction of the 0-acetyl-protected 0glucosyl trichloroacetimidate [222, 2231. Reaction with trimethylsilyl cyanide gave the a-glycoside, a valuable intermediate in the synthesis of carboxylic acid derivatives or other C-glycosides, in quantitative yield. The reaction of trichloroacetimidates with silylenol ethers is also well established (2211. Another interesting example, reaction of benzyl-protected galactosyl trichloroacetimidate with a silyl enol ether, which can be considered as a homoalanine carbanion equivalent [224], led to the a-glycoside; as expected from previous results [221] it was formed with high stereoselectivity. The P-glycoside could be obtained by base-catalyzed isomerization. Further transformations were performed to obtain the a- and P-C--galactosyl-L-serinederivatives.
2.7 Conclusion and Outlook 0-Glycosyl trichloroacetimidates have proven to be valuable glycosyl donors for efficient syntheses of various types of glycosides. Their ease of formation and their high glycosyl donor properties released under mild acid catalysis has led to extraordinarily wide application. Not surprisingly, 0-glycosyl trichloroacetimidates have often afforded the best results in demanding glycosylation studies. The trichloroacetimidate method will, therefore, continue to advance efficient synthesis, particularly of the complex glycoconjugates which are in demand for biological and clinical studies. Thus, the trichloroacetimidate method will also contribute strongly to the future development of glycoscience.
References 1. R. R. Schmidt, Angew. Chem. 1986, 98,213-236; Angew. Chem. Int. Ed. Engl. 1986,25, 212235. 2. R. R. Schmidt, in Stereochemistry of Organic and Bioorganic Transformation-Workshop Conferences Hoechst (W. Bartmann, K. B. Sharpless, Eds), Vol. 17, pp 169-189, VCH, Weinheim, 1987). 3. R. R. Schmidt, Pure Appl. Chem. 1986,61, 1257-1270. 4. R. R. Schmidt in Comprehensive Organic Synthesis, Vol. 6, B. M. Trost, I. Fleming, E. Winterfeld, Eds., Pergamon Press, Oxford 1991, pp. 33-64. 5. R. R. Schmidt, in Carbohydrates-Synthetic Methods and Application in Medicinal Chemistry, A. Hasegawa, H. Ogura, T. Suamin, Eds., Kodanasha Scientific, Tokyo 1992, pp. 66-88. 6. K. Toshima, K. Tatsuta, Chem. Rev. 1993, 93, 1503-1531. 7. R. R. Schmidt, W. Kinzy, Adv. Carbohydr. Chem. Biochem. 1994, 50, 21-123. 8. R. R. Schmidt in Modern Methods in Curbohydrate Synthesis, S. H. Khan, R. A. O’Neill, Eds., Hanvood Academic Publishers, Chur, Schweiz, 1996, pp. 20-54. 9. R. R. Schmidt, K.-H. Jung in Preparative Carbohydrate Chemistry, S. Hannessian, Ed. Marcel Dekker, Inc., New York, 1997, pp. 283-312.
54
2 Trichloroacetimidates
10. R. R. Schmidt in Glycosciences: Status and Perspectives, H.-J. Gabius, S. Gabius, Eds., Chapman and Hall, Weinheim, 1997, pp. 31-53. 11. R. R. Schmidt, Pure Appl. Chem. 1998, 70, 397-402, 12. R. R. Schmidt, J. C. Castro-Palomino, 0. Retz, Pure Appl. Chem., in print. 13. F. Barresi, 0. Hindsgaul in Modern Synthetic Methods, B. Ernst, C. Leumann, Eds., Verlag Helvetica Chimica Acta, Basel, 1995, pp. 283-330. 14. R. R. Schmidt and J. Michel, Angew. Chem. 1980,92, 763-764; Angew. Chem., Znt. Ed. Engl. 1980, 19, 731-732. 15. R. Hoos, J. Huixin, A. Vasella, P. Weiss, Helo. Chim. Acta 1996, 79, 1757-1784. 16. V. J. Patil, Tetrahedron Lett. 1996, 37, 1481-1484. 17. S. P. Douglas, D. M. Whitfield, J. J. Krepinsky, J. Curbohydr. Chem. 1993, 12, 131-136. 18. G. G. Cross, D. M. Whitfield, Synlett 1998, 487-488. 19. H. Waldmann, G. Boehm, U. Schmid, H. Roettele, Angew. Chem. 1994,106,2024-2025 (See also Angew. Chem., Int. Ed. Engl., 1994,33, 1994-1996). 20. U. Schmid, H. Waldmann, Liebigs Ann.lRec1. 1997, 2573-2577. 21. A. Lubineau, B. J. Drouillat, J. Curbohydr. Chem. 1997, 16, 1179-1186. 22. U. Schmelzer, Dissertation, Universitat Konstanz, 1997. 23. U. Huchel, C. Schmidt, R. R. Schmidt, Eur. J. Org. Chem. 1998, 1353-1360. 24. A. Lubineau, K. Basset-Carpentier, C. Auge, Curbohydr. Res. 1997, 300, 161-167. 25. H. Paulsen, S. Peters, T. Bielfeldt, M. Meldal, K. Bock, Curbohydr. Res. 1995, 268, 17-34. 26. M.-Z. Liu, H.-N. Fan, Z.-W. Guo, Y.-Z. Hui, J. Carbohydr. Chem. 1996,15, 1139-1145. 27. T. Tsuda, S.-I. Nishimura, Chem. Commun. (Cambridge) 1996, 2779-2780. 28. J. A. L. M. van Dorst, C. J. van Heusden, A. F. Voskamp, J. P. Kamerling, J. F. G. Vliegenthart, Carbohydr. Res. 1996,291, 63-83. 29. T. Nishimura, F. Nakatsubo, Carbohydr. Res. 1996, 294, 53-64. 30. G. Limberg, G. C. Slim, C. A. Compston, P. Stangier, M. M. Palcic, R. H. Furneaux, Liebigs Ann. 1996, 1773-1784. 31. Ch. Li, B. Yu, M. Liu, Y. Hui, Curbohydr. Res. 1998, 306, 189-195. 32. S. Deng, B. Yu, Y. Hui, Tetrahedron Lett. 1998, 39, 6511-6514. 33. J. C. Castro-Palomino, R. R. Schmidt, Tetrahedron Lett. 1995,36, 5343-5346. 34. M. Lergenmuller, Y. Ito, T. Ogawa, Tetruhedron 1998, 54, 1381-1394. 35. H. Paulsen, B. Halpap, Curbohydr. Res. 1991, 216, 195-212. 36. W. Dullenkopf, J. C. Castro-Palomino, L. Manzoni, R. R. Schmidt, Carbohydr. Res. 1996, 296, 135-147. 37. W.-C. Liu, M. Oikawa, K. Fukase, Y. Suda, H. Winarno, S. Mori, M. Hashimoto, S. Kusumoto, Bull. Chem. SOC.Jpn. 1997, 70, 1441-1450. 38. M. Oikawa, A. Wada, H. Yoshizaki, K. Fukase, S. Kusumoto, Bull. Chem. SOC.Jpn. 1997, 70, 1435-1440. 39. K. Fukase, Y. Fukase, M. Oikawa, W.-C. Liu, Y. Suda, S. Kusumoto, Tetrahedron 1998, 54, 4033-4050. 40. X. Qian, 0. Hindsgaul, Chem. Commun. 1997, 1059-1060. 41. S. G. Bowers, D. M. Coe, G.4. Boons, J. Org. Chem. 1998,63,4570-4571. 42. J. C. Castro-Palomino, Dissertation, Universitat Konstanz, 1998. 43. M. R. E. Aly, J. C. Castro-Palomino, El-Sayed I. Ibrahim, El-Sayed H. El-Ashry, and R. R. Schmidt, Eur. J. Org. Chem. 1998, 2305-2316. 44. E. Meinjohanns, M. Meldal, A. Schleyer, H. Paulsen, K. Bock, J. Chem. SOC.,Perkin Trans. 1 1996, 985-993. 45. G. Blatter, J.-M. Beau, J.-C. Jacquinet, Carbohydr. Res. 1994,260, 189-202. 46. R. Kosmol, L. Hennig, P. Welzel, M. Findeisen, D. Muller, A. Markus, Van J. Heijenoort, J. Prukt. Chem. Chem.-Ztg. 1997,339, 340-358. 47. 0. Ritzeler, L. Hennig, M. Findeisen, P. Welzel, D. Mueller, Tetrahedron 1997, 53, 16651674. 48. R. U. Lemieux, R. M. Ratcliffe, Can. J. Chem. 1979,54, 1244-1251. 49. A. Vasella, C. Witzig, J.-L. Chiara, Helv. Chim. Acta 1991, 74, 2073-2077. 50. I. Toth, G. Dekany, B. Kellam, PCT Znt. Appl., WO 9838197 A1 19980903, 42 pp., (Chem. Abstr., 1998, 129, 216855).
References
55
51. H. Kunz, W. Gunther, Angew. Chem. 1988, 100, 1118-1119; Angew. Chem. Int. Ed. Engl. 1988,27, 1086-1087. 52. J. Seifert, C. Unverzagt, Tetrahedron Lett. 1996, 37, 6527-6530. 53. S. Weiler, R. R. Schmidt, Tetrahedron Lett. 1998, 39, 2299-2302. 54. A. Furstner, I. Konetzki, Tetrahedron Lett. 1998, 39, 5721-5724. 55. S. David, A. Malleron, C. Dini, Carbohydr. Res. 1989, 188, 193-200. 56. R. Preuss, R . R. Schmidt, Synthesis 1988,694-697. 57. W. R. Roush, D. P. Sebesta, R. A. James, Tetrahedron 1997,53, 8837-8852. 58. J. C. Castro-Palomino, Schmidt, R. R., Synlett 1998, 501-503. 59. M. Gelin, V . Ferrieres, D. Plusquellic, Carbohydr. Lett. 1997, 2, 381-388. 60. A. K. Choudhury, N. Roy, Carbohydr. Res. 1998, 308, 207-211. 61. H. Sekljic, N. Wimmer, A. Hofinger, H. Brade, P. Kosma, J. Chem. Soc., Perkin Trans. 1 1997, 1973-1982. 62. M. Izumi, 0. Tsuruta, S. Harayama, H. Hashimoto, J. Org. Chem. 1997, 62, 992-998. 63. H. Yuasa, S Matsuura, H. Hashimoto, Bioorg. Med. Chem. Lett. 1998, 8, 1297-1300. 64. E. P. Dubois, A. Neszmelyi, H. Lotter, V. Pozsgay, Tetrahedron Lett. 1996, 37, 3627-3630. 65. V. Pozsgay, E. P. Dubois, H. Lotter, A. Neszmelyi, Carbohydr. Res. 1997, 303, 165-173. 66. M. Ludewig, D. Lazarevic, J. Kopf, J. Thiem, J. Chem. Soc., Perkin Trans. 1 1998, 17511752. 67. R. Windmueller, R. R. Schmidt, Tetrahedron Lett. 1994, 35, 7927-7930. 68. A. Toepfer, ‘W. Kinzy, R. R. Schmidt, Liebigs Ann. Chem. 1994, 449-464. 69. L. Lay, L. Manzoni, R. R. Schmidt, Carbohydr. Res. 1998,310, 157-171. 70. L. Manzoni, L. Lay, R. R. Schmidt, J. Carbohydr. Chem. 1998,17,739-758. 71. N. L. Pohl, I-. L. Kiessling, Tetrahedron Lett. 1997, 38, 6985-6988. 72. A. Toepfer, IR. R. Schmidt, J. Carbohydr. Chem. 1993, 12, 809-822. 73. G. Hummel, R. R. Schmidt, Tetrahedron Lett. 1997,38, 1173-1 176. 74. U. Sprengard, G. Kretzschmar, E. Bartnik, C. Huels, H. Kunz, Angew. Chem., Int. Ed. Engl. 1995, 34, 990-993. 75. G. Kuznik, W. Hoersch, G. Kretzschmar, C. Unverzagt, Bioorg. Med. Chem. Lett. 1997, 7, 577-580. 76. G. Kretzschmar, W. Stahl, Tetrahedron 1998, 54, 6341-6358. 77. S. J. Danishefsky, J. Gervay, J. M. Peterson, F. E. McDonald, K. Koseki, D. A. Griffith, T. Oriyama, St. P. Marsden, J. Am. Chem. Soc. 1995, 117, 1940-1953. 78. L. Lay, R. Windmueller, S. Reinhardt, R. R. Schmidt, Carbohydr. Res. 1997, 303, 39-49. 79. J. Bartek, R. Muller, P. Kosma, Carbohydr. Res. 1998, 308, 259-273. 80. M. Izumi, 0. Tsuruta, H. Hashimoto, S. Yazawa, Tetrahedron Lett. 1996, 37, 1809-1812. 81. D. Manning, C. R. Bertozzi, S. D. Rosen, L. L. Kiessling, Tetrahedron Lett. 1996, 37, 19531956. 82. R. Roy, W. K. C. Park, 0. P. Srivastava, C. Foxall, Bioorg. Med. Chem. Lett. 1996, 6, 13991402. 83. H. Ishida, H. Hosokawa, H. Kondo, M. Kiso, A. Hasegawa, Carbohydr. Res. 1997,303, 131133. 84. G. Thoma, F. Schwarzenbach, R. 0. Duthdler, J. Org. Chem. 1996, 61, 514-524. 85. G. Kretzschmar, A. Toepfer, C. Huels, M. Krause, Tetrahedron 1997, 53, 2485-2494. 86. A. Toepfer, G. Kretzschmar, S. Schuth, M. Sonnentag, Bioorg. Med. Chem. Lett. 1997, 13171322. 87. G. Kretzschmar, Tetrahedron 1998, 54, 3765-3780. 88. T. Ikeda, T. Kajimoto, H. Kondo, C.-H. Wong, Bioorg. Med. Chem. Lett. 1997, 7, 24852490. 89. U. Greilich, R. Brescello, K.-H. J u g , R. R. Schmidt, Liebigs Ann. 1996, 663-672. 90. T. Stauch, U. Greilich, R. R. Schmidt, Liebigs Ann. 1995, 2101-2111. 91. J. C. Castro-Palomino, G. Ritter, S. R. Fortunato, S. Reinhardt, L. J. Old, R. R. Schmidt, Angew. Chern., Int. Ed. Engl. 1997,36, 1998-2001. 92. Y. Isogai, T. Kawase, H. Ishida, M. Kiso, A. Hasegawa, J. Carbohydr. Chem. 1996,15, 10011023. 93. L. F. Tietze, C. 0. Janssen, J.-A. Gewert, Eur. J. Org. Chem. 1998, 1887-1894.
56
2 Trichloroacetimidates
94. J. M. Lassaletta, K. Carlsson, P. J. Garegg, R. R. Schmidt, J. Org. Chem. 1996, 61, 68736880. 95. J. M. Lassaletta, R. R. Schmidt, Tetrahedron Lett. 1995, 36, 4209-4212. 96. J. M. Lassaletta, R. R. Schmidt, Liebigs Ann. 1996, 1417-1423. 97. N. Mathieux, H. Paulsen, M. Meldal, K. Bock, J. Chem. Soc., Perkin Trans. I 1997, 23592368. 98. D. Qiu, Koganty, R. Rao, Tetrahedron Lett. 1997, 38, 961-964. 99. D. Qiu, Koganty, R. Rao, Tetrahedron Lett. 1997, 38, 45-48. 100. Y. Nakahara, Y. Nakahara, Y. Ito, T. Ogawa, Tetrahedron Lett. 1997, 38, 7211-7214. 101. Y. Nakahara, Y. Nakahara, Y. Ito, T. Ogawa, Curbohydr. Res. 1998,309, 287-296. 102. C. Unverzagt, Angew. Chem. 1994, 106, 1170-1173 (See also: Angew. Chem., Int. Ed. Engl., 1994,33, 1102-1 104. 103. C. Unverzagt, Angew. Chem., Int. Ed. Engl. 1996, 35, 2350-2353. 104. C. Unverzagt, Angew. Chem., h t . Ed. Engl. 1997, 36, 1989-1992. 105. H. Paulsen, M. Springer, F. Reck, E. Meinjohanns, I. Brockhausen, H. Schachter, Liebigs Ann. 1995, 53-60. 106. Z. Szurmai, L. Janossy, Curbohydr. Res. 1996,296, 279-284. 107. Y. Nakahara, S. Shibayama, Y. Nakahara, T. Ogawa, Curbohydr. Res. 1996,280, 67-84. 108. L. Rochepeau-Jobron, J.-C. Jacquinet, Curbohydr. Res. 1998, Volume Date 1997, 305, 181191. 109. L. Rochepeau-Jobron, J.-C. Jacquinet, Carbohydr. Res. 1997,303, 395-406. 110. H. Uzawa, M. Gerz, Y. Ito, T. Ogawa, Curbohydr. Lett. 1996,2, 197-204. 111. G. Blatter, J.-C. Jacquinet, Curbohydr. Res. 1996,288, 109-125. 112. K. W. Neumann, J.4. Tamura, T. Ogawa, Glycoconjugute J . 1996, 13, 933-936. 113. J.-i. Tamura, K. W. Neumann, T. Ogawa, Liebigs Ann. 1996, 1239-1257. 114. J. Kovensky, Ph. Duchaussoy, M. Petitou, P. Sinay, Tetrahedron: Asymmetry 1996, 7, 31193128. 115. A. Helmboldt, M. Petitou, J.-M. Mallet, J.-P. Herault, J.-C. Lormeau, P.-A. Driguez, J.-M. Herbert, P. Sinay, Bioorg., Med. Chem. Lett. 1997, 7 , 1507-1510. 116. R. C. Buijsman, W. H. A. Kuijpers, J. E. M. Basten, E. Kuyl-Yeheskiely, G. van der Marel, C. A. A. van Boeckel, J. H. van Boom, Chem. Eur. J. 1996,2, 1572-1577. 117. J. E. M. Basten, C. M. Dreef-Tromp, B. De Wijs, C. A. A. Van Boeckel, Bioorg. Med. Chem. Lett. 1998, 8, 1201-1206. 118. J. C. Castro Palomino, M. Hernandez Rensoli, V. Verez Bencomo, J. Curbohydr. Chem. 1996, 15, 137-146. 119. W. Liao, D. Lu, Curbohydr. Res. 1996,296, 171-182. 120. G. Schuele, T. Ziegler, Tetrahedron 1996, 52, 2925-2936. 121. G. Schuele, T. Ziegler, Liebigs Ann. 1996, 1599-1607. 122. M.-J. L. Thijssen, M. H. G. Bijkerk, J. P. Kamerling, J. F. G. Vliegenthart, Curbohydr. Res. 1998,306, 111-125. 123. W. Zou, H. J. Jennings, Bioorg. Med. Chem. Lett. 1997, 7 , 647-650. 124. W. Zou, J.-R. Brisson, Q.-L. Yang, M. van der Zwan, H. J. Jennings, Curbohydr. Res. 1996, 295, 209-228. 125. F.-I. Auzanneau, F. Forooghian, B. M. Pinto, Curbohydr. Res. 1996,291, 21-41. 126. V. Pozsgay, Angew. Chem., Int. Ed. 1998,37, 138-142. 127. V. Pozsgay, J. Org. Chem. 1998,63, 5983-5999. 128. T. Ziegler, E. Eckhardt, D. Keller, J. Curbohydr. Chem. 1997, 16, 719-738. 129. A. Banaszek, Curbohydr. Res. 1998, 306, 379-385. 130. M. K. Gurjar, S. Adhikari, Tetrahedron 1997, 53, 8629-8634. 131. I. Bajza, K. E. Kover, A. Liptak, Curbohydr. Res. 1998, 308, 247-258. 132. Y. Ogawa, P. Kovac, Glycoconjugute J. 1997, 14,433-438. 133. T. G. Mayer, R. R. Schmidt, Liebigs Ann./Recl. 1997, 859-863. 134. B. Kratzer, T. G. Mayer, R. R. Schmidt, Eur. J. Org. Chem. 1998, 291-298. 135. T. G. Mayer, B. Kratzer, R. R. Schmidt, Angew. Chem. 1994, 106, 2289-2293. 136. J. Ye, W. T. Doerrler, M. A. Lehrman, J. R. Falck, Bioorg. Med Chem. Lett. 1996, 6, 17151718.
References
57
137. W. Frick, G. Mueller, Ger. Offen., 1998, DE 19649350 A1 19980604,20 pp. (Chem. Abstracts 129 : 54536). 138. R. R. Schmidt and P. Zimmermann, Angew. Chem. 1986,98,722-723; Angew. Chem., Int. Ed. Engl. 1986,25, 725-726. 139. P. Zimmerniann, R. Bommer, T. Bar, R. R. Schmidt, J. Curbohydr. Chem. 1988, 7, 435452. 140. H. Ishida, H . Ando, H. Ito, H. Ishida, M. Kiso, A. Hasegawa, J. Curbohydr. Chem. 1997, 16, 413-428. 141. A. Hasegawa, H. Ishida, Y. Isogai, H. Ishida, M. Kiso, J. Curbohydr. Chem. 1993, 12, 12171222. 142. H. Ando, H. Ishida, M. Kiso, A. Hasegawa, Curbohydr. Res. 1997, 300, 207-217. 143. H. Ishida, H . Ishida, M. Kiso, A. Hasegawa, Curbohydr. Res. 1994, 260, ClLC6. 144. H. Ishida, H . Ishida, M. Kiso, A. Hasegawa, Tetrahedron; Asymmetry 1994, 5, 2493-2512. 145. K. Hotta, H. Ishida, M. Kiso, A. Hasegawa, J. Curbohydr. Chem. 1995, 14, 491-506. 146. H. Ishida, R. Miyawaki, M. Kiso, A. Hasegawa, J. Curbohydr. Chem. 1996, 15, 163-182. 147. Y. Isogai, H. Ishida, M. Kiso, A. Hasegawa, J. Curbohydr. Chem. 1996, 15, 1119-1137. 148. K. Yoshino. H. Ohmoto, N. Kondo, H. Tsujishita, Y. Hiramatsu, Y. Inoue, H. Kondo, H. Ishida, MI. Kiso, A. Hasegawa, J. Med. Chem. 1997, 40, 455-462. 149. M. Wilstermann, L. 0. Kononov, U. Nilsson, A. K. Ray, G. Magnusson, J. Am. Chem. Soc. 1995,117,4742-4754. 150. M. Wilstermann, G. Magnusson, J. Org. Chem. 1997,62, 7961-7971. 151. S. Fujita, M. Numata, M. Sugimoto, K. Tomita, T. Ogawa, Curbohydr. Res. 1994,263, 181-196. 152. M. Iida, A. Endo, S. Fujita, M. Numata, K. Suzuki, S. Nunomura, T. Ogawa, Glycoconjugute J. (1996), 13,203-21 1. 153. N. Chida, N. Sakata, K. Murai, T. Tobe, T. Nagase, S. Ogawa, Bull. Chem. Soc. Jpn. 1998, 71, 259-272. 154. T. Sugata, Y. Kan, Y . Nagaregawa, T. Miyamoto, R. Higuchi, J. Curbohydr. Chem. 1997,16, 917-925. 155. A. Hasegawa, N. Suzuki, H. Ishida, M. Kiso, J. Curbohydr. Chem. 1996,15,623-637. 156. Y . Wada, T. Saito, N. Matsuda, H. Ohmoto, K. Yoshino, M. Ohashi, H. Kondo, H. Ishida, M. Kiso, A. Hasegawa, J. Med. Chem. 1996,39,2055-2059. 157. H. Ishida, H Hosokawa, H. Kondo, M. Kiso, A. Hasegawa, Curbohydr. Res. 1997,303, 131133. 158. E. Tanahashi, K. Murase, M. Shibuya, Y. Igarashi, H. Ishida, A. Hasegawa, M. Kiso, J. Curbohydr. Chem. 1997, 16, 831-858. 159. T. Ikami, H. Hamajima, T. Usui, T. Mitani, H. lshida, M. Kiso, A. Hasegawa, J. Curbohydr. Chem. 1997,16, 859-875. 160. T. Ikami, N. Tsuruta, H. Inagaki, T. Kakigami, Y. Matsumoto, N. Tomiya, T. Jomori, T. Usui, Y. Suzuki, H. Tanaka, D. Miyamoto, H. Ishida, A. Hasegawa, M. Kiso, Chem. Phurm. Bull. 1998, 46, 797-806. 161. K. Adachi, Y. Yamada, H. Wada, A. Kameyama, H. Ishida, M. Kiso, J. Curbohydr. Chem. 1998, 17, 595-607. 162. 2. Zhang, G. Magnusson, J. Org. Chem. 1995,60, 7304-7315. 163. 2. Zhang, G. Magnusson, J. Org. Chem. 1996,61, 2383-2393. 164. P. I. Kitov, C:. Railton, D. R. Bundle, Curbohydr. Res. 1998, 307, 361-369. 165. S. Kotter, U. Krallmann-Wenzel, S. Ehlers, T. K. Lindhorst, J. Chem. Soc., Perkin Truns. I 1998,2193-2200. 166. R. R. Schmidt, K. Jankowski, Liebigs Ann. 1996, 867-879. 167. W. Kinzy, R R. Schmidt, Curbohydr. Res. 1987,164,265-276. 168. X.-T. Chen, ID. Sames, S. J. Danishefsky, J. Am. Chem. Soc. 1998, 120, 7760-7769. 169. D. Sames, X.-T. Chen, S. J. Danishefsky, Nature (London) 1997,389, 587-591. 170. J. E. Yule, T. C. Wong, S. S. Gandhi, D. Qiu, M. A. Riopel, Koganty, R. Rao, Tetrahedron Lett. 1995, 36, 6839-6842. 171. D. Qiu, S. S. Gandhi, Koganty, R. Rao, Tetrahedron Lett. 1996, 37, 595-598. 172. U. K. Saha, R. R. Schmidt, J. Chem. Soc., Perkin Truns. 11997, 1855-1860.
58
2 Trichloroacetimidates
173. A. Schleyer, M. Meldal, R. Manat, H. Paulsen, K. Bock, Angew. Chem., Int. Ed. Engl. 1997, 36, 1976-1978. 174. K. M. Depew, S. M. Zeman, S. H. Boyer, D. J. Denhart, N. Ikemoto, S. J. Danishefsky, D. M. Crothers, Angew. Chem., Int. Ed. Engl. 1997, Volume Date 1996,35, 2797-2800. 175. S. H. Olson, S. Danishefsky, Tetrahedron Lett. 1994, 35, 7901-7904. 176. K. C. Nicolaou, K. Ajito, H. Komatsu, B. M. Smith, P. Bertinato, L. Gomez-Paloma, Chem. Commun. (Cambridge) 1996, 1495-1496. 177. K. C. Nicolaou, F. L. van Delft, S. R. Conley, H. J. Mitchell, Z. Jin, R. M. Rodriguez, J. Am. Chem. SOC.1997,119,9057-9058. 178. K. C. Nicolaou, H. J. Mitchell, F. L. Van Delft, F. Rubsam, R. M. Rodriguez, Angew. Chem., Int. Ed. 1998, 37, 1871-1874. 179. T. Kaneko, K. Takahashi, M. Hirama, Heterocycles 1998, 47, 91-96. 180. A. G. Myers, J. Liang, M. Hammond, P. M. Harrington, Y. Wu, E. Y. Kuo, J. Am. Chem. SOC.1998,120, 5319-5320. 181. K. C. Nicolaou, F. L. van Delft, T. Ohshima, D. Vourloumis, J. Xu, S. Hosokawa, J. Pfefferkorn, S. Kim, T. Li, Tianhu, Angew. Chem., Int. Ed. Engl. (1997), 36, 2520-2524. 182. K. C. Nicolaou, T. Oshima, S. Hasokawa, F. L. van Delft, D. Vourloumis, J. Y. Xu, J. Pfefferkorn, S. Kim, J. Am. Chem. SOC.1998, 120, 8674-8680. 183. W. R. Roush, K. Briner, B. S. Kesler, M. Murphy, D. J. Gustin, J. Org. Chem. 1996, 61, 6098-6099. 184. Z.-H. Jiang, R. R. Schmidt, Liebigs Ann. Chem. 1994, 645-651. 185. Z.-H. Jiang, A. Geyer, R. R. Schmidt, Angew. Chem., Int. Ed. Engl. 1995,34, 2520-2524. 186. D. P. Larson, C. H. Heathcock, J. Org. Chem. 1997,62, 8406-8418. 187. D. P. Larson, C. H. Heathcock, J. Org. Chem. 1996, 61, 5208-5209. 188. R. T. Brown, S. P. Mayalarp, J. J. Watts, Chem. Soc., Perkin Trans. 11997, 1633-1637. 189. R. R. Schmidt, B. Wegmann, and K.-H. Jung, Liebigs Ann. Chem. 1991, 121-124. 190. R. R. Schmidt, H. Braun, K.-H. Jung, Tetrahedron Lett. 1992, 33, 1585-1588. 191. T. J. Martin, G. Dufner, B. Kratzer, R. R. Schmidt, GlycoconjugateJ . 1996, 13, 547-553. 192. R. Wang, D. H. Steensma, Y. Takaoka, J. W. Yun, T. Kajimoto, C.-H. Wong, Bioorg. Med. Chem. 1997,5,661-672. 193. J. Rademann, R. R. Schmidt, J. Org. Chem. 1997, 62, 3650-3653. 194. J. Rademann, A. Geyer, R. R. Schmidt, Angew. Chem., Znt. Ed. 1998,37, 1241-1245. 195. J. Rademann, R. R. Schmidt, Tetrahedron Lett. 1996,37, 3989-3990. 196. L. Jiang, R. C. Hartley, T.-H. Chan, Chem. Commun. (Cambridge) 1996, 2193-2194. 197. A. Heckel, E. Mross, K.-H. Jung, J. Rademann, R. R. Schmidt, Synlett 1998, 171-173. 198. M. Adinolfi, G. Barone, L. De Napoli, A. Iadonisi, G. Piccialli, Tetrahedron Lett. 1998, 39, 1953-1956. 199. H. Shimizu, Y. Ito, 0. Kanie, T. Ogawa, Bioorg. Med. Chem. Lett. 1996, 6, 2841-2846. 200. C. M. Dreef-Tromp, H. A. M. Willems, P. Westerduin, P. van Veelen, C. A. A. van Boeckel, Bioorg. Med. Chem. Lett. 1997, 7, 1175-1180. 201. H. Paulsen, A. Schleyer, N. Mathieux, M. Meldal, K. Bock, J. Chem. SOC.,Perkin Trans. 1 1997,281-293. 202. Y. Ding, J. Labbe, 0. Kanie, 0. Hindsgaul, Bioorg. Med. Chem. 1996,4,683-692. 203. Th. Eisele, R. R. Schmidt, Liebigs Ann.jRecl. 1997, 1303-1313. 204. Th. Eisele, A. Toepfer, G. Kretzschmar, R. R. Schmidt, Tetrahedron Lett. 1996, 37, 13891392. 205. Th. Eisele, R. Windmuller, R. R. Schmidt, Carbohydr. Res. 1998, 306, 81-91. 206. Th. Eisele, R. R. Schmidt, Liebigs Ann.jRecl. 1997, 865-872. 207. T. Skrydstrup, 0. Jarreton, D. Mazeas, D. Urban, J.-M. Beau, Chem. Eur. J. 1998, 4, 655671. 208. D. Urban, T. Skrydstrup, J.-M. Beau, J. Org. Chem. 1998, 63, 2507-2516. 209. K. Michael, V. Wittmann, W. Konig, J. Sandow, H. Kessler, Int. J. Pept. Protein Res. 1996, 48, 59-70. 210. L. Kaesbeck, H. Kessler, Liebigs Ann./Recl. 1997, 165-167. 211. E. Bozo, S. Boros, J. Kuszman, Carbohydr. Res. 1998, 311, 191-202. 212. H. C. Hansen, G. Magnusson, Carbohydr. Res. 1998,307, 243-251.
References
59
213. A. Dan, M. Lergenmuller, M. Amano, Y. Nakahara, T. Ogawa, Y. Ito, Chem. Eur. J. 1998, 4, 2182-2190. 214. A. Eschenmoser, C. Miculka, N. Windhab, H.-U. Hoppe, PCTZnt. Appl., 1998, WO 9825943 A1 19980618, 26 pp., (Chem. Abstracts, 129: 67987). 215. R. Hoos, J. Huixin, A. Vasella, P. Weiss, Helv. Chim. Acta 1996, 79, 1757-1784. 216. J.-A. Mahling, R. R. Schmidt, Synthesis, 1993, 325-328. 217. J.-A. Mahling, K.-H. Jung, R. R. Schmidt, Liebigs Ann. 1995, 461-466. 218. E. El Telbani, S. El Desoky. M. A. Hammad, A. R. H. A. Rahman, R. R. Schmidt, Eur. J. Org. Chem. 1998,2317-2322. 219. E. El Telbani, S. El Desoky, M. A. Hammad, A. H. Abdel Rahman, R. R. Schmidt, Carbohydr. Rex 1998, 306, 463-461. 220. J. C. Castro-Palomino, R. R. Schmidt, Liebigs Ann. 1996, 1623-1626. 221. M. Hoffmdnn, R. R. Schmidt, Liebigs Ann. Chem. 1985, 2403-2419. 222. M. Shiozaki, T. Mochizuki, T. Wakabayashi, S.4. Kurakata, T. Tatsuta, M. Nishijima, Tetrahedron Lett. 1996, 37, 7271-7274. 223. M. Shiozaki, S.4. Kurakata, T. Tatsuta, H. Maeda, M. Nishijima, Tetrahedron 1997, 53, 16041-1 6060. 224. A. Dondoni, A. Marra, Chem. Commun. (Cambridge) 1998, 1741-1742.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
3 Iterative Assembly of Glycals and Glycal Derivatives: The Synthesis of Glycosylated Natural Products and Complex Oligosaccharides Lawrence J. Williams, Robert M. Garbaccio, and Samuel J. Danishefsky
3.1 Introduction Polypeptides, oligonucleotides and oligosaccharides are three classes of oligomers that play key roles in transacting the processes of life. While the functions of proteins and DN,4 have been appreciated, at least qualitatively, for over fifty years, basic information as to the vital missions of carbohydrates, beyond that of energy storage, are still being elucidated. Thus, the roles of oligosaccharides in cellular adhesion, protein folding, cell signaling, fertilization, pathogen binding and inflammation have been recognized only recently. The detection, purification and structure determination of complex oligosaccharides have been critical in the emergence of glycobiology. The continuing maturation of this science has added to the incentives for preparing complex oligosaccharide conjugates in homogeneous form. Among these incentives is the possibility of evaluating vaccines comprised of synthetic, cell free, carbohydrate-based tumor antigens for enhancing immune response to cancers which bear such cell surface epitopes. In addition to the necessity for such homogeneous constructs, the preparation of complex oligosaccharides poses no small challenge to the science of chemical synthesis. While the successes achieved in oligonucleotide and oligopeptide synthesis arose only after long term and exacting research, the issues associated with oligosaccharide synthesis are of a much higher level of complexity. In assembling either amide bonds for polypeptide construction, or internucleotide phosphate bonds in DNA synthesis, no new stereochemical issues are encountered. By contrast, each glycosylation reaction in oligosaccharide construction involves a question of stereochemical outcome at the anomeric site (a or P-linkage, Figure 1). Moreover, every glycosylation event requires identification of a specific hydroxyl group to serve as an acceptor center (Scheme 1, A). In a typical hexose, the pinpointing of an acceptor site obliges one to identify one of the five potential hydroxyl groups to serve as the reaction locus for glycosylation. In some instances, this specificity or selectivity of action may reflect inherent reactivity margins between free
62
3 Iterative Assembly of Glycals and Glycal Derivatives
Oligopeptides
Oligonucleotides
Oligosaccharides
RO
Glycoside Bond
1-
RO RO RO
OR
Figure 1.
hydroxyl groups. Alternatively and much more commonly, oligosaccharide construction involves extensive reliance on the use of protecting group strategies. For purposes of reiteration, a specific protecting group at the reducing end of an oligosaccharide, having been brought in at the glycosyl acceptor stage, would then be converted into a donating group. In addition to the requirement for manipulating the acceptor and donor character of the terminal hexose with each elongation cycle, the need for inclusion of N-acetyl functions, as well as occasional C-methyl, 0-methyl, 0-acyl, or sulfated alcohol groups, not to speak of sialic acid units, serves to further complicate an already daunting set of challenges. It was with these considerations in mind that we began to explore the possible advantages of a new strategy, ie. glycal assembly, to facilitate oligosaccharide construction [l]. While the use of glycals as direct glycosyl donors or precursors of glycosyl donors had been practiced long before our entry to the field in 1988, it seemed that several potentially interesting and promising synthetic opportunities had not been recognized. Upon casual inspection, it appeared that the complexities associated with differential hydroxyl protection would be significantly reduced in a glycal. Thus, in a hexose glycal, only three hydroxyls need be distinguished (Scheme 1, B). Further-
3.1 Introduction
63
A
1
1
a
-
O
H _L Po
HO
--+bop :bop OP
PO
OH
+
PO
Po
OP
OP
HO
Ohgosaccharide
4
X PO
OP Po
OP
B Po
E+
PO Po
Po
Reiterate Ohgosaccharide
4
Po
Scheme 1.
more, each hyclroxyl could well differ from the others in its expected reactivity, since one is primary.,one is allylic, and the other is a more hindered secondary alcohol. It seemed, therefore, that the need for selective protection could be lessened and those protections which are needed could be simplified in the context of glycals. The most significant point of departure of ‘glycal assembly’ originated from the use of glycals as both glycosyl donors and glycosyl acceptors. This two modality
64
3 Iterative Assembly of Glycals and Glycal Derivatives
usage sets up the possibility of synthesizing polysaccharides by the assembly of glycals in an iterative fashion. In this approach, the activated, glycal-derived donor would be captured by a glycal acceptor in a glycosylation reaction (Scheme 1, B). The glycal linkage of the newly generated disaccharide, though initially introduced as a glycosyl acceptor, could now serve as a donor for the next elongation cycle. Thus, glycals would have the potential advantage that coupling maneuvers could be implemented, and reiterated, with minimal protecting group manipulation. It is well to emphasize that the success of the glycal paradigm could not have been assumed in advance of experiment. Thus, employment of a glycal-housed hydroxyl as the glycosyl acceptor means that the sensitive enol ether linkage, bearing a potential allylic leaving group at C3, must withstand the action of the promoter required for the glycosylation event. Yet, glycal linkages bearing such leaving groups are often subject to Ferrier rearrangement under conditions similar to those that would be used in glycosylation. Hence, it was necessary to find promoter conditions that were effective for glycosylation while consistent with survival of the acceptor glycal. Through the continued investigation of these ideas, we have developed a broad based approach to synthesize complex oligosaccharides, including glycosylated natural products, via glycals. In the context of specific target structures, the program has served as an incentive for exploring new issues of chemical reactivity. For the purpose of this review, the program description will be confined to examining, and demonstrating by example, the feasibility of these theorems in the context of selected total syntheses of polysaccharides and their conjugates. While there is a unified theme governing this chapter, i.e. that of fostering progression of the methodology and strategy of synthesis of complex glycans, the discussion is divided into several parts. For purposes of presentation, each of the first four sections introduces a key methodological advance in glycal technology, typically under the umbrella of the natural product that inspired them. The oligosaccharide targets discussed in the last three sections demonstrate the integrated application of these methodologies, and the iterative glycal assembly strategy, to access tumor-associated oligosaccharides used in our expanding anti-cancer vaccine program.
3.2 Ciclamycin 0 The ciclamycin complex of antibiotics possesses high inhibitory activity against experimental tumors [2] and is claimed to be of value against human neoplasia [3]. These antibiotics are largely or entirely composed of q- and 8-pyrromycinones joined at C7 by glycosidic linkages to variously permuted carbohydrate domains [4, 51. One component of this complex, which is particularly difficult to obtain, is ciclamycin 0. At the time of our investigation, the structural assignment for 1 was based solely on spectroscopic arguments. Our strategy focused on construction of an appropriately protected trisaccharide and its use as a donor to achieve glycosylation of the anthracyclinone nucleus.
3.2 Ciclamycin 0
OH 0
OH
65
P
e &eM
1 Ciclamycin 0
0
Figure 2.
Given our interest in iterative assembly, a trisaccharide glycal was envisioned to be the target. The requirement for a-linked 2-deoxy hexoses prompted the utilization of the iodoglycosylation reaction developed by Lemieux [6] and Thiem [7] (Scheme 2). Adapting this methodology to iterative glycal assembly required strict control of the roles of glycals, since substochiometric amounts of iodine could, in principle, react with either the donor glycal or the acceptor glycal resulting in coincident symmetrical coupling and possibly polymerization. It had long been known that acyl protecting groups lower the reactivity of glycosy1 donors [gal, but the idea of exploiting this effect in the coupling of two potential glycosyl donors had not been conceptualized until the seminal experiments of Fraser-Reid [Sb]. Consequently, it was hoped that the nature of such a coupling could be controlled by the resident substituents on two different glycals [9]. Glycal6 has its oxygen atoms alkylated, while those of 7 are acylated. Consistent with expectations, glycal 6 proved to be more reactive towards the iodonium electrophile, thereby generating the active glycosyl donor selectively in the presence of acceptor 7 (Scheme 3). Glycal 7 has a free hydroxyl group and, therefore, can act as the only acceptor. Iodinative coupling of 6 and 7, via trans-diaxial attack, gave rise to iodoglycoside 8 (58%). The C3 hydroxyl, allylic to the double bond of the glycal, is the most obvious position where the electronic difference between and acyloxy and an alkoxy group might be manifested. However, the clean and successful coupling of 6 and 9 to give 10 (76%) indicates that even without an acyloxy group at C3, subtle effects can be responsible for an ordered glycosylation.
2
Scheme 2.
3
4
66
3 Iterative Assembly of Glycals and Glycal Derivatives
B BnOn
O
g
+
no
O
S
/
+
O
e
b -
I+ 58%
BZos HO
6
B BnOn
O
7
6
BB n
HBZO
9
e
+
8
10
-
BZO
11
Scheme 3.
A true test for this strategy of controlling glycal-glycal coupling arrived with the total synthesis of ciclamycin 0 [lo], which embodied the additional feature of containing 2-deoxyhexoses (Scheme 4) [l 11. Coupling of benzoylated acceptor 14 to benzylated donor 13, followed by Ph3SnH reduction of the resultant iodide gave the 2-deoxy-disaccharide 15 in 57% yield. Following protecting group adjustment, silylated donor 16 was coupled to the same benzoylated acceptor 14 followed again by Ph$nH reduction, in 54% overall yield. Manipulation of the resulting trisaccharide into a suitable glycal donor was followed by a successful coupling to Epyrromycinone (19), which gave a mixture of a-linked iodides 20 in 60% overall yield. Protecting group removal and iodide reduction afforded ciclamycin 0. Up to this time, the direct coupling of an antrhacyclinone to a fully synthetic oligosaccharide en route to a naturally occurring antibiotic had not been accomplished
WI. 3.3 Allosamidin Allosamidin (21, Figure 3) was identified as a selective and powerful chitinase inhibitor in 1987 [13]. As such, it generated interest as a potential insecticide and fungicide. Allosamidin is composed of a 3,3’-epi-chitobiose P-linked to a novel aglycone sector termed ‘allosamizoline’ [ 141. This unique structure is believed to impart to it the function of a transition-state mimic for the hydrolysis of chitin [15]. The possibility that the electrophilic nature of the oxazoline linkage could allow it
3.3 Allosamidin
>
-&
67
->
OH Bno
14
13
MePo --
1. I+(symcoll)*cIo,&
2. PhaSnH, 54%
BnO 16
15
14
BnO OH 0
Mw 0
I+(symcoll)2c104-
>
19
18
20
60% M0
e
d
o
Scheme 4.
to function as a potential locus for covalent attachment to the chitin-degrading enzyme was also considered [16]. The combination of the interesting structure, valuable biological activity and scarcity in nature of allosamidin prompted our attempts at its total synthesis. In the contlext of carbohydrate synthesis, a key feature of this effort included a novel and efficient synthesis of axial glycal derivatives utilizing a Ferrier-type rearrangement [ 171 followed by a [2,3]-sigmatropic rearrangement [18, 191. The allosamidin effort also spurred development of the method of sulfonamidoglycosylation [20] for the construction of P-linked 2-aminohexoses. Following the invention and
21 Allosamidin
Figure 3.
3 Iterative Assembly of Glycals and Glycal Derivatives
68
50% overall SPh 22
OPC
23
25
24
Scheme 5.
application of methods addressed to these ends, an expeditious synthesis of allosamidin was enabled. A general objective of the efforts in this area was an efficient route to glycals bearing C3 axial alcohol derivatives in the context of a range of substituents at C4. Glycals of this type had been difficult to synthesize [21] and rapid access to such derivatives would broaden the expanding collection of glycosyl donors and acceptors that could be incorporated into the assembly program. We found that Ferriertype rearrangement of tri-0-acetyl-D-gluca122 with thiophenol yielded 23 (Scheme 5) [22]. Oxidation of the a-(pheny1thio)pseudoglycal with DMDO and exposure of the resultant sulfoxide (24) to diethylamine triggered a [2,3]-sigmatropicrearrangement which resulted in the desired allal 25 in 96% yield. This sequence was expanded to include both allal and gulal derivatives as well as a range of substituents at C4. The issue of access to 2-aminohexose moieties has been impressively addressed by the azidonitration protocol of Lemiuex [23] and relies on the character of the C2 substituent to direct the outcome of any subsequent glycosylation. In this regard, azidonitration, while far from ideal in terms of stereoselection, remains one of the most useful methods to access the 2-amino-a-linkage. We, however, sought a new, direct method for introducing equivalent functionality through which stereoelectronic factors, rather than local steric factors, would determine the outcome of subsequent glycosylation. To this end, a key reaction, sulfonamidoglycosylation, was developed [22] (Scheme 6).
I(syn-Colli)zCi04
RRO o
> -HzNSO2P or h BrZNSOzPh
e
NH41
1
Scheme 6.
LTMP, AgOTf > -
RO
Ph02SNH 27 X = Br, I
26
28
R:q
29
30
3.4 KS-502 and Rebeccamycin
Bny% BnO
69
OSugar Ph02SNH 31
PhOzSNH
32 52%
34
64%
23%
Scheme 7.
This method, which involves trans diaxial addition of an N- halobenezesulfonamide to a glycal, leads to the 1-aminohexose 27. Alternatively, the action of IDCP (iodonium syn-collidine perchlorate) followed by benzenesulfonamide provides the same type of 2-halo-1-aminohexose. Treatment of 27 with base, likely proceeds through the 1,2-sulfonyloaziridine 28, which then reacts quickly and selectively with acceptors to form a P-linkage at the anomeric carbon. The scope of this general method was shown to include glycal- and nonglycal-containing carbohydrates as potential acceptors. Donors could include di- or higher saccharides terminating in a glycal linkage (Scheme 7). An additional benefit of this method was the finding that the intermediate halosulfonamides were often stable crystalline solids. The efficiency of coupling was later enhanced by the combination of stannylalkoxides in the presence of AgBF4 for the actual glycosylation reaction [24]. Furthermore, an iodosulfonamide can be readily converted to a corresponding ethyl thioglycoside, which in turn functions as a useful and alternative glycosyl donor. The adaptability of the sulfonamidoglycosylation to the iterative mode (glycals as donors and acceptors) made it particularly attractive for the synthesis of allosamidin (Scheme 8) [25]. The route to allosamidin utilized the sulfonamidoglycosylation for both glycosylation reactions, as well as the [2, 31 sigmatropic rearrangement methodology for both hexoses. Bromosulfonamidation of 36 (63%) followed by direct rollover with KHMDS in the presence of glycal 38 gave the P-linked disaccharide 39 in 81% yield. In turn, 39 then served as a donor in a second azaglycosylation with the aglycon derivative 41 to afford, after several steps, allosamidin (21). The efficiency of this synthesis served to reflect the power of iterative glycal assembly.
3.4 KS-502 and Rebeccamycin Glycosylated natural products, and other carbohydrate conjugates, present a challenging problem to synthetic chemists who must modify traditional carbohydrate
3 Iterative Assembly of Glycals and Glycal Derivatives
70
BnO
B n O KHMDS, 30%
-
-w
21 Allosarnidin
Scheme 8.
domain methodology to tolerate greater diversity present in the non-carbohydrate sector. In principle, incompatibility of synthetic methods to reach one domain can often limit options along a proposed route. KS-502 (43) and rebeccamycin (44) posed precisely this challenge and became the targets of our synthetic efforts in 1992-1 993. KS-502 (43) was isolated in 1989 [26] and identified as a specific inhibitor of calmodulin-dependent cyclic-nucleotide phosphodiesterase (CaM-PDE). The important roles of CaM and CaM-PDE in cultured cells and living systems underscored the utility of KS-502, perhaps as a valuable biological tool. Its structure comprises an interesting aryl furanoside for which new methodology was devised. Rebeccamycin (44)was shown to have antitumor activity against P388 leukemia, L1210 leukemia and B16 melanoma implanted in mice [27]. It is believed to impart this effect through the single strand cleavage of DNA. Water-soluble analogues Ho*
n-C7H 15
OH 42 KS 501 R = H 43 KS 502 R = C02H
Figure 4.
3.4 KS-502 and Rebeccamycin
71
have shown increased potency and are currently advancing through clinical trials [28]. The rebeccamycin structure consists of a symmetrical indolocarbazole chromophore P-N-linked to a 4-0-methylglucose residue. Synthetic efforts had concentrated on the aglycon portion of rebeccamycin, and an effective general methodology for the glycosylation step was sought to facilitate the total synthesis of this family of antitumor agents. While a unifying solution was pursued for these two glycoconjugate problems, total synthesis was not the sole inspiration for such efforts. Though sulfonamidoglycosylation provided routes from glycals to 2-aminoglycosides [ 14, 161, there was a more general need for a reliable and far-reaching route that would serve to convert glycals to common glycosides of glucose, galactose and mannose. The trichloroacetimidate donor methodology advanced by Schmidt [29], probably the single most powerful glycosylation method available, can often provide selective access to either a- or P-glycosides. We alternatively focussed on the application of glycal epoxides to tackle these and other problems. Such epoxides can participate in iterative strategies and can provide, in some circumstances, strikingly direct routes to polysaccharides. The existing record of a-epoxides as stereoselective glycosylating reagents was, at best, unimpressive when we launched our study. Access to such 1,2-anhydrosugar derivatives was awkward and their applications to glycosylations were beset by serious problems in stereoselectivity [30]. We first sought a direct and reliable route to 1,2-anhydrosugars. Furthermore, we hoped to gain advantages at the level of stereoselectivity by careful selection of resident functions. In particular, it was felt that resident groups bearing neighboring group participating functions, would tend to undercut clean formation of P-glycoside from a-epoxides. A major advance toward this goal came with the recognition that a variety of glycals react smoothly with 3,3-dimethyldioxirane (DMDO) [311. Glucal derivative 45 reacted with DMDO to give near quantitative yields of 46 and with >20: 1 aselectivity (Scheme 9). Galactal derivative 47 yielded the same result. In the allal series, glycal 49 underwent selective epoxidation instead from the P-face, while the gulal derivative 51 yielded a 1 : I mixture of epoxides. With these results in hand, the glycosyl donating abilities of these epoxides was explored (Scheme 10). Considerable experimentation indicated that ZnClz promoted selective p-glycosylation with alcohol acceptors, (e.g. 54, 55) whereas basic conditions promoted effective P-glycosylation only with moderately acidic acceptors (e.g. 56), such a s phenols. In some instances stannyl derived acceptors, conveniently generated in situ, afforded superior results. Subsequent to these investigations, it was found that oxirane 46 is actually among the poorest of donors [32], and that glycosylation yields could be improved by constraining the C3-C4 or C4-C6 oxygens into a cyclic motif. An interesting and useful variation on this procedure arises from the AgBF4 promoted reaction of an epoxide with a stannyl ether, which gives a-linked product. For example, treatment of 46 with 57 in the presence of AgBF4, followed by acetylation, cleanly affords 58 [33]. This method is presently limited to primary alcohol acceptors. These constraints underscore the need for a comprehensive solution to the challenging problem of stereospecifically generating a-glucosides.
3 Iterative Assembly of Glycals and Glycal Derivatives
72
0%
> -
>95%
45
0 % > -
TBSO
>95%
47
a:D
> 20:l
0% > -
>95%
TBSO
TBSO
49
Ph 0% > I
TBSO 51
>95%
50 > 2O:l P:a
?G
TBSO
52 1:l 8:a
Scheme 9.
R O *RO
OH
0-sugar
53
88%
42%
55
56
""d-""' /
Scheme 10.
67% (KzC03,18-C-6)
3.4 KS-502 and Rebeccamycin
73
Roao
1 .DMSO AqO
RO
pyridine F
RO 59
RO
61
60
Scheme 11.
The glycal epoxide method of glycosylation results in the placement of a free hydroxyl group at the 2-position of the hexose following coupling. This feature of the method enabled two alternative and useful advances (Scheme 11). First, the unique C2-hydroxyl group could be removed by the free-radical reduction of the derived pentafluorophenyl thionocarbonate (Barton deoxygenation). This sequence provides a synthesis for 2-deoxy-~-glycosides(60) [32, 341. Alternatively, the C-2 hydroxyl group could be inverted via an oxidation (AclO/DMSO)/reduction (NaBH4) protocol furnishing P-mannosides (61) [33, 351. Furthermore, the efficient and reliable route to a-glycal epoxides, facilitates numerous other methodologies leading to established donor functions (Scheme 12). Of particular importance is the fluoro-sugar type (cf. 66). Following benzylation of the uniquely free C2 hydroxyl group, derivatives such as 67 are produced. This method provides access to glycosyl donors ready for the powerful Mukaiyama promoter system [36]. We have often applied this protocol to allow for the stereoselective introduction of a-glycosides, as in 68.
62
63
BnNb
TBAF
<:
ZnCl2.71%
/
6
Bno BnO
NHB
OH 64
I
B\%c V
65
Scheme 12.
HX
X
Bno& BnO
AR'OH S & OR
66R=H 67 R = Bn
74
3 Iterative Assembly of Glycals and Glycal Derivatives " 5 ; H
nC5H11
184-6
H % H
q$(c!F?: En0
81% 69
"OH
15
,q
Rn6,H
OH
71
42 KS 501 R = H 43 KS 502 R = COfl
3 --L
48% 72
73
Rebeccamycin
Scheme 13.
The finding that the acceptor for a glycal epoxide can itself be a glycal established the basis for an iterative strategy for the synthesis of repeating p-glycosides. This concept has been a central component of our approach to oligosaccharide synthesis and has been adapted to solid support strategies as well [37]. The glycosidic bonds present in KS-502 and rebeccamycin provided especially attractive contexts for evaluation of the glycal epoxide method (Scheme 13). Treatment of the furanose glycal with DMDO gave the furanose epoxide 69 stereoselectively [38].Reaction of this epoxide with the phenol 70 afforded an 81% yield of the desired aryl-glycoside 71. Subsequent steps completed the synthesis of KS501 (42) and KS-502 (43). These furanoid epoxides were also used in the synthesis of nucleosides [39]. The extension of this procedure to the glycosylation of indoles was pursued in the context of rebeccamycin [40]. In this example, deprotonation of the differentiated indolocarbazole 72 followed by exposure of the resultant anion to glycal-epoxide 73 gave a 48% yield of the glycosylated product 74. Subsequent 2,2'-bis indole bond formation and deprotection yielded the glycoconjugate rebeccamycin (44).
3.5 Extension to Thioethyl Donors Galactal derived epoxy donors, such as 48, are particularly effective in enabling pgalactosylation. By contrast, glucal derived epoxides, lacking a comparable inter-
3.5 Extension to Thioethyl Donors
75
Lewis Acid
OH RO 75
77
RO 76
Scheme 14.
locking pattern between C3 and C4, are considerably more labile to typical Lewis acid promoters. This heightened instability brings with it significant limitations to the scope of their effectiveness as p-glycosyl donors. When the acceptor reactivity of the hydroxyl group in ROH is diminished, the yields of the glycosylation are compromised. In addition, the glycal epoxide method had not provided an efficient route to mannosides. For these reasons, we sought to extend the glycal epoxide method and turned to the use of thioethyl donors (75) for glycosylation and mannosylation (Scheme 14). Thioethyl and thioaryl glycosyl donors have previously been shown, in impressive studies by Garegg [41], to provide access to PI-4 glucosyl linkages in good yield. Donor activation is accomplished by either heavy metal salts or, directly and more efficiently, by other thiophilic reagents. The stereoselectivity of the glycosylation reaction is determined by the nature of the protecting group on C2. Indeed, it was found that glycals can be smoothly converted to thioethyl donors via glycal epoxides [42] (Scheme 15). The thioethyl glycosides thus produced, could be converted into the mannosides (75 + 85, Scheme 16). This overall inversion of configuration at C2 proceeded via an oxidation-reduction sequence to give, exclusively, the mannose derivative. The
EtSH cat. TFAA
B nO
71%
>95% 70
BnO& B nO
>95% w
BnOO 46
Scheme 15.
EtSH
-*cat. TFAA 78%
s OH . 81
EtSH cat. TFAA
03
BnO 6 BnO
U
-0TIPS
82
OH
80
79
E
45
BnO
-
U
84
76
3 Iterative Assembly of Glycals and GlycaI Derivatives 1. protect
R RO
asEt
O
OH
2. MeOTf DTBP 4A MS
75 1. ACZO DMSO pyridine 76 % 2. NaBH4
77
R
RO
O
MeOTf b -
F&SEt
85 R' = Ac, Bz, PIV
R O = L
S
* rR RO
DTBP 4A MS
a7
RORO -
Scheme 16.
resultant C2 hydroxyl groups can be protected with a variety of groups of known participatory capacity. The pivalate (Piv) derivatives were generally found to be the neighboring groups of choice for installing the desired linkage in that they provide very good margins of p-stereoselection without incurring problems of orthoester formation in competition with glycosylation (Scheme 17) [42].
3.5 Lewisy The antigenic blood group determinant LewisY (Ley) [43] is over-expressed in glycolipid and mucin glycoprotein form on many human tumor cells including those found in colon, lung, breast, and ovarian cancers. With this in mind, we set out to fashion vaccines based on the lipid and protein forms of Ley (Figure 5) [44]. To achieve these goals, we settled on the following target systems: (i) the natural product Ley-ceramide 94, (ii) a conjugate form for vaccination purposes 95, and a (iii) mucin mimic en route to a polypeptide-based vaccine (vide infra). The Ley tetrasaccharide determinant consists of a bis-fucosylated N-acetyl lactosamine. We reasoned that the iodosulfonamidation methodology could be used to install the amine function and would favor the subsequent incorporation of additional glycal units. It also seemed likely that both the Ley-ceramide target and the Ley-conjugate could be prepared from a common, but late, glycal epoxide intermediate. Concise construction of the Ley tetrasaccharide 98 commenced with lactal (96, Scheme 18) [44, 451. Protection of the primary alcohols, followed by engagement of the syn-vicinal diols as a cyclic carbonate, provided the lactal acceptor 97. Stereoselective introduction of the fucose moieties was achieved by applying a modifica-
77
3.5 LewisY MeOTf OTIPS B BnO n O
a
Brio*
DTBP
s E OPlV
t
OTIPS
OSugar
BnO
b *
OPlV
88
89
91
80%
BnO
MeOTf TIPSO n o % ,., BnO
4AMS DTBP ~
~
Bno
TIPSO BnO
93
92
0-Sugar
78%
91%
76%
Scheme 17.
tion of the original Mukaiyama conditions, using either 99 or 100. By starting with the glycal, the protecting group manipulations were minimal (2 steps). It remained however to convert the glycal function to the required lactosamine and to effect glycosylation to the pentasaccharide. In the event, 98 could be cleanly converted, by application of the iodosulfona-
OH
Figure 5.
94; R = Ceramide 95 R = Ally1
78
3 Iterative Assembly of Glycals and Glycal Derivatives
I
V
IDCP, 94% Oh
OBn
&
OB"
p
PhS02HN
102
75%
Scheme 18.
midation protocol, to 101 bearing the anomeric sulfonamide in excellent yield (94%). The latter underwent rearrangement to 102, using the 3-stannyl ether of 6Tips galactal, in 75% yield. This product contained the necessary lactosamine core linked in the desired p-configuration to the terminal galactal unit and constituted a protected Ley-pentasaccharide. After conversion of 102 to peracetate 103, the stage was set for divergent pathways to the ceramide (94) as well as to the ally1 glycoside (95). The later was the form we selected for conjugation to protein carriers, bovine serum albumin (BSA) and keyhole limpet hemocyanin (KLH). These were obtained from 103 upon treatment with DMDO followed by addition of the corresponding acceptors to the glycal epoxide. By application of iodosulfonamidation and the glycal epoxide methodologies, the targets were readily obtained. These syntheses paved the way for preclinical, and eventually clinical, investigations based on the Ley-pentasaccharide [46]. In these first generation vaccines, clinical evaluation of the immune response revealed that the antibodies displayed an apparent selective binding to Leyglycolipids, while relatively low antibody reactivity was noted with Ley-mucin pro-
3.5 Lewis”
79
teins. Implicitly, these immunological results posed the challenge of designing and synthesizing a closer mimic of a Ley-bearing m u c h glycoprotein. Before any effort to construct a mucin mimic could be launched, we were faced with the issue of glycoprotein-targeted vaccine design. We set out, therefore, to construct the simplest, and yet most realistic, molecules that would serve to direct the immune response against mucins. However, unlike Ley-ceramide, which is a discrete molecule, glyco-patterns in mucins may vary along the backbone and from one protein to the next. Hence, the uncertainties regarding glycoprotein structure render it quite difficult to design, with high confidence, molecules that will stimulate an immune response which targets mucin bearing cancer cells. The complexities of mucin structure notwithstanding, we proposed to synthesize several Ley glycopeptide-based vaccine candidates [45b, c]. To achieve this, we elected to incorporate the key common features of a mucin [43d, 471, including a primary structure characterized by sequentially arranged, and glycosylated, serine or threonine residues. The first carbohydrate of the glycodomain is a-0-linked GalNAc, and a number of intervening Gal and GlcNAc spacer units may insulate the core from the terminal region, which may bear, for instance, a blood group determinant like Ley. The glycopeptide domains of three of our mucin vaccines are shown in Figure 6 (104-106). Clustered pentasaccharide 104 satisfies the minimum requirements of the Ley tetrasaccharide attached to GalNAc-a-0-Ser. The GalNAc-P-0-Ser derivative 105, containing unnatural p-linkages, was also prepared. We wondered if such a ‘non-self’ structure might prove more immunogenic than 104 and still elicit antiLey antibodies. The hexasaccharide cluster, 106, has the natural GalNAc-a-0-Ser core as well as a spacer Gal between the Ley tetrasaccharide and the first carbohydrate and as such might be considered an even more realistic mimic of mucin structure.
B
R
Figure 6.
R
80
3 Iterative Assembly of Glycals and Glycal Derivatives
Having settled on a design, we returned to our original synthesis of Ley and identified the pentasaccharide 103 as a subgoal. If this pentasaccharide could be converted to the appropriate glycosyl amino acid bearing constructs, derivatives 104-106 should be accessible through standard solution phase peptide coupling. While iodosulfonamidation of glycals is often high yielding, and the adduct ready for subsequent glycosylation, removal of the 2-benzenesulfonamido group requires Birch (metal/ammonia) reduction conditions. This protocol was developed to accommodate the glycal moiety in the context of a complex oligosaccharide and achieves removal of benzyl groups without recourse to hydrogenation. However, dissolving metal reduction is not compatible with the base- and acid-sensitive glycoamino acid function. Thus, the iodosulfonamidation method could not be appropriated for the synthesis of Ley pentasaccharide bearing glycopeptides. To gain access to 104 and 105, pentasaccharide 103 would have to be transformed to a suitable donor substrate. The latter would be combined with the amino acid core for eventual peptide coupling (Scheme 19). In practice pentasaccharide 103 was subjected to Lemeiux azidonitration [48].This transformation was followed by reduction of the nitrate ester, and conversion of the resultant anomeric alcohol to the Schmidt trichloroacetimidate [49]. Donor 107, thus prepared, was ready for coupling with a serine acceptor. Compound 107 and FmocSerOBn (108), under promotion by TMSOTf, afforded the desired O-linked glycoamino acid, albeit in a ratio of 2.7: 1, a(109):b(ll0). After considerable experimentation, it was found that by altering the solvent composition, the relative ratio could be reversed to 1 : 1.7, a(109) :P(ll0). Thus both building blocks necessary for the construction of 104 and 105 were obtained from a common donor. Further transformations afforded the acids 111 and 112, respectively. These products were ready now for peptide coupling. In several instances, as illustrated above, glycosylation of elaborated 2-azido galactosyl donors seemed to be dependent upon the nature of the donor and the identity of the serine or threonine acceptor. In this context an acceptor such as 113 was preferred (Scheme 20). Assuming pentasaccharide 103 could be converted into a suitable donor, the glycoamino acid required for assembling hexasaccharide cluster 106 could be obtained by direct glycosylation with acceptor 113 [50]. To prepare 119, pentasaccharide glycal 103 was subjected to the action of DMDO, generating the glycal-epoxide donor. However, under normal promotion conditions acceptor 113 failed to couple with the donor epoxide. In order to enhance the reactivity of the donor, the epoxide was converted to the thioethyl donor 114, which, after protecting the newly generated 2-hydroxyl group as the benzoate (116), was coupled with 113 under MeOTf promotion. The glycosylation did proceed, but in low yield and was accompanied by N-methylation of the acetamide functionality, affording both the desired 118, as well as, the undesired methyl imidate. Thus, methyl triflate promotion of thioethyl donors, found to be fully compatible with glycal acceptors, becomes complicated in the presence of acetamides and this difficulty appears to be general. Though the imidate, formed as byproduct in the methyl triflate reaction, could be hydrolyzed back to the acetamide, other more direct methods were pursued. In this regard, it was found that in the presence of NIS/TfOH [51], the desired coupling was achieved in minutes and in excellent yield. Following further optimization, the preferred route to 118 is as follows. Pen-
3.5 LewisY
81
1. CAN, Nab, (66%) b
103
OCNHCCI,
2. PhSH, iP?!NEt (92%)
3. CbCCN. I(,CO,
(89%) 107
> 75%, 1:1 .7 109:110
75%#,2.7~1
WFmcc 108
0 109
AcHN
NiFmoc
OAc
,&&
OAc d 0 r A c
0 OAc
112
ACOoAc
Scheme 19.
tasaccharide glycal 103 was subjected to the action of DMDO and subsequently converted to the pentenyl donor 115 [52]. The 2-hydroxy glycoside was then protected as the benzoate. In the event, NIS/TfOH-promoted coupling of donor 117 with acceptor 113 cleanly afforded the desired serine-O-linked hexasaccharide 118. After manipulating the protecting groups, the glycoamino acid 119 was obtained. Sequential peptide coupling of 111, 112, and 119 provided the glycopeptide domains 104--106 and eventually several vaccine candidates. It was gratifying in this regard to find that preclinical immunological evaluation demonstrated that we had indeed constructed a vaccine which provokes an antibody response targeting
82
3 Iterative Assembly of Glycals and Glycal Derivatives
oac Ho
oms
-%J--.
1. DMDO
103
2. HSEI or 4-penlen-1-01
AcHN
'NHF~~:
113
116 w117
f
3
114: R ,
= SEI. & = H
115: R ,
= 0-4-Penteny1, R,= H
BzCl
(45% from 103)
116 RI = SEt. R2 = Bz
1
117 R1 = 0-4-Penteny1, R, = Bz
Scheme 20.
Ley-mucin bearing cells. Clinical evaluation of an anti-cancer vaccine based on 106 is currently well underway.
3.7 Globo H The hexasaccharide designated globo H (120, Figure 7) has been identified on human prostate, breast and small cell lung carcinomas, as well as in a restricted number of normal epithelial tissues [53]. Globo H was originally isolated in ceramide form from the human breast cancer cell-line MCF-7 1541. The enhanced expression of antigens assumed to be globo H on both primary and metastatic prostate cancer specimens drew us to the possibility of initiating a globo H-based vaccine strategy in prostate and other cancers 1551. At the planning stage of the synthesis 1561 it seemed reasonable that, similar to the Ley synthesis, late stage divergence from a hexasaccharide glycal epoxide could pro-
83
3.7 Globo H
H type 111 trisaccharide
Globotriose
120: R = Ceramide 121: R = Ally1
Figure 7.
vide both the ceramide form and the ally1 glycoside (121) for immuno-conjugation to KLH. As i1.s name indicates, globo H contains globotriose and an H type I11 module. The chemical union of two trisaccharides, a globo trisaccharide acceptor with an H type donor, would constitute a highly convergent approach this complex target. The galactosamine unit constituted added complexity, since iodosulfonamidation had not been attempted on a galactal unit. Additionally, a final 3 3 coupling using a galactal donor derived from an iodosulfonamide could serve to provide a dramatic demonstration of the method. The galactosyl units, including the galactosamine, might be derived from a common galactal building block. Similarly, the glucose unit of the lactosyl ceramide would come from glucal, while the terminal fucose could be installed at any convenient stage in the synthesis. Compound 122 serves as an excellent acceptor, due in part to its allylic nature, and does not suffer from competing glycosylation at the axial 4-position (see 123, Scheme 21) [56].In keeping with our earlier precedents, conversion of 122 to cyclic carbonate 124 followed by treatment of 124 with DMDO generates the expected glycal epoxide 125. This intermediate was coupled with 122 and 127 to give disaccharide building blocks 126 and 128 respectively. Disaccharide 125 was carried forward without protecting group manipulations and was fucosylated under Mukaiyama conditions (125 + 130, Scheme 22). This step required selective functionalization of the unreactive C2'-hydroxyl over the,
+
(123) (not observed)
Scheme 21.
126
128
84 0
3 Iterative Assembly of Glycals and Glycal Derivatives 0
OTPS
OH 0T1p
-
AgC104, SnCl2
\ k k & O e OH 125
130: R = H
48%
131: R = Protecting Group
BnO BnO
129
Scheme 22.
presumably less reactive, axial C6hydroxyl. In the event, under optimized conditions, the desired product was obtained in 48% yield. The undesired regioisomer was a minor byproduct. Still, this sequence provided the most direct route to the target trisaccharide glycal (a total of four steps from the common precursor, 6-Tips galactal). Protection of the lone hydroxyl was achieved in a straightforward manner (130 -+ 131). As will be shown, an even more serious unanticipated turn of events arose in attempting to use this glycal as a donor. The acceptor for the projected 3 + 3 coupling was prepared from fluoro-donor 134 and lactal 132 (Scheme 23). Compound 127 was readily converted to lactal acceptor 132, while 134 was prepared from selectively protected galactal 133. It seemed prudent to convert 133 to the more reactive 0-derivative 134 by direct opening of the epoxide with TBAF, followed by protection of the resultant hydroxyl. Mukaiyama coupling of 132 with 134 proved troublesome. While the yield was high (72%), the major impediment was selectivity ( a :p, 3: 1). The desired
I33
I27
OH OBn
o&
BnO-
Bno Bm 134
135
132
AgC104, SnCI2 132
+ I34 M 135
(88%, l O : l , a:p)
136: R = PMB 137: R = H
Scheme 23.
3.7 Globo H
85
product was obtained in only about 50% yield. Furthermore, as the reaction proceeded the reactive p-derivative 134 was slowly converted into the unreactive aderivative 135. Eventually, application, with slight modification, of the Suzuki [ 571 promotion procedure, using the superiluorophile CplZr(OTf)2, provided the desired product in 10: 1 ratio in an overall yield of >80% [ 5 8 ] . In fact, this promoter allowed for smooth conversion of either 134 or 135 to the desired product. This finding simplified the sequence considerably, in that treatment of 133 with DMDO could be followed by HF:pyridine effecting clean and rapid conversion to a-derivative 135. After removing the protecting group from 136, the acceptor trisaccharide (137) was ready for the key glycosylation reaction. As depicted in Scheme 24, compound 131 underwent smooth conversion to iodosulfonamide 138 in good yield. However, in practice, 138 failed to couple with trisaccharide 137. Undaunted we pursued the complementary route. Thus iodosulfonamide 138 was converted to the thioethyl donor 139. The 3 3 coupling of 139 and 137 was effected using methyl triflate in excellent yield, particularly considering the sophistication of the j’oined partners. A small quantity of an isomeric hexasaccharide was obtained. At the stage of hexasaccharide (140) it was difficult to rigorously assign the structure of the product, and it was primarily precedence upon which we base the structural assignment. Only after eventual conversion of oligosaccharide 140 to the deprotectetl ceramide, 141, did we realize that in the 3 + 3 coupling gave the undesired a-glycoside. This unpredicted turn of events is actually rather general [591. It was found that even monsaccharides consisting of 2-sulfonamide thioethyl galactosides resulted in glycosylation with poor selectivity. In the case at hand, modifying the protecting group on the C4-hydroxyl (see ‘R’ in 139) failed to reverse selectivity. Eventually, we would conduct the key glycosylation without any protecting group on the C4-position. However, C4 deprotection of 139 to give 143 (see Schemes 24 and 25) proved to be no simple matter, regardless of the identity of the protecting group. After considerable experimentation, it appeared as if the iodosulfonamidation and the ‘roll over’ reaction would have to be performed on the unprotected glycal 130.
+
131
IDCP L (82%. PhSqNH,) W
O
B
BnO OBn
13’,
Hexasaccharlde
139 b
60%
140
... Scheme 24.
UH
n 139
86
3 Iterative Assembly of Glycals and Glycal Derivatives 0 OTIpS 130
IDCP, PhSO N 2
50%
2
Oxo&-
p
LHMDS, HSEt L
PhS0,NH
79x
oy*o& p
om SEt !‘hSO,NH
143
I43
137, MeOTf L
70 - 85%
L
L
I20 and121
Scheme 25.
While this approach did eventually succeed, the path was not traversed without further obstacles (Scheme 25). Treatment of 130 with IDCP gave the desired iodosulfonamide. However, unlike similar derivatives, the immediate product was not stable to purification or storage and isomerized to an unproductive stereoisomeric donor. Usually, instead of attempting to purify the intermediate, 142 was taken on to the ‘roll over’ step providing the desired donor 143. Remarkably, methyl triflate promoted coupling of 143 with 137 gave the desired product in good yield. This compound was accompanied by a small amount of an undesired isomer. That the major product was indeed hexasaccharide 144 was demonstrated ultimately by its conversion to the natural product. In the end, 144 was converted to a peracetate glycal, which was converted to globo H-ceramide 120 and the ally1 glycoside 121. Compound 121 was taken on to vaccination trials in the form of a KLH conjugate and continues to be studied in various clinical settings with encouraging early results. While hard won, the lessons garnered in the globo H venture paid large dividends, in that a number of related carbohydrates were accessible using ‘unprotected 4-hydroxy’ donors similar to 143. Thus, asialoGM1 [59], GM1 [60], and the demanding fucosyl GM1 [61] have each been prepared for study. In retrospect, globo H was synthesized by implementing most of the glycal methods available, including glycal donors, acceptors, epoxides, and fluorides, as well as by extension of the iodosulfonamidation protocols.
3.8 KH-1 KH-1 (145, Figure 8) is one of the most formidable carbohydrate-based tumor antigens characterized to date [ 621. This nonasaccharide has been identified on the cell surfaces of all human adenocarcinoma cells thus far studied. Furthermore, its pres-
3.8 KH-1
P
AcHN
LeY
OH
AcHN
LeX
OH
87
OH
Lactose
145: R = Ceramide 146: R = Ally1
Figure 8.
ence has never been detected in normal colonic extracts. In keeping with our program goals we targeted both natural ceramide (145) and ally1 glycoside (146) forms for total synthesis [63]. Structurally. KH-1 can be viewed as a lactosyl ceramide bearing the Lewis' (Le') trisaccharide upon which is mounted the Ley tetrasaccharide [62]. In the synthetic planning stage, however, we set, as the cardinal strategic objective, maximum relief from blocking group manipulations. In an effort to implement a highly convergent route, we hoped to introduce all three fucosyl units in a single operation. The subgoal would then consist of assembling, in an iterative fashion, glycal disaccharides to form the hexasaccharide core. Importantly, the three oxygen centers to be fucosylated would be formulated such that a single deprotection maneuver would serve to unveil them concurrently. Pursuit of the target began with glycal building blocks 147-149, and from these three glycals, and fucosyl donor 150, the entire molecule would be constructed (Scheme 26). TES ether protection, for example resident in 151, was chosen as the mask for sites of eventual fucosylation. Thus, treatment of 147 with DMDO cleanly afforded the a-epoxide, to which the acceptors 148 and 149 could be added, in the usual way, in a parallel sequence giving glycals 151 and 152. Reiteration of glycal assembly was attempted after independently applying either the acetate (153) or an additional TES ether (154) to the newly generated hydroxyl of 151. Compound 152 was destined to house the lactose bearing the ceramide unit and was therefore converted to trio1 155. While 155 has several potential glycosylation sites, in principle, the C3' hydroxyl is the most reactive position. A suitable donor could be fashioned by subjecting 153, the core of the Le' sector, to IDCP iodosulfonamidation (see 156). Addition of ethanethiolate effected the roll over reaction and the donor was ready for coupling with 155. In the event, treatment of 157 with 155 under promotion of MeOTf did, in fact, give the desired tetrasaccharide 158, albeit often in low yield (35-55%). In addition, the reaction mixture appeared to contain significant quantities of at least one regioisomer.
88
3 Iterative Assembly of Glycals and Glycal Derivatives
\KO&
?go& m, TESO
OH En0
\,
151
KzC03
80%
I,,":
c
c
LHMDS. HSEt OYKo%
-*
TESO TESO PhS0,HN I60
PhS02HN 156
I59 R=SnBu,
oy&q Eso
Aco
SEl PhSO,HN
91%
1
I 159, AQEF,
V
I55 R = H
81%
/
157
155, MeOTf
35 - 55%
Scheme 26.
We wondered if the donor, one of our most activated, might be so reactive as to be insensitive to the subtle differences in the other hydroxyl units of the acceptor. To test this hypothesis, we attempted the union of 156 with the tin ether of the triol, 159. Direct 'roll over' proved to be quite slow, but very sensitive to the differences in hydroxyl reactivity in the acceptor. This chemistry constituted a very pleasing and attractive solution to the assembly of 158. The next plateau to be reached was seen to be that of the full hexasaccharide core (Scheme 26). In an effort to maintain the tempo of the synthesis, we proceeded on to another assembly iteration with minimal protecting group-based transformations. The use of a thioethyl donor was explored; however, in the end the direct roll over route proved more reproducible. To this end, iodosulfonamidation of 154 afforded 160. While presumably even more demanding than the earlier transformation, we opted to attempt the selective coupling of the tin ether of pentaol 161 with 160 (Scheme 27). This acceptor was readily available from the tetrasaccharide by simple removal of the cyclic carbonate (158 +.161). The reaction between 160 and 162 was performed, as before, in the form of the tin ether acceptor. Fortunately, the required
3.8 KH-1
89
161 R = H
162 R = SnBu,
163 R=H.R,=TES 164 R = A c . R ( = H
\\,
Scheme 27.
coupling could be executed in good yield (>60% of 163) and without detectable formation of side product arising from glycosylation at alternative acceptor sites. The culminating stage of the synthesis was confronted after protecting the four remaining hydroxyls and removing the silyl ether protection (163 + 164). The critical step, threefold fucosylation, was indeed accomplished through the reaction of 150 and 164. An exemplary yield (60%) of the nonasaccharide 165 was obtained. In the final steps, direct epoxide opening with ally1 alcohol proved optimal for preparing 146. Our alternative procedure was used to install the ceramide. Thus, thiolation of the epoxide generated from the reaction of 165 with DMDO. This was followed by protection of the resultant free hydroxyl to give donor 166 ready for coupling with a suitable acceptor. Following this sequence, the glycal was successfully taken on to the natural product (145). Conjugation of 146 to KLH provided a vaccine and pre-clinical immunological testing is under way. Application of iterative glycal methodologies requires glycal compatible promotion systems, and these promoters are now available. The ability to attenuate donor reactivity with the acceptor is frequently desirable, especially in the search for optimized routes, and was used to effect in the case at hand. By applying the full complement of glycal technology the target was obtained in a highly convergent and efficient way. In summary, the synthesis of KH-1 serves as a cogent demonstration
90
3 Iterative Assembly of Glycals and Glycal Derivatives
of glycal assembly and the minimized protecting group manipulations that this strategy can provide.
3.9 Concluding Remarks As shown above the logic of glycal assembly has been field tested in a variety of different contexts. While our focus here has been on the total syntheses themselves, clearly, much has been learned about the chemistry of glycals and their amenability to iterative coupling with suitable acceptors and donors. At this stage, with the required compounds in hand our attention is directed toward vaccine performance, with respect to eliciting active immunity against various cancers.
Acknowledgments
This work was supported by the National Institutes of Health (Grant Numbers: CA28824, A116943, HL25848). Postdoctoral Fellowship Support is gratefully acknowledged by L.J.W. (Grant Number: NIH: F32CA79120) and R.M.G. (NIH F32CA85894). We gratefully acknowledge Dr. George Sukenick of the SloanKettering Institute’s NMR Core Facility (Grant Number: CA-08748) for NMR and Mass Spectral Analyses. References 1. S. J. Danishefsky, M. T. Bilodeau, Angew. Chem. Int. Ed. 1996, 35, 1380-1419. 2. V. Q. G. De Lima, C. A. Albert, 0. G. De Lima, An. Acad. Bras. Cienc. 1964,36, 317. 3. J. J. Afora, C. F. Santana, 0. G. De Lima, Ann. CVIIth Int. Congr. of Hematology and Hemotherapy, Paris, 1978, p. 165. 4. (a) 0. G. Da Lima, F. D. Monache, I. L. D’Albuquerque, G. B. Marini-Bettolo, Tetrahedron Lett. 1969,471; (b) L. W. Bieber, A. A. Da Silva Filho, J. F. De Mello; W. Von Der Saal, 0. G. De Lima, Rev. Znst. Antibiot. (Recife) 1982-1983, 21, 27. 5. T. Oki, Anthracycline Antibiotics [Pap,. Symp. On Anthracylines, Aug 24-25,1981, New York] Khakeme, E. Ed.; Academic Press: New York, 1982; p. 75. 6. (a) R. U. Lemiew, S. Levine, Can. J. Chem. 1964,42, 1473; (b) R. U. Lemieux, A. R. Morgan, ibid 1965, 43, 2190. 7. (a) J. Thiem, H. Karl, J. Schwentner, Synthesis 1978, 696; (b) J. Thiem, H. Karl, Tetrahedron Lett. 1978, 4999; (c) J. Thiem, P. J. Ossowowski, Carbohydr. Chem. 1984, 3, 287; (d) J. Thiem, A. Prahst, I. Lundt, Justus Liebigs Ann. Chim. 1986, 1044; (e) J. Thiem, W. Klaffke, J. Org. Chem. 1989, 54, 2006; (f) J. Thiem, Trends in Synthetic Carbohydrate Chemistry, D. Horton, L. D. Hawkins, G. L. McGarvey (Eds.), ACS Symposium Series #396, American Chemical Society, Washington, D. C., 1989, Ch. 8. 8. (a) H. Paulsen, Angew. Chem. Int. Ed. Engl. 1982,21, 155; (b) D. R. Mootoo, P. Kondradsson, U. Udodong, B. Fraser-Reid, J. Am. Chem. SOC.1988,110, 5583. 9. (a) R. W. Friesen, S. J. Danishefsky, J. Am. Chem. SOC.1989,111, 6656; (b) R. W. Friesen, S. J. Danishefsky, Tetrahedron 1990, 46, 103. 10. K. Suzuki, R. Friesen, G. Sulikowski and S.J. Danishefsky, J. Am. Chem. SOC. 1990,112,8895.
References
91
11. J. Gervay and S.J. Danishefsky, J. Org. Chem. 1991, 56, 5448. 12. For an even more concise synthesis of ciclamycin 0, see: S. Raghavan, D. Kahne, J. Am. Chem. Sac. 1993, 11'5,1580. 13. (a) D. Koga: A. Isogai, S. Sakuda, S. Matsumoto, A. Suzuki, S. Kimura, A. Ide, Agric. Biol. Chem. 1987, 51, 471; (b) S. Sakuda, A. Isogai, S. Matsumoto, A. Suzuki, J. Antibiot. 1987, 40, 296. 14. B. M. Trost, D. L. Van Vranken, J. Am. Chem. SOC.1990, 112, 1261. 15. M. L. Sinnot, Chem. Rev. 1990, 90, 1171. 16. (a) T. Iversen, D. R. Bundle, J. Chem. SOC.Chem. Commun. 1981, 1240; (b) L. J. Mathias, Synthesis 1979, 561; (c) L. A. Clizbe, L. E. Overman, Org. Synth. 1978, 58, 4. 17. R. J. Ferrier, N. Prasadi, J. Chem. Soc. C 1969, 570. 18. (a) P. Bickart, F. W. Carson, J. Jacobus, E. G. Miller, K. Mislow, J. Am. Chem. Soc. 1968, 90, 4869; (b) R. Tang, K. Mislow, J. Am. Chem. Soc. 1970, 92, 2100. 19. (a) D. A. Evans, G. C. Andrews, C. L. Sims, J. Am. Chem. SOC.1971, 93, 4956; (b) D. A. Evans, C. A. Bryan, C. L. Sims, J. Am. Chem. SOC.1972, 94, 2891; (c) D. A. Evans, G. C. Andrews, Arc. Chem. Res. 1974, 7, 147. 20. D. A. Griffith, S. J. Danishefsky, J. Am. Chem. Soc. 1990, 112, 5811. 21. S. J. Danishefsky, Chemtracts Org. Chern., 1989,2, 273. 22. M. D. Wittman, R. L. Halcomb, S. J. Danishefsky, J. Org. Chem. 1990, 55, 1979. 23. R. U. Lemieux, T. L. Nagabhushan, Can. J. Chem. 1968,46,401. 24. S. J. Danishefsky, K. Koseki, D. A. Griffith, J. Gervay, J. M. Peterson, F. E. McDonald, and T. Oriyama, J. Am. Chem. SOC.1992, 114, 8329. 25. D. A. Griffith and S. J. Danishefsky, J. Am. Chem. Soc. 1991, 113, 5863. 26. Isolation: (a) S. Nakanishi, K. Ando, I. Kawamoto, W. Kase, J. Antibiot. 1989, 42, 1049; Structure determinaton: (b) T. Yasuzawa, Y. Saitoh, H. Sano, J. Antibiot. 1990, 43, 336. 27. J. A. Bush, B. H. Long, J. J. Catino, W. T. Bradner, K. Tomita, J. Antibiot. 1987, 40, 668. 28. (a) T. Kaneko, H. Wong., J. Utzig, T. W. Doyle, J. Antibiot. 1990, 43, 125; (b) A dibromo analogue of rebeccamycin has been prepared K. Sing Lam, D. R. Schroeder, J. M. Veitch, J. A. Matson, S. Forenza, J. Antibiot. 1991, 44, 934. 29. R. R. Schmtdt, Angew. Chem., Znt. Ed. Enyl. 1986, 25, 212. 30. (a) W. J. Hickinbottom, J. Chem. SOC.1928, 3140; (b) E. Hardegger, J. de Pascual, Helv. Chim. Acta 1948, 31, 281; (c) L. L. Klein, W. W. McWhorter, Jr. S. S. KO, K.-P. Pfaff, Y. Kishi, J. Am. Chern. SOC. 1982, 104, 7362; (d) V. Bellosta, S. Czernecki, J. Chem. Soc. Chem. Commun. 1989, 199. 31. S. J. Danishefsky, R. L. Halcomb, J. Am. Chem. Soc. 1989, 111, 6661. 32. (a) C. M. Timmers, G. A. van der Marel, J. H. van Boom, Red. Trau. Chim. Pay-Bus 1993, 112, 609; (b) Ibid. Chem. Eur. J. 1995, I , 159. 33. K. K.-C. Liu, S. J. Danishefsky, J. Org. Chem. 1994,59, 1892. 34. (a) A direct method for the formation of 2-deoxy-P-glycosides from glycals employing Ph3P.HBr has been developed: V. Bolitt, C. Mioskowski, S.-G. Lee, J. R. Falck, J. Org. Chem. 1990, 55, 5812; (b) D. H. R. Barton, J. C. Jaszberepyi, Tetrahedron Lett. 1989, 30, 2619. 35. H. B. Boren, G. Ekborg, K. Eklind, P. J. Garegg, A. Pilotti, C.-G. Swahn, Acta. Chem. Scand. 1973,27, 2639. 36. T. Mukaiyama, Y. Murai, S. Shoda, Chem. Lett. 1981, 431. 37. (a) J.T. Randolph and S.J. Danishefsky, Angew. Chem. Int. Ed. 1994, 33, 1470; (b) K. F. McClure, J. T. Randolph, R. Ruggeri, S. J. Danishefsky, Science 1993, 260, 1307; (c) S. J. Danishefsky, J. Y. Roberge, Pure Appl. Chem. 1995,67,-1647;(d) J. Y. Roberge, X. Beebe, S. J. Danishefsky, Science 1995,269, 202. 38. R. G. Dushin, S. J. Danishefsky, J. Am. Chem. Soc. 1991, 114, 655. 39. K. Chow, S. J. Danishefsky, J. Org. Chem. 1990, 55, 4211. 40. M. Gallant, J. T. Link, S. J. Danishefsky, J. Org. Chem. 1993, 58, 343. 41. P. Fugedi, P. J. Garegg, H. Lonn, T. Norberg, Glyccoonj. J. 1987, 4, 97. 42. P. Seeberger, M. Eckhardt, C. Gutteridge and S.J. Danishefsky, J. Am. Chem. SOC.1997, 119, 10064. 43. (a) K. 0. Lloyd, E. A. Kabat, E. J. Layug, F. Gruezo, Biochem. 1966, 5, 1489; (b) T. Kaizu, S. B. Levery, E. Nudelman, R. E. Stenkamp, S. Hakomori, J. Biol. Chem. 1986, 261, 11254; (c) S. B. Levery, E. Nudelman, N. H. Anderson, S. Hakomori, Carbohydr. Res. 1986, 151, 311;
92
3 Iterative Assembly of Glycals and Glycal Derivatives
(d) B. W. T. Yin, C. L. Finstad, K. Kitamura, M. J. Federici, M. Welshinger, V. Kudryashov, W. J. Hoskins, S. Welt, K. 0. Lloyd, Int. J. Cancer 1996, 65, 406. 44. V. Behar, S. J. Danishfefsky, Angew. Chem. Int. Ed. Engl. 1994, 33, 1468. 45. (a) S. J. Danishefsky, V. Behar, J. T. Randolph, J. Am. Chem. SOC.1995, 117, 5701; (b) P. W. Glunz, S. Hintermann, J. B. Schwarz, S. D. Kuduk, X.-T. Chen, L. J. Williams, D. Sames, S. J. Danishefsky, V. Kudryashov, K. 0. Lloyd, J. Am. Chem. SOC.1999, 121, 10636; (c) Glunz, P. et al. Unpublished Results. 46. (a) V. Kudryashov, H. M. Kim, G. Ragupathi, S. J. Danishefsky, P. 0. Livingston, K. 0. Lloyd, Cancer Imunol. Immunother. 1998, 45, 281; Clinical data: (b) P. Sabbatini, V. Kudryashov, S. Danishefsky, P.O. Livingston, G. Ragupathi, W. Bornmann, M. Spassova, A. Zatorski, D. Spriggs, C. Aghajanian, S. Soignet, M. Peyton, C. O’Flaherty, J. Curtin, K.O. Lloyd, International Journal of Cancer, In Press. 47. (a) I. Brockhausen, in Glycoproteins, Montreuil, J., Schachter, H., Vliegenthart, J.F.G. Eds.; Elsevier: Netherlands 1995; pp. 201-259; (b) K. 0. Lloyd, J. Burchell, V. Kudryashov, B. W. T. Yin, J. T. Taylor-Papadmitrou, J. Biol. Chem. 1996,271, 33325; (c) S. Muller, S. Goletz, N. Packer, A. Gooley, A. M. Lawson, F. G. Hanisch, J. Biol. Chem. 1997, 272, 24780; (e) P. M. Rudd, R. A. Dwek, Crit. Rev. Biochem. Mol. Bio. 1997, 32, I ; (f) I. Carlstedt, J. R. Davies, Biochem. SOC.Trans. 1997,25, 214; (g) P. van den Steen, P. M. Rudd, R. A. Dwek, G. Opdenakker, Crit. Rev. Biochem. Mol. Bio. 1998, 33, 151. 48. R. Lemieux, R. M. Ratcliffe, Can. J. Chem. 1979, 57, 1244. 49. R. R. Schmidt, Angew. Chem. Int. Ed. 1986, 25, 212; R. R. Schmidt, W. Kinzy, Adv. Carbohydr. Chem. Biochem. 1994, 50, 84. 50. (a) X.-T. Chen, D. Sames, S. J. Danishefsky, J. Am. Chem. SOC.1999, 121, 10636; (b) J. B. Schwarz, S. D. Kuduk, X.-T. Chen, D. Sames, P. W. Glunz, S. J. Danishefsky, J. Am. Chem. SOC.1999,121, 2662. 51. (a) G. H. Veeneman, S. H. van Leeuwen, J. H. van Boom, Tetrahedron Lett. 1990, 31, 1331; (b) P. Konradsson, U. E. Udodong, B. Fraser-Reid, Tetrahedron Lett. 1990, 31, 4313. 52. D. Gordon, S. J. Danishefsky, Curb. Rex 1990,206, 361. 53. (a) P. 0. Livingston, Cancer Biol. 1995, 6, 357; (b) S. Zhang, C. Cordon-Cardo, H. S. Zhang, V. E. Reuter, S. Adluri, W. B. Hamilton, K. 0. Lloyd, P. 0. Livingston, Int. J. Cancer 1997, 73, 42; (c) S. Zhang, H. S. Zhang, V. E. Reuter, S. F. Slovin, H. I. Scher, P. Livingston, Clin. Cancer Res. 1998, 4 , 295. 54. (a) R. Kannagi, S. B. Levery, F. Ishijamik, S. Hakomori, L. H. Schevinsky, B. B. Knowles, D. Solter, J. Biol. Chem. 1983, 258, 8934; (b) E. G. Bremer, S. B. Levery, S. Sonnino, R. Ghidoni, S. Canevari, R. Kannagi, S. Hakomori, J. Biol. Chem. 1984,259, 14773. 55. (a) G. Ragupathi, S. F. Slovin, S. Adluri, D. Sames, I. J. Kim, H. M. Kim, M. Spassova, W. G. Bornmann, K. 0. Lloyd, H. I. Scher, P. 0. Livingston, S. J. Danishefsky, Angew. Chem. Int. Ed. 1999, 38, 563; (b) S. F. Slovin, G. Ragupathi, S. Adluri, G. Ungers, K. Terry, S. Kim, M. Spassova, W. G. Bornmann, M. Fazzari, L. Dantis, K. Olkiewicz, K. 0. Lloyd, P. 0. Livingston, s. J. Danishefsky, H. I. Scher, Proc. Natl. Acad. Sci. USA 1999,96, 5710; (c) Z.-G. Wang, L. J. Williams, X.-F. Zhang, A. Zatorski, V. Kudryashov, G. Ragupathi, M. Spassova, W. Bornmann, S. F. Slovin, H. I. Scher, P. 0. Livingston, K. 0. Lloyd, S. J. Danishefsky, Proc. Nat. Acad. Sci., 2000, 97, 2719. 56. T. K. Park, I. J. Kim, S. Hu, M. T. Bilodeau, J. T. Randolph, 0. Kwon, S. J. Danishefsky, J. Amer. Chem. SOC.1996, 118, 11488. 57. (a) K. Suzuki, H. Maeta, T. Suzuki Tetrahedron Lett. 1989, 30, 6879; (b) T. Matsumoto, H. Maeta, K. Suzuki, G. Tsuchihashi, G. Tetrahedron Lett. 1988, 29, 3567; (c) K. Suzuki, H. Maeta, T. Matsumoto, G. Tsuchihashi Tetrahedron Lett. 1988,29, 3571. 58. J. R. Allen, J. G. Allen, X.-F. Zhang, L. J. Williams, A. Zatorski, S. J. Danishefsky Eur. J. Chem. 2000, 1366. 59. 0. Kwon, S. J. Danishefsky, J. Am. Chem. SOC.1998,120, 1588. 60. S. K. Bhattacharya, S. J. Danishefsky, J. Org. Chem. 2000, 65, 144. 61. J. Allen, S. J. Danishefsky, J. Am. Chem. SOC.1999, 121, 10875. 62. E. Nudelman, S. B. Levery, T. Kaizu, S . 4 . Hakomori, J. Biol. Chem. 1986,261, 11247. 63. (a) P. P. Deshpande, S. J. Danishefsky, Nature 1997, 387, 164; (b) P. P. Deshpande, H. M. Kim, A. Zatorski, T.-K. Park, G. Rapgupathi, P. 0. Livingston, D. Live, S. J. Danishefsky, J. Am. Chem. SOC.1998,120, 1600.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
4 Thioglycosides Stefan Oscarson
4.1 Introduction Thioglycosides are not very common in nature, only a few simple alkyl and aryl thioglycosides have been found as constituents of antibiotics from Streptomyces species [ 1-31. Although not really thioglycosides, glucosinolates with an anomeric sulfur are also well-known natural compounds [4, 51. In spite of their low natural abundance, thioglycosides and their chemistry have for a long time been a field of interest for many researchers, the first thioglycoside was synthesized in 1909 [6], and their basic chemistry was early reviewed [4, 71. But it is only recently that their excellent glycosyl donor qualities have been recognized. In the beginning of the seventies the first successful synthesis of a disaccharide was performed [S], but it was not until the middle of the eighties that enough efficient promoters were discovered to make glycosylation with thioglycoside donors into a general and accepted method. Since then and in a very short time-span, thioglycosides have become one of the most used type of donors, especially in glycosylations with oligosaccharide donors (block synthesis). There are already some reviews on the subject [9-121, and in the most recent complete list of published glycosylations during one year (1994) [ 131, thioglycosides were used as donors in about 20% of all reported glycosylations and in 35% of all couplings using oligosaccharide donors, competing well with the long established halide donors and the trichloroacetimidate method. The strength of thioglycosides as glycosyl donors is their easy synthesis (Section 4.2) combined with their stability, withstanding most reaction conditions used in e.g. protecting group manipulations and glycosylations, and at the same time their effective acth ation using chemoselective thiophilic promoters (Sections 4.3.2 and 4.4.1) and also easy transformation into other types of glycosyl donors (Section 4.3.1). Hence, they are perfectly fitted to be used in block synthesis of oligosaccharides, where stable oligosaccharide donors are a prerequisite. Their stability also makes them very apt in synthetic schemes where a number of chemical modifications are performed just prior to the glycosylations, as, e.g., in various types of
94
4 Thioglycosides
internal glycosylations (Section 4.4.2). In this article the synthesis of thioglycosides and their activation and application as glycosyl donors are discussed.
4.2 Synthesis of Thioglycosides Obviously one important feature of attractive glycosyl donors are their easy availability. The synthesis of the most used types of thioglycoside donors, mainly simple alkyl and aryl glycosides, is straight-forward and can be performed on a large scale using cheap chemicals. Often the product is crystalline and can be purified without chromatography. Numerous ways to prepare thioglycosides have been described [4, 10, 111, but for the easy production of thioglycosides mainly two synthetic methods are commonly employed, namely the reaction of a peracetylated glycose with a thiol in the presence of a Lewis acid and the reaction of an acylated glycosyl halide with a thiolate.
4.2.1 From Anomeric Acetates As depicted in Scheme 1 an anomeric acetate is usually easily substituted with a thiol under Lewis acid promotion to give thioglycosides. A standard procedure is to react a peracetylated aldose dissolved in CH2C12 with a slight excess of a thiol using a hard Lewis acid, e.g. BF3-etherate [ 141 or FeC13 [ 151, as promoter to give high yields of the 1,2-truns-product (neighbouring group participation). @-Acetateprecursors usually react faster [16], which is an advantage since this diminish the formation of 1,2-cis-impurities through Lewis acid catalysed anomerisation of the pthioglycoside [ 171. a/p-Mixtures are often obtained with less reactive mercaptans [18-201 or glycosyl acetates [21, 221. Variations of this method include the utilization of trialkylstannyl [23] or silyl derivatives [24] of the mercaptan. Advantages are less odor and sometimes higher yields owing to less formation of (1,Zcis- and dithioacetal) side products, disadvantages are the accessibility and cost of the reagents. Also, thioacetic acid can be used as nucleophile in this approach, to give anomeric thioacetates, which, in a two step-procedure, can be selectively S-deacylated and alkylated to give thioglycosides, a sequence often used in the synthesis of thiooligosaccharides [251.
A AcOc
O
Scheme 1.
GOAC OAc
BF3-Et20 EtSH CHC13 83%
AcO
SEt OAc
4.2 Synthesis of Thioglycosides ,OAc
95
,OAc
5 Et,O/H,O ACO*~~,, AcO AcO Br
84%
OAc
Scheme 2.
4.2.2 From Glycosyl Halides
This is a classical way to thioglycosides [6], in which an acylated glycosyl halide is reacted with a thiolate in a polar solvent to yield the 1,2-truns-thioglycoside (Scheme 2). Both alkyl-and aryl mercaptans have been used and good yields are usually obtained, although, especially with the more basic alkyl thiolates, a reacetylation step is often necessary prior to work-up and isolation of the product. If controlled conditions and an aprotic polar solvent are used, a S~2-typedisplacement reaction can be performed to give the inverted anomeric configuration in the product, e.g. a-glycosides from p-glycosyl halide precursors, in spite of the presence of participating 2-@protecting groups (Scheme 3) [26, 271. A modification is the use of phase-transfer conditions, allowing the use of free thiols and non-polar solvents in the reaction [28-301. Also with glycosyl halides as donors, alkylstannyl derivatives of the thiol have been employed as acceptors [23]. The lower reactivity of these reagents necessitates the use of elevated temperature or a promoter in these reactions, which often result in alp-product mixtures. Syntheses of thioglycosides are sometimes possible using Koenigs-Knorr glycosylation conditions (Scheme 4) [3l], although mercury(I1) salts have been rather frequently used to promote glycosylation with thioglycoside donors (Section 4.3.2). Indirect formation of thioglycosides can also be performed from glycosyl halides. Reaction with thiourea yields a pseudothiouronium salt, which (as the thioacetate above) can be hydrolyzed under mild, selective conditions to give the anomeric thiol, which then can be alkylated in an efficient, non-smelling procedure to produce
Scheme 3.
Scheme 4.
96
4 Thioglycosides
czA,& " 3 S
Aito+
acetone A c o ~ r 80%
AcO OAc
+
H20
Aco&sH AcO
YNH2 90% NH2
OAc
Scheme 5.
I IV
OH
Scheme 6.
thioglycosides (Scheme 5) [321, however hampered by the believed carcinogenity of thiourea (and alkyl halides).
4.2.3 Protecting Group Manipulations in Thioglycosides All common protecting groups such as esters (acetates, benzoates, pivaloates, leviulonates), ethers (benzyl, allyl, trityl, silyl), acetals (isopropylidene, benzylidene, cyclohexane-l,2-diacetal (CDA)) and orthoesters can effectively be introduced, manipulated and removed. Problems encountered are the removal of benzyl groups using catalytic hydrogenolysis (when Birch reduction conditions is an alternative), since the sulfur generally contaminates the catalyst, and sometimes participating of the anomeric sulfur to the 2-position and a consecutive 1 -+ 2-migration of the thio group. In an attempt to introduce a 2,3-orthoester into a rhamnose thioglycoside, using a trimethylorthoester in CH3CN the methyl 2-thio-2-deoxy glycoside was found to be the main product (Scheme 6) [33]. This migration could be avoided by changing the solvent to DMF or by removing the methanol formed in the reaction [34]. Similar 1,Zmigrations of thioglycosides have been utilized to give 2-thio-2-deoxy glycosides or glycosyl fluorides, which can be processed to 2-deoxy-glycosides (Scheme 7) [35, 361. Most functional group conversions abundantly used on carbohydrate derivatives are also compatible with thioglycosides, e.g. SN~-displacementsof sulfonates [37], azo transfer [ 381, and deoxygenation by tributyltin hydride reduction of thiocarbonyl carbamates [39]. During oxidations care has to be taken so not to obtain the sulfoxide derivative (if that is not desired). DMSO and also PDC-mediated oxidations can usually be performed without problems [40]. The thioglycosides are also stable towards various metal reagents such as Grignard and alkyl lithium reagents, which allows carbon elongation of carbohydrates and is a handy way to higher carbon sugar donors [41-431.
4.3 Glycosylutions with Thioglycoside Donors
97
1) i.BupSnO
MeOH
ii. BnBr, CsF
DMF, 66%
SEt
OH
SEt
2)CIC(S)OPh DMAP, CH$N
80% BnO BnO
OMe OBn
OMe OBn
Scheme 7.
4.3 Glycosylations with Thioglycoside Donors The stability of thioglycosides was early recognized, as was also their very mild conversion into glycosyl halides. Hence, thioglycosides were initially used as convenient precursors for various glycosyl halides, which then could be used as donors in glycosylation reactions. Later, it was found that thioglycosides could be activated directly by the use of chemoselective thiophilic promoters, and this discovery opened up a new field of applicable donors and glycosylation reactions. 4.3.1 A Two-step Procedure: Transformation of Thioglycosides into Other Types of Glycosyl Donors Glycosyl bromides and chlorides are effectively produced using bromine [44, 451 (also I-Br [46]) or chlorine [47] (also I-Cl [46]), nowadays often with CHzClz as solvent. Under these mild conditions even unstable glycosyl halides can be produced, as e.g. furanosyl [48, 491 and alkylated oligosaccharide bromides [50], and various acid-labile protecting groups are allowed as well. Glycosyl fluorides have been produced using numerous reagents [51], e.g. dimethyl(methy1thio)sulfonium tetrafluoroborate (DMTSB) [52], NBS-DAST [ 531, HF/pyridine [53]and Selectfluor [54]. Glycals can be obtained from phenyl thioglycosides by treatment with lithium naphtalenide under conditions compatible with acid4abile groups [ 551. Trichloroacetimidates may be obtained by the hydrolysis of the thioglycoside followed by the subsequent transformation into the imidate using trichloroacetonitrile and a base [56). Hydrolysis have been performed using various promoters (e.g. AgNO:3 [57], DMTSB [58],NOBF4 [59], and DMTST (dimethyl(methylthi0)sulfonium triflate) [43])in the presence of water. Sulfoxides are formed by oxidation using MCPBA or dimethyldioxirane [60].
98
4 Thioglycosides
/
J
-cl S
L
-0Ac o
7
W
N
O HN&I,
Scheme 8.
Anomeric esters have been synthesized through the action of silver [61] or mercuric [62]carboxylates, NOBF4/Ac20 [59] or NIS/RCOOH [63] on a thioglycoside. Since it is possible to activate a thioglycoside in the presence of a pentenyl glycoside [12], it should be possible to transform a thioglycoside into a pentenyl glycoside, but to my knowledge this sequence has not been explored. This convertibility is still an attractive feature of thioglycosides and the two-step glycosylation procedure is continuously being used in glycosylations and is a good complement to the direct one-step activation of thioglycosides. It is an important asset, since in oligosaccharide synthesis it is difficult to foresee which donor that will function best in which coupling, and, furthermore, since it allows thioglycosides as the sole source of donors, which simplifies the synthesis and diminishes the number of precursors that has to be made. Of the possible donors it is mainly the halides that have been employed. As mentioned, this was the original way to use thioglycosides in glycosylation reactions and already in the early publications on the conversion to halides, alkyl 0-glycosides was prepared by activation of the formed halide by a halide promoter, Ag2C03 [49] or Hg(CN)2 [64], in the presence of an alcohol (Scheme 9) both in situ and after purification of the halide. Both procedures have been used also with glycosyl acceptors. The in situ procedure [65-671 is advantageous if the isolation of the glycosyl halide is a problem. However, with the mild conditions used, this is normally not a problem and the halide is most often crudely purified before the glycosylation reaction, which, at least in our experience, generally gives a cleaner coupling reaction with less side-products. The in situ method, of course, also disqualifies thioglycoside acceptors in orthogonal couplings to build thioglycoside blocks (Section 4.4.1).
1) clp, CHpCIp
Ac N
SEt
benzene 69%
Scheme 9.
~
2) EtOH,Hg(CN)P
OEt
4.3 Glycosylutions with Thioglycoside Donors
99
BnO
SMe
BnO
OBn
OBn
AgOTf 63%
BnO
OBn
Scheme 10.
These transformations are possible to perform on complex oligosaccharides and a number of large, biologically important structures have been synthesized with this approach both using bromides 1501 (Scheme 10) and fluorides [68]. Treatment of perbenzylated thioglycosides with iodine in the presence of an acceptor gave good yields of 0-glycosides, but there is no discussion of glycosyl iodides as intermediates in the reactions [69]. No additional promoter was necessary, and acetylated thioglycosides were not affected.
4.3.2 Direct .4ctivation of Thioglycoside Donors Heavy Metal Salt Promotors The first efforts to use thioglycosides directly as donors were performed mainly with mercury(I1) salts as promoters. It was early recognized that mercury had a high affinity for the sulfur functionality [4], and effective hydrolysis of 1-thiofuranosides using mercuric chloride as catalyst instead of a mineral acid was reported [70]. An early attempt to methanolyze ethyl tetra-0-acetyl-1 -thio-P-D-mannopyranoside using HgC12 was nct successful [71], but when Ferrier et al. tried alcoholysis on unprotected phenyl 1-thio-D-glucopyranosides using Hg(0Ac)z or HgClz/HgO as promoters good yields of the corresponding alkyl glycosides with inverted anomeric configuration was obtained [8, 721. Using the perbenzylated phenyl 1-thio-p-Dglucopyranoside as donor and HgS04 as promoter the first disaccharide was produced (Scheme 11) 181. Other similar promoters were then tried, i.a. HgC12 1731, Hg(0Bz)z [74] and PhHgOTf [75],but especially with less reactive donors, e.g. peracylated, the yields were low and it was difficult to find a general high-yielding glycosylation method.
100
4 Thioglycosides
HgS04 THF, reflux, 3h 54%
Scheme 11.
However, if a heterocyclic aryl thioglycoside was used as donor a more efficient activation with metal salts was accomplished. HgClz-promoted methanolysis of phenyl thioglycosides had been found to be faster than for the corresponding ethyl thioglycoside (compare Section 4.4.1), and with the heteroatom in the aryl system an even faster metal-ion promoted reaction took place, indicating that the heteroatom facilitated the activation of the thioglycoside (remote activation concept [76]). Mainly 2-pyridyl thioglycosides have been employed (Scheme 12), but also e.g. 2-pyrimidinyl [771 and benzothiazole thioglycosides have been used, and also different metal salt promoters, e.g. Hg(N03)Z [78], CuOTf2 [79], Pb(C104)2 [80, 811, or AgOTf [80, 82, 831. More recently, Mereyela et al. in a number of papers introduced methyl iodide as a promoter for 2-pyridyl thioglycosides [ 84-87]. Halonium, sulfonium and carbonium type promoters Owing to the many advantages of thioglycoside donors and the promising but so far limited success with heavy metal salt promoters a quest for more generally effective promoters was launched. Starting with NBS in 1983 [88] and especially with methyl triflate (MeOTf) in 1984/5 [89-911 and DMTST in 1986 [92] a seemingly never ending spectrum of new promoters has been reported (Table l), most examples variation of a common theme, soft electrophile reagents (halonium or sulfonium) with non-nucleophilic counter ions (triflate or perchlorate), but also electrochemical and radical activation have been described (Section 4.3.2). In spite of the array of different promoters and thioglycoside donors developed and reported, only a few have so far been proven generally useful in more extensive oligosaccharide synthesis. Among donors the methyl, ethyl and phenyl thioglycosides are by far the most utilized, and MeOTf, IDCP(T), DMTST and NIS alone or
OBn Ac0-q
Me0
scheme 12.
N
+ HO& Me0
Pb(C'04)2 THF,8h 59%, C@J 3:l
MeO
4.3 Glycosylations with Thioglycoside Donors
101
Table 1. Thiophilic promotors. Promoter
References
MeOTf DMTST/DMTSB NBS N BS/TfOH NBS/BQNOTf NBS/LiC104 NIS, NIS/TfOH or TMSOTf NIS/AgOTf or I3t3SiOTf IDCP IDCT Methylsulfenium triflate/bromide (MeSOTf/Br) 12
IBr Iz/AgOTf PhSOTf PhSeOTf NOBF4 N-PhSePhthOTf S02C12/TfOH PhIO-Tf20
in combination with triflic acid or silver triflate cover more or less completely the spectrum of more frequently used promoters. Numerous highly complex structures have been synthesized using these combinations verifying the impressive utility of the method. A few examples, exhibiting the synthesis of structures from N-linked glycans [91], the Salmonella LPS-core [ 1lo], and gangliosides [ 1111, are shown in Schemes 13-15. The methodology are compatible with almost all types of donors, and thioglycoside donors of e.g. furanoses, heptoses, Kdo, neuraminic acid, deoxy sugars, and uronic acids have been prepared and used successfully in glycosylation reactions. These promoters also complement each other nicely regarding reactivity, from the least reactive IDCP and MeOTf through the intermediate DMTST up to the most reactive NIS/TfOH. This promoter reactivity difference in combination with the different reactivity of donors and acceptors allows many practical orthogonal couplings between thioglycosides (Section 4.4.1). Notwithstanding the efficiency of these promoters and thioglycosides as donors, problems can, of course, be encountered in various applications. The problems are connected both to the type of thioglycoside used as well as to the promoter employed. Regarding the promoter, MeOTf can, if the acceptor is unreactive, give methylation instead of glycosylation of the acceptor hydroxyl group [ 1121, although this alkylation is normally surprisingly slow in the absence of base. The use of DMTST together with N-acetamido sugars might lead to thiomethylation of the amide group [113]. NIS, due to the
102
4 Thioglycosides
MeOTf
1
W OH L n OBn
Scheme 13.
nucleophilicity of the counter ion (as compared to triflate in most other promoters), can, once more in the case of unreactive acceptors, give rise to N-succinimide glycosides of the donor as the major product (Scheme 16) [39, 114-1 161. The obvious considerations that soft nucleophilic centres, like e.g. double bonds, should not be allowed in the donor/acceptor, since these would compete with the sulfur in its reaction with the electrophilic promoter, is not as adamant as perhaps expected. Accordingly, glycosidation with thioglycoside donors and allyl and pentenyl groups both in the donor and the acceptor have been performed using controlled conditions (Scheme 17) [ 117-1 191. Problems have been encountered when the acceptor hydroxyl group is adjacent to an allyl protecting group, which have resulted in internal addition to an activated double bond being the major reaction (Scheme 18) [97, 1131. Common problems in oligosaccharide synthesis not directly related to the method of direct activation of thioglycoside donors, such as for example transacylations or formation of orthoesters, when using 2-0-participating groups, or glycal or 1,6-anhydro derivative formation of the donor can, of course, also be encountered [90, 1lo]. As is the case with most glycosylation reactions, i.a. because of their complexity, no truly thorough mechanistic investigation has been performed on the direct activation of thioglycosides. An obvious general mechanism (Scheme I9), very similar to the one suggested for other types of glycosylation reactions, in which the soft
4.3 Glycosylutions with Thioglycoside Donors
OSP
BnO
BnO
BnO
OAc SM.3
BzO BzO
AcO
EtNBr, 90%
HO
OAc
OH
0
Scheme 14.
103
104
4 Thioglycosides
BzO
BzO?'$ko&sph BzO
:
r
s
E
e
OBn OBn
OB+oBn
'f
1) NlSRfOH 86%
NPhth
OBn
2) NaOMe
OBn
OBn
OBn
SMe
OAc
') DMTST 2) Ac20, pyridine, DMAP 3) CAN 55%
OBz
AcO
OBn
NlSKfOH 60%
OBn
OBn
OBn OBn
Scheme 15.
SE = CH2CH2SiMe3
OBn
4.3 Glycosylutions with Thioglycoside Donors
105
BnO
BnO O(CH2)&02Me NHAc
HO
Scheme 16.
DMTST
EQO OSE
92% dp 2.3/1 or NIS/AgOTf (75%)
N3
OBn
Scheme 17.
DMTST
OBn
SEt
SEt
Scheme 18.
X-
A
+HX
Scheme 19.
electrophile E+ (e.g. I+ (NIS, NBS, IDCT(P), Mef (MeOTf) or MeS+ (DMTST(B), MeSOTf(Br)) attaches to the sulfur functionality, whereafter the anomeric carbonsulfur bond is broken to generate an oxycarbenium ion (with some counterion X-, e.g. TfO- or C104), which then is attacked by the acceptor nucleophile, sometimes preceded by the formation of a cyclic acyloxonium ion if 2-0-participating groups are used, to give the 0-glycoside, is more or less commonly accepted, but the intrinsic details, like the exact nature of the intermediates or rate-determining step, are not known, and are also probably very different for different combinations of acceptor/donor/promoter and reaction conditions. No kinetic studies have been performed. Some studies of the anomerisation of alkyl p-D-thioglycosides with IDCP have been carried out showing this to be an intermolecular process and bulky
106
4 Thioglycosides
SEt
DMTST Et20 76% a
OMe
OMe
OBz
63% p
Scheme 20.
alkyl groups to be inert (however possible to use as donors) [17, 1201, and Crich and Sun have identified a stable anomeric triflate as intermediate when activating thioglycosides with PhSOTf at -78 “C [loll, but mostly mechanistic suggestion are based on product structure and distribution in combination with the understanding about mechanisms in similar reactions. Since the major outlines of the mechanism is the same, with common intermediates, as for most other types of glycosylations, much of the knowledge about the stereochemical outcome of glycosylation reactions with other type of donors is valid also for thioglycoside donors. Consequently, the use of 2-0-participating groups will lead to 1,2-trans-glycosides, if there is not a mismatch between donor and acceptor [ 1211. Furthermore, with non-participating groups, the use of diethyl ether as solvent will improve the alp-ratio [ 1221, whereas the use of acetonitrile will give predominantly P-glycosides, especially with primary acceptors [ 1231. Boons et al. advocate the use of toluene/dioxane as an even better a-directing solvent mixture [124].The stereochemical outcome is also dependent on the promoter used and on the protecting group pattern (even apart from the 2-0-protecting group). To generalize, IDCP and DMTST are good a-directing promoters whereas MeOTf and NIS are not (Scheme 20 [ 1251) [ 124, 1261. Bulky groups at 0-6 of glucopyranoside donors [ 1271 and electron-donating ester groups at 0-4 of galactopyranosides [ 1281 have been shown to increase the a//p-ratio in the disaccharide products due to steric and participating effects, respectively. As so often in glycosylation reactions these findings are only guidelines, and an optimisation is normally necessary for each specific reaction. Single-Electron Activation
Thioglycosides can also be activated by a single-electron transfer mechanism (Scheme 21). In these reactions a radical cation is generated by the transfer of a single electron from the sulfur to a suitable electron acceptor. Subsequent heterolytic cleavage of the anomeric linkage yields a sulfur radical, which dimerize to a disulfide, and an oxocarbenium ion, which, as in “normal” glycosylations, can undergo a nucleophilic attack by an alcohol to generate an 0-glycoside. The single electron transfer can be accomplished either photochemically [ 1291, electrochemically [130, 1311, or chemically [132]. Aryl thioglycosides in CH3CN/MeOH (9: 1) were irradiated in the presence of
107
4.3 Glycosylutions with Thioglycoside Donors single-
+I12 RSSR Scheme 21.
1,4-dicyanonaphtalene to give methyl glycosides [ 1291. However, the choice of protecting groups was a problem. Benzyl protected thioglycosides gave mainly other products, whereas acetylated derivatives were not activated. Hence, only permethylated compounds were compatible with the reaction conditions to give the desired glycosides. Electrochemical glycosylation was possible using both phenyl (also p-methoxyphenyl [131]) and ethyl thioglycosides [130, 1331. Acetylated donors were found to be less efficient whereas both benzylated and benzoylated donors gave good yields of disaccharides (Scheme 22). Acylated donors gave, as expected, exclusively the 1,2-truns-products, and also the benzylated donors gave predominantly the pproducts, probably due to the use of CH3CN as solvent in the reactions. Chemically, single electron transfer glycosylations have been effected by the use of the commercially available electron transfer reagent tris(4-bromo-pheny1)ammoniyl hexachloroantimonate (BAHA) as promoter. Once more, both ethyland phenyl thioglycosides, benzylated and benzoylated, could be utilized as donors to afford disaccharides in high yields (Scheme 23) [ 1321. The efficient synthesis of a trisaccharide have been reported [134],but, in spite of the promising results no other oligosaccharide syntheses have been published using this methodology. Glycosylations performed with an electron transfer quencher (1,2,4,5-tetramethoxybenzene) present showed that the reactions are completely quenched in CH2C12 but only partly in CH3CN (28% yield), giving evidence for the single-electron transfer mechanism, but also suggesting alternative mechanistic possibilities in CH3CN [135].
BBnOn
O
a SPh + OBn
Brio%
HO
anodic oxidation BBnO n -e__.___) CH3CN
BnO OMe 73%, a/p 1.3
OBz Bzo*SPh BzO
OBz
+
anodic oxidation -eCH&N
&fo&$,
~
BnoOMe
Scheme 22.
45%
O BnO
G
o
6
BnO BnoOMe
B5:oGo&$, BzO
Brio
BnoOMe
108
4 Thioglycosides
Q BnO& BnO
SPh +
OBn
::o
4
*+
SbCIi
a
Br B r * BBnO n BnO CH3CN OMe 86%, a l p 1 :2
OBz BzO Bzo$&SEt
OBz
O BnO
G 0 BnO
G BnoOMe
OBz
+
:to+
CH&N BAHA BnoOMe
81%
Bi!o&oG BzO
Brio BnoOMe
Scheme 23.
Other Types of Donors With an Anomeric Sulfur
Although not proper thioglycosides, various other derivatives with an anomeric sulfur have been synthesized and used as glycosyl donors. Kotchetkov et al. introduced, as a complement to their cyanoorthoester 1,2-truns glycosylation method, (3-glucopyranosyl thiocyanates as 1,2-cis-glycosyl donors with tritylated acceptors and trityl perchlorate as promoter [136]. With a nonparticipating group at 0-2 the glycosylation is completely stereoselective to give the inverted anomeric configuration in the product (Scheme 24). Also polycondensation using this approach has been reported [137]. Anomeric carbodithioates, both 0-and N-linked have been investigated. The 0ethyl derivative, i.e. the ethyl S-glycosyl xhantate, of 2-azido-galactopyranoseswas found to be an efficient donor activated by either Cu(0Tf)z or DMTST [138, 1391, but especially the same type of derivative of N-acetyl-neuraminic acid has proven to be an excellent alternative as a sialic acid donor, which has found general applications activated by thiophilic promoters as DMTST or MeSOTf (Scheme 25) [ 1401421. The analogous anomeric N-piperidine derivatives could be activated not only by thiophilic promoters (MeOTf and DMTST) but also by AgOTf and various Lewis acids, e.g. SnC14 and FeC13, to allow orthogonal couplings with thioglycosides and the building of thioglycoside donor blocks (Scheme 26) [ 1431.
AcoG,,, + Tf)o&oAc
AcO
OBn
= +
OAc
CHzC12 77%
OAc AcO
Scheme 24.
4.4 Applications of Thioglycosides OAc OAc COOMe - - ...-
-
Acol*%
OEt
'dS\
AcHN AcO
MST CH3CN CH&
~
109
AcHN AcO
Scheme 25.
AAcOc
O
a
s
OAc
K
Ng CH& 79%
'OBn
Scheme 26.
Also anomeric phosphorothioates have been explored. Perbenzylated S-glycosyl phosphorodiamidimidothioates promoted by 2,6-lutidinium p-toluenesulfonate gave good yields of disaccharides and 1,2-cis-selectivity(Scheme 27) [ 1441. If BQNI was used as an additive in the glycosylation even higher 1,2-cis-selectivitywas obtained. S-glycosyl phosphorodithioates as glycosyl donors are discussed in a number of articles, both 2-deoxy derivatives activated by AgF [ 1451 or iodonium promoters [ 1461 to give alp-mixtures of disaccharides, and 2-0-pivaloyl derivatives activated by MeOTf to give 1,2-trans products (Scheme 27) [147]. Although stable under some reaction conditions, none of the donors above exhibit the superior stability of thioglycosides and are therefore not an alternative to
BnO toIuene 85%
'OBn
BnO
BnO& BnO bpiv
Scheme 27.
-
MeOTf
CH2CIz 55%
-
PivO
110
4 Thioglycosides
thioglycosides, e.g. in block synthesis, but rather an addition to the spectrum of less stable glycosyl donors, which might find use in applicable cases.
4.4 Applications of Thioglycosides The stability of thioglycosides in combination with their easy activation makes them highly suitable as donors in most glycosylation strategies and technologies, e.g. in block synthesis as well as in techniques where the donor is manipulated before glycosylation and in solid phase synthesis. 4.4.1 Block Syntheses, Orthogonal Glycosylations The concept of orthogonal glycosidations will be discussed in more detail in other chapters in this book, but since the possibility of oligosaccharide block syntheses is one of the major advantages of thioglycosides, a few aspects of orthogonal glycosidations involving thioglycosides will complementarily be discussed also here. Thioglycosides as Acceptors
Most other donors can effectively be used in couplings to thioglycoside acceptors to give thiooligosaccharide blocks, e.g. halide donors using halide-assisted conditions or heavy-metal salt promoters, trichloroacetimidate donors with hard Lewis acid promoters, selenoglycosides with AgOTf activation and sulfoxide donors with TMSOTf or triflic anhydride as promoter. The efficiency in the couplings are dependent on the reactivity of the acceptor and donor and complications can arise especially with activated thioglycoside acceptors, when transglycosylation of the thiol is an often observed side reaction (Scheme 28) [89, 148, 1491. Taking the knowledge of the protecting group influence on the reactivity into account, it is normally possible to design an efficient way to thiooligosaccharide blocks, however, the protection group manipulations might be tedious and surprises might be encountered (Scheme 29) [ 1171.
Hg(CN)z or HgBr2 or . . O M B r OAc
Scheme 28.
+
A c O W S E t toluene
OAc 6O-8OYo
4.4 Applications of Thioglycosides
I
tES,&&ThTBDMSO P
11 1
bBZ
c AgOTf --
BnO
SEt
Br
OBz
Scheme 29.
Thioglycosides as Both Donors and Acceptors
The protecting group pattern is known to have a large influence on the reactivity of the donor (and of the acceptor). This has been recognized for a long time, but only been possible to utilize effectively in synthetic schemes with the event of stable donors, which has allowed the formation of donors with almost any type of protecting groups and pattern. This reactivity difference can be utilized to permit orthogonal glycosylations between thioglycoside donors (reactive, alkyl protecting groups) and acceptors (inreactive, acyl protecting groups) exploiting less reactive promoters, which only react with activated thioglycosides (Scheme 30) [97]. The idea of using the reactivity difference more systematically was developed by Fraser-Reid et al. in orthogonal couplings between armed and disarmed pentenyl glycosides [ 1501. This concept has been further developed by quantitative determination of the reactivity of donors by the use of competitive glycosylations [ 1511. Recently, fifty thioglycosides with different protecting group patterns have been synthesized and their reactivity has been determined. Through this approach it has been possible lo assign a reactivity constant to each thioglycoside, which allows the planning of synthetic schemes involving orthogonal glycosylations between thioglycosides and one-pot synthesis of oligosaccharides and also a possible computation of the selection process [ 1521. However, with thioglycosides there is an additional possibility to change the reactivity, namely through changing the alkyl (aryl) group of the anomeric thiol. This anomeric activation/deactivation prospect might be compared to the reactivity dif-
B"O& BnO
SEt
OBn
Scheme 30.
&-
+BB'po
IDCP
OBz
SEt (CICH2)2/Et20 84%
BnO SEt OBz
1 12
4 Thioglycosides OBn
SEt + H O
BnO OBn
OBn
72%
Scheme 31.
ference between halide sugars (iodo > bromo > chloro > fluoro) and the inactivation of pentenyl glycosides by bromination of the double bond, but the spectra allowed with thioglycosides are broader. Both steric effects and electronic effects can be utilized to influence the reactivity. Hence, thioglycosides of sterically crowded mercaptans, as e.g. 1,l-dicyclohexylidenemethyl mercaptan, show lower affinity for thiophilic promoters and can be used as acceptors in condensations with ethyl thioglycosides as acceptors [ 201. The higher stability of phenyl thioglycosides as compared to ethyl thioglycosides with IDCP as promoter has been shown [ 1191, and this reactivity difference have been further proved by glycosylations with p-tolyl thioglycoside donors and thioethyl acceptors using NIS/TfOH as promoter (Scheme 31) [153]. By introducing various substituents into the aromatic ring of phenyl thioglycosides a variety of reactivity can be obtained. On the extreme of the unreactivity side is the p-nitrophenyl thioglycoside, where activation does not have to be feared even with efficient promoters. On the contrary, sometimes deactivation of the acceptor hydroxyl group might be a problem and for continued use as donor the p-nitrophenyl group has to be activated as the acetamido analogue. This sequence has been investigated [ 154, 1551, but problems often arise and the scope of the methods appears to be limited, however effective with sialyl derivatives [ 1561. p-Halogenphenyl thioglycosides might be interesting compounds of intermediate reactivity. With methyl triflate or DMTST as promoters, acylated derivatives are not activated, whereas the use of NIS/triflic acid smoothly allows the use of these derivatives as glycosyl donors [ 1571. 4.4.2 Intramolecular Glycosidations To obtain desired regio- and stereoselectivity in glycosidations, new glycosylation methodology are continuously developed. Recently two intramolecular variants, internal acceptor delivery and intramolecular glycosidations of spacer prearranged glycosides, were described. In both of these approaches a number of reactions are performed on the acceptor/donor pair before the glycosidation reaction, and therefore thioglycosides as donors have been an obvious choice. In the internal delivery approach the acceptor hydroxyl group is linked to the 2-OH of a thioglycoside donor via a labile acetal linkage. Activation of the thioglycoside also breaks the acetal bond and the acceptor is transferred stereospecifically to the anomeric site of the activated donor to give high yields of
References
1 13
1,Zcis-glycosides, also the otherwise difficult obtainable P-mannopyranosides and P-fructofuranosides [ 158-1621. In the intramolecular glycosidation using prearranged glycosides the donor and acceptor are linked through a spacer via other hydroxyl groups than the ones that will be part of the new glycosidic linkage. By using various attachment sites and various spacers, regio- and stereoselective glycosylations can be performed [ 1631651. 4.4.3 Solid Phase Synthesis Since thioglycosides are possible to synthesize with almost any type of protecting group pattern and since they are shelf-stable, they should be very suitable as donors in a commercial approach to solid-phase synthesis of oligosaccharides. That they are excellent donors also in a solid-phase approach is shown, i.e., by the publications from Nicolaou et al. in which up to dodecasaccharides are synthesized on polystyrene beads using thiophenyl and thiomethyl glycoside donors and DMTST as promoter [1166, 1671. References 1. R. R. Herr, J , Am. Chem. Soc., 1967,89,2444-2447. 2. H. Hoeksema, B. Bannister, R. D. Birkenmeyer, F. Kagan, B. J. Magerlein, F. A. MacKellar, W. Schroecler, G. Slomp, R. R. Herr, J. Am. Chem. Soc., 1964,86, 4223-4224. 3. H. Hoeksema, J. Am. Chem. Soc., 1964,86, 4224-4225. 4. D. Horton, D. H. Hutson, Adv. Carbohydr. Chem., 1963, 18, 123-199. 5. V. Gil, A. J. MacLeod, Phytochern., 1980,19, 2071. 6. E. Fischer, K. Delbriick, Ber., 1909, 42, 1476-1482. 7. A. L. Raymond, Adv. Carbohydr. Chem., 1945, 1, 129-145. 8. R. J. Ferrier, R. W. Hay, N. Vethaviyasar, Carhohydr. Res., 1973, 27, 55-61. 9. P. Fugedi, P. J. Garegg, H. Lonn, T. Norberg, Glycoconj. J., 1987, 4, 97-108. 10. T. Norberg, in S. H. Khan, R. A. O’Neill (Eds.): Modern methods in Carbohydrate Synthesis, Harwood Academic Publishers 1995, p. 82-106. 11. P. J. Gareg,g, Adti. Carbohydr. Chem. Biochem., 1997, 52, 179-205. 12. G. H. Veeneman, in G.-J. Boons (Ed.): Carbohydrate Chemistry, Blackie Academic & Professional 1998, p. 98-174. 13. F. Barresi, 0. Hindsgaul, J. Carbohydr. Chem., 1995, 14, 1043-1087. 14. R. J. Ferrier, R. H. Furneaux, Carbohydr. Res., 1976, 52, 63-68. 15. F. Dasgupta, P. J. Garegg, Acta Chem. Scand., 1989,43,471-475. 16. R. U. Lemieux, Can. J. Chem., 1951,29, 1079-1091. 17. B. Lindberg, B. Erbing, Acta Chem. Scand., 1976, B30, 611-612. 18. W. V. Dahlhoff, Liebigs Ann. Chem., 1990, 1025-1027. 19. S. A. Galema, J. B. F. N. Engberts, H. A. v. Doren, Carbohydr. Res., 1997, 303, 423-434. 20. G. J. Boons, R. Guertsen, D. Holmes, Tetrahedron Lett., 1995, 31, 6325-6328. 21. P. J. Garegg, L. Olsson, S. Oscarson, J. Org. Chem., 1995, 60, 2200-2204. 22. T. Nakano, Y. Ito, T. Ogawa, Tetrahedron Lett., 1990, 31, 1597-1600. 23. T. Ogawa, M. Matsui, Carbohydr. Res., 1977, 54, C17-C21. 24. V. Pozsgay, H. Jennings, Tetrahedron Lett., 1987,28, 1375-1376. 25. H. Driguez, Topics Curr. Chem., 1997, 187, 85-1 16. 26. M. Appani, M. Blanc-Muesser, J. Defaye, H. Driguez, Can. J. Chem., 1981, 59, 314-320.
114 27. 28. 29. 30.
4 Thioglycosides
M. Blanc-Muesser, J. Defaye, H. Driguez, Carbohydr. Res., 1978, 67, 305-328. J. Bogusiak, W. Szeja, Pol. J. Chem., 1985, 59, 293-298. F. D. Tropper, F. 0. andersson, C. Grand-Maftre, R. Roy, Synthesis, 1991, 734-736. S. J. Meunier, F. 0. andersson, M. Letellier, R. Roy, Tetrahedron: Assymetry, 1994, 5, 23032312. 31. M. Gerz, H. Matter, H. Kessler, Angew. Chem. Znt. Ed. Engl., 1993, 32, 269-271. 32. D. Horton, Methods Carbohydr. Chem., 1963,2,433-437. 33. F. I. Auzanneau, D. R. Bundle., Carbohydr. Res., 1991, 212, 13-24. 34. V. Pozsgay, Carbohydr. Res., 1992,235, 295-302. 35. K. C. Nicolaou, T. Ladduwahetty, J. L. Randall, A. Chuchulowski, . I Am. Chem. Soc., 1986, 108. 2466-2467. 36. H. M. Zuurmond, P. A. M. v. d. Klein, G. A. v. d. Marel, J. H. v. Boom, Tetrahedron Lett., 1992,33, 2063-2066. 37. M. Nilsson, T. Norberg, J. Chem. Soc. Perkin Trans. I , 1998, 1699-1702. 38. T. Buskas, P. J. Garegg, P. Konradsson, J.-L. Maloisel, Tetrahedron: Asymmetry, 1994, 5, 2187-2194. 39. S. Oscarson, U. Tedebark, D. Turek, Carbohydr. Res., 1997, 159, 159-164. 40. P. J. Garegg, L. Olsson, S. Oscarson, J. Org. Chem., 1995, 60, 2200-2204. 41. G. J. P. H. Boons, G. A. v. d. Marel, J. H. v. Boom, Tetrahedron Lett., 1989, 30, 229-232. 42. G. J. P. H. Boons, Doctoral thesis, Leiden University 1991. 43. C. Bernlind, S. Oscarson, J. Org. Chem., 1998,63, 7780-7788. 44. W. A. Bonner, J. Am. Chem. Soc., 1948, 70,770-772. 45. F. Weygand, H. Ziemann, H. J. Bestmann, Chem. Ber., 1958,91,2534-2537. 46. R. Kartha, R. Field, Tetrahedron Lett., 1997, 38, 8233-8236. 47. M. L. Wolfrom, W. Groebke, J. Org. Chem., 1963,28, 2986-2988. 48. M. L. Wolfrom, P. McWain, R. Pagnucco, A. Thompson, J. Org. Chem., 1964,29, 454-457. 49. F. Weygand, H. Ziemann, Liebigs Ann. Chem., 1962, 657, 179-198. 50. K. Ekelof, S. Oscarson, J. Org. Chem., 1996, 61, 7711-7718. 51. P. J. Card, J. Carbohydr. Chem., 1985, 4 , 451-487. 52. L. Blomberg, T. Norberg, J. Carbohydr. Chem., 1992, 11, 751-760. 53. K. C. Nicolau, R. E. Dolle, D. P. Papahatjis, J. L. Randall, J. Am. Chem. Soc., 1984, 106, 4189-4192. 54. M. D. Burkart, Z. Zhang, S.-C. Hung, C.-H. Wong, J. Am. Chem. Soc., 1997, 119, 1174311746. 55. A. Fernandez-Mayoralas, A. Marra, M. Trumtel, A. Veyriirres, P. Sinay, Carbohydr. Res., 1989,188,81-95. 56. R. R. Schmidt, W. Kinzy, Ado. Carbohydr. Chem. Biochem., 1994,50, 21-117. 57. P. J. Garegg, H. Hultberg, C. Lindberg, Carbohydr. Res., 1980, 83, 157-162. 58. S. Nilsson, H. Lonn, T. Norberg, Glycoconjugate J., 1991, 8, 9-16. 59. V. Pozsgay, H. Jennings, J. Carbohydr. Chem., 1990, 9, 333-343. 60. D. Kahne, S. Walker, Y. Chang, D. v. Engen, J. Am. Chem. Soc., 1989,111,6881-6882. 61. C . Pederson, H. G. Fletcher, J. Am. Chem. Soc., 1960, 82, 3215-3217. 62. R. J. Ferrier, S. R. Haines, Carbohydr. Res., 1984, 127, 157-161. 63. G. H. Veeneman, S. H. v. Leeuwen, J. H. v. Boom, Tetrahedron Lett., 1990,31, 1331-1334. 64. M. L. Wolfrom, H. G. Garg, D. Horton, J. Org. Chem., 1963,28, 2989-2992. 65. J. 0. Kihlberg, D. A. Leigh, D. R. Bundle, J. Org. Chem., 1990, 55, 2860-2863. 66. S. Sato, M. Mori, Y.lto, T.Ogawa, Carbohydr. Res., 1986, 155, C6-C10. 67. F. 0.Andersson, P. Fugedi, P. J. Garegg, M. Nashed, Tetrahedron Lett., 1986,27, 3919-3922. 68. K. C. Nicolau, H. Ueno, in S. Hanessian (Ed.): Preparative Carbohydrate Chemistry, Marcel Dekker, Inc. 1997, p. 314-338. 69. R. Kartha, M. Aloui, R. Field, Tetrahedron Lett., 1996, 37, 5175-5178. 70. M. L. Wolfrom, K. Anno, J. Am. Chem. SOC.,1953, 75, 1038-1039. 71. J. Fried, D. E. Waltz, J. Am. Chem. SOC.,1949, 71, 140-143. 72. L. Hough, M. I. Taha, J. Chem. Soc., 1956,2042-2048. 73. T. Y. R. Tsai, H. Jin, K. Wiesner, Can. J. Chem., 1984,62, 1403-1405. 74. J. W. v. Cleve, Carbohydr. Res., 1979, 70, 161-164.
References
1 15
75. P. J. Garegg, C. Henrichson, T. Norberg, Curbohydr. Res., 1983, 116, 162-165. 76. S. Hanessian, in S. Hanessian (Ed.): Preparative Carbohydrate Chemistry, Marcel Dekker, Inc. 1997, p. 381-387. 77. Q. Chen, F. Kong, Carbohydr. Rex, 1995,272, 149-157. 78. S. Hanessian, C. Bacquet, N. Lehong, Carbohydr. Rex, 1980, 80, C17-C22. 79. T. Mukaiyama, T. Nakatsuki, S. Shoda, Chem. Lett., 1979, 487-490. 80. R. B. Woodward, e. al., J. Am.Chem. Soc., 1981, 103, 3215-3217. 81. P. G. M. U'uts, S. S. Bigelow, J. Org. Chem., 1983, 48, 3489-3493. 82. K. Tsuboyama, K. Takeda, K. Torii, M. Ebihara, J. Shimizu, A. Suzuki, N. Sato, H. Furuhata, Chem. Pharm. Bull., 1990, 38, 636-638. 83. Z. Zhiyuan, G. Magnusson, Carbohydr. res., 1994,262, 79-101. 84. G. V. Reddy, V. R. Kulkarni, H. B. Mereyala, Tetrahedron Lett., 1989, 30, 4283-4286. 85. H. B. Mereyala, G. V. Reddy, Tetrahedron, 1991, 47, 6435-6448. 86. H. B. Mereyala, V. R. Kulkarni, D. Ravi, G. V. M. Sharma, B. V. Rao, G. V. Reddy, Tetrahedron, 1992, 48, 545-562. 87. H. B. Mereyela, V. R. Gurijala, Curbohydr. Rex, 1993,242, 277-280. 88. K. C. Nicolaou, S. P. Seitz, D. P. Papahatjis, J. Am. Chem. Soc., 1983, 105, 2430-2434. 89. H. Lonn, Chem. Commun. Stockholm Univ., 1984, No2, 1-30. 90. H. Lonn, Carbohydr. Res., 1985, 139, 105-113. 91. H. Lonn, C'arbohydr. Rex, 1985, 139, 115-121. 92. P. Fugedi, P. J. Garegg, Curbohydr. Res., 1986, 149, C9-Cl2. 93. M. Sasaki, K. Tachibana, H. Nakanishi, Tetrahedron Lett., 1991, 32, 6873. 94. K. Fukase, A. Hasuoka, S. Kusumoto, Tetrahedron Lett., 1993, 34, 2187-2190. 95. K. Fukase, A. Hasuoka, Kinoshita, S. Kusumoto, Tetrahedron, 1995, 51, 4923-4932. 96. P. Konradsson, U. E. Udodong, B. Fraser-Reid, Tetrahedron Lett., 1990, 31, 4313-4316. 97. G. H. Veeneman, J. H. v. Boom, Tetruhedron Lett., 1990,31, 275-278. 98. G. H. Veeneman, S. H. v. Leeuwen, Z. H, J. H. v. Boom, J. Curbohydr. Chem., 1990,9, 783796. 99. F. Dasgupta, P. J. Garegg, Curbohydr. Rex, 1988, 177, C13-Cl7. 100. F. Dasgupta, P. J. Garegg, Curbohydr. Rex, 1990,202, 225-238. 101. D. Crich, S. Sun, J. Am. Chem. SOC.,1998,120,435-436. 102. D. Crich, S. Sun, Tetrahedron, 1998, 54, 8321-8348. 103. Y. Ito, T. Ogawa, Tetrahedron Lett., 1988, 29, 1061-1064. 104. Y. Ito, T. Ogawa, M. Numata, M. Sugimoto, Carbohydr. Rex, 1994,202, 165-175. 105. V. Pozsgay, H. J. Jennings, J. Org. Chem., 1987, 52, 4635-4637. 106. H. Shimizu, Y. Ito, T. Ogawa, SynLett, 1994, 535-536. 107. H. Shimizu, SynLett., 1998, 84-86. 108. H. Lonn, Glycoconjugute J., 1987, 4, 117-118. 109. K. Fukase, A. Hasuoka, I. Kinoshita, S. Kusumoto, Tetrahedron Lett., 1992, 33, 7165-7168. 110. S. Oscarson, H. Ritztn, Carbohydr. Rex, 1994,254, 81-90. 111. A. Hasegawa, M. Kiso, in S. Hanessian (Ed.): Preparative Carbohydrate Chemistry, Marcel Dekker, Inc. 1997, p. 357-379. 112. P. J. Garegg, S. Oscarson, M. Szonyi, Carbohydr. Res., 1990,205, 125-132. 113. A.-K. Tiden, Doctoral thesis, Stockholm University 1991. 114. L.-X. Wang, N. Sakiri, H. Kezukura, J. Chem. SOC.Perkin I , 1990, 1677-1682. 115. C. Krog-Jensen, S. Oscarson, J. Org. Chem., 1996, 61, 1234-1238. 116. M. Anderson, P. Konradsson, S. Oscarson, L. Oberg, J. Carbohydr. Chem., in press. 117. M. Alpe, S. Oscarson, unpublished results. 118. S. K. Das, R. Ghosh, N. Roy, J. Carbohydr. Chem., 1993, 12, 693-701. 119. H. M. Zuurmond, S. C. v. d. Laan, G. A. v. Marel, J. H. v. Boom, Carbohydr. Res., 1991, 215, clLc3. 120. G.-J. Boons, T. Stauch, Synlett, 1996, 906-908. 121. N. M. Spijker, C. A. A. van Boeckel, Angew. Chem. Znt. Ed. Engl., 1994, 30, 180-182. 122. H. Lonn, J. Curbohydr. Chem., 1987, 6, 301--306. 123. I. Braccini, C. Derouet, J. Esnault, C. H. d. Penhoat, J. M. Mallet, V. Michon, P. Sinay, Curbohydr. Rex, 1993,246, 23-41.
1 16
4 Thioglycosides
124. A. Demchenko, T. Stauch, G.-J. Boons, Synlett, 1997, 818-820. 125. P. Svahnberg, Licenciate Thesis, Stockholm University 1999. 126. P. J. Garegg, S. Oscarson, H. Ritztn, M. Szonyi, Carbohydr. Rex, 1992, 228, 121-128. 127. S. Houdier, P. J. A. Vottero, Carbohydr. Res., 1992,232, 349-352. 128. A. V. Demchenko, E. Rousson, G.-J. Boons, Tetrahedron Lett., 1999, 40, 6523-6526. 129. G. W. Griffin, N. C. Bandara, M. A. Clarke, W . 3 . Tsang, P. J. Garegg, S. Oscarson, B. A. Silwanis, Heterocycles, 1990,30, 939-947. 130. C. Amatore, A. Jutand, J.-M. Mallet, G. Meyer, P. Sinay, J. Chem. SOC.Chem. Commun., 1990, 718-719. 131. G. Balavoine, A. Greg, J.-C. Fischer, A. Lubineau, Tetrahedron Lett., 1990,31, 5761-5764. 132. A. Marra, J.-M. Amatore, C. Amatore, P. Sinay, SynLett., 1990, 572-574. 133. J.-M. Mallet, G. Meyer, F. Yvelin, A. Jutand, C. Amatore, P. Sinay, Carbohydr. Rex, 1993, 244,237-246, 134. Y.-M. Zhang, J.-M. Mallet, P. Sinay, Curbohydr. Res., 1992, 236, 73-88. 135. S. Mehta, B. M. Pinto, Carbohydr. Rex, 1998, 310, 43-51. 136. N. K. Kochetkov, E. Klimov, N. N. Malysheva, Tetrahedron Lett., 1989,30, 5459-5462. 137. N. K. Kochetkov, N. N. Malysheva, E. Klimov, A. V. Demchenko, Tetrahedron Lett., 1992, 33, 381-384. 138. A. Marra, L. K. S. Shun, F. Gauffeny, P. Sinay, Synlett, 1990, 445-448. 139. A. Marra, F. Gauffeny, P. Sinay, Tetrahedron, 1991, 47, 5149-5160. 140. A. Marra, P. Sinay, Carbohydr. Res., 1990, 195, 303-308. 141. H. Lonn, K. Stenvall, Tetrahedron Lett., 1992, 33, 115-116. 142. U. Ellervik, G. Magnusson, J. Orq Chem., 1998, 63, 9314-9322, 9223. 143. P. Fugedi, P. J. Garegg, S. Oscarson, G. Rosen, B. A. Silwanis, Carbohydr. Res., 1991, 211, 157-1 62. 144. S. Hashimoto, T. Honda, S. Ikegami, Tetrahedron Lett., 1990,31, 4769-4772. 145. H. Bielawska, M. Michalska, J. Curbohydr. Chem., 1991, 10, 107-1 12. 146. L. Laupichler, H. Sajus, J. Thiem, Synthesis, 1992, 1133-1136. 147. 0.J. Plante, P. H. Seeberger, J. Org. Chem., 1998,63, 9150-9151. 148. D. A. Leigh, J. P. Smart, A. M. Truscello, Carbohydr. Res., 1995, 276, 417-424. 149. F. Belot, J.-C. Jaquinet, Carbohydr. Res. 290, 1996, 290, 79-86. 150. D. R. Mootoo, P. Konradsson, U.Udodong, B. Fraser-Reid, J. Am. Chem. Soc., 1988, 110, 5583-5584. 151. N. L. Douglas, S. V. Ley, U. Lucking, S. L. Warriner, J. Chem. SOC.Perkin Trans. I , 1998, 51-65. 152. Z. Zhang, I. R. Ollman, X.-S. Ye, R. Wischnat, T. Baasov, C.-H. Wong, J. Am. Chem. SOC., 1999.121. 734-753. 153. A. K. Choudhury, I. Mukherjee, B. Mukhopadhyay, N. Roy, J. Carbohydr. Chem., 1999,18, 361-367. 154. S. Cao, F. Hernandez-Mateo, R. Roy, J. Carbohydr. Chem., 1998, 17, 609-631. 155. L. A. J. M. Sliedregt, K. Zegelaarjaarsveld, G. A. van den Marel, J. H. van Boom, Synlett, 1993, 335-337. 156. R. Roy, F. 0. Andersson, M. Letellier, Tetrahedron Lett., 1992, 33, 6053-6056. 157. M. Lahmann, S. Oscarson, unpublished results. 158. F. Baressi, 0. Hindsgaul, J. Am. Chem. SOC.,1991, 113, 9376-9377. 159. M. Bols, J. Chem. SOC.Chem. Commun., 1992,913-914. 160. F. Baressi, 0. Hindsgaul, Can. J. Chem., 1994, 72, 1447-1465. 161. Y. Ito, T. Ogawa, Ang. Chem. Int. Ed. Engl., 1994,33, 1765-1767. 162. C. Krog-Jensen, S. Oscarson, J. Org. Chem., 1998, 63, 1780-1784. 163. T. Ziegler, R. Lau, Tetrahedron Lett., 1995, 36, 1417-1420. 164. S. Valverde, A. M. Gomez, J. C. Lopez, B. Herradon, J. Chem. SOC.Chem. Commun., 1995, 2005-2006. 165. M. Miiller, U. Huchel, A. Geyer, R. R. Schmidt, J. Org. Chem., 1999, 64, 6190-6201. 166. K. C. Nicolaou, N. Winssinger, J. Pastor, F. DeRoose, J. Am. Chem. Soc., 1997,119,449-450. 167. K. C. Nicolaou, N. Watanabe, J. Li, J. Pastor, N. Winssinger, Angew. Chem. Int. Ed. Engl., 1998,37, 1559-1561.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
5 Glycosylation Methods: Use of Phosphites Zhiyuan Zhang and Chi-Huey Wong
5.1 Introduction
In the chemical synthesis of oligosaccharides, the overall procedure can be divided into five steps: 1) design and preparation of the building blocks (glycosyl donors and acceptors) with selected protecting groups; 2) manipulation of the protecting groups to free the desired hydroxyl group; 3) separation of products after each step; 4) stereo-controlled glycosylation; and 5 ) deprotection and product purification. Of those steps, 1-3 are the most time-consuming. Considerable efforts have thus been made to facilitate the synthetic process. For example, many methods have been developed to reduce the number of steps in the synthesis of building blocks. Recently developed orthogonal protection-deprotection strategies [ 1, 21 provide the efficiency and flexibility for the preparation of highly diversified carbohydrate libraries [ 1, 31. To minimize protecting group manipulation, the armed-disarmed strategy [4], the orthogonal glycosylation strategy [ 5 ] , and the one-pot multiglycosylation strategy [6-81 have been developed. In terms of simplifying the purification procedure, solid-phase synthesis [91 and one-pot solution-phase synthesis [7d, 81 have shown great promise. Glycosylation is the central reaction of carbohydrate chemistry. Because low yield, low regioselectivity, and low stereoselectivity are frequently encountered, it is this step which requires special attention. Though chemists might be able to synthesize any oligosaccharide [lo],there is no universally employed glycosylation [ lOc, 111. Therefore, development of new and efficient methods of glycosylation is still quite challeng,ing.
118
5 Glycosylation Methods: Use of Phosphites
The use of glycosyl phosphite as a glycosyl donor was initiated in 1992. The Wong group [12] and the Schmidt group [13] independently reported that trimethylsilyl triflate (TMSOTf) could activate glycosyl phosphites in glycosylation reactions. An advantage of this method is that it provides a very high yield and aselectivity in the synthesis of sialic acid derivatives [12, 131. This method has been further developed by these two groups and others. Different promoters have been investigated and some unusual stereoselectivities have been reported. The purpose of this chapter is to review recent developments in phosphite glycosylation and its applications.
5.2 Preparation of Glycosyl Phosphites Glycosyl phosphites are prepared from their corresponding anomeric hydroxyl compounds, either by treatment with dialkyl N,N-diethylphosphoramidites (DDP) and 1,2,4-triazole [ 141 or with dialkyl phosphorochloridites in the presence of Hunig's base (Figure 1) [ 13, 151. Most phosphitylation reagents are commercially available, and both methods give glycosyl phosphites in high yield. Examples of glycosyl phosphites that can be prepared by these methods are dibenzyl phosphites, diethyl phosphites, and dimethyl phosphites. Bicyclic glycosyl phosphites such as 4 and 5 can also be prepared. In the synthesis of an N-acetyllactosamine (LacNAc) derivative it has been shown that 4 gives better yields than the corresponding trichloroacetimidate [ 14fl. When the sugar moiety is highly electron-donating, such as the 2,6-di-deoxy derivative, its diethyl phosphite derivative could not be prepared.
Et2N-P-(OBn),
PCH
A . d
I
COOMe
1,2,4-triazole AcO
1
CI-P-(0Et)2 EtNiPr2
4
3 97%
AcO
Figure 1.
COOMe
5.3 Glycosylotion using Glycosyl Phosphites
1 19
The corresponding pinacol phosphite 5 was, however, stable enough to be isolated in 67% yield [16]. Several phosphitylation reagents are now available for the preparation of glycosyl phosphites with desirable reactivity. Factors known to affect the anomeric ratio in the preparation of glycosyl phosphites include the anomeric composition of the starting hydroxyl compound. For example, a mixture of galactosyl phosphite with an a : ratio of 90 : 10 was obtained with an a : ratio of 90: 10. A different from 2,3,4,6-tetra-O-benzylgalactopyranose anomeric mixture ( a :p = 61 :39) of the phosphite product was, however, generated from an anomeric mixture of the galactopyranose ( a :p = 56:44) [17].2,3,4,6-Tetra0-acetylgalactose ( a :p = 3 : 1) and glucose ( a :p = 2: 1) gave the phosphite p isomer as a predominant product ( a :p = 1 :4 and 1 :2 respectively), whereas the 2-acetamido-2-deoxy sugar gave mainly the a-phosphite ( a :p > 7: 1) [18]. Surprisingly, only phosphite p isomers (2 and 3) were observed when sialic acid (NeuSAc) derivatives were phosphitylated using either phosphitylation method [ 12, 131. Solvents also affect the product composition. Phosphitylation of 2,3,4,6-tetra-Oacetylgalactose in CH2C12 gave the dibenzyl phosphite with an a : p ratio of 1 :2; this increased to 1 : 6 when THF was used as solvent [ 181. Effects of Lewis acid [ 191 and mild base [20] on the anomeric ratio of the phosphite have also been reported. In general the anomeric ratio of the phosphite does not have a large effect on the glycosylation step in terms of reactivity and stereoselectivity [ 171. It might, however, be a factor in the synthesis of phosphate derivatives from phosphites (see Section 5.4).
5.3 Glycosylation using Glycosyl Phosphites 5.3.1 Mechanism The first catalyst used to activate the glycosyl phosphite was TMSOTf [ 12, 131. The mechanism of the reaction has been investigated [12]. As shown in Figure 2, path a applies when TMSOTf is mixed with donor first or added into the mixture of donor and acceptor. In this method, the formation of a substantial amount of phosphonate orthoester is a considerable problem. As illustrated in path b, however, a premixed solution of TMSOTf and acceptor spontaneously generates triflic acid (TfOH). TfOH then activates the glycosyl phosphite added subsequently to give the desired product, and reduces the formation of the orthoester. The most facile method is through path c where TfOH directly catalyzes the glycosylation, and also reduces the nucleophilicity of the hydrogen phosphonate. TfOH is actually involved in each of the pathways, and is potentially the real catalyst even when TMSOTf is used [21]. In general, TfOH-catalyzed glycosylation gives the highest yield. Notably, addition of glycosyl phosphite to premixed TMSOTf and acceptor gave a better yield than addition of TMSOTf to the mixture of donor and acceptor [22]. In practice, however, it is more convenient to use TMSOTf than TfOH as catalyst. Interestingly, BF3.Et20-promoted [ 171 and TMSOTf-promoted [ 12, 13, 211 gly-
120
5 Glycosylation Methods: Use of Phosphites
a. TMSOTf reacts with donorfirst
-
TMSO-P(OBn)2
TMSOTf
HOTf
b. TMSOTfreacts with acceptorfirst
TMSOTf
TMSOR HO-P(OBn),
-
R
HP(OBn),
c. When TfOH was used
Figure 2.
cosylations using perbenzylated phosphites at low temperature (-78 "C), as reported by Hashimoto et al., gave similar stereoselectivity and yields. Both activation gave high 1,2-trans-P-selectivityeven without a participating group at the C-2 position of the glycosyl donor [ 171. BF3.EtzO-promoted glycosylations, furthermore, gave even better P-selectivity. The reason for this, and the nature of the intermediate involved in the BF3 .Et20-promoted glycosylation, remain unclear [ 171. More recently, the Hashimoto group developed a new activation method using 2,6-di-tert-butylpyridinium iodide (DTBPI) and tetrabutylammonium iodide (BmNI), and achieved high a-selectivity. The intermediate was found to be the aglycosyl iodide, which is first anomerized to the P isomer then glycosylated through a S~2-likemechanism (Figure 3). In general, a-selectivity is predominant in glycosylations using the phosphite method. Watanabe et al. [22] have investigated other Lewis acids as promoters of glycosylation using glycosyl phosphites; they found that ZnCl2, ZnCl2-AgC104, 12, and other Lewis acids can efficiently activate perbenzylated phosphites at room temperature. The stereoselectivities are generally lower than when reaction is performed out at low temperature. The effect of promoters on the glycosylation of perbenzylated glucosyl phosphite 6 with a primary alcohol was studied (Figure 4).ZnClz-
5.3 Glycosylation using Glycosyl Phosphites
121
60-95% cc/B > 90110
8 donors
Figure 3.
promoted glucosylation gave predominantly the P product, 7. In contrast, 12promoted reactions mainly gave the a product, whereas almost no selectivity was observed with the ZnClz-AgC104-promoted glycosylation [ 17b]. It is unclear why the addition of AgC104 would increase the efficiency and reduce the selectivity of the reaction. The origin of selectivity remains to be investigated [17b]. Metal perchlorate salts (e.g. LiC104), which act as weak Lewis acids, activate the glycosyl phosphite and yield the glycosyl perchlorate ion pair which leads to the product [23] This method provides a useful alternative when acid-labile protecting groups, or acid-sensitive glycosyl linkages are involved. The low yield and low stereoselectivity are potential drawbacks of this method. 5.3.2 Low Temperature-Dependent Stereoselectivity The stereoselectivity of a glycosylation is controlled by many factors, including the substituent at the C-2 position, the leaving group on the anomeric center, temperature, solvents, and the structural features both of glycosyl donor and acceptor. In the construction of a glycosyl P-linkage, a participating group at the C-2 position of the glycosyl donor is normally required. In the absence of this participating group the thermodynamically favorable a-glycosyl bond is generally formed because of the anomeric effect [24]. Thus, the stereo-controlled synthesis of the P-linkage is considered to be difficult. A solution to this problem is to use acetonitrile as solvent to provide the nitrile effect [25] to aid the formation of the P-linkage. In addition, some newly developed glycosylatioii methods using glycosyl donors with a non-participating group at the C-2 position at low temperature gave very high p-selectivities [17, 21, 22b, 26, 271. These methods include the use of sulfoxides activated with Tf20 [27, 281, glycosyl bromides activated with AgOTf [27], thioglycosides activated with RSCl-AgOTf
Lewis acid
Ph(CH&OH
B
%B
Lewis acid
6
+
BB& O(CH.&Ph
OP(O5h 7
Figure 4.
Yield
1,
92% 81/19
ZnCI,
88% 20180
ZnCI2-AgC1O4 92% 52/48
122
5 Glycosylation Methods: Use of Phosphites
I
Temp.
-78OC
-5OOC
8
3:97
8:92
11:89
1:99
2:98
3:97
-3oOc
8
BF3-Et20
Ratio
ROH
9
(b.) Watanabe eta/.
6 I
LNlS/TfOtJ CH2C12
-\
B"4 BnO 7
O(CH&Ph
3°C
-21 "C -45°C -68"C
Figure 5.
[27, 291 and glycosyl phosphites activated with TMSOTf [17, 211 or BF3.Et20 [17, 261. Low temperature seems to promote P-selectivity; the thermodynamically stable a-glycosyl triflate is formed and further reacted in a S~2-likemechanism with the added glycosyl acceptor to give the P-glycoside [27b, c]. Figure 5 shows the temperature-dependent selectivity reported by Hashimoto et al. [17] and Watanabe et al. [22b] Further study showed that P-selectivity was almost independent of solvent [ 171, although slightly dependent on the anomeric composition of the donor [17, 211. This is exemplified by the synthesis of Pgalactosyl-, P-glucosyl-, and P-glucuronyl linkages [ 14e, 16, 17, 21, 26, 301. Fucosyl phosphites behave differently under these conditions, however, and a-selectivity is predominant [21, 30, 311. The stereoselectivity issues of glycosyl phosphites will be discussed later.
5.3.3 Glycosylation of Sialyl Phosphites Sialylated oligosaccharides are often expressed on the surfaces of cells, including cancer cells, and are very attractive targets for investigation. Sialylation [ 321, especially yield and stereoselectivity, is, however, still a major problem in oligosaccharide synthesis. Synthetic difficulties stem from the tertiary-hindered anomeric center and the lack of an electron-demanding group adjacent to the anomeric center of sialylic acid (NeuAc) [ 121. Sialylation using sialyl phosphites is currently one of the most successful synthetic methods, and many sialylated compounds have been prepared by this strategy. Compounds 10-11 and 13 (see Figure 6) were prepared from dibenzyl phosphite 2 [12], and compounds 10, 12 and 13-14 were synthesized from diethyl phosphite 3 [13]. In the synthesis of sialyl Lewisa (sLea), a regioselec-
5.3 Glycosylution using Glycosyl Phosphites A
123
OOMe
AcO 10 80% (a:p6:1) from 2 70% (cx:p4:1) from 3
OMe
Ho
PcHN 11 48% only cx from 2
OpC
Ph
13 44% (a:p6:1)from 2
12 38% cx from 3
Figure 6.
tive coupling of the sialyl moiety to the 3-OH on the Gal unit of partially protected Lea gave the desired compound 14 in low yield (18%). Also isolated was 35% of the lactonized product 15 [33]. This lactone side-product 15 was isolated as the only product (37%) when the same acceptor and a different glycosyl donor and promoter were used [34].
5.3.4 Glycosylation of C-2-Acylated Glycosyl Phosphites In general, the 1,2-tuans P-glycosyl linkage is formed by use of a C-2 participating protecting group. Catalytic amounts of TMSOTf (0.5 equiv.) can promote the glycosylation of peracetylated glycosyl phosphites. Donors utilized include fucosyl phosphites, galactosyl phosphites, glucosyl phosphites, glucuronyl phosphites, and glucosaminyl phosphites. Reaction of these donors with different glycosyl acceptors yield the desired P-product with yields of 40-70% [21, 351. T h s method has also been used for the synthesis of some P-C-glycosides and P-S-glycosides [21, 301. A few side-reactions are, however, encountered when using this method; examples include the formation of glycosyl phosphonate orthoesters [21, 30, 361, which are less reactive intermediates, the ester transformation problem [21, 361, and in some instances the formation of silylated acceptor byproducts [21]. Side-reactions often depend on the type of catalyst used in the reaction. Perbenzoylated glucosyl phos-
124
5 Glycosylution Methods: Use of Phosphites
Oh
BzO 16 R' =Bn, R2,R3 = PhCH, %YO, a only 17 R' =R3 = Bn, R2= H, 73%, a:p1:1 18 R' =R2=H, R3= Bn 88%, p only
BzO 19
loo%, aonly
20 93%, ponly
Figure 7.
phite in the presence of BF3.Et20 (1 equiv.), for example, gave 24% of the phosphonate orthoesters whereas use of 1 equiv. TMSOTf at -50°C gave the desired 1,2 trans-p product in excellent yield, with no traces of the aforementioned byproducts [36]. Other promoters such as ZnC12, ZnC12-AgOTf, and metal perchlorate salts have not been used for the activation of acylated glycosyl phosphites. Stereo-controlled synthesis of fructofuranosides is regarded as a difficult problem, even with a participating group next to the anomeric center [36]. The study of TMSOTf-catalyzed glycosylation using fructofuranosyl phosphite showed that a- or P-selectivity was highly dependent on the properties of the acceptor (Figure 7) [36]. 5.3.5 Glycosylation with C-2-0-Benzylated Glycosyl Phosphites
The glycosylation reaction using glycosyl phosphites without a participating protecting group at C-2 is more complicated. The stereoselectivity seems to depend on many factors, including reaction temperature, the core structure of the glycosyl phosphite, and the properties of acceptors and the promoters. Glycosylation using Glucosyl Phosphites with a Benzyl Group at C-2 Glycosylution with 0-selectivity Glucosylation of a primary hydroxyl acceptor with perbenzylated glucosyl phosphite, promoted by TMSOTf at -78"C, gave nearly exclusively the P-product 8 (66%, a : P = 1 :99 in CH2C12; and 78%, a : P = 2: 99 in EtCN). The P-selectivity decreased, giving the anomeric mixture 9 (83%, a : P = 32: 68 in CH2C12; and 65%, a : f3 = 17:83 in EtCN) when a secondary alcohol acceptor was used [17, 211. Of the many promoters studied, BF3.Et20 gave products in the highest yield and Pselectivity, even with secondary hydroxyl acceptors (a:P = 5 :95) [ 171. ZnC12-promoted glucosylations [22] gave lower 0-selectivity than the lowtemperature procedure (TMSOTf or BF3.EtzO), most probably because of the higher reaction temperature. ZnC12-AgC104 [22] was, however, successfully employed in the total synthesis of paeoniflorin reported by Corey and Wu [37]. In this instance glycosylation of the very hindered acceptor 22 (a mixture of enantiomers) with perbenzylated glucosyl phosphite 21 gave the enantiomerically pure 23 (1 8%,
5.3 Glycosylation using Glycosyl Phosphites
'& B
TIP93
22
OP(OMe)2
BnO
BB *
ZnC12-AgClO4
21
x
Y
@
BnO
M
125
e TIPS3
23 18% enantiomeric pure p
Figure 8.
see Figure 8). This process was more successful than any other glycosylation method, including the TMSOTf-catalyzed phosphite procedure. Glycosylation with a-selectivity From the results discussed above it seems that glycosyl phosphites are not adequate glycosyl donors for the formation of a-glycosyl linkages. Despite this, glycosylations using the perbenzyl glucosyl phosphite donor, in the presence of I2 at room temperature, gave the a-glycosylated compounds 7 and 24 as major products. Glucosylation of a hindered acceptor with the glucosyl phosphite, under ZnCl2AgC104 conditions, gave 9 (88%, a : p = 7 : 3, see Figure 9) with lower selectivity [22b]. The best a-selectivity was achieved under DTBPI-Bu4NI conditions to give product 9 in a 95 : 5 a : p ratio. Glycosylation using Galactosyl and Fucosyl Phosphites with a Benzyl Group at C-2
0-Galactosylation using perbenzylated galactosyl dibenzyl phosphite, in the presence of TMSOTf or BF3.Et20 at -78"C, gave nearly pure products [17, 211. In addition, ZnC12-promoted galactosylation of secondary and primary alcohols using galactosyl diethyl phosphite gave the 0 product with selectivity (a: p 1 :4)in the same range as that obtained in glucosylations [22]. To achieve high a-selectivity, 60-acetyl galactosyl dibenzyl phosphite 25 was used to regioselectively glycosylate the 3-0-position of a partially protected acceptor in CH2C12 at - 15"C to give predominantly product 26 in 60% yield (Figure 10) [30]. Fucosylation using fucosyl dibenzyl phosphites has been reported in the synthesis of Lea and Ley catalyzed by TfOH [31]. The phosphite and trichloroacetimidate
s
-
BnO B
O(CH&Ph 7 92%, a l p = 81/19
Figure 9.
24 91%, a@ = 81/19
9 88%, cr/p = 69/31
5 Glycosylution Methods: Use of Phosphites
126
TMSOTf
(0.2equiv) -15"C
B
oP(ow2
25
BB g E # %
b
OBl
60%
HO 26 cL/p = 85/15
Figure 10.
methods have been compared for the synthesis of Lea (Figure 1 1 ) [35].It was concluded that 0-glycosyl phosphites nicely complement 0-glycosyl trichloroacetimidates, especially in the higher reactivity range where trichloroacetimidates tend to be less stable. In the course of development of sialyl LewisXmimetics, the Wong group conducted extensive studies on the fucosylation of more than ten different N-acylated p-hydroxyl amino acids. They found that fucosylation of benzyl-protected amino acids with perbenzylated fucosyl phosphite gave a greater yield and better aselectivity than the corresponding ethyl-protected amino acids [30, 311. Figure 12 depicts representative syntheses of these compounds. In addition, the DTBPIBWNI-promoted fucosylation reported by Hashimoto's group also gives excellent a-selectivity [ 17b]. Glycosylation using other Glycosyl Phosphites with a Benzyl Group at C-2
The perbenzylated glucuronyl phosphite has shown preference for 1,2-truns-Pselectivity, similar to that of glucosyl phosphite, when coupled with primary and secondary alcohols in the presence of BF3.Et20 at -65°C [22]. Synthesis of the fi-mannosyl linkage is a major problem in glycosylation reactions. Many efforts have been made to improve the p-selectivity of mannosylation using, for example, insoluble Ag silicate in glycosylation [38],inversion at the C-2 of p-glucosyl derivative after glycosylation [391, and intramolecular glycosylation methods [40].Recently, p-mannosylation was also acheved at low temperature through the triflate intermediate which is generated by different activation methods [27](see Section 5.3.2). A similar result might be expected from activation of mannosy1 phosphite with TMSOTf or TfOH, because it might proceed through the
A Ac
E
x TMSOTf
____)
Et20 A
AcO 27 X = OP(OCH$CI& 28 X = OC(NH)CC13
Figure 11.
sRq
OBl
TMSOTf
___)
OTDS EtpO
opc 29 64%
88%
N3
AC
Opc
30 88% 78%
5.3 Glycosylution using Glycosyl Phosphites
127
3 2 H 2 1 L
-
BocH
31
CHJ
TfOH, CH2Cl2, -15 "C
OP(QW2
33 84% R= Bn, c(/p=99/1 34 72% R= Et,dD=71/29
Figure 12.
glycosyl triflate intermediate. Indeed, the Hashimoto group recently showed that 2,3-di-O-benzyl-4,6-O-benzylidine-mannosyl diethylphosphite preferentially gives P-selectivity [ 17~1,although ZnCl2- or ZnClz-AgC104-catalyzed mannosylations from the glycosyl phosphites have been reported to afford a- or P-selectivity. When the perbenzylated mannosyl phosphite is glycosylated with cyclohexanol under ZnCl2 conditions, slight preference for the p-anomeric linkage (83%; a : P 2 : 3 ) is obtained. When the glycosylation is performed with methyl 2,4,6-tri-O-benzyla-glucopyranoside and ZnClz-AgCIO4 activation, the product is pure a-linked disaccharide in 76% yield [22]. The phosphite method has also been used in the synthesis of di-mannosylated phosphatidylinositol with two a-glycosyl linkages, as shown in Figure 13. In the first glycosylation, ZnClz-AgC104 was used to activate perbenzylated glycosyl phosphite to give a 2: 1 alp mixture of 36a. The same conditions gave the pure a product 36b in 70% yield when a 2-OAc mannosyl phosphite was used as donor. Interestingly, at room temperature TMSOTf-catalyzed mannosylation using the perbenzylated phosphite gave a-glycoside 37 in 74% yield [41]. It is not clear how the temperature affects the stereoselectivity of glycosylations employing benzylated mannosyl phosphite with TMSOTf or TfOH as promoter. N
5.3.6 Glycosylation with 2-Deoxy Glycosyl Phosphites The synthesis of 2-deoxy oligosaccharides with controlled a- or P-selectivity remains a difficult problem [lOc, 421 and many methods have been developed to solve it.
@-
ZnC12AgC104
P
"'OH Et20
TIP<S
OH RO
35
Figure 13.
oh
36a: R=Bn 65% a:p2:1 36b: R=Ac 70% a only
5 Glycosylation Methods: Use of Phosphites
128
D-A Temp. Yield a:O
38-43 -78 "C 91% 19:81 38-43 -50
"C
87% 29:71
39-43 -94
"c
93% 13187
40-43 -94
"C 97% 1436
41-43 -94 "C 95% 23:77 42-43 -94 "C 96% 67:33 41-44 -94 "C 86% 69:31
38
39
40
Bn9 ,ow B
w (EtOhPO F
41
OH
BBs%q
43
44
B* OMe
Figure 14.
Examples include electrophile-mediated addition of acceptor to glycals [42a], and glycosylation of the C2-substituted glycosyl donor [42b] followed by reductive removal of the C-2 substituent [ lOc]. Direct stereoselective glycosylation using a 2deoxy glycosyl donor is, however, desirable. 'Remote-controlled' [43] activation of the 2-pyridine-thioglycoside by AgOTf [44]provides a very efficient method for the a-selectively controlled synthesis of 2-deoxy glycosides. Use of insoluble silver silicate for the activation of 2-deoxy glycosyl halides provides one of the best methods of preparation of 2-deoxy-0-glycosides [45]. The Schmidt group has studied the glycosylation reaction using 2-deoxy glycosyl phosphites activated by BF3 .Et20 in CH2Cl2-hexane or in Et20 [36]. Preferential a-selectivities were obtained in the formation of both the 2-deoxy galactosyl linkage and the 2-deoxy glucosyl linkage. In the glycosylation of 2-deoxy glycosyl phosphites under TMSOTf activation [12, 131, the Hashimoto group [26] recently found that the reaction temperature is crucially important in the control of P-selectivity (see Figure 14, entries 1-3). The selectivities were not significantly affected by the reactivities of the glycosyl donors (entries 1, 4, and 5; a :P = 10: 90, 13 :87, and 14: 86, respectively). They are, however, affected by other structural properties of the glycosyl donors. For example, compounds 41 and 42 with axial substituents at C-4 gave lower p-selectivities (entry 6, a : P = 23: 77 and entry 7, a : P = 67: 33). The p-selectivity was observed to increase with increasing reactivity of the acceptor alcohol (compare entries 6 and 8). In the synthesis of the hexasaccharide fragment of landomycin A, the desirable p-glycosyl linkage of the 2,6-dideoxy unit was successfully formed by use of the TMSOTf-promoted phosphite method (Figure 15) [ 161.
5.4 Other Applications of Glycosyl Phosphites Besides the 0-,S-, C-glycosylations and 0-glycopeptide synthesis discussed above, other applications of glycosyl phosphites are discussed below.
5.4 Other Applications of Glycosyl Phosphite
129
TMSOTf -94 OC AcO
HO
c,
AcO
47
&*+
OMP
TMSOTf, Toluene,
0 M
48 R' = AC (42%, a:p 5 2 ~ 8 )
A d
Figure 15.
5.4.1 Synthesis of CMP-NeuAc
The synthesis of CMP-NeuSAc remains of interest, because it is a glycosyl donor in glycosyltransferase-catalyzed reactions. Three chemical synthetic methods based on the phosphite strategy have been reported. The Wong group employed a glycosyl phosphoamidate (Figure 16A) [12], whereas the Halcomb group used a nucleoside phosphoamidate in the synthesis of CMP-NeuSAc (Figure 16B) [46a]. The latter method seems preferable, because of its high-yielding deprotection step. The Schmidt group utilized the diethyl glycosyl phosphite and the nucleoside phosphate 53 in its acid form to give P-CMP-NeuAc 54 in 50% yield (Figure 17) [46b].The reaction seems to be mediated by acid catalysis.
5.4.2 Synthesis of GDP-Fucose
GDP-fucose is a glycosyl donor substrate for fucosyltransferases. Although it has been synthesized by many chemical methods [47],the use of fucosyl phosphite in the synthesis seems the most practical [48]. A recent improvement of this method includes the addition of tetrazole to the morpholidate coupling reaction [48b]. This method might also be applicable to the synthesis of other nucleoside diphosphate sugars (Fig-ure 18). Together with the synthesis of CMP-NeuSAc discussed above, glycosyl phosphite chemistry indeed provides a useful alternative for the synthesis of natural and unnatural donor substrates for the study of enzyme-catalyzed glycosylations.
130
5 Glycosylation Methods: Use of Phosphites NHBz 1. HOR 2. BuOOH 12 Yo
50
51
52
Figure 16.
HQ
PH
HAC
CH ,CN/TH FIDMF
0 3
AcO 01% 53
(1:l:l)
-15 "C to rt.,
AcO AcO 54 50%
Figure 17.
1.30% H202 2. H2IPd-C F 0 $ O N a h AcO F z z B n 0 ) 2 3. NaOWH20 HO OH 55 63% 54 cr:p =1:10 tetrazolej Pyr
57 91%
Figure 18.
Ho
OH
OPC
5.4 Other Applications of Glycosyl Phosphite Ac
131
TMSOP(OMe)* TMSOTf CHzCI2
PCH
A d
ACQ 58 63 Yo Ponly
3 Figure 19.
5.4.3 Formation of Glycosyl Phosphonate Glycosyl phosphites can be converted by Arbuzov-type fragmentation to the corresponding glycosyl phosphonates as analogs of glycosyl phosphates [49]. Glycosyl phosphonates are more stable than glycosyl phosphates [50]and can act either as inhibitors [5I] or as regulators in metabolic processes where natural phosphates function. Figure 19 shows the synthesis of a sialyl phosphonate starting from the glycosyl phosphite [52]. 5.4.4 Transformation to other Types of Glycosyl Donor Phosphate Oxidation [I 8, 481 of glycosyl phosphites provides glycosyl phosphates which have been used for the synthesis of oligosaccharides [53] and sugar nucleosides [12, 81 (Figure 20).
rA-OP(ORh
oxidation
8
* a O F ' ( O R h
HOR TMSOTf
OR
Figure 20.
Phosphorimidate More recently, a new glycosylation method has been reported using glycosyl phosphorimidates as glycosyl donors. Thus, compounds were prepared from the corresponding glycosyl phosphites (Figure 21) 1541.
Figure 21.
132
5 Glycosylation Methods: Use of Phosphites
In summary, use of glycosyl phosphites in a new glycosylation method has received much attention for several reasons, including the high reactivity of the glycosyl phosphite at low temperature, the high efficiency of glycosylation with stereo-hindered donors or acceptors, and the high stereoselectivity of the reaction. Applications of glycosyl phosphites in the synthesis of glycosyl phosphates and glycosyl phosphonates are, furthermore, especially useful. Applications and modifications of glycosyl phosphite chemistry will surely be of interest to many chemists in the future.
References 1. Wong, C.-H., Ye, X.-S., and Zhang, Z . J. Am. Chem. SOC.1998,120, 7131. 2. T. Wubberg, C. Kallus, T. Opatz, S. Henke, W. Schmidt, H. Kunz, Angew. Chem. Int. Ed. Engl. 1998,37, 2503-2505. 3. Random glycosylation: (a) 0. Kanie, F. Barresi, Y. Ding, J. Labbe, A. Otter, L. S. Forsberg, B. Ernst, 0. Hindsgaul, Angew. Chem. Znt. Ed. Engl. 1995, 34, 2720. (b) Y. Ding, J. Labbe, 0. Kanie, 0. Hindsgaul, Bioorg. Med. Chem. 1996, 4, 683. Latent-active glycosylation: G.-J. Boons, B. Heskamp, F. Hout, Angew. Chem. Znt. Ed. Engl. 1996,35,2845. Solid-phasemethod: R. Liang, L. Yan, J. Loebach, M. Ge, Y. Uozumi, K. Sekanina, N. Horan, J. Gildersleeve, C. Thompson, A. Smith, K. Biswas, W. C. Still, D. Kahne, Science 1996,274, 1520. 4. (a). D. R. Mootoo, P. Konradsson, U. Udodong, B, Fraser-Reid, J. Am. Chem. SOC.1988,110, 5583; (b). B, Fraser-Reid, Z. Wu, U. D. Udodong, H. Ottosson, J. Urg. Chem. 1990, 55, 60686070. 5. Y. Ito, T. Ogawa, J. Am. Chem. SOC.1994,116, 12073. 6. (a). S. Raghavan, D. Kahne, J. Am. Chem. SOC.,1993,115, 1580. (b) H. Yamada, T. Harada, H. Miyazaki, T. Takahashi, Tetrahedron Lett., 1994, 35, 3979. (c) H. Yamada, T. Harada, T. Takahashi, J. Am. Chem. SOC.1994, 116, 7919. (d) H. K. Chenault, A. Castro, Tetrahedron Lett., 1994, 35, 9145. (e) R. Geurtsen, D. S. Holmes, G.-J. Boons, J. Urg. Chem. 1997, 62, 8145. 7. (a) S. V. Ley, H. W. M. Priepke, Angew. Chem. Int. Ed. Engl. 1994,33,2292. (b) P. Grice, S. V. Ley, J. Pietruszka, H. W. M. Priepke, E. P. E. Walther, Synlett 1995, 781. (c) P. Grice, S. V. Ley, J. Pietruszka, M. I. Osborn, W. M. Henning, H. W. M. Priepke, S . L. Warriner, Chem. Eur. J. 1997, 431. (d). N. L. Douglas, S. V. Ley, U. Lucking, S. L. Warriner, J. Chem. Soc. Perkin Trans. 1, 1998, 51. (e). L. Green, B. Hinzen, S. J. Ince, P. Langer, S. V. Ley, S . L. Warriner, Synlett. 1998, 4, 440. 8. Z. Zhang, I. R. Ollmann, X.-S. Ye, R. Wischnat, T. Baasov, C.-H. Wong, J. Am. Chem. SOC. 1999, 121, 734. 9. Solid-phase synthesis: For review see, (a). P. H. Seeberger, S. J Danishefsky, Acc. Chem. Res. 1998, 31, 685. (b). J. S. Fruechtel, G. Jung, Angew. Chem., Int. Ed. Engl. 1996, 35, 17; For general articles see, (c) M. J. Sofia, Comb. Chem. Mol. Diversity Drug Discovery, Eds: E. M. Gordon, J. F. Kenvin Jr., Wiley-Liss, New York, 1998, 243-269; (d). R. R. Schmidt, Pure Appl. Chem. 1998, 70, 397-402; (e). Z.-W. Guo, Y. Nakahara, Y. Nakahara, T. Ogawa, Angew. Chem., Znt. Ed. Engl. 1997, 36, 1464; (f). R. Rodebaugh, S. Joshi, B. Fraser-Reid, H. M. Geysen, J. Org. Chem. 1997, 62, 5660; (8). S. J. Danishefsky, K. F. McClure, J. T. Randolph, R. B. Ruggeri, Science 1993,260, 1307; For enzymatic methods see, (h). M. Schuster, P. Wang, J. C. Paulson, C.-H. Wong, J. Am. Chem. SOC.1994,116, 1135. 10. For the recent review articles on oligosaccharide synthesis see: (a) H. Paulsen, Angew. Chem. Int. Ed. Engl., 1990, 29, 823; (b) J. Banoub, Chem Rev. 1992, 92, 1167; (c) K. Toshima, K. Tatsuta, Chem. Rev. 1993, 93, 1503. (d) R. R. Schmidt, W. Kinzy, Adv. Carbohydr. Chem. Biochem. 1994, 50, 21. (e) S. J. Danishefsky, M. T. Bilodeau, Angew. Chem. Int. Ed. Engl., 1996,35, 1380. 11. H. Paulsen Angew. Chem. Int. Ed. Engl., 1982,21, 155.
References
133
12. H. Kondo, Y. Ichikawa, C.-H. Wong, J. Am. Chem. SOC.1992,114, 8748. 13. T. J. Martin, R. Schmidt, Tetrahedron. Lett. 1992, 33, 6123. 14. (a). L. I. Smirnova, M. A. Malenkovskaya, D. A. Predvoditelev, E. E. Nifantev, Zh. Org. Khim. 1980, 16, 1170; (b). J. W. Perich, R. B. Johns, Tetrahedron Lett. 1987,28, 101; (c). K. L. Yu, B. Fraser-Reid, Tetrahedron Lett. 1988, 98, 979; (d). R. L. Pederson, J. Esker, C.-H. Wong, Tetrahedron, 1991, 47, 2643; (e). S. Aoki, H. Kondo, C.-H. Wong, Methods in Enzymology, 247, 193; (f). Qiao, B. W. Murray, M. Shimazaki, J. Schultz, C.-H. Wong, J. Am. Chem. SOC. 1996, 118, 1653. 15. H. Paulsen, H. Tietz, Carbohydr. Res. 1984, 125, 47. 16. Y. Guo and G. A. Sulikowski, J. Am. Chem. SOC.1998,120, 1392. 17. (a). S. Hashimoto, K. Umeo, A. Sano, N. Watanabe, M. Nakajima, and S. Ikegami, Tetrahedron Lett., 1995,36, 2251; (b). H. Tanaka, H. Sakamoto, A. Sano, S. Nakajima, S. Hashimoto, Chem. Commun., 1999, 1259; (c). unpublished result, personal communication with Prof. Hashimoto. 18. M. M. Sim, H. Kondo and C.-H. Wong, J. Am. Chem. SOC.1993,115,2260. 19. R. R. Schmidt, B. Wegmann, K.-H. Jung, Liebigs Ann. Chem. 1991, 191, 121. 20. S. Sabesan, S. Neira, Carbohydr. Res. 1992, 223, 169. 21. H. Kondo, S. Aoki, Y. Ichikawa, R. L. Halcomb, H. Ritzen, C.-H. Wong, J. Org. Chem. 1994, 59, 864. 22. (a). Y. Watanabe, C. Nakamoto, S. Ozaki, Synlett, 1993, 115-116; (b). Y. Watanabe, C. Nakamoto, T. Yamamoto, S. Ozaki, Tetrahedron, 1994, 50, 6523. 23. H. Schene, H. Waldmann, Eru. J. Ory. Chem. 1998, 1227. 24. Lemieux Abstr: Pap. Am. Chem. SOC.1958. 133, 31N; (b) J. P. Praly, R. U. Lemieux, Can. J. Chem. 1987,65,213. 25. (a). R. R. Schmidt, M. Behrendt, A. Toepfer, Synlett, 1990, 694; (b). A. J. Ratcliffe, B. FraserReid, J. Chem. Soc., Perkin Trans. 1 1990, 747; (c). P. Sinay, Pure Appl. Chem. 1991, 63, 519. 26. S. Hashimoto, A. Sano, H. Sakamoto, M. Nakajima, Y. Yanagiya, S. Ikegami, Synlett., 1995, 1271. 27. (a). D. Crich, S . Sun, J. Ory. Chenz. 1996, 61, 4506-4507: (b). D. Crich, S . Sun, J. Am. Chem. SOC.1997, 119, 11217; (c). D. Crich, S. Sun, J. Am. Chem. SOC.1998, 120, 435. 28. (a). D. Kahne, S. Walker, Y. Cheng, D. V . Engen, J. Am. Chem. SOC.1989, 111, 6881; (b). L. Yan, D Kahne, J. Am. Chem. SOC.1996, 118, 9239. 29. (a). V. Martichonok, G. Whitesides, J. Am. Chem. SOC.1996, 118, 8187; (b). V. Martichonok, G. Whitesides, J. Org. Chem. 1996, 61, 1702. 30. C.-C. Lin, M. Shimazaki, M.-P. Heck, S. Aoki, R. Wong, T. Kimura, H. Ritzen, S. Takayama, S.-H. Wu, G. Weitz-Schmidt, C.-H. Wong, J. Am. Chem. SOC.1996, 118, 6826. 31. S.-H. Wu, M. Shimazaki, C.-C. Lin, L. Qiao, W. J. Moree, G. Weitz-Schmidt, C.-H. Wong, Angew. Chem. Int. Ed. Engl., 1996, 35, 88. 32. For sialylations see ref. 12, and 13 and references therein. 33. N. E. Nifantev, Y. E. Tsvetkov, A. S. Shashkov, L. 0. Kononov, V. M. Menshov, A. B. Tuzikov, a n d N . V. Bovin, J. Carbohydr. Chem. 1996, 15, 939. 34. U. Sprengard, G. Kretzschmar, E. Bartnik, C. Hiils, and H. Kunz, Angew. Chem. lnt. Ed. Engl. 1995, 34, 990. 35. T. Muller, G. Hummel, R. R. Schmidt, Liebigs Ann. Chem. 1994, 325. 36. T. Miiller, R. Schneider, R. R. Schmidt, Tetrahedron Lett. 1994,35,4763. 1993, 115, 8871. 37. E. J. Corey, Y.-J. Wu, J. Am. Chem. SOC. 38. H. Paulsen, 0. Lockhoff, Chem. Ber. 1981,114, 3102. 39. H. Kunz, W . Giinther, Angew. Chem. Int. Ed. Engl. 1988, 27, 1086. 40. (a). F. Barresi, 0. Hindsgaul, J. Am. Chem. SOC.1991, 113,9376; (b). G. Stork, G. Kim, J. Am. Chem. SOC.1992, 114, 1087; (c). Y. Ito, T. Ogawa, J. Am. Chem. Soc. 1994, 116, 12073. 41. (a). Y. Watanabe, T. Yamamoto and S. Ozaki, J. Org. Chem. 1996,61, 14-15; . (b). Watanabe, T. Yamamoto and S. Ozaki, Tetrahedron, 1997, 53, 903. 42. (a). J. Thiem, W. KlaKke, Top. Curr. Chem. 1990, 154, 870; (b). William Roush and Chad E. Bennett J. Am. Chem. SOC. 1999, 121, 3541. 43. S. Hanessian, C. Bacquet, N. Lehong Carbohydr. Res. 1982, 80, c17. 44. Z. Zhang, G. Magnusson, Carbohydr. Res. 1994,262, 79.
134
5 Glycosylation Methods: Use of Phosphites
45. (a). P. J. Garregg, S. Kopper, P. Ossowski, J. Thiem, J. Carbohydr. Chem. 1986, 5, 59; (b). R. W. Binkley, D. J. Koholic, J. Org. Chem. 1989, 54, 3577; (c). Z. Zhang, G. Magnusson, J. Org. Chem. 1996,61, 2383. 46. (a). M. D. Chappell, R. L. Halcomb, Tetrahedron 1997, 53, 11109; (b). T. J. Martin, R. R. Schmidt, Tetrahedron Lett. 1993, 34, 1765. 47. U. B. Gokhale, 0. Hindsgaul, M. M. Palcic, Can. J. Chem. 1990, 68, 1063; (c). R. R. Schimidt, B. Wegmann, K.-H. Jung Liebigs. Ann. Chem. 1991,191, 121. 48. (a). Y. Ichikawa, M. M. Sim, C.-H. Wong, J. Org. Chem. 1992, 57, 2943-2946; (b). V. Wittmann, C.-H. Wong J. Org. Chem. 1997, 62, 2144. 49. R. Meuwly, A. Vasella, Helv. Chim. Acta. 1986, 69, 25. 50. (a). J. Gao, V. Martichonok, G. M. Whitesides, J. Org. Chem. 1996, 61, 9538; (b). T.-H. Chan, Y.-C. Xin J. Org. Chem. 1997, 62, 3500; (c). B. Miiller, T. J. Martin, C. Schaub, R. R. Schmidt, Tetrahedron Lett. 1998, 39, 509; (d). M. J. Ruira, M. J. Pkrez-Pkrez, Jan. Balzaerini, M. Camarasa, Synlett. 1998, 177. 51. C. L. White, M. N. Janakiraman, W. G. Laver, C. Philippon, A. Vasella, G. M. Air, M. Luo, J. Mol. Biol. 1995,245, 623. 52. M. Imamura, H. Hashimoto, Tetrahedron Lett. 1996, 37, 1451. 53. S. Hashimoto, T. Honda, S. Ikegami, J. Chem. Soc. Chem. Commun. 1989, 685. 54. (a). S. Pan, H. Li, F. Hong, B. Yu, K. Zhao, Tetrahedron. Lett. 1997, 38, 6139; (b). H. Li, M. Chen, K. Zhao, ibid., 1997,38, 6143.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
6 Glycosylation Methods: Use of n-Pentenyl Glycosides Bert Fraser-Reid, G. Anilkumar, Mark R. Gilbert, Subodh Joshi, and R a y Kraehmer
6.1 Introduction The use of protecting groups in syntheses of complex oligosaccharides is currently inevitable. Electronic or steric factors may enable regioselectivity between competing hydroxyl groups, but for the foreseeable future such interventions will be exceptions to the rule. If protecting groups are to be necessary, it would be advantageous for them to serve additional roles. In our laboratory this concept was first appreciated with the development of n-pentenyl glycosides (NPG), in which the pentenyl group protects the sensitive anomeric center, and survives (most of) the chemical transformations tolerated by normal alkyl glycosides. Treatment with an electrophile converts the pentenyl moiety into a leaving group, however, thereby enabling replacement reactions to occur at the anomeric center. This dual protection/activation role is a fundamental property of NPGs, and this duality was heavily exploited in the synthesis of the rat brain Thy-1 glycosylphosphatidylinositol membrane anchor by the retrosynthetic plan outlined in Scheme 1 [I]. NPGs have, however, also been valuable in mechanistic investigations designed to probe events at the anomeric center. For example, NPGs were instrumental in the demonstration that protecting groups can profoundly alter reactivities of glycosyl donors-and glycosyl acceptors. These observations indicated the importance of fine-tuning reactivity of donor/acceptor partners, and efforts to contribute to this insight continue in our laboratory.
6.2 Fundamental Reactions A summary of (most of) the fundamental reactions of n-pentenyl glycosides is presented in Scheme 2 [ 2 ] .Being simple alkyl glycosides, NPGs can be readily prepared
6 Glycosylation Methods: Use of n-Pentenyl Glycosides
136
V
0
HO
HO-
-
Thy1 Anchor from Rat Brain
OBn BnO
Scheme 1. Retrosynthetic plan for rat brain Thy-1 GPI membrane anchor.
by standard Fischer glycosidations, e.g., 4 4 5 . Where one or other anomer is specifically required, however, the standard procedures used for simple alkyl glycosides can be applied as summarized in Scheme 3. The role as glycosyl donor proceeds through the standard oxocarbenium intermediate 2, which can be trapped by nucleophiles. In aqueous medium the product is an aldose (1, Nu = OH). When such oxidative hydrolyses were probed it was found that with C2-OAcyl substrates the reaction occurs much more slowly than with C2OAlkyl. This observation has been introduced into a chemoselective coupling [3] protocol that has come to be known as the 'armed-disarmed strategy.' [4]Thus as illustrated in Scheme 4a, the partners 10 and 11 gave disaccharide 12a, with no evidence of the product of self-coupling of 11, nor of further reaction of disaccharide 12a with the acceptor 11, both of which would have occurred at disarmed anomeric
RN"& p-1 Y
1 Nu = OR, OSugar, Br, NHCOPeptide
/
6.2 Fundamental Reactions
p!
Y
Y
2
3 furanylium ion
glycosyl donor
137
J
6
free sugar
NPG
I disaccharide or higher
Zn or Smlp or Nal
glycosyl acceptor
cyclic halonium
I
+
XsNu'
9 bromohydrin Nu=OH dibromide Nu=Br
Scheme 2. The chemistry of n-pentenyl glycosides (NPG).
centers. Transformation to the armed counterpart 12b, however, enabled ready formation of trisaccharide 13 [3]. Glycoside hydrolysis is of immense mechanistic interest because of the insight it provides into enzymatic cleavage or assembly of oligosaccharides [5, 61. The option of oxidative hydrolysis meant that the anomeric center of NPG could be cleaved without the use of acids and, furthermore, that acid-labile protecting groups could be employed to enhance mechanistic insight. In the course of such investigations, it became clear that frequently used cyclic protecting groups could also deactivate glycosyl donors [7]. Thus, the advantage of simultaneous protection of two hydroxyl groups might be offset by the torsional constraints imposed upon the bicyclic array. Theoretical calculations supported this finding [81.
138
(Hqn-k>0H
6 Glycosylution Methods: Use of n-Pentenyl Glycosides
camphor Pent-OH
(HO)n{>O(/
sulfonic acid
I
OPent
Scheme 3. Preparative routes to NPG.
The reality of torsional armed-disarmed effects was demonstrated (Scheme 4b) with the glucosides 14 and 15, which gave the product of cross coupling, 16, but with no evidence of 17, the product from self-coupling of 15 [9].
6.3 Determination of Relative Reactivities The evidence in Scheme 4 makes it clear that success in oligosaccharide coupling can be dependent on protecting groups present on the reactants. A procedure for direct comparison of glycosyl donor reactivity would be advantageous, and such a methodology was therefore developed [lo]. As shown in Figure 1, eq. (l), equivalent amounts of the two NPGs, S1 and S2, compete for one equivalent of NBS in aqueous solvent. As eq. (2) shows the method requires that the NBS must be completely consumed, and so the reaction must continue until a negative test is obtained with potassium iodide-starch paper, shown independently to be accurate to within 2%. Because both anomers are in the same reaction medium, the NBS terms in the rate expressions cancel out, leading eventually to eq. (3). After work-up, the ratio of unreacted NPGs in the crude reaction product is determined, eq. (4). For our purposes, HPLC was convenient for these determinations. All the terms in eq. ( 5 ) are now known or knowable. And so the relative reactivity, kl/k2, of the two NPGs can be obtained by solving eq. (3). Some comparisons of reactivities, as determined by the procedure in Figure 1, are assembled in Scheme 5. That the choice of anomeric configuration of the donor might be important is apparent from the results for 18 and 19. In both the anomer
6.3 Determination of Relative Reactivities
k M
J+
0
,""I?,..
b T a
9
139
6 Glycosylation Methods: Use of n-Pentenyl Glycosides
140
S1
+ S2 + NBS
H20 ,Products CH,CN
(1 eq. of each) [end point determined by negative KI-starch test paper]
Figure 1. Calculating the relative rates of two glycosyl donors.
reacts more quickly, the higher ratio in 19 being evidence of anchimeric assistance resulting from neighboring-group participation in the cleavage reaction [ 1I]. The classical electronic armed-disarmed situation (Scheme 4a) is seen with the p glycosides 20a and b. Notably, 20b is less reactive than 20a despite the anchimeric assistance which must be occurring in the ester (cf 19). The results for the p glucoside pairs 20 and 21 confirm that the benzylidene ring can be disarming-much more so for acyl protecting groups (21, R = Ac compared with R = Bn). Couplings involving benzylidinated or acetonated donors might, therefore, occur less readily than with their torsion-free counterparts.
BnO
OPent
AcO
,OAc
OBn 18
OPent
R:)ope,,
= 1.59
OR
20
19
18p/ M a = 1.70
20d20b
\
a R=Bn
19p/ 19a= 5.19
b R=Ac R RO o R ~ O F ’ e n pl
~
~
~
~2 0 d 2 l a O = 1.6
\
OR
OK
20
21
a R=Bn
a R=Bn
b R=Ac
b R=Ac
Scheme 5. Relative reaction rates for some glycosyl donors.
F20b/ 2lb m= 10.4
6.4 n-Pentenyl Orthoesters as Glycosyl Donors
141
Although these results are based on NPG, they should apply equally to any pair of donors, irrespective of the anomeric leaving group. This assumption is based on general relevance of armed-disarmed factors across the spectrum of glycosyl donors 1121.
6.4 n-Pentenyl Orthoesters as Glycosyl Donors Alkyl orthoesters, 23, like cyclic acetals, also simultaneously protect two hydroxyl groups. Their use in carbohydrate synthesis [13] is greatly enhanced by a regioselective, acid-catalyzed rearrangement, whereby the resulting ester is axial rather than equatorial [ 141. Glycosyl (or 1,2-) orthoesters are readily prepared by Lemeiux’s procedure, in which acylated glycosyl bromides are treated with a hindered base, 22 23 [ 151 (Scheme 6). Mild acid treatment causes a rearrangement, 23 -, 26, in which the alkoxy unit is transferred to the anomeric center [16] (Scheme 6). The group so transferred can vary from simple alkoxy to an oligosaccharide [ 171. n-Pentenyl orthoesters (23, S = Pent) have an additional mode of reaction. Thus, treatment with an electrophile leads to furanylium ion 27, and thence to the same dioxolenium intermediate 25 (Scheme 6). The discharged entity, 28, however, is not nucleophilic and so a nucleophile present in the reaction medium faces no competition for reaction with 25 leading to 29. Notably, 26c and 29a are both higher saccharides, but the strategies for their preparation offer a choice between protonand electrophile-driven reactions. Again, n-pentenyl orthoester 23 (S = Pent) and NPG 26b are both glycosyl donors, but they have very different properties, notably towards acids. Further donor versatility comes from the fact that titration with bromine affords bromide 29b, the classic glycosyl donor, ready for reaction with silver triflate [ 181. To go from one glycosyl bromide to another, 22 +29b, might seem redundant, but the advantage of this possibility is readily appreciated from Scheme 7, which summarizes two approaches to trimannan 36, a building block for a synthesis of the Thy-1 GPI rnembrane (see Scheme 1) [19]. The tetrabenzoyl mannosyl bromide 30 was converted into orthoester 31a [15]. One portion was silylated [20] and then benzylated to give 31b, which upon treatment with bromine led to the glycosyl bromide 32. The remainder of 31a was completely benzylated, and one portion of the resulting material, 31c, was processed to give the acceptor 33a whereas the other was brominated to give glycosyl bromide 35. Coupling of 32 and 33b, led to 34a and debenzoylation to acceptor 34b which reacted with 35 to give trimannan 36 P11. Glycosyl bromides 32 and 35, being unstable, are best generated and used in situ. An approach to the same trimannan in which all reacting participants are stable is, however, outlined in Scheme 7b. The pathway takes advantage of some of the unique aspects of n-pentenyl glycosides. Thus, a major development in NPG chemistry emanated from mechanistic investigations which showed that intramolecular reaction of the cyclic halonium compound leading to the furanylium ion, --f
142
6 Glycosylation Methods: Use of n-Pentenyl Glycosides
v1
c
x 8 *
8
%
Scheme 7. Two routes to a trimannan from an n-pentenyl orthoester. (i) Pent-OH, 2,6-lutidine, CHzC12; (ii) tBuPh2SiCl; imidazole, THF; (iii) BnBr, NaH, B U ~ N ITHF; , (iv) 31b or c; Br2, CH2C12, 0 ° C ; (v) 31c CSA, CH2C12, 5OoC, 6 h; (vi) NaOMe; (vii) AgOTf, CHzCl,; (viii) TBAF, THF then Ac20, DMAP, CHzC12; (ix) Br2, Et4NBr, CH2C12; (x) NIS, Et&OTf, CH2Clz;(xi) NH3, MeOH; (ClAc)20, Et3N, CH2CI2; (xii) Zn, BQNI, EtOH then (ClAc)20.
U
144
6 Glycosylation Methods: Use of n-Pentenyl Glycosides
(Scheme 2, 6-+3), could be offset by the bimolecular process leading to 9, in the presence of excess nucleophile. For example, added bromide ion led to a vicinal dibromide; because, however, reductive elimination readily restored the double bond, dibromides 7 and 8 represent ‘sidetracked’ forms of an NPG. Nevertheless, the sidetracked material enables an hydroxyl group to be liberated, thereby creating a glycosyl acceptor 8 capable of reacting with a donor to give 7. The utility of the ‘sidetracked’ capability is illustrated in Scheme 7b. A portion of NPG 33a was debenzoylated to 33b and then ‘sidetracked’ to give acceptor 38. Coupling with orthoester 37 then gave dimannan 39a, which underwent routine protecting-group alterations to produce disaccharide acceptor 39b. Coupling with the remainder of 33a now produced 40. The major process for obtaining the previously described 36 now involved reductive debromination to regenerate the pentenyl moiety [22].
6.5 n-Pentenyl Orthoesters as Latent C2 Esters It is apparent from Scheme 7 that proton- or electrophile-driven reaction leads to specifically C2-0-acylated products. C2-Esters are important for controlling stereoselectivity in anomeric replacement reactions [ 1 11, and so the use of orthoesters as specific latent C2-esters has been explored in the context of a protected form of the ‘linkage region’ of proteoglycans, which contains four 1,2-trans diequatorial linkages (Scheme 8). For the C2-acylated galactoside acceptor, a simple route begins with galactosyl bromide 41 (Scheme 8a). Orthoester formation required the use excess pentenyl alcohol and tetrabutylammonium iodide as promoter to obtain 42a and thence 42b. Acid-catalyzed reaction of the latter in the presence of excess pentenyl alcohol, then benzylidination gave acceptor 43a. The xylosyl acceptor 46b (Scheme 8b) was obtained similarly from bromide 44 via the n-pentenyl orthoester 45. Diol 46a was subjected to selective silylation at C4, paving the way to 46b. It was expected that the uronate moiety would be problematic because such 2-0acyl donors usually lead preponderantly to orthoester products. Of several donors that were tested, trichloroacetimidates were found to give the least amount of orthoesters. Donor 49 was therefore prepared from commercially available benzyl P-glucoside, 47a, via methyl ester 48 as summarized in Scheme 8c. Coupling with galactoside 43a gave the disaccharide donor 50 as the major product, the orthoester (51) being minimized at 25%. The disaccharide acceptor 52b was obtained by coupling the ‘sidetracked’ xyloside 46b to the n-pentenyl galactoside 43b, followed by dechloroacetylation. The latter reaction was problematic and required use of the van Boeckel procedure [23]. The two disaccharides were then coupled to give 53a. Iodide-induced debromination [24], a milder alternative to the zinc reduction used for 42 -+ 38 in Scheme 7, afforded 54b. Finally, coupling with the protected serine 54 [25] occurred smoothly to give the desired ‘linkage region’ construct 55 [26].
6.5 n-Pentenyl Orthoesters as Latent C2 Esters
145
146
6 Glycosylution Methods: Use of n-Pentenyl Glycosides
6.6 Protecting Groups The presence of one free amine and four amide functions in the Thy-1 anchor (Scheme 1) sensitized us to the need for new, easily removable amine protecting groups. The n-pentenyl based protecting groups for alcohols and vicinal diols shown in Scheme 9a-c are outgrowths of NPG chemistry [27], all of which are cleaved or installed with halogen electrophiles. Notably, the acetalating agents in Scheme 9d can be put in place without disturbing acid-sensitive groups already present [28]. Glycosyl n-pentenoyl esters (Scheme 9e) have been found to be good glycosyl donors, a circumstance that provides evidence for ready loss of the pentenoyl group [29]. In the light of these precedents, amine protection via n-pentenoylation was examined. As shown in Scheme 9f installation for primary and secondary amines is routine, and cleavage occurs readily with iodine. C2-N-acylated sugars are, however, prone to give oxazolines as shown in Scheme 9g. Lemieux showed that this problem could be obviated by employing phthaloyl protecting groups [30], de-Nphthaloylation being effected by prolonged treatment with hydrazine or ethylenediamine. It has been shown [31, 321 that electron-withdrawing groups on the aromatic ring facilitate cleavage remarkably, as indicated by the difference for X = C1 and X = H in Scheme 9h. Cleavage of pentenoyl or tetrachlorophthaloyl (TCP) groups is therefore seen to occur very readily, and the very different procedures employed enable these protecting groups to be used orthogonally. This aspect was explored in the context of lipid A, 56 [33] (Scheme lo), a molecule in which all sites on the disaccharide need to be differentiated. The glucosaminide functions would be protected by either npentenoyl or TCP; but use of the former on the acceptor rather than the donor was dictated by the oxazoline problem noted in Scheme 9g. In the event Koenigs-Knorr coupling of 57 or 58 produced 59 in good yield. Comparison with lipid A, 56, shows that differentiation between the three acetylated sites of 54 would be necessary for a more useful precursor. Deacetylation with base would not be tolerated by the TCP group; but acid-catalyzed deacetylation was successful and benzylidination led to the more attractive counterpart 60 with the five critical sites being differentially accessible. With respect to the protected amines, treatment with ethylenediamine liberated one and iodine the other, giving 61 and 62 respectively [34]. Notably, cleavage of the benzylidine group with sodium cyanoborohydride [35] is tolerated by TCP [36].
6.7 Solid-Phase Iterative Couple-Deprotect-Couple Strategy Scheme 2 shows that NPGs have the potential to function as glycosyl donors and also as glycosyl acceptors via sidetracked intermediates (e.g. 8). The latter property
6.7 Solid-Phase Iterative Couple-Deprotect-Couple Strategy
h I
QfB I
1 (0 0
+ I '
8
q)==J 0
I
\
141
148
6 Glycosylation Methods: Use of n-Pentenyl Glycosides
Lipid A /OAC
AcO AcO BzO
Ad3
BzO
Oknt
-
57
DIFFERENTIALLY ACCESSIBLE SITES
Scheme 10. Orthogonal protecting groups for a lipid a precursor. (i) AgOTf; (ii) Me2CO/H20/HCl, 60"C, 20 h; PhCH(OMe)2/PF'TS/dioxane, reflux; (iii) ethylenediarnine, 60°C 2 h; (iv) 12 (2 equiv.), HzO/THF, 8 min.
was instrumental in a process used to assemble the high-mannose nonamannan component via an iterative deprotect-couple-deprotect strategy [37]. A solid-phase version of this procedure would have the advantage of minimizing transformations on the bound saccharide(s). An attempt to develop such an iterative strategy, shown in Scheme 11, used the known [38], bound pentenyl alcohol 63 for a KoenigsKnorr reaction with 64, by following the procedure developed by Frechet and Schuerch [39]. To test the viability of the cleavage reaction, compound 65 was treated with benzyl alcohol and NIS/TESOTf in wet dichloromethane to afford the
R=Bn
b)
OPent
+
BnO
ii
70
BnO
BnO
65
BnO
66
BnO
71
BnO
67
Scheme 11. A solid-phase version of NPG. (i) THF; (ii) pyridine, CH2C12; (iii) BnOH, NIS, TESOTf, wet CH2C12, 2 h; (iv) NaOMe/MeOH/THF; Br2, Et4nBr, CH2C12; (v) NIS, TESOTf, CHzC12, 2 h; (vi) 0.1 M SmI2, THF, 30 mix; (vii) thiourea, methoxyethanol, 8 0 T , 6 h.
68 R=Ac
a)
Bn?% BnO
63
OH
u
P
150
6 Glycosylation Methods: Use of n-Pentenyl Glycosides
benzyl glycoside 66 in 60% yield and, presumably, the bound iodomethyltetrahydrofuran 67. For a more elaborate test case, the sidetracked acceptor 69 was prepared and coupling with 68a proceeded smoothly to give a dibromide. Zinc could not be used to restore the double bond, because of the solid-solid interaction that would be involved. A one-electron reductive elimination with samarium I1 iodide [24] proceeded smoothly, however, to give 70. Unfortunately, efforts to cleave the disaccharide from the solid support to produce 71 were unsuccessful. FTIR data suggested that addition to the double bond overwhelmed the cyclization reaction. The above results meant that the oxidative cleavage worked sometimes (65-+ 66), but not always (70 -+ 71). The use of sidetracked acceptors such as 69 might, therefore, be unreliable. Nevertheless, use of NPG as donors was still an option and so the oxidative cleavage aspect was replaced by a photocleavable alternative (Scheme 12). Thus, the Rich-Guwara linker 72A [40]was investigated. Success was had with a linear trimannan, and so a branched trimannan was tested, because this would be more demanding in that chemoselectivity deprotection would be required. This study is reported in Scheme 12. The Rich-Guwara photolinker was successfully coupled with diester 73, and chemoselective deacylation of the bound monosaccharide, 74a +74b, was found to proceed smoothly. Although coupling with donor 68b gave 15a in only moderate yields, ranging from 58-76'3'0, subsequent deprotection of the C6 acetate and iterative coupling with donor 68b proceeded without incident to give the resin-bound trisaccharide 76. Irradiation of the A-series trisaccharide required three days for recovery of optimum (10%) amounts of trisaccharide 77. On the assumption that this low yield was because of problems with the photocleavage reaction, a new linker, 72B, was designed. Advantages over 72A were (a) that linkage to the sugar was through a secondary, rather than primary, hydroxyl, and (b) that the electron-withdrawing amide carbonyl was remote from, rather than attached to, the benzene ring, where its deactivating effect would be irrelevant to the photocleavage event [41]. Indeed with 76B, the cleavage was very much faster and the overall yield of 77 was very much better [41]. An effort has also been made to synthesize polymer bound, fully deprotected saccharides by the couple-deprotect-couple iterative strategy [42]. A crucial problem with this approach was, however, that normally used benzyl groups, as in Scheme 12, would be problematic, because hydrogenolytic cleavage would involve a solid-solid interaction. Dissolving metal options were examined, but were also problematic in that the resin backbone apparently acted as an electron sink. Ester protecting groups were the only other option, and so concerns about their deactivating affect on the NPG donors were dispelled by success with preliminary experiments. The plan in Scheme 13 was therefore tested. The known NPG 78 [43] was converted into the differentially protected counterpart 79, and coupling with the polystyrene resin, A, led to 80a which reacted with 81 to give disaccharide 82a. Another decouple-couple iteration led to trisaccharide 83. Global deprotection by treatment with ethylenediamine in methoxyethanol at 70 "C for 4 h gave the fully deprotected bound trisaccharide 84.
6.7 Solid-Phase Iterative Couple-Deprotect-Couple Strategy
wI . -
P OD
\ .-
'-
I
+
Q
t
15 1
152
6 Glycosylution Methods: Use of n-Pentenyl Glycosides
..-
.-.-
;.i d
4 >
P
d 0 \
?
References
153
The sequence could be repeated with the tentage1 macrobeads, B, and the grafted crown linkers, C. The last was particularly valuable in that acetylation followed by photocleavage afforded 85, the identity of which was verified by independent synthesis [42].
References 1. For a concise review see: A. J. Bridges in Chem Tracts-Organic Chemistry, 1996, 9, 215-223. 2. B. Fraser-Reid, U. E. Udodong, Z. Wu, H. Ottosson, R. Merritt, C. S. Rao, C. Roberts and R. Madsen. Synlett, 1992, 927-942. 3. D. R. Mootoo, P. Konradsson, U. Udodong and B. Fraser-Reid. J. Am. Chem. SOC.,1988, 110, 5583-5584. 4. G. J. Boons, Tetrahedron, 1996, 52, 1095-1121. 5. M. L. Sinnott. Advances in Physical Organic Chemistry. 1988, 24, 113-204. 6. P. Deslongchamps. Stereoelectronic Efsects in Organic Chemistry. Pergamon, Oxford. 1983. 7. A. J. Ratcliffe, D. R. Mootoo, C. W. Andrews and B. Fraser-Reid. J. Am. Chem. Soc., 1989, I l l , 7661-7662. 8. C. W. Andrews, J. P. Bowen and B. Fraser-Reid. J. Chem. SOC. Chem. Comm., 1989, 19131916. 9. B. Fraser-Reid, Z. Wu, C. W. Andrews, E. Skowronski and J. P. Bowen. J. Am. Chem. Soc., 1991, 113, 1434-1435. 10. B. G. Wilson and B. Fraser-Reid. J. Org. Chem, 1995, 60, 317-320. 11. H . L. Frush and H. S. Isbell, J. Res. Nutl. Bur. Stds, 1941, 27, 413. 12. S. Oscarson and U. Tedebark. J. Carbohydr. Chem., 1996, 15, 507-512. 13. G. J. Boons, Contemp. Org. Syn., 1996, 3, 173-200. 14. J. F. King and A. D. Allbutt. Can. J. Chem., 1970, 48, 175441767, 15. R. U. Lemieux and N. J. Chu. Abstracts of Papers. Am. Chem. SOC., 1958,133, 31N. 16. A. F. Bochkov and G. E. Zaikov. Chemistry of the 0-Glycosidic Bond, Chapter 2, Pergamon Press, Oxford, 1979. 17. For some recent examples see: W. Wang and F. Kong. J. Org. Chem. 1998, 63, 5744-5745. L. Snaidman, C. Johnson, C. Crasto and S. M. Hecht. J. Org. Chem., 1995,560,3942-3943. 18. S. Hanessian and J. Banoub. Carbohydr. Res. 1977, 53 C13-Cl6. 19. A. S. Campbell and B. Fraser-Reid. J. Am. Chem. SOC.,1995,117, 10387-10388. 20. S. Hannesian and P. Lavallee, Can. J. Chem. 1975, 53,2975-2978 21. C. Roberts, C. L. May, and B. Fraser-Reid. Carbohydr. Lett., 1994, 1, 89-93. 22. C. Roberts, R. Madsen and B. Fraser-Reid. J. Am. Chem. Soc., 1995,117, 1546-1553. 23. C. A. A. van Boeckel and T. Beetz. Tetrahedron Lett. 1983,24, 3775. 24. J. R. Merritt, J. S. Debenham and B. Fraser-Reid. J. Carbohydr. Chem., 1996, 15, 65-72. 25. M. Meldal and K. J. Jensen. J. Chem. SOC.,Chem. Commun., 1990,483-484. U. K. Saha and R. R. Schmidt. J. Chem. Soc., Perkin I, 1997, 1855-1858. 26. J. G. Allen and Bert Fraser-Reid. J. Am. Chem. Soc., 1999, 121, 468-469. 27. Z. Wu, D. R. Mootoo and B. Fraser-Reid. Tetrahedron Lett., 1988, 29, 6549. 28. R. Madsen and B. Fraser-Reid. J. Chem. SOC.,Chem. Commun., 1994, 749-750. R. Madsen and B. Fraser-Reid. J. Org. Chem., 1995, 60, 772-779. 29. J. C. Lopez and B. Fraser-Reid. J. Chem. SOC.Chem. Comm., 1991, 159-161. 30. R. Lemieux, T. Takeda, B. Chung. Synthetic Methods for Carbohydrates (Ed. H. S. El Khadem, ACS Symposium Series, Washington, D. C., 1976, vol. 39, pp. 90-115. 31. J. S. Debenham, R. Madsen, C. Roberts, B. Fraser-Reid. J. Am. Chem. Soc., 1995,117, 33023303. 32. J. C. Castro-Palomino, R. R. Schmidt, Tetrahedron Lett. 1995, 36, 5343-5346. H. Shimizu, Y . Ito, Y. Matsuzaki, H. Iijima, and T. Ogawa. Biosci. Biotech. Biochem, 1996, 60, 73-76. 33. C. R. H. Raetz. J. Bacteriology, 1993, 175, 5745-5753. 34. S. L. Griffiths, R. Madsen, and B. Fraser-Reid. J. Org. Chem., 1997, 62, 365443658,
154
6 Glycosylation Methods: Use of n-Pentenyl Glycosides
35. P. T. Garegg. Pure Appl. Chem., 1984, 56, 845-858. 36. J. S. Debenham, S. D. Debenham, and B. Fraser-Reid. Bioorg. Med. Chem., 1996, 4, 19091918. 37. J. R. Merritt, E. Naisang and B. Fraser-Reid. J. Org. Chem., 1994,59, 4443-4449. 38. M. Bernard and W. T . Ford. J. Org. Chem., 1983, 48, 326-332. 39. J. M. Frechet and C. Schuerch. J. Am. Chem. Soc., 1972, 94,604-609. 40. D. H. Rich and S. K. Guwara. Tetrahedron Left., 1975, 16, 301-304. 41. R. Rodebaugh, B. Fraser-Reid and H. M. Geysen. Tetrahedron Lett., 1997,38,7653-7656. 42. R. Rodebaugh, S. Joshi, B. Fraser-Reid and H. M. Geysen. J. Org. Chem., 1997, 62, 56605661. 43. J. S. Debenham, R. Rodebaugh and Bert Fraser-Reid. Liebigs AnnJRecueil, 1997, 796-802.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
7 Glycosylidene Diazirines Andrea Vasella, Bruno Bernet, Martin Weber, and Wolfgang Wenger
7.1 Introduction Glycosylidene diazirines are useful as precursors of glycosylidene carbenes [I]. These cyclic, amphiphilic singlet alkoxycarbenes enable the formation of two bonds at the anomeric center. Although many aspects of the chemistry of pyranosylidene diazirines have been reviewed [2-51, furanosylidene diazirines [ 61 are hardly known because of their high reactivity and concomitant poor stability. Furanosylidene carbenes or carbenoids have been generated from the readily available lactone sulfonyl hydrazones [7].Whereas diazirines generate carbenes either thermally or photochemically under mild conditions, and simultaneously only produce dinitrogen, sulfonyl hydrazones, however, also generate potentially reactive side-products [7, 81. Glycosylidene carbenes, like other intermediates, are reactive and versatile. Their precursors-glycosylidene diazirines (= 1-azi-l,5-anhydroglycitols)-are thus expected to be useful. They have been devised primarily to serve as universal, highly reactive glycosylating reagents responding to the kinetic acidity of glycosyl acceptors, irrespective of their nucleophilic properties and of steric hindrance, and requiring no promoter. Their advantages will be discussed, as will their two disadvantages, uiz. their restricted stability, and their preparation which requires five (generally high-yielding) steps from hemiacetals.
7.2 Synthesis of Glycosylidene Diazirines The original route, depicted in Scheme 1, for the synthesis of the benzylated glucosylidene diazirine 7 [6, 91, is still used with minor variations, as evidenced by the preparation of the GlcNAc-derived analog 14 [3]. Details have been published, and some comments will prove useful.
156
7 Glycosylidene Diuzirines
BnO% BnO
i_ 1
B BnOn
B BnO n O
6 OBn N O
B BnO .
H
0
6 NHOH
BnO OH
O
h
2
%,
y
v
BnO BnO
BnO
Brio
7
iv
BnO& BnO BnO
NH
6
vi
BnO% BnO AcHN
NH
B BnO n 0
6
iii
N
O
H
AcHN
OH
8
/OR
E45 RR==MHs
B BnOn
~
N
O
6NHOH
AcHN
10
9
vii
c xF*n:niE
BnO% BnO y
ix
AcHN
AcHN
14
13
NH
&: ;B AcHN
/OR N
Scheme 1. Reagents: (i) Na, 96% EtOH, then NH3OH C1 and 1 , 7 h at 60°C, quant. (variable ratio of 2/3, ( E / Z )cu 65:35); (ii) NaOAc, NaI04, EtOH/H20, 19 h at 70°C, 79% from 1; (iii) pyridine, MsCl, CH2C12, 12 h at 0 ---t room temp., 87%; (iv) sat. NH3 in MeOH, CH2C12, 5 days at room temp., 76%; (v) MqN, 12, CH2C12, 2-5 h at -5O"C, 83%; (vi) Na, MeOH, then NH30HCl and 8, 33 h at 60"C, quant. (variable ratio of 9/10); (vii) NaOAc, NaI04, EtOH/H20, 18 h at 50°C, 70% from 8; (viii) Et3N, TsCl, CHzC12, 3 h at O T , 100%; (ix) sat. NH3 in MeOH, MeOH, 7 days at -2O"C, ca 70%; (x) Et3N, 12, CH2C12/MeOH, 3 h at O T , 68% from 12.
Formation of the oximes-generally obtained as ( E / Z )isomers and, most clearly [ 10, 111 (see also Ref. [ 121) and 2-acetamidofor 2,3,4,6-tetra-0-benzyl-~-glucose 3,4,6-tri-O-benzyl-2-deoxy-~-glucose oximes [3], as a mixture with the cyclic p-Dpyranosyl hydroxylamines (3 and 10, respectively)-proceeds readily at a pH of ca 6. Yields are high. The best procedure for the oxidation to the hydroximolactones depends on the structure of the oximes. Several oxidants have been tested. MnOz is generally reliable when prepared according to Attenburrow [ 131; commercial Mn02 has led to low yields [ 141.Oxidation is slightly exothermic. Fresh batches of MnOz might have to be added during the oxidation of large batches. Starting material and product have very similar RF values, but stain differently (H2S04, mostain) on the TLC
7.2 Synthesis of Glycosylidene Diazirines
157
plate. If the structure of the starting material enables formation of 1,4- and 1,5hydroximolactones, both might be formed [ 141. The relative amounts occasionally depend upon the oxidant (e.g. Hg(OAc)2 us 02/CuC12 in pyridine). NCS in the presence of DBU at low temperatures (ca -40°C) is a faster oxidant than MnO2. Low temperature is required for the oxidation of 0-acetyl oximes to avoid migration of the acetyl group from OC(4) to OC(5) [15]. Occasionally, NaI04/NaOAc has proven useful. 0-Sulfonylation of the hydroximolactones is unproblematic. Sulfonyl chlorides or anhydrides in the presence of base react rapidly at low temperature. The products often crystallize readily. A range of sulfonates has been tested [3]. Mesylates are often well suited; triflates are sometimes too unstable, although Beckmann rearrangement does not readily occur [ 141. Formation of the diaziridine requires a protic solvent; MeOH has been used as a standard. The concentration of NH3 required for the transformation depends on the nature of the sulfonyl group and on the acceptor properties of the 0-protecting groups. For the transformation of benzyl-protected sulfonates, MeOH must be saturated with NH3 at 0 "C in a thick-walled flask (the volume increases by ca 30% and the solution contains ca 24.5 g NH3 in 100 mL MeOH; it is stable for months at -20 "C); the reaction is performed at 22-26 "C, and the pressure is maintained by closing the flask with a septum; alternatively an autoclave may be used. The 2,3,4,60-pivaloylated glucose-derived hydroximomesylate reacts much faster [3]. The 2,3,4,6-0-acetylated glucose-derived hydroximotriflate reacted with half-saturated NH3/MeOH/CH*C12 at -20 "C without deacetylation [ 151. The diaziridines are ninhydrin-positive, stain dark blue with mostain, and oxidize KI to 12 in the presence of acid. They often crystallize from EtzO, and their purification by crystallization leads to improved overall yields. They are acid-labile; chromatography on silica gel must be rapid. It has led to slight losses even in the presence of base (Et3N); acetone is often a suitable (co)solvent. Crystalline diaziridines can be kept for months at -2O"C, and their solutions are stable in the absence of acid and oxygen. At 25°C and in the air, the diazirines turn red, presumably by forming increasing amounts of tetrazines. The diaziridines are usually obtained as mixtures of the two trans diastereoisomers, readily equilibrating in the presence of traces of acids. The ratio of the isomers depends on the glycosylidene residues. Oxidation of the diaziridines to diazirines requires some skill; anhydrous conditions are mandatory. Oxidation with I2 in the presence of a tertiary amine is the standard procedure. It was originally performed in MeOH and in the presence of 1-azi-glucose 7 crysEt3N. Under these conditions, the 2,3,4,6-tetra-O-benzylated tallizes from solution and can be isolated by filtration under anhydrous conditions [9]. For some diazirines, however, it is advantageous to perform the oxidation in CH2C12, using I2 and Me3N. The reaction corresponds to a titration, it is stopped when the solution remains slightly yellow. Because excess 12 is not readily removed, it is best to add a small amount of the starting diaziridine to discharge the color of the solution. The product is purified by filtration through a short Si02 column and thoroughly dried under high vacuum; a low boiling solvent (CH2C12) is recommended. On TLC, the diazirines stain purple with a solution of 4-(4-nitro-
158
7 Glycosylidene Diazirines
benzy1)pyridine; this is a specific and useful way of detection. Acyl-protected diazirines are more stable than their benzylated analogs and can be purified by flash chromatography at 0°C; solvents should be low boiling. The fractions are cooled, and condensation of water must be avoided [3, 151. The diazirines remain in the crystalline form or in solution for several weeks at -78 "C; 0-acyl protected diazirines can be stored at -20 "C.
7.3 Stability of the Glycosylidene Diazirines The stability of l-azi-l,5-anhydrohexitols depends on the substituents (protecting groups), the configuration, and the conformational flexibility. It has been quantified by thermolysis of the glycosylidene diazirines in methanol at 298 K [3, 15, 161; the results are given in Table 1. Their relative stability has been rationalized by postulating sequential (non-synchronous) heterolytic breakage of the C (1),N bond, leading to a cationic intermediate and requiring a conformational change of the pyranose ring whereby the pseudoequatorial C-N bond becomes pseudoaxial. Hence, acceptor substituents should increase the stability, and so should conformational locking, as by an intramolecular NH. . .OC(1) hydrogen-bond in the N-acetylallose derivative 25.
7.4 Glycosidation by Glycosylidene Diazirines 7.4.1 General Aspects Deprotonation of the glycosyl acceptor by a glycosylidene carbene ought to lead to a highly reactive, initially unsolvated ion pair, where both donor and acceptor are activated and should react readily with each other. Stereoelectronic effects dictate that deprotonation of the acceptor occurs in the a-plane of the carbene (more or less equivalent to the plane of the pyranoside ring), whereas addition of the oxyanion to the oxycarbenium cation has to occur in the mplane, i.e. more or less perpendicular to the a-plane, and preferentially from the axial direction (illustrated in Scheme 2 by the carbene derived from the benzylated glum-diazirine 7). Glycosidation by diazirines has been studied by using equivalent amounts of glycosyl acceptor and donor. It is expected that yields of the products increase with excess diazirine; occasionally this was shown to be so. Solvents have not systematically been optimized, but toluene, THF, and dioxane seem well suited; early experiments were mostly performed in dichloromethane. The course of the glycosidation of hydroxy compounds by glycosylidene diazirines is mainly dictated by four factors:
159
7.4 Glycosidution by Glycosylidene Diazirines Table 1. Kinetic parameters for the thermolysis of 1-aziglycosesin MeOH.
Diazirine
Compound
logA
15 R = Piv 16 R = AC 17 R = 4-F-Bn
23.0 25.0 21.6 23.7
13.4 14.1 11.6 13.8
33 202 181 43
22.4 24.0 21.0 23.1
1.7 4.8 -7.5 2.7
18
22.2
12.4
110
21.7
-3.2
19 R = B n 20 R = A c
23.2 20.5
14.2 11.4
I 45
22.6 19.9
5.5 -8.2
20.0
11.4
23
19.5
-8.2
-3.1
7 R = Bn
N
MeophT%r
BnO
AH= AS= (298 K) (298 K) [kcal [cal mol-' mol-'1 K-'1
E, [kcal mol-'1
z
(298 K) [min]
N
r *FCR
N
*::kS+yr
ND"
ND"
23
ND"
ND"
24
ND"
ND"
l4
22.6
12.6
112
22.0
25
28.1
15.1
4159
27.2
26 R = B n
20.8 22.3
11.4 12.1
88 202
20.2 21.7
BnO N OPiv PivO
Bn$Ss-,,
N
AcHN N
8.1
N
//"
RO OR
Ro
N
27 R = A c
-8.4 -5.0
160
7 Glycosylidene Diuzirines
Table 1 (continued) Diazirine
Compound
B n O - ' BnO f $ O a , N OBn BnO BnO
logA
21.8
12.2
T
(298 K) [min]
79
BnO BnO
OH
AS= (298 K) [cal mol-I K-'1
21.2
-4.8
- 100°C.
&:]
BnO% BnO
AH= (298 K) [kcal mol-'1
28
"Not determined. bVery unstable; slow decomposition even at
BnO
E, [kcal mol-l]
-
BnO
p-D
1
11 (OBn
(OBn
7
I
1
/OBn
I
OBn
BnO BnO
30
BnO
Scheme 2. Glycosidation of alcohols by gl ycosylidene carbenes derived from diazirines.
7.4 Glycosidution by Glycosylidene Diazirines
161
1) The kinetic acidity of the hydroxy group (inversely proportional to its basicity and more or less proportional to its nucleophilic properties). A sufficiently high kinetic acidity ensures rapid protonation of the carbene. Even an oxyanion derived from a poorly nucleophilic alcohol reacts readily with the simultaneously generated oxycarbenium cation. This property constitutes a major advantage of glycosylidene diazirines in the glycosidation of acids [ 171, phenols [9], highly fluorinated alcohols [ 181, hydroperoxides 1191, and hydrated Ti02 [ 151. Slow protonation of the carbene by a hydroxy compound of low kinetic acidity will lead to increasing amounts of azines (30 in Scheme 2), resulting from the interaction of the carbene with the starting diazirine. The azines are formed as a mixture of diastereoisomersthat are converted, by mild acid catalysis, mostly to the ( Z / Z )isomer [3]. 2) Hydrogen-bonding (a) Hydrogen-bonding influences the kinetic acidity of hydroxy groups. H-bond acceptors (that are not simultaneously hydrogen-bond donors) have enhanced kinetic acidity, whereas H-bond donors have reduced kinetic acidity; intramolecular hydrogen-bonding might thus determine the regioselectivity of deprotonation by the glycosylidene carbene-one of the conditions to be fulfilled for regioselective glycosidation. This effect has led to the regioselective glycosidation of dihydroxybenzene derivatives, where one of the phenolic OH groups functions as an intramolecular H-bond donor to a carbonyl group and is thereby glycosylated more slowly (Figure la) [9]. (b) Intra- or intermolecular hydrogen-bonding can also influence or determine the regio- and stereoselectivity of the ion pair combination resulting from deprotonation of the glycosyl acceptor. Whereas the kinetically most acidic OH group is deprotonated in the o-plane, a second OH group, hydrogen-bonded to the deprotonated OH group (and now linked by a H-bond to the newly generated oxyanionic center, Scheme 2) might be located in or close to the n-plane and attack the intermediate oxycarbenium ion. This effect has been demonstrated for a range of diols [20-251 and triols [26-291. It has also been evidenced for N-acetylglucosamine-deriveddiazirines, for which hydrogen-bonding to the NH group lead to the predominant formation of a-D-glycosides of sufficiently strong hydrogen-bond-accepting alcohols [301. 3) Solvation of the intermediate oxycarbenium ion. Solvation of the initially unsolvated or at least not optimally solvated oxycarbenium ion might involve the solvent, the glycosyl acceptor, or the C(2) substituent of the carbenium ion [18]. In an inert solvent, such as dichloromethane, solvation will occur by the (nucleophilic) glycosyl acceptor (inter- or intramolecularly) if it is sufficiently nucleophilic, or by the C(2)O-benzyl group for the less nucleophilic glycosyl acceptors such as phenols (as evidenced by studying the 2-deoxy analog 24 [31]). THF seems to solvate the carbenium ion well, and preferentially from the axial direction, leading preponderantly to equatorial glycosides. 4) In contradistinction to other methods of glycosidation, steric hindrance does not seem to be an important factor, as long as the hydroxy group is sufficiently acidic [ 18, 291. The following transformations illustrate these points.
162
A
7 Glycosylidene Diazirines
a)
b)
C)
F
3
C
Y
OGlcBn.,
/ OGlcBn, toluene: 79%, 8915
,,11..'
'...,,,
OGlcBn( toluene: 79%. 8416
CHzCI2: 34% (hv: 55%), 5050
dioxane, 25 "C: 76%. 95: 5 DME,25 "C 80%. 98: 2 EtCN, 25 "C: 5446,9010(16% of 31) EtCN, hv, -60 "C: 25%. 8713 (55% of 31)
31
e) CF3(CF2)5CH2CH~-OGlcBn, dioxane, 25 "C: 68%. 76:24
CHF~(CFZ)&H~~GICBQ toluene: 73% 6733
dioxane: 90%. 1:l mixture of p-D-isomers only
dioxane, 25 "C: 79%,78:22
CICH2CH2CI: 62%. 65:35 (4% of regioisorner, 4555)
Figure 1. Products of the reactions of the diazirine 7 with mono- and dihydroxy compounds: solvents, total yields, and p-D/a-D ratios (GlcBm = 2,3,4,6-tetra-0-benzyl-~-glucopyranosyl).
7.4.2 Glycosidation of Strongly Acidic Hydroxy Compounds Glycosidation of Phenols Glycosidation by the 1-azi-tetra-0-benzylglucopyranose7 proceeds in yields of approximately 75-80%, and leads mostly to the P-D-anomers [9]; presumably, the use of THF will increase the anomeric selectivity (Figure la, b). The analogous I -azimannose 19 yielded mostly a-D-mannosides, but has not been thoroughly studied [9].
7.4 Glycosidation by Glycosylidene Diazirines
163
The influence of intramolecular hydrogen-bonding is evident from the regioselective monoglycosidation of methyl orsellinate (Figure 1a) [9]. Upon addition of a second equivalent of diazirine, the chelated OH group is also glycosylated (67%,),although with less anomeric selectivity, as also observed for 4-nitrophenol. The insensitivity to steric hindrance is seen from the result of glycosylating 2,6-di(tert-butyl)-4-methylphenol(Figure 1b) [9]. Thiophenols react similarly to phenols; 4-methoxythiophenol yielding 70% of a D of thioglucosides [32]. 43 : 57 C X - D / ~ - mixture Glycosidation of Fluorinated Alcohols The influence of pK and the insensitivity to steric hindrance is seen from the glycosylation of fluorinated alcohols (Figure lc) [ 181. Glycosidation of trifluoroethanol, hexafluoro-2-propanol, and hexafluoro-tert-butanol by one equivalent of the diazirine 7 in dioxane yielded the anomeric glycosides in yields of 70-74% and in ~-D/.-D ratios ranging from 77:32 to 95:5. The solvent has an influence on the anomeric ratio, which changes for hexafluoro-tert-butanol from ~ - D / C X -= D 84 : 16 (75%) in dichloromethane to 98:2 (80%) in DME. The formation, upon glycosidation in propionitrile at -60°C, of the imino ether 31 (55%) besides the glycosides evidences the formation of an oxycarbenium cation, its solvation, and the preferred axial attack of the solvent. 3,3,4,4,5,5,6,6,7,7,8,8,-tridecafluorooctanoland of Glycosidation of CHF2(CF2)&H*OH by 7 in dioxane proceeded similarly and yielded 68 and 79% of the glucosides (P-D~CX-D = 76: 24 and 78 :22).
7.4.3 Glycosylation of Weakly Acidic Hydroxy Compounds Glycosidation of Monovalent Alcohols In contradistinction to the glycosylation of hexafluoro-tert-butanol, tert-butanol is glycosylated in only 34% yield under similar conditions (Figure 1d). Glycosylation at low temperature (photolytic generation of the carbene) doubled the yield, but had no influence on diastereoselectivity (IX-D/~-Dca 1 : 1). This dependence on the mode of generating the carbene is not general, and might reflect the temperaturedependence of the association of tert-butanol. Monoalcohols with o-acceptor substituents (such as partially protected monosaccharides) are expected to be more highly acidic and, hence, be glycosylated in higher yields. Indeed, 1,2: 5,6-di-0isopropylidene-a-D-glucofuranose yielded 75% of a 2 : 3 mixture of the corresponding anomeric disaccharides (Figure 1e) [ 181. The influence of acidity is also seen in the successful glycosidation of (hydrated) Ti02 [15]. The OH groups of hydrated Ti02 are characterized by pK values of 3.8 and 6.6. Exposure of Ti02 to benzylated or acetylated azi-glucoses 7 and 16 led to formation of a monomolecular layer of strongly (presumably covalently) bonded glucosyl residues (density 1.5 0.9 glucosyl moieties nm-2). The acetylated glucosyl moieties were deacetylated without removing the glucosyl moiety.
164
7 Glycosylidene Diazirines
Glycosidation of Diols and Triols Efsects of hydrogen-bonding The enhancing influence of an (intramolecular) hydrogen-bond on the kinetic acidity of the H-bond-accepting hydroxy group is seen from the (concentration independent!) result of the glycosidation of the myo-inositol-derived orthoester (Figure I f ) [26]. The two diastereoisomers derived from monoglycosylation of one each of the two enantiotopic hydrogen-bonded OH groups were obtained regio- and diastereoselectively in high yields. The low reactivity of the isolated equatorial OH group is because of a (bifurcated) hydrogen-bond. This effect also operates in the regioselective glycosidation at OC(2) of methyl 4,6-O-benzylidene-a-~-altropyranoside, yielding 62% of a 2: 1 mixture of anomers in dichloroethane (Figure lg) [20]. As expected, the intramolecular hydrogen-bond reduces the kinetic acidity of HOC(3), but enhances its nucleophilicity and leads to glycosylation at OC(3) bromide under Lemieux halogenwith 2,3,4,6-tetra-O-benzyl-a-~-glucopyranosyl exchange conditions, resulting in 68% of the a-D-configured, 1,3-1inked disaccharide (and 10% of the anomeric regioisomers). Combined efsects of intramolecular hydrogen-bonding and of stereoelectronic control The intramolecular hydrogen-bond between HOC(3) and the anomeric methoxy group in methyl 4,6-0-benzylidene-a-~-allopyranoside is expected to lead to protonation of the carbene mostly by the kinetically more highly acidic HOC(2) (Figure 2a) [22]. This is presumably taking place. In contradistinction to the altro-diol, however, the two OH groups are cis-oriented. Stereoelectroniccontrol requires that protonation occurs in the o-plane of the carbene and attack of the nucleophile on the ensuing oxycarbenium cation proceeds in the n-plane. HOC(3) is much closer to the x-plane than -0C(2). Glycosylation in toluene at 70 "C or in dioxane at 24 "C indeed leads mostly to the a-D-anomer of the 1,3-linked disaccharide [22]. The predominant a-D-configuration reflects the stereoelectronic control. Replacing these solvents by the more nucleophilic THF at -85°C leads to participation of the solvent. THF competes with
toluene: 79%.
C(2)0/C(3)0 2080 (68:32and 28:72)
CH2C12,-85", hv: 81%, C(2)0/C(3)O/C4)05:81:14(60:40,23:77, and 3664)
THF, -85", hv: 79%, C(2)0/C(3)0 7228 (9010and 3367)
Figure 2. Glucosylation of (a) methyl 4,6-O-benzylidene-a-~-allopyranoside and (b) benzyl ribopyranoside by the diazirine 7:solvents, total yields. regioselectivity, and p-D/a-Dratios.
p-D-
7.4 Glycosidution by Glycosylidene Diuzirines
165
Scheme 3. Preferred formation of the 3-O-P-~-glucosylateddimer in the reaction of 7 with benzyl (3-D-ribopyranoside.
HOC(3) in attacking the intermediate oxycarbenium ion, again from the axial di~ of the 1,2-1inked disaccharide! rection, and the main product is now the p - anomer Similarly for benzyl P-D-ribopyranoside, preferential protonation of the carbene ought to occur by either HOC(2) or HOC(4), but HOC(3) is in a better position to attack the carbenium ion from an axial direction. The main product, isolated in 65% yield, is the a-Danomer of the 1,3-linked disaccharide (Figure 2b) [28]. The rationalization of that reaction is illustrated in Scheme 3. In THF at -78 "C, however, the main products are the p-D anomers of the 1,2- and 1,4-linked disaccharides. More examples of regioselective glycosidations resulting from the combination of these factors have been published and a particularly striking effect of hydrogenbonding is provided by the glycosidation of the N-phthaloylated allosamine derivatives. The dominant product of the a-D-configured diol is the 1,3-linked disaccharide (Scheme 4a) [23]. The anomeric configuration suggests that the regioselectivity is the result of a protonation by HOC(4) and an interception of the carbenium ion by HOC(3); indeed, ' H NMR analysis showed the presence of a very strong hydrogen-bond between HOC(3) and the phthaloyl group, and a weaker hydrogen-bond between HOC(4) and HOC(3). The former hydrogen-bond strongly reduces the kinetic acidity of HOC(3), the latter orients the protonating HOC(4) close to OC(3). In agreement with this, the a-D-configured monoalcohol is unreactive (Scheme 4b), whereas its anomer is glycosylated (Scheme 4c), because the N-phthaloyl group now adopts a different conformation; also the anomeric diol is no longer regioselectively glycosylated (Scheme 4d). The results of the glycosylation of the a-D-configured trio1 (Scheme 4e) evidences that the weakest hydrogen-bond is that from HOC(4) to HOC(3)! Intramolecular solvation of the intermediate oxycarbenium ion has also been postulated to explain the high-yielding glycosylation of the tertiary hydroxy group
7 Glycosylidene Diazirines
166 a)
80%, 90:lO
no reaction
C)
*Thp
OAll Bn4Glc0
NFt
66%,2 1 3 9
4
CHIOBn H
o ~ HO
O
A
l
l
B n 4 G l f : N OAll + OH BqGlcO
29%.a 6 5
33%,39:61
B n 4 G l c ? !
HO FNOAll
HO FtNOAll 9%,(only p-D)
OAll NFt
+
E N Bn4Glc0 FtN OAll 51%. 72:28
Scheme 4. Glucosylation of anomeric N-phthaloylatedallosamine derivatives by one equivalent of the diazirine 7 in dioxane at room temperature: total yields and a-D/p-D ratios.
of 10-0-acetylginkgolide A by a lactose-derived diazirine (Scheme 5a) [29]; intramolecular participation of the carbonyl group of the adjacent lactone ring is presumably playing an important role also in the glycosylation of ginkgolide B, where reaction with 3.5 equivalents of 7 in THF yielded 42% of the all-P-D-configured trisaccharide (Scheme 5b) [29]. Thus, regioselective glycosidation of diols and triols by diazirines is feasible and often strikingly successful. Its course can be predicted, even if the factors are complex, if the hydrogen-bonds in the glycosyl acceptor are known. Analytically, the results of glycosylation provide important clues to the location of hydrogen-bonds (Figure 3a), e.g. evaluation of the influence of weak intramolecular OH. . .F-C hydrogen-bonding in apolar solvents (Figure 3b) [24, 251. There is yet another factor, operative in carbenes derived from 1-azi-2-acetamido-2-deoxy sugars that can occasionally be exploited. Efect of intermolecular hydrogen-bondingfrom the diazirine to the acceptor Surprisingly, thennolysis of the N-acetylglucosamine- and N-acetylallosamine-derived diazirines 14 and 25 in methanol led predominantly to the 1,2-cis-configured
7.4 Glycosidation by Glycosylidene Diuzirines
Me
167
28 (1.3 equiv.)
HO 3
THF, 25°C
0 92% of B-D-anomer
b'
0
N
Me
-
Me
7 (3.5 equiv.) THF, 25°C
42%
R = GlcBn, p+l p+3 p-10
15% p+l p-3 a+10 13% a+l p+3 p-10 24% diglucosides no rnonoglucosides
Scheme 5. (a) Lactosylation of 10-0-acetylginkgolide A by 28. (b) Glucosylation of ginkgolide B by 7.
a-r>-glycosides; similarly, glycosidation of 2-propanol (2 equivalents) by 14 at -84 "C yielded 70% of the a-D-glycoside (no 0-D-anomer observed) and 13% of the dihydrooxazole [30]. At 26 "C, the dihydrooxazole (resulting from protonation of the carbene by the acetamido group) became the major product. Under analogous conditions (-84 "C), hexafluoro-2-propanol yielded between 58 and 9 1% glycosides, depending on the excess of alcohol (1 to 10 equivalents), the anomeric ratio varying between 72 :28 and 80 :20. Glycosidation at 26 "C of 2 equivalents of hexafluoro-2propanol led to higher yields of glycosides (72 compared with 59%) and higher anomeric selectivity (.-D/~-D = 88 : 12). Glycosidation of the still more highly acidic
Figure 3. Intramolecular hydrogen-bonding in (a) ally1 2-deoxy-2-phthalimido-a-~-allopyranoside and in (b) L-4-deoxy-4-fluoro-1,3,5-pentyIidyne-myo-inositol.
168
7 Glycosylidene Diazirines
Figure 4. Intermolecular hydrogen-bonding of the carbene derived from the GlcNAc diazirine 14.
4-nitrophenol (2 equiv., 26 "C) reduced the anomeric selectivity to 58 :42 (87% yield). These and analogous observations made by glycosylating the A11NAc-derived diazirine 25 were rationalized by postulating an intermolecular hydrogen-bond from the NHAc group to the glycosyl acceptor (Figure 4). This hydrogen-bond locates the alcohol below the plane of the pyranose ring, well positioned to attack the oxycarbenium and form a-D-glycosides. The hydrogen-bond depends on the conformation of the acetamido group and on the basic properties of the alcohol (higher selectivity with the less acidic, i. e. more strongly basic 2-propanol). The conformation of the acetamido group, in its turn, depends on the configuration (glum or d o ; for a detailed study of the conformational behavior of the NHAc group see Ref. [33]) and the temperature (higher c ~ - D / ~ - Dratio at a higher temperature). The temperature-dependent association of the alcohols explains the much higher yield of the glycosidation of 2-propanol at lower temperature, influenced by the higher basicity of 2-propanol than hexafluoro-2-propanol (better H-bond acceptor) and by the greater acidity of the hydrogen-bond-accepting terminal alcohol unit in a chain of associated alcohols properly located in the o-plane of the carbene (Figure 4). Protonation of the carbene, which depends on the kinetic acidity of the alcohol, is reflected by the higher yields of the hexafluoro-2-propanol- and 4-nitrophenol-derived glycosides. These results confirm the strong influence of hydrogen-bonds on the result of glycosidation and characterize these diazirines as subtle, reactivity-based tools for the investigation of intra- and intermolecular hydrogen-bonds and their influence on regio- and diastereoselective glycosidation.
7.5 Synthesis of Spirocyclopropanes Glycosylidene diazirines react with electrophilic alkenes, e.g. N-phenylmaleimide, acrylonitrile, dimethyl fumarate, dimethyl maleate, and C&-fullerene, to yield spirocyclopropanes with formation of two C-C bonds at the anomeric center [34-371.
7.5 Synthesis of Spirocyclopropanes
R = Bn: So%, 9O:lO R = Piv: 60%. 92: 8
RO
RO
.
,
"'v
169
RO
y0N , P h
Ph
0
22%
BBnO n BnO
70%
Bn ~O- n 0
BnO
52
BnO
BnO Me02C
BBnO n BnO NC
6
,,C02Me
fCo2r"72%
q ..*>CN
CN
Brio%,
C)
O
O OBn CN q BnO
11
31
Brio%. %
Brio BnO
Me BnO BnO
,,C02Me BnO BnO
BnO C02Me
,
C02Me
BnO Me02C
60
40
-
-
29
20
33
1s
le02C
C02Me
[
: 60%
C02Me
d'
z:*,
RRO O+]. RO
'%O
R=Bn:
33%
R=Piv:
72%
Me
24%
'$o
b 9%
14%
Figure 5. Products of the reaction of the benzylated and pivaolyated diazirines 7 and 15 with (a) N-phenylmaleimide, (b) acrylonitrile, (c) dimethyl fumarate and dimethyl maleate, and (d) dihyand 2,3-dihydro-4H-pyran. drofuran, 5-methyl-2,3-dihydrofuran,
Yields depend on the protecting groups; substitution of benzyl by pivaloyl groups reduces yields, in agreement with a reduction of the predominantly nucleophilic character of these ambiphilic carbenes. As illustrated by the cyclopropanation of N-phenylmaleimide, the diastereoisomer with the substituents oriented syn to the glycosyl ring oxygen are formed preferentially (ratio = 90: 10; Figure 5a); cyclopropanation with acrylonitrile confirms this (ratio = 58 :42) and shows that the nitrile function is predominantly located below the average plane of the pyranosyl ring (ratio = 83: 17; Figure 5b). This is confirmed by cyclopropanation of ethyl acrylate [38]; the major isomer, formed in ca 47% yield, has the same configuration as the major isomer resulting from addition to acrylonitrile. The stereoselectivity of cyclopropanation with dimethyl fumarate and dimethyl maleate (Figure 5c) suggests that formation of the two C-C bonds is not concerted; it is rationalized by
170
BnO
7 Glycosylidene Diuzirines
-
BnO BnO
EN pseudoaxial approach
pseudoequatorial approach
Scheme 6. Pseudoaxial and pseudoequatorial. ring closure of the zwitterion obtained upon reaction of acrylonitrile with the carbene derived from 7.
postulating a two-step process with a preferred pseudoaxial trajectory in the formation of the second C-C bond, as illustrated in Scheme 6. In agreement with the ambiphilic character of alkoxycarbenes, glycosylidene carbenes form cyclopropanes also with electron-rich alkoxyalkenes, provided their reactivity is enhanced by ring tension, such as that in dihydrofuran, that leads to 33% (benzylated diazirine 7) or 72% (pivaloylated diazirine 15) of a single isomer (Figure 5d) [39]. Steric hindrance by an additional methyl group reduces the yield. Dihydropyran reacts less readily, and open chain enol ethers lead to low yields only. The 0-benzyl protected diazirines gave lower yields than the 0-pivaolylated analogs. The transition states of the cycloadditions leading to the isomeric cyclopropanes have been calculated (semiempiric program AM1). At the beginning of the reaction, the interaction of the HOMO of the carbene with the LUMO of the enol ether dominates, but in the transition state the interaction between the LUMO of the carbene and the HOMO of the enol ether is more important. The cyclopropanation yields of the pivaloylated diazirine 15 are higher because of the more efficient competing reaction of the 0-benzylated (as compared to the 0-pivaloylated) carbene with the starting diazirine. Azines are isolated as the main by-products. Calculations show that a pseudoaxial attack of the carbene is favored.
7.6 Addition to Aldehydes and Ketones Addition of the benzylated aziglucose 7 to acetone or cyclohexanone, used as solvents, gave 63 and 78%, respectively, of a mixture of anomeric spiroepoxidesin P-D/ a-Dratios of 70 :30 and 65 : 35, respectively (Figure 6a), whereas addition of the A11NAc-derived diazirine 25 to cyclohexanone at 60°C gave 83% of the a-Dconfigured spiroepoxide and a dihydrooxazine formed either directly from the intermediate oxycarbenium cation or by opening the anomeric oxirane (Figure 6b) [40].Similarly, addition of 25 to a threefold excess of benzaldehyde gave a mixture of &,trans-related a-D-spiroepoxides (27%), epimeric dihydrooxazoles (1 2%; formally derived from the epimeric epoxides), and a range of by-products. Such spiroepoxides have been prepared in a convenient way by Nicotra and Panza [41-431.
7.7 Explorutory Use of Diuzirines
171
b)
a)
BnO
78%
83%
35
60
40
Me
Figure 6 . Products of the reactions of (a) the diazirine 7 and (b) the diazirine 25 with cyclohexanone: total yields and product ratios.
7.7 Exploratory Use of Diazirines: Formation of Glycosyl Phosphines, Stannanes, N-Sulfonylamines, Esters, Boranes, and Alanes, and of 1,l-Difluorides The high reactivity of glycosylidene carbenes enables rapid access to the first representatives of intermediates or products for which more convenient routes might subsequently be developed. Thus, the benzylated aziglucose 7 reacted with Ph2PH under thermal conditions to give anomeric glycosyl phosphines that were isolated as the corresponding phosphine oxides in yields of up to 76% (Figure 7a) [44]. The u - D / ~ - Dratios of anomers varied between 5 1 :49 and 67 :33. Generation of the carbene at -78 "C (hv) led in 55% yield to a 74:26 mixture of anomers. Glycosyl phosphine oxides were then prepared from glycosyl acetates by TMSOTf-catalyzed treatment with MeOPPh2 and subsequently reduced to glycosyl phosphines [45].
b)
a)
BnO BnO
PPhZ OBn
16%, 55:45
Bd-0-
SnBuj
SdU3
OBn
70%. 84:16
53%. 4357
C)
."in-&+,
BnO BnO &NH.,.s OBn
50%. 2575
BnO
45%
oyyco"n ri'
NH-Bw
Figure 7. Products of the reactions of (a) the diazirine 7 with diphenylphosphine, (b) the diazirines 7 and 19 with tributylstannane, and (c) the diazirine 7 with 4-toluenesulfonamide and N-Bocasparagine benzyl ester: yields and a-D/p-D ratios.
172
7-
7 Glycosylidene Diazirines
BR3
Scheme 7. Expected primary products of the reaction of diazirine 7 with trialkylboranes.
The benzylated aziglucose 7 and azimannose 19 react with tributyl or triphenyl stannane and yield glycosyl stannanes (Figure 7b) [46]. The 1,2-cis-configured anomers are formed preferentially and the 1-C-stannylated 2-0-benzyloxyglucal is the main by-product. Advantageous preparations of glycosyl stannanes-useful precursors of glycosyl lithium compounds-from glycosyl halides have been described by Kessler et al. [47-501. Exploratory experiments have shown that glycosylidene carbenes also insert into the N-H bond of sulfonamides, whereas carboxamides yield esters, as illustrated by the product obtained from an N-Boc-asparagine benzyl ester (Figure 7c) [32]. Presumably, such esters are formed by hydrolysis of the initially formed imino ether 0-glycosides, suggesting that glycosylidene diazirines may be used for (selective?) cleavage of peptidic bonds. Nucleophilic addition of glycosylidene carbenes to triligated boron compounds followed by migration of a boron substituent to the glycosidic center should lead to anomeric glycosyl boranes (Scheme 7).
Bnb,!,, BBu3:
""
Bnd
45%
20%
62%
-
C>
B-Bu:
1) BEt3.2) : ' H
B BnO n
O
h
E
t BnO
BnO HO' 'Et 1 ) BEt, 2) PPh3.02: 25%
BnO BnO
0
46%
WC'
4
C)
'Et
55%
46
:
BnO
54
Figure 8. Products of the reaction of diazirine 7 with (a) tributylborane and 2-butyl-l,3,2-dioxaborinane, (b) triethylborane followed by acid treatment, (c) triethylborane followed by oxidation, and (d) the 9-BBN-derived phenethylborinate.
7.7 Exploratory Use of Diazirines -0Bn
D*OnB R = Me: R = 'Bu:
BnO
,OBn
BnOR
30% 20%
173
R
30% 25%
Figure 9. Products of the reaction of diazirine 7 with trialkylalanes followed by deuterolysis.
Reaction of 7 with tributyl-, triethyl-, and triphenylborane, followed by oxidation, yielded the C (1)-alkylated hemiacetals and the C(1)-alkylated glycals (Figure 8a) [51, 521. The cyclic 2-butyl-l,3,2-dioxaborinane led only to the hemiacetal. The presence of the intermediate (anomeric?) diethyl glycosyl boranes was evidenced by treating them with trifluoroacetic acid, leading to migration of a second ethyl group to C(l) and formation of a cyclic stable borinate (Figure 8b). Oxidation of the intermediate glycosyl borane by oxygen in the presence of excess triphenylphosphine led to a stable ethylglycosylborinic acid (Figure 8c). We took advantage of the higher resistance to oxidation of borinates, compared with boranes, by treating the aziglucose 7 with the 9-BBN-derived phenethyl borinate (Figure 8d). The configurations of the anomeric borinates was assigned on the basis of NOE data. Trialkylalanes, similarly to trialkylboranes, react with the aziglucose 7 leading to bona jide glycosyl alanes, as was proved by deuteration to yield the axial C-glycoside and the C(1)-alkylated glucal (Figure 9) [52]. Finally, because the 1,l-difluoro-1-deoxy derivative of cellobiose was required for kinetic studies of cellobiohydrolases, we examined the reaction of the corresponding diazirine with XeF2. A method for the synthesis of monosaccharide-derived anomeric difluorides has been described by Praly [53,541, but seems unsuitable for the synthesis of disaccharide-derived analogs. Treatment of the cellobiose-derived diazirine 26 with XeF2 in dichloromethane yielded a maximum of 22% of the desired difluoride and large amounts of azines (Scheme 8) [ 161. In the presence of small amounts of MeOH, these azines were replaced by, mostly, the equatorial monofluoride. These results show that the reactivity of glycosylidene carbenes towards XeF2 is not pronounced, but sufficient to furnish the amounts of the desired difluoride required for the enzymatic tests [ 5 5 ] .
26
R=F R=H
22%
Scheme 8. Reaction of the cellobiose-derived diazirine with xenon difluoride.
174
7 Glycosylidene Diuzirines
Acknowledgments
We thank the Swiss National Science Foundation and F. Hoffmann-La Roche AG, Basel, for continuous generous support.
References 1. A. Vasella, K. Briner, N. Soundararajan, M. S . Platz, . I . Org. Chem., 1991, 56, 4741-4744. 2. A. Vasella, in Topics in Bioorganic and Biological Chemistry (Ed.: S. M. Hecht), Oxford University Press, Oxford, 1999, pp. 56-88. 3. A. Vasella, C. Witzig, C. Waldraff, P. Uhlmann, K. Briner, B. Bernet, L. Panza, R. Husi, Helv. Chim. Acta 1993, 76, 2847-2875. 4. A. Vasella, Pure Appl. Chem. 1993, 65, 731-752. 5. A. Vasella, Pure Appl. Chem. 1991, 63, 507-518. 6. K. Briner, A. Vasella, Helv. Chim. Acta 1989, 72, 1371-1382. 7. S. E. Mangholz, A. Vasella, Helv. Chim. Acta 1995, 78, 1020-1035. 8. S. E. Mangholz, A. Vasella, Helv. Chim. Acta 1991, 74, 2100-2111. 9. K. Briner, A. Vasella, Helv. Chim. Acta 1990, 73, 1764-1778. 10. B. Aebischer, Thesis, Universite de Fribourg No. 854, 1983. 11. D. Beer, Thesis, Universitat Zurich, 1989. 12. P. Finch, Z. Merchant, J. Chem. SOC.,Perkin Trans. I 1975, 1682-1690. 13. J. Attenburrow, A. F. B. Cameron, J. H. Chapman, R. M. Evans, B. A. Hams, A. B. A. Jansen, T. Walker, J. Chem. SOC.1952, 1094-1111. 14. D. Beer, A. Vasella, Helv. Chim. Acta 1985, 68, 225442274, 15. M. Weber, A. Vasella, M. Textor, N. D. Spencer, Helv. Chim. Acta 1998, 81, 1359-1372. 16. M. Weber, Thesis, ETH-Zurich No. 13212, 1999. 17. K. Briner, A. Vasella, unpublished results. 18. K. Briner, A. Vasella, Helv. Chim. Acta 1992, 75, 621-635. 19. K. Briner, Thesis, Universitat Zurich, 1995. 20. E. Bozo, A. Vasella, Helv. Chim. Acta 1992, 75, 2613-2633. 21. P. R. Muddasani, E. Bozo, B. Bernet, A. Vasella, Helv. Chim. Acta 1994, 77, 257-290. 22. P. R. Muddasani, B. Bernet, A. Vasella, Helv. Chim. Acta 1994, 77, 334-350. 23. K. Briner, B. Bernet, J. L. Maloisel, A. Vasella, Helv. Chim. Acta 1994, 77, 1969-1984. 24. A. Zapata, B. Bernet, A. Vasella, Helv. Chim. Acta 1996, 79, 1169-1191. 25. M. A. Biamonte, A. Vasella, Helv. Chim. Acta 1998, 81, 695-717. 26. P. Uhlmann, A. Vasella, Helv. Chim. Acta 1992, 75, 1979-1994. 27. E. Bozo, A. Vasella, Helv. Chim. Acta 1994, 77, 745-753. 28. P. Uhlmann, A. Vasella, Helv. Chim. Acta 1994, 77, 1175-1 192. 29. M. Weber, A. Vasella, Helv. Chim. Acta 1997, 80, 2352-2367. 30. A. Vasella, C. Witzig, Helv. Chim. Acta 1995, 78, 1971-1982. 31. Y. Takahashi, A. Vasella, Helv. Chim. Acta 1992, 75, 1563-1571. 32. T. Rajamannar, A. Vasella, unpublished results. 33. P. Fowler, B. Bernet, A. Vasella, Helv. Chim. Acta 1996, 79, 269-287. 34. A, Vasella, C. A. A. Waldraff, Helv. Chim. Acta 1991, 74, 585-593. 35. A. Vasella, P. Uhlmann, C. A. A. Waldraff, F. Diederich, C. Thilgen, Angew. Chem. 1992,104, 1383-1385. 36. P. Uhlmann, E. Harth, A. B. Naughton, A. Vasella, Helv. Chim. Acta 1994, 77, 2335-2340. 37. U. Jonas, F. Cardullo, P. Belik, F. Diederich, A. Gugel, E. Harth, A. Herrmann, L. Isaacs, K. Mullen, H. Ringsdorf, C. Thilgen, P. Uhlmann, A. Vasella, C. A. A. Waldraff, M . Walter, Chem. Eur. J. 1995, I , 243-251. 38. C. Bluchel, A. Vasella, unpublished results. 39. C. Waldraff, B. Bernet, A. Vasella, Helv. Chim. Acta 1997, 80, 1882-1900. 40. A. Vasella, P. Dhar, C. Witzig, Helv. Chim. Acta 1993, 76, 1767-1778.
References 41. 42. 43. 44. 45. 46. 41. 48. 49. 50. 51. 52. 53. 54. 55.
115
L. Lay, F. Nicotra, L. Panza, G. Russo, G. Sello, J. Curbohydr. Chem. 1998, 17, 1269-1281. L. G. Lay, F. Nicotra, L. Panza, G. Russo, Synlett 1995, 167-168. F. Nicotra, L. Panza, G. Russo, Tetrahedron Lett. 1991, 32,4035-4038. A. Vasella, G. Baudin, L. Panza, Heteroat. Chem. 1991, 2, 151-161. A. Lopusinski, B. Bernet, A. Linden, A. Vasella, Helv. Chim. Acta 1993, 76, 94-112. P. Uhlmann, D. Nanz, E. Bozo, A. Vasella, Helu. Chim. Acta 1994, 77, 1430-1440. F. Burkhart, M. Hoffmann, H. Kessler, Tetrahedron Lett. 1998, 39, 7699-7102. M. Hoffmann, H. Kessler, Tetrahedron Lett. 1997, 38, 1903-1906. M. Hoffmann, H. Kessler, Tetrahedron Lett. 1994, 35, 6067-6070. 0. Frey, M. Hoffmann, V. Wittmann, H. Kessler, P. Uhlmann, A. Vasella, Helu. Chim. Actu 1994, 77, 2060-2069. A. Vasella, W. Wenger, T. Rajamannar, Chem. CommuM. 1999, 2215-2216. W. Wenger, A. Vaseila, Helv. Chim.Acta, submitted. J. P. Praly, L. Brard, G. Descotes, L. Toupet, Tetrahedron 1989, 45, 4141-4152. J. P. Praly, G. Descotes, Tetrahedron Lett. 1987, 28, 1405-1408. D. Becker, K. S. H. Johnson, A. Koivula, M. Schiilein, M. L. Sinnott, Biochem. J. 2000, 345, 315-319.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
8 Glycosylation Methods: Alkylations of Reducing Sugars Jun-ichi Tamuru
8.1 Introduction Methods for glycosylation generally contain a procedure for activation of the anomeric position. Scheme 1 shows two possible procedures for formation of a glycosidic linkage. Conventionally, the specific affinity of a leaving group at the anomeric position and a suitable promoter accelerates a coupling reaction, as described in the previous chapter. As a result a glycosidic bond is formed by nucleophilic attack of aglyconic oxygen (b). We can also form the glycosidic linkage by nucleophilic attack of anomeric oxygen (a). This glycosylation procedure retains the anomeric oxygen (a) in the glycosidic bond. The glycosylation methods referred to in the title of this chapter are based on the latter concept. R. R. Schmidt and P. Kovac introduced the reverse concept to glycosylation methods and developed ‘anomeric O-alkylation’ and ‘glycosylation via locked anomeric configuration’, respectively. These methods, of which general procedures are shown in Scheme 2, use different approaches for stereocontrol of glycosidic bond formation. The ‘anomeric O-alkylation’ method utilizes an anomeric sodium or potassium alkoxide as a nucleophile. Stereoselectivity in the coupling reaction depends on the different reactivities of the anomeric c1 and anions. Factors controlling the reactivity, such as conformation of the nucleophile, are described in detail in Section 8.2. Review articles by Schmidt have described the ‘anomeric O-alkylation’ method [ 1-31. ‘Glycosylation via locked anomeric configuration’ employs a stannylene acetal at the 1,2-position of the hemiacetal. The stannylene acetal fixes the anomeric oxygen in the 1,2-cis configuration and leads to 1,2-ci.stype glycoside. Details are described in Section 8.3.
8.2 Anomeric O-Alkylation In addition to temperature, solvent, promoter, and additive, we must consider the configuration of the nucleophile (including roles of substituent groups) and the re-
178
8 Glycosylution Methods: Alkylations of Reducing Sugars
Electrophile (Donor)
Nncleophile (Acceptor)
L
_-___-__ . I
Scheme 1. Conventional and new glycosylation methods. L = leaving group; M = metal atom.
11
m* Electrophile
Nucleophile
I
"Anomeric0-alkylation"
Nucleophile
1
Glycoside
I
J
I "GlycosylationvM locked anomeric configuration" I Scheme 2. General procedures for 'anomeric 0-alkylation' and 'glycosylation via locked anomeric configuration'.
activity of the electrophile (primary compared with secondary triflates). This section describes the factors that lead to the stereoselective coupling reaction.
8.2.1 Anomeric 0-Alkylation of Ribofuranose with Primary Triflates: Effect of the Protecting Group at 0 - 5 of Ribofuranose Schmidt and Reichrath performed the anomeric 0-alkylation of ribofuranose with primary triflates with complete stereocontrol as shown in Scheme 3 [4]. Ribofur-
8.2 Anomeric 0-Alkylation
179
TBDMSCI 0 0
KOtBu ITHF
I
0'
.\c (2)
I (4) 89 %
(5) 86 %
I
.\c
\I*
Scheme 3. Stereoselective syntheses of Ribfu and p(1-5)Ribf and plausible complex formation. S = solvent molecule; TBDMS = t-butyldimethylsilyl.
anose 2,3-O-isopropylidene acetal (1) and the corresponding 5-0-TBDMS derivative (2) were selected as nucleophiles. Reaction of these nucleophiles with primary triflate (3) stereoselectively gave a- and P-linked disaccharides (4 and 5), respectively, in high yields. Promoter and solvent seem not to have a significant effect on the reaction, although the substituent group at 0 - 5 of ribofuranose might be important for stereocontrol. Plausible intermediates explaining this specific reaction are shown in Scheme 3, bearing in mind the steric influence of the protecting group. 8.2.2 Anomeric 0-Alkylation of Mannofuranose with Primary Triflates: The Crown Ether Effect
Schmidt et al. reported an effect of a crown ether on the glycosylation of mannofuranose. Scheme 4 shows an example of stereoselective coupling of mannofuranose 2,3 : 5,6-di-O-isopropylideneacetal (6)with primary triflate (7) in the presence of NaH [5, 61. The coupling reaction in the absence of the crown ether afforded only P-glycosides (9). Mannofuranose is different from ribofuranose in having two endo hydroxyl groups at the 2,3-positions. Thus mannofuranose could be a better ligand for complex formation. In particular, the p anion preferentially forms a complex
180
8 Glycosylution Methods: Alkylutions of Reducing Sugars
r
1
1
(10)
Scheme 4. Anomeric 0-alkylation of mannofuranose and effect of crown ether.
(11) as speculated in Scheme 4. Addition of 18-crown-6 afforded the a-glycoside (8) exclusively. It is thought that the crown ether inhibited endo-type complex formation and equilibrium moved to favor the a-anion (lo), yielding the a-glycoside. Addition of NaI resulted in an equimolar anomeric mixture of the glycosides. Perhaps partial complex formation of 18-crown-6with the sodium ion was suppressed. 8.2.3 Anomeric 0-Alkylation of Gluco- and Galactopyranoses with Primary Triflates: High fl-Selectivity as a Result of the Reactive Anomeric p-Anion Schmidt et al. examined the reactivities of electrophile and nucleophile in the anomeric 0-alkylation of 2,3,4,6-tetra-0-benzylglucose (12) [7]. Alkylation at room temperature gave decanoyl a- and P-glucosides in 7 :3 ratio, which reflected the anomer ratio of the hemiacetal in THF (2: 1). On the other hand, alkylation with less reactive methyl triflate at room temperature afforded a 2: 8 mixture of methyl a- and P-glycosides. When the reaction is performed with the less reactive electrophile, stereoselectivity reflects the reactivities of the nucleophiles instead of the original anomer ratio of hemiacetals. The effect of complex formation was also examined. Coupling of 12 and methyl triflate in THF or toluene at -50 "C gave the a/P mixture in the ratios 8 :2 and 1 :8, respectively. Addition of crown ether to the THF or toluene at the same temperature did not significantly alter the a/P ratio of
8.2 Anomeric O-Alkylution Primary triflate (3,15-19), t-BuOK or NaH OH
OBn
/ THF or dioxane
181
Bno&oR2
*
BnO OBn
(12) R' = Bn (13) R' = H (14) R ' = MTr
EL Bg% BnO
0'
I
BnO
(17) X= OBn (15) X= >CMe2 (16) X = > C H C ~ H I ~ (18) X=OMe
1) 17, NaH I THF B BnO n
O OBnGOH
(14)
2) H+
67 %
B BnOn
O
B
4
n
O
q
(20) BnO BnoOBn
Scheme 5. Anomeric O-alkylation of glucopyranoses and primary triflates. MTr = p-methoxytrityl
the products. In contrast with the two furanose examples described above, complex formation could not explain the stereoselectivity of the coupling of pyranoses with gluco and galacto configurations. Anomeric O-alkylation of benzylated glucopyranoses 12-14 with several primary triflates (3,15-19) gave p-glucosidesexclusively as shown in Scheme 5 [7,8].These pselectivities are a result of the greater reactivity of the anomeric p-anion. Although steric hindrance by the substituent group at 0-6 (MTr, Bn and H), solvent (THF and dioxane) and promoter (NaH and t-BuOK) did not affect the P-selectivity, there is one exception-glycosylation of 14, which has MTr at 0-6, with triflate (17) unexpectedly afforded the a-linked isomaltoside (20) only. Steric factors and the different reactivity on the triflates are primarily responsible for this behavior. The product from coupling of tribenzylglucopyranose (13) and triflate (19) was converted into lipoteichoic acid carrier fragment (21). Scheme 6 demonstrates the result and excellence of this anomeric O-alkylation method for the synthesis of a complex natural product (22) [2]. 8.2.4 Anomeric O-Alkylation of Acyl-Protected Nucleophiles with Primary Triflates
Despite of the presence of NaH, neither acyl migration nor orthoester formation occurred during the anomeric O-alkylation of alkaline labile acetyl-protected sugars (hemiacetal). Scheme 7 shows the results. Klotz and Schmidt reported preferential
182
8 Glycosylution Methods: Alkylutions of Reducing Sugars
Scheme 6. Synthesis of lipoteichoic acid fragment via anomeric 0-alkylation.
p-selectivities in the anomeric 0-alkylation of tetraacetylgluco- (23) and -galactopyranose (24), and of heptaacetyllactopyranose (25) with decyl triflate at room temperature [9]. This procedure is applicable to the synthesis of p-lactosyl ceramide. Klotz and Schmidt converted heptaacetyllactose (25)with the triflate and nonaflate(nonafluorobutane-sulfonate)of azidosphingosine (31 and 32) into plactosyl ceramide (33)in 49 and 42% yield, respectively (Scheme 8). In contrast, tetraacetylmannopyranose (26)was converted to decyl a-mannopyranoside (20) stereoselectivelyat -40 "C as shown in Scheme 7. This a-selectivity arises as a result of steric and electronic effects of the nucleophile and the reduced reactivity of the p-
(OAc
Scheme 7. Stereoselective anomeric 0-alkylation of acyl-protected sugars. DME = 1,2-dimethoxyethane.
8.2 Anomeric 0-Alkylution
183
y3
NaH I 1,2-diethoxyethane
OTBDMS
(31) R = Tf
Scheme 8. Synthesis of Iactosylceramide from acyl-protected lactose via anomeric 0-alkylation.
anion. When the temperature was reduced a-glycosides were also obtained from 23-25. This tendency is similar to that observed with alkyl-protected glucoses. Lubineau et al. reported an anomeric 0-alkylation (a-stereoselective Michael (GlcNAc) (34) on P-tosyloxaddition) of 3,4,6-tri-O-acetyl-N-acetylglucosamine yacrolein, as shown in Scheme 9 [lo]. The coupling product (35) was successfully converted into the corresponding glycoside with an a-linked spacer (36). The acetylprotected disaccharide nucleophile Galp( 1-3)GalNAc is also applicable to the above procedure. Very recently Lubineau et al. also accomplished stereoselective synthesis of aand P-glycosides [ 111. Allylation of acetylated GlcNAc (34) with ally bromide in the presence of NaH in CH2C12 gave the P-anomer (37b) exclusively. In contrast, addition of Bu4NI to the reaction mixture gave an a-glycoside (37a) only. This complete stereoselectivity is because the anomeric tetrabutylammonium alkoxide was
A AcO c O
L
,
,
NHAc
A AcO c
NaH, 18-crown-6 I THF
83%
O
G
AcHNo+%o (35)
1 ) Ph3P=CHCOzEt I THF 2) H2 I Pd 3) NaOEt
i
Scheme 9. Anomeric 0-alkylation of GlcNAc (a-selective Michael addition)
184
8 Glycosylation Methods: Alkylations of Reducing Sugars
A AcO c
a
,
NHAc
r
bco&ONa AcO
NHAc
AC& AcO
AcmONa
1 I
AllBr
/OAc A cAcO O-&$, AcHNOAll
Scheme 10. Stereoselective ally1 glycosylation of GlcNAc via anomeric 0-alkylation.
more reactive than its sodium counterpart and thus the alkylation occurred more rapidly than epimerization to the p-anomer (Scheme 10). 8.2.5 Anomeric 0-Alkylation of Mannopyranose with Primary Triflates: Possibility of Intramolecular Complexation of the Nucleophile The reactive p-anion of gluco- and galacto-type nucleophiles explains the pselectivities of anomeric 0-alkylations. Schmidt et al. examined this for the mannose-type nucleophile which is a C-2 epimer of glucose. Temperature-dependent stereoselectivity was observed in the coupling of tetrabenzyl- and tetraacetylmannopyranose with triflated solketal (15), behavior similar to that of the gluco and galacto series [6, 91. Alkylation of 2,3,4-tri-O-benzyl-6-O-p-methoxy-trityl mannopyranose (39) with 15 in THF, however, gave the P-mannopyranoside exclusively at temperatures in the range -30 to 0 "C [ 6 ] .Obviously, steric hindrance by the substituent group at 0 - 6 leads to P-selectivity. This result is different from that for glucopyranose. Tamura and Schmidt explained this selectivity by use of several mannopyranosyl nucleophiles with different types of protecting group [ 121. Stereoselectivities were found to be a result of the formation of more reactive intermediates derived from substituent groups on the nucleophiles. Variation of the substituent group at 0 - 6 evidently changes the yield and stereoselectivity. Coupling of triflate 15 and mannopyranosyl nucleophiles 39-41 with bulky substituent groups (R = MTr, Tr, and TBDPS) at 0 - 6 resulted in higher pselectivity [R = MTr; a : 15, f3 :74, R = Tr; a :3, p: 62, R = TBDPS; a : 14, p: 73 (%)I in THF than for the 6 - 0 unprotected nucleophile (38) [ a : 6 ,p:21 (%)I. The
8.2 Anomeric O-Alkylation
BnO
R 1-
f."
OH
I
185
(38)R=H (39) R = MTr (40)R = Tr (trityl) (41) R = TBDPS (t-butyldiphenylsilyl) rOTf
NaH
Ic) Scheme 11. Plausible complex formation (A-D) and coupling reaction of mannopyranose. Ln = solvent molecule as ligand.
equilibrium between the 1C (A) and C1 (B) conformations in Scheme 11 explained these results. In general, for thermodynamic reasons, mannopyranose with a bulky group at 0 - 6 tends to shift towards the C1 form (complex B) in the equilibrium. Thus, P-mannosides are afforded uia the reactive intermediate B. The 6-0unprotected mannopyranose, on the other hand, prefers 1C-type stable complex A formation, which leads to a lower yield in THF. MEM (methoxyethoxymethyl) at 0 - 2 certainly forms a highly stable crown ether-like complex C in THF, as depicted in Scheme 11. In addition, THF is an obligatory ligand in the formation of the above MEM ether complex. No coupling product was obtained in THF whereas alkylation in polar CH3CN afforded a- and p-mannopyranosides in yields of 23 and 48%, respectively. The combination of solvent and protecting group also affects yield and stereoselectivity. Anomeric O-alkylation of mannopyranose with isopropylidene acetal at in 0-2,3 afforded the corresponding mannopyranoside in high yield [a:9, P :61 ("A)] THF. The same reaction in CHZC12 resulted in a lower yield (27%) but exclusive pformation. Isopropylidene acetal at 0-2,3 contributed to the formation of a stable skew-boat-type complex D and CHZC12 acted as more suitable ligand than THF. 8.2.6 Anomeric O-Alkylation of KDO with Primary Triflates a-Glycosylation with KDO (2-keto-3-deoxyoctonate= 3-deoxy-~-manno-octulosonic acid) glycosyl halides, with the exception of fluorides, by the Koenigs-Knorr
186
8 Glycosylation Methods: Alkylations of Reducing Sugars
@ R' +R2 = (CH&, @=Solvent
Figure 1. Possible reaction intermediate leading to KDO glycoside.
method often gives unsatisfactory results because of elimination of the hydrogen halide and accompanying P-glycoside formation, whereas anomeric O-alkylation accomplishes a-selective KDO glycosylation [ 131. To increase the nucleophilicity of KDO it is better to convert it into the corresponding 4,5 :7,sdicyclohexylidenecarboxamide beforehand, because this compound can take a boat conformation and keep the 0 - 2 reactive quasi-equatorial configuration. Deprotonation of the carboxamide possibly stabilized the dianion species by complexation with metal ion as depicted in Figure 1. KDO glycosylation with some primary triflates gave the desired a-glycosides exclusively in 48-83% yields [ 141. Rembold and Schmidt accomplished the synthesis of lipid A with the [KDOa(2-6)GlcNP(26)GlcNI sequence which contains fatty acid and phosphate moieties [ 151. Anomeric O-alkylation can be a powerful method for KDO glycosylation. 8.2.7 Anomeric O-Alkylation of Partially Protected Aldoses with Primary Triflate Long-chain alkyl glycosides have become increasing important as surfactants because of their good physical performance and low toxicity. A shorter synthetic route for these compounds and use of simple materials are required by industry. Schmidt's group proved it was possible to reduce the substitution on the nucleophile in the anomeric O-alkylation. The use of 6-O-unprotected tribenzylglucopyranose (13) and P-selective coupling has been described in Section 8.2.3. Scheme 12 shows p-selective coupling of 2-O-unprotected tribenzylglucopyranose (42) with decyl triflate in CHzC12 to afford 43 in 60% yield [16]. These reactive positions (2- and 6OH) proved not to need protection. The amount of promoter was shown to control the generation of the regioisomer. The use of one equivalent of NaH in the alkyla(44)with triflate 15 afforded P-glycoside tion of 3,4,6-tri-O-benzylmannopyranose (45) exclusively, whereas the undesired 1,2-disubstituted isomer (46) was also obtained when two equivalents of NaH were used (Scheme 12) [ 121. TBDPS ether was employed at the primary position of glucose and lactose to increase solubility in common organic solvents [ 161. Scheme 13 shows the results of alkylation of these nucleophiles (47 and 50). Anomeric O-alkylation of the glucose (47) in toluene afforded a- and P-decylglycoside (48a, b) in 58% yield. The coupling of lactose (50) needed additional crown ether to yield the glycoside (51a, b). THF was not suitable because of formation of an undesired product (49a, b).
8.2 Anomeric 0-Alkylation ,OBn H O* ' " BnO
187
CioH21OTf NaHCH2CIZ (1.1 eq) OH
BnO& BnO o-
on
,/rb NaH ( 1 eq)
BnOB
X
o
BnO
H
(44)
I CHpClp
NaH (2 eq)
Scheme 12. Anomeric 0-alkylation of partially protected nucleophiles.
c1&210Tf, NaH
H HOo
g
o
-
OH
(48a,b)
HO
58 % (a$= 1:4)
OTBDPS
C,oH,,OTf, NaH
(47)
THF
- -&H
OH
(49a,b)
OTBDPS
(50)
OH
toluene 15-crown-5
55 %(a$ = 1:2)
- 0
OTBDPS
(51a,b)
OH
56 % (a$= 1:l)
Scheme 13. Anomeric 0-alkylation of less protected nucleophiles.
8.2.8 Anomeric 0-Alkylation of Unprotected Aldoses with Primary Triflate, Bromides, and Cyclic Sulfates We can also use 0-unprotected aldoses for anomeric 0-alkylation by employing an aprotic dipolar solvent, namely N,N'-dimethylhexahydropyrimidin-2-one (DMPU) [17]. DMPU is essential to accomplish the anomeric 0-decylation of 0-
188
8 Glycosylation Methods: Alkylations of Reducing Sugars
unprotected aldoses with didecyl sulfate in the presence of NaH. Glucose, mannose, N-benzoylglucosamine, and xylose are converted to the corresponding decylglycopyranosides in moderate yields as anomeric mixtures ( a :p = 2 : 1-1 : 10). Decyl agalactofuranoside and a/p-arabinofuranoside ( 1 : 3) were obtained exclusively from galactose and arabinose, respectively, under these reaction conditions. Benzyl and allyl glycosides of these aldoses can be obtained by this procedure with benzyl and allyl bromides. This is a much more effective method than conventional Fisher glycosylation for simple glycoside synthesis. Cyclic sulfates can be used as potent electrophiles [ 181. Cyclic sulfates with longer alkyl chains which are suitable for the lipophilic part of the surfactants were condensed with free glucose in the same manner as above in 65-72% yields (Scheme 14). Anomeric 0-alkylation of the unprotected sugar is a potent procedure for producing amphiphilic molecules in short steps and contributes to saving materials and energy.
Scheme 14. Anomeric 0-alkylation of free glucose with cyclic sulfates.
8.2.9 Anomeric 0-Alkylation with Secondary Triflates and Nonaflate Glycosidic linkage to secondary positions [e.g. Man( 1-4)GlcI can be accomplished via anomeric 0-alkylation with secondary triflate. This procedure is accompanied ~ by inversion of the stereochemistry of the electrophile as a result of the S N reaction. The use of promoters (15-crown-5and NaH) in toluene accomplished coupling of tetrabenzylglucose (12) with aliphatic triflates (isopropyl and 1-methylnonyl triflate) in 70% yield ( a :p = 1 :9) and quantitative yield respectively, whereas condensation of 12 and the triflate of 1,6-anhydr0-2,3-di-O-benzylgalactose (52) failed under these conditions. Schmidt’s group has recently employed aprotic dipolar solvent systems; DMF-HMPT and HMPT-THF to solve the problem (Scheme 15) [ 191. These new solvent systems gave the desired glycosides in high yields with a/S
(a),
(52) R = Tf (53) R = Nf
Scheme 15. Anomeric 0-alkylation procedure with secondary electrophiles. P = protecting group; HMPT = hexamethylphosphoric triamide.
8.3 Glycosylation via the Locked Anomeric Conjiguration
189
selectivities of 2 : 1-5 : 1. Exclusive a-selectivity was observed during formation of the sequence Mana( 1-4)Glc from tetrabenzylmannose and triflate 52. The corresponding nonaflate (53) is more effectivethan the triflate for this glycosylation [12].
8.3 Glycosylation via Locked Anomeric Configuration V. K. Srivastava and C. Schuerch have first introduced 1,2-0-dibutylstannylene acetal-mediated methyl and allyl P-glycosylation for mannopyranose in 1979 [20]. This has recently been successfully developed by G. Hodosi and P. Kovai: as a titled concept; Scheme 2 shows the general procedure used. Synthetically difficult 1,2-cis P-glycosides, especially p-mannopyranosides, can be obtained by this method. The possibilities and problems of the locked anomeric configuration method are discussed in this section.
8.3.1 Synthesis of Methyl, Allyl, and Benzyl Glycosides via Stannylene Acetals Stannylene-mediated glycosylation was first applied to alkyl mannosylations. Condensation of the stannylene acetal of 3,4,6-tri-O-benzylmannopyranose (44) with methyl iodide and allyl bromide in DMF afforded the desired methyl and allyl P-mannopyranosides, respectively, in almost quantitative yields [20]. Benzyl Smannopyranoside was obtained from the same stannylene acetal by use of BwNI in benzene [21].The electrophilic leaving group is responsible for the reactivity and stereoselectivity. Methylation with methyl tosylate and dimethyl sulfate gave methyl mannopyranosides as anomeric mixtures at temperatures above 75 "C [20]. Glycosylation of 3,4,6-tri-O-benzylglucopyranose (42) via stannylene acetal with methyl iodide resulted in the production of the 2-O-methyl ether (70%) and amethyl glycoside (30%) [20], presumably as a result of the formation of stannylene acetal on axial 0 - 1 and equatorial 0 - 2 . The equatorial 0 - 2 is more reactive during stannylene complexation, which leads the 2-O-methyl ether. Thus stannylenemediated alkylation of 6-O-tritylmannose and methyl a-mannopyranoside afforded 3-O-alkylated products because of the greater reactivity of the equatorial oxygen at the 3-position [20]. The reaction mechanism is discussed in detail in Section 8.3.4.
8.3.2 Epimerization at C-2 by Locked Anomeric Configuration Method Epimerization at C-2 sometimes occurs during dibutylstannylene complex formation. In particular, 1,2-cis P-per-O-acylation for formation of the stannylene acetals of free mannose, rhamnose, and lyxose is accompanied by epimerization [22]. Scheme 16 shows the epimerization process schematically. This unwanted phenomenon can be minimized by conducting the acetalation under milder conditions-brief reaction in dilute solution at low temperatures [23].
190
8 Glycosylution Methods: Alkylations of Reducing Sugars
OR I
Bu
Scheme 16. Epimerization of mannose by stannylene acetylation. P = protecting group.
We can, on the other hand, take advantage of this epimerization to obtain rare sugars. Hodosi and KovaC converted 6-0-tritylgalactose to talose in 70% yield by stannylene acetalation and C-2 epimerization [241.
8.3.3 Locked Anomeric Configuration Method for Rhamnosyl Stannylene Acetal
Locked anomeric configuration method results in complete p-selectivity in oligosaccharide synthesis [25, 261. The electrophile must be converted to an active triflate and inactive bromide, iodide and mesylate cannot be used. Addition of CsF or Bu4NF into the reaction effectively increases the solubility of the stannylene acetals. Accompanying formation of the formate derived from the primary triflate at room temperature can be suppressed by reducing the reaction temperature to -5°C. Table 1 shows results from coupling the 1,243-stannylene acetal of rhamnose (54) with primary and secondary triflates (57 and 58) in the presence of CsF.
8.3.4 Locked Anomeric Configuration Method for Mannosyl Stannylene Acetal: Isomerization of Acetal [25, 261
Anomeric oxygen on the analogous acetal of mannose is less reactive than that of rhamnose. Coupling of the stannylene acetal of mannose 55 with the primary triflate 57 in DMF at 0-25 "C afforded the desired p-linked disaccharide together with the (3-6) linked pseudo-disaccharide in 40 and 23% yield, respectively. Changing the solvent to CH3CN and addition of BQNF increased the yield and regioselectivity. Because of the low solubility and reactivity of the stannylene acetal in CH3CN, other aprotic polar solvents (e.g. DMF, abbreviated in Table 1. DMA, and DMSO) were employed for the coupling of secondary triflate 58, as shown in
8.3 Glycosylution via the Locked Anomeric Conjiqurution Table 1. Results from glycosylation via the locked anomeric configuration. DMA = N , N-dimethylacetamide. I I
191
I
Manp( 14)Glc
RhafJ( 1-4)Glc CsF I DMF 25 "C, 78 %
B z0
Tf+
Bzo OMe (58)
DMF DMA DMSO
40% 52% 59% 25 "C
Mang( 1-4)Glc CsF I DMF 25 "C, 67 %
Table 1. The secondary triflate did not react with 3-0 of the nucleophile; the anomeric oxygen was more reactive. Isomerization of the stannylene acetal from 1,2-0 to 2,3-0 and the resultant equatorial 3-0 activation rationalize the formation of the pseudo-disaccharide as shown in Scheme 17. The epimerization of the stannylene acetal seems much faster than alkylation. The use of the analogous 3-O-benzyl ether effectively prevented the unfavorable coupling. Thus, generation of the etheric by-product was suppressed in the reaction with the same primary triflate (57).
8.3.5 Locked Anomeric Configuration Method for Stannylene Acetal with the Glucose Configuration 125,261 Scheme 18 shows results obtained from use of the locked anomeric configuration method for coupling of gluco-type stannylene acetals with triflates. Reaction of the stannylene acetal of maltose (59) and the secondary triflate (58) resulted in generation of an ether (2-4) linked pseudo-trisaccharide similar to the result with glucose. It is worthy of note that the coupling of the stannylene acetal of 4,6-O-benzylideneglucose (61) with the primary triflate (57) predominantly afforded the p( 1-6) disaccharide (62). The stereochemistry of the main product is inconceivable, however; it is plausible that the anomer oxygen favored a configuration close to equatorial orientation as the most stable 1,2-0-stannylene complex conformation.
192
8 Glycosylation Methods: Alkylations of Reducing Sugars
Scheme 17. Isomerization between 1,242- and 2,3-O-stannylene acetals of mannose.
HO
H$%o&
1) BuzSnO / MeOH (59) HO
Ho
OH
HO
-$ .*
2) 58, B u g F / DMF OHO + L -
(60) 75 %
BzO
OMe
Scheme 18. Coupling reaction via locked anomeric configuration for maltose and glucose moieties.
8.4 Conclusion The advantages of the ‘anomeric 0-alkylation’ and ‘glycosylation via locked anomeric configuration’ methods are:
1) primarily we can use unprotected sugars as nucleophiles; 2) we can also perform glycosylation reactions under non-acidic conditions; 3) we can also use polar solvents such as DMF, DMA, and DMSO, and even CH3CN; and 4) inversion of configuration occurs at the 0-glycosidically linked carbon of the electrophile. These points reduced the limits of glycosylation reactions and often result in increased coupling yields, compared with conventional glycosylation. With regard to point 4,we can avoid the use of poorly reactive hydroxyl groups, which reduce coupling yields in Koenigs-Knorr-type glycosylation. ‘Anomeric 0-alkylation’ and ‘locked anomeric configuration method’ can be alternatives to traditional KoenigsKnorr-type glycosylation.
References
193
References I . R. R. Schmidt, Angew. Chem. Int. Ed. Engl., 1986,25, 212-235 (197 references). 2. R. R. Schmidt, Pure Appl. Chem., 1989, 61, 1257-1270 (69 references). 3. R. R. Schmidt, Modern Methods in Carbohydrate Synthesis, Hanvood Academic publishers, 1996, pp. 20-54 (57 references). 4. R. R. Schmidt and M. Reichrath, Angew. Chem. Int. Ed. Engl., 1979, 18, 466-467. 5. R. R. Schmidt, M. Reichrath, and U. Moering, Tetrahedron Lett., 1980,21, 3561-3564. 6. R. R. Schmidt, U. Moering, and M. Reichrath, Chem. Ber., 1982, 115, 39-49. 7. R. R. Schmidt, M. Reichrath, and U. Moering, J. Carbohydr. Chem., 1984,3, 67-84. 8. R. R. Schmidt, U. Moering, and M. Reichrath, Tetrahedron Lett., 1980, 21, 3565-3568. 9. W. Klotz and R. R. Schmidt, J. Curbohydr. Chem., 1994, 13, 1093-1101. 10. A. Lubineau, H. Bienayme, and J. Le Gallic, J. Chem. Soc., Chem. Commun., 1989, 19181919. 1 1. A. Lubineau, S. Escher, J. Alais, and D. Bonnaffk, Tetrahedron Lett., 1997, 38, 4087-4090. 12. J. Tamura and R. R. Schmidt, J. Curbohydr. Chem., 1995,14, 895-911. 13. R. R. Schmidt and A. EDwein, Angew. Chem. Int. Ed. Engl., 1988,27, 1178-1180. 14. A. EBwein, H. Rembold, and R. R. Schmidt, Curbohydr. Rex, 1990,200, 287-305. 15. H. Rembold and R. R. Schmidt, Carbohydr. Res., 1993,246, 137-159. 16. R. R. Schmidt and W. Klotz, Synlett, 1991,3, 168-170. 17. W. Klotz and R. R. Schmidt, Liebigs Ann. Chem., 1993, 683-690. 18. W. Klotz and R. R. Schmidt, Synthesis, 1996, 687-689. 19. Y. E. Tsvetkov, W. Klotz, and R. R. Schmidt, Liebigs Ann. Chem., 1992, 371-375. 20. V. K. Srivastava and C. Schuerch, Tetrahedron Lett., 1979,35, 3269-3272. 21. A. Dessinges, A. Olesker, and G. Lukacs, Carbohydr. Res., 1984, 126, c6-c8. 22. G. Hodosi and P. KovaE, XVIII Int. Carbohydr. Symposium Abstr., Milano, 1996, pp. 394. 23. G. Hodosi and P. KovaE, Carbohydr. Rex, 1997,303, 239-243. 24. G. Hodosi and P. KovaE, J. Carbohydr. Chem., 1998,17, 557-565. 25. G. HodosiandP. KovaE, J. Am. Chem. SOC.,1997,119,2335-2336. 26. G. Hodosi and P. KovaE, Curbohydr. Res., 1998, 308, 63-75.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
9 Other Methods of Glycosylation Luigi Panza and Luigi Lay
Introduction and Summary The biological relevance of oligosaccharides in molecular recognition phenomena stimulated the development of many synthetic approaches for the formation of glycosidic linkage, which is the main problem in oligosaccharides chemistry. Besides the most popular methods, which will be discussed in other chapters of the book, many other protocols have been explored.
X = "exotic"leaving group
New methods should, in general, have advantages towards the existing protocols (e.g. to be more efficient, simpler, to use cheaper reagents, to allow better selectivity, to be environmental friendly etc). This chapter will also deal with common methods not covered by other chapters, but more attention will be given to those approaches which fulfil, at least in part, the above advantages. Highlights
1. Use of a protecting group at the anomeric position easily transformed into a good leaving group.
196
9 Other Methods of Glycosylution
Example: an anomeric 2-butenyl glycoside can be isomerized into a propenyl ether which has been demonstrated to be an efficient leaving group. Introduction of the concept of latent-active glycosyl donor
OBn 78%, a/p 1:20 TMSOTf CH$N
BnO
G . J. Boons, S. Isles, Tetrahedron Lett., 1994,35, 3593.
BnO
Scheme B.
Advantages: a protecting group can be easily converted into an efficient leaving group. Disadvantages: use of mercury salts for the preparation of 2-butenyl glycosides.
2. Use of an heterogeneous acid catalyst. Example: an heteropoly acid act both as promoter and dehydrating agent in the glycosylation of an anomeric hydroxy sugar donor
Go&$ 82%, pia 1090
BnO BnO
N3
K. Toshima, H. Nagai, S. Matsunnua, Synlett, 1999, 1420
Scheme C.
Advantages: the procedure is simple, the promoter is reusable and environmental friendly. Disadvantages: the method seems to work well only with reactive donors; only a few examples available so far.
3. New achievement with old methods. Example: use of ortho esters as intermediates in regio- and stereoselective glycosylations.
9.1 En01 Ethers
197
TMSOTf CH2C12
W. Wang, F. Kong, J. 0%.Chem., 1998,63,5744.
Scheme D.
Advantages: good regioselectivity with partially protected acceptors, multiple glycosidic bond formation. Disadvantages: multistep procedure; requires AgOTf for the ortho esters formation.
N. B. Because of the vast scope of this chapter, it is almost impossible to include all the pertinent literature. The authors have made an attempt to organise in a logical manner most of the multifarious approaches for the formation of the glycosidic linkage not included in other chapters. They apologize for any omission they can have made unintentionally.
9.1 Enol Ethers This section will examine the preparation of glycosides and oligosaccharides starting from sugar derivatives which contain a double bond substituted with an ethereal or an acetalic oxygen atom. The term enol ether is therefore used in this context in a quite broad sense. The following general scheme summarizes the approaches which have been explored for synthesis of glycosides from enol ethers. The different approaches can be classified into two main groups: a) strategies in which the starting compound is an hydroxy enol ether already containing the aglycone moiety and from which the glycoside is obtained by cyclization with an electrophile (from endo-en01 ethers, retrosynthetic pathway d and exo-enol ethers retrosynthetic pathway e, Scheme 1); b) approaches in which a properly activated donor, which can be derived from or still contain an en01 ether function, reacts with a glycosyl acceptor (a sugar alcohol) in a more classical way (from exo-glycals, retrosynthetic pathway a, endo-glycals, retrosynthetic pathway b and vinyl glycosides, retrosynthetic pathway c, Scheme 1).
198
9 Other Methods of Glycosylation
&'
from exoenol ethers
I
H'R' endo-enol ethers
Y
PO
S
O
R
Scheme 1. Retrosynthetic pathways for the synthesis of glycosides from enol ethers.
Furthermore, in some of these approaches the diverse structure of the starting compound may be used to obtain different kinds of glycosides, namely aldosides or ketosides (endo-en01 ethers and endo-glycals), whereas other starting structures allow only the formation of ketosides (exo-enol ethers and exo-glycals). The following examples illustrate some applications of the different approaches shown above.
9.1.1 Endo-Enol Ethers The first example of the use of sugar hydroxy enol ethers was originally proposed by Suzuki and Mukaiyama [ 11 in 1982 for the synthesis of 2-deoxyglycopyranosides (Scheme 2). The enol ether precursors 3 were obtained by a Horner-Wittig reaction 1 with an alkoxymethyldiphenylphosphine of 2,3,5-tri-O-benzyl-~-arabinofuranose oxide 2, as an E/Z-mixture separated by careful chromatography. Treatment of each diastereoisomer with various electrophiles (Hg(I1) salts, PhSeX, NIS) induced a 6-endo cyclization. The substituent at position 2 was then reductively removed to give the desired 2-deoxydisaccharides 4 and 5. It is noteworthy that, in such an approach, the factors which control the stereochemistry of the process are completely different from those of a classical glycosylation and will be discussed later. In this case the stereochemistry at the anomeric position was dependent on the stereochemistry of the double bond and on the electrophile used.
Scheme 2. Reagents: i) LDA, THF, -78 to 0°C; ii) KH, THF, 40"C, 75% Z/E 2: 1; iii) Hg(I1) salts, PhSeCl or NIS; iv) NaBH4, Bu3SnH or LiAlH4 respectively.
9.1 En01 Ethers
199
BnO ?Me COOMe
BnO v
f- l o R =
CHO
i,ii BnO
COOMe
9
yo OMe 6
I
i,iii BnO
BnO
BnO
16
BnO
COOMe
I
14
BnO
COOMe
17 R = HgCl 19R=H
IZOR=H
Scheme 3. Reagents: i) THF, NaH, O T , 90%, Z/E 2: 1 from 7, 85%, Z/E 2 : 3 from 14; ii) CF3COOH/H20 9 : 1, 0 "C, 80%; iii) MeONa in MeOH, 95%; iv) Hg(OCOCF3)2,THF, 0 "C then KCI, 80%; v) Ph3SnH, anhydrous AcONa, toluene, 90%.
Sinay and Paquet cleverly applied this approach to the synthesis of sialic acid [2] and Kdo [3] glycosides (Scheme 3). The enol ether precursors were prepared in good yield by a Horner-Emmons reaction between the properly protected aldehydo sugar derivative 7 or 14 and the sugar containing alkoxyphosphono acetate 8, and the E/ Z mixtures obtained were easily separated by chromatography. With respect to the previous procedure, the double bond in the starting enol ether was trisubstituted. In the case of sialic acid disaccharides, the mercuriocyclization of the E diastereisomer
200
9 Other Methods of Glycosylation
9 gave the a-linked mercurioderivative 11 with complete stereo- and regioselectivity. Likewise, cyclization of the Z isomer 8 afforded exclusively the p-linked mercurioderivative 10. The remarkable stereoselectivity of the reaction has been rationalized in terms of the attack of the electrophile from the less hindered side of the double bond controlled by the adjacent stereocenter and by the chair conformation in the cyclization transition state. Treatment of the mercurioderivatives with triphenyltin hydride and sodium acetate gave the p- and a-sialic acid disaccharides 12 and 13. When the procedure was applied to the Kdo enol ether precursors 15 and 16, the cyclization was again highly stereoselective affording, after demercuriation, the a-linked Kdo disaccharide 19 from the Z isomer and the p-linked Kdo disaccharide 20 from the E isomer. The reaction was also applied to a phosphonoacetate linked to the 3 position of a properly protected N-acetylgalactosamine. In this case the Horner-Emmons reaction was completely stereoselective giving exclusively the E isomer, from which the P-linked Kdo disaccharide was easily obtained in good yield. The above procedures allow access to 2-deoxyglycosidesand oligosaccharides. In an analogous way such an approach could be used for the synthesis of 2-hydroxy glycosides provided that the enol ether is allowed to react with an electrophilic oxygen. Russo et al. developed this idea [4]through the epoxidation of hydroxy enol ethers followed by ring closure of the intermediate epoxides to give the desired 2-hydroxy glycosides. It is worthy of mention that these compounds, having a single free hydroxyl group, allow further manipulation at position 2. The starting enol ethers E- and 2-3were prepared according to Mukaiyama [ 11, through a Horner-Wittig reaction of alkoxymethylphosphine oxides with 2,3,5-tri0-benzyl-D-arabinose (Scheme 4) and ribose. After separation of the E/Z mixture, epoxidation with MCPBA of the of the hydroxy enol ethers, either in Z or E configuration of the double bond, gave directly the cyclized products 21-24 owing to the high reactivity of the intermediate enol ether epoxides, with a 6-endo-tet ring closure. As mentioned above, the stereochemical course of this process is controlled by factors which are completely different from those of a “classical” glycosylation reaction. In this case two new stereocenters are introduced, at the anomeric position and at C-2, and the stereochemical outcome of the process depends on the config-
OR
BnO
-
Bno&OR BnO
E-3
BBnOn
O 2 3
+
21 OH
e OR
B,,O
OR 23
Scheme 4. Reagents: i) MCPBA, CH2C12, Na2HP04.
+
BBnOn
I
22
OR
O
3
24 HoOR
9.1 En01 Ethers
201
uration of the double bond, on the stereoselectivity of the epoxidation reaction, and on the stereospecificity of the ring closure. While the epoxidation was not highly stereoselective, the ring closure was stereospecific in all cases studied except for the formation of mannosides with oxygenated aglycons. The same procedure was also applied to the synthesis of ketofuranosides [5] such as fructosides and psicosides, where the starting enol ethers contained an extra hydroxymethyl group linked to the double bond. This structural feature allowed the use, besides the MCPBA epoxidation, of the Sharpless epoxidation. Although the epoxidation reaction gave, in general, better results than in the former case, it is noteworthy that the stereoselectivity of the Sharpless epoxidation was somewhat unpredictable. This is probably due to the presence of a second hydroxyl group which can coordinate the titanium and, in some cases, counteract the normal course of the Sharpless epoxidation. 9.1.2 Exo-Enol Ethers
The synthesis of glycosides from exo-enol ethers through a iodoetherification reaction was initially proposed by Barrett [6].It is obvious that from this method only glycosides of ketosugars can be obtained. In this approach a hydroxy group of a sugar either at the anomeric carbon or at another position is initially converted into an ester with a series of carboxylic acids containing a silyloxy function. Different procedures were used for the esterification step at the anomeric position in order to obtain a- or p-anomeric esters with good stereoselectivity. The esters were methylenated using the Tebbe reagent, desilylated and iodocyclized to give reducing and nonreducing disaccharides in fair to good yield. It is noteworthy that, in the case of iodocyclization of anomeric vinyl ethers, the anomeric C-0 bond is not cleaved at the intermediate iodonium ion stage. The stereochemical outcome of the cyclization reaction is strongly dependent on the structure of the vinyl ether, and the stereoselectivity varies from modest to excellent. Barrett applied this procedure to the synthesis of sucrose (Scheme 5) [7]. 2,3,4,6Tetra- 0-benzyl-D-glucopyranose was converted into the a-glucopyranosyl ester 25 with excellent selectivity. In this case the methylenation using Tebbe reagent was unsuccessful. However, the Nozaki-Takai protocol gave the desired vinyl ether. After desilylation, the cyclization reaction gave only the (3-fructofuranosyl derivative 26 when the reaction was performed in the presence of silica. The sN2 displacement of the iodide proved to be difficult, as expected for a neopentylic position. The problem was solved using a radical-mediated substitution with TEMPO, and final deprotection afforded sucrose 29. A similar approach was used by Sinay (Scheme 6) for a new synthesis of Kdo glycosides and disaccharides [8]. The mannose derived hydroxyheptenitol 30 was converted into the exo-enol ether 31 through a multistep synthesis. Cyclization was achieved using potassium t-butoxide and iodine at low temperature to give the disaccharidic derivative 32 as the only product of the reaction. The transformation of the iodomethyl appendage into the carboxylic group required a difficult nucleo-
202
BnO B n
9 Other Methods of Glycosylation
OBn QSiPh;Bu
Bn007/yvoB -
O
25
q
0
...
i,u
OBn
111
26
Bnd
27
: \OBn
28
Bn6
Acb
bAc
OBn
HV
OH
Scheme 5. Reagents: i) TiC14, TMEDA, Zn, CH2Br2, THF (Nozaki-Takai protocol), 68%; ii) TBAF on silica, THF, then tBuOK, 12, THF, 25 "C, 65%; iii) TEMPO, Bu3SnH, benzene, hv, 80%; iv) Na, NH3, THF then Ac20, pyridine, 90%; v) MeONa, MeOH, 96%.
philic displacement with cesium acetate in HMPA at 140 "C followed by a standard sequence of deprotection, oxidation and esterification with diazomethane to afford the final disaccharide 33. 9.1.3 Endo-Glycals Glycals have been extensively used for the synthesis of 2-deoxyglycosides and oligosaccharides since Lemieux introduced their reaction with simple alcohols in the presence of iodine and silver salts [9]. Interesting results were obtained using elec-
COOMe
30
BnO BnO 31
BnoOMe
Scheme 6. Reagents: i) PivC1, pyridine, DMAP, 98%; ii) 9-BBN, THF, then H202, NaOH, 71%; iii) RuC13/NaI04 CC14, CH3CN, H20, 92%; iv) Methyl 2,3,4-tri-O-benzyl-a-~-glucopyranoside, DCC, DMAP, 78%; v) Tebbe reagent, 81%; vi) LiAlH4, THF, O"C, 93%; vii) tBuOK, then 12, THF, 92%; viii) CsOAc, HMPA, 130 "C, 68%; ix) MeONa in MeOH, rt, 98%; x) (COC1)2/DMSO then Et3N; NaC102, H202, CHjCN; xi) CHzN2 Et20, 68%.
9.1 En01 Ethers
BnO BnO
203
Bz0 34
35 31 39 R=H
Scheme 7. Reagents: i) IDCP, CH2C12,4A ms, 58% of 36, 79% of 38; ii) Ph-iSnH, AIBN, benzene, 94%.
trophiles such as IDCP, again described by Lemieux, and more recently exploited by Danishefsky [lo], who applied the concept of armed-disarmed glycosylation (Scheme 7). In this manner it was possible to use the benzyl protected glycal34 as a donor, and the benzoyl protected glycal35 as an acceptor, in the initial glycosylation reaction. The product still contains a glycal function which can be submitted for a second glycosylation reaction to finally give compound 39. Other groups have proposed the use of NIS and NBS as electrophiles. For example, Tatsuta introduced [ l l ] and applied the NBS mediated glycosylation to the synthesis of natural 2deoxyglycoside containing compounds [ 121. Thiem [ 131 developed the NIS-mediated glycosylation, which was applied by many groups to the synthesis of various natural products containing oligosaccharidic chains [ 141. Thiem has improved the method for the synthesis of disaccharide side chains of antracyclines by the transformation of the alcohol acceptor into the corresponding tin-alkoxide to enhance its reactivity, as in the glycosylation of 41 (Scheme 8) 1151. All these 2-deoxy-2-haloderivatives can be reductively dehalogenated to give the desired 2-deoxyglycosides. Sinay introduced the use of phenylselenyl chloride as an electrophile [ 161, which was exploited by Barrett in the avermectin a-disaccharide synthesis [ 171. Ogawa
HOIt..
/ 0
i ____)
40
42 R=I
41 43 R=H
AcO
Scheme 8. Reagents: i) (Bu3Sn)z0,benzene then NIS, CH3CN 65%; ii) BqSnH, AIBN, 76%.
204
9 Other Methods o j Glycosylation (OBn
BnO& BllO
t
34
ratio
Scheme 9. Reagents: i) (PhS)2, PhSC1, SbC15, CH2Cl2 -6O"C, 70°h
described [ 181 the addition of phenylsulfenate esters to glycals in the presence of TMSOTf while Franck performed the reaction using phenylbis (pheny1thio)sulfonium salts, in which the alcohol acceptor was again presented as the stannyl ether (see e.g. the glycosylation of 34 with 44, Scheme 9) [19]. The addition product was reduced to the 2-deoxyglycosides by using either Raney-Ni or a tin hydride. In these glycosylation reactions the use of NIS, NBS or phenylselenyl chloride afforded mainly the a-glycosides, whereas other electrophiles gave the pglycosides as the main products with moderate selectivity. Franck studied quite extensively the face-selectivity of his reaction and found that it depends on the arylthio substituents, on the nucleophile and on the glycal structure. Such an approach was also applied to the synthesis of glycofuranosides starting from furanoid glycals. Some examples of activation of glycals using protons as electrophiles are also described. PTSA [20], CSA [21], triphenylphosphine hydrobromide [22], and AG50 WX2 resin [23] were used as promoters allowing the direct formation of 2-deoxyglycosides with predominant a-anomeric configuration. For example, Tatsuta effected the glycosylation of a glycal in the presence of CSA for the synthesis of the antibiotic elaiophylin [21]. The main drawback of this procedure is the possible occurrence of a competitive Ferrier rearrangement. Glycals were also transformed into the corresponding epoxides and used as glycosy1 donors, as will be described in another chapter. 9.1.4 Exo-Glycals
The NIS-mediated glycosylation introduced by Thiem, was applied not only to endo-glycals, but also to exo-glycals for the preparation of glycosides of ketosugars. It was initially studied [24] on the exo-glycal of leucrose 48 (Scheme 10) which was obtained in a classical way by reductive elimination on the glycosyl bromide 47 with zinc dust. Treatment of 48 with NIS in the presence of the sugar acceptor 49 gave the trisaccharide 50 in about 50% yield as a single anomer. Similarly, van Boom obtained [25] a iodomethyl disaccharide by treatment of the exo-glycal derived from tetra- 0-benzybglucopyranose with IDCP in the presence of a protected mannose acceptor.
205
9.1 End Ethers
i
OBz 41
BzO
OAc
Br 48
49
50
OAc
Scheme 10. Reagents: i) Zn, CH3COONa, CH3COOH/H20 1 : I, -10"C, 65%; ii) NIS; 3 A ms, CH3CN, rt, 47%.
Attempts to apply an analogous approach to the synthesis of Kdo disaccharides were made by Sinay by reaction of an exo-glycal obtained from 2,3,5,6-di-O-isopropylidene-D-mannose with iodine and potassium t-butoxide, but after a positive result with methanol, sugar acceptors did not give the expected disaccharides [8]. Russo [26] developed the use of exo-glycals such as 51 and 54 for the synthesis of glycosides of ketosugars by epoxidation with dimethyldioxirane followed by a Lewis acid-mediated glycosylation (Scheme 11). In this way glycosides and disaccharides of fructose 56 and gluco-heptulose 53 were obtained. The stereoselectivity of the reaction was dependent on the diastereomeric ratio of the epoxides in the case of simple alcohol acceptors, while less reactive sugar alcohols gave moderate selectivity for fructose glycosides and good to excellent a-selectivity in the case of pyranosyl derivatives. The method was further developed for the synthesis of Kdo disaccharides (Scheme 12) [27]. The exo-glycal57, previously synthesized by Sinay [8], was epoxidized with dimethyldioxirane and glycosylated with different alcohol and sugar acceptors to give the hydroxymethyl derivatives such as 59. The advantage of this approach with respect to the NIS mediated glycosylation is the direct introduction of the oxygenated function at C-1 of the Kdo moiety thus avoiding the cumbersome substi-
OBn
9
ABn
55 a/!3 11:
Scheme 11. Reagents: i) DMD, CH2C12, 0°C; ii) ZnCl2 'OEt2, CH2C12, -78"C, (55-90%)
206
xF--
9 Other Methods of Glycosylation
OH
11
*
5 8 "
57
Scheme 12. Reagents: i) DMD, CH2C12, -78 "C; ii) ZnCl2 . OEt,, CHzC12, -78 "C, 82%).
tution of the iodide. The reaction occurs with excellent a-selectivity and its stereochemical outcome has been rationalized by means of semiempirical calculations. 9.1.5 Vinyl Glycosides This method may be seen as a more classical approach as vinyl ethers can act as leaving groups in glycosylation reactions (Scheme 13a). The method was again introduced by Sinay [28] and derived from the observation that mercury(I1) chloride induces cyclization of 1-propenyl glycosides of 2-acetamido sugars to give oxazoline derivatives. Some initial positive results using 1-propenyl glycosides promoted the
a)
BBnO n 0
G o BnO
T
+
BBnO n 0
i
BnoOCH3
60
b,
Ho% BnO
BBnO n O BnO G o Brio $
62
61
G
o
H
-
+
N3
63
BBnO
n N3
65
64
OBn
Brioo
Pia 5:1
BnoOCH3
~ p/a 6.611 o BnoOCH3
OBn
BnO
66
61
Scheme 13. Reagents: i) BF3. OEt,, CH3CN, -25°C; ii) TMSOTf, CH3CN, -25"C, 75% of 76 and 92% of 79.
~
9.1 En01 Ethers
207
development of isopropenyl glycosides as glycosyl donors. Thus, treatment of the anomeric acetates of tetra-0-benzylglucopyranose with the Tebbe reagent gave the desired isopropenyl glucoside 60. The donor was activated both with TMSOTf and BF3 . OEt2. The glycosylation reactions were performed both in acetonitrile and in dichloromethane. The first solvent gave the better yield and p-selectivity, as expected, owing to the formation of transient a-nitrilium species. The rationalization of the process in terms of an intermediate mixed acetal formation led to the hypothesis that the glycosylation should also occur by reverting the reacting partners, namely using a vinyl derivative of the acceptor with a 1-hydroxy sugar. This reversed approach was successfully applied to the glycosylation of 2-azido derivative 63, for which the isopropenyl glycoside itself is not available as the Tebbe reagent is not compatible with the azido group (Scheme 13b). Finally the same paper describes the use, as donors, of isopropenyl glycosyl carbonates, easily obtained from hemiacetal derivatives with the commercially available isopropenyl chloroformate (Scheme 13c). Strictly speaking these donors are anomeric carbonates but are included here as their efficiency as glycosyl donors derives from the presence of the double bond. The glycosylation reaction of donors such as 66 with various acceptors occurred smoothly in the presence of TMSOTf to give disaccharides in good to excellent yield. Isopropenyl glycosides were also studied by Chenault [29] with particular attention to mechanistic aspects (Scheme 14). The stereoselective synthesis of pivaloyl protected isopropenyl glycosides was achieved either using Tebbe reagent or by reaction of glycosyl halides with bis(acetony1)mercury. The glycosylation reaction (e.g. of 69 with 37) was promoted with various electrophiles the best results being obtained with Tf20, TMSOTf and NIS/TfOH. The use of IDCP gave electrophilic addition to the double bond with subsequent formation of acetals such as 71. Chenault found that there are interesting influences of the reaction conditions on the reactivity of the isopropenyl glycosides towards transglycosylation or electrophilic addition on the double bond. (OPiv
(OPiv
69
Scheme 14. Reagents:
NIS, TfOH, CH3CN, 70'31, ii) IDCP, CH3CN or CH2C12, 53%
208
9 Other Methods of Glycosylation
AcO
Br 12
-Ai:030n i
BnO BnO+
AcO
iv
13
74
Bno$&oF+ OBn
B;:oGonv
BnO active
BnO
6'
latent
BnO
BnO
l5
op-
latent
& $-on.
alp 1:20
BnO BnO BnO 71
Scheme 15. Reagents: i) 3-Buten-2-01, Hg(CN)Z,HgBrz, CH3CN, 70%; ii) MeONa in MeOH, 90%, iii) NaH, BnBr, DMF, 820/0; iv) (Ph3P)3RhCl/nBuLi,THF, 92%; v) TMSOTf, CH3CN, 78%.
Boons developed the use of vinyl glycosides as donors by modifying the leaving group and exploiting the concept of latent-active glycosylation strategy (Scheme 15) [30]. In his approach a sugar was protected at the anomeric position as 3-buten-2-yl glycoside (72+ 74). The isomerization of the double bond with Wilkinson's catalyst in the presence of a base afforded 2-butene-2-yl glycosides such as 75, which can undergo a Lewis acid catalyzed glycosylation reaction. The activation of the butenyl group with TMSOTf promoted the glycosylation reaction giving disaccharides in good yield. Such interesting procedures exploit the easy transformation of a protecting group at the anomeric position into an efficient leaving group. The use of an allyl glycoside (e.g. 76) as acceptor allows the reiteration of the procedure. Moreover, the stability of the allyl glycoside allows quite a simple and flexible protecting group strategy. This latent-active glycosylation strategy was also applied by Boons to the synthesis of libraries of both linear [31] and branched trisaccharides [32]. The main drawback of the procedure is the preparation of the starting glycosides [ 301: These are obtained from acetobromosugars and 3-buten-2-01 using HgBr2/ HgCN2 as promoters, a procedure quite laborious and problematic from a health and environmental point of view. Boons recently tried to improve the preparation of substituted allyl glycosides by enzymatic means, using almond 0-glycosidase [33]. Another type of vinyl glycoside was used by Schmidt for glycosylation reactions [34].The glycosyl donor was obtained by treatment of a sugar hemiacetal with ethyl phenyl propiolate and sodium hydride. Glycosylation reactions were performed using TMSOTf as a promoter and demonstrated that such vinyl ethers at the anomeric position are not very good leaving groups. Surprisingly, Takeda et al. proposed very recently the same method [35] without mentioning Schmidt's paper. The only difference is the procedure used for the preparation of the vinyl glycosides which are obtained by addition of tetra-0-benzylD-glucopyranose to some alkynyl esters and ketones in the presence of tributylphosphine. The glycosylation reaction is carried out in the presence of molecular sieves and requires 10 equivalents of TMSOTf, thus confirming the relatively low reactivity of such donors.
9.2 I-Hydroxy Sugars
...
111
209
B n O & o q BnO
Me0
MeoOCH3
81
82
Scheme 16. Reagents: i) Nysted reagent, THF, CH2C12, 75%); ii) TfOH, CHZC12, -2O”C, 67%; iii) Raney-Ni, yield very much batch-dependent (50-90%).
Finally, some examples of the use of bicyclic systems in which the anomeric vinyl ether is partly or completely contained in a ring are present in the literature [36]. Franck described the use of cycloaddition products obtained from glycals as donors (e.g. 78, Scheme 16). Compound 78 exhibited low reactivity, so it was modified by introduction of a second double bond using the Nysted reagent. The vinyl glycoside 79 so obtained revealed excellent glycosyl donor properties when activated with Lewis or protic acids, the best results being obtained with TfOH, to give fair to good yields of P-linked products. Raney-Ni desulfurization afforded the 2-deoxy-Pglycosides. The difference in reactivity between 78 and 79 introduces the possibility of using these compounds as latent-active donors [ 3 7 ] . Hetch et al. used cyclic ketene acetals for glycosylation reactions [38]. The donor is obtained from a glycosyl bromide by treatment with silver oxide and Hunig’s base and is activated with CSA in the presence of the acceptor to give an orthoester. TMSOTf catalyzed rearrangement of such an ortoesther gave the desired disaccharide.
9.2 l-Hydroxy Sugars The use of l-hydroxy sugars for the formation of glycosidic bonds would be one of the most fascinating glycosylation methods, owing to the simplicity and the stability of the “donor”. Of course, the OH group itself is not a good leaving group and needs to be activated. The simplest way for the activation of the OH group is the use of an acidic promoter in order to have a water molecule as leaving group. In this case, better results are obtained in the presence of a water scavenger. Other methods involve the transformation of the anomeric hydroxy group into an intermediate glycosyl donor, which is used without isolation in glycosylation reactions.
210
9 Other Methods of Glycosylation
\ OTBDPS
+ +
i
Ho H AllO O%
OTBDPS
OH 83
,O 84
AllO H O &
*'
Scheme 17. Reagents: i) pyridinium p-toluenesulfonate, CH2C12, azeotropic dehydration, 73%.
Finally, the activation at the anomeric position through the Mitsunobu reaction will be considered. Besides the 1-hydroxy sugars, the l-O-silylated glycosides used as glycosyl donors will also be included in this section. The classification into the following sub-sections is in some senses questionable, but may help the reader to find a selected topic more easily. 9.2.1 Acidic Activation Apart from the obvious problems given by the water formed during the reaction, the direct transformation of l-hydroxy sugars into glycosides suffers of two other main problems: Their poor reactivity as donors and the strong tendency to selfcoupling to give trehalose like structures. Such problems are easily overcome in the formation of simple glycosides when the alcohol acceptors can be used in large excess (i.e. they are sufficiently volatile and cheap). After Fischer proposed the acid catalyzed formation of simple glycosides [39], some other methods appeared in literature [40]. Wessel [41] used the triflic acid for the formation of simple glycosides using the alcohol as solvent. The method was improved by Fraser-Reid [42] for the preparation of pentenyl glycosides, easily obtained using DMSO as solvent, TfOH and only 3 equivalents of alcohol. Glycosylation with sugar acceptors was performed on the Kdo precursor 83 as donor (Scheme 17), using pyridinium p-toluenesulfonate as acid catalyst and removing water either with 4 A ms or by azeotropic dehydration [43]. Glucuronic acid glycosides have been obtained by treatment of the methyl 2,3,4tri-O-acetyl-D-glucuronate with alcohols and phenols in the presence of TMSOTf [44]. According to the reactivity of the alcohol, the reaction is believed to proceed via a SN2 like reaction on a silylated hydroxy group to give a-glycosides, or by formation of an oxonium ion to produce the P-anomers owing to the participation of the acetate. The reactivity of N-acetyl-D-glucosamine and galactosamine in hydrogen fluoride solutions was also studied [45]. More recently Mukaiyama developed a method for the glycosylation of 2,3,5-triO-benzyl-D-ribose using a catalytic amount of trityl salts as promoters, in the pres-
9.2 I-Hydroxy Sugars
BnO&o, BnO
f
B
$
o
G
Brio OMe 87
86
2 11
pla ,6,84
BnBnO O&%O+ Brio OMe
88
Scheme 18. Reagents: i) 5 mol%,TrB(CsF5)4, benzene/toluene 10: 1, Drierite, O T , 83%.
ence of Drierite as a dehydrating agent [46]. Without any additive the p-anomer was formed preferentially, while the presence of lithium salts reversed the stereoselectivity leading to a large predominance of the a-anomer. The same procedure was also applied to the synthesis of 2-deoxy-~-glucopyranosides88 (Scheme 18)
PI.
A very interesting possibility has arisen in recent years, namely the use of solid acidic catalysts both as a promoter and as a dehydrating agent. Such an approach, when applicable, has many advantages: The procedure is simple and environmental friendly, the catalysts are usually quite cheap and reusable. The main limitation to be noted from literature data, is that these catalysts are efficient in glycosylation of donors which form the oxonium ion quite easily, such as deoxysugars. Montmorillonite K-10 was used for glycosylation reactions with simple alcohols and sugar acceptors [48]. In particular, olivosides were obtained using this Lewis acid solid catalyst. Another recent example of dehydrative glycosylation of 1-hydroxy sugars was performed [49] using an heteropoly acid, H4SiW12040 (Scheme 19). Different glycosyl donors and acceptors were used, with the following results: The glycosylation of deoxy sugars requires small amounts of promoter, whereas a larger amount is needed for benzylated hexoses; the formation of simple glycosides is described for all the donors, while the preparation of disaccharides, such as 90, is performed only on deoxy sugars; the glycosylation reaction is usually quite stereoselective giving mainly the a-anomer. Although at an initial stage, such promoters deserve attention because they can also be used for the activation of less reactive glycosyl donors. Acidic Activation With Additional Reagents
Several other attempts to activate 1-hydroxy sugars with cationic activators or Lewis acid catalysts have appeared in literature, in which a second active species is generated in situ by additional reagents. Inanaga et al. used catalytic amounts of Yb(OTf)3 and methoxyacetic acid as promoters to obtain glycosides of perbenzylated glucose 91 and ribose in excellent
&
BnO BnO
OH
86
+
8'
N3
HO
89 N3
p/a 10:90
90
Scheme 19. Reagents: i) heteropoly acid (H4SiW12040) 10 Wt%, CH3CN, 2 5 T , 82%.
212
9 Other Methods of Glycosylation
BnO%oH BnO
BBnOn OBn
91
O
q
-
67 Brio OMe
i
BBnO n o &OBn o ~ o M ~
2515
68
Scheme 20. Reagents: i) Yb(OTf)s, CH30CH2COOH 10 mol% each, CH2C12, ms 4A, reflux, 99%.
yield (Scheme 20) [50]. In the same conditions it is also possible to obtain thioglycosides. Although the role of methoxyacetic acid is not clear, no reaction took place when it was absent. The use of trimethylsilyl derivatives in the presence of various Lewis acids is also described. Susaki has published [51] a glycosylation reaction promoted with TMSCl and Zn(0Tf)Z. In a similar way, Mukaiyama described [ 521 successful catalytic glycosylations using (TMS)20 in combination with four different Lewis acids, again studying the effect of LiC104 on the stereoselectivity of the reaction. In a further development the same group studied the use of Sn(0Tf)z with various trimethylsilyl derivatives [ 531. 9.2.2 Dehydrative Glycosylation
Starting from observations from different fields of organic synthesis, Mukaiyama introduced the use of dehydrating reagents for the activation of the anomeric position of 1-hydroxy sugars. In a first example [54]a diphosphonium salt, generated by reaction of a trialkyl- or a triarylphosphine oxide and triflic anhydride allowed the synthesis of 1,2-cis-ribofuranosides when used in combination with a base (Hunig's base),1!4 ms and an appropriate acceptor. Similar results were obtained when the phosphine oxide was substituted with an oxotitanium derivative [ 551 in conditions similar to those previously described. A third paper [56] introduced the use of a different metal derivative in order to improve the already good yields and selectivities on the glycosylation of 2,3,5-tri-O-benzylribose92. Excellent results were obtained with diphenyltin sulfide and triflic anhydride in the presence of LiC104, with CsF as an acid scavenger, using alcohols or trimethylsilyl ethers as acceptors (Scheme 21). It is worthy of note that most of the tin reagent is recovered at the end of the reaction.
Scheme 21. Reagents: i) diphenyltin sulfide, Tf20, CsF, CH2C12, 0°C; ii) -23"C, 99%.
1% 1
6-
RO RO
OH
1
OR 91R=Bn 94R=Bz
- OTf
RO
OR
9.2 I-Hydroxy Sugars
RROO
&
o GBnO OR
213
Brio OMe
96a R = Bn, 85%, alp 6040 %b R = Bz, 72%, p only
Scheme 22. Reagents: i) diphenylsulfoxide, Tf20, 2-chloropyridine, toluene/CHzClz 3 : 1, -78123 "C.
More recently, Gin et a]. developed [57] the glycosylation with 1-hydroxy sugars using diphenyl sulfoxide and triflic anhydride to give glycosides and disaccharides in good yields. Benzyl or benzoyl protected glycosyl donors (91 and 94, Scheme 22) reacted equally well, and the procedure even allowed the N-glycosylation of an amide. In the Presence of the Acceptor From the Beginning
In spite of their efficiency, the above procedures involve the prior activation of the anomeric position of the donor, followed by the addition of the acceptor. The selective activation of the donor in the presence of the acceptor would allow the presence of both reagents from the beginning of the reaction. Koto et al. introduced [ 5 8 ] a glycosylation reaction in which a mixture of both the donor and the acceptor, together with CoBr2, is treated with methanesulfonic acid to give the glycoside or disaccharide through the in situ formation of a glycosyl bromide. The procedure was further modified, in order to obtain a better control of the stereoselectivity, with Et4NC104 [59] or using TMSBr instead of methanesulfonic acid [60]. The same group also introduced the activation of the anomeric position of 1-hydroxy sugars with p-nitrobenzenesulfonyl chloride, silver triflate and Et3N, probably through the formation of an anomeric sulfonate intermediate [61]. From this procedure glycosides and disaccharides were obtained with predominant pconfiguration. The addition of N,N-dimethylacetamide reverted stereoselection to predominant formation of the a-anomer of 68 (Scheme 23) [62]. A simplified procedure, described by Szeja [63], involves in situ formation of glycosyl sulfonates under phase-transfer conditions followed by alcoholysis to give mainly a-glycosides. Recently Koto et al. proposed [64] another dehydrative glycosylation in which DAST is used to activate the donor in the presence of the acceptor, Sn(OTf)Z,
BBnOn O g o H OBn 91
+
BBnOn 61
O q Brio OMe
alp 13:2?
Scheme 23. Reagents: i) NsCl, AgOTf, dimethylacetamide, Et3N, CHzCIz, -40 "C, 88%.
214
9 Other Methods of Glycosylation
B n o S O H BnO
1
95
"Me,
B n*;BnO C -) Brio OMe ma
alp 82:18
Scheme 24. Reagents: i) (CCl3CO)20, TMSC104, ms 5A,EtzO, rt, 91%.
Bu4NC104, and Et3N. The reaction probably proceeds through the formation of a glycosyl fluoride, which is further converted into an anomeric triflate and eventually to the glycoside. Finally, an even more recent paper [65] described a novel dehydrative glycosidation using trifluoro- or trichloroacetic anhydride in the presence of TMSC104 and 5 A ms with preferential formation of the a-anomer of 96a (Scheme 24). The anhydride is believed to form the anomeric ester with the essential contribution of the 5 A ms as acid scavengers. The ester is then activated by the TMSC104 to give glycosides such as 96. 9.2.3 Mitsunobu Glycosylation
A particular method for the anomeric activation of 1-hydroxy sugars is the Mitsunobu reaction which has mainly found application for the synthesis of aryl glycosides. Following from an earlier publication [66], Garegg described [67] the use of this reaction for synthesis of aryl 8-D-mannopyranosides. Lubineau studied [68] the influence of the pK, of the acid or phenol in the formation of aryl and acylglycosides derived from three pesticides. Mitsunobu reaction was employed by Ogawa in the synthesis of the antibiotic hygromycin A [69]. A more extensive study was made by Roush in a project devoted to the synthesis of analogs of aureolic acid family [70], which contain oligosaccharidic chains constitutes of 2,6-dideoxypyranosides. It was observed that the glycosylation reaction proceeds through an SN2 mechanism, as the stereochemistry of the products reflect the anomeric configuration of the starting hemiacetal. Moreover, the reaction was also performed using 2-thiophenyl derivatives as donors (Scheme 25). The preferential formation of 8-glycosides, such as 99, seems to be due to the effect of 2-thiophenyl substituent on the anomeric ratio of 97 and not to a neighboring group-participation. In a recent paper, Dondoni et al. obtained [711
Scheme 25. Reagents: i) 2-naphthol, Ph3P, DEAD, toluene, O T , 74%.
9.2 1-Hydroxy Sugars
AcO 100
OSiMe,
OMe
BzO
OAc
101
i h -
215
AcO
OBz
OAc lo'
BzO
OMe OBz
Scheme 26. Reagents: i) TMSOTf, CH2C12: -50&-30"C, 76%.
glycosylated calix[4]arenes under Mitsunobu conditions. Mitsunobu reaction was also performed between glucuronic acid and phenolic chromium tricarbonyl complexes [72]. The phenol complexation is used in order to increase its acidity. Glycals ' of were shown to react under Mitsunobu conditions through an S N ~displacement the allylic hydroxy group [73]. Anomeric 0-acylation was exploited by Amos Smith within the synthesis [74] of phyllanthoside, an antineoplastic glycoside containing a (3-glycosyl ester linkage. Mitsunobu reaction has rarely been used for the synthesis of alkyl glycosides owing to the lower acidity of an alcoholic hydroxy group. An old example describes the formation of alkyl glycosides and disaccharides using, besides triphenylphosphine and DEAD, a mercuric salt [75].Descotes et al. studied the reactivity of 1,3,4,6-tetra-O-benzoyl-~-fructofuranose in Mitsunobu conditions with different nucleophiles including alcohols [ 761. Finally, Nicolaou exploited [77] the Mitsunobu reaction for the formation of an 0-glycosyl hydroxylamine in the synthesis of calicheamicin oligosaccharides. 9.2.4 1-0-Silyl Glycosides
The use of 1-0-silylglycosides as glycosyl donor was introduced by Tietze for the synthesis of aryl glycosides [78].Reaction of trimethylsilyl glycosides, protected as ester or benzyl ethers, with phenyltrimethylsilyl ethers in the presence of TMSOTf gave good yields of aryl glycosides. The method was developed by Glaudemans (Scheme 26) [79],in a version in which the hydroxy group of the acceptor was protected as a t-butyldiphenylsilyl ether (e.g. 101) and the 1-0-TMS derivative or galactopyranose 100 used as a donor, in the of 2,3,4,6-tetra-O-acetyl-~-glucopresence of TMSOTf. In this way it was possible to avoid the prior deprotection of the acceptor. Simple glycosides were obtained from the reaction between 1-0-TMS glycosides and alcohols using BF3 . OEt2 as promoter [go]. Kolar et al. used silylated glycosyl donors derived from 2,6-dideoxysugars for the synthesis of anthracycline derivatives [811. 2-Deoxy- and 2,6-dideoxy silylglycosides were also studied by Priebe et al. [82], particularly for their conversion into other synthetically useful anomeric derivatives. Starting from the observation of a transient anomeric silyl derivative in a previously studied glycosylation reaction on 1-hydroxy sugars [56], Mukaiyama applied a similar procedure (a catalytic amount of a Lewis acid, diphenyltin sulfide) to 1-0-silyl derivatives of furanoses and pyranoses. Thus 1,2-
2 16
9 Other Methods of Glycosylution
trans- or 1,243 ribofuranosides, and 1,2-& glucopyranosides were obtained using the appropriate reaction conditions [83]. Finally, a very recent work described the synthesis of spacer-linked neodisaccharides of L-daunosamine [ 841. In a glycosylation reaction on a monosilylated 1,4-butanediol the main product obtained was the monoglycosylated unprotected diol. This may suggest either a desilylation due to the acidic conditions or a higher reactivity of the silylated oxygen of the acceptor.
9.3 Esters and Related Derivatives 9.3.1 Esters
Among the various glycosyl derivatives, glycosyl esters are the most readily accessible and frequently serve as useful precursors for other glycosyl donors such as glycosyl halides. Despite the impressive development of new and efficient glycosylation methods, described in other chapters of this book, glycosyl esters still offer a good alternative as glycosyl donors, especially when more classical procedures fail. In this section, we will focus mainly on the 1-0-acyl donors introduced in the last decade, even if some older methods are mentioned. For a more detailed description of the most classical methods we refer to ref. [86] and references cited therein. The first example of 1-0-acyl sugars as glycosyl donors dates back to 1933, when Helferich described the stereoselective syntheses of both a and fl glucosides using zinc chloride or p-toluenesulfonic acid as a catalyst and 1,2,3,4,6-penta-O-acetyl-BD-glucose as a donor [85]. Since then, several applications of 1-0-acyl donors, namely 1-0-acetyl, l-O-(2’-substituted)-acetyland 1-0-benzoyl sugars, have appeared in the literature [86]. Furthermore, many different Lewis acids have been used as effective promoters in the glycosylation reactions, including tin tetrachloride [87], ferric chloride [88], BF3 . OEtz [89] and TMSOTf [90]. Mukaiyama made an extensive contribution to this area. First, he showed that 2,3,4,6-tetra-0-benzyl-l-O-bromoacetyl-fl-~-glucopyranose could be stereoselectively converted into a-glucosides with primary and secondary alcohols using trityl perchlorate as an activator [91]. This method was further developed using different promoters and applied to the synthesis of 0-glycosides and disaccharides from 1-0acyl donors and 0-trimethylsilylated nucleophiles (Scheme 27).
cat.
PO
PO
X=H, halogen, -OR
Scheme 27. Glycosylations from 1 -0-acyl donors and 0-trimethylsilylated acceptors.
9.3 Esters and Related Derivatives
r (OBn
103
1
I
L
I
,OBn B BnOn
B BnO n o S O A c BiO
217
O
G
BnO 105
ClO,
J
TMSOR
(OBn
TMSOAc
TMSCIO,
Scheme 28. General mechanism proposed for glycosylation reactions promoted by MCI,-AgC104 systems.
Thus, D-ribofuranosides with high a-stereoselectivity were obtained with the combined use of a catalytic amount of tin tetrachloride and tin(I1) triflate and a stoichiometric amount of lithium perchlorate using 1-0-acetyl-2,3,5-tri-O-benzylp-D-ribofuranose as a glycosyl donor and 0-trimethylsilylated alcohols or monosaccharides as acceptors [92]. Moreover, highly stereoselective syntheses of 1,2-cis glucosides from 1-0-acetyl glucose having a non participating group have been achieved using a combination of a Lewis acid and silver perchlorate; several Lewis acids corresponding to the general formula MCl, have been screened, tin tetrachloride and gallium trichloride ~ being the most effective [93]. The reaction was assumed to proceed via the S N type mechanism and the species such as 104 were postulated to be the active catalyst, generated from MCl, and silver perchlorate (Scheme 28). The activation of the anomeric acetoxy group affords the intermediate oxonium ion 105 stabilised by the perchlorate ion. This large counter anion blocks the p-face of the anomeric centre; silyl alkoxide attack from the a-face predominantly produces the a-glucoside along with trimethylsilyl perchlorate and MCln-lOAc, from which the catalyst is regenerated. Further, the effect of the leaving group on the stereoselectivity of this glycosylation reactions was explored by screening various l -0-acyl-2,3,4,6-tetra0-benzyl-D-glucopyranoses with tin tetrachloride and silver perchlorate as the catalytic system. The 2-(2-methoxyethoxy)acetoxygroup gave the best results. Using this glycosyl donor, either c1 or p glucosides were obtained with high stereoselectivity [94], depending on the catalytic system. Mainly a-glucosides were formed employing tin tetrachloride and silver perchlorate in diethyl ether at -5 "C (a/p ratios ranging from 96/4 to 98/2), whereas P-glucosides were achieved with tetra-
218
9 Other Methods of Glycosylution
chlorosilane-silver perchlorate in acetonitrile at - 10 "C (a/p ratios ranging from 9/91 to 1/99). It is worthy of note that the stereoselectivities of the above glycosylations were demonstrated to be independent on the a/p ratio of the starting substrate, confirming the formation of the oxocarbenium cation intermediate and therefore the S N type ~ mechanism. Mukaiyama's protocol was successfully applied to the synthesis of other pglycopyranosides and a or p furanosides. 1-0-Acetyl-2,3,4,6-tetra-0-pivaloyl-~-~-glucopyranose afforded in high yields 0glucosides and disaccharides using methyltrichlorosilane and silver perchlorate as the catalytic system [95].2-Amino-2-deoxy-~-~-glucoand galactopyranosides, which are still challenging tasks in carbohydrate chemistry, have been prepared in high yields from the corresponding 1-0-acetyl donors either using trichlorotin triflate or tin(I1) triflate as a catalyst, the latter being more advantageous for practical reasons [96]. The p configuration of the newly formed glycosidic linkages was ensured by exploiting the participation of N-2,2,2-trichloroethoxycarbonyl(Troc) as a neighbouring group. 2,3,5-Tri-O-benzyl-l-O-iodoacetyl-~-ribofuranose (alp = 1/5) gave the best results as a glycosyl donor in the synthesis of both a and p ribofuranosides: the combined use of silver perchlorate and lithium perchlorate afforded ribofuranosides with high a stereoselectivity. On the other hand, P-ribosides were obtained with different catalytic systems, such as the combination of silver perchlorate and diphenyltin sulfide. The milder tin(I1) chloride also gave high yields of p-ribosides, when used in combination with 2 mol% of tetrachlorosilane [97]. More recently, Reynolds described the syntheses of D-arabinofuranosides, Dgalactofuranosides and L-rhamnopyranosides viu tin tetrachloride mediated glycosylation of the corresponding l -0-acetyl donors [98]. Finally, the combination of dimethyl dichlorosilane and silver perchlorate was proved to be efficient in the a-sialylation of the 6-OH of a P-D-galactopyranosyl acceptor, using a 2-0-acetylated sialyl donor: the disaccharide product, a useful building block for the synthesis of sialyloligosaccharides, was obtained in 76% yield in propionitrile at -28 "C (alp = 83/17) [99]. As stated above, the 1-0-acetyl donors may still be an effective alternative to classical methods, as recently witnessed by the synthesis of a kedarcidin chromophore analogue (Scheme 29). In a key step of the synthesis, the alcohol 106 was
Kedarosamine
106
Scheme 29. Reagents: i) BF3 . OEtz, toluene, 0 "C, 80%.
107
9.3 Esters and Related Derivatives
219
AcO OAc
OAc 108
109
110
Scheme 30. Reagents: i) Lewis acid, CH2C12, -1O"C, 76-950/0; ii) Bu3SnH, AIBN, toluene, 1 lO"C, 80-95%.
glycosylated with the N-protected 1-0-acetyl kedarosamine and afforded the uglycoside 107 in 80% yield. It is worthy of note that the use of either the glycosyl imidate or the phenyl thioglycoside of the kedarosamine led to lower yields of the product [ 1001. An efficient synthesis of P-glycosides of 2,6-dideoxy sugars, occurring in many biologically relevant antibiotics such as digitoxin, venturicidins and olivomycin, was achieved using acetyl 3,4-di-O-acetyl-2,6-anhydro-2-thio-~-~-altropyranoside 108 as a glycosyl donor (Scheme 30). Several Lewis acids promoters and solvents were examined. The best results, in terms of yields and stereocontrol, were obtained when the glycosylations were carried out in dichloromethane in the presence of TMSOTf (1.1 eq.) at -10°C. The 2,6-anhydro-2-thio bridge of the obtained p-glycosides can be easily converted into the corresponding 2,6-dideoxy derivatives either by standard hydrogenolysis using Raney-Ni as a catalyst or by the treatment with tributyltin hydride and AIBN in toluene at 110 "C [loll. Lanthanoid(II1) triflates catalyzed glycosylations with 1-O-methoxyacetyl sugars as donors have been described. After a preliminary screening of many lanthanoid triflates, ytterbium triflate, Yb(OTf)3, was selected as the catalyst and acetonitrile as the reaction solvent. By this method, a variety of glycosides including disaccharides and thioglycosides were prepared in good to excellent yields. The stereoselectivities are sometimes modest, but they could be improved by increasing the reaction temperature to 53 "C. Under these conditions, the thermodynamically more stable p-isomer was obtained as the major product [102]. The Nicholas reaction [lo31 has been successfully exploited for a novel glycosylation reaction based on the internal delivery concept (Scheme 3 la). The suitably O-protected glycopyranoside derivative 111 bearing the cobalt-complexed 4-alkoxy6-phenyl-5-hexynoate residue at the anomeric position affords the corresponding propynyl cation 112 under Lewis acid catalysis. The intermediate 112 collapses spontaneously to form the oxonium ion delivering the cobalt-complexed y-lactone 113. The oxocarbenium cation is then intramolecularly captured by its counter alkoxy anion leading to glycoside 114. The rather laborious preparation of the starting substrates for the investigation of this glycosylation reaction is described in Scheme 3 1b. Various glycosyl "donors" of the gluco, galacto and manno series were synthesized, either O-benzylated or 67 was O-benzoylated, whereas only methyl 2,3,4-tri-O-benzyl-cr-~-glucopyranoside used as nucleophile.
220
9 Other Methods of Glycosylation
Ph b)
TBDPSO
BnO
115 61
117
(OBn
pp. (CO)co
k$co)3
0
118
alp 9191
113
Scheme 31. Reagents: i) CO~(CO)S, Et20, rt; ii) 67,BF3 . OEt2, CH2C12, 0°C; iii) CAN, MeOH, rt, overall yield 74%; iv) TBAF, THF, rt, 92%; v) PDC, DMF, rt; vi) 91, DCC, DMAP, HOBt, Et3N, CH2C12, rt; vii) Co2(CO)8, Et20, rt; viii) TMSOTf, EtCN, -65 "C, 740/0.
As an illustrative example, compound 117 was synthesized from 115 and 116. The treatment of 117 with Coz(C0)~gave the corresponding cobalt complexes, which afforded the disaccharide 118 with high p-stereoselectivity. The y-lactone 113 was also recovered in high yields [ 104). The p-nitrobenzoate was successfully employed as a leaving group in the synthesis of the biologically important disaccharide H, 2-O-(a-~-fucopyranosyl)-~-galactopyranose. Although the p-nitrobenzoate is not an excellent leaving group, in the case of the highly reactive 2,3,4-tri-O-benzyl-t-fucosyl derivative (a/p = 5/1) it was efficiently activated using BF3 . OEt2 in the presence of 1,3,4,6-tetra-O-acetyl-a-~galactopyranose as a glycosyl acceptor. The disaccharide H was obtained in yields
9.3 Esters and Related Derivatives
221
OBn-p-CI p-CBnO p-CBnO p-ClBnO 119
HO 0
BnO OMe
87
p-CBnO p-CBnO 120
alp 6194
Scheme 32. Reagents: i) TrB(C6F5)d 10 mol%, BuCN-CH2C12 2/1, Drierite, -23 "C, 97's.
ranging from 70Y0to 79%, and with a a/p ratio of 11 : 1, from which crystallization gave pure a anomer in 40-50% yield [ 1051. 9.3.2 Sugar Carbonates and Derivatives Glycosyl phenyl carbonates as glycosyl donors have been reported by Mukaiyama et al.: after activation in the presence of an acceptor, the leaving group decomposes into carbon dioxide and phenol. First, efficient a-sialylation of a 6-O-unprotected galactosyl acceptor was achieved with a 2-O-phenyl carbonate derivative of Nacetylneuraminic acid, under dimethyl dichlorosilane-silver perchlorate catalysis [99]. Glucosyl phenyl carbonate 119 was activated with a catalytic amount of trityl tetrakis(pentafluoropheny1)borate to provide (1 +6), (1'3) and (1+4)-linked disaccharides in very good yields and with excellent p-stereoselectivity (Scheme 32). Interestingly, the reaction can be performed on alkyl thioglycoside as acceptors, giving access to one-pot sequential syntheses of trisaccharides upon the activation of the alkyl thio group of the disaccharide product [ 1061. An attractive strategy for the formation of glycosidic bonds is based on bridging donor and acceptor. The decarboxylative glycosylation method, developed by Ikegami and co-workers [107], is an example. Donor and acceptor are connected through a carbonate bridge and the glycosidic bond is obtained by internal removal of carbon dioxide promoted by a Lewis acid (Scheme 33a). The mixed carbonates were prepared as a mixture of anomers by l-O-acylation of pyranoses (glucose and galactose) with activated alkyl carbonates of alcohols or monosaccharides (glucose and galactose). The decarboxylative glycosylations were best promoted by stoichiometric amounts of trialkylsilyl triflates (TMSOTf or TBDMSOTf) and the choice of the solvent was crucial. The highest yields were obtained using toluene and mesitylene. The stereoselectivities were only modest when a non participating group was present, furthermore, the anomeric ratio of the starting materials was not reflected in the products, as deduced starting from anomerically pure carbonates. Although an intramolecular mechanism was initially claimed, Schmidt et al. [ 1081 demonstrated with competition experiments that the acid promoted decarboxylative glycosylation follows an intermolecular course. Good a-stereoselectivities were obtained applying a catalytic version of the decarboxylative glycosylation, using the combinations of tin tetrachloride-silver perchlorate or CpzHfCl&ver perchlorate as promoters and EtzO as a solvent [ 1091. On the other hand, very high p-stereoselectivities were reported in the decarboxylative glycosylations of O-acyl protected
222
9 Other Methods of Glycosylation
BnO
123 121
Scheme 33. Reagents: i) K2C03, toluene, 88%; ii) TMSOTf, CH2C12, rt, 89%, only 0.
sugar carbonates. Using 0-acyl protected donors (i.e. 94), selective synthesis of mixed P-carbonates (122) was achieved from 0-succinimidyl-activated carbonates of various acceptors, mainly in the presence of K2CO3 (Scheme 33b) [ 1lo]. Ley and co-workers reported that various carbohydrate derivatives react readily via the C-1 hydroxy group with carbonyldiimidazole or thiocarbonyldiimidazole in ether (EtlO or THF) to give the 1-imidazolylcarbonyl-or thiocarbonyl- glycosides 124 and 125, respectively (Scheme 34). These compounds were reacted with alcohols and glycosyl acceptors in the presence of zinc bromide or silver perchlorate affording the corresponding glycosides in good yields. When the reactions were carried out in diethyl ether as a solvent, a glycosides were obtained with good stereoselectivity, whereas in dichloromethane the quantity of the anomers was increased, suggesting that a SN1 type process is operating [ 1111.
124
0
Scheme 34. 1 -1midazolylcarbonyl- (1 24), 1 -imidazolylthiocarbonyl- (125) and 2-pyridincarboxyl(126) glycosides.
9.3 Esters and Related Derivatives
223
The authors suggest that the displacement is facilitated by chelation of the metal atom to the carbonyl oxygen (or thiocarbonyl sulfur) and the N-3 of the imidazolyl group. This can be related to the remote activation concept originally introduced by Hanessian [ 1121. The same strategy, i.e. the introduction of a bidentate ligand as a leaving group, was exploited using glycosyl2-pyridinecarboxylates 126 as glycosyl donors (Scheme 34). These compounds were easily prepared by esterification of the anomeric hydroxy group of the sugar with picolinoyl chloride in the presence of triethylamine. Mild Lewis acids, such as tin(I1) triflate and copper(I1) triflate, were found to be more effective than other reagents, such as BF3 . OEtz and TMSOTf, in promoting the activation of the 2-pyridinecarboxy group. Interestingly, in preliminary experiments carried out both with alcohols and monosaccharides as nucleophiles, the stereochemical outcome of the glycosylation reactions was observed to be dependent on the choice of the metal salt and the solvent [ 1 131. This procedure was further applied to more glycosyl donors, namely mannopyranose, 2-azido-2-deoxy-glucopyranose and 2-deoxy sugar derivatives [ 1141, demonstrating that the stereochemistry of the glycosylation reactions was influenced not only by the promoter and the solvent, but also by the structure of the donor. Glycosyl 2-thiopyridylcarbonates have been described as effective glycosyl donors by Hanessian et al. However, this leaving group has been used mainly for the formation of N-glycosidic linkages in the synthesis of 1,2-cis and 1,2-trans nucleosides [115]. A new glycosylation method based on the mild activation of 0-glycosyl N-ally1 carbamates with soft electrophiles has been described by Kunz and co-workers [ 1161. This activation is similar to that of 0-glycosyl4-pentenoyl esters reported by Fraser-Reid [ 1171, with formation of an oxazolidinone instead of a lactone as the leaving group at the anomeric center. Anomeric N-ally1 carbamates were readily obtained from anomerically unprotected carbohydrates by reaction with ally1 isocyanate. Among the electrophiles used for the activation, the methyl bis-methylthiosulfonium hexachloroantimonate (TMTSB) was found to be the most effective in promoting the glycosylations. Various alcohols and O-unprotected sugars were used as acceptors, and the corresponding glycosides were obtained in good yields but with low stereoselectivity, except in cases where neighbouring-group participation occurred. The authors applied this procedure to the synthesis of the mucine core trisaccharide 131 showing that many different protecting groups, such as alkyl and silyl ethers, esters and acetals, remain unaffected under the mild conditions required for the TMTSB-promoted activation of 0-glycosyl N-ally1 carbamates (Scheme 35) [ 1 181. 9.3.3 Orthoesters and Oxazolines The first appearance of 1,2-0-orthoesters as glycosyl donors in glycosylation reactions was in 1971. Kochetkov et al. reported a teut-butyl orthoester as a donor upon activation with 2,6-dimethylpyridinium perchlorate [ 1 191 and extensively
224
9 Other Methods of Glycosylation Ph
I
127
A c AcO
C
Y
128
&
oms+ yo
&
AcO
N3
129
NPht
130
129
... AcO
0
oms AcO AcO 131
Scheme 35. Reagents: i) TMTSB, CH2C12, -15°C + -5"C, 67%; ii) HS(CH&$H, CH2C12, 83%; iii) TMTSB, CH2C12, -29°C + -1O"C, 48%.
PTSA,
explored this methodology introducing the 1,2-0-(1-cyanoethylidene) group, whose activation occurred under milder conditions. Other orthoester derivatives, such as 1,2-0-[1-(p-methylpheny1)thiolethylideneand 1,2-0-[I-"-( 1-phenylethylidene)amino]oxyl]-2,2-dimethylpropylidene, were also described [861. A recent useful application of the 1,2-0-(1-cyanoethylidene) glycosyl donors was found in the highly regioselective glycosylations of primary-secondary ditrityl ethers acceptors. Quite surprisingly, secondary trityl ethers were essentially more reactive than the primary ones, using cyanoethylidene derivatives of mannose and galactose as donors. All glycosylations gave the 6-0-trityl derivatives of 1-2 and 1-4 linked disaccharides in good yields and very high regioselectivity, even in the case of such a poorly reactive acceptor as the 0-4 of galactose. This unusual regioselectivity was attributed to the formation of an early transition state, where electronic factors, i. e. electron density at oxygen, rather than steric factors will become determinant [ 1201. There are two main advantages in using 1,2-0-orthoesters as glycosyl donors: -
-
the orthoester ring directs the nucleophilic attack to the back side, leading to 1,2trans glycosides; the nucleophilic opening of the orthoester ring (for example, with a simple alcohol) allows the contemporary and orthogonal protection of the C-1 and C-2 functionalities of the sugar.
During the last decade sugar orthoesters have been widely employed. However, their use has been more as protecting groups rather than as glycosyl donors in oligosaccharide synthesis. The new achievements concerning glycosylation reactions with sugar orthoesters exploit their chemistry in a quite different way. Recently, Kong and co-workers reported a new methodology for highly regio-
225
9.3 Esters and Related Derivatives
(OAC
/OH
(OAc AcO AcO Br 72
+
AcO
Ho%
BnO
AcO
-
Me (OH
OMe
y:o%
132
OMe
OMe
ii
7
E
133: R = H 1 3 4 R = Ac
AcO
TMSOTf
OMe 135:R=H 136R=Ac
Scheme 36. Reagents: i) AgOTf, 2,4-lutidine, CH2C12, rt, 95%; ii) AczO, pyridine, qu.; iii) TMSOTf, CH2C12, O"C, 78%.
and stereoselective synthesis of oligosaccharides. The procedure is based on the preparation of sugar-sugar orthoesters by coupling acetobromosugars with unprotected or partially protected sugar acceptors in high yields and excellent selectivity. Subsequent rearrangement in the presence of catalytic quantities of TMSOTf yields the oligosaccharide [121]. In a typical example, acceptor 132 was regioselectively converted into orthoester 133 from acetobromoglucose 72 using silver triflate in the presence of 2,4-lutidine (Scheme 36). Rearrangement of 133 with a catalytic amount of TMSOTf selectively afforded disaccharide 135 in 78% yield. The same compound was obtained in 80% yield when 4-0-acetylation of 133 ( 4 134) was performed prior to the rearrangement. These results are mainly due to steric factors (Scheme 36). The less hindered hydroxy group attacks the acyloxonium carbon of A generating orthoester 133 which, during the following rearrangement, is fragmented again into A and the trimethylsilylated acceptor: finally, a regio- and stereoselective backside attack on the C-1 of A gives disaccharide 135. Many other examples reported by the authors demonstrate the excellent regioselectivity in the orthoester formation. This new method was widely explored and applied to the synthesis of complex branched oligosaccharides. First, a variety of acetobromosugars and glycosyl acceptors were examined in terms of their ability to afford regioselective orthoester formation. It was found that both the regio and the stereoselectivity were dependent on the acceptor rather than the donor structure, according to the mechanism depicted in Scheme 36. Indeed, acetobromo derivatives of glucose, galactose,
226
9 Other Methods of Glycosylation
mannose, rhamnose, lactose and maltose invariably led to highly regioselective formation of the orthoesters with various acceptors [121], [122], [123]. On the other hand, the behaviour of the glycosyl acceptors under the orthoester formation conditions can be summarized as follows: n,6-unprotected acceptors (n = 2-4) always afford 1+6 linked oligosaccharides; selective 3-0-glycosylations were achieved with 2,3- and 3,4-unprotected sugars (glucose, mannose); - the rhamnose behaviour is an exception: Very high 3-regioselectivity was observed both with 3,4- and 2,3,4-unprotected sugars, whereas the 2,3-diol gave good 2-regioselectivity, further demonstrating that orthoester formation is very sensitive to steric factors [ 1221. -
The mannose-type acceptors gave particularly interesting results: The 3,4,6unprotected derivatives can be directly converted into the corresponding 3,6diorthoester with very high regioselectivity. Thus, using acetobromomannose 137 as a donor, the biologically important 3,6-branched mannotriose 141 was obtained in 88% yield by rearrangement of the diorthoester 140 (Scheme 37). When acetobromolactose was employed as a donor, a 3,6-branched pentasaccharide diorthoester was formed (88%)) the rearrangement of which afforded the 3,6-branched pentasaccharide in 740/0yield [124], [125]. Likewise, both 2,3,4- and 3,4-unprotected mannosyl acceptors gave 3-0-glycosylation regioselectively, by rearrangement of the corresponding orthoesters, leading to various very important mannose-based building blocks [ 1251. The versatility of this methodology was demonstrated by its application to the synthesis of biologically relevant branched oligosaccharides, such as the phytoalexin elicitor hexasaccharide [ 1241, [ 1261, [ 1271, a trisaccharide part of the Escherichia coli K26 antigen [ 1221 and the trisaccharide repeating unit of the 0 2 antigen occurring in the lipopolisaccharide of Stenotrophomonas maltophilia [ 1281. A related approach to the stereoselective synthesis of glycosidic linkages is based on the reductive cleavage of cyclic orthoesters generated by two sugar moieties. A C O y OAC ACOAAcOc
OAC O
q
+
Br 137
138
ii
139: R = H
141: R = Ac
140: R = Ac
Scheme 37. Reagents: i) AgOTf, 2,4-lutidine, CH2C12, rt, 91%; ii) Ac20, pyridine, qu.; iii) TMSOTf, CH2C12, -30 "C, 88%.
9.3 Esters and Reluted Derivatives
227
i
OMe
R, = H, R4= OBn R2=OBn, R, =OBn, % = H
142: R, = OBn, R, = H,
143: R, = H ,
OMe 144: 82% 145 74%
Scheme 38. Reagents: i) LiAlH4, AlC13, CH>ClZ/Et20, rt.
Such cyclic orthoesters are available from the corresponding sugar lactones through reaction with sugar diols in the presence of methoxy trimethylsilane and TMSOTf [ 1291. Using this procedure, 4,6-O-glucopyranosylidene, galactopyranosylidene and mannopyranosylidene acetals were synthesized. For the cleavage of the orthoesters, the combination LiAlH4-AlC13 was found to be the most effective reducing agent. The treatment of glycopyranosylidene acetals afforded the p-(1-+4)-linkeddisaccharides, with high regio- and stereoselectivity [ 1301. It is worthy of note that only the most sterically congested p-(1-4) glycosides were obtained, and not one of the other possible isomers, i.e. the p-(1-6) glycosides andfor a-glycosides, were detected in the reaction mixture, even in the case of the mannose-type donor. This high selectivity was attributed to the predominant coordination of the less hindered oxygen atom followed by the axial attack of a hydride. The procedure was further extended to the synthesis of glycosyl-galactosides (Scheme 38). The corresponding glycopyranosylidenes 142 and 143 were reductively cleaved with LiAlH4-AlC13 giving the highly congested p-( 1-4)-glycosyl-galactosides 144 and 145 with excellent regio- and stereoselectivity [131]. The mannose 1,2,6-orthoester 147 was recently reported as a potential glycosylating agent; after protection of the remaining hydroxy groups, the ring opening of the orthoester with nucleophiles could be achieved with 2-OH and 6-OH groups being orthogonally protected (Scheme 39). Compound 147 was prepared by mannose 1,2-0rthoester 146 using tert-butyldiphenylsilyl chloride and imidazole in DMF or, better, pyridinium triflate in pyridine. O-Acetylation or O-benzylation of 147 gave orthoesters 148a,b. The glycosylating properties of 148b were then examined: BF3 . OEtz was found to be the best promoter, and glucoside 149 was chosen as an exemplificative acceptor (Scheme 39). Disaccharide 150 was obtained in good yields. Interestingly, when an excess of orthoester was used, the partial formation of the trisaccharide due to the glycosylation of the 6’-hydroxy group took place [ 1321. Sugar tricyclic orthoesters, such as glucose 1,2,4-0rthoester derivatives, were used as monomers in polymerization reactions to afford cellooligosaccharides with a degree of polymerization up to 20. The 1,2,4-0rthopivalate was found to be the best monomer, both in terms of regio- and stereoregularity of the polymer produced, and it was successfully employed in the first chemical synthesis of cellulose [133].
228
9 Other Methods of Glycosylation
HO
HO HO
RO
146
147
148a:R = Ac 148b:R = En
-
+ En0
BnO 149
yo Bn0-&&/ooct BnO
148b
En0 150
Scheme 39. Reagents: i) Py . TfOH, pyridine, 89%; ii) AczO, pyridine, qu. or BnCI, raH, DMF, qu.; iii) TMSOTf, ClCH2CHzC1, 0 "C, 68%.
The electronic and steric effects of diverse orthoester groups (orthopropionate, orthoacetate and orthobenzoate) on the ring-opening polymerization of glucose 1,2,4-0rthoester were then explored. Although these compounds gave stereoregular polysaccharides, having only P-glucosidic bonds, the polysaccharides were not regioregular, consisting of a mixture of P-(1+4) and P-(1+2) units. These results were largely attributed to steric rather than electronic effects [ 1341. The 1,2-O-orthoesters derivatives of D-mannose and D-galactose were used as glycosyl donors to incorporate a-mannoside and P-galactoside branches at C-6 of chitin and its N-deacetylated form chitosan [ 1351 employing a N-phthaloyl 6-0-trimethylsilylated chitosan derivative as an acceptor. Sugar oxazolines are obtained from 2-acylammido-2-deoxy glycosyl donors through deprotonation of the oxazolinium ion intermediate 151 (Scheme 40). They are quite stable and can be used as glycosyl donors in 1,2-truns glycosylation reactions. In acidic media, the oxazoline 152 generates the oxazolinium ion, which can react with a suitable acceptor to provide the corresponding P-glycoside 153.
151
R
Y 153
Scheme 40. Glycosylation via oxazolinium ion intermediate.
9.3 Esters and Related Derivatives
229
Due to the strongly acidic conditions usually required for their activation, the oxazoline method has not found broad application in the field of oligosaccharide synthesis. They are incompatible with acid-sensitive protecting groups and with poorly nucleophilic acceptors. Their employment seems to be restricted to reactive acceptors. Despite these severe limitations, some important examples of the application of the oxazoline method have been reported [ 1361. Recently, Martin et al. described the synthesis of a- and P-u-glucosamine and galactosamine 1-phosphates using the more reactive trifluoromethyl oxazolines and dibenzyl phosphate [ 1371. was used as a monomer The methyl oxazoline of 3,6-di-O-benzyl-~-glucosamine for the synthesis of a dibenzylchitin-type polysaccharide. The polymerization under 1O-camphorsulfonic acid catalysis provided the polyaminosaccharide [ 1381. was used as a glycosyl The methyl oxazoline of 3,4,6-tri-O-acetyl-~-glucosamine donor in the glycosylation of ally1 6-O-pivaloyl-~-~-galactopyranoside: the reaction took 3-4 days in the presence of BF3 . OEtl and afforded the (3-(1+3)-disaccharide as the only regioisomer in 45-500/0 yield [ 1391. Finally, the unprotected methyl oxazoline of D-glucosamine was used as a substrate for the glycosidase chitinase in the synthesis of the N,N’-diacetylchitobiose, the repeating unit of chitin, via an enzymatic glycosylation [ 1401. 9.3.4 Phosphorus and Sulfur Derivatives Various glycosyl donors incorporating a variety of phosphorus-containing leaving groups have been developed and used for the stereoselective construction of a- and p- glycosidic linkages, with or without a neighbouring participating group [ 861. Besides the classical glycosyl phosphates and phosphites, the use of glycosyl diphenylphosphineimidates [141] and glycosyl phosphorodiamidimidothioates [142] as glycosyl donors have been described. New achievements in this area include both pentavalent and trivalent phosphorous-containing leaving groups. Glycosyl phosphoroamidates, phosphorimidates and phosphoramidimidates come into the first category. prepared from Shelf-stable glycosyl N,N,N’,N’-tetramethylphosphoroamidates, both O-benzyl and O-benzoyl-protected D-glucopyranose and D-galactopyranose, were coupled with a variety of primary and secondary glycoside alcohols in propionitrile in the presence of TMSOTf at -78 “C. In all cases, the chemical yields were very high as well as the p-stereoselectivities, demonstrating the considerable versatility of this glycosylation method [ 1431. Recent reports detail how this methodology has been extended. The high stability of glycosyl N,N,N’,N’-tetramethylphosphoroamidates allowed the reactivity to be modulated either by varying the nature of the protecting groups (benzyls or benzoyls) or the glycosylation conditions (Lewis acid and reaction temperature). This feature was exploited in a novel application of the “armed-disarmed” principle introduced by Fraser-Reid [ 1441. Thus, the “armed” donor 154 was coupled with the “disarmed” acceptor 155 under the aforementioned conditions to provide disaccharide 156 in 86% yield ( a :(3 = 5:95). Switching the protective groups from
230
9 Other Methods of Glycosylation
alp 5/95
BnO RO 154
155
156: R = Bz
ii
(OBn
158
0
oP@Me2)
157: R = Bn
BzO 0PWez)z
Scheme 41. Reagents: i) TMSOTf, EtCN, -78”C, 86%; ii) NaOMe, MeOH, then BnBr, NaH, DMF, 89% overall yield; iii) 155, TMSOTf, EtCN, -78 “C, 83%.
benzoyls to benzyls, the “armed” donor 157 was obtained, the glycosylation of which with acceptor 155 gave trisaccharide 158 in 83% yield (a: = 13:87, Scheme 41) [145]. On the other hand, the “armed-disarmed’’ concept was applied to glycosylation reactions between glycosyl N,N,N’,N‘-tetramethylphosphoroamidates and glycosides bearing other phosphorous-containing leaving groups, such as glycosyl diphenylphosphinimidates, phosphites and phosphorodiamidimidothioates, on each occasion taking advantage of their well documented reactivity differences [ 1451. An illustrative example is showed in Scheme 42. Fully benzylated glycosyl phosphites, such as 159, are known to be readily activated with BF3 . OEt, even at -78 “C giving very high p-selectivities, whereas the corresponding tetramethylphosphoroamidates were unreactive at below - 10 “C in the same conditions. The “armed” donor 159 was therefore coupled with
BnO
alp 3197
BnO
160
BnO
Bn 157 159
161
OMe
67
Scheme 42. Reagents: i) BF3 OEtl, CH2C12, -7O”C, 66%, only -78”C, 88%.
p; ii) 67, TMSOTf, EtCN,
9.3 Esters and Related Derioatives
-T&
T&OP(OEtJ2
-,
NF’h
II
0 OEt P-OEt
23 1
promoter ROH
(OMe
alp 1/30 0-P-OEt Me0
Me0 162
I
OEt
+
BnO I63
BnO BnO
164
Scheme 43. Reagents: i) PhN3, benzene, 50°C; ii) TMSOTf, EtCN, -78”C, 7 5 %
the “disarmed” acceptor 160 at -70°C giving disaccharide 157 in 66%1 yield (only p-anomer). This was subsequently activated with TMSOTf at -78°C in the presence of the alcohol 67 to afford the trisaccharide 161 (88%) a : p = 3 :97). Glycosyl N-phenyl diethylphosphorimidates were prepared by the treatment of the corresponding glycosyl phosphites with phenyl azide and used without further purification in glycosylation reactions with primary and secondary glycosyl acceptors. The chemical yields were generally good, whereas the stereoselectivities ranged from fair to only moderate. Scheme 43 shows an overview of the procedure and a selected example [ 1461. Interestingly, the authors observed a general a-selectivity using lutidinium ptoluensulfonate/tetrabutylammonium iodide as promoters and P-selectivity in the presence of TMSOTf. Moreover, the highest selectivities (a or p ) were obtained for the primary acceptor alcohols, whereas the ratios drop with secondary acceptors. The treatment of glycopyranoses with ethoxy bis(diisopropy1amino)phosphine in the presence of diisopropylammonium tetrazolide afforded the corresponding glycosy1 phosphoramidites 166 which, by reaction with phenyl azide, gave the glycosyl phosphoramidimidates 167. When they were used as glycosyl donors with various acceptors, the corresponding disaccharides were obtained, but only with modest pstereoselectivities. The best results were achieved with primary alcohols in propionitrile, in the presence of TMSOTf or BF3 . OEt2 (Scheme 44)[ 1471. The glycosyl phosphoramidites themselves were explored as glycosyl donors. First, 2-deoxyglycosidic linkages were synthesized. Excellent a-stereoselectivities were achieved using 2-deoxyglycosyl N,N-diisopropylphosphoramiditesas glycosyl donors, even in the case of hindered alcohol acceptors [148]. Compounds 169 were then coupled with selected primary and secondary glycosyl acceptors to give disaccharides with high p : a ratios, either using TMSOTf or BF3 . OEt2 as promoters. Propionitrile was found to be the best solvent for achieving high selectivities as well as good chemical yields (Scheme 45) [149]. Glycosyl diphenylphosphinyl donors were recently explored for the synthesis of D-ribofuranosides: the couplings were conducted with 1.5 eq. of TMSOTf at low temperature using simple alcohols or a-aminoacids as acceptors. Although only
232
9 Other Methods of Glycosylation
Me0 Me0
\
Me0
-
NPf, 165
(OMe
0-P-OEt
I
Me0
NPr',
166
...
167
ci/p 1/20
111
.___)
/OH
67
BnO
OMe
168
Scheme 44. Reagents: i) EtOP(NPr2')2, diisopropylammonium tetrazolide, CH2C12, rt; ii) PhN3, benzene, 45 "C; iii) 67,TMSOTf, EtCN, -78 "C, 73%.
modest stereoselectivities were observed, the glycosides were obtained in excellent yields. However, when a glucosyl acceptor was tested, the formation of a complex mixture of products was observed [ 1501. Glycosyl phosphorodithioates 171 (Scheme 46), prepared by action of 0,Odialkylphosphorodithioic acid on the corresponding glycals, were employed as glycosy1 donors in the synthesis of 2-deoxydisaccharides. The glycosylations of primary and secondary acceptors in the presence of silver fluoride proceeded sluggishly (from 7 to 12 days) and the a-stereoselectivities were somewhat low, except in a few cases [151]. This procedure was extended by the preparation of the related glycosyl phosphorothioates 172 and phosphoroselenoates 173 (Scheme 46). They were used as donors in the glycosylation of Diclofenac, a non-steroidal agent with potent antiinflammatory, analgesic and antipyretic activity, in order to produce new Diclofenacderived prodrugs. The corresponding glycosyl esters were obtained in good yields (72-84%) and with excellent P-selectivity, using silver carbonate as a promoter [ 1521. Among the sulfur containing leaving groups, glycosyl sulfonates have been described 11.531, but they were not further developed in recent years, although glycosyl triflates have been claimed as reactive intermediates in many glycosylation reaction 11541. Only one example of the use of glycosyl sulfates as glycosyl donors has been reported [ 1551. They were easily prepared from gluco- and mannopyranoses and em-
0-P
BnO BnO 169
BnO
9/91
BnO 95
Scheme 45. Reagents: i) BF3 . OEtz, EtCN, -78 "C, 92%.
170
References
171
172
233
173
Scheme 46. Glycosyl phosphorodithioates (171), glycosyl phosphorothioates (172) and glycosyl phosphoroselenoates (173).
ployed in the glycosylation of simple alcohols as well as primary and secondary sugar acceptors, providing the corresponding glycosides in fair to good yields, but with low stereoselectivities. Finally, sugar 1,2-cyclic sulfites have been investigated as glycosyl donors. Preliminary studies exploited these compounds for the introduction of reactive nucleophiles, such as azide or benzoate, on the anomeric position [156]. Effective b-0glycosylations with sugar cyclic sulfites were then reported, under catalysis of lanthanide(II1) triflates. However, the reactions required high temperatures (from 80 to 100 "C),limiting their application only to simple alcohols [ 1571. References I. a) K. Suzuki, T. Mukaiyama, Chem. Lett., 1982, 683-686; b) K. Suzuki, T. Mukaiyama, Chem. Lett., 1982, 1525-1528. 2. a) J. M. Beau, R. Schauer, J. Haverkamp, L. Dorland, J. F. G. Vliegenthart, P. Sinay, Curbohydr. Rex, 1980, 82, 125-129; b) F. Paquet, P. Sinay, Tetrahedron Lett., 1984, 25, 30713074. 3. F. Paquet, P. Sinay, J. Am. Chem. Soc., 1984, 106, 8313-8315. 4. a) F. Nicotra, L. Panza, F. Ronchetti, G. Russo, L. Toma, Tetrahedron Lett., 1985, 26, 807808; b) F. Nicotra, L. Panza, F. Ronchetti, G. Russo, L. Toma, J. Chem. SOC.,Perkin Trans. 1987, I , 1319-1324. 5. A. Boschetti, L. Panzd, F. Ronchetti, G. Russo, J. Chem. Soc., Perkin Trans. 1988, 1 , 33533357. 6. A. G. M. Barrett, B. C. B. Bezuidenhoudt, A. F. Gasiecki, A. R. Howell, M. A. Russell, J. Am. Chem. Soc., 1989, I l l , 1392-1396. 7. A. G. M. Barrett, B. C. B. Bezuidenhoudt, L. M. Melcher, J. Org. Chem., 1990; 55, 51965197. 8. A. Haudrechy, P. Sinay J. Org. Chem., 1992, 57, 4142-4151. 9. R. U. Lemieux, S. Levine, Can. J. Chem., 1%4,42, 1473-1480. 10. R. W. Friesen, S. J. Danishefsky, J. Am. Chem. SOC.,1989, 111, 6656-6660. 1 1 . K. Tatsutd, K. Fujimoto, M. Kinoshita, S. Umezawa, Carbohydr. Rex, 1977, 54, 85-104. 12. a) K. Tatsuta, T. Yamauchi, M. Kinoshita, Bull. Chem. Soc. Jpn. 1978, 51, 3035-3038; b) K. Tatsuta, Y . Amemiya, Y. Kanemura, H. Takahashi, M. Kinoshita, Tetrahedron Lett., 1982, 23, 3375-3378; c) K. Toshima, K. Tatsuta, M. Kinoshita, Tetrahedron Lett., 1986, 27, 47414741. 13. a) J. Thiem, H. Karl, J. Schwenter, Synthesis, 1978, 696-699; b) J. Thiem, H. Karl, Tetrahedron Lett., 1978, 4999-5002. 14. a) J. Thiem, S. Kopper, Tetrahedron, 1990, 46, 113-138; b) D. Horton, W. Priebe, M. Snaitzman, Carbohydr. Res., 1989, 187, 149-153; c) B. Abbaci, J. C. Florent, C. Monneret, J. Chem. Soc., Chem. Commun., 1989, 1896-1897; d) S. J. Danishefsky, H. G. Selnick, D. M. Annistead, F. E. Wincott, J. Am. Chem. Soc., 1987, 109, 8119-8120.
234 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.
9 Other Methods of Glycosylation
J. Thiem, W. Klafie, J. Org. Chem., 1989, 54, 2006-2007. G. Jaurand, J. M. Beau, P. Sinay, J. Chem. Soc., Chem. Commun., 1981, 572-573. A. G. M. Barrett, T. A. Miller, Tetrahedron Lett., 1988,29, 1873-1874. Y. Ito, T. Ogawa, Tetrahedron Lett., 1987, 28, 2723-2726. G. Grewal, N. Kaila, R. W. Franck, J. Org. Chem., 1992, 57, 2084-2092. C. J. Tu, D. Lednicer, J. Org. Chem., 1987, 52, 5624-5627. See e.g. K. Toshima, K. Tatsuta, M. Kinoshita, Bull. Chem. Soc. Jpn., 1988, 61, 2369-2381. V. Bolitt, C. Mioskowski, S. G. Lee, J. R. Falck, J. Org. Chem., 1990, 55, 5812-5813. S. Sabesan, S. Neira, J. Org. Chem., 1991, 56, 5468-5472. J. Thiem, M. Kleeberg, Carbohydr. Rex, 1990, 205, 333-345. D. Noort, G. H. Veeneman, G. J. P. H. Boons, G. A. van der Marel, G. J. Mulder, J. H. van Boom, Synlett, 1990,205-207. 26. F. Nicotra, L. Panza, G. Russo, Tetrahedron Lett., 1991, 32, 4035-4038. 27. L. Lay, F. Nicotra, L. Panza, G. Russo, G. Sello, J. Carbohydr. Chem., 1998,17, 1269-1281. 28. A. Marra, J. Esnault, A. Veyrieres, P. Sinay, J. Am. Chem. Soc., 1992, 114, 6354-6360. 29. a) H. K. Chenault, A. Castro, L. F. Chafin, J. Yang, J. Org. Chem., 1996, 61, 5024-5031; b) H. K. Chenault, L. F. Chafin, J. Org. Chem., 1998,63, 833-840. 30. a) G. J. Boons, S. Isles, Tetrahedron Lett., 1994, 35, 3593-3596; b) G. J. Boons, S. Isles, J. Org. Chem., 1996, 61, 4262-427 1. 31. G. J. Boons, B. Heskamp, F. Hout, Angew. Chem. Int. Ed. Engl., 1996,35, 2845-2847. 32. M. Johnson, C. Arles, G. J. Boons, Tetrahedron Lett., 1998, 39, 9801-9804. Perkin Trans. 1997, I , 3357-3360. 33. R. R. Gibson, R. P. Dickinson, G. J. Boons, J. Chem. SOC., 34. Y. D. Vankar, P. S. Vankar, M. Behrendt, R. R. Schmidt, Tetrahedron, 1991,47, 9985-9992. 35. Y. Osa, K. Takeda, T. Sato, E. Kaji, Y. Mizuno, H. Takayanagi, Tetrahedron Lett., 1999, 40, I53 1-1 534. 36. C. H. Marzabadi, R. W. Franck, J. Chem. Soc., Chem. Commun., 1996, 2651L2652. 37. R. W. Franck, C. H. Marzabadi, J. Org. Chem., 1998,63, 2197-2208. 38. M. L. Sznaidman, S. C. Johnson, C. Crasto, S. M. Hetch, J. Org. Chem., 1995, 60, 39423943. See also the preparation of disaccharides by cycloaddition to sugar cyclic ketene acetals: S. C. Johnson, C. Crasto, S. M. Hetch, J. Chem. Soc., Chem. Commun., 1998, 1019-1020. 39. E. Fischer, Chem. Ber., 1893, 2400-2412. 40. See e.g.: a) A. A. Pavia, J. M. Rocheville, S. N. Ung-Chhun, Carbohydr. Res., 1980, 79, 7989; b) A. A. Pavia, S. N. Ung-Chhun, Can. J. Chem., 1981, 59, 482-489 (synthesis of trehaloses); c) A. Lubineau, J. C. Fischer, Synth. Commun., 1991,21, 815-818 (glycoside synthesis). 41. H. P. Wessel, J. Carbohydr. Chem., 1988, 7, 263-269. 42. P. Konraddson, C. Roberts, B. Fraser-Reid, Reel. Trav. Chim. Pays-Bas, 1991, 110, 23-24. 43. S. Horito, M. Tada, H. Hashimoto, Chem. Lett., 1991, 117-120. 44. B. Fischer, A. Nudelman, M. Ruse, J. Herzig, H. E. Gottlieb, J. Org. Chem., 1984, 49, 49884993. 45. J. Defaye, A. Gadelle, C. Pedersen, Carbohydr. Rex, 1989, 186, 177-188. 46. a) H. Uchiro, T. Mukaiyama, Chem. Lett., 1996, 79-80; b) H. Uchiro, T. Mukaiyama, Chem. Lett., 1996, 271-272 47. K. Takeuchi, S. Higuchi, T. Mukaiyama, Chem. Lett., 1997, 969-970. 48. a) K. Tomooka, Y. Nakamura, T. Nakdi, Synlett, 1995, 321-322; b) T. Jyojima, N. Miyamoto, Y. Ogawa, S. Matsumura, K. Toshima, Tetrahedron Lett., 1999, 40, 5023-5026. 49. K. Toshima, H. Nagai, S. Matsumura, Synlett, 1999, 1420-1422. 50. J. Inanaga, Y. Yokoyama, T. Hanamoto, J. Chem. Soc., Chem. Commun., 1993, 1090-1091. 51. H. Susaki, Chem. Pharm. Bull., 1994, 42, 1917-1918. 52. T. Mukaiyama, K. Matsubara, M. Hora, Synthesis, 1994, 1368-1373. 53. H. Uchiro, K. Miyazaki, T. Mukaiyama, Chem. Lett., 1997, 403-404. 54. T. Mukaiyama, S. Suda, Chem. Lett., 1990, 1143-1146. 55. S. Suda, T. Mukaiyama, Chem. Lett., 1991, 431-434. 56. T. Mukaiyama, K. Matsubara, S. Suda, Chem. Lett., 1991, 981-984. 51. B. A. Garcia, J. L. Poole, D. Y. Gin, J. Am. Chem. Soc., 1997,119, 7597-7598. 58. S. Koto, N. Morishima, S. Zen, Chem. Lett., 1976, 1109-1110. 59. S. Koto, N. Morishima, S. Zen, Bull. Chem. Soc. Jpn., 1982, 55, 1543-1547.
References
235
60. S. Koto, N. Morishima, C. Kusuhara, S. Sekido, T. Yoshida, S. Zen, Bull. Chem. Soc. Jpn., 1982,55,2995-2999. 61. S. Koto, T. Sato, N. Morishima, S. Zen, Bull. Chem. Soc. Jpn., 1980, 53, 1761-1762. 62. a) N. Morishima, S. Koto, S. Zen, Chem. Lett., 1982, 1039-1040; b) S. Koto, N. Morishima, M. Owa, S. Zen, Carhohydr. Res., 1984, 130, 73-83. 63. W. Szeja, Synthesis, 1988, 223-224. 64. M. Hirooka, S. Koto, Bull. Chem. Soc. Jpn., 1998, 71, 2893-2902. 65. M. Wakao, Y. Nakai, K. Fukase, S. Kusumoto, Chem. Lett., 1999, 27-28. 66. G. Grynkiewicz, Carbohydr. Rex, 1977, 53, CI 1-C12. 67. a) P. J. Garegg, T. Iversen, T. Norberg, Carbohydr. Res., 1979, 73, 313-314; b) K. Akerfeldt, P. J. Garegg, T. Iversen, Acta Chem. Scand. B, 1979, 33, 467-468. 68. A. Lubineau, E. Meyer, P. Place, Carbohydr. Rex, 1992, 228, 191-203. 69. a) N. Chida, M. Ohtsuka, S. Ogawa, Chem. Lett., 1988, 969-972. 70. a) W. R. Roush, X. F. Lin, J. Org. Chem., 1991,56,5740-5742; b) W. R. Roush, X. F. Lin, J. Am. Chem. Soc, 1995,117, 2236--2250. 71. A. Dondoni, A. Marra, M. C. Scherrmann, A. Casnati, F. Sansone, R. Ungaro, Chem. Eur. J., 1997, 3, 1774-1782. 72. G. T. Badmdn, D. V. S. Green, M. Voyle, J. Oryanomet. Chem., 1990, 388, 117-121. 73. A. Sobti, G. A. Sulikowski, Tetrahedron Lett.: 1994, 35, 3661-3664. 74. a) A. B. Smith, 111, K. J. Hale, R. A. Rivero, Tetrahedron Lett., 1986, 27, 5813-5816; A. B. Smith, 111, R. A. Rivero, K. J. Hale, H. A. Vaccaro, J. Am. Chem. Soc, 1991, 113, 2092-2112. 75. W. A. Szarek, H. C. Jarpell, J. K. N. Jones, Curhohydr. Res., 1977, 57, C13-CI6. 76. A. Bouali, G. Descotes, D. F. Ewing, A. Grouiller, J. Lefkidou, A. D. Lespinasse, G. Mackenzie, J. Curbohydr. Chem., 1992, 11, 159-169. 77. K. C. Nicolaou, R. D. Groneberg, J. Am. Chem. Soc, 1990, 112, 4085-4086. 78. L. F. Tietze, R. Fischer, H. J. Guder, Tetrahedron Lett., 1982, 23, 4661L4664. 79. E. M. Nashed, C. P. J. Glaudemans, J. Org. Chem., 1989, 54, 6116-6118. 80. D. X. Qiu, F. Y. Wang, M. S. Cai, Syntli. Coinmun., 1989, 19, 3453-3456. 81. a) C. Kolar, G. Kneissel, Angew. Chern. Int. Ed. Engl., 1990, 29, 809-810; b) C. Kolar, G. Kneissel, U. Knodler, K. Dehmel, Curhohydr. Rex, 1991, 209, 89-100. 82. W. Priebe, G . Grynkiewicz, N. Neamati, Tetruhrdron Lett., 1991, 32, 2079-2082. 83. T. Mukaiyama, K. Matsubara, Chem. Lett., 1992, 1041- 1044. 84. A. Kirschning, G. Chen, Tetrahedron Lett., 1999, 40, 4665~-4668. 85. B. Helferich, E. Shimitz-Hillebrecht, Chem. Ber., 1933, 66, 378--383. 86. K. Toshima, K. Tatsuta, Chem. Rev.,1993, 93, 1503-1531 87. a) R. U. Lemieux, W. P. Shyluk, Can. J. Chem., 1953, 31, 528-535; b) S. Hanessian, Y. Banoub, Curhohydr. Res., 1977, 59, 261-267. 88. a) M. Kiso, L. Anderson, Carhohydr. Res., 1979, 72, C15-CI7; b) L. M. Lerner, Carhohydr. Rex 1990,207, 138-141. 89. J. Dahmen, T. Frejd, G. Magnusson, G. Noori, Carbohydr. Res., 1983, 114, 328-330. 90. T. Ogawa, K. Beppu, S. Nakabayashi, Carbohydr. Res., 1981, 93, C6--C9. 91. T. Mukaiyama, S. Kobayashi, S. Shoda, Chem. Lett., 1984, 907-910. 92. T. Mukaiyama, T. Shinipuku, T. Takashima, S. Kobayashi, Chem. Lett., 1989, 145-148. 93. a) T. Mukaiyama, T. Takashima, M. Katsurada, H. Aizawa, Chem. Lett., 1991, 533-536; b) T. Mukaiyama, M. Katsurada, T. Takashima, Chem. Lett., 1991, 985-988. 94. K. Matsubara, T. Sasaki, T. Mukaiyama, Chem. Lett., 1993, 1373-1376. 95. T. Mukaiyama, M. Katsurada, Y. Iida, Chem. Lett., 1992, 2105-2108. 96. T. Mukaiyama, K. Matsubara, Chem. Lett., 1992, 1755-1758. 97. a) T. Mukaiyama, N. Shimomura, Chem. Lett., 1993, 781-784; b) N. Shimomura, M. Saitoh, T. Mukaiyama, Chem. Lett., 1994, 433-436. 98. A. K. Pathak, Y. A. El-Kattan, N. Bansal, J. A. Maddry, R. C. Reynolds, Tetrahedron Lett., 1998,39, 1497-1500. 99. T. Mukaiyama, T. Sasaki, E. Iwashita, K. Matsubara, Clzem. Lett., 1995, 455--456. 100. T. Vuljanic, J. Kihlberg, P. Somfai, J. Org. Chem., 1998, 63, 279--286. 101. K. Toshima, Y . Nozaki, S. Mukaiyama, K. Tatsuta, Tetrahedron Lett., 1992, 33, 1491-1494. 102. Y. Inanaga, Y. Yokoyama, T. Hanamoto, Tetrahedron Lett., 1993,34, 2791-2794.
236 103. 104. 105. 106.
9 Other Methods of Glycosylation
K. M. Nicholas, Acc. Chem. Res., 1987, 20, 207-214. C. Mukai, T. Itoh, M. Hanaoka, Tetrahedron Lett., 1997, 38, 4595-4598. F. Nicotra, L. Panza, A. Romano, G. Russo, J. Curbohydr. Chem., 1992, 11, 397-399. a) T. Mukaiyama, K. Miyazaki, H. Uchiro, Chem. Lett., 1998, 635-636; b) T. Mukaiyama, Y. Wakiyama, K. Miyazaki, K. Takeuchi, Chem. Lett., 1999, 933-934. 107. T. Iimori, T. Shibazaki, S. Ikegami, Tetruhedron Lett., 1996, 37, 2267-2270. 108. G. Scheffler,R. R. Schmidt, Tetrahedron Lett., 1997,38, 2943-2946. 109. T. Iimori, I. Azumaya, T. Shibazaki, S. Ikegami, Heterocycles, 1997, 46, 221-224. 110. I. Azumaya, T. Niwa, M. Kotani, T. Iimori, S. Ikegami, Tetrahedron Lett., 1999, 40, 46834686. 111. a) M. J. Ford, S. V. Ley, Synlett, 1990, 255-256; b) M. J. Ford, J. G. Knight, S. V. Ley, S. Vile, Synlett, 1990, 331-332. 112. S. Hanessian, C. Bacquet, N. Lehong, Curbohydr. Res., 1980, 80, C17-C22. 113. K. Koide, M. Ohno, S. Kobayashi, Tetrahedron Lett., 1991, 32, 7065-7068. 114. H. Furukawa, K. Koide, K. Takao, S. Kobayashi, Chem. Pharm. Bull., 1998,46, 1244-1247. 115. a) S. Hanessian, J. J. Conde, B. Lou, Tetrahedron Lett., 1995, 36, 5865-5868; b) S. Hanessian, J. J. Conde, H. H. Khai, B. Lou, Tetrahedron, 1996, 52, 10827-10834. 116. H. Kunz, J. Zimmer, Tetrahedron Lett., 1993, 34, 2907-2910. 117. J. C. Lopez, B. Fraser-Reid, J. Chem. Soc., Chem. Comrnun., 1991, 159-161. 118. H. Herzner, J. Eberling, M. Schultz, J. Zimmer, H. Kunz, J . Carbohydr. Chem., 1998, 17, 759-776. 119. N. K. Kochetkov, A. F. Bochkov, T. A. Sokolovskaya, V. J. Snyatkova, Curbohydr. Rex, 1971,16, 17-27. 120. Y. E. Tsvetkov, P. I. Kitov, L. V. Backinowsky, N. K. Kochetkov, Tetrahedron Lett., 1993, 34, 7977-7980. 121. W. Wang, F. Kong, J. Org. Chem., 1998,63, 5744-5745. 122. Y. Du, F. Kong, J. Carbohydr. Chem., 1999,18,655-666. 123. W. Wang, F. Kong, J. Curbohydr. Chem., 1999, 18, 451-460. 124. W. Wang, F. Kong, J. Org. Chem., 1999,64, 5091-5095. 125. W. Wang. F. Kong, Angew. Chem. Int. Ed. Engl., 1999,38, 1247-1250. 126. W. Wang, F. Kong, Tetrahedron Lett., 1998, 39, 1937-1940. 127. W. Wang, F. Kong, Curbohydr. Rex, 1999, 315, 117-127. 128. W. Wang, F. Kong, J. Curboydr. Chem., 1999,18, 263-273. 129. T. Iimori, H. Ohtake, S. Ikegami, Tetrahedron Lett., 1997,38, 3413-3414. 130. T. Iimori, H. Ohtake, S. Ikegami, Tetrahedron Lett., 1997,38, 3415-3418. 131. H. Ohtake, T. Iimori, S. Ikegami, Synlett, 1998, 1420-1422. 132. S. Hiranuma, 0. Kanie, C.-H. Wong, Tetrahedron Lett., 1999, 40, 6423-6426. 133. F. Nakatsubo, H. Kamitakahara, M. Hori, J. Am. Chem. Soc., 1996,118, 1677-1681. 134. M. Hori, H. Kamitakahara, F. Nakatsubo, Macromolecules, 1997, 30, 2891-2896. 135. a) K. Kurita, M. Kobayashi, T. Munakata, S. Ishii, S. Nishimura, Chem. Lett., 1994, 20632066; b) K. Kurita, H. Akao, M. Kobayashi, T. Mori, Y. Nishiyama, Polymer Bull., 1997,39, 543-549; c) K. Kurita, K. Shimada, Y. Nishiyama, M. Shimojoh, S. Nishimura, Mucromolecules, 1998, 31, 4764-4769. 136. a) L. Lay, L. Panza, G. Russo, D. Colombo, F. Ronchetti, E. Adobati, S. Canevari, Helu. Chim. Actu, 1995, 78, 533-538; b) D. Colombo, L. Panza, F. Ronchetti, Curbohydr. Rex, 1995,276, 437-441; c) J. Banoub, P. Boullanger, D. Lafont, Chem. Rev., 1992, 92, 1167-1 195 and references cited therein. 137. P. Brusca, 0. R. Martin, Tetrahedron Lett., 1998, 39, 8101-8104. 138. a) J. Kadokawa, Y. Watanabe, M. Karasu, H. Tagaya, K. Chiba, Mucromol. Rapid Commun., 1996,17, 367-372; b) J. Kadokawa, S. Kasai, Y. Watanabe, M. Karasu, H. Tagaya, K. Chiba, Macromolecules, 1997, 30, 8212-8217. 139. J. Xia, C. F. Piskorz, R. D. Locke, E. V. Chandrasekaran, J. L. Alderfer, K. L. Matta, Bioorg. Med. Chem. Lett., 1999, 9, 2941L2946. 140. S. Kobayashi, T. Kiyosada, S. Shoda, Tetrahedron Lett., 1997, 38, 21 11-21 12. 141. S. Hashimoto, T. Honda, S. Ikegami, Tetruhedron Lett., 1991, 32, 1653-1654. 142. S. Hashimoto, T. Honda, S. Ikegami, Tetrahedron Lett., 1990, 31, 4769-4772.
References
237
143. S. Hashimoto, Y. Yanagiya, T. Honda, H. Harada, S. Ikegami, Tetrahedron Lett., 1992, 33, 3523-3526. 144. D. R. Mootoo, P. Konradsson, U. Udodong, B. Fraser- Reid, J. Am. Chem. Soc., 1988, 110, 5583-5584. 145. S. Hashimoto, H. Sakamoto, T. Honda, S. Ikegami, Tetrahedron Lett., 1997,38, 5181-5184. 146. S. Pan, H. Li, F. Hong, B. Yu, K. Zhao, Tetrahedron Lett., 1997, 38, 6139-6142. 147. M.-J. Chen, K. Ravindran, D. Landry, K. Zhao, Heterocycles, 1997, 45, 1247- 1250. 148. H. Li, M.-J. Chen, K. Zhao, Tetrahedron Lett., 1997, 38, 6143-6144. 149. D. Niu, M.-J. Chen, H. Li, K. Zhao, Heterocycles, 1998, 48, 21-24. 150. G. Singh, I. Tranoy, Carbohydr. Lett., 1998, 3, 79 -84. 151. H. Bielawska, M. Michalska, J. Carbohydr. Chem., 1991, 10, 107-112. 152. J. Borowiecka, Liebigs Ann., 1997, 2147-21 50. 153. R. Eby, C. Schuerch, Carbohydr. ReJ., 1982, 102, 131-138. 154. D. Crich, W. Cai, J. Ory. Chem., 1999, 64, 4926-4930. 155. L. Cipolla, L. Lay, F. Nicotra, L. Panza, G. Russo, Tetrahedron Lett., 1994,35, 8669-8670. 156. A. Meslouti, D. Beaupere, G. Demailly, R. Uzan, Tetrahedron Lett., 1994, 35, 3913-3916. 157. W. J. Sanders, L. L. Kiessling, Tetrahedron Lett., 1994, 35, 7335-7338.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
10 Polymer-Supported Synthesis of Oligosaccharides Jiri J. Krepinsky and Stephen P. Douglas
10.1 Introduction Among the variety of biological activities of oligosaccharides, antiinfective effects are the reason some oligosaccharides are close to a practical use [ 11, provided that the oligosaccharides can be cleanly made on large scale. Glycosylations by enzymic (cf. Part 3 of this Volume) and chemical methods compete to meet this challenge. Classical chemical methods of oligosaccharide synthesis invariably require extensive purification of the desired reaction products, mostly by chromatography, which makes chemical syntheses of oligosaccharides very laborious and expensive. Similar problems encountered in the syntheses of oligopeptides and oligonucleotides were dealt with successfully using polymer-supported synthetic and purification techniques. A polymer support facilitates quick and efficient purification of products assuming that these reactions proceed to completion. Although some indications suggest that polymer supports may influence the glycosylation reaction itself [2], the primary purpose of the polymer support is rapid purification after every step in the reaction sequence leading to the formation of an oligosaccharide [ 3 ] . Therefore, polymer supports will not make the chemistry of oligosaccharide synthesis any simpler which is unfortunate since this chemistry is complex and many details important for the synthesis remain unknown despite the rapid progress in this area during recent years documented, for instance, in this Volume. However, because of the significantly simpler purification, synthetic sequences become enormously faster. Furthermore, for syntheses of combinatorial oligosaccharide libraries, polymer support is essential since it allows purifications of the desired intermediates or products as groups, which is difficult to accomplish otherwise. Four factors combine to make oligosaccharide synthesis challenging: i) several hydroxyl groups of similar reactivity are present in monosaccharides, from which oligosaccharides are generally constructed and which almost invariably require laborious protection-deprotection schemes; ii) difficulty to form anomerically pure glycosidic linkages; iii) acid lability of the glycosidic linkages (hemi-acetals); iv)
240
10 Polymer-Supported Synthesis of Oligosaccharides
incompletely known intramolecular interactions. The most common glycosylation technique utilizes a nucleophilic substitution at the anomeric carbon with a hydroxyl group (“acceptor”) of the aglycon. In order to prevent other hydroxyls from taking part in this reaction, they must be protected [4] (and subsequently deprotected) by methods compatible with the labile glycosidic linkages [ 5 ] . As is common for acetal formation reactions, glycosylations are often both incomplete and accompanied by numerous side-reactions, so that chromatography is usually needed to clean desired products after each synthetic step. Therefore, the synthesis of an oligosaccharide in solution is extremely time consuming, costly, and environmentally unfriendly in part because of the large volumes of solvents used. Despite the ability of this methodology (c.f. this Volume) to provide almost any oligosacaccharide [6], if the chromatographic purification is the bottleneck step in the chemical oligosaccharide syntheses, removal of the requirement for chromatography may make chemical synthesis of oligosaccharides significantly more practical. Methods employing enzymes have been developed to overcome these difficulties (cf. Part 3 of this Volume). Glycosyltransferases [7] and glycosidases working in reverse [S], or combined enzyme and chemical strategies [9, 101 have been examined. Syntheses employing glycosyl transferases using soluble polymer supports, such as water-soluble polyvinyl alcohol [ 1I] and polyacrylamide [ 121, to which a monosaccharide was bound via a photolabile (O-nitrobenzyl) linker, also established that a spacer between the polymer support and the oligosaccharide must be of certain length so that the reactions can occur [13]. Despite considerable advantages, such as not needing a hydroxyl protection-deprotection scheme, enzymic syntheses have their own problems. One problem is the requirement of glycosyl transferases to employ relatively unstable and difficult to prepare intermediates. Furthermore, transferases cannot as yet be used to synthesize oligosaccharides unknown in Nature.
10.2 General Reflections Polymer supported designs of oligosaccharide synthesis began to be explored thirty years ago encouraged by the solution of analogous problems in the syntheses of oligonucleotides and oligopeptides by polymer-supported strategies. The spectacular success of solid-state syntheses of oligopeptides and oligonucleotides on polystyrene resins and controlled-pore glass is due not only to the polymer support but also to specifically designed chemical procedures to be performed on solid state support. The main benefit of procedures involving solid state reactants, i.e. the purification of desired intermediates from by-products, excess reagents and products of their decomposition by simple washing, is combined with prompt quenching of the reaction and fast work-up, eliminating overexposure of reaction products to excess reagents and by-products. The first polymer-supported oligosaccharide synthesis by Schuerch’s group employed the Koenigs-Knorr reaction and its variants, using an immobilized glycosyl acceptor on an organic solid polymer (Scheme 1) [ 141. It was followed by inves-
10.2 General Reflections
+
J
t
P
& O Q ! ! j
P
24 1
242
10 Polymer-Supported Synthesis of Oligosaccharides
tigations of an inorganic support, namely porous glass, zirconized and silanized [ 151. The results were not satisfactory; among the problems were decreased reaction rate and lower yields compared to solution strategies, incomplete coupling, and poor anomeric specificity. These problems seemed to be attributed to the solid state rather than its incompatibility with certain aspects of Koenigs-Knorr chemistry. Unfortunately, these difficulties stymied further progress in polymer-supported glycosylations, although the method was not entirely abandoned by van Boom [16, 171 and others [ 181, providing various oligosaccharides using Merrifield cross-linked polystyrene and controlled-pore glass. Although it was suggested that the use of a soluble polymer could improve the results [19], it took another twenty years until the first report in 1991 from Krepinsky’s group on successful1 applications of polyethyleneglycol as soluble support was published (Scheme 1) [20]. Since both Koenigs-Knorr (promoted by silver triflate) and trichloroacetimidate chemistry gave comparable results on polyethylene glycol, it seemed to confirm the belief that it was the solid state support, i.e. two phase condition, which failed glycosylations [20]. That this conclusion was erroneous was demonstrated when the glycal method of glycosylation developed by Danishefsky’s group 1211, was exploited by that group for the synthesis of oligosaccharides using immobilized glycosyl donors on polystyrene support 1221. This scenario is reported to provide excellent anomeric specificity although an exception was noticed [23, 241, which was subsequently remedied by a modification of the procedure 1251. Oligosaccharide chains can be extended (i) from the reducing end to the nonreducing terminus (using polymer-supported acceptors), as well as (ii) from the nonreducing end to the reducing terminus (using polymer-supported donors) (Scheme 2). Both strategies have serious problems due to incomplete glycosylations and/or donor decomposition. Assuming that a small portion (realistically perhaps 3-5%) of polymer-supported monosaccharide M1 acceptor fails to be glycosylated to a disaccharide D1, the disaccharide D1 in the subsequent step is extended to a trisaccharide T1, but a portion of the previously unglycosylated monosaccharide M 1 gives disaccharide D2. These second round glycosylations are again incomplete. After the third glycosylation the following compounds will be polymer-bound in addition to the desired tetrasaccharide: the monosaccharide M 1, disaccharides D 1, D2, and D3, and trisaccharides T1 and T2. After each additional glycosylation the mixture becomes even more complicated, yet all impurities originating from the donors are absent. Thus the desired product must be separated from the truncated sequences (= failures), after detachment from the polymer support. To alleviate the situation, excess glycosylation reactants are recommended to be applied in portions (Scheme 2), as is practiced to obtain “difficult sequences” in polymer-supported oligopeptide syntheses. Incomplete anomeric specificity causes further complications. Starting the synthesis from the nonreducing terminus, i.e. using a polymer-bound donor presents the same problem of truncated sequences. Since donors are usually quite reactive, a portion of a donor may be transformed into molecules which do not react anymore with acceptors and since they are fastened to the polymer, they are carried through the synthetic sequence (Scheme 2) and must be removed by chromatography at the end of the synthesis. This will remain problematic unless
10.2 General Reflections
k
\
4
I
4
0
\
243
244
10 Polymer-Supported Synthesis of Oligosaccharides
Glycal Assembly
0
2 Self-policingFailure
ZnClDHF Due to their relatively higher polarity the "failures" are easily separated after the synthesis by simple Chromatography.
Scheme 3. Two-phase polymer supported oligosaccharide synthesis using galactal donor attached
to an insoluble polymer.
stable donors are employed, which react specifically with acceptors. One solution has been developed by Danishefsky's group (cf. Schemes 3 and 4) [26]. Failures are self-policing in that they are easily removed at the end of the synthesis by simple chromatography owing to their highly polar nature [26]. An ingenious solution of this problem was developed by Ogawa's group (Scheme 5 ) [27]. The monosaccharide added last to the sequence is equipped with a hydrophobic group, such as 2-(trimethylsilyl)ethyl, which after detachment from the polymer permits isolation of the sequence containing the hydrophobic group by reversephase chromatography (investigated by Hindsgaul's group) [28], thus separating it from most failure sequences. As already mentioned, solid-state oligopeptide and oligonucleotide syntheses required the development of synthetic methods compatible with the solid state processes. This should be equally or more indispensable for oligosaccharide synthesis since syntheses of oligosaccharides are an even more daunting task than the syntheses of oligonucleotides or oligopeptides. This is actually happening, since in spite of this complexity, the progress in designing glycosylation methods is steadily accelerating.
10.2 General Reflections
245
Glycal Assembly
B
> opiv
0BnO 1
MeOTUDTBP
OPiv
BnO
OBn
I
=Bn*
--
BnO
,o& BnO
OH
opiv
Scheme 4. Two-phase polymer supported oligosaccharide synthesis using on-support transformation of glucal into thioglycoside donor attached to insoluble polymer.
+
E
" L & Y
Repeat
-
r
Y
Non-tagged impurities easily removed by chromatography Scheme 5. Polymer supported oligosaccharide synthesis utilizing hydrophobic handle attached in the final stage of the synthetic sequence. The handle allows to separate most of failure sequences accumulated during the synthesis.
246
10 Polymer-Supported Synthesis of Oligosuccharides
10.3 Polymer Supports Automated two phase polymer-supported syntheses of oligopeptides and oligonucleotides provide their products in micromolar to millimolar quantities because of the combination of loading capacity (the loading capacity is a number of anchoring sites in millimoles per gram of a support) of solid polymers (cross-linked polystyrene, controlled-pore glass, Tentagel, Argogel) and the limit of the support volume (mass) practically usable. Aside from mechanical stability of the support, the most important consideration is the chemical stability of both the product and the supporting system (i.e. the polymer and the tether). Since prolonged contacts with the reagents may induce decomposition of the polymer-bound product and the supporting system itself, the reactions should be as fast as possible. Given a small scale of the synthesis, the reagents can be used in a huge excess (e.g. hundredfold) to ensure rapid reactions. However, to synthesize larger quantities of shorter sequences it became necessary to employ polymeric supports which can be practicably handled in substantially larger volumes than the solid polymers, such as polyethyleneglycol o-monomethylether (MPEG) of m.w. = 5,000-6,000 [29, 301, albeit the loading capacity of MPEG at approximately 0.2 mmol/g is not much different from loading capacities of polystyrene-based supports. MPEG is soluble in most solvents including water and thus the polymer-bound synthon can be dissolved under many reaction conditions. After completion of the reaction an ether such as methyl t-butyl ether is poured into the reaction mixture and the polymer-bound product precipitates because MPEG is insoluble in ethers. The remaining components of the reaction mixture can be removed from the desired MPEG-bound product by simple filtration (Scheme 1). One of the original arguments for using a soluble polymer support for oligosaccharide syntheses was based on the necessity to obtain oligosaccharides in larger quantities because biological effects are only observed in higher concentrations. Furthermore, it was assumed that glycosylations on soluble supports are more easily controlled than those immobilized on insoluble matrices, and that all reactants in solution would allow reaction kinetics and anomericity control similar to those observed in solution chemistry [19, 20, 311. Fortunately these arguments did not prevent solid supports from being investigated. Thus it was shown in quick succession that almost any solid support used previously for oligopeptide and oligonucleotide syntheses could be employed for oligosaccharide syntheses (Table 1): polystyrene crosslinked with divinylbenzene (1-2%) [21] usually derived from “Merrifield and Wang resins” [32] or Rink resin [33], polystyrene with grafted polyethylene glycol (TentaGel [34], Argogel [35]), and controlled-pore glass (CPG) [36], as well as novel supports such as polystyrene crown [37], in combination with a suitable chemistry and a tether between the polymer and oligosaccharide chain. Now it appears that there are no significant differences between glycosylations performed in solution without a polymer support, with a soluble polymer support, or using a solid polymer support [38], although some variations were observed 12). It should be noted, of course, that insoluble catalysts are much more likely to be
10.4 One-Phase Systems (Syntheses in Solution)
247
Table 1. Polymer supports showing the functionality to which linkers are attached. Supports for one-phase (solution) syntheses are marked with an asterisk (*); the remainder are supports for twophase syntheses. Except more heavily crosslinked macroporous polystyrene, all supports for two phase syntheses are 1-2% cross-linked polystyrene-divinylbenzene.The first four resins (Merrifield, Wang, Rink, aminomethyl) have loading capacity 0.1-0.3 mmol/g, and swell in solvents such as dichloromethane, methanol, dimethylformamide, and pyridine. Macroporous polystyrene has loading capacity 0.3-0.6 mmol/g, and maintains constant pore size in most solvents, including water. In both Tentagel and Argogel, swelling in organic solvents, polyethylene glycol is grafted on 1-2% cross-linked polystyrene-divinylbenzene with loading capacity 0.3 mmol/g. Argogel is claimed to be more chemically stable. Soluble Polvstvrene* Aminornew
Polvethvlene Glvcol*
Merrfield
\, TentaGelTM
X= OH, NHz, others
effective in association with a soluble support, which can be considered a protecting group, as exemplified by the removal of boronate diester from 4,6-positions of galactose by Amberlite IRA-743 [39].
10.4 One-Phase Systems (Syntheses in Solution) In the synthesis of oligosaccharides, a soluble polymer is bound to the first member of the oligosaccharide chain by a linker permitting removal of the polymer from the
248
I 0 Polymer-Supported Synthesis of Oligosaccharides
completed oligosaccharide, after the completion of the synthesis. Most physical properties of such synthons, are dictated by the polymer and they are not much different from those of the polymer, unless oligosaccharides on the polymer become very large. The polymer can be linked to either the acceptor or the donor. Polyethyleneglycol o-monomethylether (MPEG) and linear polystyrene have been investigated, although the latter has been used only infrequently (Table 1). Polyethyleneglycol = ,-monomethylether (MPEG)
MPEG of different average molecular weights is commercially available [40]. MPEG of average m.w. = 5,000 are most frequently used, but m.w. = 2,000 and m.w. = 12,000 were used as well. Depending on size, MPEG forms rigid rod-like structures stabilized by intramolecular interactions [41]. It is soluble in most apolar solvents (benzene, toluene, methylene chloride, chloroform, acetonitrile, acetone) as well as in water, and insoluble in hexane, diethylether and t-butylmethyl ether, and cold ethanol. It can be dissolved in hot tetrahydrofurane and ethanol, therefore it is commonly purified by reprecipitation from ethanol. Otherwise it is purified by precipitation with an ether from its solutions in the organic solvents cited above. Although MPEG is strongly hygroscopic, the adsorbed water can be almost completely removed by azeotropic distillation, e.g. with toluene or benzene; wet MPEG precipitates with ether only reluctantly. t-Butylmethyl ether is safer to use for the precipitation than diethylether since its boiling and flash points are higher [42]. Recovery of MPEG derivatives is usually no less than 95%. This recovery is substantially lower when MPEG is complexed with metal cations (e.g. mercury, chromium, cerium). Therefore many common metal-containing reagents cannot be used with substrates containing MPEG. MPEG also precipitates less easily and is more difficult to filter if it carries higher molecular weight synthons. Precipitations with alcohols (e.g. methanol) give similar results. To improve precipitation properties of MPEG when the synthesized oligosaccharide extends beyond five monosaccharides and its protected form approaches m.w. 5,000, a suitable remedy is to use MPEG of average mw = 12,000 [43]. Also, shorter MPEG of average mw = 2,000 was used in some cases, and instead of the precipitation a filtration through a silica gel column was used for purification [44]. Although MPEG is stable enough under many reaction conditions that it can be considered a protecting group, some conditions cause its partial degradation [29]; for instance pyridinium salts peel off one or more ethylene glycol units [45]. Furthermore, MPEG cannot be located directly on the anomeric carbon. Treatment of an MPEGyl glycoside, e.g. 2,3,4,6-tetra-O-acetyI-~-D-galactoside with a glycosylating agent such as acetobromomannose causes a glycosyl exchange to take place is formed, and soluble glycolyl and MPEGyl 2,3,4,6-tetra-O-acetyl-a-D-mannoside or (bis)glycolyl 2,3,4,6-tetra-O-acetyl-p-D-galactoside can be isolated from the mother liquors [46] after the MPEG-bound saccharide is precipitated (Scheme 6). The solubility of MPEG in organic solvents allows monitoring of the progress of the synthesis by high field 'HNMR spectroscopy using the signal of its single OCH3 group (6 = 3.380 ppm) as internal standard. Since the glycol peak of MPEG at
10.4 One-Phase Systems (Syntheses in Solution)
249
AcO-7
Ace% AcO
OAc
OCH~C€IZ(OCH~CH~),,OCH~+
AcoA%AcO X
1 AcoA* AcO
Promoter
+
Scheme 6. An example of instability of polyethylene glycol directly attached to the anomeric carbon.
6 = 3.640 ppm is very strong, it must be suppressed by irradiation for 3.0 s to increase the dynamic range and obtain a good quality spectrum. Despite this, sufficient structural information to identify the product can be usually obtained from its 'HNMR spectrum. Linear Polystyrene As with MPEG, linear polystyrene is soluble in benzene, toluene, chloroform, and insoluble in alcohols (methanol) and ethers (e.g. 2-methoxyethyl ether). The first monosaccharide was linked via thioglucoside to chloromethylated polystyrene (prepared from the polymer using chloromethyl methyl ether in the presence of stannic chloride) in benzene solution, and subsequent glycosylations were also performed in benzene. Polymer bound disacharides were isolated by precipitation using 2-methoxyethyl ether or methanol. Although the original report seemed unpromising [ 18b], an application of recent methods of oligosaccharide synthesis is desirable since linear polystyrene should be stable under conditions incompatible with the use of MPEG. Linear polystyrene to which a protected monosaccharide was bound directly was also prepared by copolymerization of styrene with four different derivatives of protected glucose substituted on C-6 with 4-vinylbenzoate, and some reactions have been investigated [ 19a].
250
10 Polymer-Supported Synthesis of Oligosaccharides
10.4.1 Linkers First members of growing oligosaccharide chains usually are not bound to polymeric supports directly but through a linker. Two major reasons for this arrangement are that a too short span between the carbohydrate and the support may create interactions detrimental to the course of the synthesis (however, some reactions may be improved [2]). Thus the tether maintains a safe distance between the support and the growing oligosaccharide chain. The major reason for inserting a tether is that it allows for an easy release of the carbohydrate from the polymer support. Usually the direct bond between the saccharide and the support would be stronger than the glycosidic bonds between two monosaccharides making impossible the removal of the oligosaccharides from the supports. Should the bond between a saccharide and the polymer involve the anomeric carbon, this bond often would be too labile to tolerate all reactions required by the synthetic sequence without a partial decomposition of this bond. Several linkers were examined for one phase systems (Table 2). The choice of a linker is determined by the reactions employed in the reaction sequence and on the form of the final oligosaccharide, which further depends on the eventual use of the oligosaccharide and the possible utilization of the linker or a part of it. As in any organic chemical synthetic scheme, reagents and protecting groups must act in harmony; soluble polymers-linkers can be regarded as special protecting groups. The following examples will illustrate such criteria. Succinoyl Diester The extensive use of succinoyl diester in polymer-supported solution syntheses of oligopeptides and oligonucleotides invited its application in polymer-supported synthesis of oligosaccharides. Since it is essential that both the protective groups and the linker are stable during the synthetic sequence, it is obvious that the baselability of ester linkages prohibits temporary protection with ester groups to be removed before the completion of the synthesis. Particularly unstable are ester groups located on anomeric carbons. Moreover, suitably located succinoyl-MPEG groups are prone to migrations in acidic environment, for instance 2-O-SuMPEG+ 3-0-SuMPEG in acceptor P-mannosides with OH-3 unprotected [47]. Despite these restrictions the acceptor-bound succinoyl-MPEG [48] was found to permit glycosylations with glycosyl trichloroacetimidates promoted by boron trifluoride, triflic anhydride, and trimethylsilyl or triethylsilyl triflates, with glycosyl halogenates promoted by silver triflate, and with thioglycosides promoted by the iodonium ion. It is compatible with ester long-term protection, with allylic, benzylic type (including benzylidene-type acetals), and certain silicon-based protecting groups, and deprotection with hydrogenolysis. To synthesize an oligosaccharide, MPEG is bound to a hydroxyl of the first monosaccharide through the succinoyl diester linker (Scheme 1). Next, another hydroxyl of this monosaccharide is deprotected, glycosylated, temporary protection on the second monosaccharide is removed and the above process is repeated until the required oligosaccharide is synthesized. Although incomplete glycosylation can
0
-S-Polystyrene
-0CHz \ /-CHzO-PEG
CH&BnOH
3) Acetolytic PEG Removal with Sc(OTO3/AczO
Removal
2) Hydrogenolytic PEG-DOX
1) Reductive PEG Removal
'Abbreviations. Donors: A= Trichloroacetimidate; B= Bromide; C= Sulfoxide; D= ThioalkyVaryl; E= Fluoride
7) Thio
6 ) DOX
Removal
!) Hydrogenolysisfor Complete
Removal
18b
55
54
54
27
0
) NaOMdMeOH for PEG
5 ) Benzylphenol
53
TEA/DCM
44
Dioxirane oxidation then NaOMmeOH
4) Fluorene
2,209 49, 51,52,91
Reference
Base-catalysed (DBU; NaOMe)
Removal
44
Donor(s) Used'
Hydrogenolysis
-0CHZ \ 1-O(CH&COO-PEG
-OCO(CHz)zCO-PEG
Structure
3) Phenylacetamide
2) Thioethanol
1) Succinoyl
Linker
~
Table 2. Major linkers used in one-phase (solution) oligosaccharide synthesis showing chemical structure of the linker, glycosylating agent. and removal methods used.
252
10 Polymer-Supported Synthesis of Oligosaccharides
be minimized by using glycosylating agents in excess, it is probably more efficient to add an excess of the glycosylation agent in several portions. The synthesis of the methyl heptaglucoside 1 is an excellent example of a regioselective glycosylation of the primary hydroxyl in 4,6-diol using succinoylated MPEG and iodonium promoted glycosylations with thioglycosides [49]. After the glycosylation, the unreacted OH-4 was capped by acetylation. A comparison with a classical solution synthesis [50]confirmed, as expected, that the MPEG-supported synthesis was significantly faster. Another example of a synthesis using MPEG-Subound acceptor are syntheses of heparan sulphate-like oligomers up to a dodecamer [51]. MPEGSu- can be also linked to a donor as shown in the synthesis of an oligomer of N-acetylneuraminic acid [521. Succinoyl diester has been further modified by linking to 9-hydroxymethylfluorene (Table 2) and using amino MPEG (i.e. MPEG terminating with amino group). Increasing sensitivity of the linker towards bases, this modification made it possible to distinguish between base-labile long-term protecting groups such as pivaloyl on the oligosaccharide while removing it from the polymer-linker with a mild base [53]. Dioxyxylyl Diether (DOX) The base-lability of succinoyl diester linker severely limits the selection of protecting groups available for an oligosaccharide synthesis, so a more versatile tether was required. A diether bond of a,a'-DiOxyXylyl diether, - O C H Z C ~ H ~ C H ~(DOX) O-, [54] is not limited by restriction of the succinoyl linker: (i) bound via a hydroxyl or as an 0-glycoside, it is stable under many reaction conditions including glycosylation; (ii) it is easily removable from a finished oligosaccharide by hydrogenolysis under mild acidic condition; (iii) using suitable hydrogenolytic conditions MPEG only is removed and DOX is converted into 4-methylbenzyl protecting group; (iv) ,using scandiumtictriflate/aceticanhydride[551 DOX can be converted into 4(acetoxymethy1)benzyl protecting group by removal of MPEG only (Scheme 7); (v) it is easily accessible from a,a'-dichloro-p-xylene. The MPEG-DOX grouping is readily synthesized from MPEG-OH and c1,a'dichloro-p-xylene by Williamson ether synthesis yielding MPEG-DOX-CI, and, after hydrolysis, the alcohol MPEG-DOX-OH [ 541. MPEG-DOX-OH can be
10.4 One-Phase Systems (Syntheses in Solutiorz)
Carbohydrate Portion
253
QiQxy Xylyl Portion u MetbylPEG Portion
Carbohydrate-OCH
HJH+
AczO/DCM Carbohydrale-OH
I?$?
CH,C,
,CCH,
Carbohydrate-OCH,
Scheme 7. An example of a versatile linker that can be transformed during or at the end of a synthetic sequence into other synthetically useful forms.
glycosylated to attach a saccharide at the anomeric position (an impossible connection through succinoyl tether [20, 461) to create an MPEG-supported acceptor. In a practical example (Scheme l), the MPEG-DOX-OH is glycosylated by 2-0acety1-3,4,6-tri-O-benzyl-a-D-mannopyranosyl trichloroacetimidate. The 2-0-acetyl group as a short-term protecting group can be easily deacetylated to a hydroxyl acceptor for the next glycosylation; it also serves for steric control of glycosylation via neighbouring group participation. Repetition of the glycosylation and hydrolytic steps gave the expected protected MPEG-DOX-bound pentasaccharide. Standard deprotection including the removal from the polymer support yielded the desired pentamannopyranoside [Manp(al-2)]4Manp(2). Target oligosaccharides are often contaminated with impurities originating in deprotection steps which usually can be removed by chromatography of peracetylated oligosaccharides. Peracety-
254
10 Polymer-Supported Synthesis of Oligosaccharides
lated oligosaccharides are commonly more suitable for final purification by chromatography than deprotected compounds; after deacetylation pure oligosaccharides are obtained readily [56]. The attachment of MPEG-DOX to a carbohydrate hydroxyl other than the anomeric one is achieved by the reaction of such hydroxyl with MPEG-DOX-Cl by Williamson ether synthesis [541. Syntheses of oligosaccharides on MPEG, tethered with dioxyxylyl (DOX) and related 4-oxybenzyloxy diethers, can be performed even more speedily using orthogonal glycosylation [27] and multicomponent carbohydrate [ 571 coupling strategies, which are described in detail elsewhere in this Volume. 10.4.2 Chemistry Investigations In addition to its use as polymer support for oligosaccharide syntheses, MPEGDOX-OH established itself as a useful aide for investigations of problems associated with glycosylations. This is possible because electrophiles competing for nucleophilic acceptors with glycosylating agents can be readily identified after precipitating products of reaction between such electrophiles and MPEG-DOX-OH. These competing reactions complicate results of glycosylations including those polymer supported; therefore it is urgent to suppress them as much as possible. Better understanding of the competing reactions makes this objective realizable. Thus it was established that the trans (p) anomeric specificity of some 2-acetamido glycosylating agents does not require oxazoline intermediacy, and that formation of oxazolines represents a side reaction. It was further found that oxazoline cannot be an obligatory intermediate since MPEG decomposes under the conditions required
10.5 Two-Phase Systems (Syntheses on Solid Supports)
I
BnTBnO
OBnOBn
O
A
’h3q
TBSO
TBSO
SMe
Ph-
255
BnO SMe
OBnOBn
OH
Scheme 8. “Reverse” use of a polymer support for the synthesis of P-mannoside linkage. The synthetic helper arm (MPEG) is removed from the desired product by simple precipitation.
by oxazoline intermediacy [58]. The origins of acyl transfer from 2-acyloxy glycosylating agents to acceptors [59a] have been studied in combination with calculations based on density functional theory [59b] in order to eliminate this very common side reaction. Novel reactants in glycosylation reactions can be efficiently evaluated using MPEG-DOX-OH as illustrated by the investigation of dimethylboron triflate as a promoter [60]. The formation of the P-mannoside linkage by intramolecular aglycon delivery [61] was modified to be performed on MPEG support: in the last step of the synthesis the j3-mannoside remains in solution after precipitation of MPEG-bound byproducts (Scheme 8) [62].
10.5 Two-Phase Systems (Syntheses on Solid Supports) Polymers used as a support for oligosaccharide synthesis should be commercially available or easily modifiable, for instance by introduction of spacers and linkers. The supports should be both mechanically and chemically stable, and have reasonable loading capacity. Three main types of solid support for oligosaccharide syntheses were essentially “borrowed” from oligonucleotide and oligopeptide solid state syntheses (cf. Table 1): (i) functionalized controlled pore glass (CPG); (ii) functionalized polystyrene cross-linked with divinylbenzene (e.g.: Merrifield, Wang, Rink resins); and (iii) crosslinked polystyrene resin with grafted functionalized polyethylene chains (TentaGel, Argogel). The last two types of supports (ii and iii) were compared as to their reaction kinetics. It was concluded that, contrary to the
256
I0 Polymer-Supported Synthesis of Oligosaccharides
common belief, reactions on PEG-containing beads do not proceed faster and that the kinetics depends on the reaction rather than on the support [63]. Compared to one-phase syntheses, monitoring progress of the synthesis of twophase syntheses presents a problem because of the insolubility of the support. Usually the product is detached from a portion of the polymer-bound material and analyzed by convential techniques, such as TLC, NMR spectroscopy and mass spectrometry. Monitoring of the progress of the synthesis without the loss of material by high resolution magic angle spinning NMR spectroscopy of solids is an important development [641. Controlled Pore Glass This has become the support of choice for oligonucleotide syntheses. It is mechanically quite sturdy, does not change its volume in different solvents, and it withstands fairly strong chemical conditions to which it is exposed for a short time (for instance, in an oligonucleotide synthesizer. However, it becomes unstable on prolonged exposure to bases and acids [65] and has a very low loading capacity. Cross-Linked Polystyrene This support was well tested for oligopeptide syntheses, and this led to its adoption for solid state synthesis of oligosaccharides. It has to be preswollen before the synthesis, and it also changes its volume during the synthesis as the synthesized chain becomes more voluminous. It is acid and base resistant, and can be used with linkers cleavable by photolysis, oxidation, fluorolysis, and several other conditions (Table 3). However, it does not swell in some solvents, for instance diethylether. Macroporous highly cross-linked poly(styrene-co-divinylbenzene) resins [661, such as Argopore [67],maintain a constant volume in organic solvents, including protic solvents, and even water. Polyethyleneglycol Grafts on Cross-Linked Polystyrene While the above mentioned supports are rigid, functionalized polyethyleneglycol chains on polystyrene in TentaGel [68] and Argogel [69] are highly mobile and can be considered to be in “solution” [70]. TentaGel (Table 1) is used extensively for oligonucleotide and oligopeptide syntheses; because of the polyethyleneglycol nature of the support, somewhat diminished stability towards acids would be expected. Because of the polystyrene matrix, Tentage1 should be swollen before use; swelling of its beads was determined to be higher or comparable to 2% cross-linked polystyrene-divinylbenzene beads [71]. It has been observed that the purity of oligonucleotides synthesized on TentaGel is significantly better than after synthesis on Teflon or CPG supports [65]. It is not known if this applies also to the synthesis of oligosaccharides. Mechanically TentaGel is more delicate than crosslinked polystyrene supports. Properties of Argogel are similar to those of TentaGel but it is claimed that its chemical stability is significantly improved (cf. Table 1 for its chemical structure), and also because of its double loading capacity.
~~
Linker
*
-
0 1 0 CPG 0,
O2N
= TG = PS(C)
,0(CH&O-PS
-NHCO(CH~)SCONH-AP
-0-CH2-p
CH3
-0(CHZ)~-S~-(CH~)&ONHCH~-PS
I I
CH3
-So-OCH2CONH-TG
A
-OCO(CH2)2CONHCH2COO-TG
-O-CO(CH2)2CO-NH-PS
-O-CO(CH2)2CO-NH-TG
-0-CO(CH2)2CONH-CPG
\
-S(CH2)3Si-
/
/
Structure
D. H. Rich, S. K. Gurwa, J. Am Chem. SOC.,1975,97,1575.
F
b. Rich's Linker
9) a. Pentenyl
I) Nitrobenzyl
r) p-Acylaminobenzyl
butylcarbonamide
i ) Ethoxydimethylsilyl-
acetamide
); QMercaptophenol-
I)CMercaptophenol
3) Sucanoyl-glycine
1) Succinoyl
1) Mercaptopropyl
~
Donor(s) Used
34,36b
32% Aq. Ammonia
NIA
hv /THF
DDQ/DCM, H20
NIA
Hg(OCOCF3)2 DCM, H 2 0
NaOMe/MeOH
37
32€
67
32j
S4,86
32a
82
36b
36b, 77
32%Aq. Ammonia
32%Aq. Ammonia
36a
~
Reference
NBS in THF/MeOH with DTBP
Removal
Table 3. Major linkers used in two-phase (solid state) oligosaccharide synthesis showing chemical structure of the linker, glycosylating agent, and removal methods used.
-S(CH&O-PS(M)
A
32b, 32i
3%
DMTSB-DIPEA DCM/MeOH NBSlDTBP
32e
32e
21
TrBFflCM
NaOMelMeOH
TBAFl HOAc
24,25b
22
32d
1)hvmHF for Partial Removal 2) PhSSiMeflnI2 for Conversion to Thioglycoside TBAF/ HOAc TBAM HOAc
32b
73
Reference
NaI, then NaOAc or N d
AcOH/THF/HzO
Removal
'Abbreviations. Donors: A= Trichloroacetimidate; B=Thioalkyl/aryl; C= Sulfoxide; D= n-Pentenyl; EFluoride; F= Glycal Polymers: CPG= Controlled-Pore Glass; TG= TentaGelR; PS(M)= Merrifield Resin; PS(C)= Polystyrene-Graft Crown (Cbiron Mimotopes, San Diego, CA, 92121); AP= ArgoPoreR
b. Mercaptopropanol
A
1s) a. Mercaptothiopropanol
A
F
F
F
Donor(s) Used
A
-S(CH&-S-PS(M)
R= Ph, 'Pr
R=pb R= 'fi
OZN
-TPS R
I
R
-S02(CH&-PS(M)
Et
I
-Si-(CH2)4-PS
I
Et
Structure
b. p-Oxybenzyl
14) a. p-Alkoxybenzyl
13) Diaryl(a1kyl)silyl
11) Sulfonyl
10) Diethylsilyl
Linker
Table 3 (continued)
10.5 Two-Phase Systems (Syntheses on Solid Supports)
259
10.5.1 Linkers
As in one-phase polymer supported synthesis, linkers must not interfere with the synthetic procedure, must be completely compatible with all synthetic steps, and must facilitate easy removability under conditions which harm neither the oligosaccharide nor the support. Many linkers have been adopted from other branches of solid state chemistry, others have been designed de now, and many more will be designed according to the requirements of a particular synthesis. It should be noted, of course, that a more general practical adoption of polymer-supported synthetic methodologies depends to a certain extent also on the general employability of a linker. Linkers utilized in the two-phase synthesis of oligosaccharides are surnmarized in Table 3. Dialkyl- or Diaryl-Silyl In some cases the solid support, i.e. polystyrene, has to be functionalized first, and the novel functional group represents the linker. The case of silicon-based linkers is a fitting example since it is easily removable by fluoride anions (e.g. tetrabutylammonium fluoride) and yet stable under most conditions utilized in oligosaccharide synthetic strategies. Cross-linked polystyrene-divinylbenzeneis first randomly lithiated [72],followed by reaction with diphenylsilyldichloride to create polystyrene-bound silyl chloride [24,271 which can be linked to a carbohydrate hydroxyl. The relative instability of the diphenylsilyl group to acidic conditions led to the replacement of the phenyl group with the isopropyl group [2Sb], using diisopropylsilyldichloride. Diethylsilyl functionality indirectly linked to the polymer [ 731 was investigated as a linker in a series of glycosylations under different conditions. Another promising silicon-based linker is S-(2-oxyethyldimethylsilyl)valeroyl[32JI which, in analogy with the cleavage of (trimethylsily1)ethyl glycosides [74],is transformed into an anomeric ester, e.g. propionate, by treatment with BF3.EtzO and propionic anhydride (other acyl anhydrides most likely can be used as well). From this glycosylating agents such as trichloroacetimidate can be subsequently prepared. The amidic bond between the valeric acid of the linker and the amino group of the functionalized polymer is sufficiently stable under most conditions used in oligosaccharide synthesis. Thioglycoside Linkers Thioglycosidic bonds are admirably stable under most conditions used for oligosaccharide synthesis and yet can be cleaved with many reagents, such as mercuric trifluoroacetate, N-bromosuccinimide with 1,6-di-t-butylpyridine, dimethylmethylthiosulfonium triflate or tetrafluoroborate, or oxidatively using dioxirane followed by NaOMe in methanol. Therefore cross-linked polystyrene utilizing thioglycoside [32a], or sulfide-based linkers are commonly used strategies [32h, i]. In fact, they were among the first linkers used in the two-phase oligosaccharide synthesis. 1,4Thiophenol, 1,3-propanedithiol, 1,3-thiopropanol, 2-(4-thiophenyloxy)acetamide have also been linked to the functionalized polystyrene support.
260
10 Polymer-Supported Synthesis of Oligosaccharides
1
THF J.
Reducing Oligosaccharide
ProtectedGlycoside
Scheme 9. A photochemically removable linker (top) can be combined with an ester funcionality (bottom) to increase versatility of the tether.
Linkers Cleavable by Photolysis The o-nitrobenzyl protection group has been used in oligosaccharide syntheses infrequently [14e], since it requires a long period (15-20 hours) of irradiation in a photochemical (e.g. Rayonet) reactor at 350 nm. The photoredox process can be substantially accelerated by the presence of a methoxy group in p-position to the nitrogroup [75]. However, the scope of the reaction is limited to furnishing upon photolysis a reducing saccharide when the tether is hooked to the anomeric carbon. The versatility of this type of linker, first used for an oligosaccharide synthesis by Zehavi in 1973, was recently expanded by Nicolaou's group (Scheme 9). In this novel form of the linker the nitrobenzyl moiety was connected to the anomeric carbon via 4-hydroxy benzoyl ester; therefore photolysis results in removal of the polymer producing a glycoside of p-hydroxy benzoyl ester. Alternatively, treatment with trimethylsilyl thiophenol/zinc iodide converts the complete linker assembly into a thioglycoside, an excellent glycosylating reagent.
10.6 Examples of Syntheses It would be of interest to evaluate different polymer-supported synthetic strategies for the preparation of the same oligosaccharides. To date, two oligosaccharides have been synthesized using both the one-phase system on MPEG and the two-
10.7 Combinatorial Libraries
261
phase system using crosslinked polystyrene. One example is the synthesis of the heptasaccharide 1. One method [32c] utilized phenolic polystyrene and the photolabile 5-hydroxy-2-nitro-benzyl alcohol tether, while the other [491 employed MPEG and the succinoyl linker. Because of the different synthetic designs a meaningful comparison of the two supports, such as to the advantages of one support over the other, is not possible. The second example is the synthesis of the pentamannoside [Manp(al-2)I4Manp (2). Mannose oligosaccharides are superbly suited to these studies since a-anomers are formed exclusively under appropriate glycosylation conditions. The one-phase (solution) system used MPEG and the DOX linker [54] and the two-phase system (solid state) utilized crosslinked polystyrene and a thioglycoside linker [32h]. To monitor reactions, the one-phase strategy took advantage of the support’s solubility to use NMR spectroscopy; the two-phase strategy employed cleavage of a small sample from the solid support, and analyzed the sample by either NMR spectroscopy or mass spectrometry (MALDI-TOF) [32h]. The results of these pentamannoside syntheses are not fully comparable since the perbenzylated pentamannoside obtained by the two-phase method was not deprotected. As noted before, the use of cross-linked polystyrene support for syntheses of oligosaccharides by the glycal strategy is discussed elsewhere in this Volume. In Ogawa’s group the p-alkoxybenzylether linker, used in one phase polymer supported synthesis, was utilized in combination with Wang’s resin to synthesize polylactoseamines of various lengths [ 32eI. Finally, polymer-supported synthesis has been used to synthesize uncommon monosaccharides [761. Controlled pore glass has been used to synthesize an a1-2 trimannoside [36a], several disaccharides [36b], and glycosylated oligonucleotides [77]. The examples confirm that CPG-supported oligosaccharide synthesis is feasible, although its limited loading capacity and scale-up may eventually become a restricting factor. TentaGel-N, linked via the succinoyl linker to a monosaccharide acceptor was utilized using trichloroacetimidate chemistry to prepare disaccharides [34] and trisaccharides [78]. It was observed that TentaGel is slightly unstable under the acidic conditions required by trichloroacetimidate chemistry [78], most likely due to the acid sensitivity of PEG [79]. No similar sensitivity was observed in similar applications employing Argogel [ 781.
10.7 Combinatorial Libraries Polymer-supported strategies of oligosaccharide syntheses are ideally suited to preparations of combinatorial oligosaccharide libraries since purification of all products of a particular synthetic step is done at the same time and it would not be expected that individual library members would be lost during the purification process. Both one-phase and two-phase polymer supported strategies have been examined, and a review was published recently [go]. More recent articles that were not discussed in this review include: syntheses of disaccharide [81] and trisaccharide 1821 libraries leading to the discovery of novel antibacterial agents [83], and prepa-
262
I0 Polymer-Supported Synthesis of Oligosaccharides
ration of libraries on TentaGel beads as polyvalent molecules [84]. Often carbohydrates (including oligosaccharides) form a scaffold for a library [85],and the variations in carbohydrate structure may have an effect on biological activity of the constructs in the library [86].
10.8 Capping Capping, or protection of the unreacted hydroxyl to prevent it from reacting in the next condensation step, is a very important operation in oligonucleotide automatic synthetic protocols. In oligosaccharide synthesis capping seems to be much less important, although sometimes unreacted hydroxyls are acetylated to prevent their glycosylation in the next step [49]. However, it seems more useful to repeat the glycosylation step, in analogy with syntheses of “difficult sequences” in oligopeptide synthesis, rather than to use capping. It should be noted that important variants of polymer supported synthesis of oligosacharides, such as the “self-policing” glycal strategy (Scheme 3 ) or strategies involving a hydrophobic handle (Scheme 5), do not require capping. The last mentioned strategy eventually may become more generally adopted.
10.9 Concluding Remarks The polymer-supported synthesis of protected oligosaccharides has attracted considerable attention of organic chemists during the last decade of the 20th century. Full reviews dedicated to the polymer supported techniques of oligosaccharide synthesis were published recently [26, 871, significant parts of reviews on oligosaccharide synthesis have been reserved for polymer-supported methods [5g, 88, 891 and parts of reviews on polymer-supported methodology have been dedicated to oligosaccharide syntheses [ 31. As well, a review on combinatorial carbohydratebased libraries was recently published [90]. It appears to be a suitable methodology for the synthesis of oligosaccharides in quantities required for biomedical applications; for instance, fragments of the capsular polysaccharide of Huemophilus inflenzae were synthesized both by one-phase [91] and two-phase [92] polymer supported methods. It is clear that the methodology, while not perfect, will become more widely used, as better and more efficient reaction conditions are discovered and the essential components become less expensive. References 1. D. Zopf, S. Roth, The Lancet, 1996,347, 1017-1021. 2. 0.-T. h u n g , S. P. Douglas, D. M. Whitfield, H. Y. S. Pang, J. J. Krepinsky, New J. Chem., 1994, 18, 349-363.
References
263
3. D. J. Gravert, K. D. Janda, a) Chem. Revs., 1997, 97, 489-509, b) Current Opin. Chem. Biol., 1997, I , 107-113. 4. For recent reviews, see e.g. a) K. Jarowicki, P. Kocienski, J. Chem. Soc. Perkin Trans. I , 1999, 1589-1615; b) 0. Seitz, Angew. Chem. Int. Ed. Engl. 1998,37, 3109-3111. 5. For reviews, see e.g. H. Paulsen, Angew. Chem. Int. Ed.Eng1. a) 1982,21, 155-224; b) 1990, 29, 823; c) H. Paulsen, Revs Chem. Soc. 1984, 13, 15-45; d) R. R. Schmidt, Angew. Chem. Int. Ed. Engl. 1986, 25, 212; e) P. Sinay, Pure Appl. Chem. 1991, 63, 519-528; f ) K. Toshima and K. Tatsuta, Chem. Revs., 1993, 93, 1503-1531; g) R. R. Schmidt, J. C. Castro-Palomino, 0. Retz, Pure Appl. Chem. 1999, 71, 729-744; h) R. R. Schmidt, Pure Appl. Chem. 1998, 70, 397-402. 6. E. g. Modern Methods in Carbohydrate Synthesis (Eds.: S. H. Khan, R. A. O’Neill), Harwood Academic Publishers, Amsterdam, 1996. 7. For instance: a) K. J. Kaur, G. Alton, 0. Hindsgad, Carbohydr. Res., 1991,210, 145; b) C.-H. Wong, Y. Ichikawa, T. Krach et al. J. Am. Chem. Soc., 1991, 113, 8137; c) S. Roth, US Pat. 5,180,674, Jan 19, 1993; d) Y. Ichikawa et al. J. Am. Chem. Soc., 1992, 114, 9283; (e) for a review, see Y. Ichikawa, G. C. Look, C.-H. Wong, Anal. Biochem., 1992,202, 215. 8. a) J. Thiem, B. Sauerbrei, Angew. Chem. Int. Ed. Engl., 1991, 30, 1503-1505; b) B. Sauerbrei, J. Thiem, Tetrahedron Lett., 1992, 33, 201-204. 9. For instance, G. C. Look, Y. Ichikawa, G.-J. Shen, P.-W. Cheng, C.-H. Wong, J. Org. Chem., 1993, 58,4326-4330. 1994, 116, 1135-1136. 10. M. Schuster, P. Wang, J. C. Paulson, C.-H. Wong, J. Am. Chem. SOC., 11. a) U. Zehavi, M. Herchman, Carbohydr. Rex, 1984, 128, 160-164; b) U. Zehavi, S. Sadeh, M. Herchman, Carbohydr. Res., 1983, 124, 23-34; c) U. Zehavi, M. Herchman, Curbohydr. Res., 1986, 151, 371-378. 12. T. Wiemann, N. Taubken, U. Zehavi, J. Thiem, Carbohydr. Res., 1994,257, C1LC6. 13. S.-I. Nishimura, K. Matsuoka, Y. C. Lee, Tetrahedron Lett., 1994, 35, 5657-5660. 14. a) J. M. J. Frechet, in Polymer-Supported Reactions in Organic Synthesis (P. Hodge, D. C. Sherrington, Eds.), Wiley, Chichester, 1980, pp. 293 and 407; b) J. M. J. Frechet, C. Schuerch, J. Am. Chem. Soc., 1971, 93, 492-496; c) Carbohydr. Res., 1972, 22, 399-412; d) N. K. Mathur, C. K. Narang, R. E. Williams, Polymers as Aids in Organic Chemistry, Academic Press, New York, 1980, Chapter 6; e) U. Zehavi, Adv. Carbohydr. Chem. Biochem., 1988, 46, 179-204; f ) J. M. J. Frechet, Tetrahedron 1981, 37, 663-683. 15. R. Eby, C. Schuerch, Carbohydr. Res., 1975, 39, 151-155. 16. a) P. Westerduin, G. H. Veeneman, Y. Pennings, G. A. van der Marel, J. H. van Boom, Tetrahedron Lett., 1987,28, 1557-1560; b) G. H. Veeneman, S. Notermans, R. M. J. Liskamp, G. A. van der Marel, J. H. van Boom, Tetrahedron Lett., 1987,28, 6695-6698. 17. G. H. Veeneman, H. F. Brugghe, H. van den Elst, J. H. van Boom, Carbohydr. Rex, 1990,195, C1-C4 (after p. 308). 18. a) U. Zehavi, A. Patchornik, J. Am. Chem. Soc., 1973, 95, 5673-5677; b) S.-H. L. Chiu, L. Anderson, Carbohydr. Rex, 1976, 50, 227-238; c) G. Excoffier, D. Gagnaire, J.-P. Utille, M. Vignon, Tetrahedron, 1975, 31, 549-553; d) for a review see; A. Malik, H. Bauer, J. Tschakert, W. Voelter, Chemiker Ztg., 1990, 114, 371-375. 19. a) R. D. Guthrie, A. D. Jenkins, J. Stehlicek, J. Chem. Soc. ( C ) , 1971, 2690-2696; b) R. D. Guthrie, A. D. Jenkins, G. A. F. Roberts, J. Chem. Soc. Perkin Trans. I, 1973, 2414-2417. 20. S. P. Douglas, D. M. Whitfield, J. J. Krepinsky, J. Am. Chem. Soc., 1991, 113, 5095-5097. 21. For example, J. T. Randolph, K. F. McClure, S. J. Danishefsky, J. Amer. Chem. Soc., 1995, 117, 5712-5719. 22. S. J. Danishefsky, K. F. McClure, J. T. Randolph, R. B. Ruggeri, Science, 1993,260, 1307-1309. 23. C. M. Timmers, G. A. van der Marel, J. H. van Boom, Red. Trav. Chim Pays-Bas, 1993, 112, 609-6 10. 24. J. T. Randolph, S. J. Danishefsky, Angew. Chem. Znt. Ed. Engl., 1994, 33, 1470-1473. 25. a) P. H. Seeberger, M. Eckhardt, C. E. Gutteridge, S. J. Danishefsky, J. Am. Chem. Soc, 1997, 119, 10064-10072; b) C. Zheng, P. H. Seeberger, S. J. Danishefsky, J. Org. Chem., 1998, 63, 1126-1 130. 26. P. H. Seeberger, S. J. Danishefsky, Acc Chem. Res., 1998, 31, 685-695. 27. Y. Ito, 0. Kanie, T. Ogawa, Angew. Chem. Int. Ed. Engl., 1996,35, 2510-2512. 28. a) M. M. Palcic, L. D. Heeze, M. Pierce, 0. Hindsgaul, Glycoconjugate J., 1988,5,49-63; b) P. Stangier, M. M. Palcic, D. R. Bundle, Curbohydr. Res., 1995, 267, 153-159.
264
10 Polymer-Supported Synthesis of Oligosaccharides
29. a) J. M. Harris, J. Macromol. Sci.-Rev. Macromol. Chem. Phys., 1985, C25, 325-373; b) J. M. Harris in Poly(Ethy1ene Glycol) Chemistry: Biotechnical and Biomedical Applications (J. M. Harris, Ed.) Plenum Press, New York 1992, pp. 2. 30. a) E. Bayer, M. Mutter, The Peptides (E. Gross, J. Meienhofer, Eds.), Academic Press, New York, 1980, 2, 286; b) G. M. Bonora, C. L. Scremin, F. P. Colonna, A. Garbesi, Nucl. Acids Res., 1990, 18, 3155-3159; c) G. M. Bonora, G. Biancotto, M . Maffini, C. L. Scremin, Nucl. Acids Res., 1993, 21, 1213-1217. 31. J. J. Krepinsky, S. P. Douglas, D. M. Whitfield, in Methods in Enzymology, 242, Neoglycoconjugates, Pt. A: Synthesis (Y. C. Lee, R. T. Lee, Eds.), Academic Press, San Diego, CA. 1994, pp. 280-293. 32. a) L. Yan, C. M. Taylor, R. Goodnow, Jr., D. Kahne, J. Am. Chem. Soc., 1994, 116, 69536954; b) J. A. Hunt, W. R. Roush, J. Am. Chem. Soc., 1996, 118, 9998-9999; c) K. C. Nicolaou, N. Winsinger, J. Pastor, F. DeRoose, J. Am. Chem. Soc., 1997, 119, 449-450; d) K. C. Nicolaou, N. Watanabe, J. Li, J. Pastor, N. Winssinger, Angew. Chem. Znt. Ed. Engl., 1998,37, 1559-1561; e) H. Shimizu, Y. Ito, 0. Kanie, T. Ogawa, Bioorg. Med. Chem. Lett., 1996, 6, 2841-2846; f ) K. Fukase, K. Egusa, Y. Nakai, S. Kusumoto, Mol. Divers., 1996,2, 182-188; g) J. Rademann, R. R. Schmidt, Tetrahedron Lett., 1996, 37, 3989-3990; h) J. Rademann, R. R. Schmidt, J. Urg. Chem., 1997, 62, 3650-3653; i) J. Rademann, A. Geyer, R. R. Schmidt, Angew. Chem. Znt. Ed. Engl., 1998, 37, 1241-1245; j) D. Weigelt, G. Magnusson, Tetrahedron Lett., 1998, 39, 2839-2842. 33. L. Szabo, B. L. Smith, K. D. McReynolds, A. L. Parrill, E. R. Morris, J. Gervay: J. Urg. Chem., 1998, 63, 1074-1078. This series of sialooligomers (up to the octamer) are linked by (1+ 5 ) amidic bonds between two units of neuraminic acid and not by (2+) glycosidic bonds. 34. M. Adinolfi, G . Barone, L. De Napoli, A. Iadonisi, G. Piccialli, Tetrahedron Lett., 1996, 37, 5007-5010. 35. S. P. Douglas, J. J. Krepinsky, to be published. 36. a) A. Heckel, E. Moss, K.-H. Jung, J. Rademann, R. R. Schmidt, Synlett, 1998, 171-173; b) M. Adinolfi, G. Barone, L. De Napoli, A. Iadonisi, G. Piccialli, Tetrahedron Lett., 1998, 39, 1953-1956. 37. R. Rodebaugh, S. Joshi, B. Fraser-Reid, H. M. Geysen, J. Org. Chem., 1997, 62, 5660-5661. 38. S. Manabe, Y. Ito, T. Ogawa, Synlett, 1998, 628-630. 39. G. G. Cross, D.M. Whitfield, Synlett, 1998, 487-488. 40. Suitable MPEG mw = 5,000 was obtained from Fluka AG, Buchs, Switzerland. 41. a) Technical Bulletin: Polyglycols Hoechst. Polyethylene Glycols: Properties and Applications. Hoechst, Frankfurt (1983); b) J. Dale, Zsr. J. Chem., 1980, 20, 3. 42. t-Butylmethyl ether of sufficient quality and degree of dryness for this purpose is available inexpensively in bulk quantities from A R C 0 (Atlantic Richfield) Chemicals. 43. S. P. Douglas, D. M. Whitfield, J. J. Krepinsky, unpublished observations. 44. L. Jiang, R. C. Hartley, T.-H. Chan, J. Chem. Soc. Chem. Commun., 1996, 2193-2194. 45. S. P. Douglas, D. M. Whitfield, H. Y. S. Pang, J. J. Krepinsky, unpublished observations. 46. a) D. M. Whitfield, S. P. Douglas, J. J. Krepinsky, Tetrahedron Lett., 1992, 33, 6795-6798; b) S. P. Douglas, H. Y. S. Pang, J. J. Krepinsky, unpublished results. 47. M. Y. Meah, D. M. Whitfield, S. P. Douglas, J. J. Krepinsky, unpublished results. 48. J. J. Krepinsky, in Modern Methods in Carbohydrate Synthesis (S. H. Khan, R. A. O’Neill, Eds.), Hanvood Academic Publishers, 1996, pp. 194-224. 49. R. Verduyn, P. A. M. van der Klein, M. Douwes, G. A. van der Marel, and J. H. van Boom, Recl. Trav. Chim. Pays-Bas, 1993, 112,464-466. 50. R. Verduyn, M. Douwes, P. A. M. van der Klein, E. M. Mosinger, G. A. van der Marel, J. H. van Boom, Tetrahedron, 1993, 49, 7301-7311. 51. C. M. Dreef-Tromp, H. A. M. Willems, P. Westerduin, P. van Veelen, C. A. A. van Boeckel, Bioorg. Med. Chem. Lett., 1997, 7, 1175-1180. 52. L. 0. Kononov, Y. Ito, T. Ogawa, Tetrahedron Lett., 1997, 38, 1599-1602. 53. Y. Wang, H. Zhang, W. Voelter, Chem. Lett., 1995, 213-274. 54. S. P. Douglas, D. M. Whitfield, J. J. Krepinsky, J. Am. Chem. Soc., 1995, 117, 2116-2117. 55. S. Mehta, D. M. Whitfield, Tetrahedron Lett., 1998, 39, 5907-5910. 56. D. M. Whitfield, H. Y. S. Pang, J. P. Carver, J. J. Krepinsky, Can. J. Chem., 1990,68,942-952.
References
265
57. S. Manabe, Y. Ito, T. Ogawa, Molecules Online, 1998,2, 40-45. 58. G. Hodosi, J. J. Krepinsky, Synlett, 1996, 159-162. 59. a) R. U. Lemieux, Chemistry in Canada, 1964, 16, 14; b) A. Berces, I. Chrzczanowicz, A. Obuchowska, D. M. Whitfield, J. J. Krepinsky, J. Am. Chem. Soc., 1999, manuscript to be submitted, and references therein. 60. Z . G . Wang, S. P. Douglas, J. J. Krepinsky, Tetrahedron Lett., 1996, 37, 6985-6988. 61. F. Barresi, 0. Hindsgaul, a) J. Am. Chem. Soc., 1991, 113, 9376; b) Can. J. Chem., 1994, 72, 1447- 1465. 62. Y. Ito, T. Ogawa, J. Am. Chem. Soc., 1997, 119, 5562-5566. 63. W. B. Li, B. Yan, J. Org. Chem., 1998, 63, 4092-4097. 64. P. H. Seeberger, X. Beebe, G. D. Suckenick, S. Pochapsky, S. J. Danishefsky, Angew. Chem. Int. Ed. Engl., 1997, 36, 491-493. 65. B. P. Zhao, G. B. Panigrahi, P. D. Sadowski, J. J. Krepinsky, Tetrahedron Lett., 1996, 37, 3093-3096. 66. M. Hori, D. J. Gravert, P. Wentworth, Jr., K. D. Janda, Biorg. Med. Chem. Lett., 1998, 8, 2363-2368. 67. K. Fukase, Y. Nakai, K. Egusa, J. A. Porco, Jr., S. Kusumoto, Synlett, 1999, 1074-1078. 68. Rapp Polymere, GmbH, 7072 Tubingen, Germany. 69. Argonaut Technologies, 887 Industrial Blvd., Suite G, San Carlos, CA 94070, USA. 70. a) E. Bayer, Angew. Chem. Znt. Ed. Engl., 1991, 30, 113-129; b) E. Bayer, W. Rapp in Poly (Ethylene Glycol) Chemistry: Biotechnical and Biomedical Applications (J. M. Harris, Ed.) Plenum Press, New York 1992, pp. 325-345. 71. M. E. Wilson, K. Paech, W.-J. Zhou, M. J. Kurth, J. Org. Chem., 1998, 63, 5094-5099. 72. T.-H. Chan, W.-Q. Huang, J. Chem. Soc. Chem. Commun., 1985,909-911. 73. T. Doi, M. Sugiki, H. Yamada, T. Takahashi, Tetrahedron Lett., 1999, 40, 2141-2144. 74. K. Jansson, T. Noori, G. J. Magnusson, J. Org. Chem., 1990,55, 3181-3185. 75. H. Venkatesan, M. M. Greenberg, J. Org. Chem., 1996, 61, 525-529. 76. S. Kobayashi, T. Wakabayashi, M. Yasuda, J. Org. Chem., 1998, 63, 4868-4869. 77. M. Adinolfi, G. Barone, L. De Napoli, L. Guariniello, A. Iddonisi, G. Piccialli, Tetrahedron Lett., 1999, 40, 2607-2610. 78. S. P. Douglas, J. J. Krepinsky, unpublished observations. 79. A. A. Patel, I. Chrzczanowicz, J. J. Krepinsky; manuscript in preparation. 80. Z.-G. Wang, 0. Hindsgaul, Adv. Exptl. Med. Biol., 1998, 435, 219-236. 81. D. J. Silva, H. Wang, N. M. Allanson, R. K. Jain, M. J. Sofia, J. Org. Chem., 1999, 64, 59265929. 82. T. Zhu, G.-J. Boons, Angew. Chem. Int. Ed. Engl., 1998, 37, 1898-1900. 83. M. J. Sofia, N. Allanson, N. T. Hatzenbuhler, R. Jain et al, J. Med. Chem., 1999, 42, 31933198. 84. R. Liang, J. Loebach, N. Horan, M. Ge, C. Thompson, L. Yan, D. Kahne, Proc. Natl. Acad. Sci. USA, 1997, 94, 10554-10559. 85. a) T. Wunberg, C. Kallus, T. Opatz, S. Henke, W. Schmidt, H. Kunz, Angew. Chem. Int. Ed. Engl., 1998,37,2503-2505; b) M. J. Sofia, R. Hunter, T. Y. Chan, A. Vaughan, R. Dulina, H. Wang, D. Gange, J. Org. Chem., 1998, 63, 2802-2803. 86. R. Liang, L. Yan, J. Loebach, M. Ge, Y. Uozumi, et al, Science, 1996,274, 1520-1522. 87. U. Zehavi, Reactive & Functional Polymers, 1999, 41, 59-68. 88. a) H. M. I. Osborn, T. H. Khan, Tetrahedron, 1999,55, 1807-1850; b) Y. Ito, S. Manabe, Curr. Opin. Chem. Biol., 1998,2, 701-708. 89. a) G.-J. Boons, Tetrahedron, 1996, 52, 1095-1121; b) G.-J. Boons, Contemp. Org. Synth., 1996, 3, 173-200; c) D. M. Whitfield, S. P. Douglas, Glycoconj. J., 1996, 13, 5-17. 90. M. J. Sofia, Molec. Diversity, 1998, 3, 75-94. 91. A. A. Kandil, N. Chan, P. Chong, and M. Klein, Synlett, 1992, 555-557. 92. S. Nilsson, M. Bengtsson, T. Norberg, J. Carhohydr. Chem., 1992, 11, 265-285.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
11 Glycopeptide Synthesis in Solution and on the Solid Phase Hovst Kunz and Michael Schultz
11.1 Introduction Glycopeptides are defined as fragments of glycoproteins consisting of saccharide units which are covalently linked to a peptide backbone. The structural diversity emerging from the combination of amino acid and carbohydrate functionality is immense [l-51. Table 1 summarizes the known types of natural linkage types between amino acid side-chains and the first carbohydrate residue. The most abundant carbohydrates are highlighted and the corresponding structural details are depicted. In nature, especially in eucaryotic organisms, most proteins are glycosylated. This observation suggests that such a sophisticated result of evolution should be accompanied by increased functional complexity [6]. In fact, the last decade has revealed numerous fascinating insights into glycoprotein biology. In the course of this introduction both the typical biological role of such a complex glycoprotein and the characteristic preparative problems one faces will be demonstrated by the example of P-selectin glycoprotein ligand-1 (PSGL-1) [7]. Organisms defend themselves against inflammation by deploying white blood cells (leukocytes) which infiltrate the inflamed tissue [8-121. One of the first signals for patrolling leukocytes to migrate from the blood vessel into neighboring tissue is presented on the endothelium of the vessel. This signal is the expression of the glycoprotein receptor P-selectin. Initial weak adhesion to the ligand PSGL-1 located on the surface of leukocytes to this selectin leads to retarded leukocyte movement (‘rolling’). Several subsequent adhesion processes mediated by other recognition molecules finally lead to migration of the leukocytes through the endothelium. In chronic diseases such as rheumatoid arthritis or reperfusion injury a promising treatment could involve inhibition of the aforementioned selectin-ligand interaction [ 111. In this context, the selectin ligand itself is the first lead structure. Biological investigations have revealed the critical binding segment of PSGL-1 to be a sulfated oligopeptide located in the N-terminus and carrying an oligosaccharide terminated with sialyl Lewis’ on a defined threonine residue (Scheme 1) [7, 13, 141.
268
11 Glycopeptide Synthesis in Solution and on the Solid Phase
Table 1. Linkage regions in natural glycoproteins. Amino acid
Carbohydrate
Asparagine
N-Acetylgalactosamine N-Acetylglucosamine D-Glucose L-Rhamnose
Hydroxylysine
D-Galactose
Hydroxyproline
L-Arabinose D-Galactose
Serine
Galactose Glucose Xylose Fucose
Threonine
Linkage region
I HO
HNJ 0
+O+
Fucose
HO
Serine or Threonine
N-Acetylgalactosamine N-Acetylglucosamine Mannose
AcHN
R
R=H, CH3
Tyrosine
Glucose Galactose
,O
O
11.1 Introduction
s0:so:-
I /
so?-
269
HO
I
Q A T E Y E Y L D Y D F L P E T E P I
I
kHo..0H
' T O H
OH
COH
AcHN
OH
1
Sialyl Lewis' (%ex)
1 n=2-6 Scheme 1.
If PSGL-1 is not sulfated, the molecule is no longer recognized by P-selectin but it still serves as a ligand for another adhesion molecule, E-selectin [15]. Because PSGL-1 is a glycosylated protein the over-expression of this factor in a suitable cell-line is not a possible method for the synthesis of an effective compound. Such an approach would not deliver the sulfated and glycosylated form. On the other hand, PSGL-1 cannot be isolated from natural sources as a homogeneous compound in a suitable quantity for medicinal purposes. At this point recent developments of glycopeptide synthesis justify closer retrosynthetic analysis of compound 1, although in terms of size and complexity 1 constitutes quite a challenging target. Thus, the first step is simplification of the structure in such a way that biological activity and experimental feasibility are both taken into account. Biological results suggest that mainly the terminal tetrasaccharide (sialyl Lewis") is required for recognition. Furthermore, only one of the sulfated tyrosine residues is considered essential for binding. Thus, the minimum target molecule is represented by a sulfated heptapeptide carrying sialyl Lewis" (2) (Scheme 2) [ 161. Further retrosynthetic analysis of this structure splits the target into the corresponding amino acid building blocks required for solid-phase synthesis (Scheme 2). N- and side-chain-protected amino acid derivatives are commercially available except for the glycosylated threonine. The glycosylated key building block can be substituted by the more readily available glycosylated asparagine derivative which is constructed from suitably protected monosaccharides 3-6 and an aspartic acid moiety ready for glycosylation. For the realization of such a synthetic approach a detailed knowledge of glycosylation chemistry and protecting group strategy becomes important [4, 17, 181. Fundamental methods in both fields relevant to the synthesis of 2 shall therefore be outlined in the following paragraphs. 11.1.1 Which Protecting Groups are Suitable for Carbohydrates (Table 2)?
For 0-glycosylation, at least, the answer to this question depends mainly on the desired stereochemistry of the products. Ether type hydroxy-protecting groups which do not exert a neighboring group-effect, e.g. benzyl (Bzl), in the 2-position lead almost exclusively to glycosides with axial linkages (usually a ) to the used glycosyl acceptor if respective reaction conditions are applied (see below) [ 191. The
270
I 1 Glycopeptide Synthesis in Solution and on the Solid Phase H-Tvr(S032-)-Asp-Phe-Leu-Pro-Glu-Asn-OH ,OH
/
Fmoc-Amino Acids-OH and Fmoc-Asn(SLeX)-OH ======3 SLeX-N3
Scheme 2.
target molecule 2 contains two a-linked saccharides and, indeed, the fucose donor 4 is an ideal example of the rule of preferred cis-glycosylation employing benzylprotected sugars. The formation of the a-sialoside, however, cannot be influenced by an assisting group neighboring the anomeric center. Thus, acyl type protection of hydroxy groups, which is readily introduced into the valuable neuraminic acid, can be applied after protecting the carboxy group as the methyl ester (6). The remaining donors galactose 5 and glucosamine 3 should be equipped with a combination of ether- and acyl-type protection. This pattern ensures the orthogonal stability of protecting groups during the synthetic protocol. Benzyl ethers can be removed by hydrogenation and ester type groups are cleaved by use of catalytic amounts of base (table 2).
Table 2. Common protecting groups in carbohydrate chemistry. Protecting group
Introduction
Removal ~~
CsHsCH20CH3COO-COOCH3
NaH/benzyl bromide Pyridine/acetic anhydride H+/methanol
H2/Pd-C cat. NaOMe/MeOH dil. NaOH
11.1 Introduction
27 1
11.1.2 Which Glycosylation Methods are Useful for the Formation of Glycopeptides?
Again the range of methods useful for the formation of the different types of linkage will be illustrated using compound 2 as an example [4, 18, 20-231. Formation of Asparagine N-Glycosides The asparagine-glucosamine linkage is the most common type of linkage found in natural glycoproteins. Because asparagine cannot be used directly as a glycosyl acceptor the linkage is usually formed by coupling suitable glycosylamine (saccharide) and aspartic acid derivatives. The amino function of the saccharide is accessible either from unprotected saccharides and ammonium hydrogen carbonate [24] or via the hydrogenation of an intermediate azide which is prepared, for instance, from glycosyl halides in phase-transfer-catalyzed reactions with sodium azide [25]. Coupling of the glycosylamine to aspartic acid can be accomplished by use of established peptide-coupling reagents such as isobutoxyisobutyldihydroquinoline (IIDQ, Scheme 3) or TBTU [ 171. a-Fucosylation
A very efficient introduction of a-fucosyl moieties, e.g. into the 3- or 4-position of a glucosamine acceptor, is possible via the in situ anomerization procedure developed by Lemieux and coworkers [26]. Tri-0-benzylated fucosyl bromide 7 is a useful glycosyl donor in these cases [27]. Activation of this donor by catalytic amounts of Br- results in the intermediate formation of the reactive P-halide (Scheme 4).
4 HZ Ra-Ni
Acoq Aliquat
AcHN CI NaN3
N H ~IIDQJDIPEA
N)
AcHN
AcHN
T-
Fmoc-Asp(0H)OtBu
Scheme 3.
Scheme 4.
TNH
L~
272
I I Glycopeptide Synthesis in Solution and on the Solid Phase
OBzl 5
OBzl 8
Scheme 5.
The P anomer spontaneously reacts with nucleophiles to give the desired afucoside. On the other hand, so-called disarmed donors such as Ac3FucBr carrying electron-withdrawing protecting groups do not react under these conditions. Formation of the P-Lactosamine Linkage
P-Galactosylation of the 4-hydroxy group of glucosamine is difficult because of the significantly reduced nucleophilicity of this acceptor group in the vicinity of protected or glycosylated neighboring hydroxy functions, as was shown in earlier investigations. The powerful trichloroacetimidate methodology [28, 291 is most recommendable to achieve the desired conversion (Scheme 5). Trichloroacetimidate donors are easily prepared from the corresponding 1-OH precursors and are stable towards basic conditions [30]. a-Sialylation
N-Acetylneuraminic acid contains no C-3 substituent but carries a carboxy group at position 2. Both factors complicate the stereoselective and efficient formation of the desired a-2,3-sialoside in example 2 [31]. A neighboring group at C-3, required for stereoselective synthesis, can be introduced temporarily (SPh, SePh) and removed after completion of the sialylation [32]. This strategy, however, requires additional steps and results in reduced overall yield. Recently, thioalkyl/aryl sialosides and Ssialosyl xanthates have been used to achieve regio- and stereoselective formation of a-glycosides of N-acetyl neuraminic acid derivatives (Scheme 6) [33-351. The 2-SR sialosides can be activated by use of N-iodosuccinimide (NIS) and traces of TfOH or with alkyl sulfenyl triflate [33, 36, 371. Under these conditions the desired a-product 10 can be obtained in yields above 50% without the need for purification by HPLC. The regioselectivity of the NeuNAc-SR donor for the 3-OH group of the galactose residue of the acceptor 9 is particularly advantageous [38].
11.1.3 Glycopeptides Containing Particularly Sensitive Linkages
Because of the particular sensitivity of glycopeptides, precautions must be taken during the assembly of the target molecule, in particular during the deblocking reactions.
273
11.1 Introduction AcO
Ac0w0cH3
L
SEt
AcHN 6
AcO
NIS/TfOH
N3 c
AcO
OBzl
10
NHAc
OBzl
NHAc
9
Scheme 6.
Acid Sensitivity In general, the 0-glycosidic linkages between saccharide residues and also between saccharides and amino acids are sensitive towards acids. For instance, the a-fucosidic linkage is only compatible with the trifluoroacetic acid-promoted cleavage of side-chain protection or anchoring groups if acyl protection is used (Scheme 7). In the presence of benzylated or methoxy benzylated (OMpm) carbohydrate moieties, treatment with trifluoroacetic acid can lead to extensive destruction of the corresponding saccharide linkage [39, 401. Base Sensitivity If strongly basic conditions (pH > 11) are used, the a-CH linkages of amino acids in peptides and, in particular, in peptide esters are likely to epimerize (Scheme 8). In
OBzl 1 OBzl
-0BA
A
C*o--
Peptide
I
NHAc
COOtBu
1
1
Peptide
I 0
NHAc
COOtBu
TFA TFA
Complete loss of the fucose residue
Scheme 7.
4
nNH
AcO
1
Selective cleavage of the rert-butyl ester
274
I 1 Glycopeptide Synthesis in Solution and on the Solid Phase
addition, with serine/threonine glycosides the removal of the a proton can result in complete elimination of the glycan portion by an E 1cB mechanism:
Scheme 8.
The risk of side-reactions is particularly increased if ester protecting groups (e.g. the methyl ester in 10) are cleaved using, for example, dilute sodium hydroxide. The progress of these deblocking reactions should, therefore, always be carefully monitored [41]. Fortunately, the basic conditions required for the removal of the Fmoc group (morpholine, short treatment with piperidine) and the 0-acetyl groups (catalyst NaOMe, pH 8.5) are so mild that undesired reactions can be minimized. For this reason the N-Fmoc and the 0-acetyl groups are well established as standard protecting tools in glycopeptide synthesis [ 171.
11.2 Synthesis of Glycopeptides in Solution Despite the successful development of a routine methodology for the solid-phase synthesis of peptides and glycopeptides (see below) a synthesis in solution is still the method of choice, in particular if glycopeptides with demanding saccharide components are to be synthesized. Short sequences with a relatively high content of complex saccharide residues can be prepared efficiently in solution avoiding the loss of valuable building blocks required in excess in syntheses on the solid phase. The development of new coupling reagents or protecting groups, moreover, often requires an initial verification in solution. Particularly difficult sequences are advantageously prepared in solution by coupling preformed fragments [42]. Finally, synthesis of glycopeptides in solution is required, not only if a few milligrams of the product are required for a first attempt at biological evaluation (and claiming) but also if preparative amounts of a glycopeptide with valuable biological properties are to be obtained. 11.2.1 0-Glycopeptides
Glycopeptides Carrying N-Acetylgalactosamine (Tn-Antigen)
N-Acetylgalactosamine (Tn-antigen) is usually found a-glycosidically linked to serine or threonine. The Ser/Thr(GalNAc) residue also represents the key building
11.2 Synthesis of Glycopeptides in Solution
215
block of the important glycoprotein family of the mucins [6, 43, 441. Mucins are highly glycosylated proteins produced by a wide variety of epithelial and endothelial cells. Despite their normal cell-protecting function, the appearance of mucin molecules with diminished carbohydrate chains has also been correlated with a negative prognosis in the development of tumors. In particular, Tn (aGalNAc), T (PGal-1,3-aGalNAc), STn (aNeuNAc-2,6-aGalNAc) and the regioisomeric ST [ PGal-I,3-(aNeuNA~-2,6)-aGalNAc or aNeuNAc-2,3-PGal-l,3-aGalNAc] antigens have been described as tumor-associated antigens. [8, 45, 461. The synthesis of partial structures of these biologically relevant cell surface components has, therefore, become a major target in glycopeptide synthesis. Some results of these efforts have already been applied in the fields of immunology (vaccination) and diagnosis
WI.
MUCl is the mucin which has been most thoroughly investigated [48]. It consists of numerous glycosylated tandem repeats of 20 amino acids. A bisglycosylated decapeptide of this repeating unit was synthesized in solution by use of enzymatic protecting group methodology [49]. The key building blocks for this synthesis were the Fmoc-protected Ser/Thr-aGalNAc residues which were accessible in eight steps starting from galactose via the azidonitrate 11 [50]and the thioethyl glycoside 12 [51] (Scheme 9). Because the azido group in the 2-position does not exert a neighboring-group effect, a mixture of a- and P-glycoside with prevailing a-anomer is obtained by reaction with the Fmoc-protected serine heptyl ester after activation with a soft electrophile (DMTST) [52].Subsequently, the C-terminal heptyl ester in 13 was cleaved in a lipase-catalyzed reaction [53] to give building block 14. Coupling of 14 with preformed glycosylated dipeptide 15 using water-soluble carbodiimide/HOBt gave the bisglycosylated tripeptide which was converted into the N-acetyl galactosamine derivative by use of thioacetic acid. After N-terminal enzymatic cleavage of the C-terminal heptyl ester and coupling with tripeptide 16 (+17), the Fmoc-protected N-terminus was removed and the amino component condensed with tetrapeptide 18 to give the bisglycosylated decapeptide (Scheme 10).
I ) AczOiHC104 2) PBr, 3) Zn/CuS04NaOAc AcOWH20 4) ceric ammonium AcO nitrateNaN3
Gal
-
1 ) EtOCSSK AcO ONo2 2) NaVacetone
OAC
AcO
11
AcO @SE' 12
Fmoc-Ser(OH)-OHep DMTST
\
FmocHN---r-COOHep
Aco+0213 AcO
Scheme 9.
OAc
276
I1 Glycopeptide Synthesis in Solution and on the Solid Phase 1) EDC/HOBt
FmocHN-cH-cooH II 13
*
7H2
pH7,3I0C
go
AcO OAc OAC 14
H-TF-Ala-OHep
I
aAc3GalN3
15
-
aAc3GalNHAc
I
FmocSer-Thr-Ala-Pro-Pro-Ala-OHep
I
2) CH3C(O)SH 3) LipaseM 4) EDCRIOBt aAc3Ga NHAc H-Pro-Pro-Ala-OHep 16
l7
\
1)Morpholine/Ac20 2)Ac-Pro-Ala-Pro-Gly-OH 18 3)Lipase N 4)NaOMe/MeOH GalNHAc
AcPro-Ma-Pro-Gly-Ser-Thr-Ala-Pro-Pro-Ala-OH GalNHAc
19
Scheme 10.
The target molecule 19 was obtained by lipase-catalyzed heptyl ester cleavage and employing sodium methanolate for the removal of the 0-acetyl groups. In summary, this result showed a set of three base-labile protecting groups (Fmoc, OAc, OHep) to be removable selectively and thus useful for the synthesis of complex glycopeptidic structures. A conformational investigation of the target compound 19 was conducted by use of state-of-the-art NMR techniques [49]. The results indicate that carbohydrate residues can influence the conformation of the peptide backbone. Further studies on lipase-catalyzed ester hydrolysis revealed that more hydrophilic structures such as the methoxyethoxy ethyl ester, were generally better substrates for the enzymes, in particular when the peptides and glycopeptides were sterically hindered (Scheme 11) [54, 551. A different approach to obtaining potential Tn antigen-containing vaccines is to assemble dimeric and trimeric structures from Ser(aGa1NAc) building blocks, which are equipped with an aminobutyric acid spacer molecule: Ac-Ser (aGalNAc)-Ser (aGalNAc)-Ser (aGalNAc)-CO-NH-( CH2) X O O H The resulting dimeric antigen was coupled to a lipopeptide and was successfully used to stimulate antibody (IgM and IgG) production in mice [ 5 6 ] . Glycopeptides Carrying the T-Antigen (Gal-GalNAc)
The key steps in the formation of the T antigen linked to serine or threonine usually involve azidonitrate 11 which is converted into the methyl glycoside, then deacety-
lipase
Aloc-Ser(GalNAc)-Val-Phe/O~O~O\
Scheme 11.
Aloc-Ser(GalNAc)-Val-Phe
277
11.2 Synthesis of Glycopeptides in Solution Ph
I
AcO
&q OAc
\n
2) 1) AcOH NIS/TfOH
I+
.+
Hok&
3) AczO/pyridine 4)ACZ"'H2SU4 * 5 ) TiBr4
N3
20
&;% OAc
21
N3
Br
Scheme 12.
lated and regioselectively benzoylated at the 4- and 6-OH groups [57]. This acceptor molecule can be efficiently and stereoselectively galactosylated in position 3 to give the desired disaccharide. Conversion into a suitable donor can be achieved after acetolysis of the anomeric position using TiBr4. Important disadvantages of glycosyl halides as donor molecules are their instability and the need for silver or mercury salts as activating agents. In this context an improvement was achieved by introduction of thioalkyl/aryl donors which can be stored and easily activated by means of a soft electrophile [58]. A combination of thioglycoside methodology with pentenyl glycoside activation [59] led to thiopentenyl glycosides as glycosyl donors (e.g 20) [60]. Attack of iodonium reagents at the double bond and subsequent exo-trig cyclization forms a thiotetrahydrofuran as the leaving group, which can be substituted by the glycosyl acceptor. After conversion of the intermediate methyl glycoside into a bromo glycoside 21, glycosylation of the Fmoc-protected threonine phenacyl (Pac) ester, reduction of the azido group and reductive elimination of the Pac ester [61], a building block for convergent glycopeptide fragment condensations was obtained (Scheme 12). Use of phenacyl protection avoids the application of harsh conditions for the liberation of the C-terminal carboxy group (e.g. tBu/CF3COOH). Coupling of T antigen derivative 22 with pentapeptide 23 gave a fully protected fragment of the MUCl repeating unit. Removal of all remaining protecting groups was achieved in high overall yield by standard deblocking procedures. ROESY NMR analysis suggests that glycosylation of the threonine induces a cis peptide bond between Thr and Arg in 24 (Scheme 13) Fmoc-Thr-OH 1) IIDQ
H-Arg(Pmc)-Pro-Ala-Pro-Gly-OtBu 23
AcO&O& I ) Fmoc-Thr-OPac AgOTf 2)CHxCOSH f
AcA coAc 22
\
2) morpholine 3) AcZOlpyridine 4) TFA 5 ) NaOMeMeOH
Ac-Thr-Arg-Pro-Ala-Pro-GI y-OH 21
H
O OH
Scheme 13.
OH
e
O g 24 OH OH
218
I1 Glycopeptide Synthesis in Solution and on the Solid Phase
AcO
1) dimethyldioxirane 2) 6-O-triisopropylsilyIgalactal/ZnC12 3)AC20, EtjN, DMAP
-
1) TBAF
R=TIPS
26
3) NaN3lCAN 4) PhSWDEA 5 ) CCI&N/KzCO,
AcO
gNH \
2) TMSOTO27 _, ....-3TO27
OAc
0
OAc
N3
CCI3
28
27
Scheme 14.
Glycopeptides Carrying the Sialyl T Antigen (NeuAca2,6[Gal~l,3]GalNAc) The synthesis of a nonapeptide carrying three sialyl T residues was recently reported [62]. As for the preparation of Tn and T antigens, glycal and azidonitrate intermediates play key roles throughout the synthetic strategy. The glycal 25 was subjected to epoxidation. Activation of the epoxide with ZnC12 and reaction with a 6-0-silyl protected galactal gave disaccharide 26. After fluoride-mediated removal of the TIPS group, the 6-OH groups were sialylated by use of sialyl phosphite 27 [63, 641 and activated with TMSOTf. The resulting protected sialyl T glycal was converted into the azidonitrate which was subsequently hydrolyzed. The resulting hemiacetal can be transformed into the trichloroacetimidate 28 by treatment with K2C03 and trichloroacetonitrile (Scheme 14). Upon activation with BF30Et2 the P-trichloroacetimidate 28 gave high yields of predominantly a-glycosylated Fmoc-protected serine benzyl ester. Subsequent to the reduction of the 2-azido groups and the deprotection of the carboxy group the serine and threonine building blocks 29 carrying the sialyl T antigen were introduced into a convergent peptide synthesis. The peptide couplings were achieved with IIDQ, and Fmoc removal was performed with potassium fluoride in the presence of 18-crown-6 [65]. Finally, complete deprotection by standard procedures gave the mucin-type target 30 (Scheme 15). To date, the immunoreactivity of this glycopeptide has not been reported. A similar strategy also using glycals as the crucial carbohydrate building blocks was used in the successful synthesis of glycopeptides containing regioisomeric sialyl (2, 3) T antigen carbohydrate side-chains [66]. An alternative sequence of reactions also based on an early introduction of the sialic acid was applied for the construction of a sialyl T antigen serine/threonine building blocks [67].
279
11.2 Synthesis of Glycopeptides in Solution 28
1 ) BF30Etz/Fmoc-Se/Thr-OBzl 2) CH3COSH 3) Hz/lO% Pd-C AcO
t
a2,6NeuNAc(p I ,3Gal-aGalNAc)
-
'
I
AcPro-Ile-Val-Ser-Th-Ser-Asp-Pro-Val-OH a2,6NeuNAc(p 1,3Gal-aGalNAc)
I
30
I
Scheme 15.
Glycopeptides Carrying 0-GlcNAc The existence of natural glycoproteins containing 0-linked N-acetylglucosamine was unknown until 1984 [68]. Since then, numerous investigations have focused on this ubiquitous structural motif [69]. Recently, it was found that the 0-GlcNAc moiety might be connected with important phenomena such as Alzheimer's disease. The development of methods for a straightforward and efficient approach towards 0-GlcNAc-containing peptides followed evaluation of the biological background. The major synthetic problem is the formation of the b-configured GlcNAc-Ser/Thr linkage. Although the 2-N-acetamido group has the desired neighboring effect, the intermediate cation has a high tendency to form the rather unreactive oxazolidine ring (Scheme 16). To overcome these drawbacks the trichloroethyloxycarbonyl (Tcoc) group was used for temporary protection of the 2-amino group [70]. In combination with the anomeric thioethyl group an efficient donor for the construction of p-GlcNAc-Ser/ Thr building blocks was provided [71].The reactivity of 31 was even high enough to enable the glycosylation of dipeptides in acceptable yields. In particular, the sensitive tert-butyl ester as the C-terminal protection remained unaffected during both the mild activation procedure and the subsequent conversion of the Tcoc group into the acetamide (Scheme 17). With 32 in hand, the orthogonal stability of the tert-butyl and the Z protecting groups facilitates extension of the peptide chain in the N-terminal direction.
Nu-
Scheme 16.
+
""yo
Nu-
280
A AcOc
11 Glycopeptide Synthesis in Solution and on the Solid Phase
ONH G S E31 ~
1)DMTST/ZSerAlaOtBu 1) H2/Pd-C 2) AcOWZn 2) Z-Ala 3)Ac20/pyridine EDCBIEA 2-Ser-Ala-OtBu
-
I
PAC~GICNAC32 0~OCH*CCI,
NHAc
Scheme 17.
Other glucosamine donors based on the N-Tcoc and N-tetrachlorophthaloyl protecting groups in combination with the anomeric trichloroacetimidate also enabled other efficient syntheses of GlcNAc-Ser/Thr building blocks [72]. Despite these achievements the 0-GlcNAc-containing building blocks for the solution-phase synthesis of a bisglycosylated and phosphorylated hexapeptide from RNA polymerase I1 were obtained by the activation of the oxazoline 34 in the presence of urethane-protected serine 33 (Scheme 18) [73]. The unusual N-terminal protection used herein facilitates the subsequent deblocking of the N-terminus by penicillin G acylase under exceptionally mild conditions. For assembly of the target peptide the GlcNAc-Ser derivative 35 had to be modified: elongation at the C-terminus led to glycodipeptide 36 and introduction of the tert-butyl ester and liberation of the terminal a-amino function gave building block 37. After coupling of both GlcNAc compounds (-38)) N-terminal deprotection, condensation with a suitable Pro-Thr moiety, and another N-terminal deprotection, a phosphorylated serine residue was introduced at the N-terminus (-39). Simultaneous removal of the amino and phosphate protecting groups using Pd(0) ally1 transfer followed by standard deblocking procedures gave target 40: 11.2.2 N-Glycopeptides N-Glycopeptides Carrying Natural Saccharide Side-Chains Natural N-glycoproteins usually carry a central (‘core’) pentasaccharide 41 linked to aspartic acid (Scheme 19). The Asn-GlcNAc unit is, therefore, an ideal model compound for methodological investigations towards the preparation of N-glycopeptides. That is, it served for the development of C-terminal protecting groups cleavable by lipases [ 5 5 ] . This class of enzyme was chosen because it does not cleave peptide or other linkages present in glycopeptides. For the synthesis of 0-glycopeptides, heptyl esters as lipase substrates had already been successfully used (see above) [54].In this context it was found that the hydrophobicity of heptyl esters is not a prerequisite for a sufficient conversion. Instead, rates of hydrolysis by lipases were distinctly higher for esters derived from ethylene glycol.
281
11.2 Synthesis of Glycopeptides in Solution
OH
BF3-OEtz
1)
PhAcOZ-Ser-OH
iPrN F O t B u iprm HSer-OtBu
2) penicilline G acylaseMHS03
I
c
A & % !AcO
AcHN
PAc~GIcNAc
35
31 1 ) EDC/HOB~-Pro-OAll 2) Pd(O)/morpholine
EDC/HOBt
PhAcOZ-Ser-Pro-OH
I
PAc~GIcNAc 36
Ph AcOZ-Ser-Pro-Ser-0th
I
PAc~G~cNAc
OPO(OH),
PGlcNAc
\
38
OPO(OAlI),
PAc3GlcNAc
I
I
Aloc-Ser-Pro-Thr-Ser-Pro-Ser-OtBu PGlcNAc
40
Scheme 18.
Mana 1,6
\
Manal,3 /
Man-Pl,4-GlcNAc-P1,4-GlcNAc-P-Asn
\
/ " Z e " :41
Scheme 19.
I
PAc~GIcNAc
39
282
-
I 1 Glycopeptide Synthesis in Solution and on the Solid Phase
PeptdieyO-o,n-O ,. ,
Peptide O
v
OMEE
K 0
O 0’-
OME
42
Scheme 20.
COOMe Tcoc ,
H 43
1
l)AcjGlcNAc-NHZ45 IIDQ Tcoc-Asn-Ser-Ala-OH 2)Lipase N Lipase A6 Tcoc , I N COOMe m-Ser-Ala-OMe PAc3GlcNAc H IIDQ OME 44 4)Lipase A6 46
Scheme 21.
The methoxyethyl (ME) esters (e.g. 42, Scheme 20) combined high substrate acceptance with optimum purification properties and high solubility in aqueous solution. As a model target in the area of N-glycopeptides glycotetrapeptide 46 was synthesized. Starting from the bis-ME aspartic acid ester 43 the p-ester was selectively cleaved in a lipase-catalyzed reaction. Condensation of compound 44 with AqGlcNAc-NH2 45 gave the fully protected asparagine derivative. Upon treatment with a different type of lipase, the a-carboxy group could be deblocked and condensed with a dipeptide ME ester. After elongation at the N-terminus the resulting ME ester again could be cleaved by use of a lipase (+46, Scheme 21). In summary, the selectivity and mildness of the lipase-catalyzed reactions was established as a tool applicable to multifunctional and sensitive glycopeptide targets in particular. Variants of the ‘core’ 41 shown in Scheme 19 with increased structural complexity were found, which can be related to pathogenic phenomena. As a basis for further biological investigations, the complete N-glycan of the so-called complex type carrying a pentapeptide was recently synthesized using a chemoenzymatic strategy (Scheme 22) [741. This molecule contains several preparative challenges that warrant more detailed description. The synthetic strategy applied can be summarized as:
I
xxx
NeuSNAca-2,6-Galp-l,4-GIcNAc~-I ,2-Mana-l,
\
I
Man-P-l,4-GlcNAc-P 1,4ClcAc-P-Asn 2,6-Galp- 1,4-GlcNAc~-1,2-Mana-1,4
I
xxx I
Scheme 22.
283
11.2 Synthesis of Glycopeptides in Solution
1) 2) 3) 4)
assembly of heptasaccharide 47 [75]; introduction of aspartic acid at the anomeric center; selective deprotection and extension of the peptide chain; and sequential enzymatic transfer of galactose and neuraminic acid on to the nonreducing ends of the glycan chain
The synthesis started from chitobiosyl azide 48 which is available in 11 synthetic steps. The azido group herein is essential for the chosen strategy because it: 1) remains stable during the applied reaction conditions, and 2) enables the generation of the anomeric amino function required for introduction of aspartic acid [76]. Next, a P-mannoside linkage must be established and two major problems must be overcome because commonly used neighboring groups, and the anomeric effect lead to the formation of the undesired a-mannosides. During the past 20 years several efficient solutions to this key problem have been provided [22].The strategy used here is based on the epimerization of the 2-hydroxy group subsequent to the formation of a p-glucosidic linkage (Scheme 23) [77, 781. To this end, a phenylcarbamoyl group at 3-OH and a leaving group at 2-OH (OTf) were introduced (49). An intramolecular s N 2 reaction (DMF, 40-50 "C) resulted in
NPth
\
OBzl
AcO
+
48
&
o+
6"
OAc
49
1) BFtOEt,
2) K2603kIeOH 3) dimethoxytoluene/H+
phqo
4) 5 ) TfiOlpyridine DMFipyridine
0HO a B
6 ) AcOWdioxane 7) NaOMdMeOH
O
e
:
z
An-, UDLl
&
NPth N3 50
I
\ OAc
47
Scheme 23.
g
1) A-trichloroacetimidate BF30Et2 2) AcOW80"C 3) B-trichloroacetimidate BF30Et2 I
NPth OBzl
284
11 Glycopeptide Synthesis in Solution and on the Solid Phase
the formation of the 2,3-cis carbonate which upon treatment with NaOMe/MeOH gave the trisaccharide acceptor 50. Advantageously, the 2”,3”-diol 50 was now selectively glycosylated with the disaccharide donor A at position 3”. After removal of the 4”,6/’-benzylidene group a second selective glycosylation with B gave heptasaccharide 47 in high yield. This glycan was now subjected to a manipulation of protecting groups followed by the conversion of the azido group into the P-configured amino function, by use of propanedithiol, and condensation with a fully protected aspartic acid derivative. All remaining protecting groups were subsequently removed by Pd-catalyzed hydrogenation and the Fmoc group was introduced at the terminal amino function. The efficiency and selectivity of modern peptide-coupling reagents thereafter facilitated the extension of the peptide chain at C- and N-terminal functional groups in the presence of the totally deblocked heptasaccharide side-chain. All remaining protecting groups were removed before the final enzyme-catalyzed transfer of galactose and neuraminic acid on to both non-reducing ends of the glycan chains (Scheme 24). During these transformations the addition of alkaline phosphatase was essential to prevent product inhibition by UDP [79, 801. These final steps yielded the target undecasaccharide glycopentapeptide 51 in 91% yield for the last two steps. It is obvious why the authors of the aforementioned synthesis have preferred peptide assembly in solution to the solid-phase approach:
1) the synthetic effort required for peptide assembly is small compared with the construction of the saccharide chain
YOOHHo
I
s,, I
Ser
I
Ser I Asn
I
Phe
HO
Scheme 24.
51
11.2 Synthesis o j Glycopeptides in Solution
285
2) on the solid phase, a molar excess of building blocks must usually be used to ensure high coupling yields. For the saccharide-asparagine moiety this would have entailed tedious re-isolation of this valuable compound. In general, therefore, glycopeptides with a comparably short peptide sequence and complex and valuable carbohydrate parts can be efficiently prepared in solution. This is also true for N-glycopeptides carrying Lewis antigen-type side-chains. As an alternative to the chemical and chemoenzymatic routes, N-glycopeptides can be obtained by a convergent strategy [ 8 1] using glycans isolated from natural Nglycoproteins or oligosaccharides and presynthesized peptides 1821. The unprotected saccharide having the reducing terminal N-acetylglucosamine is converted into a glycosylamine by use of excess ammonium hydrogen carbonate [24] and then coupled with a peptide [82]. By this strategy, large oligosaccharide side-chains can be introduced in the molecules. This is also true for the construction of N-glycopeptides from N-acetylglucosamine-containing glycopeptides and extension of their glycan portion by trans glycosylation-catalyzed saccharide transfer from N-glycoproteins [ 831. Despite the possible microheterogeneity of the natural glycoprotein glycans, the limitation of these reactions is that only analytical amounts (<1 pmol) are obtained. N-Glycopeptides with Lewis-Type Saccharide Side-Chains
The biological importance of Lewis-type saccharide structures, and typical methods for their chemical preparation, have already been outlined in the introduction. In particular, the preparation of peptide backbones carrying a high density of Lewis type antigens is of increasing interest. Such clustered structures are often more immunoreactive than the corresponding monovalent antigens [84]. This observation was the motivation for a chemoenzymatic synthesis of the trivalent N-glycopeptide 52 (Scheme 25) [85]. Selectively protected GlcNAc-Asn units were prepared as suitable precursors and conjugated via an amino acid of variable length. The trivalent glycopeptide was converted into the corresponding tris-sialyl LewisX glycoconjugates by stepwise elongation with galactosyltransferase/UDP-galactose, sialyltransferase/CMP-sialic acid, and fucosyltransferase/GDP-fucose. By this approach, 20 mg of target compound 52 were obtained and used for E-selectin binding assays. An inhibition four times stronger than that of the monovalent SLe"-Asn residue was achieved. Without the restrictions of enzymatic reactions, such as the strict specificity of glycosyltransferases and the limited availability of enzymes and nucleotide substrates, a purely chemical approach has its advantages in generating a broader structural variety and larger product quantities. For this purpose, the SLeXiaazides are ideal precursors (see Scheme 6). They can be: 1) produced on a gram scale; 2) easily converted into the corresponding P-glycosylamino derivatives and 3 ) efficiently coupled to form stable amide linkages.
286
I1 Glycopeptide Synthesis in Solution and on the Solid Phase 0
HO
0
HO
J
HO
J
Scheme 25.
In this manner, SLe" amine 53 was combined with a tetrapeptide partial sequence from fibrinogen required for binding to the integrin receptor GpIIb [29]. For this conjugation the C-terminal alanine of RGDA derivative 54 served the as coupling position (Scheme 26). The fully deprotected SLeX-RGDA glycopeptide 55 inhibited P-selectin binding with high affinity (1Cso 25 pmol), but had no affinity for E-selectin. Structurally even more sophisticated is the trivalent cycloheptapeptide 57 (861. The synthesis of this complex molecule involved the preformation of the cyclic peptide 56 containing three aspartic acid residues (Scheme 27). After removal of the tert-butyl side-chain protecting groups simultaneous condensation with three equivalents of SLeX-lactone amine 53 using HATU/HOAT/DIEA gave the cycloheptapeptide carrying three SLeX side-chains. This was fully deprotected in two further steps. The resulting trivalent antigen proved to inhibit the binding of human tumor cells (HL60) to E-selectin in the low millimolar range (IC50 0.35 mM), but in contrast with 55 had no affinity for P-selectin.
11.3 Glycopeptide Synthesis on the Solid Phase In contrast with the construction of glycopeptides in solution, automated assembly on solid supports promises rapid access to the desired target compounds [87-891.
11.3 Glycopeptide Synthesis on the Solid Phase
53
OH
287
I OBzl
nn-1
OH
NH
I
Scheme 26.
This only holds true, however, if certain structural and methodological prerequisites are fulfilled: 1) glycosylated amino acids must be provided in sufficient quantity as N-Fmocprotected compounds containing a free a-carboxylic function and 2) reaction conditions and polymer-anchoring systems applied during the solidphase procedure and the cleavage and deprotection steps must be adapted to the sensitive glycosylated compounds. Throughout the following section, solutions of these problems will be presented in connection with recently published solid-phase 0- and N-glycopeptide syntheses. 11.3.1 O-Glycopeptides Glycopeptides Carrying N-Acetylgalactosamine (Tn-Antigen)
The biological relevance of a-GalNAc-carrying glycopeptides has been outlined above. For the preparation of a glycopeptide from the homophilic recognition domain of Epithelial Cudherin 1 (E-CAD, 58, Scheme 28) the particular importance of the anchoring structure and of adapted coupling conditions can be demonstrated [901.
288
I 1 Glycopeptide Synthesis in Solution and on the Solid Phase
51
Scheme 27.
58
Scheme 28.
11.3 Glycopeptide Synthesis on the Solid Phase
289
58
Scheme 29.
The depicted target efficiently was constructed with the help of the allylic anchoring principle and a newly developed coupling reagent. To this end, the starting amino acid (Glu) was linked to the polymeric support (Tentagel@)via the Hycron anchor comprising the internal standard p-alanine, a flexible and polar oligo(ethy1ene glycol) spacer, and a 1,4-dihydroxybut-2-ene unit (59, Scheme 29). Subsequent initial attempts to synthesize the E-CAD partial structure with the help of TBTU [91] as coupling reagent gave only low yields although TBTU had already proved efficient in glycopeptide synthesis. Obviously, this sequence is prone to back-folding, and particularly efficient activation of the corresponding amino acids building blocks had to be achieved. This problem was finally overcome by the development of a new coupling reagent: PfPyU ( N ,N , N ’ , N’-bis(tetramethylene)-O-pentafluorophenyluronium hexafluorophosphate) 60 [92]. Compared with TBTU, activation with PfPyU in model reactions resulted in eightfold increased reaction rates. This observation was confirmed during the E-CAD synthesis. First, Fmoc-AlaVal-OH was coupled to the starting glutamic acid. This step was advisable to prevent diketopiperazine formation on the stage of the resin-bound dipeptide. All coupling steps were conducted using Pf PyU and were followed by morpholine-
290
-
11 Glycopeptide Synthesis in Solution and on the Solid Phase "Rink"
BzO Fmoc-He-Ser-Gly-Ile-GI
OH
NH
2) TFA/H20
Fmoc-Ile-Ser-Gly-lle-Gly-NH2
I
aBz3GalN3
Scheme 30.
mediated Fmoc removal. Finally, the fully assembled glycopeptide was detached from the resin by Pd(0)-catalyzed ally1 transfer to N-methylaniline and gave the glycododecapeptide 58a in an overall yield of 55%. After removing all acid labile protecting groups, the 0-acetyl and Fmoc groups were cleaved by Zemplen transesterification. The high overall yield and purity of 0-glycododecapeptide 58a demonstrated the efficiency of the Hycron/Pf PyU methodology. The approach described above employed conventional insertion of a preformed GalNAc-Ser building block into the peptide chain to be synthesized. A method presented recently performs solid-phase glycosylation subsequent to peptide assembly [93]. Obviously, with this strategy the construction of glycopeptide libraries containing different glycan side-chains becomes feasible. The improvement achieved by the authors concerned the nature of the polymer support. A polyoxyethylenepolyoxypropylene (POEPOP) resin with balanced polarity and hydrophobicity was developed. In combination with an acid-labile RINK linker [94] this system served for the synthesis of pentapeptide 62 containing one serine residue (Scheme 30). Moreover, the properties of the POEPOP resin enabled the solid-phase glycosylation of the serine hydroxy group using (among others) perbenzoylated 2-azidogalactosyl donor 61. The target Tn glycopeptide 63 was obtained in high yield after detachment from the resin. 0-Glycopeptides Carrying the T Antigen (Gal-GalNAc)
Similar to the above mentioned on-resin glycosylation of hydroxy amino acids, the solid-phase elongation of saccharide chains linked to peptides has also been described. With the help of this method the preparation of T antigen carrying MUC2
11.3 Glycopeptide Sjmthesis on the Solid Phase
291
Ac-Pro-Thr(tBu)-Thr-Thr(tBu)-Pro-ile-Ser(tBu)-Thr(tBu) -RINK-
BIo%0Yc13 BzO 64
NH
Ho+
G-" Ph
Ac-Pro-Thr-Thr-Thr-Pro-Ile-Ser-Thr-NH2
66
HO OH
Scheme 31.
fragments was achieved [95]. To this end, a suitably protected a-azidogalactosyl threonine unit was used in a solid-phase glycopeptide synthesis (Scheme 31). In the resulting glycooctapeptide 65 the unsubstituted 3-OH group of azidogalactose served as an acceptor for a solid-phase P-galactosylation using galactosyl trichloroacetimidate 64 [30]. After optimization of the reaction conditions and reagent purity, the disaccharide octapeptide 66 was obtained in 67% yield after cleavage from the resin with TFA and complete deprotection. It should again be noted that the resin employed had a decisive influence. 0-Glycopeptides Carrying the Sialyl Tn Antigen (NeuNAc-a2,6-GalNAc)
The sialyl Tn antigen is considered to be one of the most important tumor-associated carbohydrate antigens [46, 961. Because of its immunoreactivity the application of clustered STn conjugates as cancer vaccines is already being investigated in clinical studies [47]. Because it is assumed that both the peptide and carbohydrate parts of the corresponding conjugates are important for a tumor-selective cellular and humoral immune response [97], the synthesis and application of tumor related glycopeptides are other major areas currently being investigated. Only recently has the successful incorporation of a suitable STn-Ser/Thr building block into a solidphase glycopeptide synthesis been achieved [98]. The key element of the strategy used here consisted in the construction of the disaccharide threonine unit 70. This was achieved starting from the known GalNAc-Thr moiety already carrying the required Fmoc and the tert-butyl ester protecting group (Scheme 32). Despite the base-sensitive Fmoc group, the removal of the 0-acetyl groups was selectively accomplished by careful treatment with NaOMe in MeOH at pH 8.2.
292
I1 Glycopeptide Synthesis in Solution and on the Solid Phase
YOAc
COOtBu HNFmoc
I ) AC20 1 p p d ~ n e 88% , 2) TFA,anaole, quant
69
B
OAc
OAc
70
Scheme 32.
The resulting trio1 acceptor 68 was then sialylated by use of sialyl xanthate 67 [34] and methylsulfenyl triflate/AgOTf [37] as the promoter. At -62 "C the reaction proceeded almost diastereoselectively and regioselectively and the glycosylated threonine derivative 69 was obtained in 32% yield after purification. The desired building block 70 for solid-phase synthesis was provided after O-acetylation and cleavage of the tert-butyl ester group. Glycosyl threonine 70 was subsequently incorporated into a partial sequence of the MUCl repeating unit known to be tumorrelevant [99]. To this end, the starting amino acid proline attached to the Hycron anchor (see above) was amino-deprotected and coupled with Boc-Ala-OH. Changing to the Boc methodology at this stage prevented diketopiperazine formation on the resin-linked dipeptide strongly favored for an Ala-Pro sequence (Scheme 33). This intermittent change of protecting groups was possible because of the stability of the anchor towards acids and bases. Further chain extension and Pd (0)-catalyzed detachment from the resin gave the protected glycoundecapeptide. After acidolytic removal of the side-chain protecting groups the O-acetyl groups and the methyl ester were cleaved by carefully optimized treatment with dilute aqueous NaOH in MeOH and aqueous NaOH (pH 11.5) (-71). At pH < 11.5 no methyl ester cleavage was achieved whereas above this pH epimerization and b-elimination of the carbohydrate occurred. The attention paid to the preparation of STn and other sialic acid-containing glycopeptides of biological importance became apparent with the appearance of another publication reporting the successful synthesis of a STn-carrying decaheptapeptide from HIV at almost the same time [loo].
11.3 Glycopeptide Synthesis on the Solid Phase
293
aAcgNeu(OMe)-(2-6)-aAc2GalNac
I
Ac-Ala-Pro-Pr~Ala-His(Trt)-Gly-Val-Thr-Ser(tBu)-Ala-Pro-HYCRON-~Ala-AMPS
I)[Pd(PPh3)4], DMSO I DMF, rnorpholine 2) TFA, anisole, EtSMe 3) NaOH, MeOH 4) 5 mM NaOH
H
*’HO HO
71
Scheme 33.
0-Glycopeptides Carrying the 2,3-Sialyl T Antigen
Another biologically important saccharide found in nature and resulting from incomplete glycosylation (see Tn, T, STn) is the sialyl T antigen [ 1011. Only recently, a 27 amino acid peptide carrying the 0-glycosidically linked a2,3NeuNAcGalpl, 3GalNAc structure was synthesized [102]. Again, a glycosylated serine had to be prepared before the solid-phase procedure (Scheme 34). Starting with di-0-benzyl galactose residue 72 sialylation of the 3-OH was achieved by use of the perbenzylated sialic acid donor 73 [103]. This donor was temporarily equipped with a thiophenyl residue in the 3 position to ensure high stereoselectivity in the mercury salt-promoted a-sialylation. Upon desulfurization and treatment with DBU a disaccharide was obtained wherein the lactone serves as an internal protecting group. After oxidative cleavage of the anomeric methoxyphenyl group and conversion into the trichloroacetimidate 74, the fully protected trisaccharide serine was obtained by BF3 .OEt2-mediated reaction of 74 with TBDMS-protected azidogalactosyl serine residue 75. Protecting group manipulation and carboxy deprotection using Pd(O)/N-methylaniline gave building block 76 which was used for the solid-phase assembly of the peptide employing dicyclohexylcarbodiimide/HOBt as coupling reagent and an acid-labile anchoring group.
294 HO HO
I 1 Glycopeptide Synthesis in Solution and on the Solid Pliase OBzl
Bzlo
BzlO
72
+
BzlO
W AcHN C
’
OMe
O
BzlO
O
BSPh z
I
73
I ) HgO)
2) Ph3SnH. AIBN 3) DBWTHF 4) CAN 5 ) CCI3CNDBU I ) BF30Etz
76
75
Fmoc-Ser-OAll
NeuNAca2,3Gal!31,3GalNAca 1
I
H-Thr-Val-Val-Gln-Pro-Ser-Val-Gly-Ala-~a-Ala-Gl~~o-Val-Va~-Pro-Pro-Cys-Pro-Gl~~g-Ile-~g-His-Phe-Lys-Val-OH
77
Scheme 34.
Cleavage from the resin was followed by an unusual debenzylation step which was performed using TMSOTf and TFA [104]. By this procedure the hydrogenolytic cleavage of benzyl ethers-often found difficult in such complex molecules-was avoided. Finally, opening of the lactone ring with NaHC03/D20 and treatment with 1,4-dithiothreitol to reduce the undesired disulfide bridges gave the target glycoheptacosapeptide 77. 0-Glycopeptides Carrying 0-GlcNAc Side-Chains
The biological importance of 0-GlcNAc-carrying peptides has already been mentioned. Again, a prerequisite for the solid-phase approach is the availability of Fmoc-Ser/Thr(GlcNAc)-OH building blocks. Doubtlessly, the most rapid synthesis of these derivatives was achieved by Lewis acid-promoted activation of peracetylated glucosamine in the presence of Fmoc-Ser or Fmoc-Thr. Including the preparation of the starting compounds only three steps gave the desired glycosylated amino acid. In this instance the sugar oxazoline is the reactive intermediate [ 1051. A similar conversion can be performed starting from the corresponding N-Tcoc derivative [ 1061. The number of protecting-group manipulations can also be reduced by use of
11.3 Glycopeptide Synthesis on the Solid Phase
ACO&O&OQ; AcO
H *t
HNAC
R
O
F
R=Y CH3
/
295
OH
H-Ala-Val-Ser-Thr-GIu-Pro-Phe-Gly-Arg-NH~ I
I
F 78
79
HO HO HNAC
Scheme 35.
Fmoc-Ser/Thr pentafluorophenyl esters [ 1071. The latter remain stable during the glycosylation process and so suitable solid-phase building blocks 78 are obtained after the generation of the N-acetamido residue. Solid-phase synthesis with Pfp esters was performed on a poly(ethy1ene glycol)-derived resin equipped with an acidlabile linker [108]. By following a standard protocol for the coupling reactions, detachment from the resin, and deprotection, the bisglycosylated nonapeptide 79 was obtained and used for detailed NMR investigations (Scheme 35). A chemoenzymatic approach to O-GlcNAc peptides with less general applicability has recently been reported [ 1091. O-Glycopeptides Carrying O-Linked Fucose
Nothing was known about the appearance of O-fucosylated proteins until the early 90s. Then, the uncommon Fuc-Ser/Thr motif was found in the growth factor domains of several coagulation and fibrinolytic proteins [ 1101. The structure also occurs in a small serine protease inhibitor isolated from Locustu migratoriu (83; Scheme 36) [ 1 I I]. In the latter both the anomeric configuration of the fucose and its biological function remain unclear. To evaluate the influence of the single fucose moiety on the conformation and activity of this protein, the fucosylated and unfucosylated form were synthesized [ 1121. The required a-fucosylated threonine building block (which was assumed to be the more likely a isomer) was prepared in four steps from a suitable Mpm etherprotected 1-thioalkyl fucose 80 and the Fmoc/tBu-protected threonine derivative 79 [71]. Activation of the donor 80 was achieved with DMTST and resulted in the formation of the a-configured glycoside 81. Because of the acid-sensitivity of the afucosidic linkage, 2,6-di-tert-butylpyridine had to be added to the reaction mixture. The Mpm ethers were removed by oxidation with ceric ammonium nitrate followed by O-acetylation and cleavage of the tert-butyl ester with formic acid. The intermittent exchange of the hydroxy protecting groups was essential to enhance the acid stability of the fucoside before liberation of the terminal carboxy group and final peptide deprotection [40]. By use of the building block 82, the solid-phase synthesis was performed by means of an acid-sensitive resin and BOP [ 1 131 as coupling reagent. Release from the solid support and side-chain deprotection was achieved by
296
I1 Glycopeptide Synthesis in Solution and on the Solid Phase
-
Fmoc-Thr(0H)-OH tBuOWDCC CUCl Fmoc-Thr(0H)-OEJu
MpqFucSEt 80
Fmoc/'?OtBu
1 DMTST 2,643 -ferfbutylpyridine
@O ' MOMpm pm MpmO
1) CAN 2) AczO/pyridine
3, HCooH 81
-
Fmoc-Thr(aAc,Fuc)-OH 82
Fucose
I
HOOC -
Scheme 36.
treatment with TFA. Finally, disulfide formation and 0-acetyl removal were performed simultaneously in diethylmethylamine in the presence of air. Careful HPLC purification gave the monofucosylated peptide 83 containing 36 amino acids and three disulfide bridges. A straightforward synthesis towards the fucosyl threonine block 82 was performed by direct activation of peracetylated fucose with BF3.0Et2 in the presence of Fmoc-Thr-OH [114]. Small amounts of 0-configured impurities had to be removed from the crude product by HPLC, however. A more laborious procedure for the construction of building block 82 and its incorporation into a undecapeptide has also been reported [ 1 151. 0-Glycopeptides Carrying a Sialyl Lewis Antigen Structure The first synthesis of a glycopeptide carrying 0-linked SLe" was recently described 11161. The major achievement therein consisted of the enzymatic assembly of the SLe" tetrasaccharide on a suitable solid support. This approach promises a rapid and variable construction of complex glycopeptides. The starting point was the preparation of the corresponding O-GlcNAc-containing octapeptide 85 linked to controlled-pore glass (CPG) via the aforementioned Hycron linker [ 1171 by use of standard protecting groups and coupling conditions. Because the ally1 ester unit herein proved unstable under the typical conditions used for the deacetylation of sugar side-chains, a building block with unprotected sugar
11.3 Glycopeptide Synthesis on the Solid Phase
%
OAc
AcO AcO
1) BF30Et*/Z-Thr 2) NaOMehleOH 3) Pd-C/MeOH 4) Fmoc-OSu
Fmoc-Thr-OH
NHAc
84
" - h o
/ w
NHAc
--
Ac-Lys-Pro-Pro-Asn-Thr-Thr-Ser-Ala-HYCRON-CPG ,H0&0,
297
85
Ac-Lys-Pro-Pro-Asn-Thr-Thr-Ser-Ala-HYCRON-CPG
H6p NHAc
NHAc
\
HO
Scheme 37.
hydroxy groups (84) had to be provided according to the pathway depicted in Scheme 37. After removal of the tert-butyl and trityl side-chain-protecting groups with TFA, galactosylation, sialylation, and fucosylation were successively performed using the corresponding glycosyltransferasesand donor substrates. Finally, the advantages of mild and reliable detachment from the solid support by use of the Hycron/Pd(O) system was demonstrated with this complex and sensitive structure (86).
11.3.2 N-Glycopeptides Compared with O-glycopeptides, access to N-glycopeptides on the solid phase is more readily possible via two different pathways [42]-conventional sequential incorporation of preformed and suitably protected building blocks into the solidphase synthesis or assembly of the peptide chain, selective removal of the aspartic acid side-chain protecting groups, and formation of the amide linkage between glycosyl amines and the peptide backbone [81] on the solid phase. The simplicity of amide formation and access to a great variety of glycosyl amines enables the use of the second method for the synthesis of N-glycopeptides whereas only single examples of solid-phase O-glycosylation have been reported in the synthesis of O-glycopeptides.
298
11 Glycopeptide Synthesis in Solution and on the Solid Phase H-Q(GkNAc)YGGFL-OH H-Q(Fuc)YGGFL-OH H-Q(GlcNAc(p 1,~GICNAC)YGGFL-OH H-Q(GalP 1,4Glc)YGGFL-OH H-Q(GalP 1,3(Fuca 1,4)GlcNAcP1,3GalB1,3Glc)YGGFL-OH H-Q(Gal6SO3)YGGFL-OH
f
H-N(Ga1)YGGFL-OH H-N(Glca 1,4Glcal,4Glca 1,4Glca 1,4Glca 1,4Glcla I,4Glc)YGGFL-OH H-N(GlcN2,6(S0332)YGGFL-OH H-N(Ga16SOi)YGGFL-OH H-N(GaL4)YGGFL-OH
Fmoc-E(OAIl)Y(tBu)GGFL-SASRIN
’/
’
Fmoc-D(OAII)Y(tBu)GGFL-SASIUN
-
Fmoc-Y(tBu)GE(OAII)GFL-SASRIN
H-Y GO(GlcNAcEFL-OH H-YGQ(Gal)GFL-OH H-YGQ(Glca 1,4Glc)GFL-OH
Fmoc-Y(tBu)GGFLE(OAII)-SASRM
H-QGQ(GIc!~I,~GIc)GFL-OH
\
Fmoc-Y(tBu)GGFLD(OAlI)-SASRIN \
H-QGY(GICNAC~SO~~)GFL-OH
\ H-YGGFLQ(MmNAc)-OH H-Y GGFLQ(Glc)-OH H-YGGFLQ(GalP 1,4Man)-OH H-YGGFLQ(Glca 1,4Glca 1,4Glca 1,4Glca 1,4Glca 1,4Glc1a 1,4Glc)-OH H-Y GGFLN(G1cNAc)-OH H-YGGFLN(Ga1NAc)-OH H-YGGFLN(Lm)-OH
Scheme 38.
The Construction of N-Glycopeptide Libraries on the Solid Phase The development of efficient methodology for on-resin N-glycopeptide formation has been described as a starting point toward the preparation of glycopeptide libraries (Scheme 38) [ 1181. Employing an acid-labile anchoring system (Sasrin resin [ 1191) five different hexapeptides containing either glutamic or aspartic acid allyl ester residues were synthesized. Pd(0)-catalyzed selective cleavage of the allyl ester and subsequent formation of the corresponding pentafluorophenyl esters enabled conversion into N-glycopeptides by use of unprotected glycosyl amines and a mixture of DIEA and HOBt. Detachment from the resin and removal of tert-butyl side-chain protecting groups were achieved by treatment with TFA. The combination of mono- and oligoglycosidic amines with five peptides resulted in 23 different N-glycopeptides (see Scheme 38) including interesting products with sulfated glycosyl residues [ 1201. Problems were encountered with fucosylated glycosides-this 0-glycosidic linkage was cleaved even by dilute TFA. Another difficulty that prevents general applicability is the tendency of activated aspartic acid derivatives to rearrange to aspartimides. A solution to this problem was provided by appropriate partial protection of
11.3 Glycopeptide Synthesis on the Solid Phase
299
the peptide backbone. With the help of the N-2-acetoxy-4-methoxybenzyl (AcHmb) group on the alanine residue, aspartimide formation throughout the convergent solid-phase synthesis of a chitobiosylated hexapeptide was suppressed [ 1211. The AcHmb group can be removed in two steps-hydrazine-mediated deacetylation and subsequent treatment with TFA. Sequential N-Glycopeptide Synthesis on the Solid Phase with Oligosaccharides from Natural Sources The use of oligosaccharide residues from natural sources in solid-phase peptide synthesis requires two steps, isolation in sufficient quantity and conversion into suitable building blocks. Whereas conversion of the reducing end of very complex glyco-structures into glycosyl amines has already been achieved [24, 1221, the primary step of detaching sufficient quantities of N-linked oligosaccharides from their natural precursors still requires simplification. Very recently a modified hydrazinolysis procedure has enabled the isolation of di- and trianntenary structures from fetuin and high-mannose type compounds from ribonuclease B on a 100-500-mg scale (Scheme 39) [123]. Upon treatment with aqueous ammonium bicarbonate the corresponding p-configured glycosylamines were obtained and coupled to activated Fmoc-Asp-OtBu. After tert-butyl removal the resulting building blocks were successfully used in solid-phase synthesis. Compounds such as the undecaglycodecapeptide 87 were obtained.
Val
I I Thr I Ile
OH
I<
OH O 'H 0,
OH AcHN
OH
flnu 5
P " LOH O H
Scheme 39.
87
300
I1 Glycopeptide Synthesis in Solution and on the Solid Phase
Certainly, this methodology holds great potential for particular saccharide sidechains that can be isolated from natural sources and are not readily accessible via chemical synthesis.
11.4 Conclusion The synthetic methods reviewed in this chapter and in earlier survey articles now make available glycopeptides of considerable complexity. These methods have been provided by the development of distinctly chemo-, regio-, and stereoselective chemical reactions supplemented by efficient techniques adopted from biochemistry and biotechnology. The step-by-step assembly of glycopeptides in solution is still the method of choice if larger quantities of the target compounds ( 2 1 mmol) are required. The synthesis of glycopeptides containing important but complex saccharide residues remains a major task of glycopeptide chemistry. The introduction of sialic acid residues-considered to be a limitation in the past-can now be achieved efficiently. Remarkable progress has also been made in the use of enzymatic methods. The latter have been shown to be of particular advantage for the assembly of carbohydrate side-chains in the context of linear or convergent synthetic strategies. In combination with improvements in solid-phase technology, e.g. adapted resins and coupling reagents, further progress in the preparation of biologically important glycopeptides can be expected. Abbreviations AgOTf All
Bn Boc BOP Bz Bzl CAN DCC DIEA DMAP DMF DMSO DMTST EEDQ
Fmoc
silver triflate ally1 benzyl t-butyloxycarbonyl benzotriazole-1-yl-oxy-tris(dimethylamin0)phosphoniumhexafluorophosphate benzoyl benzyl ceric ammonium nitrate N ,N' -dicyclohexylcarbodiimide
diisopropylethylamine dimethylaminopyridine dimethylformamide dimethylsulfoxide dimethylthiomethylsulfonium triflate 2-ethoxy-1-ethoxycarbonyl-1,2-dihydrochinoline 9-fluorenylmethoxycarbonyl
References
301
Fuc Gal GalNAc GlcNAc HATU
fucose galactose 2-acetamido-2-desox y-galactose 2-acetamido-2-desox y-glucose 2-( 1H-9-azobenzotriazole-1-yl)-l,l,3,3-tetramethyluronium hexafluorophosphate hydroxyazabenzotriazole HOAt hydroxybenzotriazole HOBt Sheptadecanoyl HYCRON (E)-17-hydroxy-4,7,1O,I3-tetraoxa-l IIDQ 1-isobutyloxycarbonyl-2-isobutyloxy1,2-dihydrochinoline lactose Lac mannose Man p-methoxyphenylmethy1 MPm N-acetyl neuraminic acid NeuNAc PEGA polyethylene glycol-dimethylacrylamide co-polymer pentafluorphenyl PfP PhAcOZ p-(phenylacetoxy)benzyloxycarbonyl POEPOP polyoxyethylene-polyoxypropylene copolymer rotating frame nuclear overhauser spectroscopy ROESY super acid sensitive resin SASRIN sialyl-Lewis-)< SLe" 2-(1H-benzotriazole-1-yl)- I, 1,3,3-tetramethyluroniumtetrafluoro-borate TBTU 'Bu tert-butyl Tcoc N-trichloroethoxycarbonyl TFA trifluoroacetic acid TMSOTf trimethylsilyl triflate benzyloxycarbonyl Z
References H. Lis, N. Sharon, Eur. J. Biochem., 1993, 218, I . A . Varki, Glycohiol., 1993, 3 , 97. R. A. Dwek, Chem. Rev., 1996, 96, 683. G. Arsequell, G. Valencia, Tetrahedron Asym., 1997, 8, 2839. B. K. Hayes, G. W. Hart, Curr. Opin. Struct. Biol., 1994, 4, 692. N. Jentoft, TZBS, 1990, 15, 291. D. Sako, K. M. Comess, K. M. Barone, R. T. Camphausen, D. A. Cumming, G. D. Shaw, Cell, 1995, 83, 323. 8. T. A. Springer, Cell, 1994, 76, 301. 9. M. L. Phillips, E. Nudelman, F. C. A. Gaeta, M. Perez, A. K. Singhal, S.4. Hakomori, J. C. Paulson, Science, 1990, 250, 1130. 10. M. Larkin, T. J. Ahern, M. S. Stoll, M. Shaffer, D. Sako, J. O'Brien, C.-T. Yuen, A. M. Lawson, R. A. Childs, K. M. Barone, P. R. Langer-Safer, A. Hasegawa, M. Kiso, G. R. Larsen, T. Feizi, J. Biol. Chem., 1992, 267, 13661. 11. E. E. Simanek, G. J. McGarvey, J. A. Jablonowski, C.-H. Wong, Chem. Rev, 1998, 98, 833. 12. H. Lis, N. Sharon, Chem. Rev., 1998, Y8, 637. 13. W.-j. Liu, V. Ramachandran, J. Kang, T. K. Kishimoto, R. D. Cummings, R. P. McEver, J. Biol. Chem., 1998, 273, 1078.
I. 2. 3. 4. 5. 6. 7.
302
11 Glycopeptide Synthesis in Solution and on the Solid Phase
14. P. P. Wilkins, R. P. McEver, R. D. Cummings, J. Biol. Chem., 1996, 271, 18732. 15. F. Li, H. P. Erickson, J. A. James, K. L. Moore, R. D. Cummings, R. P. McEver, J. Biol. Chem., 1996,271, 6342. 16. M. DeLuca, L. C. Dunlop, R. K. Andrews, J. V. Flannery, R. Ettling, D. A. Cumming, G. M. Veldman, M. C. Berndt, J. Biol. Chem., 1995, 270, 26734. 17. H. Kunz, M. Schultz, in D. G. Large, C. D. Warren (Eds.): Glycopeptides and Related Compounds, Marcel Dekker. Inc., New York 1997, p. 23. 18. H. G. Garg, K. v. d. Bruch, H. Kunz, Adu. Carbohydr. Chem. Biochem., 1994,50,277. 19. H. Paulsen, Angew. Chem. Int. Ed. Engl., 1982, 21, 155. 20. H. Kunz, Angew. Chem. Znt. Ed. Engl., 1987,26, 294. 21. K. Toshima, K. Tatsuta, Chem. Rev., 1993, 93, 1503. 22. 0. Lockhoff, in H. Hagemann, D. Klamann (Eds.): Methoden der Organischen Chemie, Vol. E14a/3, G. Thieme, Stuttgart 1992, p. 621. 23. R. R. Schmidt, Angew. Chem. Znt. Ed. Engl., 1986,25, 212. 24. L. M. Likhosherstov, 0. S. Novikova, V. A. Derevitskaja, N. K. Kochetkov, Carbohydr. Rex, 1986, 146, C1. 25. H. Kunz, H. Waldmann, Angew. Chem. Znt. Ed. Engl., 1985,24, 883. 26. R. U. Lemieux, J. I. Haymi, Can. J. Chem., 1965,43,2162. 27. H. Lonn, Carbohydr. Res., 1985, 139, 105. 28. R. R. Schmidt, J. Michel, Angew. Chem. Znt. Ed. Engl., 1980, 19, 731. 29. U. Sprengard, G. Kretzschmar, E. Bartnik, C. Huh, H. Kunz, Angew. Chem. Znt. Ed. Engl., 1995, 34, 990. 30. R. R. Schmidt, in H. Ogawa, A. Hasegawa, T. Suami (Eds.): Carbohydrates-Synthetic Methods and Applications in Medicinal Chemistry, Verlag Chemie, Weinheim 1992, p. 66. 31. R. R. Schmidt, ASC Symposium Series, 1994, 560, 276. 32. K. Okamoto, T. Goto, Tetrahedron, 1990, 46, 5835. 33. A. Hasegawa, H. Ohki, T. Nagahama, H. Ishida, M. Kiso, Carbohydr. Res., 1991,212, 277. 34. A. Marra, P. Sinay, Carbohydr. Res., 1989, 187, 35. 35. H. Lonn, K . Stenvall, Tetrahedron Lett., 1992, 33, 115. 36. T. Murase, H. Ishida, M. Kiso, A. Hasegawa, Carbohydr. Res., 1988, 184, C1. 37. F. Dasgupta, P. J. Garegg, Carbohydr. Rex, 1988, 177, C13. 38. W. Stahl, U. Sprengard, G. Kretzschmar, D. W. Schmidt, H. Kunz, J. Prakt. Chem., 1995, 337, 441. 39. H. Kunz, C. Unverzagt, Angewj. Chem. Znt. Ed. Engl., 1988, 27, 1697. 40. C. Unverzagt, H. Kunz, Bioorg. Med. Chem., 1994,2, 1189. 41. B. Liebe, H. Kunz, Helv. Chim. Actu, 1997, 80, 1473. 42. D. G. Large, I. J. Bradshaw, in D. G. Large, C. D. Warren (Eds.): Glycopeptides and Related Compounds, Marcel Dekker, Inc., New York 1997, p. 295. 43. P. L. Devine, I. F. C. McKenzie, BioEssays, 1992, 14, 619. 44. J. J. R. Gum, Biochem. SOC.Trans., 1995, 23, 795. 45. I. Brockhausen, J.-M. Yang, J. Burchell, J. T.-Papadimitriou, Eur. J. Biochem., 1995, 233, 607. 46. S. H. Itzkowitz, M. Yuan, C. K. Montgomery, T. Kjeldsen, H. K. Takahashi, W. L. Bigbee, Y. S. Kim, Cancer Rex, 1989, 49, 197. 47. R. R. Koganty, M. A. Reddish, B. M. Longenecker, in D. G. Large, C. D. Warren (Eds.): Glycopeptides and Related Compounds, Marcel Dekker, Inc., New York 1997, p. 707. 48. S. Gendler, J. Taylor-Papadimitriou, T. Duhig, J. Rothbard, J. Burchell, J. Biol. Chem., 1988, 263, 12820. 49. P. Braun, G. M. Davies, M. R. Price, P. M. Williams, S. J. B. Tendler, H. Kunz, Bioorg. Med Chem., 1998, 6, 1531. 50. R. U. Lemieux, R. M. Ratcliffe, Can. J. Chem., 1979, 57, 1244. 51. H. Paulsen, W. Rauwald, U. Weichert, Liebigs Ann. Chem., 1988, 75. 52. M. Ravenscroft, R. M. G. Roberts, J. G. Tillett, J. Chem. Soc., Perkin Trans., 1982, 1568. 53. A. Reidel, H. Waldmann, J. Prakt. Chem., 1993, 335, 109. 54. J. Eberling, P. Braun, D. Kowalczyk, M. Schultz, H. Kunz, J. Org. Chem., 1996, 61, 2683. 55. M. Gewehr, H. Kunz, Synthesis, 1997, 1499.
References
303
T. Toyokuni, S.4. Hakomori, A. K. Singhal, Bioorg. Med. Chem., 1994, 2, 1119. H. Paulsen, M. Paal, Carbohydr. Res., 1984, 135, 71. P. Fugedi, P. J. Garegg, H. Lonn, T. Norberg, Glycoconjugate J., 1987, 4, 97. D. R. Mootoo, P. Konradsson, B. Fraser-Reid, J. Am. Chem. Soc., 1989, 111, 8540. M. Leuck, H. Kunz, J. Prukt. Chern., 1997,339, 322. B. Liining, T. Norberg, J. Tejbrant, Glyeoconj. J., 1989, 6, 5. D. Sames, X.-T. Chen, S. J. Danishefsky, Nature, 1997, 389, 587. T. J. Martin, R. Brescello, A. Toepfer, R. R. Schmidt, Glycoeonj. J., 1993, 10, 16. M. M. Sim, H. Kondo, C. H. Wong, J. Am. Chem. Soc., 1993,115,2260. J. Liang, W.-R. Li, M. Joullie, Synth. Conzmun., 1994, 24, 187. J. B. Schwarz, S. D. Kuduk, X.-T. Chen, D. Sames, P. W. Glunz, S. J. Danishefsky, J. Am. Chem. SOC., 1999, 121, 2662. 67. S. Komba, M. Meldal, 0.Werdelin, T. Jensen, K. Bock, J. Chem. Soc., Perkin Trans. 1,1999, 415. 68. G. W. Hart, R. S. Haltiwanger, G. D. Holt, W. G. Kelly, Annu. Rev. Biochem., 1989, 58, 785. 69. G. W. Hart, D. M. Snow, Int. Rev. Cytol.; 1998, 181, 43. 70. J. Banoub, P. Boullanger, D. Lafont, Chem. Soc. Rev., 1992, 92, 1167. 71. M. Schultz, H. Kunz, Tetrahedron Asymmetry, 1993, 4, 1205. 72. U. K. Saha, R. R. Schmidt, J. Chem. Soc., Perkin Trans. I , 1997, 1855. 73. T. Pohl, H. Waldmann, J. Am. Chern. Soc., 1997, 119, 6702. 74. C. Unverzagt, Tetrahedron Lett., 1997, 38, 5627. 75. C. Unverzagt, Angew. Chem. Int. Ed. Engl., 1994: 33; 1102. 76. C. Unverzagt, H. Kunz, J. Prakt. Chem., 1992,334, 570. 77. H. Kunz, W. Giinther, Angew. Chern. Int. Ed. Engl., 1988, 29, 1068. 78. W. Gunther, H. Kunz, Curbohydr. Rex, 1992, 228, 217. 79. C. Uuverzagt, S. Kelm, J. C. Paulson, Carbohydu. Rex, 1994, 251, 285. 80. C. Unverzagt, H. Kunz, J. C. Paulson, J. Am. Chem. Soc., 1990, 112, 9308. 81. S. T. Cohen-Anisfeld, P. T. Lansbury, J. Am. Chem. Soc., 1993, 115, 10531. 82. S. Y. C. Wong, G. R. Guile, W. T. Rademacher, R. A. Dwek, Glycoconj. J., 1993, 10, 227. 83. M. Mizuno, K. Haneda, R. Iguchi, I. Muramotot, T. Kawakami, S. Aimoto, K. Yamamoto, T. Inazu, J. Am. Chem. Soc., 1999, 1999, 284. 84. T. Feizi, Curr. ,Opin. Stuuct. B i d , 1993, 3, 701. 85. G. Baisch, R. Ohrlein, Angew. Chem., Int. Ed. Engl., 1996, 35, 1812. 86. U. Sprengard, M. Schudok, W. Schmidt, G. Kretzschmar, H. Kunz, Angew. Chem., Int. Ed. Engl., 1996, 35, 321. 87. S. J. Danishefsky, J. Y. Roberge, in D. G. Large, C. D. Warren (Eds.): Glycopeptides and Reluted Compounds, Marcel Dekker, Inc., New York 1997, p. 245. 88. M. Meldal, Curr. Opin. Struct. Biol.,1994, 4, 710. 89. G. Barany, N. Kneib-Cordonier, D. G. Mullen, Int. J. Peptide Protein Res., 1987, 30, 705. 90. J. Habermann, H. Kunz, Tetrahedron Lett., 1998, 39, 265. 91. R. Knorr, A. Trzeciak, W. Bannworth, D. Gillessen, Tetrahedron Lett., 1989, 30, 1927. 92. J. Habermann, H. Kunz, J. Prukt. Chem., 1998, 340, 233. 93. A. Schleyer, M. Meldal, M. Renil, H. Paulsen, K. Bock, Angew. Chem., Int. Ed Engl., 1997, 36, 1976. 94. H. Rink, Tetrahedron Lett., 1987,28, 3787. 95. H. Paulsen, A. Schleyer, N. Mathieux, M. Meldal, K. Bock, J. Chem. Soc., Perkin Trans. I , 1997, 281. 96. S. H. Itzkowitz, E. J. Bloom, W. A. Kokal, G. Modin, S.4. Hakomori, Y. S. Kim, Cancer, 1990, 66, 1960. 97. H. Kunz, in C. H. Schneider (Ed.): Peptide3 in Immunology, J. Wiley and Sons, Ltd., ChiChester 1996, p. 10. 98. B. Liebe, H. Kunz, Helv. Chim. Acta, 1997, 80, 1473. 99. T. R. E. Stadie, W. Chai, A. M. Lawson, P. G. H. Byfield, F . G . Hanisch, Eur. J. Biochenz., 1995,229, 140. 100. M. Elofsson, L. A. Salvador, J. Kihlberg, Tetrahedron, 1997, 53, 369. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66.
304
11 Glycopeptide Synthesis in Solution and on the Solid Phase
101. J. U. Yang, J. C. Byrd, B. B. Siddiki, Y. S. Chung, M. Okumo, M. Sowa, Y. S. Kim, K. L. Matta, I. Brockhausen, Glycobiology, 1994, 4, 873. 102. Y. Nakahara, Y. Nakahara, Y. Ito, T. Ogawa, Tetrahedron Lett., 1997,38, 721 1. 103. Y. Nakahara, H. Iijima, T. Ogawa, Tetrahedron Lett., 1994, 35, 3321. 104. N. Fujii, A. Otaka, 0. Ikemura, K. Akaiji, J. Chem. Soc., Chem. Comm., 1987, 1987, 274. 105. G. Arsequcll, C. Krippner, R. A. Dwek, S. Y. C. Wong, J. Chem. Soc., Chem. Commun., 1994, 1994, 2383. 106. L. A. Salvador, M. Elofsson, J. Kihlberg, Tetrahedron, 1995, 51, 5643. 107. E. Meinjohanns, M. Meldal, K. Bock, Tetrahedron Lett., 1995, 36, 9205. 108. U. K. Saha, L. S. Griffith, J. Rademann, A. Gcyer, R. R. Schmidt, Carbohydr. Rex, 1997, 304, 21. 109. K. Witte, 0. Seitz, C.-H. Wong, J. Am. Chem. Soc, 1998, 120, 1979. 110. R. J. Harris, M. W. Spellman, Glycobiology, 1993, 3, 219. 111. N. Nakakura, H. Hietter, A. v. Dorssclear, B. Luu, Eur. J. Biochem., 1992, 204, 147. 112. H. Hietter, M. Schultz, H. Kunz, Synlett, 1995, 1219. 113. D. Nguyen, A. Heitz, B. Castro, J. Chem. Soc., Perkin Trans. I , 1987, 1915. 114. M. Elofsson, S. Roy, L. A. Salvador, J. Kihlberg, Tetrahedron Lett., 1996, 37, 7645. 115. S. Peters, T. L. Lowary, 0 . Hindsgaul, M. Meldal, K. Bock, J. Chem. Soc., Perkin Trans. I, 1995, 3017. 116. 0. Seitz, C.-H. Wong, J. Am. Chem. Soc, 1997, 119, 8766. 117. 0. Seitz, H. Kunz, Angew. Chem. Int. Ed. Engl., 1995, 34, 803. 118. D. Vetter, D. Tumelty, S. K. Singh, M. H. Gallop, Angew. Chem., Znt. Ed. Engl., 1995,34, 60. 119. M. Mergler, R. Tanner, J. Gostcli, P. Grogg, Tetrahedron Lett., 1988, 29, 4009. 120. C. Galustian, R. A. Childs, C. T. Yucn, A. Hasegawa, M. Kiso, A. Lubineau, G. Shaw, T. Feizi, Biochem., 1997, 36, 5260. 121. J. Offer, M. Quibell, T. Johnson, J. Chem. Soc., Perkin Trans. I , 1996, 175. 122. A. Lubineau, J. Auge, B. Drouillat, Carhohydr. Rex, 1995, 266, 211. 123. E. Meinjohanns, M. Meldal, H. Paulsen, R. A. Dwck, K. Bock, J. Chem. Soc., Perkin Trans. I, 1998, 549.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
12 Glycolipid Synthesis Hideharu Ishida
12.1 Introduction Glycolipids are classified into three groups, glycosphingolipids, glycoglycerolipids and others which include lipid A of lipopolysaccharide. Gangliosides, sialylated forms of glycosphingolipids, are most intensively studied because of their biological as well as chemical interest. As shown Table 1, gangliosides are further classified into several subclasses depend on the structures of oligosaccharide backbone. In the first section, the synthesis of ganglioside G D l a as a example of ganglioseries ganglioside is described, including the topics which are common even to other series gangliosides, for example preparation of building blocks and transformation of oligosaccharides into glycolipids. In the following section, the synthesis of polysialo ganglio-series gangliosides is described as the extension of this methodology.
12.2 Synthesis of Ganglio-Series Gangliosides In this section, synthesis of ganglioside G D l a as the example of a-series gangliosides which carry sialic acid at 0-3’ of lactose, is described. 12.2.1 Retrosynthetic Analysis of Ganglioside GDla
Retrosynthetic analysis of ganglioside G D l a (Scheme 1 [l]) shows two import concepts which can be applied for all of gangliosides in common. First, lipid part should be incorporated to the sialo-oligosaccharide after complete construction of the sugar chain. When the lipid is coupled with the glucose,
306
12 Glycolipid Synthesis
Table 1. Fundamental carbohydrate structure of gangliosides. hematoside-series Galp(l+I)Cer Galp(1 +4)GlcP( 1+1)Cer ganglio-series
GalNAc~(1+4)Gal~(l+4)Glc~(l+l)Cer Galp( 1+3)GalNAcP( 1+4)Galp( 1-+4)Glcp(1+1)Cer lacto- and neolacto-series [GalP(l+3)GlcNAc~(l+3)]nGal~(1+4)Glc~(l+l)Cer
[Gal~(l+4)GlcNAc~(l+3)]nGal~(l-+4)Glc~(l+l)Cer globo- and isoglobo-series
GalNAc~(l+3)Gala(l+4)Gal~(l+4)Glc~(l+l)Cer GalNAc~(l+3)Gala(l+3)Gal~(l+4)Glc~(l+l)Cer
which is the reducing terminal of oligosaccharide, at the first stage of the synthetic route, the resulting glucosyl ceramide has too low reactivity to serve as a glycosyl acceptor for further elongation of oligosaccharide chain, because mainly of the steric hindrance caused by the lipid. Second, sialic acid should be introduced to the oligosaccharide in the form of building block, e.g. sialylgalactose or sialyllactose. Because of the structural feature, sialic acid affords the corresponding glycoside in good yields only on the coupling with reactive acceptors. When reacted with less reactive acceptors, like oligosaccharides, sialic acid is easily transformed into the 2,3-dehydro derivative which is the major by-product of the reaction and lower the yields. The detail of this matter is discussed separately in this book. It is also of importance to note that the stereochemistry of the anomeric center was used to be determined by the chemical shifts of NMR of H-3ax, H-3eq, H-4 by the empirical rule [2]. This means that the stereochemistry of a novel sialoside synthesized by a coupling of sialic acid and complex sugar chain can not be determined absolutely by NMR. This problem can be solved by employing the building block, the stereochemistry of which has already been determined. 12.2.2 Preparation of Sialylgalactose Donor as Building Block
The sialylgalactose 1 [2] synthesized by the established procedure can be readily transformed into the corresponding glycosyl donor as the building block which is required to synthesize gangliosides in a tailor-made manner (Scheme 2 [3]). Compound 1 was converted by (1) benzoylation of the remaining hydroxyls, followed by either (2) selective transformation of 2-(trimethylsily1)ethyl (SE) group at the reducing end into the corresponding p-acetate by treatment [4] with boron trifluoride etherate-acetic anhydride and (3) introduction of the methylthio group by treatment [51 with (methy1thio)trimethylsilane and boron trifluoride, or (4) chemo-
12.2 Synthesis of' Ganglio-Series Gangliosides
301
,."* OH
AcHN HO "'OH HO
I OBZ
HO-Cl3H2,
HN
x-
C17H35
or AcO
OAc
AcO
OBz
"OAc AcO
HO+CI3H2, N3
SMe
AcO
OAc AcO
I .
n
__
< - s S M e NPhth AcO AcO
"'OAc
Scheme 1.
selective deprotection of SE group by treatment [4]with trifluoroacetic acid and (5) activation as trichloroacetimidate [6] to the desired donors, respectively. The use of resulting donors excludes any possibility of the P-sialyl glycoside formation. The same procedure can be applied for the conversion of disialylgalactose to the corresponding donor (see 3-(2)).
308
12 Glycolipid Synthesis ACO
OAc
OSE
AcHN AcO
’1
AAcOc O U 0 + OAc C02Me OSE
AcHN AcO
2)s 3)
BzO
OBz
2
1 AcO A c
OAc C02Me O U O *
SMe
AcHN
BzO
OBz
3
4 1) Bz,O, DMAP, Pyr., 2) Ac20, BF3*OEt2,3) TMSSMe, TMSOTf, 4) TFA, 5 ) CCISCN, DBU
Scheme 2.
12.2.3 Construction of Oligosaccharide The most important synthetic problems in the systematic synthesis of a-series ganglio-series gangliosides is a formation of GM2 structure, GalNAcb1-3(NeuAca23)Galbl-4Glc, synthesis of which has been found to be difficult because of its compact and lipid structure [7]. As shown in Fig. 1, retrosynthetic analysis of GM2 gives two synthetic strategies; (A): GA2 + NeuAc and (B): GM3 GalNAc. Because introduction of NeuAc into the isolated hydroyxl at 0-3’ of GA2 trisaccharide was found to be difficult by the conventional procedures reported, the strategy B is recommended for use.
+
A GalNAcP1, 4 GalPl+4Glc 3
’
NueAca2
GM2
GalNAcp,
P
4 GalPl+4Glc
NeuAc
+
GalNAc
GA2
\ B NueAca2’
Gal01-4Glc 3 GM3
Figure 1.
+
309
12.2 Synthesis of Gunglio-Series Ganyliosides A
OAc
C
O
AcHN
S
OSE
0 En0 AcO
~
&ty
OAc
~
SMe NPhth
5 A
~
-k
BnO
OBn
HO
~
6 C
AcHN
O
~
0 BnO OBn
~
~
~
,OSE OBn
NPhth
7
BzO 4cHN
+
OBz SMe
ACO""' AcO
C02Me
OBz
OAc
3
OAc
9
1) NIS-TfOH, 2) Lil, 3) N2H4*H20,4) Ac20, Pyr., 5) CH2N,,
6)80% aq. AcOH, 7) NIS-TfOH Scheme 3.
For the systematic synthesis of ganglio-series gangliosides, sialyllactose derivative 5 [8] and galactosamine derivative 6 [8] were selected as the glycosyl acceptor and the glycosyl acceptor, respectively (Scheme 3). The glycosylation of 5 with 6 in the presence of NIS-TfOH as the glycosyl promoter [9]gave the desired P-glycoside 3 in 68% yield. Methyl ester of the sialic acid was cleaved by treatment with LiI [ 101 to give the carboxyl group free derivative to present the side-product formation during
310
12 Glycolipid Synthesis
the treatment with hydrazine monohydrate to cleave the phthaloyl group. Schmidt et al. [ 1 I] employed the N-trichloroethoxycarbonyl group in place of phthaloyl group to protect the galactosamine donor to skip this tedious steps. After conversion of phthalimide into acetamide, isopropylidene group was removed to give the tetrasaccharide acceptor 8. The glycosylation of 8 with 3 in the presence of dimethyl(methy1thio)sulfonium triflate [ 121 gave the expected hexasaccharide in 66% yield.
12.2.4 Transformation of Oligosaccharide into Glycolipid Transformation of oligosaccharides into glycolipids can be accomplished be following our standard methods (Scheme 4). First, benzyl group used in oligosaccharide must be replaced with acetyl group, because final deprotection must be accomplished without affecting the double bond of ceramide. Hydrogenolysis over Pd/C, followed by treatment with Ac2O in pyridine led to replacement of all 0benzyl groups by 0-acetyl groups. Second, SE group at the anomeric position is removed by treatment with trifluoroacetic acid in CH2C12 (41 in a quantitative yield. When the anomeric position was protected with acetyl group, converted form benzyl group as described abve, treatment with hydrazine monoacetate (N2H4.AcOH) [ 131 cleaves the anomeric 0-acetyl group with high regioselectivity. And the following treatment with trichloroacetonitrile in the presence of 1,8-diazabicycl0[5.4.0]undec7-ene (DBU) 1141 afforded the hexasaccharide donor. The resulting glycosyl donor was then coupled with ceramide [15] or azidosphingosine [ 16, 171 which is a synthetic precursor of ceramide. Although the former strategy can shorten the synthetic route, the yields of coupling are rather low. Ogawa et al. [ 161 and Schmidt et al. [ 101 were successful in increasing the yield by employing the pivaloyl group for the protection of hydroxyl at the C-2 of the reducing terminal glucose, in place of acetyl group which causes the formation of the corresponding orthoester to decrease the coupling yield. The latter strategy, the coupling with azidosphingosine, is now popularly employed because of the good yield for coupling and the versatility to accept a series of fatty acids as components of ceramide, although the resulting glycosyl azidosphigosine nessitates the further transformation, reduction of azido to amine and introduction of fatty acids. The reduction was performed mainly by two procedures, treatment with H2S in aqueous pyridine [I81 or treatment with triphenylphoshine [19].When the hydroxyl at C-3 of azidosphingosine is protected with benzoyl group, both of the procedures often cause the migration of benzoyl from hydroxyl to amine formed. This problem can be solved by having the temperature during the reduction or by employing tertbutyldiphenylsilyl group [ 191 for the protection in place of benzoyl group. Finally, the complete deprotection of the acyl group, e.g. acetyl, benzoyl or pivaloyl group, followed by the saponification affords the desired gangliosides. Scheme 4 illustrates our standard methods to transform oligosaccharides into the corresponding gangliosides. When the silyl protection was employed, treatment [20] with tert-butylanmonium fluoride was required before the deacylation.
12.3 Synthesis of Polysiulo Gunglio-Series Gangliosides
3 11
OSE AcHN
OBn OAc
9 1) H2, Pd(OH)2I MeOH 2) AczO, Pyr.
3) CFjCOzH I CHzCl2 4) CCIICN, DBU I CH2CIz
TMSOTf I CH2C12
5) H O & C , ~ H ~ ~ OTBDPS 6) PPh3,HzO I C6Hs
7) stearic acid, WSC I CH2C12 8)l.OM TBAF I CH&N
9) MeONa I MeOH, 0.2M KOH V
OH
\ no-
\
OH
L..
C02H
GD1 a Scheme 4.
12.3 Synthesis of Polysialo Ganglio-Series Gangliosides Polysialo gangliosides are common in nature, having a(2+8)glycosidic linkage between two sialyl residues. In this section, the synthesis of ganglioside G Q l b as an example of polysialo ganglio-series gangliosides is described in comparison with that of GDla.
3 12
12 Glycolipid Synthesis
u1 \
W u)
?
12.3 Synthesis of Polysiulo Gunglio-Series Gangliosides
3 13
12.3.1 Retrosynthetic Analysis of GQlb An efficient method for obtaining disialyl glycoside is the use of dimeric sialic acid donor, which can be readily prepared from naturally available colominic acid. As shown in Scheme 5 [21], ganglioside GQlb can be synthesized in almost same procedure as that of G D l a by employing dimeric sialic acid in place of monomeric sialic acid, except for the construction of GD2 which is the key intermediate for the synthesis of GQlb. It is of worth to note that the dimeric sialic acid donor is enough reactive to glycosylate the isolated hydroxyl at 0-3' of GA2, which can not be glycosylated with the monomeric sialic acid. This surprising result made the synthetic route of GQlb short and practical.
12.3.2 Preparation of Building Block The disialylgalactose obtained by the glycosylation of the galactose acceptor with the dimeric sialic acid donor, was converted into the glycosyl donor as the same procedure as described for the sialylgalactose donor Scheme 6 [22].
11
Bz20, DMAP, Pyr., 2) Ac20, BF3*OEt2,3) TMSSMe, TMSOTf Scheme 6.
3 14
12 Glycolipid Synthesis OBn
AcO 4 0 A c
,OBn
co
+ OBn OBn
12 13
Aco O ,Ac
co
14
/
AcO o,Ac
co
/
11
3,
15 1) NIS-TfOH, 2) 80% aq. AcOH, 3) DMTST
Scheme 7.
12.3.3 Construction of Oligosaccharide
GD2 pentasaccharide acceptor 14 was selected as the key compound to achieve the systematic synthesis of polysialo ganglio-series gangliosides (Scheme 7). Glycosylation of GA2 acceptor 12 with a-sialyl-(2+8)-sialic acid donor 13 in the presence of NIS-TfOH in acetonitrile at -25 "C, afforded the desired pentasaccharide in 50% and no p-isomer was isolated. De-0-isopropylidenation of the pentasaccharide with aqueous 80% acetic acid afforded the acceptor 14 in 91% yield. It is noteworthy that the key intermediate 14 was prepared by one step, de-0-isopropylidenation, after the construction of GD2 pentasaccharide, in contrast to the synthesis of G D 1a. Glycosylation of 14 with sialyl a(2-8)sialyl galactose donor 11 by use of
315
12.4 Conclusion
+
Glc-Cer GaCGlc-Cer
GaCGlc-Cer Ned5Ac
GalNAc-Gal-Glc-Cer
GA2
+
+
Gal-GalNAc-GaCGlc-Cer
+
+ +
Gal-GalNAc-GaCGlc-Cer Ned5Ac
J.
NeyCAc Gal-GalNAc-Gal-Glc-Cer Ned5Ac GDclx I I
I' I/
GM2
+
GaCGalNAc-GaCGlc-Cer N e e Neu5Ac GDlb
GM1
f Gal-GalNAc-Gal-Glc-Cer Ned5Ac Ne@ GTlb Neu5Ac
GDla
I
I
f
GaCGalNAc-GaCGlc-Cer Ne&Ac NedsAc I
,
1
I I
I'
Gal-GalNAc-Gal-Glc-Cer Nei5Ac Ned5Ac Neu5Ac
GT1a
[a-series]
+
Gal-GalNAc-Gal-Glc-Cer Ned5Ac Ned5Ac Ned5Ac Ned5Ac
,
I
I
GT1act
Gal-Glc-Cer Nel(5Ac Neu5Ac GD3
GalNAc-GaCGlc-Cer Ne@ Neu5Ac GD2
Gal-GalNAc-Gal-Glc-Cer NedSAc
GM l b
I I I
GM3
GalNAc-Gal-Glc-Cer Ned5Ac
GA1
I I I I
~
I
GQ1b
[b-series)
NeukAc
GQlbcx Figure 2.
DMTST in CHZCl2 gave the octasaccharide 15 in 52% yield, which was transformed into the final compound by our standard methods described in the previous section.
12.4 Conclusion In this chapter, the methodology to synthesize a series of a-series as well as b-series ganglio-series gangliosides is described. By combining the two procedures, all of
3 16
12 Glycolipid Synthesis
ganglio-series (Fig. 2), GM2 [8, 91, GM1 [l, 91, G D l a [l],G T l a [23], GD3 [22, 241, GD2 [25], G D l b [Zl], G T l b [21], and G Q l b [21]. In addition, a series of novel ganglio-series gangliosides, GTlacl [27] and GQlbcl [28] (also see, Fig. 2) was successfully synthesized as the application of the procedures. Other series of gangliosides, sialyl Le" [29], sialyl dimeric Le" [30, 311, and sialyl Lea [32] of lacto- and neolacto-series gangliosides, and sialyl globopentasaccharide (SSEA-4) [33, 341 and sialyl isoglobopentaosyl ceramide [ 351 of globo- and isoglobo-series gangliosides in a tailor-made manner, respectively.
References 1. A. Hasegawa, H. K. Ishida, T. Nagahama, M. Kiso, J. Curbohydr. Chem. 1993,12, 703-718. 2. T. Murase, A. Kameyama, K. P. R. Kartha, H. Ishida, M. Kiso, A. Hasegawa, J. Curbohydr. Chem. 1989, 8, 265-283. 3. A. Kameyama, H. Ishida, M. Kiso, A. Hasegawa, Curbohydr. Res. 1990, 200, 269-285. 4. a) K. Jansson, T. Frejd, 3. Kihlberg, G. Magnusson, Tetrahedron Lett. 1986, 27, 753-756; b) K. Jansson, S. Ahlfors, T. Frejd, J. Kihlberg, G. Magnusson, J. Dahmen, G. Noori, K. Stenvall, J. Org. Chem. 1988, 53, 5629-5647. 5. V. Pozsgay, H. J. Jennings, Tetrahedron Lett. 1987,28, 1375-1376. 6. a) R. R. Schmidt, J. Michel, Angew. Chem. Int. Ed. Engl. 1980, 19, 731; b) R. R . Schmidt, J. Michel, M. Roos, Liebigs Ann. Chem. 1984, 1343. 7. Y.-T. Li, S.-C. Li, A. Hasegawa, H. Ishida, M. Kiso, A. Bernardi, P. Brocca, L. Raimondi, S. Sonnino, J. Biol. Chem. 1999,274, 10014-10018. 8. A. Hasegawa, T. Nagahama, H. Ohki, M. Kiso, J. Carbohydr. Chem. 1992, 11, 699-714. 9. a) P. Konradsson, U. E. Udodong, B. Fraser-Reid, Tetrahedron Lett. 1990, 31, 4313-4316; b) G. H. Veeneman, S. H. van Leeuwen, J. H. van Boom, Tetrahedron Lett. 1990, 31, 13311334. 10. M. Sugimoto, M. Numata, K. Koike, Y. Nakahara, T. Ogawa, Curbohydr. Rex 1986, 156, cl. 11. J. C. Castro-Palomino, G. Ritter, S. R. Fortanato, S. Reinhardt, L. J. Old. R. R. Schmidt, Angew. Chem. Int. Ed. Engl. 1997, 36, 1998-2001. 12. P. Fiigedi, P. J. Garegg, Curbohydr. Res. 1986, 149, c9-cI2. 13. G. Excoffer, D. Gagnaire, J. P. Utille, Curbohydr. Res. 1975, 39, 308-373. 14. R. R. Schmidt, J. Michel, Angew. Chem. Znt. Ed. Engl. 1980, 19, 731. 15. K. Koike, Y. Nakahara, T. Ogawa, Glycoconjugate J. 1986, 1, 107-109. 16. Y. Ito, M. Kiso, A. Hasegawa, J. Curbohydr. Chem. 1989,8,285-294. 17. R. R. Schmidt, P. Zimmermann, Angew. Chem. Znt. Ed. Engl. 1986,25, 725-726. 18. T. Adachi, Y. Yamada, 1. Inoue, M. Saneyoshi, Synthesis 1977, 45-46. 19. M. Mori, Y. Ito, T. Ogawa, Curbohydr. Res. 1990. 195, 199-224. 20. T. Ehara, A. Kameyama, Y. Yamada, H. Ishida, M. Kiso, A. Hasegawa, Curbohydr. Res. 1996.281.237-252. 21. H.-K. Ishida, H. Ishida, M. Kiso, A. Hasegawa, Tetrahedron: Asymmetry. 1994, 5, 24932512. 22. H.-K. Ishida, Y. Ohta, Y. Tsukadd, M. Kiso, A. Hasegawa, Carbohydr. Rex 1993,246, 75-88. 23. H.-K. Ishida, H. Ando, H. Ito, H. Ishida, M. Kiso, A. Hasegawa, J. Curbohydr. Chem. 1997, 16, 413-428. 24. Y. Ito, M. Numata, M. Sugimoto, T. Ogawa, J. Am. Chem. SOC.1989, 111, 8508. 25. Y. Matsuzaki, S. Nunomura, Y. Ito, M. Sugimoto, Y. Nakahara, T. Ogawa, Carbohydr. Res. 1993,242, cl. 26. H.-K. Ishida, Y. Ohta, Y. Tsukada, Y. Isogai, H. Ishida, M. Kiso, A. Hasegawa, Curbohydr. R ~ s 1994,252, . 283-290. 27. H. Ito, H. Ishida, M. Kiso, A. Hasegawa, Curbohydr. Res. 1997, 304, 187-190. 28. K. Hotta, H. Ishida, M. Kiso, A. Hasegawa, J. Curbohydr. Chem. 1995, 14, 491-506.
References
3 17
29. A. Kameyama, H. Ishida, M. Kiso, A. Hasegawa, J. Curbohydr. Chern. 1991, 10, 549-560. 30. A. Kameyama, T. Ehara, Y. Yamada, H. Ishida, M. Kiso, A. Hasegawa, J. Curbohydr. Chem. 1995, 14, 507-523. 31. G. Hammel, R. R. Schmidt, Tetrahedron Lett. 1997, 38, 1173-1 176. 32. A. Kameyama, H. Ishida, M. Kiso, A. Hasegawa, J. Curbohydr. Chem. 1994, 13, 641-654. 33. H. Ishida, R. Miyawaki, M. Kiso, A. Hasegawa, J. Curbohydr. Chern. 1996, 15, 163-182. 34. J. M. Lassaletta, K. Carlsson, P. J. Garreg, R. R. Schmidt, J. Urg. Chem. 1996,61, 6873-6880. 35. H. Ishida, R. Miyawaki, M. Kiso, A. Hasegawa, Carhohydr. Res. 1996, 284, 179-190.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
13 Stereoselective Synthesis of P-Mannosides Vince Pozsguy
13.1 Introduction P-D-Mannopyranosyl residues are frequently found in oligosaccharides and glycoconjugates in living systems. This unit is omnipresent in N-linked glycoproteins that contain a pentasaccharide core (1) in which the P-linked mannose residue can carry additional sugars [I] (Scheme 1). P-Mannopyranose moieties are often part of the 0-specific polysaccharides (OSPs) of Gram-negative bacteria [2-8] and of bacterial capsular and em-polysaccharides [6, 9-15]. P-Linked mannopyranose is also a common residue in plant polysaccharides [ 161. Further examples of the occurrence of the P-mannosidic linkage are the glycosphingolipids in wheat flour [ 171 and in the freshwater bivalve Hyriopsis schlegelii [ 181. Interestingly, this residue is absent from mammalian glycolipids [19]. Cell-wall phosphomannans of yeasts contain linear chains of up to fourteen P-1,2-linked mannose residues [20, 211. P-Mannopyranose units also occur in secreted small molecules. Examples include the phospholipase inhibitor caloporoside [22] and free oligosaccharides detected in a patient with mannosidosis [23] and isolated from ripening tomatoes [24]. The related P-linked 2-acetamido-2deoxy-D-mannopyranose also frequently occurs in OSPs [ 8, 251 and in capsular polysaccharides [26-3 11. The corresponding mannuronic acid is present in the enterobacterial common antigen [32] and in other bacterial polysaccharides [6, 331. The occurrence of the 0-mannosyl unit in many biological systems creates a biological rationale for its synthesis. Whereas the simple methyl [34-381 and phenyl PD-mannopyranoside [ 391 have been known for many years, the synthesis of complex oligosaccharides containing this unit is still a challenge for two reasons [40-441. First, the anomeric effect facilitates the formation of the 1,2-truns-mannopyranosyl (a) linkage thus it disfavors the P-linkage and, second, the 1,2-cis arrangement of the equatorial aglycone and the axial functionality at C-2 in P-mannopyranosides harbors repulsive steric effects [40]. Glycosyl donors with a participating group at 0 - 2 and halide-ion catalytic conditions promote the exclusive formation of (Y man-
320
13 Stereoselective Synthesis ojP-Mannosides
a-D-Man
1: fa
p-D-Man-(1 -+4)-p-D-GlcNAc-( 14 4 )--pD-GlcNAc-(1+Asn
1
a-D-Man
Scheme 1.
nosides. Once formed, the P-mannosidic linkage might be prone to anomerization under acidic conditions adding another factor to be considered in synthetic sequences involving this linkage [45, 461. Whereas anomeric pairs of galacto- and H-2 coupling constants, glucopyranosides can easily be differentiated by their 3JH-1. this is not so for mannopyranosides. Because of the gauche relationship between the H-1 and H-2 protons in either anomer, the 3JH-1,H-2 couplings (- 1-3 Hz) are not suitable for diagnostic purposes [47]. Reliable information can be gained from in, c-1 heteronuclear coupling traresidual NOE data [48] and from the one-bond, constants [49]. The chemical shift of the H-5 proton (- 3.3-3.4 ppm) is also of diagnostic value [50, 511. The specific rotation can also be used for configurational assignment, if both anomers are available [51]. This chapter reviews methods for the construction of P-mannosidic linkages. Together with the more popular protocols, less widespread and emerging approaches are also covered in the hope they will stimulate further progress.
'&-,
13.2 Chemical Methods 13.2.1 Glycosylation with Mannosyl Donors Mannosylation using Insoluble Promoters Silver cations on the surface of insoluble catalysts anchor mannosyl bromides from the a-side [52]. As a consequence, highly reactive alcohols can attack the anomeric center from the P-side in an S N type ~ reaction, thus leading to P-mannosidic linkages. The first application of this principle was reported by Perlin [53] in 1961 who condensed the cyclic carbonate-protected mannosyl bromide [541 2 and the reactive aglycone 3 under silver oxide activation to afford the first synthetic disaccharide (4) that contains a P-mannopyranosidic linkage (Scheme 2). Because of the strain imposed on the pyranose ring by the fused carbonate moiety in 2, the formation of a planar oxocarbonium ion from it is disfavored and an S N ~ type anomeric displacement predominates [ 551 when strongly nucleophilic primary [56]and secondary alcohols [57-591 are used as the aglycones. Condensations with moderate nucleophiles proceed in lower yields and with reduced P-selectivity. Interestingly, AgzO-assisted reaction of the carbonate 2 with weak nucleophiles gives, exclusively, a-mannosides [ 60, 6 11.
321
13.2 Chemical Methods FHzOAc
&
AcO
Br
CHzOH
HOH& 1 . Ag20.1,
2.Ba(OWzb -t
AcO OAc
2
3
Scheme 2.
+
H
O
OBn
c1 5
~
Ag2C03 O CH2CI2 Yield: 64%
~
E@&&o, Bn
6
7
Scheme 3.
The use of silver carbonate as the promoter has been successful for p-selective condensation between the 0-benzylated donor 5 and the reactive, non-carbohydrate primary alcohol 6 to give 7 (Scheme 3) [62] but it is less practical in other combinations [62-661. Because activation by Koenigs-Knorr promoters produces an equimolar amount of water, loss of the donor as a result of hydrolysis is difficult to prevent even in the presence of drying agents [52]. High yields have been achieved with Paulsen's silver silicate activator precipitated on aluminum oxide that can also act as a water scavenger 1641. A threshold level of acceptor reactivity is necessary for satisfactory p-stereoselectivity-whereas coupling of mannosyl bromide 8 with the anhydro derivative 9 proceeds in an excellent yield and high p-selectivity [67] to afford 10, mannosylation of the less nucleophilic disaccharide 11 with the same donor affords the trisaccharide 12 as a 1 : 1 a/P anomeric mixture (Scheme 4) [68].
-
10 R r -.
8
+ $ G e ONPhth B - & $CH20Bn J ) B n
-
AAll
c
~
&
o
~
o
B
n
All CH20Bn
NPhth
11
Scheme 4.
Yield: 76%
0 CHzOBn
12. cr/D - I
Bn NPhth
13 Stereoselective Synthesis of p-Mannosides
322
1
Br 13
-55 to -30°C Yield: 87%
14
Scheme 5.
+ Br 15
0~~ pphth 16
Ag silicate CH,CI, -15 to25 OC Yield: 22%
Bn
17
OMP NPhth
Scheme 6.
A more predictable approach to this problem has become possible by van Boeckel’s recognition that an electron-withdrawing, acyl substituent at 0-4 of mannosyl donors promotes P-selectivity whereas such substituents at 0 - 3 and 0-6 reduce it [52]. Highly reactive donor-acceptor combinations seem exempt from this rule [69, 701-silver silicate-promoted condensation of the mannosyl donor 13 with n-octanol affords the P-linked mannoside 14 in excellent yield even without an acyl substituent at 0-4 (Scheme 5) [69]. Paulsen’s activator has been used with success to synthesize a large number of P-mannosides related to N-glycoproteins [61, 62, 64, 67-79], glycolipids [ 80-831, and other natural products 1451. Among other insoluble promoters proposed for P-mannoside synthesis van Boeckel’sporous silver silicate [84] has emerged as a most efficient activator [85, 861. This catalyst even promotes 0-selective mannosylation by donor 15 at 0-4 of the disaccharide 16 to afford the 4,6-branched trisaccharide 17 that corresponds to an Asn-linked core structure (Scheme 6) 1861. Garegg’s silver zeolite [87] is also a powerful promoter [82, 88-90] and has been reported to afford increased yields and P-selectivity over silver silicate 182, 911. Other activators, including siluer imiduzolute [92], silver sulicylate [93], and thallium zeolite [88] have also been investigated. The Sulfonate Approach
The leaving group properties of sulfonate groups were first exploited in p-mannoside synthesis by Perlin [94, 951. In the first step of this approach mannofuranose 18 is treated with Tf20 to afford the anomeric triflate 19. This is converted in situ to the bromide 20 by Bu4NBr. Subsequent addition of MeOH led to a 2: 3 mixture of the methyl a- and P-mannosides 21 without heavy metal promoters (Scheme 7). Schuerch has synthesized O-benzyl-protected mannosyl brosylate, tosylate, tresylate, and triflate derivatives 23 from the corresponding chloride 22 and the respective silver sulfonate [96]. On exposure to MeOH, the sulfonates 23 give a nearly 1 : 1 mixture of methyl a- and P-mannosides 24 (Scheme 8).
13.2 Chemical Methods r
323
1
Scheme 7.
Scheme 8.
Scheme 9.
Improved P-selectivity has been obtained by using a mesyl [97-991 or a benzylsulfonyl group [I001 at 0 - 2 of the donor. Such groups are non-participating and by their electron-withdrawing capacity they disfavor anomeric oxocarbonium ion formation, thus favoring direct substitution. For example, reaction of the mesyl derivative 25 with cyclohexanol affords the p-mannoside 26 in 75% yield with excellent p-selectivity (Scheme 9) [99]. Condensation of the benzylsulfonyl-protected donor 27 with the moderately nucleophilic alcohol 28 has led to mannobiose 29 in 49% yield, in a 1 :2 alp ratio (Scheme 10) [loo]. In these studies Schuerch has shown that to achieve optimum pselectivity complete formation of the mannosyl sulfonate is essential before addition of the aglycone component [ 1001. A major development in the sulfonate approach was Crich's serendipitous observation that Kahne-activation of the mannosyl sulfoxide 30 with Tf20 in the presence of DTBMP, at -78 "C in CHzC12, followed by exposure of the intermedi-
z q n +f : Bn CF,CH2S020 27
Scheme 10.
!
M&N
HO
OSO2Bn
< )' ::
Yield:49%
Bn
OMe
28
R = p-BrC&
2 9 . a / p = 1:2
OMe
324
13 Stereoselective Synthesis ojp-Mannosides
phTq
Tf20, DTBML CH?CI?
phTw +
Bn
I
CF3S020
30
u
thenodC
AcOAMe
A20
31
I
OMe
rll
32
Yield: 95%
33, p/a > 25
Scheme 11.
ate triflate 31 to the alcohol 32 led to the disaccharide 33 in an excellent yield and very high P-selectivity (Scheme 11) [51].The a anomeric stereochemistry of the triflate 31 has been demonstrated by low-temperature NMR experiments [101, 1021. Hindered secondary and tertiary alcohols are also suitable acceptors [51, 1031071. Mannosyl triflates can also be prepared from thiomannosides and mannosyl bromides by reaction with PhSOTf, or silver triflate, respectively. As in Schuerch’s method [ 1001, the order of the addition of the reactants is critical for the success. The intermediate triflates should be prepared first before addition of the alcohol to prevent attack by the latter at the initially formed oxocarbonium ion. Additional requirements are the presence in the mannosyl donors of non-participating (benzyl) groups at 0 - 2 and 3 and a benzylidene-acetal moiety at 0 - 4 and 0-6. Crich’s method is likely to enjoy widespread popularity because of its simple experimental protocol combined with high yields and excellent P-stereoselectivity.
Intramolecular Mannosylation The previous examples demonstrate that intermolecular mannosylations proceed with high P-selectivity only with donors that are structurally disposed to direct sN2type substitution at the anomeric carbon and disfavor the formation of oxocarbonium ion intermediates. Because of the large number of variables, the steric outcome of intermolecular mannosylations is predictable only in closely related systems. Stork has proposed a conceptually new, intramolecular mannosylation protocol that has initially been implemented in the first stereocontrolled synthesis of a p-Cmannoside [ 1081. The starting compound is the phenylseleno mannoside 34 in which a 2-phenylethynyl group is anchored to 0 - 2 through a silicon tether. Radicalinitiated removal of the anomeric phenylseleno group (+35) in concert with cyclization of the phenylethynyl group on to the anomeric radical provides 36 from which fluoride-mediated detachment of the silicon connector affords 0-C-mannoside 37 (Scheme 12).
34
Scheme 12.
35
36
37 EK= 10.1
325
13.2 Chemical Methods
Me\
I.BuLi. THF, , S i ,C I 2. Me2SiC12
ye Bn SPh Yield: quant.
Yield: quant.
38
39
OMe
SPh 41
40
MCPBA -78 10 20 "c
-25 to 0 'C
OMe
S(0)Ph Yield: 73%
42
43
Scheme 13.
The principle of intramolecular C-mannosylation has been extended to the synthesis of 0-linked P-mannosides [ 1091. In the first step, a chlorodimethylsilyl moiety is introduced to the primary hydroxyl group of the 0-benzylated aglycone 38 to afford compound 39. Next, chlorosilane 39 is connected to the thiomannoside 40 to give the silaketal intermediate 41. Subsequent conversion to the sulfoxide 42 followed by Kahne-activation [ 1 101 affords the P-linked disaccharide 43 stereospecifically (Scheme 13). This version of the temporary connection method is not suitable for mannosylation of secondary alcohols because of the failure to fashion a silaketal tether between HO-2 of a thiomannoside and a weak nucleophile. More recently, Stork has demonstrated that the silaketal 46 involving a secondary alcohol can be prepared in a one-pot process by using equimolar amounts of the mannosyl sulfoxide 44 instead of the parent thiomannoside, the sugar 45 to be tethered and dichlorodimethylsilane [ l l l ] . Treatment of 46 with Tf20 affords the P(1-3) linked disaccharide 47 in a high yield (Scheme 14). A problem of the Stork method is signaled by a serendipitous finding-that Kahne-activation of the tethered sulfoxide 48 gives mostly the 1,6- (49) instead of the targeted 1,4-linked disaccharide [ 11 11 (Scheme 15). This observation indicates that the silicon-tethered oxygen is not always the most nucleophilic for reaction with the activated anomeric carbon. Hindsgaul has proposed the isopropylidene acetal as an alternative, reactive tether between 0 - 2 of the mannosyl donor and the linkage oxygen of the aglycone B n M Tf20, CH2CIz
OMe -100 10 20°C* %:& BnO nB
Bn
BnO
OMe
S(0)Ph
Yield 88%
44
Scheme 14.
45
B nO OMc
Yield 82% 46 S(O)Ph
41
326
13 Stereoselective Synthesis of p-Mannosides
Scheme 15.
[ 112, 1 131. In the first phase the vinyl ether 51 is prepared from the acetate 50 by methylene-transfer from the Tebbe-reagent. Next, the thioglycoside 51 and the aglycone 52 are connected by acid-catalyzed addition of the latter to the vinyl ether moiety to afford the acetal-tethered intermediate 53. In the final phase, NIS/TfOHinduced anomeric activation proceeds in concert with the attack of the tethered oxygen at the anomeric carbon to afford the P-linked disaccharide 54 (Scheme 16) [114]. Whereas both primary and secondary carbohydrate alcohols are suitable for the linking, the yield is very sensitive to structure and to experimental conditions. A further limitation is the acid-sensitivity of the acetal-tether; this can lead to hydrolysis and transacetalization [ 1141. Ogawa has introduced an interresidual 4-methoxybenzylidene-acetal moiety as the stereocontroling element for intramolecular P-mannosylation [ 1 151. This tether can be fashioned by oxidation of the 4-methoxybenzyl group at 0 - 2 of the mannosy1 fluoride 55 with DDQ to give an intermediate (56) that is stabilized by the aglycone alcohol 57 to form the mixed acetal 58 as a single stereoisomer (Scheme 17) [ 1161. Interestingly, oxidative coupling of the fluoride 59 and the methoxybenzyl ether 60 furnished 58 as a 3 :2 mixture of diastereoisomers [ 1161. The configuration of the acetal carbon atom is dictated by kinetic control. Anomeric activation of the acetal58 afforded the P-mannosyl disaccharide 61 in a stereospecific process that is rather insensitive to the configuration of the acetal (Scheme 17) [116]. The 4-methoxybenzyl-assisted P-mannosylation is also suitable for the blockwise synthesis of higher-membered oligosaccharides [ 117, 1181. For example, DDQ-
Cp2TiCHzClAlMez
SEI
Yield: 90% ,OBn
54
Scheme 16.
Bn'O I OMe
NIS. TfOH
p-TsOH
THF. C6H5CH,
+
DTBMP
OMe
S El 51
~
Bn
52
Yield: 57%
SE1
53
Yield: 77'0
13.2 Chemical Methods
327
1 DBMP
55
MeCN
%aYn B"
Bn
OUII
OBn
Yield: 67% 61
b
59
Bn
OBII
60
Scheme 17.
Scheme 18.
induced attachment of the branched trimannoside thioglycoside 62 to the chitobiose acceptor 63 followed by MeOTf-mediated anomeric activation afforded the pentasaccharide 64 in 42% overall yield (Scheme 18) [ 1181. This compound corresponds to the core oligosaccharide of Asn-linked glycoproteins and represents the most complex setting for which stereospecific pmannosylation has been achieved. The methoxybenzylidene-acetal procedure has also been implemented on a polymer support [ 1 191. In a related approach, studied by Ziegler, the precursor is a 'prearranged glycoside' (e.g. 65) constructed by connecting the mannosyl donor and the acceptor alcohol by means of a persisting succinyl bridge [120, 1211. Activation of 65 by NIS/ TMSOTf gives a 2: 1 mixture of the a- and the P-linked mannosyl disaccharide 66 (Scheme 19) [120]. Anomeric selectivity in this approach is sensitive to the activation procedure and also depends on the glycosyl acceptor component [122]. In contrast with the intramolecular mannosylation methods discussed above, in Ziegler's method the hydroxyl group to be mannosylated is unprotected and the tether remains intact in the glycosylation step.
Other Mannosyl Donor-Based Methods Starting from the epoxide 67 Kochetkov prepared the mannosyl thiocyanate 68 [123] which reacts with the trityl ether 69 in the presence of trityl perchlorate to afford the disaccharide 70 with an a/p ratio of 2 : 1 (Scheme 20) [ 1241.
328
13 Stereoselective Synthesis of P-Mannosides
66, a/p = 2:l
65
Bzo'OBn
Scheme 19.
1. NH4SCN, 18-crown-6 C6HS.acetone, -20 'C 2. CCI3COBr,CSH5N -20 OC Yield: 40%
Bn
CHf.212 SCN
Bn
OMe Yield: 58%
67
68
70, a/p = 21
69
Scheme 20,
Inazu's procedure [125] uses the mannosyl phosphinothioate 72 as the mannosyl donor; it can be prepared from the hemiacetal71 with Me2P(S)C1under basic conditions. Condensation of 72 with the alcohol 73 in the presence of iodine and trityl perchlorate affords the mannosyl disaccharide 74 in 45% yield, with an a/P ratio of 1 : 3 (Scheme 21). Tatsuta has examined the glycosyl donor properties of thiomannosides under NIS/TfOH activation and found that the best P-selectivity can be obtained with the bulky donor 75 that in the presence of the alcohol 76 has afforded the mannoside 77 as a 3 : 1 mixture of the P and a anomers (Scheme 22) [22]. Hashimoto found that low-temperature activation of mannosyl chlorides with silver triflate in the presence of alcohols leads to varying ratios of a- and P-linked mannosides [45, 126, 1271. A study of the factors controlling stereoselectivity in silver triflate-mediated mannosylation has been published [ 1281. Activation of 0benzyl-protected mannosyl fluorides by a combination of Sn(OTf)2 and La(C104)3 has been reported by Shibasaki to afford mixtures of a- and P-mannosides [129]. The large proportions of a-mannosides formed in the homogeneous reactions outlined in this section indicate the involvement of oxocarbonium-ions enabling the anomeric effect to dictate the steric outcome. Because the stereoselectivity of these approaches is crucially dependent on the reacting partners and conditions their scope remains limited.
I . BuLi
OH THF. -30 "C 71
Scheme 21.
Yield: 75%
BnO Bn
-z w
2. Me2P(S)CI
Bn
OP(S)Me2+
72
73
13.2 Chemical Methods OBn
+
gBn
H
o OBn
BnO
329
OTBDPS
NIS,TfOH ~
____C
Bn
OBn QBn
CHzCl2,40 O C Yield: 88%
BnO 15
16
77,a/p = I :3
Scheme 22.
13.2.2 Epimerization of P-Glucopyranosides at C-2 The Oxidation-Reduction Approach Extending Theander's finding [ 1301 that reduction of hexopyranosiduloses proceeds with high stereoselectivity, Lindberg has developed a powerful approach to Pmannosidic linkages [ 13I]. In the first phase, the glucopyranosyl donor 78 that carries a selectively removable, participating group at 0 - 2 is condensed with the aglycone to give the P-glucoside 79. Subsequently, the HO-2 group is deprotected (i 80). Glucal 1,2-epoxides can also be precursors to such intermediates [ 1321. In the next step C-2 is oxidized to give uloside 81. In the final phase, the uloside is reduced to afford the p-manno product 82 together with usually
Direct Inversion Intermolecular nucleophiles The first one-step conversion of P-glucosides to P-mannosides has been reported by Miljkovic [ 1501. In his approach a mesyloxy group at C-2 of glucoside 83 is displaced by a benzoyl group to afford the mannoside 84 (Scheme 24). In a related reaction 2-deoxy-2-fluoro-~-mannoside86 has been prepared [ 15I] by fluoride displacement of the 2-triflyloxy group in 85 (Scheme 25).
78
Scheme 23.
13 Stereoselective Synthesis of p-Munnosides
330 % o T & h +p
BzOK. DMF
Me
OMe
7
MsO
OMe
Yield: 62%
83
84
Scheme 24.
phTow BudNF
BZ
OMe
TfO
p BzOh TOMes
EiGzT Yield: 3540%
86
85
Scheme 25.
These substitutions proceed in moderate yields because of repulsive electrostatic interaction between the incoming nucleophile and the lone pair of the ring oxygen atom [I521 and non-bonded steric interactions in the transition state [150]. Extension of the repertoire of leaving groups has alleviated these problems and made the direct displacement a powerful approach to P-mannoside synthesis. David's variation uses an imidazolylsulfonyloxy group as the leaving group at C2 and Bu4NOBz as the nucleophile [153]. In a similar strategy, a C-2 triflyloxy group is displaced by a combination of CH3C02Cs and 18-crown-6 under sonication to give P-mannosides [ 1541. An analogous method reported by Furstner employs BmNOAc as the nucleophilic reagent in conjunction with ultrasound [ 155, 1561. David has also reported the conversion of double-triflylated P-galactosides to P-mannosides [157, 1581. According to this procedure, Tf0-4 of P-galactoside 87 is displaced by BuNOBz at room temperature to give the gluco compound 88. In the second phase heating at 100 "C causes displacement of Tf0-2 to afford mannoside 89 (Scheme 26). A variation of this approach uses CH3COzCs and CF3C02Cs in combination with 18-crown-6 [ 159, 1601. An improved protocol for the P-galacto--,p-manno conversion and identification of by-products has been reported by Sinay [161]. Intramolecular nucleophiles
Kunz has taken advantage of an entropically favored, intramolecular substitution reaction [ 1621 to construct P-mannosides by configurational inversion at C-2 of P-
Bz
OMe
All
TfO 87
Scheme 26.
25
OC
TfO
sn
Bu~NOBZ * OMe C6H3Me I00 O C Yield: 64%
89
13.2 Chemical Methods
'yo Ac%
ROH AgOTf AcO
C6%
OR 1 , K2C03. MeOH
7
Z.C&CH(OMe): 3 Tf20 Yldd ca 75%
Ac&NHPh -3OOC Yield: 65-808
NHPh
331
90
91
PhTO OO& o l 75 "C Yield: 65-808 93
NHPh 92
OK
PhT& i&, 94
Scheme 27.
glucosides [152, 1631. The first phase is assembly of a P-glucopyranosyl donor (90) with a urethane-type group at C-3. This donor is then condensed with the aglyconic component to form the P-glucopyranoside 91. Subsequently, a benzylidene-acetal moiety is installed at 0 - 4 and 0 - 6 and HO-2 is triflylated (+ 92). In the final stages, thermally induced intramolecular attack of the carbamoyl oxygen at C-2 affords the P-mannoside 93 with small amounts of the iminocarbonate 94 (Scheme 27). The acetal moiety is essential for the success of this approach. Using this procedure, Unverzagt has synthesized the core-region trisaccharide P-D-Manp-1,4-P-DGlcpNAc-l,4-P-~-GlcpNAcin multigram quantities [ 1641. C-2 epimerization by intramolecular substitution has also been achieved with the 3-0-benzoyl analog of the 0-glucoside 92 [165]. 13.2.3 The 2-Ulosyl Donor Method Introduced by Lichtenthaler [42, 1661, this method utilizes the limited tendency of 2-glycosulosyl a-bromides to form oxocarbonium ions upon anomeric activation, thus favoring an S N type ~ reaction at C-I. In the first phase the arabino-hexulosyl bromide 96 is prepared by exposure of the readily available acyloxyglycal 95 to NBS/EtOH [48, 166-1711. Next, the bromide 96 is activated by an insoluble silver promoter in the presence of the aglycone. Because the 2-0x0 function not only supports the sN2 mechanism by its electron-withdrawing capacity but also is a nonparticipating group, the product is almost exclusively the P-glycoside 97. The use of soluble promoters with bromide 96 or with the corresponding pentenyl and thioglycosides yields substantial amounts of a-linked products [42, 1671. In the final step, the 0x0 group is reduced as in Lindberg's method to afford the p-mannoside 98 (Scheme 28) [167]. The low anomeric reactivity of 2-ulosyl bromides can be a limitation when moderately nucleophilic alcohols are used as aglycones. Attempts
Yield: 92% 95
Scheme 28.
Yield: 93%
Yicld:Yi% 96
97
98
332
13 Stereoselective Synthesis of P-Mannosides
Scheme 29.
to overcome t h s problem include the use of ulosyl iodides and 0-benzyl protected ulosyl donors [48, 169, 1721 in combination with van Boeckel's silver silicate promoter [84]. 13.2.4 Anomeric 0-alkylation
Alkylation of 1-0-Metal Complexes Schmidt has developed a technically simple route to p-manno furanosides and pyranosides in which the sodium or potassium salt of mannose hemiacetals is reacted with strong alkylating agents derived from the aglyconic component [ 173-1781. For example, treatment of the sodium complex of mannofuranose 99 with the triflate 100 gives the corresponding mannofuranoside 101 as a 1 :2 ratio of the ct and the P anomers (Scheme 29) [ 1751. It is believed that intramolecular complexation between the alkali metal and the sugar oxygens locks the anomeric oxygen in the P position before the mannosyloxy anion attacks the aglycone. The Stannylene Acetal Method Five-membered cyclic dibutylstannylene acetals formed on vicinal cis-axialequatorial pairs of hydroxyl groups selectively enhance the nucleophilicity of the equatorial oxygen in 0-alkylation reactions [179, 1801. On the basis of this rule Schuerch surmised that alkylation of the tin complex 103, having the anomeric oxygen locked in the equatorial position, should lead to P-mannosides [181]. This assumption was proved by treating the mannose diol 102 with BuzSnO (+ 103) followed by in situ exposure to alkyl halides whereby P-mannopyranosides 104 were formed stereospecifically (Scheme 30) [ 1811.
B Bn :
102
Scheme 30.
103
Yield: 94-100%
~
o
= M e Or A" 104
R
13.2 Chemicul Methods
%fNH
333
Yield: 82%
Bn
Scheme 31.
107
108
OMe Yield: 67%
109
Scheme 32.
This strategy [99, 1821 has been used by Nicolaou for stereospecific synthesis of trehalose analogs [50]. The procedure involves reaction of the complex 103 with the a-mannosyl donor 105 under TMSOTf activation to furnish the 0,a-linked mannosy1 disaccharide 106 without the a,a-connected stereoisomer (Scheme 3 1). In another application of Schuerch's concept, 0-triflyl derivatives of monosaccharides are used as electrophiles [183]. For example, reaction of the 4-0-triflyl guluctoside 108 with a sixfold molar excess of the partially protected dibutylstannylene complex 107 in DMF affords the p-1,4-linked mannosyl-glucoside 109 in 67% yield. The scope of this protocol is limited by configurational inversion at the linkage carbon and solubility problems (Scheme 32) [ 1831. 13.2.5 Miscellaneous Methods Radical Inversion of the Anomeric Chirality of a-D-Mannopyranosides This approach was first reported by Kahne in 1988. He found that hydrogen atom transfer to hexopyranosyl radicals having an alkoxy group at C-1 occurs, preferentially, from the a-side giving rise to P-glycosides with high stereoselectivity [ 1841. For example, the anomeric radical 111 generated by photolysis of the mannose hemithio orthoester 110 is converted under reducing conditions to the mannoside 112 with an impressive 18 : 1 ratio of the and a anomers (Scheme 33).
zqsMe
Bu3SnH, AIBN C6H5Me
Bn
bMe 110
Scheme 33.
30 "C Yield: 85% 111
112,cdp=l:18
334
13 Stereoselective Synthesis of p-Mannosides
Scheme 34.
In Curran’s method 11851 the first step is homolytic cleavage of the Br-C bond in the radical translocating group in 113 to produce the aryl radical 114. Subsequent intramolecular 1,6-hydrogen transfer from the anomeric carbon translocates the radical to C-1 (-115). In the final phase the radical is quenched from the a side by intermolecular hydrogen transfer from Bu3 SnH to afford the P-mannoside 116 (Scheme 34). A major problem with this approach is epimerization at C-2 through 1,5-hydrogen abstraction producing the a-yluco isomer 117. Another competing reaction is the intermolecular replacement of bromine by hydrogen in the radical translocating group. Additional approaches capitalizing on stereospecific hydrogen translocation to the anomeric radical have been described by Crich 1186, 1871. Despite excellent stereoselectivity, the radical approach is “rendered impractical by the circuitous methods needed for generation of the key anomeric radical” [51]. Reductive Cleavage of Cyclic Orthoesters Inspired by Liptak’s regioselective transformations of hexopyranose 4,6-benzylidene acetals by hydrogenolysis [ 1881, Ikegami has developed a new entry to glycosidesynthesis 11891. In the first phase the mannopyranosylidene acetal 120 is prepared from lactone 118 and the diol 119. In the subsequent step the interresidual spiro orthoester is reduced with LiAlH4/AlCl3 to produce the P-linked disaccharide 121 regio- and stereospecifically (Scheme 35). This method is limited to the preparation of P(1-4) linkages.
De novo Syntheses Tietze has synthesized ethyl P-D-mannopyranoside from non-carbohydrate precursors [ 1901. The procedure involves [4 + 21 hetero Diels-Alder cycloaddition of the enol ether 122 and the chiral oxazolidinone 123 under promotion by MeZAlC1 to give the dihydropyran 124 in very high diastereoselectivity. Reductive cleavage of
118
Scheme 35.
119
Yield: 77%
120
Yield 98%
121
13.2 Chemical Methods
''3
V
+
Ac
op N
A
o
1. Ac,O, NaOAc 2. BH3 . Me2S
/
Yield: 95%
U"O ~.
Yield: 85%
Ulld
123
I22
335
Bn6
Yield: 82%
Bn6
126
125
I24
Scheme 36.
the oxazolidinone moiety (+125) followed by hydroboration with oxidative workup has provided P-mannoside 126 (Scheme 36). In a similar approach, Boger has prepared enantiomeric mixtures of P-mannopyranosides using achiral precursors (Scheme 36) [191].
13.2.6 2-Acetamido-2-deoxy-~-~-mannopyranosides Two main directions have emerged to this group of compounds. In the first an azido group is employed as the precursor to the acetamido group [ 192-1941. In Paulsen's method the 2-azido-mannosyl donor 127 is used as the ManNAc synthon that is condensed with the aglycone component under silver silicate activation to afford the P-linked mannoside 128 (Scheme 37). In the alternative route [159, 195, 1961 a P-glucopyranoside is synthesized first, followed by azide-displacement at C-2 to provide the 2-azido-2-deoxy-Pmannopyranoside unit in a process as reviewed for the parent compound in Section 13.2.2. The second direction is based on 2-ulosyl intermediates. In one approach [ 197, 1981 an uloside e.y. 129 is synthesized first, as reviewed in Section 13.2.2, then converted to the oxime 130. Subsequent reduction and N-acetylation affords the Plinked N-acetyl-mannosaminyl disaccharide 131 (Scheme 38).
$,
25'C
Yield: 83%
127
128, cc/p = 2:3
Scheme 37.
E - d k 2 O H .
0 129
Scheme 38.
&;?*::
HCI 0CH20Bn Yield: 80%
Bn
LiAIH,,
2. AczO
Bn
'IHL
:-;&
CH20Bn
HO"
130
BnOoMe
Bn 0 CHzOBn 131
13 Stereoselective Synthesis of p- Mannosides
336
\
B z d h Br
OBz Yield: 74%
132
BzON Yield: 93%
133
134
135
Scheme 39.
In Lichtenthaler's method [33, 42, 199-2011 the oximinoglycosyl bromide 133 is used as the p-mannosamine precursor that can be prepared from the benzoylated hydroxyglycal 132. Condensation of 133 with MeOH using A g ~ C 0 3as the promoter has provided the p-linked glycoside 134 from which N-acetyl mannosaminide 135 can be obtained by BH3 reduction and N-acetylation [199] (Scheme 39). This approach is also suitable for glycosylating less reactive, secondary hydroxyl groups and for blockwise oligosaccharide synthesis [42]. 13.2.7 Aryl P-D-mannopyranosides P-Mannopyranosides with a chromo- or fluorogenic aromatic aglycone, e.g. a 4nitrophenyl, naphthyl, or 4-methylumbelliferyl group, are valuable enzyme substrates. The 4-nitrophenyl derivative is also employed as a mannosyl donor in glycosidase-catalyzed P-mannosylation. These compounds can be prepared in 4070% yields by reaction of the mannosyl bromide 2 [54] with sodium [202] or potassium phenolates [203, 2041. Garegg has synthesized aryl P-mannosides in 50-650/0 yields by condensation of dicyclohexylidene-mannopyranose with phenols in the presence of diethyl azodicarboxylate and triphenylphosphine [44, 2051. Epimerization of C-2 of aryl P-D-glucopyranosides by the Lindberg protocol (Section 13.2.2) has also been described [206]. 13.2.8 1-Thio-P-D-mannopyranosides 4-Nitrophenyl 1-thio-P-D-mannopyranoside has been synthesized by reaction of acetochloromannose [207] or the mannosyl bromide 2 [202] with the sodium salt of 4-nitrothiophenol. A similar reaction between sodium thiophenoxide and acetobromomannose in HMPT gives phenyl 1-thio-P-D-mannopyranoside quantitatively [208]. In a related reaction the sodium salt of 1-thio-(3-D-mannopyranosehas been alkylated with benzyl bromide as in Schmidt's approach (Section 13.2.4) to give benzyl P-thiomannoside in 86%) yield [209]. P-Linked 1-thiomannopyranosides can also be prepared by BF3-catalyzed anomerization of the readily available a-anomers [46]. The Lindberg protocol has also been used for the synthesis l-thioP-D-mannopyranosides [ 146, 1491. Aryl 1-thio-p-mannofuranosides have been syn-
13.3 Enzymatic Synthesis
136
137
331
138
Scheme 40.
thesized by reaction of di-isopropylidene-mannofuranose 99 and diary1 disulfides in the presence of triethylphosphine [210]. 13.2.9 P-D-Mannopyranosylamines The only N-glycoside that contains a P-linked mannopyranosyl residue was synthesized by palladium-catalyzed condensation of the amine 136 with the chloropurine 137 to afford the nucleoside antibiotic analog 138 together with the a-linked isomer in a 6: 1 ratio [211]. It is of interest to note the exclusive formation of the P amine 136 upon catalytic reduction of the corresponding azide, with no trace of the c1 anomer (Scheme 40).
13.3 Enzymatic Synthesis Although chemical methods have provided access to many oligosaccharides containing the p-mannopyranoside linkage, their overall efficiency is quite low because of numerous protecting group manipulations and glycosylation steps. As an alternative enzymatic methods have been receiving increasing attention. Both exomannosidases (glycosyl hydrolases) [47, 196, 2 12-2 161 and mannosyl transferases [21, 217, 2181 might be suitable biocatalysts for stereospecific p-mannoside synthesis. Although mannosiduse-catalyzed P-mannosylation (reverse hydrolysis) uses relatively readily available enzymes and mannosyl donors, e.g. p-nitrophenyl p-Dmannopyranoside or even mannose [216],overall yields are low [215].An additional difficulty is that when a saccharide is used as the acceptor alcohol the regioselectivity is not always predictable [47]. In the mannosyl transjerase approach a pmannosyl unit is transferred from GDP-mannose to the acceptor in a regio- and stereospecific manner. The high cost of GDP-mannose, the restrictions that might be imposed by substrate specificity, and instability and limited availability and of mannosyl transferases can be serious difficulties that require considerable investment before this method can be used for preparative-scale synthesis of 0-mannosides. A more detailed account of the enzymatic approach can be found in Chapters 23 to 30.
338
13 Stereoselective Synthesis of P-Mannosides
13.4 Conclusions Numerous developments in the last decade have rendered the construction of the problematic P-mannoside unit a less formidable challenge. Many years after its introduction, Paulsen’s silver silicate procedure is still often used for P-mannoside synthesis. Of the new methods Crich’s version of the sulfonate approach stands out because of its scope, operational simplicity, high overall yields, and P-selectivity. The more laborious inter- and intramolecular inversions of the C-2 chirality of P-glucosides are also powerful methods but require more steps that reduce the overall yield. Of the intramolecular glycosylation approaches Ogawa’s 4-methoxybenzylidene-assisted intramolecular glycosylation is a generally applicable method that is compatible with a variety of manipulations and is uniquely suitable for block-synthetic operations. Schuerch’s stannylene acetal approach has proved particularly useful for the stereoselective synthesis of trehalose analogs. Rapid advances in biotechnology are likely to open up efficient biocatalytic approaches to P-mannosyl oligosaccharides.
References 1. J. F. G. Vliegenthart and J. Montreuil, in Glycoproteins (Eds. J. Montreuil, J. F. G. Vliegen-
thart, H. Schachter), Elsevier, Amsterdam, 1995, Vol. 1, pp. 13-28. 2. D. R. Bundle, Topics. Curr. Chem., 1990, 154, 1-37. 3. C. Erbing, L. Kenne, B. Lindberg, S. Hammarstrom, Curbohydr. Res., 1978, 60, 400-403. 4. B. Lindberg, K. Leontein, U. Lindquist, S. B. Svensson, G. Wrangsell, A. Dell, M. Rogers, Curbohydr. Rex, 1988, 174, 313. 5. M. Betley, J. W. Redmond, N. H. Packer, D. Liu, P. Reeves, Curbohydr. Rex, 1997, 303, 313-318. 6. L. Kenne, B. Lindberg, in The Polysucchurides (Ed. G. 0. Aspinall) Academic Press, New York, 1983, Vol. 2, pp. 287-363. I . L. Galbraith, S. G. Wilkinson, Carbohydr. Rex, 1997, 303, 245-249. 8. U. Zahringer, H. Rettenmaier, H. Moll, S. N . Senchenkova, Y. A. Knirel, Curbohydr. Res., 1997,300, 143- 151. 9. V. Gloagen, L. Vebret, L. Hoffmann, H. Morvan, Y. Plancke, J.-M. Wieruszeski, G. Strecker, XVIII Int. Curbohydr. Symp., 1996, Milan, Italy, AP075. 10. D. Garozzo, G. Impallomeni, E. Spina, L. Sturiale, XVIII Int. Carbohydr. Symp., 1996, Milan, Italy, A0017. 11. P. L. Hackland, H. Parolis, L. A. S. Parolis, Curbohydr. Res., 1988, 172, 209-216. 12. Y.-M. Choy, G. G. S. Dutton, A. M. Zanlungo, Can. J. Chem., 1973,51, 1819-1825. 13. S. Kalelkar, J. Glushka, H. Van Halbeek, L. C. Morris, R. Cherniak, Curbohydr. Res., 1997, 299, 119-128. 14. D. Garazzo, G. Impallomeni, E. Spina, L. Sturiale, Curbohydr. Rex, 1998, 307, 113-124. 15. N. A. Paramanov, L. A. S. Parolis, H. Parolis, I. F. Boan, J. Anton, F. Rodriguez-Valera, Carbohydr. Res., 1998, 309, 89-94. 16. J. F. Kennedy, C. A. White, in Carbohydrate Chemistry (Ed. J. F. Kennedy), 1988, Clarendon Press, Oxford, pp. 220-226. 17. M. Ohnishi, S. Ito, Y. Fujino, Agric. Biol. Chem., 1985, 49, 3609-3611. 18. M. Sugita, Y. Nakano, T. Nose, 0. Itasaka, T. Hori, J. Biochem. (Tokyo), 1984, 95, 47-55. 19. N. Hada, T. Takeda, Y. Ogihara, Carbohydr. Rex, 1994, 258, 93-104. 20. P. A. Trinel, G. Lepage, T. Jouault, G. Strecker, D. Poulain, FEBS Let., 1997, 416, 203-206.
References
339
21. A. Suzuki, N. Shibata, M. Suzuki, F. Saitoh, H. Oyamada, H. Kobayashi, S. Suzuki, Y. Okawa, J. Biol. Chem., 1997,272, 16822-16828. 22. K. Tatsuta, S. Yasuda, Tetrahedron Let., 1996, 37, 2453-2456. 23. N. Sharon, H. Lis, H. in Glycosciences (Eds. H.-J. Gabius, S. Gabius) 1997, Chapman and Hall, London, pp. 133-162. 24. B. Priem, R. Gitti, C. A. Bush, Plant. Phys., 1993, 102, 445-458. 25. N. A. Kocharova, 0. V. Bystrova, S. A. Borisova, A. S. Shashkov, A. S, Y. A. Knirel, XVIII Znt. Curbohydr. Symp., 1996, Milan, Italy, AP008. 26. P. Branefors-Helander, L. Kenne, B. Lindberg, K. Petersson, P. Unger, Carbohydr. Res., 1981, 97,285-291. 27. F.-P. Tsui, R. Schneerson, R. A. Boykins, A. D. Karpas, W. Egan, Carbohydr. Rex, 1981, 97, 293-306. 28. P. Branefors-Helander, L. Kenne, B. Lindberg, K. Petersson, P. Unger, Curbohydr. Res., 1981, 88, 77-84. 29. F.-P. Tsui, R. Schneerson, W. Egan, Carbohydr. Rex, 1981, 88, 85-92. 30. C. Adlam, J. M. K. Knights, A. Mugridge, J. C. Lindon, P. R. W. Baker, J. E. Beesley, B. Spacey, G. R. Craig, L. K. Nagy, J. Gen. Microbiol., 1984, 130, 2415-2426. 31. H. J. Jennings, Curr. Top. Microbiol. Zmmunol., 1990, 150, 97-127. 32. C. Lugowski, E. Romanowska, L. Kenne, B. Lindberg, Curbohydr. Res., 1983,118, 173-181. 33. E. Kaji, Y. Osa, K. Takahashi, M. Hirooka, S. Zen, F. W. Lichtenthaler, Bull. Chem. SOC. Jpn., 1994, 67, 1130-1140. 34. J. K. Dale, J. Am. Chem. Soc., 1924,46, 1046-1051. 35. H. S. Isbell, H. L. Frush, J. Research Natl. Bur. Standards, 1940,24, 125-151. 36. H. G. Bott, W. N. Haworth, E. L. Hirst, J. Chem. SOC.,1930, 2653-2659. 37. B. Helferich, G. Duve, Chem. Ber., 1958, Y I , 1790-1793. 38. W. A. Bonner, J. Am. Chem. SOC.,1958,80, 3372-3379. 39. B. Helferich, S. Winkler, Chem. Ber., 1933, 66, 1556-1558. 40. H. Paulsen, Angew. Chem. Znt. Ed., 1982, 21, 155-173. 41. P. J. Garegg, Chemtracts, Org. Chem., 1992, 5, 389-393. 42. E. Kaji, F. W. Lichtenthaler, Trends. Glycosci. Glycotechnol., 1993, 5, 121-143. 43. F. Barresi, 0. Hindsgaul, in Modern methods in carbohydrate synthesis (Eds. S. H. Khan, R. O’Neill), Harwood Academic Publishers, Amsterdam, 1996, pp. 25 1-276. 44. B. V. McCleary, Methods Enzymol., 1988, 160, 515-518. 45. J. Tamura, S. Horito, H. Hashimoto, J. Yoshimura, Curbohydr. Rex, 1988, 174, 181-199. 46. D. T. Connolly, S. Roseman, Y. C. Lee, Curbohydr. Res., 1980, 87, 227-239. 47. N. Taubken, B. Sauerbrei, J. Thiem, J. Curbohydr. Chem., 1998,12, 651-667. 48. F. W. Lichtenthaler, T. Schneider-Adams, J. Org. Chem., 1994, 59, 6728-6734. 49. R. Kasai, M. Okihara, J. Asakawa, K. Mizutani, 0. Tanaka, Tetrahedron, 1979, 35, 14271432. 50. K. C. Nicolaou, F. L. van Delft, S. R. Conley, H. J. Mitchell, Z. Jin, R. M. Rodriguez, J. Am. Chem. SOC.,1997,119, 9057-9058. 51. D. Crich, S. Sun, Tetrahedron, 1998, 54, 8321-8348. 52. C. A. A. van Boeckel, T. Beetz, S. F. van Aelst, Tetruhedron, 1984, 40,4097-4107. 53. P. A. J. Gorin, A. S. Perlin, Can. J. Chem., 1961,39, 2474-2485. 54. T. J. Tewson, M. Soderlind, J. Curbohydr. Chem., 1985, 4, 529-543. 55. M. A. E. Shaban, I. E. Ary, D. A. Jeanloz, R. W. Jeanloz, Carbohydr. Rex, 1975,45, 105-1 14. 56. R. K. Jain, K. L. Matta, Curbohydr. Rex, 1996, 282, 101-111. 57. G. M. Bebault, G. G. S. Dutton, Curbohydr. Res., 1974, 37, 309-319. 58. V. I. Betaneli, M. V. Ovchinnikov, I. V. Backinowsky, N. K. Kochetkov, Curbohydr. Rex, 1980,84,211-224. 59. N. K. Kochetkov, V. I. Torgev, N. N. Malysheva, S. A. Shashkov, Tetrahedron, 1980, 36, 1099-1105. 60. M. A. E. Shaban, R. W. Jeanloz, Carbohydr. Rex, 1976, 52, 103-114. 61. M. A. E. Shaban, R. W. Jeanloz, Curbohydr. Rex, 1976,52, 115-127. 62. T. Sugawara, K. Irie, H. Iwasawa, T. Yoshikawa, S. Okuno, H. Watanabe, T. Kato, M. Shibukawa, Y. Ito, Curbohydr. Rex, 1992,230, 117-149.
340
13 Stereoselective Synthesis of P-Mannosides
H. Paulsen, 0. Lockhoff, Tetrahedron Let., 1978, 402774030, H. Paulsen, 0. Lockhoff, Chem. Ber., 1981, 114, 3102-3114. P. J. Garegg, T. Iversen, Curbohydr. Rex, 1979, 70, C13-C14. P. J. Garegg, T. Iversen, R. Johansson, Acta Chem. Scund., 1980, B34, 505-508. H. Paulsen, M. Heume, Z. Gyorgydeak, R. Lebuhn, Carbohydr. Rex, 1985, 144, 57-70. T. Ogawa, T. Kitajama, T. Nukada, Carbohydr. Rex, 1983, 123, C5-C7. H. Paulsen, M. Springer, F. Reek, E. Meinjohanns, I. Brockhausen, H. Schachter, Justus Liebigs Ann. Chem., 1995, 53-66. 70. P. Grice, S. V. Ley, J. Pietruszka, H. M. I. Osborn, H. W. M. Priepke, S. L. Warriner, Chem. Eur. J., 1997, 3, 431-440. 71. T. Ogawa, K. Katano, K. Sasajima, M. Matsui, Tetrahedron, 1981,37, 2779-2786. 72. H. Paulsen, R. Lebuhn, Angew. Chem. Int. Ed., 1982,21,926-927. 73. T. Ogawa, T. Kitajima, T. Nukada, Curbohydr. Res, 1983, 123, C8-Cll. 74. H. Paulsen, R. Lebuhn, Justus Liebigs Ann. Chem., 1983, 1047-1072. 75. H. Paulsen, R. Lebuhn, Carbohydr. Res., 1984, 130, 85-101. 76. T. Ogawa, M. Sugimoto, T. Kitajima, K. K. Sadozai, T. Nukada, Tetrahedron Let., 1986,27, 5739-5742. 77. F. Yamazaki, T. Nukada, Y. Ito, S. Sato, T. Ogawa, Tetrahedron Let., 1989, 30, 4417-4420. 78. H. Paulsen, M. Heume, H. Nurnberger, Curbohydr. Res., 1990,200, 127-166. 79. T. Nukada, T. Kitajima, Y. Nakahara, T. Ogawa, Curbohydr. Rex, 1992, 228, 157-170. 80. K. Koike, M. Mori, Y. Ito, Y. Nakahara, T. Ogawa, Agric. Biol. Chem., 1990,54,2931-2939. 81. M. Mori, Y. Ito, T. Ogawa, Carbohydr. Rex, 1990, 195, 199-224. 82. 0. Kanie, T. Takeda, N. Hada, Y. Ogihara, J. Curbohydr. Chem., 1991,10, 561-581. 83. K. Koike, M. Mori, Y. Ito, Y. Nakahara, T. Ogawa, Biosci. Biotech. Biochem., 1993,57,698702. 84. C. A. A. van Boeckel, T. Beetz, A. C. Kock-van Dalen, H. van Bekkum, Rec. Truo. Chim. Pu~s-Bus,1987, 106, 596-598. 85. Z.-W. Guo, Y. Nakahara, T. Ogawa, Bioorg. Med. Chem., 1997,5, 1917-1924. 86. Z.-W. Guo, Y. Itoh, Y. Nakahara, T. Ogawa, Carbohydr. Rex, 1998,306, 539-544. 87. P. J. Garegg, P. Ossowski, Actu Chem. Scand., 1983, B37, 249-251. 88. D. M. Whitfield, R. N. Shah, J. P. Carver, J. J. Krepinsky, Synth. Commun., 1985, 15, 737747. 89. T. Takeda, N. Hada, Y. Ogihara, Chem. Pharm. Bull., 1992,40, 1930-1933. 90. R. N. Shah, D. A. Cumming, A. A. Grey, J. P. Carver, J. Krepinsky, Curbohydr. Rex, 1986, 153, 155-161. 91. P. J. Garegg, C. Henrichson, T. Norberg, P. Ossowski, Carbohydr. Rex, 1983, 119, 95-100. 92. P. J. Garegg, R. Johansson, B. Samuelsson, Actu Chem. Scand., 1981, B35, 635-636. 93. G. Wulff, J. Wichelhaus, Chem. Ber., 1979, 112, 2847-2853. 94. J. Leroux, A. S. Perlin, Curbohydr. Rex, 1976, 47, C8-CIO. 95. J. Leroux, A. S. Perlin, Carbohydr. Rex, 1978, 67, 163-178. 96. E. S. Rachaman, R. Eby, C. Schuerch, Carbohydr. Rex, 1978,67, 147-161. 97. V. K. Srivastava, C. Schuerch, Carbohydr. Res., 1980, 79, C13-CI6. 98. V. K. Srivastava, C. Schuerch, J. Org. Chem., 1981, 46, 1121-1126. 99. E. S. El Ashry, C. Schuerch, Carbohydr. Rex, 1982, 105, 33-43. 100. L. F. Awad, E. S. H. El Ashry, C. Schuerch, Bull. Chem. SOC.Jpn., 1986,59, 158771592, 101. D. Crich, S. Sun, J. Am. Chem. Soc., 1997,119, 11217-11223. 102. D. Crich, W. L. Cai, J. Org. Chem., 1999,64,4926-4930. 103. D. Crich, S. Sun, J. Am. Chem. Soc., 1998, 120,435-436. 104. D. Crich, S. Sun, J. Org. Chem., 1997, 62, 1198-1199. 105. D. Crich, S. Sun, J. Org. Chem., 1996, 61, 4506-4507. 106. D. Crich, Z. Dai, Tetrahedron Let., 1998, 39, 1681-1684. 107. D. Crich, Z. M. Dai, Tetrahedron, 1999, 55, 1569-1580. 108. G. Stork, H. S. Suh, G. Kim, J. Am. Chem. Soc., 1991,113,7054-7056. 109. G. Stork, G. Kim, J. Am. Chem. Soc., 1992, 114, 1087-1088. 110. L. Yan, D. Kahne, J. Am. Chem. Soc., 1996, 118, 9239-9248. 111. G. Stork, J. J. La Clair, J. Am. Chem. Soc., 1996, 118, 247-248. 63. 64. 65. 66. 67. 68. 69.
References
341
F. Barresi, 0. Hindsgaul, J. Am. Chem. Soc., 1991, 113, 9376-9377. F. Barresi, 0. Hindsgaul, Synlett, 1992, 759-761. F. Barresi, 0. Hindsgaul, Can. J. Chem., 1994, 72, 1447-1465. Y. Ito, T. Ogawa, Angew. Chem. Int. Ed., 1994, 33, 1765-1767. M. Lergenmuller, T. Nukada, K. Kuramochi, A. Dan, T. Ogawa, Y. Ito, Eur. J. Org. Chem., 1999, 136771376, 117. A. Dan, Y . Ito, T. Ogawa, J. Org. Chem., 1995, 60, 4680-4681. 118. A. Dan, Y. Ito, T. Ogawa, Tetrahedron Let., 1995, 36, 7487-7490. 119. Y. Ito, T. Ogawa, J. Am. Chem. Soc., 1997, 119, 5562-5566. 120. T. Ziegler, G. Lemanski, A. Rakoczy, Tetrahedron Let., 1995,36, 8973-8976. 121. T. Ziegler, G. Lemanski, Eur. J. Org. Chem., 1998, 163-170. 122. T. Ziegler, G. Lemanski, Angew. Chem. Int. Ed., 1998, 37, 3129-3132. 123. N. K. Kochetkov, N. N. Malysheva, A. V. Demchenko, N. G. Kolotyrkina, E. M. Klimov, Bioorg. Chem. (Russia), 1991, 17, 1655-1659. 124. E. M. Klimov, A. V. Demchenko, N. N. Malysheva, N. K. Kochetkov, Bioorg. Chem. (Russia), 1991, 17, 1660-1664. 125. T. Yamanoi, K. Nakamura, H. Takeyama, K. Yanagihara, T. Inazu, Bull. Chem. Soc. Jpn., 1994,67, 1359-1366. 126. J. Yoshimura, S. Horito, J. Tamura, H. Hashimoto, Chem. Let., 1985, 1335-1338. 127. J. Tamura, S. Horito, J. Yoshimura, H. Hashimoto, Carbohydr. Res., 1990, 207, 153-165. 128. J. Tamura, R. Terayama, T. Shimadate, J. Carbohydr. Chem., 1993, 12,893-902. 129. W.-S. Kim, H. Sasai, M. Shibasaki, Tetrahedron Let., 1996, 37, 7797-7800. 130. 0. Theander, Acta Chem. Scand., 1958, 12, 1883-1885. 131. G . Ekborg, B. Lindberg, J. Lonngren, Acta Chem. Scand., 1972,26, 328773292, 132. K. K. C. Liu, S. J. Danishefsky, J. Org. Chem., 1994, 59, 1892-1894. 133. A. Liptak, J. Hajko, Unpublished results, 1998. 134. H. Boren, G. Ekborg, K. Eklind, P. J. Garegg, A. Pilotti, C. Swahn, Acta Chem. Scand., 1973, 27, 2639-2644. 135. N. K. Kochetkov, B. A. Dmitriev, 0. S. Chizhov, E. M. Klimov, N. N. Malysheva, A. Y. Chernyak, N. E. Bayramova, V. I. Torgov, Carbohydr. Res., 1974, 33, C5-C7. 136. N. K. Kochetkov, B. A. Dmitriev, N . N . Malysheva, A. Chernyak, E. M. Klimov, N . E. Bayramova, V. I. Torgov, Carbohydr. Rex, 1975, 45, 283-290. 137. G. Ekborg, J. Lonngren, S. Svensson, Acta Chem. Scand., 1975, B29, 1031-1035. 138. E. E. Lee, G. Keaveney, P. S. O’Colla, Carbohydr. Res., 1977, 59, 268-273. 139. C. D. Warren, C. Auge, M. L. Laver, S. Suzuki, D. Power, R. W. Jeanloz, Carbohydr. Rex, 1980, 82, 71-83. 140. C. Auge, C. D. Warren, R. W. Jeanloz, M. Kiso, L. Anderson, Carbohydr. Res., 1980,82, 8595. 141. G. Ekborg, C. P. J. Glaudemans, Carbohydr. Rex, 1984,129,287-292. 142. P. J. Garegg, C. Hallgren, J. Carbohydr. Chem., 1992, 11, 425-443. 143. H. H. Lee, L. N. Congson, D. M. Whitfield, L. R. Radics, J. J. Krepinsky, Can. J. Chem., 1992, 70, 2607-2617. 144. J . Kerekgyarto, J. G. M. van der Ven, J. Kamerling, A. Liptak, J. F. G. Vliegenthart, Carbohydr. Res., 1993, 238, 135-145. 145. S. H. Khan, K. L. Matta, Carbohydr. Res., 1995,278, 351-362. 146. P. H. Seeberger, P. F. Cirillo, S. Hu, X. Beebe, M. T. Bilodeau, S. J. Danishefsky, Enantiomer, 1996, I , 311-323. 147. S. Sarbanja, A. K. Misra, N. Roy, Synth. Commun., 1998,28, 2559-2570. 148. J. Kerekgyarto, J. P. Kamerling, J. B. Bouwstra, J. F. G. Vliegenthart, A. Liptak, Carbohydr. Res., 1989, 186, 51-62. 149. S. J. Danishefsky, S. Hu, P. F. Cirillo, M. Eckhardt, P. H. Seeberger, Chem. Eur. J., 1997, 3, 161771628, 150. M. Miljkovic, M. Gligorijevic, D. Glisin, J. Org. Chem., 1974, 39, 3223-3226. 151. A. Olesker, A. Dessinges, T. T. Thang, G. Lukacs, C. R. Acad. Sci. Paris Ser. 2, 1982, 295, 575-517. 152. W. Gunther, H. Kunz, Carbohydr. Rex, 1992,228, 217-241.
112. 113. 114. 115. 116.
342
13 Stereoselective Synthesis of /3-Mannosides
153. S. David, A. Malleron, C. Dini, Curbohydr. Rex, 1989, 188, 193-200. 154. I. Matsuo, M. Isomura, R. Walton, K. Ajisaka, Tetruhedron Let., 1996, 37, 8795-8798. 155. A. Fiirstner, I. Konetzki, J. Org. Chem., 1998, 63, 3072-3080. 156. A. Fiirstner, I. Konetzki, Tetrahedron Let., 1998, 39, 5721-5724. 157. S. David, A. Fernandez-Mayoralas, Carbohydr. Res., 1987, 165, clILc13. 158. J. Alais, S. David, Carbohydr. Rex, 1990, 201, 69-77. 159. K. Sato, A. Yoshimoto, Y. Tdkai, Bull. Chem. SOC.Jpn., 1997, 70, 885-890. 160. K. Sato, H. Seki, A. Yoshimoto, H. Nanaumi, Y. Takai, Y. Ishido, J. Curbohydr. Chem., 1998, 17, 703-727. 161. Y.-M. Zhang, J.-M. Mallet, P. Sinay, Carbohydr. Res., 1992,236, 73-88. 162. H.-W. Hagedorn, R. Brossmer, Helv. Chim. Actu, 1986, 69, 2127-2132. 163. H. Kunz, W. Giinther, Angew. Chem. Znt. Ed., 1988,27, 1086-1087. 164. C. Unverzagt, Angew. Chem. Int. Ed., 1994,33, 1102-1104. 165. I. A. Ivanova, A. V. Nikolaev, J. Chem. Soc. Perkin Trans 1, 1998, 3093-3099. 166. F. W. Lichtenthaler, E. Kaji, Justus Liebigs Ann. Chem., 1985, 1659-1668. 167. F. W. Lichtenthaler, E. Kaji, S. Weprek, J. Org. Chem., 1985, SO, 3505-3515. 168. F. W. Lichtenthaler, U. Klares, M. Lergenmiiller, S. Schwidetzky, Synthesis, 1992, 179-184. 169. F. W. Lichtenthaler, T. Schneider-Adams, S. Immel, J. Org. Chem., 1994, 59, 6735-6738. 170. F. W. Lichtenthaler, U. Klares, Z. Szunnai, B. Werner, Carbohydr. Rex, 1997,305, 293-303. 171. F. W. Lichtenthaler, T. W. Metz, Tetrahedron Let., 1997, 38, 5477-5480. 172. A. Fiirstner, I. Konetzki, Tetrahedron, 1996, 52, 15071-15078. 173. R. R. Schmidt, M. Reichrath, U. Moering, Tetrahedron Let., 1980,21, 3561-3564. 174. R. R. Schmidt, U. Moering, M. Reichrath, Chem. Ber., 1982, 115, 39-49. 175. A. Terjung, K.-H. Jung, R. R. Schmidt, Justus Liebigs Ann. Chem., 1996, 1313-1321. 176. A. Terjung, K.-H. Jung, R. R. Schmidt, Curbohydr. Rex, 1997,297, 229-242. 177. J . Tamura, R. R. Schmidt, J. Curbohydr. Chem., 1995,14, 895-911. 178. W. Klotz, R. R. Schmidt, Justus Liebigs Ann. Chem., 1993, 683-690. 179. S. David, S. Hanessian, Tetrahedron, 1985, 41, 643-663. 180. David, S. in Preparative carbohydrate chemistry, (Ed. S. Hanessian), Marcel Dekker, Inc., New York, 1997, pp. 69-83. 181. V. K. Srivastava, C. Schuerch, Tetrahedron Let., 1979, 3269-3272. 182. A. Dessinges, A. Olesker, G. Lukacs, T. T. Thang, Carbohydr. Res., 1984, 126, C6-C8. 183. G. Hodosi, P. Kovac, Curbohydr. Res., 1998, 308, 63-75. 184. D. Kahne, D. Yang, J. J. Lim, R. Miller, E. Paguaga, J. Am. Chem. Soc., 1988, 110, 87168717. 185. N. Yamazaki, E. Eichenberger, D. P. Curran, Tetrahedron Let., 1994, 35, 6623-6626. 186. D. Crich, S. Sun, J. Brunckova, J. Org. Chem., 1996, 61, 605-615. 187. D. Crich, J.-T. Hwang, H. Yuan, J. Org. Chem., 1996, 61, 6189-6198. 188. A. Liptak, I. Jodal, P. Nanasi, Curbohydr. Rex, 1975, 44, 1-11. 189. T. Iimori, H. Ohtake, S. Ikegami, Tetrahedron Let., 1997,38, 3415-3418. 190. L. F. Tietze, A. Montenbruck, C. Schneider, Synlett, 1994, 509-510. 191. D. L. Boger, K. D. Robarge, J. Org. Chem., 1988, 53, 5793-5796. 192. H. Paulsen, J. P. Lorentzen, W. Kutschker, Carbohydr. Res., 1985, 136, 153-176. 193. H. Paulsen, B. Helpap, J. P. Lorentzen, Carbohydr. Rex, 1988, 179, 173-197. 194. T. Sugawara, K. Igarashi, Carbohydr. Res., 1988, 172, 195-207. 195. B. Classon, P. J. Garegg, S. Oscarson, A.-K. Tiden, Carbohydr. Rex, 1991, 216, 187-196. 196. M. Scigelova, S. Singh, D. H. G. Crout, J. Chem. SOC.Perkin I , 1999, 777-782. 197. E. Micheli, F. Nicotra, L. Panza, F. Ronchetti, L. Toma, Curbohydr. Rex, 1985, 139, C1-C3. 198. R. Andersson, I. Gouda, 0. Larm, M. E. Riquelme, E. Scholander, Curbohydr. Rex, 1985, 142, 141-145. 199. E. Kaji, F. W. Lichtenthaler, Y. Osa, K. Takahashi, S. Zen, Bull. Chem. Soc. Jpn., 1995, 68, 2401-2408. 200. Y. Osa, E. Kaji, K. Takahashi, M. Hirooka, S. Zen, F. W. Lichtenthaler, Chem. Let., 1993, 1567-1570. 201. E. Kaji, F. W. Lichtenthaler, T. Nishino, A. Yamane, S. Zen, Bull. Chem. Soc. Jpn., 1988, 61, 1291-1297.
References 202. 203. 204. 205. 206. 207. 208. 209. 210. 21 1. 212. 213. 214. 215. 216. 217. 218.
343
K. L. Mattd, J. J. Barlow, Curhohydr. Rex, 1976, 48, 294-298. M.-C. Courtin-Duchateau, A. Veyrieres, Curhohydr. Res., 1978, 65, 23-33. H. P. Kleine, R. S. Sidhu, Curbohydr. Rex, 1988, 182, 307-312. K. Akerfeldt, P. J. Garegg, T. Iversen, Actu Chem. Scund., 1979, B33, 467-468. N. Kashimura, K. Kawaguchi, Ayric. Biol. Chern., 1976, 40; 1621-1624. M. Blanc-Muesser, J. Defaye, H. Driguez, Tetruhedron Let., 1976, 4307-4310. V. Pedretti, A. Veyrieres, P. Sinay, Tetruhedron, 1990, 46, 77-88. P. L. Durette, T. Y. Shen, Carbohydr. Res., 1980, 81, 261-274. A. Fiirstner, Justus Liehigs Ann. Chem., 1993, 1211-1217. N. Chida, T. Suzuki, S. Tanaka, I. Yamada, Tetrahedron Let., 1999, 40, 2573-2576. H. Itoh, Y. Kamiyama, Journal of Fermentution and Bioengineering, 1995, 80, 510-512. H. Fujimoto, M. Isomura, K. Ajisaka, Biosci. Biotech. Biochem., 1998, 61, 164-165. T. Usui, M. Suzuki, T. Sato, H. Kawagishi, K. Adachi, H. Sano, Glycocorzj. J., 1994, 11, 105110. S. Singh, M. Scigelova, D. H. G. Crout, J. Chrm. SOC.Chem. Commun., 1996, 993-994. N. Kildemark, K. Nilsson, XIX. Znt. Curhohydr. Symp., 1998, San Diego, CA, USA, BP035. G. M. Watt, L. Revers, M. C. Webberley, I. B. H. Wilson, S. L. Flitsch, Curhohydr. Rex, 1998,305, 533-541. L. Revers, R. M. Bill, I. B. H. Wilson, G. M. Watt, S. L. Flitsch, Biochim. Biophys. Actu, 1999, 1428, 88-98.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
14 Special Problems in Glycosylation Reactions: Sialidations Mukoto Kiso, Hideharu Ishida and Hiromi It0
14.1 Introduction Sialoglycoconjugates such as gangliosides and sialoglycoproteins have been recognized to play important roles in many biological processes [ 11. Sialooligosaccharide chains on cell surfaces are exposed as ligdnds to the external environment, and are capable of various biological functions, not only serving as receptors for hormones, viruses, and bacterial toxins, but also as mediators in cell growth, cell differentiation, cell adhesion, immune responses, and oncogenesis, etc. The most representative sialic acid, N-acetylneuraminic acid (NeuSAc, 1) is usually attached to 0 - 3 or 0-6 of galactose (Gal) or 0 - 6 of N-acetylgalactosamine (GalNAc) with an a(2-3) or a(2-6) linkage, and to 0-8 of another Neu5Ac with an w(2-8) linkage, giving diverse structures and functions. Because of this diversity systematic understanding of structure-function relationships of sialoglycoconjugates at the molecular level necessitates an efficient method for regio- and a-stereoselective glycoside synthesis of sialic acids.
14.2 Sialidation by the Koenigs-Knorr Method Several successful preparations of a-sialyl glycosides have been achieved mainly by use of the classic Koenigs-Knorr method employing N-acetylneuraminyl halides 2 as sialyl donors [2]. Although coupling of 2 (X = C1) with primary sugar hydroxyls, such as 3, gave a mixture of a-glycoside (4) and 0-glycoside ( 5 ) in good yields (Scheme 1) [3], the yield and stereoselectivity of the glycosides with hindered sugar hydroxyls were generally poor. For example, the sialidation of secondary sugar hydroxyls, such as 6, afforded an a,P mixture in low yields accompanied by a major by-product 7 (Scheme 2) [4], and the similar results were also obtained in couplings
346
14 Special Problems in Glycosylation Reactions: Siulidutions HO AcHN HO\W' W
C
0
2
H
~AcHN ~
HO
~
C
O
&
AcO
1 (Neu5Ac)
2
X = CI, Br etc
+
2 ( X = CI)
BnO
sieves 4 8, CHzC12
R
3 (R = OBn, N3)
AcouOAc
AcHN
"BnO " O0F o B n
AcO
OBn BnO
4 (a-glycoside) 36 % (R = OBn) 42 % (R = N3)
5 (p-glycoside) 48 % (R = OBn) 36 % (R = N3)
Scheme 1.
2 (X = CI)
t
HO
sieves 4 8, CH2C12
OBn 6
AcHN
OBn
AcO
C02Me
AcHN AcO
a (6 -12 %)
(3 (6 -19 %)
Scheme 2.
7
l
e
14.3 Siulidution Using an Auxiliary Group at C-3
347
of 2 (X = C1 or Br) with the lactose derivatives [5a, 5b]. Some improvements in the yield and a-stereoselectivity have been achieved by employing modified KoenigsKnorr conditions [2, 5c]. Particularly annoying is the competitive elimination which results from the deoxy center at C-3, giving the 2,3-dehydro derivative 7 as the major by-product. As critical problems, there are several disadvantages to the stereoselective synthesis of a-sialyl glycosides: 1) the anomeric center (C-2) of sialic acid is not only sterically hindered, but the reactivity is also electronically disfavored by the carboxylic acid function, which destabilizes the oxonium ion intermediate necessary for glycosidation; 2) the lack of a substituent at C-3 precludes the suitable neighboring participation leading to a-glycoside; and 3) the thermodynamically favored product is the P-glycoside. These combined factors are considered to disfavor formation of the desired aglycoside.
14.3 Sialidation Using an Auxiliary Group at C-3 The use of 2-halo-3-P-substituted sialic acid derivatives 8 as sialyl donors is an efficient means of overcoming the disadvantages in a-sialyl glycoside syntheses (Scheme 3) [2].Early work employing 8 (X = Br, Y = OH, P = Ac) showed that primary sugar hydroxyls react smoothly with this sialyl donor to give the desired
7
-
PO C02Me
AcHN
p0\\lts' AcHN
PO
PO
8
9
X = Halides Y = OH, SPh, SePh P = Ac, Bn
Scheme 3.
ROH
PO*"'
348
14 Special Problems in Glycosylation Reactions: Siulidutions
+
8
H O f l0O BnO e O B n
(X = CI or Br, Y = SPh, P = Bn)
HO
OBn
OBn 11
Hg(CN)2 HgBr2
Brio
sieves Molecular cc144A
B nBnOO AcHN
OBn
C02Me k
f
64 % (X = CI)
p
OBn OBn
OBn
HO
78 % (X = Br)
Brio &
o
12
OAc 7-
PhSCl CHcC12 30 C 2 days
AcOu' W L AcHN AcO
O
z M SPh
+
e
BnO* BnO
Brio OMe
8
AgOTf
(X = CI, Y = SPh, P = Ac)
40
13
Mokcular sieves 4 A
Na2HP04 A
c OAc O U
~
&
AcHN AcO
BnO BnO BnO OMe 14
Scheme 4a.
a-glycosides 9 with modest stereoselectivity. Nevertheless, the total yield and astereoselectivity of the glycosides with secondary sugar hydroxyls were generally poor. Many attempts to solve this problem were examined, and the thiophenyl auxiliary proved to be superior, as shown in Scheme 4a [6]. In fact, coupling of 8 (X = C1 or Br, Y = SPh, P = Bn) with a benzyl-protected lactose acceptor 11 gave excellent yields of the sialyl a(2-3')lactose derivative 12 [6a]. This procedure was immediately applied to the preparation of the sialyl a(2-8)sialic acid building blocks toward the synthesis of ganglioside GD3 and GQlb (Scheme 4b) [ 7 ] . The acetyl-protected sialyl donor 8 (X = C1, Y = SPh, P = Ac) was also examined for coupling with 13 to form a-sialoside 14 in 40% yield [6b]. Additional multiple steps for preparation of glycosyl donor 8 and removal of the C-3 substituent to arrive at the desired products 10 are, however, required (see Scheme 3), even though improvements in the yield and a-stereospecificity of the glycosides are achieved.
-
14.4 Siulidution Using 2-Tltioglycosidrs
BnO 8
(X = Br, Y = SPh, P = Bn)
OH
+
349
Hg(CN)2 HgBr2 C02Me
AcHN
Molecular
sieves 4A
cc14
BnO
(64%)
."W
C02Me
Bnvn C02Me
Scheme 4b.
14.4 Sialidation Using 2-Thioglycosides, Xanthates, or Phosphites of Sialic Acids in Acetonitrile 14.4.1 Thioglycosides Facile and highly regio- and a-stereoselective sialyl glycoside syntheses have been achieved [S, 91 by using the 2-thioglycosides of NeuSAc (15, 16) or 3-deoxy-w glycero-~-gaZacto-2-nonulosonicacid (Kdn, 17) as sialyl donors, and the partially protected sugar acceptors (18-26) (Figure l), in the presence of thiophilic promoters such as dimethyl(methy1thio)sulfonium triflate (DMTST) (promoter A) [9c, lo] or N-iodosuccinimide (N1S)-trifluoromethanesulfonic acid (TfOH) (promoter B) [ l l ] in acetonitrile (Table 1, Figure 2). The sialyl donors (15-17) were readily prepared [9, 12, 131 in three steps from NeuSAc or Kdn. Early work employing methyl 2-thioglycoside 15 as the glycosyl donor and DMTST as the promoter indicated [S, 91 that glycosylation in acetonitrile gives predominantly a-sialyl glycosides. Also, the use of lightly protected sugar acceptors (Figure I), in which several OH groups are unprotected, efficiently gave the desired sialyl glycosides a(2-3)- or a(2-6)-linked to the Gal residue in 50-70% yields, even in large-scale reactions [13]. Later, it was found that the use of phenyl 2-thioglycoside 16 [ 121 as the sialyl donor, promoted by NIS/TfOH, gives higher yields (Table 1, entries 3-5). Sialidation of trisaccharide acceptors (25, 26) with 16 afforded the corresponding sLeX (36) [14] and sLea (37) [15] derivatives in good yields (Table 1, entries 18 and 19). Reaction with reactive hydroxyls, such as OH-6 of 22 or OH-3 of 24, however, resulted in anomeric mixtures (entries 12-17). A
350
14 Special Problems in Glycosylation Reactions: Sialidations
Sialyl donors:
AcHN AcO 15R=Me 16R=Ph
AcO AcO 17
Sugar acceptors:
18 R’ = H, R2 = BZ 19 R’ = R2 = Bn
21 R = B z 22 R = Bn
23
24
J @ o ) O B n NHAc OBz OBn
&
HO Me
OBz
OBn
OBn
BnO OBn 25
26
Figure 1. Sialyl donors (15-17) and sugar acceptors (18-26).
similar tendency was observed also in the reaction with OH-6 of the GalNAc residue. Thus, the gangliotriose derivative 38 reacted with 16, promoted by NIS/ TMSOTf in acetonitrile, to afford 39a and 39p in 65% and 15% yield, respectively (Scheme 5). The use of NIS/TMSOTf, instead of NIS/TfOH, as the glycosyl promoter might give higher a-selectivity for reactive hydroxyls. Modification of the leaving group at C-2 in 2-thioglycosides of sialic acid has been found to modulate the reactivity as sialyl donors [ 161. The N-acetoxyacetamido (40a) [ 171, N-benzyloxyacetamido (40b) [ 181, and Ntrifluoroacetamido (42) [ 191 derivatives of 15 or 16 were also successfully coupled
14.4 Sialidation Using 2-Thioglycosides Products:
AcopOAc
R'
R2
R3
27
H
H
Bz
28
Bn
H
Bn
29
H
30
benzylidene
z:N2$$ b ; 33R=Bz 34R=Bn
OBz
31 R = NHAc 32 R = OAC
OSE
AcO
RO
OSE
AcO
CO2Me
OH
.
c
OAc o b
p
o C02Me
AcHN AcO
0 HO
o&
BnO
OBn
OSE OBn
35
OSE
OBn OBn
Figure 2. Structures of sialidation products.
351
352
14 Special Problems in Glycosylution Reactions: Siulidations
Table 1. DMTST (A)”-or NIS/TfOH (B)b-promoted sialidation of sugar acceptors (18-26) with methyl or phenyl 2-thioglycoside of sialic acid (15-17) in acetonitrile [Sc].
Entry
Donor
Acceptor
Promoter
Isolated yield (%)
Product
a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
18 18 18 19 20 18 23 23 23 21 21 22 22 22 24 24 24 25 26
15 15 16 16 16 17 15 16 17 15 15 15 15 16 15 15 16 16 16
A B B B B B A B B A B A B B A B B B B
52 61 70 70 65 49 47 55 46 70 59 50 51 50 30 59 58 58 69
27 21 27 28 29 30 31 31 32 33 33 34 34 34 35 35 35 36 37
Reactions were performed at - 15 to -20 “ C . bReactions were performed at -30 to -40°C.
a
HO ,OH*
16 b
OBn
OBn
38
NIS / TMSOTf Molecular
sieves 3 A CH3CN -30‘C
AcHN
OSE OBn 39 a (65 %), p (15 %)
Scheme 5.
P 0 0 0 0 0
0 0 0 0 0 0 15 26 25 8 10 11 0 0
353
14.4 Sialidation Using 2-Thioglycosides
18
SPh
NIS I TMSOTf CH$N -35 c (54 %)
40a
0
41a
24
NH
'
BnO" C\
c o
SMe OAc
40b
NIS / TfOH CH&N -23"C
OAc
A
CQMe o
c
B n o Z C0y
OAc
Ho
(55 %)
~
~
20
c o
OAC
42
~
OBn
41b
NH
CF3-C:
o
BnO
OBn
Acow;: AcO
~
NIS I TfOH CH&N/CHzCI, -30 "C (61 %)
OAc
*
CF3-C;A NHc OAC 0 W 0
i
M
p OSE
0 \
Ph
43
Scheme 6.
with 18,24 or 20, promoted by NIS/TfOH in acetonitrile, providing the desired sialyl-u(2-3)Gal derivatives 41a,41b,and 43 in yields of 45, 55, and 61%, respectively (Scheme 6). The N-benzyloxycarbonyamino (Boc) derivative (44)of sialyl donor 15 reacted with the lactose derivatives 45 and 46,promoted by PhSeOTf/ TMSOTf in tetrahydrofuran, to give 47 (84%) and 48 (46%), u predominantly (Scheme 7) [20]. When this reaction was conducted in dichloroethane, the sialosides were preferred products [21]. An efficient method for obtaining oligosialyl glycosides is the use of dimeric or trimeric Neu5Ac glycosyl donors, which can be prepared from naturally available colominic acid [22]. The lactonized phenyl 2-thioglycosides of dimeric [23] or trimeric [24] sialic acids (49)were each coupled with sugar acceptors 18 and 19 by use of NIS/TfOH as the promoter in acetonitrile, as just described, giving 30-48% of the desired a-glycosides (50,51) selectively (Scheme 8) [9d]. The reactivity of monosialyl (15,16)and disialyl (52)donors for the highly hindered OH-3 of the internal Gal residue (53)was examined (Scheme 9) [25], providing di- and trisialooligosaccharides (54, 55), the key intermediates for the synthesis of u-series gangliosides GTlau [25] and GQlbu [26]. As summarized in Table 2, the lactonized disialyl donor (52)gave the best result (Table 2, entry 6; 44%, u only) promoted by NIS/TfOH, whereas the monosialyl donors (15,16) afforded 34% (entry 1) and 40% (entry 5) of the desired a-sialosides, respectively, promoted by DMTST or
o
s
E
354
-
14 Special Problems in Glycosylation Reactions: Sialidations OH
OBn
OBn
HO H o
c
OBn
45
PhSeOTf TMSOTf THF
1
g
;
*
OBn
OBn
OBn
46
AcO\\‘’ BocHN AcO
c
44
BocHN AcO OBn OBn
48 4 6 % ( a : f 3 = 8 : 1 )
47 8 4 % ( a : P = l l : l )
Scheme 7.
SPh
+
18or19
AcHN AcO
49 (n = 0,l)
N ISKfO H Molecular sieves 4 A (30-48 %)
OSE
50 R’ = H, R2 = BZ 51 R’ = R2 = Bn
Scheme 8.
14.4 Sialidation Using 2-Thioglycosides OAc AcO%
15116
AcHNAcO
2%
CO2Me
.O.
'OSE
AcNH
+
or
355
k1 '0
.t
53
O
G OBn
OBn
OAc A
c
O
k
C02Me
g
AcO * O AcO\\'''
SPh AcO
52
AcHN
Scheme 9.
Table 2. a-Glycosylation" of 53 with sialyl donors 15, 16, and 52 in acetonitrile. Entry
Donor
Promoter
Product
Isolated yield (%)
54 54
34 I
54 54 55
26 40 44
~~
1 2
15 15
DMTST NIS/TfOH
?
16
NT9ITMCnTf
4
16 16 52
NIS/TfOH NIS/TMSOTf NIS/TfOH
5
_I
6 a
All reactions were performed at
~
._I
-I-----.
- 15 "C.
9A --
in A"
356
14 Special Problems in Glycosylation Reactions: Siulidutions 60
Acow
1
RSOTf
Me
Act37 R
SR
C02Me
AcO
(4
I I
-
FH3
TMSOTf
70
+ a-sialosides
Figure 3. A proposed mechanism for a-predominant sialidation in acetonitrile.
NIS/TMSOTf. The yields and a-selectivity are quite appreciable, considering the bulkiness of the donor and the steric hindrance on the sialidation site. Taking into account all these results the critically important factors seem to be (1) the rate of generation of the oxocarbenium ion, (2) the preferred formation of the p-acetonitrilium ion intermediate, and (3) the nucleophilicity of sugar acceptors; this suggests the reaction mechanism [8c, 91 proposed in Figure 3. This highly practical sialidation method has been extensively applied [9] to the synthesis of numerous gangliosides, sialooligosaccharides, and their derivatives and analogs by using not only monosialyl (15-17, 40, 42 etc.), di- and trisialyl (49, 52) donors, but also building blocks such as 56-59 shown in Figure 4 (9; see also 'Glycolipids' in this book). 14.4.2 Xanthates and Phosphites
As alternative sialyl donors, the use of 2-xanthates 60 [27-291 or 2-phosphites 70 [30, 311 of NeuSAc has been examined (Schemes 10 and 11). Early work employing DMTST as the glycosyl promoter showed [27] that sialyl 2-xanthate 60 reacts with Gal acceptor 61 or 6 in acetonitrile to give predominantly
14.4 Siulidution Using 2-Thioglycosides AdqOAc
357
AmuoAc C02Me
C02Me
PO \op
OBz
57
56 AcO
4 0 A c
58
co
59
Figure 4. Building blocks for the systematic synthesis of sialooligosaccharides, gangliosides, and their derivatives and analogs. X = NHAc, NHCOCH~OAC,NHCOCF3, NHBoc, OAc, efc. Y = SMe, OC(= NH)CC13, etc. P = Ac, Bz, etc.
a-sialosides 63 or 64 (Scheme lo), similarly to the corresponding sialyl2-thioglycosides. Improvements in the yield and a-stereospecificity of the products were achieved [28] by using methylsulfenyl triflate [32] as the promoter and a lightly protected lactose acceptor 62 which corresponds to 23 in Figure 1 , to afford 65 in 63-82% yield. The reactive sugar hydroxyl, such as OH-6 of Gal or GalNAc, gave an a,p mixture, however, even at -70 "C [28]. Similar results were also obtained by using phenylsulfenyl triflate as promoter in acetonitrile (Scheme 11) [29]. Sialyl 2-phosphites 70 were also found to react with sugar acceptors (11, 13, 71) promoted by TMSOTf in acetonitrile (Scheme 12) [30, 311, giving a-sialosides (7274) predominantly. 14.4.3 Reaction Mechanism A possible reaction mechanism is shown in Figure 3. When the 2-thioglycosides (a) of sialic acids are specifically activated by the thiophilic promoter at low temperature, the cyclic oxocarbenium ion (d), necessary for almost all known glycosylation reactions, is formed by the initially produced intermediates (b) or (c), and subsequently, reacts with acetonitrile to generate the p-acetonitrilium ion (e), which then undergoes S Ndisplacement ~ at the anomeric center by sugar hydroxyls to give predominantly a-glycosides. The common oxocarbenium ion intermediate ( d ) is also thought to be formed by activation of 2-xanthate (60) or 2-phosphites (70), generating the p-acetonitrilium ion (e)which provides predominantly a-sialosides. It has been demonstrated that the a-D-glucopyranosyl acetonitrilium ions are stereospecifically generated from the corresponding oxocarbenium ions in dry ace-
358
14 Special Problems in Glycosylution Reactions: Siulidutions
BzO "-0~~
+
or
6
OH
AcHN
61 AcO 60
4 62
(A) DMTST CH$N -15
(6)AgOTf
c
t
MSB CH&N/CH
AcouOAc AcHN
C02Me
AcHN
A&
OMe
AcO
63 a (48 %), fi (16 %)
64 a (26%), p (4 %)
,OBz AcHN AcO
-
65 (63 82 Yo)
Scheme 10.
tonitrile [33-351. It seems most plausible, therefore, that stereospecific generation of the p-acetonitrilium ion of Neu5Ac from the oxocarbenium ion holds the key to the a-predominant formation of sialyl glycosides in acetonitrile medium as shown in Figure 3 . Thus, the reactive sugar hydroxyls have a chance to attack other reaction intermediates before the generation of the fi-acetonitrilium ion of NeuSAc is complete, giving an anomeric mixture. Coupling with highly hindered secondary alcohols, however, generally results in low yields (but high a-selectivity) of the desired sialyl glycosides with formation of 7 as a major by-product.
14.5 Furtlzev Solutions to the Problem
359
.OTBDMS
un
HO(
-0Bn
n
OBn
I OAc A AcH N
c
67
66
PhSCI/AgOTf/DTBP CH,CN/CH2CI,, -70 "C
,OTBDMS
C02Me O ~ 'OMe
AcO \OBn 68 74 % (a$ = 955)
or
35 (78%; a:p = 96:4)
Scheme 11.
14.5 Further Solutions to the Problem To solve the problem in sialidation (1) introduction of an auxiliary group at C-3 in sialyl donors, (2) specific activation of the anomeric carbon C-2 by use of thioglycosides, xanthates or phosphites in acetonitrile medium, and (3) the use of sterically less hindered sugar acceptors have been attempted and found to be highly effective for obtaining a-sialosides in a regio- and stereospecific manner. Combinations of these procedures have resulted in further improvements. 14.5.1 Combination of C-3 Auxiliary and Sterically less Hindered Sugar Acceptors Combination of C-3 auxiliary (SPh) in sialyl donor 8 and sterically less hindered sugar acceptors (75, 77) has been successfully applied to the synthesis of sialyl LeX [33] and ganglioside GM3 [34] to give the desired 76 and 78 in 63% and 72% yields, respectively (Scheme 13). This method has been extended to the synthesis of disialyl lactose derivative 81 by the coupling of 80 with 77 (Scheme 14) [35].
14 Special Problems in Glycosylation Reactions: Sialidutions
360
OAc
A AcHN C O w E : i e
+
11/13
or 0
AcO
Po
70 (R = Et, Bn)
Ph
71
TMSOTf Molecular sieves 3A
CH&N -40 "C
AcouoAc C02Me AcHN BnO
OBn BnO
OMe
72
73
70-80% (a:P= 4:l
- 5.6:l)
50-55% (aonly)
Ph
74 38 % (aonly)
Scheme 12.
14.5.2 Combination of C-3 Auxiliary and Specific Activation of the Anomeric Center C-2 Further improvements in yield and a-stereospecificity have been achieved [36, 371 by combining the C-3 auxiliary (SPh) and specific activation of the anomic carbon C-2 by use of 2-thioglycosides (82) in acetonitrile (Scheme 15). Thus the coupling of 82 with 24, promoted by (A) methylsulfenyl triflate [36] or (B) phenylsulfenyl triflate [37] in acetonitrile at -40°C, gave 83 in 71% and 83% yields, respectively. Even when coupling was performed with the sterically hindered acceptor 84, the desired a-sialoside 85 was obtained in 67% yield [36]. Coupling with 86, however, resulted in 28% yield.
14.5 Further Solutions to the Problem
8
(X = Br, Y = SPh, P = Bn)
i-
OBn
63%
1
75
Hg(CN)2kwr2 Molecular sieves 4A, CC4 40 “C, 2 days
AcHN
OBn
76
8 (X = CI, Y = SPh, P = Ac)
+ OPlV
72%
AgOTf Molecular sieves 3A,CH3CN 60 “C, 2 days
SEt
AcHN AcO OPIV
78
Scheme 13.
361
14 Special Problems in Glycosylution Reactions: Siulidations
362
AcO
I::!;
C02Me
AcHN
AgOTf Molecular sieves 4R Na2HP04 CHpC12/toluene
AcO
79
-*
AcHN
(X = CI, Y = 8SPh, P
77
AcHN AcO
80 (49 %)
AcHN
AgOTf SnC12 Molecular
SEt
AcHN
sieves 4R -AW300 Na2HP04 CH3CN
OPiV
81 (39 %)
Scheme 14. OAc AcHN A c
O
AcO%;i,i;e".OAc
CqMe S -
AcO
82
24
*
(A) MeSBdAgOTf (6)PhSCI/AgOTf CH+N -40
(R = Me, Et)
C02Me
AcHN AcO
OSE
HO
C
OBn
Brio
83 (71 %, 83 %)
OBn
84 85 (67 %)
AcHN ACo
MPMO
86 AcHN
BZO
87 (28%)
Scheme 15.
OBn
14.5 Further Solutions to the Problem
363
AcO'
Phd
90
8 8 X = CI 89 X = OP(OEt)2
(A) AgOTf, C2H4C12 (B) TMSOTf, CH3CN
AcO
AcHN A
c OAcO
C02Me G PhOBnO
AcHN AcO
OBn
Fs PhO HO
OBn
Brio
OBn
C02Me
AcHN AcO
92 (68%, 83 %)
91 (89 %, 88 %) Scheme 16.
Dramatic improvement in the yield of a-sialyl glycosides has been achieved [38] by using the 3-phenoxythiocarbonyloxy derivatives of the 2-chloride (88) and 2phosphite (89) as sialyl donors (Scheme 16). Even the chloride derivative 88 coupled smoothly with 11 or 90, promoted by AgOTf in dichloroethane, to afford 91 (89%) and 92 (68%) in high yields. Specific activation of C-2 by diethyl phosphite (89), promoted by TMSOTf in acetonitrile, improved the yield of 92 (83%).
14.5.3 Thioglycoside of N,N-Diacetylneuraminic Acid and Combination with C-3 Auxiliary A novel N,N-diacetyl sialyl donor 93, prepared by N-acetylation of 15, has been found to react with 18 more smoothly than 15, giving 94 in 72% yield (Scheme 17) [39]. This method was immediately combined with C-3 auxiliary [40]. In fact, a novel sialyl donor 95 was found to couple with 84 or 86 in higher yields than the corresponding Neu5Ac donor 82 (R = Et) shown in Scheme 15, affording 96 (83%) and 97 (44%).
14 Special Problems in Glycosylation Reactions: Siulidutions
364
18 __7
NIS/TfOH
#
AyN
Molecular 93 __
sieves 38, CH3CN -40 "C
94 (72%)
04
COzMe SEt
AcO
Molecular
I
sieves 3A CH3CN/CH&I, -45 "C
96 (83%)
97 (44 %)
Scheme 17.
References 1. (a) W. Reutler, E. Kottgen, C. Bauer, W. Gerok in Sialic Acids-Chemistry, Metabolism and Function, Cell Biology Monographs, Vol. 10 (Ed.: R. Schauer) Springer, New York, 1982, pp. 263-305; (b) S. Hakomori, J. Bid. Chem. 1990, 265, 18713-18716; (c) S. Kelm, A. Pelz, R. Schauer, M. T. Filbin, S. Tong, M.-E. de Bellard, R. L. Schnaar, J. A. Mahoney, A. Hartnell, P. Bradfield, P. R. Crocker, Curr. Biol. 1994, 4 , 965-972; (d) C. Mitsuoka, K. Ohmori, N. Kimura, A. Kanamori, S. Komba, H. Ishida, M. Kiso, R. Kannagi, Proc. Natl. Acad. Sci. USA 1999, 96, 1597-1602. 2. (a) K. Okamoto, T. Goto, Tetrahedron 1990, 46, 89-102; (b) M. P. DeNinno, Synthesis 1991, 583-593. 3. (a) H. Paulsen, H. Tietz, Carbohydr. Res. 1984, 125, 47-64; (b) H. Paulsen, U. von Deessen, H. Tietz, ibid. 1985, 137, 63-77. 4. (a) T. Ogawa, M. Sugimoto, Carbohydr. Res. 1985, 135, c5-c9; (b) M. Numata, M. Sugimoto, K. Koike, T. Ogawa, ibid. 1987, 163, 209-225. 5. (a) M. Sugimoto, T. Ogawa, Glycoconjugate J. 1985, 2, 5-9; (b) H. Paulsen, U. von Deessen, Carbohydr. Res. 1986, 146, 147-153; (c) S. J. Danishefsky, J. Gervay, J. M. Peterson, F. E. McDonald, K. Koseki, T. Oriyama, D. A. Griffith, J. Am. Chem. Soc. 1992,114, 8329-8331. 6. (a) Y. Ito, T. Ogawa, Tetrahedron Lett. 1988, 32, 3987-3990; (b) T. Kondo, H. Abe, T. Goto, Chemistry Lett. 1988, 1657-1660.
References
365
7. Y. Ito, S. Nunomura, S. Shibayama, T. Ogawa, J. Org. Chem. 1992,57, 1821-1831. 8. (a) 0. Kanie, M. Kiso, A. Hasegawa, J. Curbohydr. Chem. 1988, 7; 501-506; (b) T. Murase, H. Ishida, M. Kiso, A. Hasegawa, Curbohydr. Res. 1988, 184, cl-c4; (c) A. Hasegawa, T. Nagahama, H. Ohki, K. Hotta, H. Ishida, M. Kiso, J. Carbohydr. Chem. 1991, 10, 493-498. 9. (a) A. Hasegawa, M, Kiso in Carbohydrates-Synthetic Methods and Applications in Medicinal Chemistry (Eds.: H. Ogura, A. Hasegawa, T. Suami), Kodansha/VCH, Tokyofleinheim, 1992, pp. 243-266; (b) A. Hasegawa in Synthetic Oligosaccharides~lndispensableProbes f o r the Life Sciences (Ed.: P. Kovdc), ACS Symposium Ser. 560, ACS, Washington DC, 1994, pp. 184-197; (c) Hasegawa et al., Methods Enzymol. 1994, 242, 158-198; (d) A. Hasegawa in Modern Methods in Carbohydrate Synthesis (Eds.: S. H. Khan, R. A. O’Neill), Harwood Academic Publishers, The Netherlands, 1996, pp. 277-300; (e) A. Hasegawa, M. Kiso in Prepururive Carbohydrate Chemistry (Ed.: S. Hanessian), Marcel Dekker, Inc., 1997, pp. 357-379. 10. P. Fugedi, P. J. Garegg, Carbohydr. Rex 1986, 149, c9-cl2. 11. (a) G. H. Veeneman, S. H. van Leeuwen, J. H. van Boom, Tetrahedron Lett. 1990, 31, 13311334; (b) P. Konradsson, U. E. Udodong, B. Fraser-Reid, Tetrahedron Lett. 1990, 31, 43134316. 12. A. Marra, P. Sinay, Carbohydr. Res. 1989, 187, 35-42. 13. A. Hasegawa, H. Ohki, T. Nagahama, H. Ishida, M. Kiso, Carbohydr. Res. 1991, 212, 277281. 14. A. Hasegawa, K. Fushimi, H. Ishida, M. Kiso, J. Curbohydr. Chem. 1993, 12, 1203-1216. 15. Y. Makimura, H. Ishida, M. Kiso, A. Hasegawa, J. Curbohydr. Chem. 1996; 15, 1097-1 11 8. 16. R. Roy, F. 0. Andersson, M. Letellier, Tetrahedron Lett. 1992, 33, 6053-6056. 17. A. Hasegawa, A. Uchimura, H. Ishida, M. Kiso, Biosci. Biotech. Biochem. 1995, 59, 10911094. 18. T. Sugata, R. Higuchi, Tetrahedron Lett. 1996, 37, 2613-2614. 19. S. Komba, C. Galustian, H. Ishida, T. Feizi, R. Kannagi, M. Kiso, Angew. Chem. Int. Ed. Engl. 1999,111, 1131-1133. 20. S. Fujita, M. Numata, M. Sugimoto, K. Tomita, T. Ogawa, Carbohydr. Rex 1992, 228, 347370. 21. Y. Ito, T. Ogawa, Tetrahedron Lett. 1988, 29, 1061-1064. 22. R. Roy, R. A. Pon, Glycoconjugate J. 1990, 7, 3-12, 23. H.-K. Ishida, Y. Ohta, Y. Tsukada, M. Kiso, A. Hasegawa, Curbohydr. Res. 1993,246, 75-88. 24. (a) H.-K. Ishida, H. Ishida, M. Kiso, A. Hasegawa, J. Curbohydr. Chem. 1994, 13, 655-664; (b) H. Ando, H. Ishida, M. Kiso, A. Hasegawa, Curbohydr. Rex 1997, 300, 207-217. 25. H. Ito, H. Ishida, M. Kiso, A. Hasegawa, Carbohydr. Res. 1998, 306, 581-585. 26. K. Hotta, H. Ishida, M. Kiso, A. Hasegawa, J. Curbohydr. Chem. 1995, 14, 491-506. 27. A. Marra, P. Sinay, Curbohydr. Res. 1990, 195, 303-308. 28. H. Lonn, K. Stenvall, Tetrahedron Lett. 1992, 33, 115-1 16. 29. V. Matrichonok, G. M. Whitesides, J. Org. Chem. 1996, 61, 1702-1706. 30. T. J. Martin, R. R. Schmidt, Tetruhedron Lett. 1992,33, 6123-6126. 31. M. M. Sim, H. Kondo, C.-H. Wong, J. Am. Chem. SOC.1993,115, 2260- 2267. 32. F. Dasgupta, P. J. Garegg, Curbohydr. Res. 1988, 177, c13-c17. 33. K. C. Nicolaou, C. W. Hummel, N. J. Bockovich, C.-H. Wong, J. Chem. Soc. Chem. Commun. 1991, 870-872. 34. T. Tomoo, T. Kondo, H. Abe, S. Tsukamoto, M. Isobe, T. Goto, Carbohydr. Res. 1996, 284, 207-222. 35. T. Kondo, T. Tomoo, H. Abe, M. Isobe, T. Goto, J. Carbohydr. Chem. 1996, 15, 857-878. 36. T. Ercegovic, G. Magnusson, J. Org. Chem. 1995,60, 3378-3384. 37. V. Martichonok, G. M. Whitesides, Carbohydr. Res. 1997, 302, 123-129. 38. J. C. Castro-Palomino, Y. E. Tsvetkov, R. R. Schmidt, J. Am. Chem. Soc. 1998, 120, 54345440. 39. A. V. Demchenko, G.-J. Boons, Tetrahedron Lett. 1998,39, 3065-3068; Chem. Eur. J. 1999,5, 1278- 1283. 40. N. Hossian, G. Magnusson, Tetrahedron Lett. 1999, 40, 2217-2220.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars Aluin Veyrihes
15.1 Introduction 2-Deoxy and 2,6-dideoxy glycosides are important structural units in many natural products including antitumor drugs (anthracyclines, aureolic acids, calicheamicin, esperamicin), antibiotics active against Gram-positive bacteria (erythromycins, orthosomycins), antibiotics inhibiting platelet aggregation (angucyclines), drugs used in the treatment of cardiac insufficiency (cardiac glycosides), antiparasitic agents (avermectins). Whereas the therapeutic effect of these drugs is usually mediated by their aglycone part, the glycosidic part is essential for biological activity by regulating the pharmacokinetic properties, not only transport, but also molecular recognition (interaction with nucleic acids, for example). The role of carbohydrates in biologically active natural products has been recently reviewed [ 11. Therefore, methods for the efficient and stereoselective construction of deoxyglycosidic linkages will have useful applications in medicinal and bioorganic chemistry by allowing to understand biological mechanisms and elaborate new and less toxic drugs. The lack of a stereodirecting neighboring group adjacent to the anomeric center makes 2-deoxyglycoside synthesis a particular challenge. Moreover, the absence of electron-withdrawing substituent at C-2 makes the glycosidic bond much more acid labile, giving rise to easy hydrolysis or anomerization. Glycals soon appeared as ideal precursors especially for the synthesis of 2-deoxy-a-glycosides which are thermodynamically more stable than the P-isomers. The most frequently employed strategy for accessing 2-deoxy-~-glycosidesuses temporary equatorial blocking groups at C-2 which can be removed after glycosylation. These groups should stabilize the glycosidic bond and provide the anchimeric assistance required for a good P-selectivity. The synthetic aspects of the chemical construction of 2-deoxyglycosides have been the subject of four recent reviews [2-51.
368
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars
15.2 Electrophilic Additions to Glycals: Mechanistic Aspects and Applications to the Synthesis of 2-Deoxyglycosides Addition of an electrophile E+ to a glycal such as protected D-glucal 1 gives a positively charged species which can be viewed as an oxycarbenium ion (2 or 3) in resonance with a cyclic onium ion (4 or 5 respectively); the weight of each structure is highly dependent of the nature of the electrophile and the polarity of the solvent (Scheme 1). If the addition of an electrophile is made in the presence of an alcohol R'OH, one can easily conceive that the glycosylation stereoselectivity will be dictated by the face-selectivity of electrophilic attack, at least if a true onium ion is involved. Trans-diaxial opening of the cyclic ion will occur under assistance by a lone pair of the pyranose oxygen atom. An above-plane addition should lead to an a-D-manno pyranoside 6 (diaxial outcome); a below-plane addition should give a P-D-glucopyranoside, first as a boat conformer 7, then flipping to the usual chair conformer 8 (diequatorial outcome). If a true oxycarbenium ion is involved, one can expect that the stereoselectivity will be governed by the kinetic anomeric effect [6-81 leading to a predominant axial stereochemistry at C-1, the a/P ratio being of course modulated by steric effects (a 3-axial substituent can override the stereoelectronic effect). Oxycarbenium ions 2 and 3 lead to a-D-manno 6 and a-D-gluco 9 pyranosides respectively. Altogether, 2-deoxy-a-glycopyranosides can be predicted to be more easily attained after replacement of E by hydrogen.
RO
Scheme 1. Electrophile-promoted addition of an alcohol to protected D-glucal 1 .
-
15.2 Electrophilic Additions to Glycals Ph3P-HBr 0.05 e ROD 10
H
11
D
369
OR
12
Scheme 2. Ph3P-HBr promoted addition of a deuterated alcohol to 3,4,6-tri-0-acetyl-~-glucal.
15.2.1 Protonation of Glycals
The acid-catalyzed addition of an alcohol to an acetylated glycal appears to be the most direct method for the synthesis of a 2-deoxyhexopyranoside. However, the acid catalyst has to be carefully chosen in order to avoid the Ferrier allylic rearrangement. Bolitt et al. [9] have reported that triphenylphosphine hydrobromide allows mild and high-yield protonation, then subsequent glycosylation (Scheme 2). Glycals with all equatorial substituents, like 3,4,6-tri-O-acetyl-~-ghcal10, give mainly a-glycoside products. Sabesan and Neira [ 101 have disclosed another efficient procedure where a dehydrated strongly acid ion-exchange resin in acetonitrile is used; lithium bromide is added to generate traces of hydrogen bromide needed for the reaction. Both methods give good results only with primary alcohols and water. However, when TMS triflate-triethylamine is used [ 111 at low temperature instead of a protic acid, silylation of the secondary alcohol 14 generates in situ Et3NH+ TfO- which activates the glycal 13 and gives the 2’-deoxy-a-(1-4)-disaccharide 15 in excellent yield (Scheme 3 ) . Franck [12] has shown that the major axial stereochemistry at the anomeric center is not due to a trans addition. No strong bridging of the proton can be really expected and the oxycarbenium ion 11 should be the reactive intermediate (Scheme 2). The use of deuterated alcohols ROD with catalytic amounts of Ph3P-HBr showed that deuteron delivery occurs largely from below the plane of the double bond. The predominant axial stereochemistry at C-1 and C-I’ of the glycoside products 12 and 15 probably arises from the kinetic anomeric effect. These results are consistent with an A d ~ 2(addition-electrophilic-bimolecular) mechanism for a first step of protonation, followed by a glycosyl transfer usually directed by the kinetic anomeric effect. Theoretical calculations [ 131 have shown O h
Scheme 3. Protioglycosylation of a glycal
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars
370
16
17
18
Scheme 4. P-Galactosidase-catalyzed addition of glycerol to D-galactal 16.
that the diastereofacial selectivity in protonation of D-glycals can be explained by a polarization of the R bond below the plane, with the exception of D-allal. 15.2.2 Enzyme-Catalyzed Additions to Glycals As early as 1972, Lehmann [14,151 reported that glycals could be stereospecifically converted to 2-deoxyglycosides using glycosidase enzymes. For example, D-galactal2-d 16 was transferred to glycerol using a p-galactosidase from Escherichia coli, 18 (Scheme 4). Delivery of the affording glyceryl 2-deoxy-~-~-galactopyranoside proton to C-2 of D-galactal took place from the bottom face to afford apparent trans addition. But it has been shown [ 161 that an enzymic nucleophile E-Nu is first bound on the a-face in a cis addition similar to the above acid-catalyzed process; the a-adduct 17 thus obtained is then displaced by glycerol from the B-face. Such a mechanism is the exact reversal of the enzyme's natural and stereospecific cleavage reaction. These results have been applied by Beau [17] for the synthesis of various p-2deoxyglycosides including disaccharides. With simple a-galactosides as acceptors, only p-(17-6) linkages are formed, the highest yield (50'!40) being obtained with the methyl glycoside. Limited yields are not the consequence of competing hydration or cleavage of the glycoside, but seem to be due to mutiple transfers. 15.2.3 Halogenation of Glycals Addition of halogen to double bonds occurs by a bimolecular process, with the approach of a halogen molecule perpendicularly to the n system of the enol ether from above or below the plane of the molecule. The resulting charge-transfer complex may rearrange, in the rate-limiting step, to intermediate ions (syn ion pairs, halonium ions, or open carbocations) which, according to their relative stabilities, control the stereochemical course of the reaction. Igarashi [18] reported that chlorine addition to 3,4,6-tri-O-acetyl-~-glucal10 gave the four 1,2-dichlorides 23-26 in proportions which depended on the solvent used. In nonpolar solvents, cis-addition products 24 (p-D-manno) and 25 (a-D-gluco) were predominantly obtained through ion pairs 19 and 20 in which the chloride ion is associated on the same side of the original plane from which chlorine attack first occurred. Moreover, because of the anomeric effect, ion pair 20 and therefore compound 25 were much more favored than ion pair 19 and compound 24 (Scheme 5).
311
15.2 Electrophilic Additions to Glyculs
+
ActaACO *
CI~..CI A
c AcO
Z
S
i
slow
Clp
AcO
charge-transfer complex
AcO
ACO
23
AcO
25 'I CI
24
CI
CI
AcO
CI
26
Scheme 5. Chlorine addition to 3,4,6-tri-O-acetyl-D-glucal.
In polar solvents, trans-addition products 23 (a-D-munno)and 26 (p-D-gluco)were predominantly obtained in nearly equal amounts through chloronium ions 21 and 22. The study of bromine addition to glycals is complicated by the easy anomerization of the 1,2-di-bromides. However, Boullanger and Descotes [ 191 observed that 29 in CC14 gave predominantly an a - ~ bromination of 3,4,6-tri-O-benzyl-~-glucal glum 1,2-dibromide 31, whereas bromination of 3,4,6-tri-O-acetyl-~-glucallO in the same solvent gave mainly the CL-D-WXUZY~O 1,2-dibromide 28 (Scheme 6 ) . Horton [20]
A
c
O
S
&
6r2
LOAC ei 27 cc14
10
29
AcO & *# Br
=OAC
28
-h E ! q
30
Scheme 6. Influence of protecting groups in bromine addition to D-glUCa1.
31
Br
372
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars
demonstrated that electronic effects, rather than purely steric effects, can explain this spectacular influence of protecting groups upon the stereoselectivity of the reaction, at least in nonpolar solvents: an electron-withdrawing substituent at C-6 disfavors the formation of ion pairs or oxycarbenium ions and therefore stabilizes symmetrical bromonium ions such as 27, but has no influence upon the addition of chlorine. As seen later on for the addition of N-iodosuccinimide to glycals, preferential approach of bromine from above the plane of the acetylated glycal might result from electrostatic effects, namely orientation of the net dipole moment of the molecule. When substituents with low inductive effects (benzyl groups or 6-deoxyglycal) are present, attack from below the molecular plane leads to a syn ion pair 30 favored by the anomeric effect and readily collapsing to the a-cis isomer 31. In polar solvents, the formation of oxycarbenium ions instead of syn ion pairs will be favored and the attack by bromide ions from below the molecular plane (kinetic anomeric effect) should be easier when bromine at C-2 is above (intermediate 2 in scheme 1); higher proportions of an a-trans addition product are therefore expected whatever the protecting groups are. The results obtained in electrophilic additions of bromine (and other electro29 have been recently reexamined [21]. philes) to 3,4,6-tri-O-benzyl-~-glucal
15.2.4 Bromo- and Iodoalkoxylation of Glycals In 1977 Tatsuta et al. [22] reported that 3,4,6-tri-O-acetyl-~-glucal 10 reacted with various alcohols in the presence of N-bromosuccinimide in acetonitrile to give mainly the 2-bromo-2-deoxy-a-~-mannopyranosides in 70-90% yields. With the exception of methanol, the a-mannolP-gluco ratio was estimated to be better than 95 : 5 when the reaction was performed at 5 “C; higher temperatures increased the amount of P-glucosides. As for protonation, this procedure is limited to sugar primary alcohols such as 32 leading to an a-(1+6) disaccharide 33 in excellent yield (Scheme 7). Radical debromination was then performed with tributyltin hydride and catalytic amounts of azobis(isobutyronitri1e) (AIBN). To explain the good stereoselectivity of the glycosylation reaction, the authors postulated that the gluco bromonium ion (below-plane attack) is destabilized by the “reverse anomeric effect”. This effect, which was put forward [23] to explain the preference of anomeric cationic substituents for the equatorial position on a pyranose ring, was recently shown [241 to be not consistent with theories of molecular
NBS,CH&N % * : A AcO
+
10
% A/ AcO
OAC
32
OAc
-5”C, e 94%
AcFq A&
33 Ac*o-cAC: OAc
Scheme 7. N-bromosuccinimide-promoted glycosylation of 3,4,6-tri-O-acetyl-~-glucal.
15.2 Electrophilic Additions to Glycals
373
structures. It is more probable that in acetonitrile an intermediate oxycarbenium ion with an axial bromine atom at C-2 is preferentially attacked by an alcohol coming from the a-side (kinetic anomeric effect), as it has been postulated above for the addition of bromine. The iodoalkoxylation of acetylated glycals was initially reported by Lemieux [25] as early as 1964. As an application of well-known Prevost and Woodward methods, the reaction was promoted by a mixture of iodine, silver perchlorate and collidine (or iodonium di-collidine perchlorate [25],IDCP) in benzene. In 1978 Thiem [26] developed a very useful variation of this reaction where N-iodo-succinimide (NIS) is used as an iodonium donor. Later on, Horton [27] reported a thorough study of Thiem’s procedure where he examined the influence of the solvent, the nucleophile and the structure of the glycal in relation to the ratio of the final products. Iodonium ions being much more stable than corresponding bromonium or chloronium ions, it was not expected that the substituent at C-6 of the glycal would have a significant effect. Observing that the proportion of stereoisomers (only trans-) in acetonitrile remained the same regardless of the nucleophile used, it was concluded that in this solvent the electrophilic addition of iodine to the double bond was irreversible and the proportion of final products reflected the proportion of attack of iodine from below or above the plane of the double bond. In tetrahydrofuran (or in methanol) a complex is formed with iodine, so that the first step will be fast and reversible; then the complex collapses slowly to give the final product (Figure 1). In such a situation, the ratio of stereoisomers will be controlled by the tendency for trans-diaxial opening of the cyclic ion, and by steric factors in the glycal and the nucleophile. It appeared that in acetonitrile steric factors do not exert a major influence on formation of the iodonium ion; electrostatic efects might be responsible for the stereochemistry of the first step. 3,4-Di-O-acetyl-~-fucal34 in the favored ’H4 (L) conformation has an axial acetoxy group at C-4 and is still iodinated mainly from below the plane of the molecule. The dipole moment of the ring oxygen atom, the equatorial acetoxy group at C-3, and especially the axial acetoxy group at C-4 combine to give a net dipole moment with the negative pole below the plane, favoring electrophilic attack from this side and leading to an a-L-fucopyranoside 36 through the iodonium ion 35 (Scheme 8). 3,4-Di-O-acetyl-~-rhamnal 37 with a C-4 acetoxy group equatorial also in the preferred ’H4 (L) conformation has a smaller dipole moment and shows therefore a
3- -
R
R
\+ ,o-I
R
R
fast
0
OR’
+‘A R OH
I
Figure 1. NIS-promoted addition of an alcohol to a double bond in ether solvents.
374
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars ROH
Me
AcO
AcO
OAc
34
I
36
35
Scheme 8. NIS-promoted glycosylation of 3,4-di-O-acetyl-~-fucal.
37
10
Figure 2. Iodonium intermediates with acetylated L-rhamnal37 and D-glucal 10.
slight decrease in reaction from below the plane of the molecule. 3,4,6-Tri-O-acetylD-glucal 10 has the same relative configuration as L-rhamnal and consequently is mainly attacked by iodine from above the plane of the molecule in its 4H5 (D) conformation (Figure 2). This interpretation could be complicated by the conformational equilibrium 4H5/ 5H4 which gives to 3,4,6-tri-O-acetyl-~-glucal10, for instance, a 40% contribution of the 5H4 (D) conformer with all its substituents axial. A vinylogous anomeric effect could explain this unusual distribution [28]. However, assuming a lower energy barrier for this conformer equilibrium in comparison to the subsequent reactions, the Curtin-Hammett principle can be applied and the product ratio will be governed by the activation energy of the electrophilic addition, rather than by the actual concentration of conformers. Thiem [29] found that methyl 3,4-di-O-acetyl-~-glucuronal 38 adopts almost exclusively an inverted 5H4 (D) half-chair conformation. A 60:40 mixture of a-D-manno-pyranoside 39 [in a flattened 1,4B(D) boat conformation] and p-Dgluco pyranoside 40 was obtained when 38 was treated with cyclohexanol and N-iodosuccinimide in acetone (Scheme 9). Other methyl glycuronals (L-ribo, D-Z~XO and L - x ~ ~ oadopt ) the usual conformation with an equatorial methoxycarbonyl
Scheme 9. NIS-Promoted glycosylation of methyl 3,4-di-O-acetyl-~-glucuronal.
15.2 Electrophilic Additions to Glycals
375
group and give almost exclusively the expected a-glycoside. A “conformational effect”, where an increased ratio of inverted half-chair conformation in the glycal shifts the a : p anomer ratio towards increased P-glycosides, was suggested. In the N-iodosuccinimide-promoted procedure the succinimide anion is frequently observed to compete with a less potent nucleophile. This leads to glycosyl succinimides which can adopt predominantly an inverted C d (D) chair conformation [30]. To this respect, nucleophilicity of the alcohol can be increased by a previous 0-tributylstannylation which allows better reactivity in NIS-promoted addition to glycals (excellent results are thus obtained with the axially configurated 4-hydroxy group of various L-fucose derivatives) [3 11. Since the stereochemistry of the iodoglycosylation reaction is most often governed by trans-diaxial addition, this methodology is quite successful for the synthesis of 2‘-deoxy-a-disaccharides. The trisaccharide sequences of dihydroaclacinomycin A, an anthracycline antibiotic, and kijanimycin, a macrolide antibiotic, containing three 2,6-dideoxysugar with a-(1-4) and a-(1-3) linkages respectively, were efficiently synthesized by Thiem using the NIS-procedure [3]. The 2-deoxy function is generated from the 2-iodo derivatives most often by radical reduction (Bu3SnH or Ph3SnH and catalytic AIBN), or sometimes by hydrogenation in the presence of palladium-charcoal and a base to neutralize HI, or with NiCl2NaBH4. Applying the “armed-disarmed’’ concept, Danishefsky [32] extended the NISprocedure to the coupling of an armed glycal donor protected by benzyl(29) or silyl ethers with one hydroxyl group of a disarmed glycal acceptor (41) otherwise protected by acyl groups. N-Iodosuccinimide or sym-collidine iodonium perchlorate are the “I+ equivalent” reagents. To reiterate the procedure, it is necessary to enhance the nucleophilicity of the obtained disaccharide glycal42 toward I+, so that it would function as a glycosyl donor to the next acceptor. For this purpose, benzoate groups are exchanged to silyl ethers. The armed disaccharide glycal43 can now be coupled with the disarmed glycal 41 to give a trisaccharide glycal 44, which can react further with a sugar alcohol to give a tetrasaccharide (Scheme 10).
’
Scheme 10. Trisaccharide assembly employing the iodoglycosylation reaction.
376
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars
45
Scheme 11. An a-iodoacetate as a-glycosyl donor.
Using this strategy, Danishefsky [33] performed the synthesis of ciclamicin 0, an anthracycline antibiotic containing a trisaccharide with three a-(1-4) linked 2,6dideoxy sugars and an a-linkage to the daunomycin aglycone (See Figure 10 for structure of ciclamicin 0). This work and other uses of glycals for the assembly of oligosaccharides and glycoconjugates have been recently reviewed [ 341. Olivomycin A, a member of the aureolic acid family (See Figure 7 for structure), contains a 2,6-dideoxy branched sugar, olivomycose, which is a-(1-3) linked to another 2,6-dideoxy sugar. Instead of running a direct iodoglycosylation of olivomycal 45, Roush [35] preferred to convert this glycal into the iodoacetate 46 by action of acetic acid and N-iodosuccinimide at -78 "C; a 85 : 15 ratio of diaxial 46 and diequatorial isomers was thus obtained (Scheme 11). It appeared that iodoacetate 46 is an excellent glycosyl donor when activated by trimethylsilyl triflate at low temperatures; high a-stereoselectivity and excellent yields were obtained with warious acceptors including glycals such as 47 which gave a disaccharide glycal48 incorporated into the total synthesis of olivomycin A [361. At reflux the reaction of a glycal49 with acetic acid and NIS gives a 1 : 1 mixture of P-D-gluco 50 and a-D-manno 51 isomers. After separation, the manno isomer can be reduced back to the starting glycal with lithium iodide in tetrahydrofuran
PI.
Another route, completely stereospecific at C-2, to a p-D-glUC0 isomer 54 starts from a 1,6-anhydro compound 53 easily obtained [38] by iodocyclization of D-glucal 52 (Scheme 12). The 2-deoxy-2-iodo-~-glucopyranosyl acetates such as 55 appeared as excellent glycosyl donors with various sugar alcohols including glycals (56) using trimethylsilyl or tert.butyldimethylsilyl triflate as promoters. The 2'-deoxy-2'-iodo-Pdisaccharides (57) were obtained in high yield and with a stereoselectivity often higher than 9 : 1. The high levels of stereochemical control in glycosylations with either a-iodoacetates (46) or P-iodoacetates (55) are undoubtedly due to the fact that corresponding iodonium ions of type 4 or 5 respectively are configurationally stable under the glycosylation conditions (TMSOTf, low temperatures); no equilibration of these ions occurs.
15.2 Electrophilic Additions to Glycals
’
-Ro% -
49
Lil, THF, 8246%
317
50
1. (Bu,Sn),O
2.12, CHBCN
R0
OAc
80%
52
HO
H
T W O OAc
TWO
+
F
53
54
O h TMSOTf, -78°C
En0
80%
I
55
i
56
57
Scheme 12. a-Iodoacetates as p-glycosyl donors.
15.2.5 Epoxidation of Glycals
The efficient epoxidation of glycals was reported in 1989 by Danishefsky [34, 391. Since acyl protecting groups can participate in the ring opening reactions of the epoxide, they have to be replaced by ether or acetal functions. Moreover, glycal epoxides being highly sensitive to nucleophilic attack at the anomeric carbon, peracids commonly used for epoxidation of alkenes lead to products of heterolysis; the use of anhydrous potassium fluoride in the mchloroperbenzoic acid-mediated epoxidation of glycals was, however, reported [40] to be successful. 2,2-Dimethyldioxirane (DMDO) appeared as the proper reagent, since the by-product, acetone, is not expected to react with the 1,2-anhydrosugar. 3,4,6-Tri-O-benzyl-~-glucal 29 reacted smoothly with DMDO to give a 20 : 1 mixture of am-gluco 58 and p-D-manno epoxides in nearly quantitative yield (Scheme 13). Epoxidation of D-allal with an axial 3-O-substituent occurred selectively on the p-face, whereas D-gulal with hindering substituents on both faces of the double bond gave a 1 : 1 mixture of a- and 0-epoxides. Neat methanol reacted smoothly with the a-D-yluco-epoxide 58 to give quantitatively the methyl P-o-glucopyranoside. More complex acceptors, like sugar alcohols (59), required a promoter (anhydrous zinc chloride, or zinc triflate with stannylated alcohols) and gave predominantly p-Dglucopyranosides (60). Yields can be improved by constraining the C-3 and C-4 or C-4 and C-6 oxygen functions of the glycal epoxide into a cyclic motif (carbonate or acetal). The uniquely generated 2’-hydroxyl group arising from opening of the 1,2epoxide donor can therefore be exploited in the synthesis of 2-deoxy-~-glycosides [41].Deoxygenation into 62 was accomplished by free radical reduction of the 2’-
318
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars
0
ZnCI, , THF 50% S
29
E
W
0
77%
1
R = OC(S)OPhF561
90%
K
R=H62
Scheme 13. Synthesis of a 2’-deoxy-P-disaccharide with a glycal epoxide as glycosyl donor.
pentafluorophenylthiocarbonate 61 (Scheme 13). Moreover, the glycosyl acceptor can be a glycal which will be compatible with the deoxygenation process. Glycal epoxides also react with phenols [41, 441 in the presence of potassium carbonate and a crown-ether to give (3-aryl glycosides; opening of the epoxide by phenate ions occurs according to a mechanism of S N type. ~ Deoxygenation at C-2 as above affords aryl 2-deoxy-(3-glycosides which are found in aureolic acids and other antibiotics. However, with 2,6-disubstituted aryl rings elimination of the anomeric function competes with the deoxygenation reaction and starting glycals are obtained. Under acidic conditions (ZnC12) 1,2-anhydro sugars react with phenols to give mixtures of a- and (3-glycosides; the mechanism of these glycosylations has been recently investigated [45]. In the presence of silver tetrafluoroborate stannylated primary alcohols react with the a-D-gluco-epoxide 58 to give a-glycosides [42]. When the acceptor is a glycal such as 63, an a-disaccharide glycal 64 is obtained, upon which the process can be reiterated to give a trisaccharide 65 (Scheme 14). 1. AgBF4
2. A 5
8
0
~ ~Pyr. o, 52%
*
1. 2,2-Dimethyldioxirane 2.63, AgBF4 3. Ago. Pyr.
P 51%
0-6
Scheme 14. Synthesis of an a-linked trisaccharide with glycal epoxides as glycosyl donors.
1.5.2 Electrophilic Additions to Glycals
319
4-Penten-1-01 h0 OH
ZnCI, ,63%
50% p:a6:1
66
58
67
0
-
Brio*
F
BnO
68
OH
Scheme 15. Conversion of a glycal epoxide to other p-glycosyl donors.
Reactions with secondary acceptors proceeded in lower yields and exhibited sharply diminished stereoselectivities. It should be mentioned that 1,2-epoxides can be converted [43] to other glycosylating agents, thiophenyl glycoside 66, pentenyl glycoside 67, fluoride 68, which can be helpful after acylation at C-2 for the synthesis of 2-deoxy-P-glycosides(Scheme 15). 15.2.6 Addition of Sulfur Based Electrophiles to Glycals Phenylsulfenyl chloride. Addition of phenylsulfenyl chloride, PhSCl, to 3,4,6-tetraO-benzyl-D-glucal29 was first performed by Schmidt [46]. The adduct arising from a below-plane addition was hydrolyzed with Na2C03 (80% overall yield), then converted into an w-trichloroacetimidate which in the presence of BF,.OEt2 and alcohols delivered mainly 2-deoxy-2-(phenylthio)-~-~-glucopyranosides. As for protonation, the preferred a-face selectivity in the addition of PhS+ was corroborated by theoretical studies [13]. The addition of PhSCl was thoroughly reinvestigated by Roush [47] who came to the following conclusions: the stereoselectivity of the reactions of D-glucal derivatives with PhSCl is highly dependent on the presence of an electronegative heteroatom substituent at C-6, as well as on the functionality at C-4. The C-6 substituent strongly influences the conformational preferences of the D-glucal derivatives, and greatest stereoselectivity is obtained with those glycals that exist preferentially in the inverted 5H4 (D) half-chair conformation [29]. Addition to the glycals is slow compared to 5H4/4H5 conformational interconversion, especially if the reaction is performed at low temperature. Therefore, the Curtin-Hammett principle must apply and the product distribution will be determined by the relative energies of the competing transition states and not the distribution of ground state conformers. Addition of PhSCl to the bottom face of the 4H5 (D) conformer of D-glucal 69 leads to the episulfonium ion 71 and should be favored relative to the top face addition leading to 70, owing to non-bonded interactions of the PhS- unit with the adjacent TBS ether in the latter case. This diastereofacial selectivity, however,
380
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars
R&'
TBSO
f- TBSO
TBSO
72 a-D-Manno
S
70
O
7
GI
I
%
X
R O + ~ @
-k 'Ph 71
Ro SPh
+
*?&o PhS CI
73 a$-D-Gluco
A
Scheme 16. Stereochemistry of addition of PhSCl to D-glUCal derivatives.
should not be expected to be overwhelmingly large, according to what is observed [48] for glycals that are locked in this conformation. On the other hand, the two faces of the 5H4 (D) conformer are differentiated to a much greater extent: the bottom face is shielded only by the axial C-4 substituent. A charge-dipole interaction will stabilize the transition state when that substituent is an acyloxy group 72 > 25 : 1) was indeed obtained [27]. The best selectivity (a-D-glUCO 73/a-~-manno with bromo and chloroacetoxy substituents at C-6 and C-4 respectively (Scheme 16). This mechanistic analysis has to be, however, readjusted with glycals of other configurations, since a D-fucal derivative, which has no polar substituent at C-6, gives a bottom face addition with an excellent selectivity [49]. The 2-deoxy-2-(thiopheny1)-a,P-D-glucopyranosylchlorides such as compound 74 were hydrolyzed with Na2C03 or DBU in tetrahydrofuran-water to give lactols which can be converted [50] into aryl 2-deoxy-P-glycosides by a Mitsunobu reaction, followed by desulfurization or into a-trichloroacetimidates such as 75 which were found configurationally stable at C-2 under the TMS triflate-promoted glycosylation conditions [ 5 I]. The stereoselectivity in the glycosylation of various alcohols was found to be dependent on the substrate; the least sterically hindered alcohols gave the best selectivity for the desired P-glycosides 76 (a direct S N ~ displacement or the substitution of a tightly solvated trichloroacetamideoxycarbenium ion pair was probably occurring). The fact that the a-glycosides such as 78 comprise up to 20-50% of the product in glycosylation of hindered secondary alcohols supports the thesis that the reaction stereoselectivity is not governed by the intermediacy of episulfonium ions 77a or 77d, but rather that substitution reactions of oxycarbenium ions 77b or 77c play a dominant role (Scheme 17). The episulfonium ions have been shown [52] by theoretical methods to be less stable than the corresponding oxycarbenium ions. This subject is still a matter of high controversy [53].
15.2 Electvophilic Additions to Glycals
381
1. DBU, THF-HZO
TMSOTf, ROH CH& , -78OC
2. NaH, CC13CN
ROH Felkin-Anh control AcO
n b
SPh
PhS
nc control
76
AcO
s ' h 77d
70
OR
Scheme 17. Stereochemistry of glycosylation with a 2-thiophenyl-a-~-glucopyranosyl donor.
Desulfurization is achieved with Raney-nickel in refluxing ethanol or tetrahydrofuran at room temperature [46], or Bu3SnH-AIBN in toluene at 100°C [49]. Sulfonium salts and sulfenates. Arylbis(ary1thio)sulfonium salts were shown by Franck [48] to undergo electrophilic addition to glycals in the presence of alcohols to form principally P-glycosides. Ester-blocked glycals cannot be used since they are decomposed through a Ferrier rearrangement. The p-tolyl reagent gave the best P-selectivity, but although a major below-plane attack of sulfur onto the 3,4,6tri-0-benzyl-D-ghcal was always observed, the face selectivity was found to be highly dependent on the nature of the nucleophile. A case of double diastereodifferentiation was also uncovered with a racemic alcohol. Even 4,6-O-benzylidene or -isopropylidene D-glycals, which cannot flip to the inverted half-chair conformation, show a preference for below-plane attack. The conformational effect underlined above for iodine and PhSCl additions should therefore be discounted here. A powerful steric effect of a 3-axial alkoxy group will direct the electrophile to the opposite face. These results suggest that the reactive species associates the electrophile and the nucleophile in a dithiosulfenate structure (Figure 3). It can be seen that the aglycon and the glycal interact through space as the glycal begins to bond to the sulfur, and a face-differentiating interaction can be envisaged. The first-formed intermediate would then be a sulfurane. Franck [54] applied this methodology to the synthesis of an aureolic acid disaccharide made of two 2,6dideoxy sugars with a p-(1-3) linkage. This approach is strongly reminiscent of the reaction reported by Ito and Ogawa [55],where phenylsulfenate esters are added to
382
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars \-
OR
SAr
Arylbis(arylthio) sulfonium
Dithiosulfenate
Figure 3. Addition of an arylbis(ary1thio)sulfonium salt and an alcohol to a glycal.
glycals in the presence of TMS triflate to give mainly P-glycosides (Figure 4). But although a similar attack of the TMS-activated sulfenate on the glycal is occurring, a close interaction between aglycon and glycal is not required here. In both cases the reaction can evolve to an episulfonium ion which is trans opened by the alcohol or a nucleophilic equivalent. 15.2.7 Addition of Selenium Based Electrophiles to Glycals
Beau and Sinay [ 561 reported the glycosyloxyselenation of glycals as an approach to 2'-deoxy-a-disaccharides. Addition of phenylselenenyl chloride, then an alcohol 79 and collidine to a protected D-glucal such as 29 in acetonitrile gave 9 : 1 mixtures of a-D-manno 80 and P-D-gluco glycosides in good yields (Scheme 18). The selective formation of a 1,2-episelenonium ion above the molecular plane was explained by complexation with benzyloxy groups. Unlike sulfur addition, the selenation is reversible and attempts to produce P-2-deoxy-glycosides in a one-step procedure were therefore investigated [57].It turned out that only trans diequatorial acetoxyselenides such as 81 can be obtained with a high selectivity (P: a 9 : 1) by adding PhSeC1, then silver acetate to the glycal in toluene. The stereochemistry of this addition was strongly influenced by the solvent polarity and the nature of the glycal protecting groups (a 4-0-benzoyl group reversed the selectivity). Acetoxy-selenides (81) were able to glycosylate even hindered secondary alcohols such as 1,2 : 5,6-di0-isopropylidene-a-D-glucofuranosein the presence of TMS triflate to give the P-disaccharide 82 in excellent yield (Scheme 18). The high P-selectivity was given
Figure 4. Addition of a phenylsulfenate ester to a glycal.
15.2 Electrophilic Additions to Glycals
383
PhSeCI, CHsCN, 0°C
%q/J then
r.t.
BnO
29
collidine -
1
79 AcHN o
h
*
80
61%
PhSeCI, Tol. then AgOAc
DlPGlcf TMSOTf
BnF* Brio
29
81%
81
SePh
a-Manno : g-Gluco 1 : 9
97%
82
a-Manno : g-G/uco I : 10
T\
Scheme 18. Stereoselective syntheses of 2/-deoxy 2'-phenylseleno-a- and 0-disaccharides.
by the efficient directing effect of the phenylselenyl group at C-2 provided that diethylether was used as a solvent. Other solvents like dichloromethane induced epimerization at C-2. Roush [47, 511 reinvestigated this two-step P-glycosylation method and came to the same conclusions as for the addition of PhSCl to glycals: the stereoselectivity of the reaction of ~-glucalderivatives with PhSeCl is highly dependent on the presence of an electronegative heteroatom substituent X at C-6 (X = Br or OTs better than X = H or OBn), as well as the functionality at C-4. A possible equilibration of episelenonium species, which does not seem to have been taken into account in other studies [ 581, makes the procedure, however, slightly less stereoselective than trichloroacetimiwith PhSCl. The 2-deoxy-2-(selenenophenyl)-a-~-glucopyranosyl dates are also highly reactive, but do not give better selectivity than the sulfur analogs. Deselenation is commonly performed with a tin hydride and AIBN in toluene at 100 "C. In a novel approach to 2'-deoxynucleosides, Castillon [59, 601 reported also a thorough study of factors controlling the stereoselectivity of the selenium-mediated N-glycosylation of pyranoid and furanoid glycals by silylated bases. The organoselenium chemistry was also applied [61] to the conversion of glycals into spiroortholactones (86), a type of 2-deoxyglycoside linkage found in orthosomycins (see Figure 8 for orthosomycin structure). Glycosyloxyselenation of 3,4,6tri-O-benzyl-D-glucal 29 by alcohol 83 was followed by unmasking of the second hydroxyl group to be glycosylated, then oxidation into a selenoxide 84. Because syn-elimination away from the two oxygen atoms is not possible for stereochemical reasons, forcing conditions provide a transient and reactive ketene acetal 85 which reacts intramolecularly and under stereoelectronic control with the hydroxyl function to give a new axial a ( l h 6 ) glycosidic linkage. The isomeric ortholactone resulting from an equatorial addition was not detected in the reaction mixture (Scheme 19).
384
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars
29
Tol.-VinylOAc BnO
85
Scheme 19. A selenium-mediated synthesis of a 2-deoxy-glycosyl-spiroortholactone.
15.3 The Cycloaddition Way to Glycosyl Transfer Following the elegant synthesis of 2-aminosugar glycosides by [4+2] cycloaddition of azodicarboxylates to glycals [62], Franck [63, 641 developed new bicyclic donors for the synthesis of 2-deoxy-P-glycosides. A diacylthione 88 generated in situ from a phthalimidosulfenyl precursor 87 undergoes cycloaddition reactions with a variety of glycals (pyranoid 1 and furanoid); an excellent a-face selectivity leading to cycloadducts 89 was observed in all cases, except with the allal where the pseudoaxial substituent at C-3 directs the addition above the plane (Scheme 20). The resulting heterocycle 89 is a vinyl glycoside which should behave as a glycosyl donor by activation with an electrophile. The acyl side chain or the sulfur atom could provide neighboring group participation, making the glycosylation highly stereoselective. The cycloaddition appears to be a reaction with inverse electron demand, since the smallest differences in energy are between the HOMO of the glycal dienophile and the low-lying LUMO of the heterodiene. Acetates lower too much the HOMO of the glycal, so that no reaction was observed. When the reaction is performed in methanol, a 50% yield of methyl glycosides is obtained along with cycloadducts. This result was interpreted as a two-step process where episulfonium species or corresponding oxycarbenium ions are initially formed, then rearrange into cycloadducts or are intercepted by methanol.
lutidine CHzClz , r.t.
SNPhth
87
88
CHzClz, pyridine, r.t. 5 days, 80% (R = En)
Scheme 20. Cycloaddition of a diacylthione to a protected D-glucal.
15.4 Fluoroglycosylation of Glycals
385
1 . 2 R’OH, TfOH CH2CI2, - 20% RO 2. Raney nickel RO RO -OR r
liCI4, 76% 92
Scheme 21. Synthesis of 2-deoxy-P-glycosides with cycloadduct glycosyl donors.
The cycloadducts 89 cleanly opened in the presence of p-toluenesulfonic acid only with methanol. A further survey of the reaction [65] showed that the a-D-gluco cycloadduct exhibited in fact reversibility with its glycoside product, wheras the 0D-manno adduct did not. The differing anomeric effects exerted in the ground states were proposed as the force which explains the differences in behavior. As a consequence, alcohol acceptors should be added in large excess to the glum donor. The a-D-gluco 89 and D-galacto cycloadducts could be, however, activated by methylenation of the ketone function with the Nysted Reagent 90 and TiC14. affording dienes 91 which are now extended vinyl glycosides. Triflic acid in dichloromethane proved to be the promoter of choice for their conversion into P-glycosides in good yields and with excellent stereoselectivities, even with less nucleophilic secondary sugar alcohols (Scheme 2 1). The addition of tetrabutylammonium triflate suppressed the anomerization of the P-glycoside and also prevents the acidcatalyzed 0- to C-rearrangement of an aryl glycoside. Raney nickel desulfurization affords the 2-deoxy-~-glycosides92.
15.4 Fluoroglycosylation of Glycals Although 2-deoxy-2-fluoro-glycosides cannot be precursors of the corresponding 2deoxy-glycosides, they are highly appreciated for their biological activities and the stabilization of the glycosidic linkage against hydrolysis brought by the fluorine atom. In the field of electrophilic addition to glycals, it seems therefore justified to mention the recent work of Wong [66] where the electrophilic fluorinationnucleophilic addition reactions with Selectfluor-type reagents 93 upon glycals have been studied and optimized. Selectfluor 93 adds first to a glycal such as 10 predominantly from above the plane of the molecule to give a syn adduct, which is susceptible to a slow anomerization and is further displaced by the alcohol 59; heating improves yields and also affects anomeric configuration. A major 2’-deoxy-2’-fluoro-a-~-mannodisaccharide 94 is thus produced (Scheme 22). A judicious choice of protective groups was shown to improve the stereoselectivity of both fluorination and nucleophilic addition.
386
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars
Scheme 22. Fluoroglycosylation of 3,4,6-tri-O-acetyl-~-glucal 10 with Selectfluor 93.
15.5 Glycosyl Donors with a C-2 Heteroatom Other methods than electrophilic additions to glycals can afford glycosyl donors with a C-2 heteroatom substituent that will guide the stereochemical course of the glycosylation by anchimeric participation and will be reductively removed to provide the 2-deoxyglycoside. Most often, the glycosylating intermediate (onium ion, ion pair or oxycarbenium ion) will be identical to the one obtained from a glycal. But the nature of the promoter (metal salt, Lewis acid) can strongly affect the stereoselectivity of the glycosylation. Glycosyl donors with an equatorial C-2 heteroatom should normally give access to 2-deoxy-P-glycosides.
15.5.1 2-Bromo-2-deoxyglycosyl bromides Thiem [31 introduced the use of 2-bromo-2-deoxy-a-~-glucopyranosyl bromides 96 for the p-selective glycosylation of complex aglycones involved in aureolic acid antibiotics. These bromides were prepared by the reaction of dibromomethyl methyl 95 and gave ether [67] with a methyl 2,3-O-isopropylidene-a-~-mannopyranoside predominantly (P :a 10 : 1) 2-deoxy-2-bromo-~-glucopyranosides in their silver triflate-promoted reactions with sugar alcohols. Radical debromination afforded the 2-deoxy-P-glycosides97 in excellent yields (Scheme 23). The presence of a labile fonnyl substituent at 0 - 3 of 97 is highly valuable for the
-
R O Y & ,
95
1. R"OH, AgOTf CH3N02-Tol. Ro= -78"c OCHO
BrgF
_____)
OM9
06
Br Br 2. Bu3SnH AIBN, Tol., A
OR"
07
Scheme 23. 2-Bromo-2-deoxyglucopyranosyl bromides in the synthesis of 2-deoxy-P-glycosides.
387
15.5 Glycosyl Donors with a C-2 Heteroatom
further elongation of an oligosaccharide chain at that position, a strategy that was successfully used by Thiem for the synthesis of a P-linked trisaccharide of mithramycin, an antibiotic of the aureolic acid family [68]. 15.5.2 2-Deoxy-2-(thiophenyl)-glycosylfluorides
Nicolaou [69, 701 reported that phenylthio glycosides with a trans OH group at C-2 undergo 1,2-migration of the thiophenyl group when they are treated with (diethylamino)sulfur trifluoride (DAST) at 0 "C. The glycosyl fluorides with an inverted configuration at C-2 are obtained in high yields. A starting phenylthio a-D-mannopyranoside 98 will afford the 2-deoxy-2thiophenyl-P-D-glucopyranosylfluoride 99. Activation of the fluoride with SnC12 in diethyl ether, a tin-complexing solvent, allows the glycosylation of alcohols to give P-glycosides 100 with an excellent P-stereoselectivity through a 1,2-episulfonium intermediate. In the absence of a complexing medium, a-glucosides 101 are predominantly obtained, most probably because the promoter is then coordinated to the sulfur atom and thus prevents it from participating in the coupling reaction (Scheme 24). Desulfurization with Raney nickel affords the desired 2deoxyglycosides. This strategy was applied [711 to the synthesis of the 2,2'-dideoxy-P-disaccharide B-C found in the antibiotic everninomicin, a member of the orthosomycin family (see Figure 8 for complete structure). Phenylthio a-D-mannopyranosides 102 and 104 were converted into fluoride 103 and P-methyl glycoside 105 respectively. The P-disaccharide 106 (BC), obtained by coupling of 103 and 105 in diethyl ether and subsequent protective group adjustment, was then acylated at 0-4' by fluoride 107 (A,) and a-glycosylated at 0-3' by the evernitrose fluoride 108 (A) in dichloromethane to give the AlB(A)C fragment of everninomicin (Scheme 25). Similarly, thioglycosides with a phenoxythiocarbonyl ester group at C-2 (109) undergo 1,2-migration of the anomeric phenyl- or ethylthio group when they are activated (110) by NIS/cat. TfOH. The intermediate episulfonium ion 111 is trapped by an alcohol to give P-Dglucopyranosides 112 when the starting compound is a phenyl- or ethylthio a-D-
- EtZNSF3 CHzCIz, 0°C
98
SPh
R'OH, SnCI, EtpO, - 15°C + : R Ro
RO
Ro *R
99
I
PhS
OR'
loo
PhS
101
PhS OR'
R'OH. SnCI, CHzCI2,-15"C RO !!
Scheme 24. Stereospecific syntheses of 2-deoxy-2-thiophenyl-P- and a-glycosides via 1,2-migration of the PhS group.
388
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars
Twkq 1. DAST, CH2C12
Me
~o~~~
2. MeOH. SnCI, , EbO
104
F
Sph 3. BUdNF, THF
105
95%
SPh
1. SnCI, , Et,O. -15"C, 78% 2. Raney Ni, MeOH, 65OC 3. BnBr, NaH. Bu4NI, DMF, 0°C 4. BUdNF, THF, 25°C
Me
BnO
C Me0
109
2. DDQ 3.BF3 OEt, , CH,CI,,
NO2
95% (a-glycosylation)
Scheme 25. Synthesis of an everninomicin fragment.
mannopyranoside 109 (Scheme 26) [72]. 2'-Ethylthio disaccharides are, however, desulfurized by Raney nickel much more sluggishly than the corresponding 2'phenylthio compounds. 15.5.3 2,6-Anhydro-2-Thio-Glycosyl Donors Tatsuta and Toshima [73] designed conformationally rigid 2,6-anhydro-2-thio sugars for the stereocontrolled synthesis of 2,6-dideoxy-a- and -P-glycosides which are very commonly found in bioactive natural products. These new donors have a very rigid structure (boat conformation) and the stereoselectivity of the glycosylation should not be affected by the anomeric effect in the same manner as the more usual chair conformers.
S B
n
?
!
BnO
109
R = Ph or Et
- - -
OPhNIS
TtOH
SR
~
n
Bno
110
OAOPh ?
t
SR
q
B
n
?
!
Bno
R'OH
B n E o w Bno
111
:+
SR
112
R
Scheme 26. 1,2-Ethyl (or phenyl) thio group migration for the stereospecific synthesis of 2-deoxy-Pglycosides.
15.5 Glycosyl Donors with a C-2 Heteroatom
389
The required methyl 2,6-anhydro-2-thio-u-~-altropyranoside 113 was prepared according to known procedures [74, 751, then converted into a phenylthio glycoside 114,a fluoride 115 or a p-acetate 116.Activation of the thioglycoside 114 with Nbromosuccinimide or methyl triflate, and of the fluoride 115 with various Lewis acids (SnC12, SnC12-AgC104, SnCI2-ZnClz, Cp2 HfC12-AgC104, CpzZrC12-AgBFq, TMSOTf) allowed the glycosylation of various alcohols to give a-glycosides 117 in high yields and nearly complete stereoselectivity . Strikingly, that selectivity was independent of the solvent, the anomeric configuration of the donor and the nature of the promoter. In a drastic contrast, the P-glycosides 118 were almost exclusively obtained when the p-acetate donor 116 was activated by various Lewis acids (TMSOTf, Tf20, SnC14, TrC104); only in diethyl ether and tetrahydrofuran was the selectivity reversed, because of deactivation of the promoter. Anomerization of ainto p-glycosides by the Lewis acid alone prooved that a thermodynamic control is responsible for the high P-stereoselectivity of the glycosylation reaction (Scheme 27). Methanolysis, then hydrogenolysis with Raney nickel or radical reduction with Bu&H afforded the 2,6-dideoxy-a-(119) and ~-(120)-~-ribo-hexopyranosides. Moreover, 2,6-anhydro-2-thiomannopyranosyldonors 121 (Scheme 28) having an equatorial 3-0-substituent can be obtained by oxidation of the 3-hydroxyl group
OAc
4
OAc ‘ I ”
I
”*
OH
120
-& hF :$=
NBS.DAST
____)
AcO
AcO
OM
OAc
AcO
SPh
OAc
113
OAc
114 E2, ROH
1 . NaOMe 2.[HI E2 = SnClz or SnC12-AgCIC or SnCi2-ZnCI, OR
119
OAc
117
OR
or Cp2HfCI2-AgCIO, or Cp2ZrCI2-AgBF4 or TMSOTf E, = TMSOTf or Tf20 or SnCi, or TrCIO,
Scheme 27. 2,6-Anhydro-2-thio sugars as glycosyl donors for the stereospecific synthesis of 2deoxy-a- and P-glycosides.
390
A+4- %& 15 Special Problems in Glycosylution Reactions: 2-Deoxy Sugars
TPSO
AcO
NIS,TMSOTf
TPSO
121
SPh
Ho
122
TPSO
SPh
AcO
TPSO
123
0
SPh
1. LAH. THF 2. AQO, Pyr. 3. NBS, ROH
Scheme 28. Armed-disarmed glycosylation with 2,6-anhydro-2-thio sugars.
into a ketone, then stereoselective reduction with diisobutylaluminum hydride; they all give 2,6-dideoxy-a-~-arabino-hexopyranosides. Since the configuration of the glycosidic bond is completely independent of the configuration at the anomeric center of the glycosyl donor, these glycosylation reactions must involve an oxycarbenium ion as an intermediate. Under kinetic conditions, a repulsive electronic interaction between the sulfur atom of the 2,6bridge and the alcohol approaching from the p-face impedes the reaction and aglycosides are exclusively obtained. In contrast, under thermodynamic conditions, an a-glycoside with an axial 3-0-substituent will anomerize to the more stable pglycoside (Figure 5). When the 3-0-substituent is equatorial, no 1,3-diaxial interaction is present and the a-glycoside is thermodynamically stable. The high reactivity of 2,6-anhydro-2-thioglycosyldonors results from the electron-donating nature of the bridging sulfur atom. Indeed, the derived sulfoxides 122 and sulfones have no glycosylating power and can thus be implied in block syntheses exploiting the “armed-disarmed effect” (Scheme 28). Coupling of the armed phenylthio glycoside 121 with the disarmed acceptor 122 gives the a-disaccharide 123 in 89% yield. Activation of 123 by lithium aluminohydride (LAH) reduction of the sulfoxide group and subsequent reacetylation allows to glycosylate an alcohol in an a-selective procedure (124 is obtained in 98% yield when R = cyclohexyl). I
H
0-R
&-
AcO OAc
EZ‘
Figure 5. Repulsive interactions in glycosylation with a 2,6-anhydro-2-thio glycosyl donor.
15.5 Glycosyl Donors with a C-2 Heteroatom
HO
391
OH
OM W
O
H Cladinose
Me
SPh
Me
Erythromycin A
S
Figure 6. 2,6-Anhydro-2-thio-cladinose as a glycosyl donor in erythromycin A synthesis.
This highly stereocontrolled method of glycosylation was applied to the synthesis of erythromycin A and the C-D-E trisaccharide of olivomycin A [76]. The 3hydroxyl group of erythromycin A aglycone is sterically hindered and forms a hydrogen bond with the C-1 carbonyl group. Its glycosylation by a L-cladinose donor is a difficult task, as noticed by Woodward in his total synthesis of erythromycin A in 1981. The use of a phenyl2,6-anhydro-2-thio-~-cladinose thioglycoside activated by N-iodosuccinimide and triflic acid allowed the stereocontrolled synthesis of the desired a-glycoside in 90% yield (Figure 6). The olivomycin A trisaccharide (under the form of a P-cyclohexyl glycoside) was assembled through three 2,6-anhydro-2-thio sugars. The P-glycosidic bonds of units C and D were made (90% yields, no a-anomer) with a p-acetate donor which has an axial 3-O-substituent, the conversion to the reauired equatorial configuration being made after each coupling. The a-glycosidic bond of unit E was built in 89% yield and a fully stereoselective glycosylation reaction from a phenylthio glycoside derived [77] from olivomycose (Figure 7).
HO &eM
OAc
?Me OH
DEIPSO
D ‘PrOCO
HO E
Olivomycin A
Figure 7. Synthesis of the CDE trisaccharide of olivomycin A with 2,6-anhydro-2-thio sugars as glycosyl donors.
392
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars
15.5.4 1,2-Di-O-Acetyl-fi-Hexopyranoses and N-Formylglucosamine Derivatives An efficient, general and stereospecific preparation of 2'-deoxy-P-disaccharides reported by Sinay [78, 791 makes use of a 2-0-acetyl substituent as a stereodirecting 126 (and -D-galacto-)pyranoses group. 1,2-Di-0-acetyl-3,4,6-tri-O-benzyl-P-~-glucowere obtained from the known corresponding orthoesters 125 (they could have been also prepared from glycal epoxides). They reacted with various sugar alcohols in the presence of TMS triflate at -20 "C to give P-disaccharides in high yields (75-90%) and a total stereoselectivity attributed to the efficient anchimeric assistance of the 2-0-acetyl group. After removal of this group, the Barton-McCombie procedure allowed deoxygenation at C-2' to give the 2-deoxy-P-glycosides 127 in 70-95% yields (Scheme 29). This method could be extended to 6-deoxysugars giving thus a straightforward access to 2,6-dideoxy-P-glycosides. If necessary, a 2-0-acylpyranosyl bromide can be used in place of the 1,Zdiacetate; Konigs-Knorr conditions are then applied to obtain the P-disaccharide [80]. The only limitation lies in the impossibility of using acylated glycosyl acceptors, inasmuch as regiospecific deoxygenation at C-2' would be problematic. This can be circumvented, however, by the following procedure [78, 791. 1,3,4,6-Tetra-0-acetyl-2-deoxy-2-fo~amido-~-~-glucopyranose 128, which is readily obtained from glucosamine, can be converted to an a-chloride 129 or an atrichloroacetimidate 130. These three glycosyl donors, when properly activated at room temperature (TMS triflate for the acetate 128 and the imidate 130, silver tri131 (Scheme flate for the chloride 129),give 2'-deoxy-2'-formamido-~-disaccharides 29). The excellent stereocontrol of the glycosylation reaction results from the participating property of the N-formyl group (oxazolinium ion).
1. ROH, TMSOTf, - 20°C 2. NaOMe, MeOH 3. NaH, irn., CSz, Me1 4. Bu,SnH. AlBN OR
126
127
Me ROH 1. POC13, NEt3 2. Bu,SnH, AlBN
TMSoTf A AcO c ;
A-li
a
O
A
r.t.
c
~
AcO
128 NHCHo 13' HCONH
OR
AcO
NHCHO
t
132
1ax=cI ROH, AgOTf, r.t. 130 X = OC: NHCCI, ROH, TMSOTf, r.t.
Scheme 29. fi-stereocontrol with 2-0-acetyl or 2-N-formyl groups in the synthesis of 2-deoxy-Pglycosides.
15.6 2-Deoxyglycosyl Donors
393
Q Figure 8. Synthesis of the BCDE tetrasaccharide fragment found in orthosomycins.
When the donor is correctly adjusted to the acceptor, yields are high (64-97%). However, acidic conditions of the glycosylation reaction may induce partial anomerization at the reducing end of the obtained disaccharide. In that case, more reactive N-phthaloylglucosamine donors can be used; a sequential treatment converts then the N-phthaloyl group into the required N-formyl substituent. Radical deamination at C-2' through an isonitrile delivered 2/-deoxy-P-disaccharides 132 in 65-89% overall yields (Scheme 29). This methodology was applied to the synthesis of the B-C-D-E tetrasaccharide fragment found in orthosomycins (Figure 8) [82].* The interglycoside spiroortholactone junction between disaccharide B-C and D-E fragments was constructed by glycosyloxyselenation of the glycal, oxidation at selenium, and syn-elimination of the resulting selenoxide, according to the procedure already described [611.
15.6 2-Deoxyglycosyl Donors In the absence of an heteroatom at C-2 a pyranosyl donor is expected, when activated by a promoter, to give an oxycarbenium intermediate which will be attacked
*After completion of this chapter, this methodology has been used as a key step for the total synthesis of everninomicin 13,384-1 by Nicolaou's group (Angew. Chem. Int. Ed. 1999,38, 3334-3350).
394
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars
-
Montrnorillonite K-10 ~d-0
CH&,
25’C
Me
TBSO
67%
133
134
om
135 @a 69:31
OMe
Montrnorillonite K-10
CHPCI~, 25°C 136
137a:P 89:19
OMe
Scheme 30. Montmorillonite-promoted glycosylations of olivose.
by an alcohol preferentially on its a-face (kinetic anomeric effect) if no impeding steric interaction is present. However, in a few cases (insoluble silver silicate catalyst, anomeric phosphites), a reasonable 0-stereoselectivity can be obtained. 15.6.1 2-Deoxy-Hexopyranoses
It has been recently reported [ 831 that the stereocontrolled glycosylation of olivose (2,6-dideoxy-~-arabino-hexose) by a primary alcohol such as 134, using montmorillonite K-10 as a solid acid promoter as well as a dehydrating agent, leads predominantly to p-glycosides (135) when a 4-0-acetyl group is present (133); a participating effect of that group was postulated [ 851. With a 4-0-tert-butyldimethylsilyl substituent like in 136, a major a-glycosylation leading to 137 is observed as expected from the kinetic anomeric effect (Scheme 30). In his total synthesis of the macrolide avermectin Bla, Ley 1841 reported the in situ activation of an oleandrose unit (2,6-dideoxy-3-0-methyl-~-arabino-hexose) by carbonyldiimidazole or thiocarbonyl-diimidazole to give imidazolylcarbonyl and imidazolyl-thiocarbonyl glycosides respectively which undergo predominant aglycosylation in the presence of silver perchlorate in tetrahydrofuran. 15.6.2 Tert-Butyldimethylsilyl 2-Deoxyglycosides
Tert-Butyldimethylsilyl 2-deoxy-p-glycosides have been used for the glycosylation of primary [86]and secondary [ 111 sugar alcohols in the presence of TMS triflate at low temperatures. 2’-Deoxy-a-disaccharides are obtained in excellent yields. 15.6.3 1-0-Acyl- and Acetimidyl-2-Deoxy-Hexopyranoses
Remarkable selectivity for an attack from the a-side was observed [87a] when the anomeric p-nitro-benzoate of evernitrose 138 was reacted with the sugar alcohol
15.6 2-Deoxyglycosyl Donors
395
Scheme 31. Stereoselective a-glycosylation of the branched-chain nitro sugar evernitrose.
139 in the presence of TMS triflate at low temperature. In spite of a repulsive 1,3syn-axial interaction between the 3’-methyl branch and the anomeric substituent, a 2’-deoxy-a-disaccharide 140 (AB fragment of everninomicin) was exclusively obtained in a 64% yield (Scheme 31). As early as 1979 and as an application of the “imidate procedure”, Sinay [87b] reported the use of anomeric (N-methy1)acetimidates of 2-deoxysugars for the highly stereoselective synthesis of 2’-deoxy-a-disaccharides in excellent yields. 15.6.4 2-Deoxyglycosyl Bromides and Fluorides ~ of an Insoluble silver salts such as silver silicate favor direct S N displacement anomeric bromide by alcohols [ 881. When applied to 2-deoxy-pyranosyl bromides, this procedure can lead to 2‘-deoxy-P-disaccharideswith a good stereoselectivity. For instance (Scheme 32), bromide 141, where an ester group at 0-4 can also favor [85] P-glycoside formation, reacted with the sugar alcohol 142 in the presence of silver silicate to give the disaccharide 143 in 85% yield (P:a 5.4: 1) [89]. 2-Deoxy-a- and P-pyranosyl fluorides react with sugar alcohols in diethyl ether in the presence of TiF4 to give predominantly glycosides of the same anomeric configuration; a fluoride/catalyst/solvent complex is postulated in a “double s~~displacement”. A non-nucleophilic solvent such as hexane can neither form a complex nor stabilize an oxycarbenium ion and, therefore, in such solvents major S N ~ reactions are observed [90]. 2-Deoxyglycosyl fluorides have also been activated in neutral conditions by LiC104-diethyl ether mixtures 1911. Wiesner [92] introduced the concept of 1,3-participation of an axial 3-0substituent (p-methoxy-benzoate or N-methylcarbamate) for the stereoselective A 1,3-acyloxonium or a P-glycosylation of digitoxose, a 2,6-dideoxy-~-riho-hexose. 1,3-bridging iminium ion was proposed as an intermediate species where the a-face of the donor would be blocked during the glycosylation step (Figure 9).
+
++ozB BnO
;horn + / ; p & Silver silicate
~
BZO
BnO
05%
141
Br
142
Scheme 32. Silver silicate-promoted P-glycosylation.
143 @:a5.4:l
O M
396
R
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars
O
or
W
R
O
R"OH w
W
0.
R
O
w
o
R
.
R'COO
R' = pmethoxyphenyl
+o
or NHMe
+ OMe Figure 9. Hypothetical 1,3-~articipationof a p-methoxybenzoate or a carbamate in glycosylation.
However, several attempts [93, 941 to apply this concept with digitoxosyl halides under silver triflate activation (or thioalkyl digitoxosides under mercury salt activation) showed no reliability in enhancing the P :a ratio of 2-deoxyglycosides. 15.6.5 S-(2-Deoxyglycosyl)phosphorodithioates
Michalska [95, 961 introduced S-(2-deoxyglycosyl)phosphorodithioatesfor the synthesis of 2-deoxy-glycosides. 0,O-Dialkylphosphorodithioic acids add to acylated (144) or benzylated glycals in benzene (Scheme 33) to give predominantly crystalline a-adducts (145). Protonation at C-2 occurs from the a-face as it has been seen with Ph3P.HBr, and overall the addition is cis [ 121. Simple alkoxides react upon these a-adducts to give 2-deoxy-P-glycosides ac~ [97]. Activation by thiophilic promoters such as silver cording to a S N mechanism perchlorate in acetonitrile [96], or NIS in dichloromethane, or IDCP in acetonitrile [ 981 leads with alcohols mainly to 2-deoxy-a-glycosides. 2'-Deoxy-a-disaccharides (147) can thus be prepared in good yields, but with a : P ratios better than 9 : 1 only in a few cases (sugar alcohol 146). Anomeric a$-S-phosphorodithioates can also be obtained [99] from glycal epoxide 58. After pivaloylation at 0-2, they can be activated by methyl triflate to give with sugar alcohols exclusively 0-disaccharides in good yields. Selective deoxyge-
-
vc S
S
II
HS P(OEt);!
PhH, r.t.
BZO W
144
Bzd
OEt OEt
$ BzO
s
'
IDCP, CHSCN, 0°C
80%, a:t3 9:l
145
BZO BZO
Scheme 33. 0,O-Diethylphosphorodithioic acid-mediated a-glycosylation of a glycal.
147
15.6 2-Deoxyglycosyl Donors
391
Scheme 34. Generation of anomeric 2-deoxy-a-glycosyl phosphate donors by a radical rearrangement.
nation at 0-2’ as already described [41, 78, 791 would then afford 2’-deoxy-pdisaccharides. 15.6.6 2-Deoxyglycosyl Phosphates, Phosphoramidites and Phosphites An anomeric 2-deoxy-a-glycosyl phosphate 150 is generated by a radical 2-1 migration of the phosphate ester group, which occurs when bromide 148 is irradiated in the presence of tributyltin hydride [loo]. The intermediate anomeric radical 149 (in a boat conformation) rearranges into a C-2 radical in an usual 4C1 (D) chair conformation; an hydrogen atom is then trapped mostly from the less shielded above face (Scheme 34). The unstable phosphate 150 reacts with alcohols in the presence of magnesium perchlorate to give mainly 2-deoxy-a-glycosides 132 in good yields (a: p ratio 2.34.5 : 1). 3,4,6-Tri-O-benzyl-2-deoxy-~-arabino-hexopyranose 151, easily obtained [ 101 from 3,4,6-tri-O-benzyl-~-glucal 29, was converted into an anomeric N,Ndiisopropyl phosphoramidite 152 which, under activation by TMS triflate at low temperatures, reacted with sugar alcohols to give 2’-deoxy disaccharides 153 in high yields and excellent a-stereoselectivity (Scheme 35) [ 1011. 1,2-Cis-2-(p-methoxypheny1thio)-a-D-glycopyranosylphosphoroamidate donors, with an equatorial 2-SAr participating group and the anomeric OP:O(NMe*)Z
EtOP(NiPr,),, CH2C12 HNiPr, - Tetrazole
YEt
BnO
&lo
OH
151
o4
p .
152 a:p1.6:1
OH
BnO NiPr,
- fi 153
CIP(0Et)z
151
ROH, CHzCIZ TMSOTf, -78°C
OR
ROH. Tol. TMSOTf, -94OC
90%
OR
154 a : p 9 4 : 6
OP(OEt)p
127
Scheme 35. 2-Deoxyglycosyl phosphoramidites and diethyl phosphites for respective a- and glycosylations.
p-
398
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars
Scheme 36. 2-Deoxy (2-pyridy1)thioglycosides as a-glycosyl donors
leaving group, have been previously [ 1021 used for the P-glycosylation of alcohols. This variety of anomeric phosphates was claimed [ 1031 to be more stable than other phosphates such as 152. Diethyl phosphite 154, when activated in the same conditions (except toluene as solvent) as 152, gave predominantly 2-deoxy-~-glycosides127 (Scheme 35) [ 1041. The highest P-stereoselectivity was observed with primary alcohols (P :a 9 : 1). The glycosylation reaction was assumed to proceed through the intermediacy of a thermodynamically more stable 2-deoxy-a-~-glucopyranosyl triflate (or its tight a-ion pair). Anomeric a,P-bis(trichloroethy1) phosphites undergo mainly a-glycosylation when activated by boron trifluoride etherate or tin (11) triflate at room temperature [105]. 15.6.7 2-Deoxy Thioglycosides
2-Deoxy-(2-pyridyl)thioglycoside 155, when activated by methyl iodide, reacts smoothly with sugar alcohol 156 to give 2'-deoxy-a-disaccharide 157 in good yield and high stereoselectivity (Scheme 36) [ 1061. Tatsuta [ 1071 reported that highly a-stereoselective glycosylation of phenyl 2deoxy-3,4-0-isopropylidene- 1-thio-D-fucopyranoside 158 could be accomplished because of its rigid and unusual boat conformation. 2-Deoxy-a-glycosides (161 versus 159) were obtained with a much lower selectivity from 2-deoxyfucosyl donors 160 in an usual 4C1 (D) chair conformation (Scheme 37). It was also shown that
0
%
" QNBS
0
T SPh
99% 0 -
158
159 a:P>99:1
A d
A m ! & ,
160
97 idem %
SPh
AcO
AcO&
161 a$ 11:l
\
I
-o
0
Scheme 37. Highly stereoselective glycosylation by conformational assistance.
15.6 d-Deo,~yglycosylDonors
399
selectivity came from conjormational ussistunce, not from a 1,3-diaxial interaction due to the configuration of the 3,4-O-isopropylidene group, and was highly independent of alcohol acceptors. This effect was largely exploited in the 2,6-anhydro-2thio-glycosyl donors as already seen [73, 761. 15.6.8 2-Deoxyglycosyl Sulfoxides The “armed/disarmed concept” takes into account that the electron-donating/ withdrawing power of substituents in carbohydrates has a profound impact on the reactivity of the anomeric center. It can therefore be exploited to construct tri- and larger oligosaccharides in a one-step reaction. In 1993 Kahne [lo81 reported the synthesis of the trisaccharide of ciclamycin 0, an anthracycline antibiotic, in 25% yield by a “polymerization reaction” using two 2-deoxyglycosyl sulfoxides B and C and a phenyl2-deoxy-thioglycoside A as monomers (Figure 10). The reactivities of the sulfoxide donors and nucleophiles were tuned such that the linkage between A and B would occur first (a p-methoxybenzyl sulfoxide B is more reactive than an unsubstituted phenyl sulfoxide C), followed by the linkage to C (a silyl ether AB is a good acceptor when triflic acid is used as a promoter, but reacts more slowly than an unprotected alcohol). Methyl propiolate is used as a sulfenic acid scavenger. The trisaccharide has an anomeric phenyl sulfide on the A ring and can be readily oxidized into a sulfoxide, allowing an iterative strategy for oligosaccharide synthesis. The use of p-methoxybenzyl (PMB) groups in place of benzyl ethers was required for final deprotection of ciclamycin 0, but was found to decrease the yield of the one-step coupling reaction. Therefore, a stepwise approach to the trisaccharide was finally performed with noticeable improvements in glycosylation steps [ 1091. Activation of an anomeric sulfoxide with triflic anhydride releases phenylsulfenyl tri-
TfOH, Et20-CHZCIZ HC&CO,Me -70to-70°C 25%
c
1I
Ciclarnycin 0 b
p
C
0 ’
Figure 10. One-step synthesis of the trisaccharide of ciclamycin 0.
SPh
Me& A
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars
400
Me$sph
TO’
kSPh
-&+Me?g-7 2 ‘:SPh
HO OPMB
AcO
AcO
PhSOTf
- q m l V e n g m J
AcO
OPMB
PhSSPh
Me0 -
m
;
P
Me$sph
h
Me0
b@o OPMB
b&o AcO
-
Me@
OPMB
Me#o AcO
OPMB
OPMB
Figure 11. Formation of a trisaccharide by-product by competitive PhSOTf activation of a phenylthio glycoside.
flate, PhSOTf, which causes problems by activating thiophenyl glycosides even at low temperatures; a trisaccharide resulting from such an activation was identified among by-products (Figure 1 1). It was found that 4-allyl-1,2-dimethoxybenzene could trap efficiently the phenylsulfenyl triflate, although some trisaccharide was still present. A second strategy was then examined: a hindered sulfide (2,6-dichlorophenyl) with decreased nucleophilicity would react more slowly with PhSOTf, providing a better opportunity for the scavenger to operate more efficiently. Finally, an “inverse addition” procedure (the sulfoxide is slowly added to the mixture of acceptor and promoter) minimizes the formation of anomeric sulfenates. With these modifications disaccharide A-B, then trisaccharide A-B-C could be obtained in 82 and 68% yields respectively. Oxidation of trisaccharide A-B-C with 2,2-dimethyldioxirane gave a sulfoxide (900/,)which was coupled to the aglycone of ciclamycin 0 under the modified conditions in 75% yield (Figure 10). After removal of the p-methoxybenzyl ethers with DDQ the ciclamycin 0 was obtained in a 17% overall yield and with only six steps from the starting monomers.
15.7 Other Approaches to 2-Deoxyglycosides All the approaches which have been described so far involved glycosylation reactions, that is to say the creation of an exocyclic C-1-0 bond on the pyranose ring.
15.7 Other Approaches to 2-Deoxyglycosides
162
I
1. PhSeCl 2. Bu3SnH
401
%& OR
127
Scheme 38. Electrophile-mediated intramolecular cyclization of an acyclic enol ether.
However, it should be also possible to construct a 2-deoxy-glycoside by formation of the endocyclic C-1-0 bond from an acyclic sugar, or the C-1-H-1 bond from an anomeric radical. 15.7.1 Cyclization of Acyclic Sugars Acyclic sugars obtained by Wittig reactions. Acyclic enol ethers 163 obtained by Wittig reaction upon 2,3,5-tri-O-benzyl- D-arabinofuranose 162 undergo intramolecular cyclizations when they are treated by an electrophile. N-Iodosuccinimide gives predominantly 2-deoxy-2-iodo-a-~-mannopyranosides which can be deiodinated to give 2-deoxy-a-glycosides 153. The reaction of phenylselenenyl chloride with 163, followed by radical deselenation, gives mainly the 2-deoxy-P-glycosides 127 (Scheme 38) [110, 1111. Redox glycosylation via thionoester intermediates. Barrett [ 112, 1131 introduced the concept of “redox glycosylation”, where the usual glycosylation reaction is replaced by the esterification of an aldonic acid 164, followed by reduction to a reactive intermediate (enol ether from an ester or 0,s-acetal 166 from the thionoester 165 obtained by treatment of the ester by the Lawesson’s reagent). Cyclization of 166 by silver tetrafluoroborate gives the 2-deoxyglycoside 167 in good yield (Scheme 39).
1. ROH, DCC 2. Lawesson’s reagent
165
OL.OR S
OMe 167
Scheme 39. Redox glycosylation via thionoester intermediates.
166
II
A m 4
collidine
402
15 Special Problems in Glycosylation Reactions: 2-Deoxy Sugars 1. Lawesson's reagent 2.286-Di-t-butyl-4-methy~En~+ Mel. MeOH, 40°C
Eno+
pyridine
sMe-
BnO 168
169
' 0
81 %
En0
om
OMf
170 p:a6:1
Scheme 40. Alkoxy-substituted anomeric radicals for the construction of 2-deoxy-P-glycosides.
15.7.2 Use of Alkoxy-Substituted Anomeric Radicals The hemithio orthoester 169 was prepared from lactone 168 by treatment with Lawesson's reagent, then reaction with methyl iodide and methanol. Radical desulfurization of 169 gives predominantly a 2-deoxy-P-glycoside 170 by transfer of hydrogen to an intermediate methoxy-substituted anomeric radical preferentially from below (Scheme 40). P-Selectivity is, however, enhanced when an alkoxy substituent is present at C-2 [ 1141. Crich [ 115-1 171 generated anomeric alkoxy-substituted radicals by irradiation of "Barton esters" (0-acyl thiohydroxamate 176) in the presence of a thiol. Conversion of a protected D-galactall71 into the anomeric sulfone 172, then into the thiophenyl glycoside of a methyl 2-ulosonate 173, was followed by a classical 0-glycosylation (alcohols or phenols). The carboxylic acid 174 was coupled with the salt 175 to give the 0-acyl thiohydroxamate 176. Its photolysis in the presence of a thiol generated an anomeric radical by decarboxylation; hydrogen transfer took place stereoselec177 were tively from below as for compound 169 and 2-deoxy-~-galactopyranosides predominantly produced (Scheme 41).
RO
- -
1. HCI, tol. 2. PhSH, Et(iPr),N OR 3. MCPBA. NaHC03
1 . Buti, O:C(OMe),
2.U Naphth., (PhS)2
RO
171
172
RO
OR'
+-
177
R = tBuMe,Si
Scheme 41. Reductive decarboxylation of 3-deoxy-ulosonic acid glycosides
I I SPh
179
1 . R'OH, HgCI, or NBS
R'SH CHZC12 hv, 5°C
RO
CO2Me
RO
References
403
References 1. A.C. Weymouth-Wilson, Nut. Prod. Rep., 1997, 14, 99-110. 2. R.W. Franck, S.M. Weinreb in Studies in Natural Products Chemistry (Ed.: Atta-UrRahman), Elsevier, Amsterdam, 1989, p. 173. 3. J. Thiem, W. Klaffke, Top. Curr. Chem., 1990, 154, 285-332. 4. K. Toshima, K. Tatsuta, Chem. Reo., 1993, 93, 1503-1531. 5. A. Kirschning, A.F.-W. Bechtold, J. Rohr, Top. Curr. Chem., 1997, 188, 1-84. 6. A.J. Kirby, The Anomeric Effect and Related Stereoelectronic Effects at Oxygen, Springer Verlag, New York, 1983. 7. P. Deslongchamps, Stereoelectronic Effects in Organic Chemistry, Pergamon Press, Oxford, 1983. 8. R.W. Franck in Conformational Analysis and Stereochemistry of Six-membered Rings (Ed.: E. Juaristi), VCH Publishers, Inc., New York, 1995, Chapter 5 , p. 157-198. 9. V. Bolitt, C. Mioskowski, S.-G. Lee, J.R. Falck, J. Org. Chem., 1990, 55, 5812-5813. 10. S. Sabesan, S. Neira, J. Org. Chem., 1991, 56, 5468-5472. 11. C. Kolar, G. Kneissl, Angew. Chem. Znt. Ed. Engl., 1990, 29, 809-81 1. 12. N. Kaila, M. Blumenstein, H. Bielawska, R.W. Franck, J. Org. Chem., 1992, 57, 4576-4578. 13. R.W. Franck, N. Kaila, M. Blumenstein, A. Geer, X.L. Huang, J.J. Dannenberg, J. Org. Chem., 1993,58, 5335-5337. 14. J. Lehmann, E. Schroter, Carbohydr. Res., 1972, 23, 359-368. 15. J. Lehmann, B. Zieger, Carbohydr. Rex, 1977, 58, 73-78. 16. O.M. Viratelle, J. Yon, Biochemistry, 1980, 19, 4143-4149. 17. J.-M. Petit, F. Paquet, J.-M. Beau, Tetrahedron Lett., 1991, 32, 6125-6128. 18. K. Igarashi, T. Honma, T. Imagawa, J. Org. Chem., 1970,35, 610-616. 19. P. Boullanger, G. Descotes, Carbohydr. Res., 1976, 51, 55-63. 20. D. Horton, W. Priebe, 0. Varela, J. Org. Chem., 1986, 51, 3479-3485. 21. G. Bellucci, C. Chiappe, F. D’Andrea, G. Lo Moro, Tetrahedron, 1997, 53, 3417-3424. 22. K. Tatsuta, K. Fujimoto, M. Kinoshita, Carbohydr. Res., 1977, 54, 85-104. 23. R.U. Lemieux, A.R. Morgan, Can. J. Chem., 1965,43,2205-2213. 24. C.L. Perrin, M.A. Fabian, J. Brunckova, B.K. Ohta, J. Am. Chem. Soc., 1999, 121, 69116918 and references therein. 25. a) R.U. Lemieux, S. Levine, Can. J. Chem., 1964, 42, 1473-1480; b) R.U. Lemieux, A.R. Morgan, Can. J. Chem., 1965,43, 2190-2198. 26. J. Thiem, H. Karl, J. Schwentner, Synthesis, 1978, 696-698. 27. D. Horton, W. Priebe, M. Sznaidman, Carbohydr. Rex, 1990, 205, 71-86. 28. D.P. Curran, Y.G. Suh, Carbohydr. Rex, 1987, 171, 161-191. 29. J. Thiem, P. Ossowski, J. Carbohydr. Chem., 1984, 3, 287-313. 30. J. Thiem, S. Kopper, J. Schwentner, Liebigs Ann. Chem., 1985, 2135-2150. 31. J. Thiem, W. Klaffke, J. Org. Chem., 1989, 54, 2006-2009. 32. R.W. Friesen, S.J. Danishefsky, J. Am. Chem. Soc., 1989, 111, 6656-6660. 33. K. Suzuki, G.A. Sulikowski, R.W. Friesen, S.J. Danishefsky, J. Am. Chem. Soc., 1990, 112, 8895-8902. 34. S.J. Danishefsky, M.T. Bilodeau, Angew. Chem. Int. Ed. Engl., 1996,35, 1380-1419. 35. W.R. Roush, K. Briner, D.P. Sebesta, Synlett, 1993, 264-266. 36. W.R. Roush, R.A. Hartz, D.J. Gustin, J. Am. Chem. Soc., 1999, 121, 1990-1991. 31. W.R. Roush, C.E. Bennett, J. Am. Chem. Soc., 1999,121, 3541-3542. 38. D. Tailler, J.-C. Jacquinet, A.-M. Noirot, J.-M. Beau, J. Chem. Soc., Perkin Trans. I , 1992, 3 163-3 164. 39. R.L. Halcomb, S.J. Danishefsky, J. Am. Chem. Soc., 1989, 111, 6661-6666. 40. G. Bellucci, G. Catelani, C. Chiappe, F. D’Andrea, Tetrahedron Lett., 1994, 35, 8433-8436. 41. J. Gervay, S.J. Danishefsky, J. Org. Chem., 1991, 56, 5448-5451. 42. K.K.-C. Liu, S.J. Danishefsky, J. Org. Chem., 1994, 59, 1895-1897. 43. D.M. Gordon, S.J. Danishefsky, Carbohydr. Rex, 1990,206, 361-366. 44. R.G. Dushin, S.J. Danishefsky, J. Am. Chem. Soc., 1992, 114, 3472-3475.
404
15 Special Problems in Glycosylution Reactions: 2-Deoxy Sugars
C. Chiappe, G. Lo Moro, P. Munforte, Tetrahedron, 1997, 53, 10471-10478. R. Preuss, R.R. Schmidt, Synthesis, 1988, 694-697. W.R. Roush, D.P. Sebesta, C.E. Bennett, Tetrahedron, 1997,53, 882558836, G. Grewal, N. Kaila, R.W. Franck, J. Org. Chem., 1992, 57, 2084-2092. W.R. Roush, X.-F. Lin, J. Am. Chem. Soc., 1995, 117, 2236-2250. W.R. Roush, X.-F. Lin, J. Org. Chem., 1991, 56, 5740-5742. W.R. Roush, D.P. Sebesta, R.A. James, Tetrahedron, 1997, 53, 8837-8852. D.K. Jones, D.C. Liotta, Tetrahedron Lett., 1993, 34, 7209-7212. T.J. Dudley, I.P. Smoliakova, M.R. Hoffmann, J. Org. Chem., 1999, 64, 1247-1253. R.W. Franck, N. Kaila, Carbohydr. Rex, 1993,239, 71-83. Y. Ito, T. Ogawa, Tetrahedron Lett., 1987, 28, 2723-2726. G. Jaurand, J.-M. Beau, P. Sinay, J. Chem. SOC.,Chem. Commun., 1981, 572-573. M. Perez, J.-M. Beau, Tetrahedron Lett., 1989, 30, 75-78. A. Kaye, S. Neidle, C.B. Reese, Tetrahedron Lett., 1988,29, 2711-2714. A. El-Laghdach, M.I. Matheu, S. Castillon, Tetrahedron, 1994, 50, 12219-12234. Y. Diaz, A. El-Laghdach, S. Castillon, Tetrahedron, 1997,53, 10921-10938. G. Jaurand, J.-M. Beau, P. Sinay, J. Chem. Soc., Chem. Commun., 1982, 701-703. Y. Leblanc, B.J. Fitzsimmons, J.P. Springer, J. Rokach, J. Am. Chem. Soc., 1989, I l l , 29953000. 63. R. W. Franck, C.H. Marzabadi, J. Org. Chem., 1998, 63, 2197-2208 and references therein. 64. A. Dios, A. Geer, C.H. Marzabadi, R.W. Franck, J. Org. Chem., 1998,63, 6673-6679. 65. A. Dios, C. Nativi, G. Capozzi, R.W. Franck, Eur. J. Org. Chem., 1999, 1869-1874. 66. S.P. Vincent, M.D. Burkart, C.-Y. Tsai, Z. Zhang, C.-H. Wong, J. Org. Chem., 1999, 64: 5264-5279, and references therein. 67. K. Bock, C. Pedersen, J. Thiem, Carbohydr. Rex, 1979, 73, 85-91. 68. J. Thiem, M. Gerken, B. Schottmer, J. Weigand, Carbohydr. Res., 1987, 164, 327-341. 69. K.C. Nicolaou, T. Ladduwahetty, J.L. Randall, A. Chucholowski, J. Am. Chem. Soc., 1986, 108, 2466-2467. 70. K.C. Nicolaou, C.W. Hummel, N.J. Bockovich, C.-H. Wong, J. Chem. Soc., Chem. Commun., 1991, 870- 872. 71. K.C. Nicolaou, R.M. Rodriguez, H.J. Mitchell, F.L. van Delft, Angew. Chem. Znt. Ed., 1998, 37, 1874-1876. 72. H.M. Zuurmond, P.A.M. van der Klein, G.A. van der Marel, J.H. van Boom, Tetrahedron, 1993,49, 6501-6514. 73. K. Toshima, S. Mukaiyama, Y. Nozaki, H. Inokuchi, M. Nakata, K. Tatsuta, J. Am. Chem. SOC.,1994, 116, 9042-9051 and references therein. 74. J. Kocourek, Carbohydr. Rex, 1967, 3, 502-505. 75. A.B. Foster, J.M. Duxbury, T.D. Inch, J.M. Webber, Chem. Commun., 1967, 881-883. 76. K. Toshima, Y. Nozaki, S. Mukaiyama, T. Tamai, M. Nakata, K. Tatsuta, M. Kinoshita, J. Am. Chem. Soc., 1995,117, 3717-3727. 77. K. Toshima, T. Yoshida, S. Mukaiyama, K. Tatsuta, Carbohydr. Res., 1991, 222, 173188. 78. M. Trumtel, A. Veyrieres, P. Sinay, Tetrahedron Lett., 1989, 30, 2529-2532. 79. M. Trumtel, P. Tavecchia, A. Veyrieres, P. Sinay, Carbohydr. Rex, 1989, 191, 29-52. 80. C. Zagar, H.-D. Scharf, Carbohydr. Rex, 1993, 248, 107-118. 81. P. Tavecchia, M. Trumtel, A. Veyrieres, P. Sinay, Tetrahedron Lett., 1989, 30, 2533-2536. 82. M. Trumtel, P. Tavecchia, A. Veyrieres, P. Sinay, Carbohydr. Rex, 1990,202, 257-275. 83. T. Jyojima, N. Miyamoto, Y. Ogawa, S. Matsumura, K. Toshima, Tetrahedron Lett., 1999, 40, 5023-5026. 84. S.V. Ley, A. Armstrong, D. Diez-Martin, M. J. Ford, P. Grice, J.G. Knight, H.C. Kolb, A. Madin, C.A. Marby, S. Mukherjee, A.N. Shaw, A.M.Z. Slawin, S. Vile, A.D. White, D.J. Williams, M. Woods, J. Chem. Soc., Perkin Trans. I , 1991, 667-692. 85. C.A.A. van Boeckel, T. Beetz, S.F. van Aelst, Tetrahedron, 1984, 40, 4097-4107. 86. E. Petrakova, C.P.J. Glaudemans, Carbohydr. Res., 1995, 268, 35-46. 87a. P. Jutten, H.-D. Scharf, G. Raabe, J. Org. Chem., 1991, 56, 7144-7149. 87b. J. Szymoniak, P. Sinay, Tetrahedron Lett., 1979, 6, 545-548. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62.
References
405
88. a) H. Paulsen, W. Kutschker, 0. Lockhoff, Chem. Ber., 1981, 114, 3233-3241; b) H. Paulsen, 0. Lockhoff, Chem. Ber., 1981, 114, 3102-3114. 89. a) R.W. Binkley, D. J. Koholic, J. Org. Chem. 1989, 54, 3577-3581; b) R.W. Binkley, M.R. Sivik, J. Carbohydr. Chem., 1991, 10, 399 -416. 90. J. Jiinnemann, I. Lundt, J. Thiem, Liebiys Ann. Chem., 1991, 759-764. 91. H. Schene, H. Waldmann, Synthesis, 1999, 1411-1422. 92. K. Wiesner, T.Y.R. Tsai, H. Jin, Helu. Chzm. Acta, 1985, 68, 300-314. 93. R.W. Binkley, D.J. Koholic, J. Carhohydr. Chem., 1988, 7, 487-499. 94. J. Thiem, S. Kopper, Tetrahedron, 1990, 46, 113-138. 95. J. Borowiecka, P. Lipka, M. Michalska, Tetrahedron, 1988, 44, 2067-2076. 96. H. Bielawska, M. Michalska, Tetrahedron Lett., 1998, 39, 9761-9764. 97. M. Michalska, J. Borowiecka, J. Carhohydr. Chem., 1983, 2, 99-103. 98. L. Laupichler, H. Sajus, J. Thiem, Synthesis, 1992, 1133-1136. 99. O.J. Plante, P.H. Seeberger, J. Org. Chem., 1998, 63, 9150-9151. 100. A. Koch, C. Lamberth, F. Wetterich, B. Giese, J. Org. Chem. 1993, 58, 1083-1089. 101. H. Li, M. Chen, K. Zhao, Tetrahedron Lett., 1997,38, 6143-6144. 102. S. Hashimoto, Y. Yanagiya, T. Honda, S. Ikegami, Chem. Lett., 1992, 1511-1514. 103. S. Hashimoto, Y. Yanagiya, T. Honda, H. Harada, S. Ikegami, Tetrahedron Lett., 1992, 33, 3523-3526. 104. S. Hashimoto, A. Sano, H. Sakamoto, M. Nakdjima, Y. Yanagiya, S. Ikegami, Synlett, 1995, I27 1-1273. 105. T. MuIler, R. Schneider, R.R. Schmidt, Tetrahedron Lett., 1994, 35, 4763-4766. 106. D. Ravi, V.R. Kulkarni, H.B. Mereyala, Tetrahedron Lett., 1989, 30, 4287-4290. 107. K. Toshima, Y. Nozaki, K. Tatsuta, Tetruhedron Lett., 1991, 32, 6887-6890. 108. S. Raghavan, D. Kahne, J. Am. Chem. Soc., 1993, 115, 1580-1581. 109. J. Gildersleeve, A. Smith, K. Sakurai, S. Raghavan, D. Kahne, J. Am. Chem. Soc., 1999,121, 6176-61 82. 110. K. Suzuki, T. Mukaiyama, Chem. Lett., 1982,683-686. 111. K. Suzuki, T. Mukaiyama, Chem. Lett., 1982, 1525-1528. 112. A.G.M. Barrett, B.C.B. Bezuidenhoudt, A.R. Howell, A.C. Lee, M.A. Russell, J. Org. Chem., 1989,54, 2275- 2277. 113. A.G.M. Barrett, B.C.B. Bezuidenhoudt, L.M. Melcher, J. Org. Chem., 1990,55, 5196-5197. 114. D. Kahne, D. Yang, J.J. Lim, R. Miller, E. Paguaga, J. Am. Chem. Soc., 1988, 110, 87168717. 115. D. Crich, T.J. Ritchie, J. Chem. Soc., Chem. Commun., 1988, 1461-1463. 116. D. Crich, T.J. Ritchie, Carbohydr. Rex, 1989, 190, C3-C6. 117. D. Crich, T.J. Ritchie, J. Chem. Soc., Perkin Trans. I , 1990, 945-954.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
16 Orthogonal Strategy in Oligosaccharide Synthesis Osamu Kan ie
In the pursuit of efficient chemical synthesis of oligosaccharides, important progress has been made. Landmarks include chemoselective glycosylations and polymersupported oligosaccharide synthesis. Although polymer-supported synthesis is advantageous in respect of the isolation process, it needs to be tuned to be a more realistic prospect, especially in oligosaccharide synthesis. Also, because the final product is released along with deletion byproducts, delicate purification manipulations, e.g. by HPLC must usually be performed at the final stage. The advantage of the chemoselective activation strategy is recognized as its potential that no manipulations are required before the coupling reactions because the glycosyl acceptor has a pre-installed leaving group which also protects the anomeric center. The number of available protecting groups and chemoselective glycosylation conditions, however, limit the chemoselective sequential glycosylation strategy. This chapter addresses the problem of the chemoselective activation strategy and describes the successful introduction of a novel coupling strategy-an orthogonal glycosylation strategy in which two chemically distinct glycosylation reactions are utilized-and its application to polymer-supported oligosaccharide synthesis.
16.1 Introduction Much effort has been devoted to identification of the functions of the oligosaccharides found on the cell surface, where they covalently attached to a variety of organic molecules such as proteins and lipids. Because of the diversity of oligosaccharide structures, they are often described as the ‘finger print’ of a certain cell. The possibility that the complex structures are directly related to their functions has prompted many synthetic chemists to synthesize glycoconjugates. During the last two decades the technology for the synthesis of these structures has, therefore, improved dramatically. The key reaction in the construction of oligo-
408
16 Orthogonal Strategy in Oligosaccharide Synthesis
saccharides is, of course, the glycosylation reaction, in which stereo- and regiospecificities must be carefully controlled. There are several ways of controlling the stereochemistry of glycosidic bond formation: e.g.: 1) 1,Ztrans glycoside formation by means of a C-2 participating group [ l , 21; 2) axial glycoside formation resulting from the anomeric effect [3]; 3) equatorial glycoside formation using solvation of the carbonium cation by acetonitrile [4]; 4) axial glycoside formation by SN2 displacement of P-halides formed by halide equilibration in the presence of quaternary ammonium halide [ 51; 5) P-mannosylation by means of insoluble silver salts [6]; and 6) P-mannosylation by intramolecular aglycone delivery [71. The generalized scheme for the synthesis of oligosaccharides is shown in Figure 1 as an example of complex oligosaccharide synthesis. One must decide which method (1-6) should be applied for each coupling step before the synthesis. Also, a protecting group must be removed after each glycosylation step to release the hydroxyl function required for the next glycosylation reaction.
16.2 Analysis of the Strategic Aspects of Oligosaccharide Synthesis 16.2.1 General Aspects
To achieve efficient synthesis, both stepwise and convergent methods must be employed. A Stepwise method in which synthesis starts from the reducing end has classically been used for oligosaccharide synthesis because of the difficulty of transforming an anomeric protecting group into a leaving group. The recent development of anomeric protecting groups has enabled alternative synthesis which starts from the non-reducing end and a convergent synthesis (Figure 2). These anomeric protecting groups include the allyl, methoxyphenyl (MP), npentenyl, 2-trimethylsilylethyl (SE), t-butyldimethylsilyl, and t-butyldiphenylsilyl groups. Removal of this class of protecting groups releases the anomeric hydroxyl function to be converted into leaving groups [8].
16.2.2 The Pursuit of Efficiency in Oligosaccharide Synthesis
The other significant contribution in oligosaccharide synthesis (see Figure 3) was the introduction of ‘stable’ leaving groups which can be used as protecting groups until exposed to certain activation conditions. The thioglycosides are typical examples of this class of leaving group. Thioglycosides can be activated under alkylating
16.2 Analysis of the Strategic Aspects of Oligosaccharide Synthesis
409
Suitably protected sugar [A]
1
Anomeric deprotection
1
GI~COSYI
Activation
acceptor
Glvcosvl donor
p2d (NP) Sugar [A-B]
J Figure 1. Generalized oligosaccharide synthesis. Iterative reactions consisting of protection, deprotection, and glycosylation reaction are performed to obtain oligosaccharides. L = leaving group; P = protecting group.
or oxidative conditions but are stable under the traditional glycosylation conditions such as the Koenigs-Knorr method, so that halides can be chemoselectively activated without affecting them (Figure 3C). In addition, they can be transformed into halides or other leaving groups (Figure 3B). For this reason, thioglycosides are ideal intermediates in a flexible synthetic strategy [9]. The possibility of chemoselective activation of anomeric leaving groups has thus emerged [9-111. The armed and disarmed concept (Figure 3C), which employs a single potential leaving group, n-pentenyl glycosides, for both the donor and acceptor has been developed on the basis of the observation that the reactivity of glycosyl donors is
410
16 Orthogonal Strategy in Oligosaccharide Synthesis B
A
reducing end
m]
C
non-reducingend
product
: glyoosyl donor
Figure 2. Two stepwise syntheses (A and C ) and block condensation (B). One of the stepwise methods in which synthesis starts from the reducing end (A) has classically been used for oligosaccharide synthesis. The recent development of anomeric protecting groups has enabled alternative synthesis starting from the non-reducing end (C) and a convergent one (B).
affected by the protecting groups (i.e. ether or ester) especially at 0-2 [12a]. The utility of n-pentenyl glycosides is obvious, because small fragments of oligosaccharides can be synthesized in a very efficient manner and the n-pentenyl group can be temporarily protected as the dibromide to be used as the acceptor for block condensations. The armed and disarmed concept has proven to be applicable also to glycals [ 12b], thioglycosides [ 12~1,selenyl glycosides [ 12d], and glycosyl phosphoroamidates [ 11, 12el.
16.3 The Introduction of the Orthogonal Glycosylation Strategy 16.3.1 Limitation of Current Concepts
The advantage of the chemoselective activation strategy lies in its efficiency, because the glycosyl acceptor has a potential leaving group which also temporarily protects the anomeric center. Therefore, in principle, there is no need for deprotection or protecting group manipulations before the coupling reactions. Because the methodology relies on the relative reactivities of donor molecules, and these are controlled by the protecting groups and/or different leaving groups available, one must very carefully select the combination of leaving groups to be used, and thus this naturally imposes a limitation on this chemoselective sequential glycosylation strategy. Therefore, a different methodology which does not rely on the different reactivities of molecules is needed to reduce the complexity of the coupling process. An idea of such methodology can be derived by comparing the reactivities of glycosy1 halides and thioglycosides as an example (Figure 4).Although their reactivities are different in each group (A),there is a possibility of selective activation of halides and thioglycosides (B) in both directions, which might open a new coupling strategy.
16.3 The Introduction of the Orthogonal Glycosylution Strategy
A '$-L
41 1
B
, H O e P
O
Reactivity of leaving groups L' > L2> L3> L"
L
H O O P
Reactivity of leaving groups L1> L2(conditions A) L2> L1(conditions B)
P O G L H O G P
F
t P P O G L HO-P
O
*
G
P
t e
L
2T M S O G P
pod.,, Figure 3. Schematic representation of stepwise synthesis. (For A-D the direction of elongation is non-reducing end + reducing end and for E and F it is reducing end 3 non-reducing end. A: Most traditional method in this series (A-D) involving glycosylation reaction, deprotection of anomeric protecting group, and activation of the anomeric hydroxyl group into a donor. B: This method eliminates a deprotection step but involves direct transformation of the anomeric protecting group into a leaving group. C: This method utilizes a leaving group as the protecting group and so the number of reaction steps is minimized. This group includes the armed-disarmed and chemoselective glycosylations. D: This method also features the minimum number of steps in the synthetic scheme. It does not, however, rely on the hierarchy of the reactivities but on orthogonality and, therefore, hypothetically, eliminates the limitation of the number of coupling reactions. E: Most traditional method involving glycosylation reaction and deprotection to release the hydroxyl group for the next coupling reaction. F: This method utilizes a carefully selected combination of leaving groups and hydroxyl protecting groups, which enables the shortest, one-pot, synthetic scheme.
412
16 Orthogonal Strategy in Oligosaccharide Synthesis
labile
4
stable
silver salts
Br
l
CI
x n
5 oddative m conditions
'-Me
'\Ph
Figure 4. Comparison of the reactivities of glycosyl halides and thioglycosides as examples. Although their reactivities are different in each group (A) there is a possibility of selective activation of halides and thioglycosides (B) in both directions, which might lead to a new coupling strategy.
16.3.2 The Orthogonal Coupling Concept
The minimum requirement for this scheme would be to combine two chemically distinct glycosylation reactions in which one of the leaving groups is activated while the other behaves as a protecting group, and vice uersa (Figures 3D, 4 and 5). To fulfil the requirement for this orthogonal system, each selected leaving group should be unaffected under the conditions used to activate the other. Also, both leaving groups should tolerate routine manipulations of temporary protecting groups. For this orthogonal strategy, a set of leaving groups and activation conditions for each group-L', phenylthio group and L2, fluoride, and (a), NIS-TfOH (or AgOTf) [ 131 and (b),Cp2HfClz-AgC104 [ 141-are selected as candidates.
Figure 5. A scheme for orthogonal glycosylation. The combined use of two chemically distinct glycosylation reactions, where one of the leaving groups is activated while the other behaves as a protecting group and vice versa, enables continuous coupling without any transformations. L = leaving group; P = protecting group; a = activation condition for L1; b = activation condition for L2. N.B. L2 is stable under condition a and L' is stable under condition b.
16.3 The Introduction o j the Orthogonal Glycosylation Strategy
41 3
Figure 6. Manipulations of orthogonal protecting groups. Orthogonal protecting groups are “a set of completely independent classes of protection groups, such that each class can be removed in any order and in the presence of all other classes” as defined by Baranay and Merrifield. X, Y, Z = functional groups; A, B, C = orthogonal protecting groups; a, b, c = orthogonal deprotection conditions.
16.3.3 What is Orthogonality Anyway? The term ‘orthogonality’ is frequently used in a generalization of its mathematical meaning to describe sets of primitives or capabilities that span the entire ‘capability space’ of the system and are non-overlapping or mutually independent. The term ‘orthogonal system’ in chemistry was first defined by Baranay and Merrifield as “a set of completely independent classes of protection groups, such that each class can be removed in any order and in the presence of all other classes” (Figure 6) [ 151. Manipulations of orthogonal protecting groups are widely accepted in peptide chemistry and can also be seen in other fields [16]. Despite the importance of the idea in the synthesis of polymeric materials such as peptides, oligonucleotides, and oligosaccharides, the term is used to describe the independence of a set of protecting groups during deprotection, and is not used to describe condensation reactions. The synthesis does, therefore, require multiple steps before the coupling reactions. To reduce the number of synthetic steps, the challenge is to apply the idea of orthogonality to the reactive site of coupling. In this system, each synthon which is involved in the coupling reactions must have a functional group with orthogonal behavior in the coupling reactions, e.g. inertness and reactiveness under the given conditions. The basis of the orthogonal condensation system is the combined use of individual reactions which enable the synthesis of polymeric compounds without deprotection or activation steps, as explained above. It should, furthermore, be
414
16 Orthogonal Strategy in Oligosaccharide Synthesis
noted that the number of conditions to be employed is not limited in a system as long as the conditions are orthogonal to each other. 16.3.4 Orthogonal Glycosylation and Solid-Phase Oligosaccharide Synthesis Such systems are effectively suited to solid-phase oligosaccharide synthesis [ 171. Although the main advantage of solid-phase synthesis is the elimination of tedious purification processes, one drawback of this advantage is that each reaction conducted on the support must give nearly quantitative yield or the unreacted reaction center must be capped (again, the capping must be quantitative). These are required to ensure elimination of the possibility that there are deletion compounds in the mixture of compounds obtained. When a mixture that consists only of product and precursors for each step is given after several reaction cycles on a solid support, they can be separated by conventional purification protocols such as HPLC. If, on the other hand, the final mixture contains compounds in which some of the synthons are randomly deleted, isolation will not be a practical proposition. To avoid this problem, we usually give the above two answers; when, however, the orthogonal glycosylation strategy is applied to the solid-phase synthesis, these problems can be solved simultaneously (Figure 7), because (i) the hydrolyzed byproduct cannot be a candidate for the glycosyl donor in the next coupling cycle [17d], and (ii) the activation step is no longer necessary because the leaving group is incorporated as the protecting group for the previous reaction. Thus, in this system, one of the leaving groups is activated in the presence of hemiacetals and the other leaving group. There is, therefore, no need for capping. High yielding coupling reactions are, of course, needed but it is not necessary if one can efficiently extract the desired product from a product mixture. A tag might be incorporated at the end of the coupling reactions, because the only compound that has leaving group is the desired compound and others are presumably hydrolysis or elimination by-products formed at some stage in the orthogonal glycosylation strategy. Such tags include hydrophobic aglycones; these have proved to be extremely useful for the rapid solid-phase extraction of oligosaccharides in the assay system known as the SepPak assay [18].
16.4 The Orthogonal Glycosylation Strategy 16.4.1 Orthogonal Chain Elongation of Homo-Oligosaccharides: Synthesis of Chito-Oligosaccharides 1191 To assess the orthogonality of the above-mentioned combination of reactions, the interconversion of the two anomeric potential leaving groups phenylthio and fluoro group was performed. Thiophenyl glycoside 1 was treated under Nicolaou’s conditions [20] (u) to yield the glycosyl fluoride 2 quantitatively. The fluoride 2 could,
16.4 The Orthogonal Glycosylution Strutegy
415
Orthogonal condensation
PO Sugar [2] Polymer
-
PO Sugar [ l ] PO Sugar [3]
&O
Polymer-
Activate Y
PO
Sugar [l+]-Tag deprotection
L L O HOW*
Ho
H0-1-
L o
Ho
OX/* HO
HO
O
L o O HO w* HO
Sugar [l-n]-Tag HO etc
Figure 7. The overall idea of solid-phase oligosaccharide synthesis on the basis of the orthogonal glycosylation strategy. The orthogonal glycosylation strategy can be used in solid-phase oligosaccharide synthesis and can accelerate the process by dramatically reducing the number of reaction steps. A hydrophobic tag can be incorporated at the end of the synthesis and this is used to extract the desired compound from the accumulated by-products.
16 Orthogonal Strategy in Oligosaccharide Synthesis
416
-
OAc
.OAc
a quant
AAGO c O s S pSPh h NPhth 1
F NPhth
A&& AcO* AcO
b
2
93%
Scheme 1. An interconversion of potential donors. Reagents and conditions: (a) SnC12-AgC104PhSH in CH2C12, room temp.; (b) NBS-DAST in CHZC12, room temp.
on the other hand, be activated under Mukaiyama's conditions [21] (b)in the presence of thiophenol to give the thioglycoside 1 in 93% yield (Scheme 1). This successful interconversion reaction prompted us to investigate further. Using this orthogonal set of reactions, a set of glycosylation reactions was investigated using 1 and 2 as donors and 3 and 4 as acceptors. It should be noted here that because the selected anomeric functional groups are orthogonal, all the compounds used were synthesized from a single compound. The thioglycoside 1 was reacted with the acceptor 3 under conditions (a)to afford disaccharide 5 (90%). The fluoride 2 was also successfully activated under conditions (b) and reacted with 4, without affecting the thioglycosidic linkage, to give disaccharide 6 (78%) (Scheme 2). Self-condensed product was not detected in either sequence. These results indicate that less reactive acyl protected donors were activated in preference to the potentially more reactive ether-protected acceptors; the reactions are therefore clearly distinguishable from armed-disarmed reactions. The chosen set of reactions is therefore shown to be orthogonal; chemoselective in both directions. To show the applicability of this strategy to the synthesis of longer-chain oligosaccharides, iterative couplings were further examined by constructing the heptasaccharide 12 in which ail synthons were derived from 1. Firstly, thioglycoside donor 7 was coupled with acceptor fluoride 3 under conditions (a) to give the disaccharide fluoride 8 (85%) which was then reacted with the acceptor 4 to produce 9
Ester protected donors Ether protected acceptors
AcO AcO&Sph
NPhth
+
1
i$&&F
NPhth
8_ 90%
Ap*CO-og BnO NPhth
3
NPhth F
5
NPhth
AcO 2
+
! f o a SNPhth p h 4
&
A
NPhth BnO
NPhth ~ SPh
6
Scheme 2. A set of orthogonal reactions showing complete distinctness of the reactions used. Reagents and conditions: (a) NIS (1.3 equiv.)-AgOTf (0.1 equiv.) in CH2C12, -50 "C + room temp.; (b) Cp2HfClz (1.3 equiv.)-AgC104 (2.6 equiv.) in CH2C12, -78 "C .+ room temp.; (c) NaOMe.
16.4 The Orthogonal Glycosylution Strategy
417
[conditions (b), 72%]. Subsequent reaction of 9 with 3 [conditions (a), 65%] gave tetrasaccharide 10. Having accomplished the synthesis of a tetrasaccharide by the orthogonal strategy, we examined a block condensation approach. Tetrasaccharide acceptor 11, prepared by Zemplen deacetylation of 10, was coupled with its precursor 9 to give compound 12 [conditions (a), 670/0],which is again ready for further use as a oligosaccharide donor. The orthogonal glycosylation reactions were put together with the block condensation to show its efficiency and flexibility (Scheme 3 ) .
BnO
-+&+ -
4%
“
4
w
SNPhtP
h
-
a
65%
9
10 R = A c ‘(11 R = H
NPht
-$&++-F
12
Scheme 3. An example of the orthogonal glycosylation strategy showing only the continuous coupling reactions required to obtain the oligomer. Reagents and conditions: (a) NIS (1.3 equiv.)AgOTf (0.1 equiv.) in CH2C12, -50°C 4room temp.; (b) CpzHfClz (1.3 equiv.)-AgClOa (2.6 equiv.) in CHIC12, -78 “C + room temp.; (c) NaOMe, 91%.
4 18
16 Orthogonal Strategy in Oligosaccharide Synthesis
fj-D-GalNAc-(1 -3)-a-D-Gal-( 1-3)-p-D-Gal-( 1-3)-p-D-GalNAc-( 1-4)-fj-D-Gal-( 1-4)-fj-D-Gle( 1-)-Cer a-L-FuC(1-2)
J
13
14
Figure 8. The structure of a novel extended blood-type B glycosphingolipid (13)and the target structure (14).
16.4.2 Orthogonal Coupling for Hetero-Oligomer Synthesis [22] To verify generality of the aforementioned orthogonal system, a branched tetrasaccharide 14, a partial structure of a novel blood-type related glycosphingolipid 13 (Figure 8), was chosen as a target molecule. Four monosaccharide units 15, 16, 18, and 22 were synthesized as either glycosyl fluorides or thioglycosides to fulfil the criteria of the orthogonal coupling concept. All necessary selective protections and anomeric transformations were completed at the monosaccharide stage, thereby minimizing manipulations during elongation of the sugar chain. The protecting groups especially at C-2 position of each synthon were carefully chosen for the stereoselective glycosylation reactions (Scheme 4). The synthesis was started from the suitably protected galactosyl fluoride 15 (a : P = 1.7 : 1) which was glycosylated with GalNAc precursor 16 under conditions (a) to afford 17. The stereochemical assignment was confirmed by 'H NMR; J1,,2, % 8.5 Hz for H-1' indicated the P-D-configuration for the GalNAc residue. No anomerization of the reducing-end fluoride was observed during the glycosylation reaction, which eliminated the potential ambiguity of this strategy. The disaccharide 17 was used directly as the donor for the next glycosylation reaction with the phenylthioglycoside 18. Glycosylation under conditions (b) afforded the desired trisaccharide 19. The a configuration of the newly formed glycosidic linkage was evident from the coupling constant of the anomeric proton at 64.77 ( J = 3.6 Hz). Glycosylation of 19 with octanol was then conducted under conditions (u) to yield the octyl glycoside 20. The applicability of the current strategy was proven by showing direct orthogonal coupling reactions three times for introduction of various forms of interglycosidic linkage. Deprotection of the levulinoyl group on trisaccharide 20 by use of hydrazine acetate [23] afforded 21 (85.4%) which was then used as an acceptor for the a-
16.4 The Orthogonal Glycosylution Strutegy
419
OBn I .-Oh
AcO
SPh 18
AcO
73%
OBn 17
Octyl alcohol
19
OLev
SPh % +
Scheme 4. Synthesis of blood-type-related oligosaccharides by the orthogonal strategy. Reagents and conditions: (a) *NIS-AgOTf in CH2C12, -50 + -20°C; **NIS-AgOTf in CH2C12, 10°C; (b) Cp2HfC12-AgC104 in -20°C + room temp.; *** DMTST in benzene-CH2Cl2, 0 CH2C12-Et20, -78 "C ---t room temp.; (c) NHzNH2.AcOH; (d) (i) H2NCH2CH2NH2-EtOH, 90 "C; (ii) Ac20-MeOH-Et3N (cat.), room temp.; (e) Hz-Pd/C-MeOH, H+. ---f
420
16 Orthogonal Strategy in Oligosaccharide Synthesis
fucosylation reaction. Thus the levulinoyl group was used not only as a participating group to yield the P-glycoside but also as a selectively removable protecting group. The coupling reaction of 21 using methylthio glycoside 22 [24] gave the desired tetrasaccharide (88%). Ethylenediamine [25]treatment of the tetrasaccharide then selective acetylation of the resulting amine yielded 14 (85%) after hydrogenolysis under acidic conditions. In the same manner, compound 21 was deprotected to afford 25. We have demonstrated the applicability of the orthogonal glycosylation strategy to the synthesis of a branched oligosaccharide with four different types of glycosidic linkage. The synthesis of the target tetrasaccharide 14 was thus achieved in seven steps (38%) overall yield) from protected monosaccharide units. With the trisaccharide 25 and the known blood-type B trisaccharide available it might be possible to utilize these oligosaccharides as probes to investigate an unknown biosynthetic pathway. 16.4.3 Application to Polymer-Supported Synthesis 1261 The elongation of the sugar chain from the non-reducing end in polymer-supported oligosaccharide synthesis is less straightforward, mainly for two reasons: 1) accumulation of by-products on the support together with the desired compound; and 2) a transformation reaction must be performed to obtain the glycosyl donor. When, however, the orthogonal glycosylation strategy is applied to polymersupported synthesis, an important improvement, namely pre-installation of a potential leaving group, is achieved. This has potential to make the methodology more advantageous-because the leaving group is attached, there will be no need for the activation reaction and no need to protect the anomeric hydroxyl group produced by hydrolysis. As a result, again only one reaction step is required per cycle; this is the same as was shown for solution synthesis. In addition, because only the last introduced sugar unit has a leaving group, one can introduce a tag to ease isolation of the final product after deprotection. The recoverability of excess glycosyl acceptor used in the coupling reaction should be noted in addition to the above advantages as a common feature of polymer-supported synthesis starting from non-reducing end terminus. To test this hypothesis, the first attempt was made using ‘soluble’ poly(ethy1ene glycol) (PEG) as the polymer [17b]; the trimannoside portion (27) of the highmannose type glycoprotein (26) was chosen as the target. In addition, the 2-(trimethylsily1)ethyl (SE) group was used as a hydrophobic tag [27] in this case in addition to the temporary protecting group [8d,e] (Figure 9). Initial coupling of the polymer, PEG monomethyl ether (average MW = 5000)supported methylthiomannoside 28 with the fluoride 29 was performed in the presence of DMTST, prepared in situ from MeOSOzCF3 and MeSSMe [conditions (a)], to afford the disaccharide-PEG conjugate 30 in 89% yield. Subsequent coupling with the reducing-end mannoside was performed by using SE-glycoside 31 as aglycone under conditions (b)to afford the trisaccharide-PEG conjugates 32.
16.5 Conclusions and Prospects a-0-Man-(1-2))-a-D-Man-(l-6)
42 1
1
a-D-Man-(1-6) a-D-Man-(l-2)-a-D-Man-(l-6) p-D-Man-(l-.+p-D-GicNAc-(
1
1-.4)-B-D-GlcNAc-Asn-Protein
a-D-Man-(l-2)-a-D-Man-(l-+2)-a-D-Man-( 1 ’ 3 ) -1 26
I
27
OSE
Figure 9. The structures of the high mannose-type N-linked oligosaccharide 26 and the target tri. mannoside 27.
Having shown the construction of a trisaccharide sequence, PEG was cleaved off under basic conditions and the crude trisaccharide was hydrogenated over Pd(OH)*. The resulting mixture was then applied to a short column of CISreversed-phase silica gel, which was first eluted with water to elute all by-products. Subsequent elution with 50% MeOH gave a mixture of trisaccharides from which 27 was isolated by eluting with a linear gradient from water to 50% MeOH (Scheme 5). This strategy, featuring a novel combination of polymer-support chemistry, the concept of orthogonal glycosylation, and simple isolation by incorporating a hydrophobic tag has proven successful. It should also be noted that the aglycone in 27, which is used as a hydrophobic tag, can be selectively cleaved if one wants to conjugate the formed oligosaccharide with other structures. One expansion of this strategy can be seen in connection with the intramolecular aglycone delivery approach, where a polymer-substituted benzyl group was used as both the linker and the tether [28] (Scheme 6 ) . Thus, methylthiomannoside 33 was coupled with a fluorinated GlcNAc derivative 3 to yield only P-linked mannoside which can be used as an acceptor itself or as a donor after protection of released 2OH. The advantage of this particular use is that the only compound to be released from the polymer is the desired product.
16.5 Conclusions and Prospects An orthogonal glycosylation strategy has been developed by combined use of phenylthioglycosides and glycosyl fluorides both as donors and acceptors. By use of this
422
16 Orthogonal Strategy in Oligosaccharide Synthesis
BnO BnO BnO BnO
30
SEO :
BnO BnO
'siWo 'I
32
OSE
H O T OH
HO HO
HO
OH
HO
HO *H r HO
Wafer eluent
27
OSE
Organic (alcohollc) eluent Scheme 5. Polymer-supported synthesis of trimannoside on the basis of the orthogonal strategy. Reagents and conditions: (a) DMTST in CHzC12, room temp., (b) CpzHfClz-AgOTf in CH2CI2, 0 "C + room temp., (c) (i) NaOMe; (ii) Hz-Pd(OH)z in MeOH-EtOAc. *Yields were calculated on a weight basis.
16.5 Conclusions and Prospects
423
SMle
34
,OBn
lb TBDPSO
BnO
NPhth
36
NPhth
d 85%
35 pTBDPSO hTo0?~*+;& NPhth
N3 NPMh
37
Scheme 6. Combination of polymer-supported chemistry, P-mannosylation, and orthogonal strategy. Reagents and conditions: (a) DDQ, 4 A mol. sieves in CH2C12, room temp., (b) AczO-Pyr., 48% from 3, (c) Cp*HfClz-AgOTf in ClCH2CH2C1, -15°C + room temp.
strategy, syntheses of several oligosaccharides have been achieved. As was clearly demonstrated in the synthesis of the chito-oligosaccharide, the novel strategy is free from the effects of the protecting groups, unlike the armed-disarmed concept. The usefulness of the strategy is obvious, because by use of this method an oligosaccharide can be synthesized highly efficiently with the lowest number of reactions. The syntheses of homo- and hetero-oligosaccharides has been demonstrated successfully. Further, the strategy was also examined for polymer-supported synthesis; this showed the potential of the methodology in solid-phase oligosaccharide synthesis. A hydrophobic tag was incorporated at the end of the synthesis to demonstrate the idea of simple isolation of the final product and/or the possibility of direct use of the isolated material for biological evaluations. The orthogonal coupling strategy can be applied to the synthesis of other polymeric compounds. Several approaches have been reported for construction of polymeric and dendritic structures on the basis of the orthogonal coupling strategy [29] (Figure 10).
424
16 Orthogonal Strategy in Oligosaccharide Synthesis
conditions a
PPhB - DEAD
b
Pd(PPh,)$l2
-
- CUI
Pd(0Ac)z - P(o-tOlyl), NBu3
Pd2(dba)3- DPPF - NaOtBu
NaH
Et3N DMAP
-
Figure 10. Some examples of the orthogonal coupling strategy in the synthesis of polymers and dendnmers.
Acknowledgments
The author thanks Professor Tomoya Ogawa and Dr. Yukishige Ito for their continuous encouragement and suggestions. Professors Hans Paulsen, Ole Hindsgaul, Robert Field, Chi-Huey Wong, Makoto Kiso, Tadahiro Takeda, and Dr. Frank Barresi are also acknowledged for their encouragement. Most of all, the author would like to express his sincere respect and gratitude to the late Professor Akira Hasegawa. References 1. (a) Igarashi, K. The Koenigs-Knorr reactions. In: Adu. Curbohydr. Chem. Biochem., 1977, 34, pp. 243-283. (b) Paulsen, H. Angew. Chem. Znt. Ed. Enyl., 1982, 21, 155-175. 2. (a) Lemieux, R. U., Takeda, T., Chung, B. Y. A. C.S. Symp. Ser., 1976, 4, 90-1 15. (b) Ito, Y., Ogawa, T. Tetrahedron. 1990, 46, 89-102. (c) Okamoto, K., Goto, T. Tetrahedron. 1990, 46,
5835-5857. 3. Lemieux, R. U. In: Molecular Rearrangements, de Mayo, P. Ed., Interscience, New York, 1964, p. 709. 4. Brccini, I., Derouet, C., Esnault, J., Heme du Penhoat, C., Mallet, J.-M., Michon, V., Sinay, P. Carbohydr. Res., 1993, 246, 23-41. and references cited therein. 5. Lemieux, R. U., Hendriks, K. B., Stick, R. V., James, K. J. Am. Chem. Soc., 1975, 97,40564062. 6. (a) Paulsen, H., Lockhoff, 0. Chem. Ber., 1981,114, 3102-3114. (b) Garegg, P. J., Ossowski, P. Acta Chem. Scand., 1983, B37, 249-250. (c) Van Boeckel, C. A. A., Beetz, T., van Aelst, S. F. Tetrahedron, 1984, 40, 4097-4107. 7. (a) Barresi, F., Hindsgaul, 0. J. Am. Chem. Soc., 1991, 113, 9376-9377. (b) Stork, G., Kim, G. J. Am. Chem. Soc., 1992, 114, 1087-1088. (c) Barresi, F., Hindsgaul, 0. Synlett, 1992, 759761. (d) Bols, M. Tetrahedron, 1993, 49, 10049-10060. (e) Barresi, F., Hindsgaul, 0. Can. J. Chem., 1994, 72, 1447-1465. (f) Ito, Y., Ogawa, T. Angew. Chem. Int. Ed. Engl., 1994, 33, 116551767.
References
425
8. (a) Sugimoto, M., Ogawa, T. Glycoconjugate J., 1985, 2, 5-9. (b) Murakata, C., Ogawa, T. Carbohydr. Rex; 1992, 235, 95-1 14. (c) Fraser-Reid, B., Udodong, U. E., Wu, Z.: Ottosson, H., Merritt, J. R., Rao, C. S., Roberts, C., Madsen, R. SYNLETT., 1992, 927-942. (d) Jansson, K., Frejd, T., Kihlberg, J., Magnusson, G. Tetrahedron Lett., 1988, 29, 361-362. (e) Jansson, K., Ahlfors, S., Frejd, T., Kihlberg, J., Magnusson, G. J. Org. Chem., 1988, 53, 56295647. (f) Murase, T., Kameyama, A,, Kartha, K. P. R., Ishida, H., Kiso, M., Hasegawa, A. J. Carbohydr. Chem., 1989, 8, 265-283. (g) Kanie, O., Takeda, T., Ogihara, Y. Carbohydr. Res., 1989,190, 53-64. (h) Kinzy, W., Schmidt, R. R. Liebigs Ann. Chem., 1985, 1537-1545. (i) Nakahara, Y., Ogawa, T. Tetrahedron Lett., 1987,28, 2731-2734. 9. (a) Koto, S., Uchida, T., Zen, S. Bull. Chem. Soc. Jpn., 1973, 46, 2520-2523. (b) Nicolaou, K. C., Dolle, R. E., Papahatjis, D. P., Randall, J. L. J. Am. Chem. Soc., 1984, 106, 4189-4192. (c) Lonn, H. Carbohydr. Rex, 1985, 139, 105-113. (d) Fiigedi, P., Garegg, P. J., Lonn, H., Norberg, T. GlycoconjugateJ., 1987, 4, 97-108. ( e ) Nilsson, M., Norberg, E. Carhohydr. Rex, 1988, 183, 71-82. 10. Chemoselective glycosylation reaction in general: (a) Paulsen, H.; Tietz, H. Carbohydr. Rex, 1985,144, 205-229. (b) Friesen, R. W., Danishefsky, S. J. J. Am. Chem. Soc., 1989,111, 66566660. (c) Trumtel, M., Veyrieres, A., Sinay, P. Tetrahedron Lett., 1989, 30, 2529-2532. (d) Veeneman, G. H., van Boom, J. H. Tetrahedron Lett., 1990,31,275-278. (e) Veeneman, G. H., van Leeuwen, S. H., Zuurmond, H., van Boom, J. H. J. Carbohydr. Chem., 1990, 9, 783-796. (f) Mori, M., Ito, Y., Uzawa, J., Ogawa, T. Tetrahedron Lett., 1990, 31, 3191-3194. (g) Mehta, S., Pinto, B. M. Tetrahedron Lett., 1991, 32, 4435 4438. (h) Marra, A,, Gauffeny, F., Sinay, P. Tetrahedron, 1991,47, 5149-5160. (i) Zegelaar-Jaarsveld, K., van der Marel, G. A,, van Boom, J. H. Tetrahedron, 1992, 48, 10133-10148. (j) Jain, R. K., Matta, K. L. Carbohydr. Rex, 1992, 226, 91-100. (k) Marra, A., Esnault, J., Veyrieres, A., Sinay, P. J. Am. Chem. Soc., 1992, 114, 6354-6360. (1) Hashimoto, S., Yanagiya, Y., Honda, T., Kobayashi, H., Ikegami, S. Presented at 18th Symposium on Progress in Organic Reactions and Syntheses-Applications in the Life Sciences, Sapporo, Japan, October 1992, Abstr. pp 276-280. (m) Raghavan, S., Kahne, D. J. Am. Chem. Soc., 1993, 115, 1580-1581. (n) Mehta, S., Pinto, B. M. J. Org. Chem., 1993, 58, 3269-3276. ( 0 ) Sliedregt, L. A. J. M., Zegelaar-Jaarsveld, K., van der Marel, G. A., van Boom, J. H. SYNLETT, 1993, 335-337. (p) Yamada, H., Harada, T., Miyazaki, H., Takahashi, T. Tetrahedron Lett., 1994, 35, 3979-3982. (4) Yamada, H., Harada, T., Takahashi, T. J. Am. Chem. Soc., 1994, 116, 7919-7920. (r) Chenault, H. K., Castro, A. Tetrahedron Lett., 1994, 35, 9 145-9148. 11. Latent-active strategy: (a) Roy, R., Andersson, F. O., Letellier, M. Tetrahedron Lett., 1992, 33, 6053-6056. (b) Boons, G.-J., Isles, S. Tetrahedron Lett., 1994, 35, 3593-3596. 12. Armed-disarmed strategy: (a) Mootoo, D. R., Konradsson, P., Udodong, U., Fraser-Reid, B. J. Am. Chem. Soc., 1988, 110, 5583-5584. (b) Friesen, R. W., Danishefsky, S. J. J. Am. Chem. Soc., 1989: I l l , 6656-6666. (c) Veeneman, G. H., van Leeuwen, S. H., van Boom, J. H. Tetrahedron Lett., 1990, 31, 1331-1334. (d) Grice, P., Ley, S. V., Pietruszka, J., Priepke, H. W. M., Walther, E. P. E. Synlett, 1995, 781-784. (e) Hashimoto, S., Sakamoto, H., Honda, T., Abe, H., Nakamura, S., Ikegami, S. Tetrahedron Lett., 1997, 38, 8969-8972. 13. (a) Konradsson, P., Mootoo, D. R., McDevitt, R. E., Fraser-Reid, B. J. Chem. Soc., Chem. Commun., 1990, 270-272. (b) Veeneman, G. H., van Leeuwen, S. H., van Boom, J. H. Tetrahedron Lett., 1990,31, 1331-1334. (c) Konradsson, P., Udodong, U. E., Fraser-Reid, B. Tetrahedron Lett., 1990,31,4313-43 16. 14. (a) Suzuki, K., Maeta,; H. Matsumoto, T., late Tsuchihashi, G. Tetrahedron Lett., 1988, 29, 3571-3574. (b) Suzuki, K., Maeta, H., Matsumoto, T. Tetrahedron Lett., 1989,30, 4853-4856. 15. (a) Barany, G., Merrifield, R. B. J. Am. Chern. Soc., 1977, 99, 7363-7365. (b) Barany, G., Merrifield, R. B. J. Am. Chem. Soc., 1980, 102, 3084-3095. 16. Schelhaas, M.: Waldmann, H. Angeiv. Chem. Znt. Ed. Engl., 1996, 35, 2056-2083. 17. For the recent advances of the solid-phase ohgosaccharide synthesis, see: (a) Yan, L., Taylor, C. M., Goodnow Jr., R., Kahne, D. J. Am. Chem. Soc. 1994,116, 6953-6954. (b) Douglas, S. P., Whitfield, D., Krepinsky, J. J. J. Am. Cliem. Soc. 1994, 117, 2116-2117. (c) Schuster, M., Wang, P., Paulson, J. C., Wong, C.-H. J. Am. Chem. Soc. 1994, 116, 1135-1136. (d) Randolph, J. T., McClure, K. F., Danishefsky, S. J. J. Am. Chem. Soc. 1995, 117, 5712-5719. (e) Rademann, J., Schumidt, R. R. Tetrahedron Lett. 1996, 37, 3989-3990. (f ) Shimizu, H., Ito, Y . ,
426
18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.
16 Orthogonal Strategy in Oligosaccharide Synthesis
Kanie, O., Ogawa, T. Bioorg. Med. Chem. Lett. 1996, 6, 2841-2846. (8) Nicolaou, K. C., Winssinger, N., Pastor, J., DeRoose, F. J. Am. Chem. Soc. 1997, 119, 449-450. Palcic, M. M., Heerze, L. D., Pierce, M., Hindsgaul 0. GlycoconjugateJ. 1988, 5, 49-63. Kanie, O., Ito, Y., Ogawa, T. J. Am. Chem. Soc., 1994, 116, 12073-12074. Nicolaou, K. C., Caulfield, T. J., Kataoka, H., Stylianides, N. A. J. Am. Chem. Soc., 1990, 112, 3693-3695. Mukaiyama, T., Murai, Y., Shoda, S. Chem. Lett., 1981, 431-432. Kanie, O., Ito, Y., Ogawa, T. Tetrahedron Letters, 1996, 37, 4551-4554. Koeners, H. J., Verhoeven, J., van Boom, J. H. Rec. Trav. Chim. Pays-Bas., 1981, 100, 65-72. Sato, S., Ito, Y., Nukada, T., Nakahara, Y., Ogawa, T. Carbohydr. Rex, 1987, 167, 197-210. Kanie, O., Crawley, S. C., Palcic, M. M., Hindsgaul, 0. Carbohydr. Res., 1993, 243, 139-164. Ito, Y., Kanie, O., Ogawa, T. Angew. Chem. Int. Ed. Engl., 1996, 35, 2510-2512. Ito, Y., Paulson, J. C. J. Am. Chem. Soc. 1993, 115, 1603-1604. Ito, Y., Ogawa, T. J. Am. Chem. Soc. 1997, 119, 5562-5566. (a) Zeng, F., Zimmermann, S. C. J. Am. Chem. Soc. 1996,118,5326-5327. (b) Maddux, T., Li, W., Yu, L. J. Am. Chem. Soc. 1997, 119, 844-845. (c) Deb, S. K., Maddux, T. M., Yu, L. J. Am. Chem. Soc. 1997, 119, 9079-9080. (d) Frost, C. G., Mendonp, P. Chem. Lett. 1997, 1 159-1 160.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
17 Protecting Groups: Effects on Reactivity, Glycosylation Stereoselectivity, and Coupling Efficiency Luke G. Green and Steven V. Ley
17.1 Introduction In general, protecting groups are seen as an expedient, performing the uninteresting task of masking functionality that would otherwise interfere with a carefully planned strategy. A protecting group or protecting group strategy is usually chosen on the basis of its compatibility (in protection, deprotection and lability to other transformations), selectivity (in protection) and sequence (in deprotection when multiple protecting groups are used) [I]. Ultimately the same criteria apply in oligosaccharide assembly where the protecting group pattern is principally dictated by the target structure. Even within these constraints there are, however, options which can mean the difference between success and failure, efficiency and tedium. Protecting groups are not innocent bystanders in carbohydrate chemistry and to relegate them in the synthesis design is foolhardy. It is not the purpose of this chapter to review methods for installing and removing carbohydrate protecting groups, because this has already been covered by other texts [2]. Rather, it is intended to outline the mechanisms by which they effect glycosylations thereby enabling readers to decide which protecting group strategy best serves their purpose. Before investigating the roles various protecting groups can play it is pertinent to analyze briefly the glycosidic mechanism, because this is central to oligosaccharide assembly processes. There have been many detailed studies of the processes that occur during a glycosylation [ 3 ] but much of the evidence used to substantiate proposed inter-glycosyl coupling mechanisms is anecdotal or circumstantial. Monitoring actual glycosylation reactions is complex and often not very illuminating, given the number of factors involved and the instability of the intermediates. The mechanism presented here is not intended to be definitive but serves well as a working model.
428
I7 Protecting Groups
17.2 Glycosidic Mechanism The first rule is that glycosylations do not, under most reaction conditions, proceed through a free oxonium ion in an s N 1 manner [4]. Jencks has estimated the lifetime s, a time too short to of a glycosyl oxonium ion in water to be approximately enable solvent equilibration. It therefore seems unlikely that in typical, low-polarity glycosylation solvents the ion-pairs can be separated sufficiently to generate the free glycosyl oxonium ion as an intermediate. Hence, glycosylations can be considered as proceeding through an 'exploded' transition state in which the incipient oxacarbenium moiety is stabilized electrostatically by both entering and leaving groups [ 5 ] , as in Capand CBataken from Lemieux's mechanism (Figure 1) [6].
R.
*.
!?
X
Aa
Ba
R.
Figure 1. Lemieux's glycosidic mechanism.
17.2 Glycosidic Mechanism
429
Lemieux's mechanism (based on earlier work by Rhind-Tutt and Vernon [7], and Ishikawa and Fletcher [S]) was put forward to explain the high u-selectivity observed on alcoholysis of anomeric bromides with an ether-type protecting group on C-2. The first step involves dissociation of the glycosyl bromides to tight ion-pairs (B) [9]. This process is promoted by donation from the oxygen lone pair (n) into the carbon oxygen antibonding orbital (o*), requiring the two orbitals to be appropriately aligned [ 101. For the u-halide, with the required orbitals naturally in an antiperiplanar arrangement, this interaction only results in flattening of the pyran ring as the oxygen takes on more sp2 character. For the (3-halide, however, the sugar must reorganize to a boat conformation (AD) to place the orbitals anti-periplanar (there is mounting evidence that the reaction probably proceeds through a synperiplanar arrangement via the lower energy twist-boat formation) [ 111. The resulting ion pairs are then intercepted by the acceptor alcohol (SOH) with inversion, in the rate limiting step, leading to the protonated glycosides (E). The product from the a-halide pathway proceeds through a boat-like intermediate (Ep, whereas the (3-halide leads to a lower energy chair-like intermediate (Ea). This smaller activation barrier, coupled with the higher ground-state energy of the (3-halide (relative to the anomerically stabilized u anomer), more than compensate the energy penalty incurred in aligning the oxygen lone pair with the antibonding orbital (CurtinHammet principle) [ 121. Hence the p-bromide is more reactive than the u-bromide. Even though the u-bromide is predominant (as it is the more thermodynamically stabilized species) interconversion between u- and (3-bromides (C ) is considerably faster than glycosylation (especially when lowly nucleophilic alcohols such as glycosy1 acceptors are used and a source of bromide ion is present-% situ anomerization protocol') [6] and glycosylation occurs preferentially on the more reactive (3bromide, resulting in the u-glycoside as the major product. This is one of the earliest examples of a dynamic kinetic resolution. This mechanism is readily translated to other glycosylation systems if an activation step is added (Figure 2). The leaving group (X) is initially coordinated by the activating group (M+Y,-) to form a catalyst-coordinated ion-pair. Although this pair can be attacked by the alcohol directly, affording products with inverted stereochemistry of the initial ROU
Figure 2. Generalized glycosylation process.
430
17 Protecting Groups
leaving group, in general [ 131, this does not occur and anion exchange occurs before alcohol attack. Schuerch first demonstrated that silver triflate activation of glycosyl halides generates glycosyl triflates [ 141 and it has been shown recently that anion exchange occurs in a variety of other systems [15]. As with the bromide, any functionality sufficiently reactive to act as a leaving group will not be configurationally stable and the same mechanism for glycoside formation applies. The a-selectivity of these reactions is not, however, usually as consistent as for the 'in situ anomerization protocol' of anomeric bromides.
17.3 Electronic and Torsional Effects The first step of any glycosylation involves dissociation into ion-pairs, a process which results in reorganization of the glycoside and the development of a positive charge on the ring oxygen. Clearly, any factors that influence this process will have profound consequences on the outcome of a glycosylation. Riiber et al. noticed that the removal of the oxygen at C2 of glycosides greatly increased the rate at which the methyl glycosides were hydrolysed (Scheme 1) [ 161. Overend et al. [17], amongst others [18], further investigated the effects of deoxygenation at other positions on the rates of hydrolysis of methyl glycosides (Table 1). Richards proposed the explanation that the oxygen atoms, owing to their electronegative nature, inductively destabilize the developing positive charge required for the formation of the ion-pairs (B) [ 181. Hence, their removal results in an increase in the rate of hydrolysis, with the C2 oxygen, because it is nearest the reacting center, having the greatest effect. Even the C6 can, however, affect the reaction, as is evi-
HO
H+
I
*
&HH
OMe
OH
Scheme 1. Reagents and conditions: HCl(aql.
Table 1. Rates of hydrolysis of methyl glucopyranosides at 58 "C in 0.01 M HCI. Glucopyranoside
Relative rate
Methyl-a-D-glucopyranoside Methyl-2-deoxy-a-~-glucopyranoside Methyl-3-deoxy-a-~-glucopyranoside Methyl-4-deoxy-a-~-glucopyranoside
1 2090 20 40
17.4 Influence o j Protecting Group on Donor Reactivity
43 1
Table 2. Comparison of the rates of hydrolysis of methyl glycosides and their 6-deoxy analogs. GIycoside
Rate relative to Me-a-D-glucopyranoside
Methyl-a-D-mannopyranoside Methyl-a-L-rhamnopyranoside Methyl-a-D-galactopyranoside Methyl-6-deoxy-a-~-galactopyranoside
2.4 8.3 5.2 28.2
Figure 3. Torsional strain in the incipient oxonium ion
dent from comparing the rates for methyl-a-D-mannose and methyl-a-L-rhamnose [19], or for methyl-a-D-galactose and its 6-deoxy analog [17] (Table 2). The enhanced lability of the methyl-6-deoxygalactopyranosideis mirrored by the acid-sensitivity of a-fucosyl linkages in general [20]. Another pattern that emerges from these data is the relative reactivities of the differing hexoses-galactosides > mannosides > glucosides. Although there are many factors responsible for this sequence (ground state energies in particular) one contributing factor, as Edwards suggested [2I], arises from the eclipsing interactions that develop between the C2-C3 and C4-C5 substituents, disfavoring formation of the ion-pairs (Figure 3 ) . An axial substituent will interact less with its neighbor and therefore provide less resistance to the conformational reorganization. Overend et al. later argued that the larger substituent on C5 results in the C4-C5 interaction being more influential than the C2ZC3 interaction [ 171, purportedly explaining why galactosides were more reactive than mannosides. Although this explanation is incomplete in rationalizing the complex processes involved in glycoside activation, it was the first recognition that torsional effects could be a controlling influence in a glycosylation; an idea that did not resurface for many years [22]. Inductive effects, however, were all too evident to early researchers; Fletcher et al. demonstrated that increasing numbers of ester protecting groups reduced the reactivity of glycosyl halides under methanolysis conditions (Table 3 ) [23].
17.4 Influence of Protecting Group on Donor Reactivity The origin of t h s effect lies in the destabilizing effect of the exocyclic ring oxygens on ion-pair formation. An ester protecting group, owing to its electron-withdrawing
432
17 Protecting Groups
BnO
1
3
2
Table 3. Influence of protecting group on the rates of methanolysis of arabinosyl chlorides. Substrate
Relative rate of methanolysis
1
106
2 3
13 1
nature, increases the electronegativity of the oxygen it is protecting thereby increasing the deactivating effect of that oxygen. This deactivation effect translates to increased resistance to incipient oxonium ion formation i. e. the leaving group is stabilized. In contrast, a benzyl group does not disturb the oxygen in the same manner (not being electron withdrawing) and thus the fugacity of the leaving group is untempered. Ley et al. have investigated this effect in detail, quantifying the individual contributions of a variety of protecting groups to the different hydroxyl positions of rhamnosides and mannosides [24]. Two differently protected glycosyl donors were made to compete for a standard glycosyl acceptor (Scheme 2). A carbohydrate
Donor A 2 equiv.
Donor 0 2 equiv.
Disaccharide AC
Acceptor C (R'OH) 1 equiv.
Disaccharide BC
Scheme 2. Reagents and conditions: NIS (N-Iodosuccinimide), TfOH (cat.), 4 (CH2C1)2-Et20.
A molecular sieves,
17.4 Influence of Protecting Group on Donor Reactivity
433
Table 4. The possible protecting group patterns of ethyl 1-thio-a-L-rhamnoside. R'
R2
R3
Bn BZ Bn Bn Bz BZ Bn BZ Bn
Bn Bn BZ Bn BZ Bn BZ BZ CDA
Bn Bn Bn BZ Bn BZ BZ BZ
acceptor was specifically chosen to simulate actual glycosylation conditions because the steric profile of the alcohol can influence reaction kinetics through variations in its nucleophilicity [26]. All possible combinations of benzyl and benzoyl protecting group patterns were prepared (Table 4). The torsionally deactivating protecting group cyclohexane-1,2diacetal (CDA), introduced for the selective protection of trans diequatorial 1,2diols (Scheme 3) [25], was also included in the survey of donor activity. The rigidity imposed by the fused cyclic acetals resists the conformational changes required for glycosylation, hence the reactivity of the sugar is diminished. The ratio of the product disaccharides AC and BC was determined by 'H NMR (each disaccharide had already been prepared discretely) thus providing a quantitative analysis of reactivity of the various protecting group patterns (Table 5). Not surprisingly the amount of electronic deactivation arising from a benzoyl protecting group varies with proximity to the ring oxygen/reacting center in a manner similar to that in the deoxygenation experiments of Overend et al. (cf. Table 1); i.e. the order of importance is C2 > C4 > C3 [27]. The torsional deactivation effect from the CDA group is marked, being slightly greater than a benzoate at C2 and slightly less than the 3,4-dibenzoyl compound. As a simplifying approximation, the individual contributions from each protecting group can be treated as being independent of any other protecting groups in the system. Although the validity of this approximation is questionable [28] it does
Me0 i
55% UH
--
D
OH -. .
M e0-
Scheme 3. Reagents and conditions: 1,1,2,2-tetramethoxycyclohexane,CSA (1 0-Camphorsulfonic acid), CH(OMe)3, MeOH.
434
I7 Protecting Groups
Table 5. Selected results from competition experiments. Donor A
Donor B
Quotient AC/BC
TriBn TriBn TriBn 2,4-DiBz 2,3-DiBz 3,4-DiBz 3,4-DiBz 3,4-CDA
3-BZ 4-BZ 2-BZ TriBz TriBz TriBz 3,4-CDA 2-BZ
3.1 8.9 26.6 2.5 13.0 24.2 1.7 1.7
enable semi-quantitative prediction of donor reactivities by multiplication of the quotients or deactivation factors (DFs) for each position. For example: reactivity of 2,4-DiBz (Di-benzoyl) = 8.9 x 26.6 = 236.7 reactivity of 2,3,4-TriBz (Tri-benzoyl) = 3.1 x 8.9 x 26.6 = 733.9 therefore ratio of 2,4-DiBz/TriBz = 733.9/236.7 = 3.1 observed ratio = 2.5 That the predicted result is of similar magnitude to the observed value is indicative of the usefulness of this approximation; the difference cannot, however, be explained away merely as statistical error and should in part be attributed to the failure of the approximation. A similar set of competition experiments was performed for thiomannosides which included the second generation of diacetal protecting groups; butane diacetal (BDA) [29] (Table 6). The relative importance of the various positions is unchanged but C6 emerges as the position of most significance after C2, which could be further tuned by varying the electronics of the benzoate. Making the benzoate more electron rich with a donating p-methoxy group reduced the deactivation; conversely the
Table 6. Standardized DFs for non-Bn protecting groups on thiomannosides. Position of non-Bn group
DF
3-BZ 4-BZ 6-(p-MeOBz) 6-BZ 6-(p-N02Bz) 3,4-CDA 3,4-BDA 2-BZ
1.1 5.0 6.6 8.2 18.9 13.9 16.5 33.6
17.4 Influence o j Protecting Group on Donor Reactivity
BzO BzQ
BzO Bz&
BZO
1.o
To’ BZO
BzO Bz&
2.3
435
To1 HO
3.1
BzO
5.1
11.8
Figure 4. Relative reactivities of selected thiogalactosides.
more electron deficient p-nitro group served to increase the deactivation. Again the torsional effects of the diacetal protecting groups are amply demonstrated, reducing the reactivity of the donor by a factor of around 15. Wong et al. have investigated protecting group effects on p-methylphenyl thiogalactosides and p-methylphenyl thioglucosamines employing a similar competition strategy except with the significant difference of using methanol as the acceptor alcohol [28]. This facilitated examination of effects of hydroxyl groups on donor reactivity by suppressing homocoupling that might otherwise occur if a less nucleophilic acceptor (e.g. carbohydrate alcohol) was used. Curiously, the order of importance of the positions seems to be dramatically altered with C2 and C6 being demoted in favor of C4 and C3 (Figure 4).This result seems to be at odds with the pattern so far established in this chapter, even allowing for the variations that might be expected from changes in stereochemistry of the hydroxyl and leaving group (axial uersus equatorial). Whether this is particular to galactose [30] or is the result of comparing a benzoyl group with a hydroxyl function whose ability to hydrogen bond (providing some torsional resistance) will vary with position and adjacent protecting groups, has yet to be determined. It is, however, clear that a hydroxyl group can be considered as more activating than a benzyl protecting group (Figure 5).
pP
pP Bnb
1.o
2.8
Figure 5. Relative reactivities of a benzylated and an unprotected thiogalactoside.
436
17 Protecting Groups
Wong et al. also demonstrated that the deactivating power of the different electron withdrawing groups investigated was -=Nj> -0AcCl > -NPhth > -0Bz > NHTroc (Troc = 2,2,2-trichloroethurycurbonyl> -0Bn. It should now be clear that protecting groups profoundly affect the reactivities of carbohydrates through torsional and electronic effects. The size of this effect varies with the position and nature of the protecting group and for multiple protecting groups the effects are additive. The reactivity difference can be significant enough to enable successful competitive coupling of two sugars containing identical leaving groups (see Chapters 6(?) and 19(?)).This reactivity tuning can also influence the stereoselectivity and yield of a glycosylation.
17.5 Stereoselectivity 17.5.1 Neighboring-Group Participation The most obvious way in which a protecting group can influence the stereoselectivity of a glycosylation is through anchimeric assistance (neighboring-group participation) [31]. When there is a carbonyl functionality present at C2 [32] (e.g. ester, carbonate, phthalimide, etc.) it can attack the incipient oxonium ion to form the thermodynamically more stable dioxonium species 5 (Figure 6). The position of this equilibrium will depend upon the nucleophilicity of the carbony1 oxygen which varies with the electronics of substituent R. A trichloroacetate, for example, cannot participate whereas a rnonochloroacetate does, but with lower efficiency than a simple acetate [33]. Attack by an alcohol on 5 occurs anti to the carbonyl bond resulting in exclusive generation of the trans glycoside [34]. Hence, donors bearing a 2-acyl protecting group react to give predominantly trans glycosides as they react through the lower energy dioxonium species. If, however, there are severe steric clashes (vide infra Section 17.7) between donor and acceptor that disfavor reaction on the dioxonium species, reaction will instead occur on the incipient oxonium ion with concomitant loss of stereoselectivity [35]. This problem can sometimes be resolved by deactivating the donor; stabilizing the sugar makes intermolecular attack on the incipient
*x
?R
-
-% Lo
4
Figure 6. Anchimeric stabilization.
x-
R
5
437
17.5 Stereoselectivity
Ac
AcA&B & :sph AcO
Ac
P hthN
6
Bn
+
BnO BnO
i
*
BnO
86%
Bn BnO
Me
p:a 15:l 7
OMe
* 9
74% +
I
10
7
OMe
P:a30:l
Scheme 4. Reagents and conditions: (i) NIS, AgOTf, 4 A molecular sieve, CH2C12-toluene, -50 "C to -30 "C; (ii) AgOTf, 2,6-lutidine, 4 molecular sieve, CHzCl2-toluene, -50 " C .
A
oxonium ion a less favorable process, forcing the reaction to proceed through the dioxonium species. Such a situation arose during the synthesis of the glycans of glycodelin (Scheme 4) [36].Coupling of lactosamine donor 6 to trimannoside acceptor 7 provided pentasaccharide 8 as a 15: 1 mixture of anomers (p : a). The stereoselectivity of the coupling was improved to 30 : 1 (p : a) if the glucosamine residue was protected with ester groups instead of benzyl groups. Although it is possible that the observed improvement in stereoselectivity originates from potential alteration of the steric profile of the lactosamine donor as a result of changing the protecting groups, a completely acetylated lactosamine donor [37] gave identical selectivity to 9. 17.5.2 Reactivity Control
If, however, the C2 is functionalized by a group that cannot participate (e.g. azide, benzyl ether, etc. ) the stereoselectivity of a glycosylation is more unpredictable. The two competing pathways, reaction on the a-leaving group and reaction on the pleaving group, are of more similar activation energy than the incipient oxonium ion and the dioxonium species. The choice of protecting group can, however, still effect
438
17 Protecting Groups
Bn BnO Br
12
11
Nc%
BnoBr
14
13
02%
15
BnO
,
16
Table 7. Stereoselectivity of methanolysis of glucosyl halides (molar ratio of methanol to monomer -390: 1). Substrate
a : p Ratio
11 12
7:93 16:84 16:84 37 : 63 92:s 16:84
13 14 15 16
the stereochemical outcome electronically, as was noted by Frechet and Schuerch (Table 7) [38]. They investigated the effect of different protecting groups at C6 on the stereoselectivity of methanolysis of glucosyl halides (building on earlier work by Ishikawa and Fletcher) [ 81. A clear pattern emerged-electron-rich benzoates promoted pselectivity and the more electron-withdrawing the benzoate became (p-nitro being the most deactivating) the greater the proportion of a-glycoside formed. This can be rationalized in terms of the reactivities of the donors; from the data in Table 6 it is clear that the more electron-deficient the benzoyl, the more the reactivity of the donor is diminished (the greater the DF) owing to the inductive effects highlighted above (Section 17.3). This results in an increase in the activation energies for glycosylation of both the a- and P-bromide. The a-anomer, with its lower ground-state energy (anomerically stabilized), becomes too unreactive to glycosylate methanol at a rate that is competitive with the P-bromide pathway (4). Interconversion of the ato P-bromide is fast and the reaction therefore (for 15) generates the a-glycoside as the major product (Figure 7). If, however, the reactivity of the donor is reduced too much, as in 16, in which the leaving group is changed to the chloride (some 20-30 times less reactive), the rate of
17.5 Stereoselectivity U-D-GIY-1-X
a-D-Gly+, x-
p -~-Gly-l-X
p - ~ - G l y +X,
439
(3) MeOH
(4) M eOH
Methyl 0-D-glycopyranoside
Methyl a-D-glycopyranoside
Figure 7. Methanolysis of glycopyranosides.
interconversion of a- to P-chloride (2) is reduced (relative to glycosylation). Because the a-chloride is present at a higher concentration (because it is the thermodynamically favored arrangement) than the p-chloride, reaction proceeds through the achloride pathway (3) and the p-glycoside is the major product. Alternatively, if the donor is made sufficiently reactive (as in ll), the a-bromide is reactive enough to glycosylate methanol at a rate competitive with the lower energy p-pathway, aided by its increased concentration, such that the P-glycoside predominates. Unfortunately these results were not reproduced when carbohydrate acceptors were used; only a-glucosides were obtained, irrespective of the protecting group on C6 [39]. This is a reflection of the sensitivity of the kinetics of glycosylation-a carbohydrate alcohol is considerably less nucleophilic than methanol for reasons both of size and electronics. This reduced nucleophilicity ensures that the a- to phalide equilibration process is faster than the glycosylation and hence reaction proceeds through path (4) [40]. The reduced concentration of alcohol relative to donor (ratios of 390: 1 are not practical for synthesis of oligosaccharides) would also ensure that equilibration was faster than glycosylation, thus favoring formation of the a-glycoside [41]. The underlying principles, however, are still valid and can be successfully applied in many systems; in general, deactivation of the donor improves the a-selectivity of a glycosylation if the donor species can still anomerize. Such an occasion arose during various groups’ syntheses of glycosyl phosphatidyl inositol membrane anchors [42], in which an oligosaccharide is linked to an inositol moiety via an a-glucosamine residue. a-Linkages in gluco-configured sugars are particularly troublesome because, unlike manno-configured sugars whose axial C2 hydroxyl serves to stereoelectronically promote a-selectivity [43], the equatorial C2 imparts no particular bias. Ley’s initial approach employed glycosyl donor 17 but even under optimized conditions [44] the best selectivities obtained consisted of a 3 : 1 mixture (a : p) (Scheme 5). The in situ-generated glycosyl triflate/perchlorate was too reactive for the two glycosylation pathways ((3) and (4) in Figure 7) to be cleanly differentiated. Ley’s solution was to change the donor species to the less reactive iodide (as opposed to the triflate/perchlorate), this deactivation afforded the desired disaccharide 19 with near complete a-selectivity [42a]. Schmidt’s solution was also to deactivate the donor but instead of deactivating the active leaving group (triflate), he deactivated the donor by employing acetyl protecting groups
440
T Bn B
17 Protecting Groups
S
O
~
F
N3 I
17
+
b
60% a:p3:1
19
18
T Bn B s
o
20
G
ii
Br
+ 18
b
65%
TBs4& Bn
Al I
Bn
a$ >30:1 19
Scheme 5. Reagents and conditions: (i) Yb(OTf)Z, CaC03, Ba(C104)2,Et20, room temp; (ii) TBAI (tetra-N-butyl-ammonium iodide), CH2C12, room temp; (iii) TMSOTf, Et20, room temp. Mnt = menthyl.
(21), affording the same result [42c].The different protecting groups on the acceptor alcohol 22 may also have contributed to this improvement in selectivity (uideinfru). A similar problem was encountered during the synthesis of the glycans of glycodellin-S [36]. The synthesis of LewisY epitope 26 required bis-a-fucosylation of lactosamine moiety 24 (Scheme 6). The tri-0-benzylated fucosyl donor 25 provided the tetrasaccharide 26 with acceptable selectivity but unfortunately as an inseparable mixture. Recourse to a bromide donor under halide ion catalysis (as in the previous example) was not successful because the deactivation was so great it made glycosylation of the hindered hydroxyls prohibitively slow. Deactivating the donor species by acetylating the 3- and 4-positions (27) [45], however, increased the a-selectivity to the point that the other anomer was unobservable. There are other ways to influence the stereochemical outcome by affecting the
441
17.6 Injluence of the Protecting Group on the Acceptor
i OH
PhthN
24
*
57%
++% PhthN
+ P Bn
OBn Bn
BnO
25
i OH
PhthN
24 +
26
a$ 15:l
*
SEt
47%
F' AcO
OBn AcO
24
28
Scheme 6. Reagents and conditions: (i) AgC104, 2,6-lutidine, Et,O, THF, -35 "C.
anomerization equilibrium; lower temperatures 'freeze out' the a-donor species promoting p-selectivity, especially when the donor is deactivated [38, 461 (see Chapter 16(?) on p-mannose synthesis) and the polarity and donating ability of the solvent can alter the equilibrium [47] (see chapter on solvent effects). As has already been mentioned, the nucleophilicity of the acceptor alcohol can also effect the stereoselectivity of glycosylation. The more nucleophilic alcohols can react on the a-ion pair whereas less nucleophilic alcohols glycosylate more slowly, allowing the donor species to equilibrate to the more reactive p-ion pair, generating the a-glycoside. Hence glycosylation of the primary C6 is usually less a-selective than any of the secondary positions. The choice of protecting group can also electronically influence the nucleophilicity of the acceptor alcohol.
17.6 Influence of the Protecting Group on the Acceptor In the same way that protecting groups influence the electron density on the anomeric carbon, tuning the reactivity, they also effect the electron density of the hydroxyl groups within the sugar, adjusting their nucleophilicity. Sinay noted in the synthesis of lactosamine derivative 32 that the protecting group at C3 of the glucosamine had a dramatic effect on the glycosylation yield with galactosyl bromide 29 (Scheme 7) [48]. Although the acetylated acceptor 30 was such a poor nucleophile that the coupling with 29 only produced a 5% yield of the desired
442
I 7 Protecting Groups
Scheme 7. Reagents and conditions: (i) HgBr2, (CH2C1)2, room temp.
i A
* Ac
Ac H OBn
Br
AcO
OBn
R= Ac, 30 R= Bn,31
29 Acceptor
Yield (%)
30 31
5 87
32
product, benzyl protection increased the nucleophilicity of acceptor 31 affording 32 in 87% yield [49]. The protecting group at the anomeric position also influences hydroxyl reactivity; thioglycosides tend to be less nucleophilic than the equivalent 0-glycoside [ 501. This can result in reduced glycosylation yields for difficult couplings as was the case in the synthesis of disaccharide 36, taken from van Boom's synthesis of a pentasaccharide from Crytococcus neoformans (Scheme 8) [ ~ O C ] .Coupling of xyloside 33 with thiomannoside 34 provided disaccharide 36 in a disappointing 38% yield, methyl mannopyranoside 35, however, coupled with 33 in 68% yield. The final mechanism by which protecting groups can influence the outcome of a glycosylation is through the way they alter the steric profile of the donor and acceptor.
Scheme 8. Reagents and conditions: (i) TMSOTf, (CH2C1)2, room temp
A%*
i
AcOO
-
;&-% A
AcO
Bn
33
BnO Bn
R=SEt, 34 R=OMe, 35 Acceptor
Yield ('%)
34
38 68
35
R
36
17.7 Steric EfSects on Glycosylution
443
17.7 Steric Effects on Glycosylation Of all the ways in which protecting groups can influence a glycosylation, steric effects are perhaps the least quantifiable and yet ultimately, the most influential. Although the rate-limiting step for most glycosylations is the activation of the leaving group [51] the product determining step is the approach of the acceptor to the donor (step C in Figure 1) [52]. The different steric interactions in the two transition states CaPand CPaaffect the activation energies of each pathway, which determine the selectivity (and yield) of a glycosylation [53]. This is obvious when the protecting group is close to the reacting center, as in Scheme 9 [54].Attack by the rhamnoside 40 on the torsionally deactivated mannosyl a-triflate, preformed from 37 at low temperature by activation with triflic anhydride, proceeded only with modest P-selectivity when the C2 of mannosyl donor was protected with a bulky tert-butyldimethylsilyl (TBDMS) group. Replacement of this group with the smaller trimethylsilyl (TMS) or benzyl group greatly improved the P-selectivity (for optimized conditions see Chapter 16 (?) on p-mannose). Even remote positions, the C6 for example, have, however, been reported to have an effect on the stereoselectivity of glycosylation [55]. The issue is unfortunately clouded by the change in electronic properties which frequently accompanies a change in protecting groups. Perhaps the most dramatic example of a protecting group affecting the outcome of a glycosylation comes from a synthesis of a glycan from laminin by Zhu et al. (Scheme 10) [56].Attempted coupling of lactose donor 42 with P-methylglycoside 43p failed to produce any of desired hexasaccharide 44p. When coupling was attempted on the a-methyl glycoside 43a under identical conditions, however, this
Scheme 9. Reagents and conditions: (i) Tf20, DTBMP, Et,O, -78 "C. Me P
BnQ
i
q
+
Bn
VEt R=TBDMS,37 0 R=TMS, 38 R=Bn, 39
41
40
Donor
Selectivity (j3 : a )
37 38 39
1.6: 1
19
5.1 : 1 6.1: 1
91
Yield (%)
85
\
444
17 Protecting Groups
Scheme 10. Reagents and conditions: (i) BF3 . OEtz, CH2C12,-10°C to room temp.
Ac
AcO
Ac i
I
Ac BnO
*
Ac
44
AcO
OH
T&
BnO BnO
B -nBn
R
43 BnO
R’
A 43a 43P
R
R’
Yield (%)
Me0 H
H Me0
65 0
single stereochemical alteration was sufficient to rectify the situation, furnishing hexasaccharide 44u in 65% yield. Rationalization of this reversal of fortune lies in the different conformations of 43u and 43p. Molecular modeling demonstrated that the steric hindrance about the hydroxyl group of 43p was greater than that for 43u, making the hydroxyl less amenable to glycosylation. Add to the equation the steric hindrance about the donor site and the net ‘steric mismatch’ is enough to prevent coupling between donor 42 and 43p. It is unusual for such a controlling element to be so readily identified or modified and steric mismatch, whose likelihood increases with increasing size of coupling partners, can be the Nemesis of block coupling-based strategies [ 571.
17.8 Conclusions In the last 25 years much progress has been made in the development of glycosyl donors and activating systems [58],with few corners of the periodic table remaining untouched in the search for an ever-better system. With few exceptions [59], however, they all depend on the displacement of an anomeric leaving group (frequently a triflate) in a bimolecular fashion and are thus all subject to the influences outlined hitherto. That different donors provide varying results for the same glycosylation
445
17.8 Conclusions
i
BnO
30%
45
-
All
-
‘&OAIl
47
PhthN
46
a:P 23:l
Scheme 11. Reagents and conditions: (i) 0.07 M LiC104, CHZC12, room temp.
can be ascribed to their differing rates of activation under the conditions used. This was encountered by Waldmann and Schmid in their investigations into the synthesis of blood group determinants (Scheme 1 1) [60]. Coupling of the a-fucosyl fluoride 45 with the acceptor 46 proceeded with good stereocontrol to afford the a-( 1,4)-linked disaccharide 47 albeit in poor yield. When, however, the more reactive P-fluoride was employed under identical conditions, the desired disaccharide was obtained in better yield (68%) but with worse stereoselectivity ( a : P 4: 1). In this example even though the donors are of the same type, the difference in stereochemistry was enough to affect the course of the reaction. The reaction mechanism has not changed, merely the positions of the various equilibria. This result is not unique to a certain leaving group but to the entire system-the activator, the solvent, the temperature, the donor, the acceptor, the protecting groups, and the leaving group-potentially requiring that each individual reaction be optimized. This predicament prompted carbohydrate chemists, as early as 1973, to bemoan “unfortunately the reactivity of C1 and the stereoselectivity of its reaction depend in a complex way on the leaving group, the blocking groups, the configuration of the sugar and the structure of the molecule bearing the entering hydroxyl function” [611. Although the glycosylation reaction is by no means tamed, owing to the unpredictable interplay of these factors (steric effects in particular), its behavior has mostly been characterized, even if not entirely understood. Protecting groups, as first summarized by Paulsen in his seminal review [62], can alter the course of a reaction in a predictable manner; providing the shrewd chemist with some control over the reaction. To gain complete control of a glycosylation will require the development of a coupling strategy which evades these influences, one that does not depend on a bimolecular collision as its key step [63]. Acknowledgments
We thank Professor A. J. Kirby and Dr S. L. Warriner for helpful discussion in the preparation of this manuscript.
446
I 7 Protecting Groups
References 1. P. J. Kocienski Protecting Groups;Georg Thieme Verlag: Stuttgart: 1994. 2. (a) T. W. Greene, P. G. M. Wuts Protective Groups in Organic Synthesis, Wiley: New York: 1991 (b) ref [ I ] (c) M. Bols Carbohydrate Building Blocks, Wiley: New York: 1996 (d) S. Hanessian Preparative Carbohydrate Chemistry, Marcel-Dekker: New York: 1997 (e) G.-J. Boons Carbohydrate Chemistry, Blackie Academic & Professional: London: 1998. 3. B. Capon Chem. Rev. 1969, 69, 407. 4. M. L. Sinott, W. P. J. Jencks J. Am. Chem. Soc. 1980, 102, 2056. 5. (a) T. L. Amyes, W. P. J. Jencks J. Am. Chem. SOC.1989, 111, 7888 (b) M. L. Sinott Chem. Reu. 1990, 90, 1171. 6. R. Lemieux, K. B. Hendriks, R. V. Stick, K. James J. Am. Chem. SOC.1975,97, 4056. 7. A. J. Rhind-Tutt, C. A. Vernon J. Chem. Soc. 1960, 4637. 8. T. Ishikawa, H. G. Fletcher J. Org. Chem. 1969, 34, 563. 9. R. U. Lemieux, G. Huber Can. J. Chem. 1955,33, 128. 10. (a) P. Delongchamps, C. Moreau, D. FrCchet, P. Atlani Can. J. Chem. 1972, 50, 3402 (b) P. Deslongchamps ACS Symposium series: The Anomeric Effect and Associated Stereoelectronic effects; G. R. J. Thatcher Ed.; American Chemical Society: Washington, D. C., 1993; Vol. 549. 11. (a) A. J. Ratcliffe, D. R. Mootoo, C. Webster Andrews, B. Fraser-Reid J. Am. Chem. SOC. 1989, 111, 7661 (b) P. Deslongchamps, P. G. Jones, S. Li, A. J. Kirby, S. Kusela, Y. Ma J. Chem. SOC.,Perkin Trans. I 1997, 12, 2621. 12. D. Y. Curtin Rec. Chem. Proc., 1954, 15, 111. 13. There are of course exceptions: (a) where there is no available counter-ion e.g. BF30Et2 activation of anomeric fluorides and trichloroacetimidates but N.B. the catalyst coordinated ionpair is also not configurationally stable either. Some silver salts e.g. AgZC03, AgzO, and Ag silicate also operate via a ‘push-pull’ mechanism--(;. Wulff, G. Rohle, U. Schmidt Chem. Ber. 1972, 105, 111-however, in this case the initial bromides are not usually configurationally stable. ZnC12 mediated concerted nucleophilic opening of deactivated glycal epoxides relies on direct attack for selectivity. (b) In cases of solvolysis, as the nucleophile is present in the solvent cage, attack immediately follows activation of the leaving group, leading to product with inversion of the leaving group stereochemistry. A similar special case includes the reaction of trichloroacetimidates with carboxylic acids-R. R. Schmidt, J. Michel Angew. Chem. Znt. Ed. Engl. 1980,19,731-and dibenzyl phosphate-R. R. Schmidt, M. Strumpp Liebig Ann. Chem. 1984, 680-where the nucleophile is again present in the solvent cage at the point of activation of the leaving group. 14. (a) V. Marousek, T. H. Lucas, P. E. Wheat, C. Schuerch Carbohydr. Res. 1978, 60, 85. Anomeric perchlorates had been reported previously-K. Igarashi, T. Honma, J. Irisawa Curbohydr. Res. 1970, 15, 329. 15. D. Crich, S. X. Sun Tetrahedron 1998, 54, 8321. 16. C. N. Riiber, N. A. Ssrensen Kgl. Norske. Videnskab. Selskabs Skrifter 1938, I , 1. 17. W. G. Overend, C. W. Rees, J. S. Sequeria J. Chem. SOC.1962, 3429. 18. G. N. Richards Chem. Znd. 1955, 228. 19. M. S. Feather, J. F. Harris J. Org. Chem., 1965, 30, 153. 20. E. A. Kabat, H. Baer, A. E. Bezer, V. Knaub J. Exp. Med., 1948, 88, 43. 17. J. T. Edward Chem. Znd. 1955, 1102. 22. B. Fraser-Reid, A. Wu, C. Webster Andrew, E. Skowronski J. Am. Chem. SOC.1991, 113, 1434. 23. C. P. J. Glaudemans, H. G. Fletcher J. Am. Chem. SOC.1965, 87,4636. 24. N. L. Douglas, S. V. Ley, U. Lucking, S. L. Warriner J. Chem. SOC.,Perkin Trans. I , 1998, 51. 25. L. R. Schroeder, J. W. Green, D. C. Johnson J. Chem. SOC.B, 1966, 447. 26. (a) S. V. Ley, H. W. M. Priepke, S. L. Warriner Angew. Chem. Znt. Ed. Engl. 1994, 33, 2290 (b) P. Grice, S. V. Ley, J. Pietruszka, H. W. M. Priepke, S. L. Warriner, J. Chem. SOC. Perkin Trans. I , 1997, 351. 27. A plausible explanation for the increased importance of the C4 relative to the C3 based upon hyperconjugation with the endocyclic oxygen lone-pairs has been put forward-C. A. A. van Boeckel, T. Beeta, S. F. van Aelst Tetrahedron, 1984, 40, 4097.
References
447
28. Z. Zhang, I. R. Ollmann, X.-S. Ye, R. Wischnat, T. Baasov, C.-H. Wong J. Am. Chem. Soc. 1999,121, 734. 29. (a) N. L. Douglas, S. V. Ley, H. M. I. Osborn, D. R. Owen, H. W. M. Priepke, S. L. Warriner Synlett, 1996, 793 (b) U. Berens, D. Leckel, S. C. Oepen J. Org. Chem. 1995, 60, 8204 (c) J.-L. Montchamp, F. Tian, M. E. Hart, J. W. Frost J Org. Chem. 1996, 61, 3897. 30. That the C4 of galactose has a particular bearing has been noted by others (a) M. Miljkovic, D. Yeagley, P. Deslongchamps, Y. L. Dory J. Org. Chem. 1997, 62, 7597 (b) 0. Kwon, S. J. Danishefsky J. Am. Chem. Soc. 1998, 120, 1588. 31. R. U. Lemieux Adu. Carbohydr. Chem. 1954, 9, I . 32. Although participation from other positions has been postulated by many early investigators, there is little evidence to substantiate that this is possible. 33. R. U. Lemieux, C. Brice, G. Huber Can. J. Chem. 1955, 33, 134. 34. In truth this is a simplification; although attack at C1 of 6 leads to the thermodynamic product, kinetic attack occurs first at the carbonyl carbon leading to an orthoester product. If the reaction is performed under acidic conditions, the acid sensitive orthoester rearranges to form the trans glycoside. However if the donor is very deactivated the orthoester may require a higher acid concentration to induce rearrangement which can result in transesterification (to the acceptor alcohol) predominating, especially if an acetate is used. Destabilizing the orthoester by activating the donor or using benzoyl or pivaloyl protecting groups can resolve this problem. 35. This problem has been reported by many other authors (a) S. Oscarsson Topics in Current Chemistry: Glycoscience; H. Driguuez, J. Thiem Ed.; Springer: Berlin Heidelberg, 1997; Vol. 186 (b) A. V. Nikolaev, T. J. Rutherford, M. A. Ferguson, J. S. Brimacornbe Bioorg. Med. Chem. Lett. 1994, 4, 785 (c) T. Ziegler, B. Adams, P. Kovac, C. P. J. Glaudemans Carbohydr. Chem. 1990, 9, 135 (d) H. Jiao, 0. Hindsgaul Angew. Chem. Int. Ed. Engl. 1999, 38, 346 (e) N. Kochetkov The Synthesis of Polysaccharides in The Total Synthesis of Natural Products; J. ApSimon Ed.; Wiley-Interscience: New York, 1992; Vol. 8, p 245. 36. D. Depre, A. Duffels, L. G. Green, R. Lenz, S. V. Ley, C.-H. Wong Chem. Eur. J. 1999, 5, 3326. 37. J. Arnop, M. Haraldsson, J. Lonngren J. Chem. Soc., Perkin Trans. 1 1982, 1841. 38. J. M. Frechkt, C. Schuerch J. Am. Chem. Soc. 1972, 94, 604. 39. J. M. Frechkt, C. Schuerch Polymer Supported Reactions in Organic Synthesis; P. Hodge Ed.; Wiley: Chichester, 1980. 40. Schroeder et al. demonstrated that a-selectivity increased with increasing size of the alcoholL. R. Schroeder, J. W. Green, D. C. Johnson J. Chem. SOC.B 1966, 447. 41. J. E. Wallace, L. R. Schroeder J. Chem. Soc., Perkin Trans. 1 1976, 1938. 42. (a) D. K. Baeschlin, A. R. Chaperon, V. Charbonneau, L. G. Green, S. V. Ley, U. Lucking, E. Walter Angew. Chem. Int. Ed. Engl. 1998,37, 3423 (b) A. S. Campbell, B. Fraser-Reid J. Am. Chem. Soc. 1995,117, 10387 (c) T. G. Mayer, B. Krazer, R. R. Schmidt Angew. Chem. Znt. Ed. Engl. 1994, 33, 2177 (d) C. Murakata, T. Ogawa Curbohydr. Res. 1992,235, 95. 43. The A2 effect-R. E. Reeves Adu. Carbohydr. Chem. 1960, 15, 11-the axial oxygen at C2 serves to both to reinforce the anomeric effect, further lowering the ground state energy of the a-leaving group relative to the P-leaving group and sterically shields the p-face thus promoting a-glycosidation. 44. W. S. Kim, S. Hosono, H. Sasai, M. Shibisaki Heterocycles 1996, 42, 795-perchlorate counter-ions have been proposed to provide higher a-selectivities than triflates owing to their slightly reduced fugacity and ether type solvents at room temperature also serve to promote aselectivity (see chapter on solvent effects). 45. Others have used deactivated fucosyl donors-(a) M. Djeter, H. M. Flowers Carbohydr. Res. 1972, 23, 41 (b) V. Behar, S. J. Danishefsky, Angew. Chem. Int. Ed. Engl. 1994, 33, 1468 (c) R. Windmuller, R. R . Schmidt, Tetrahedron Lett. 1994, 65, 7927 (d) T. Kiyoi, Y. Nakai, H. Kondo, H. Ishida, M. Kiso, A. Hasegawa, Biocirg. Med. Chem. 1996, 4, 1167. 46, J. E. Wallace, L. R. Schroeder J. Chem. Soc., Perkin Trans. I1976, 1633. 47. R. Eby, C. Schuerch Carbohydr. Res. 1974, 34, 79. 48. P. Sinay Pure Appl. Chem. 1978, 50, 1437. 49. Boons and Zhu have exploited this effect for the synthesis of small oligosaccharides-G.-J. Boons, T. Zhu Synlett 1997, 809.
448
17 Protecting Groups
50. (a) V. Pozsgay Carbohydrates: Synthetic Methods and Applications in Medicinal Chemistry: Synthesis of Oligosaccharides Related to Plant, Vertebrate, and Bacterial Cell-wall Glycans: H. Ogura, A. Hasegawa, T. Suami Ed.; Kodansha, VCH: Tokyo, New York, 1992, p 188 (b) H. Paulsen, W. Rauwald, U. Weichert Liebig. Ann. Chem. 1988, 75 (c) K. Jaarsveld Zegelaar, S. A. W. Smits, G. A. van der Marcel, J. H. van Boom Bioorg. Med. Chem. 1996,4, 1819. 51. e.g. for sulfoxides D. Kahne, S. Raghavan J. Am. Chem. SOC.1993, 114, 1580. 52. For glycosyl bromides under halide catalysis this is the rate determining step [6]. 53. N. M. Spijker, C. A. Boeckel Angew. Chem. Int. Ed. Engl. 1991,30, 180. 54. D. Crich, S. X. SunJ. Org. Chem. 1997,62, 1198. 55. inter alia (a) M. Wakao, Y. Nakai, K. Fukase, S. Kusumoto Chem. Lett. 1999, 27 (b) R. Rodebaugh, S. Joshi, B. Fraser-Reid, H. M. Geysen J. Org. Chem. 1997, 62, 5660. 56. X.-X. Zhu, M . 3 . Cai, R.-L. Zhou Carbohydr. Res. 1997, 303, 261. 51, (a) Y.-M. Zhang, A. Brodzky, P. Sinay, G. Saint-Marcroux, B. Perly Tetrahedron: Asymmetry 1995, 6, 1195 (b) V. Pozsgay, E. P. Dubois, L. J. Pannell J. Org. Chem. 1997, 62, 2832. 58. K. Toshima, K. Tatsuta Chem. Rev. 1993, 93, 1503. 59. These include: (a) direct alkylation of the anomeric position-R. R. Schmidt, Angew. Chem. Int. Ed. Engl. 1986, 25, 212 (b) attempts at SNi based strategies-@) M. E. Behendt, R. R. Schmidt Tetrahedron Lett. 1993, 34, 10733; (ii) T. Iimori, T. Shibizaki, S. Ikegami Tetrahedron Lett. 1996, 37, 2267 iii) G. Scheffler, R. R. Schmidt Tetrahedron Lett. 1997, 38, 2943 iv) G. Scheffler, R. R. Schmidt J. Org. Chem. 1999, 64, 1319 (c) tethered strategies (see Chapter 16 (?) on 0-mannose) (d) redox glycosylation-A. G. M. Barrett, B. C. B. Bezuidenhoudt, A. F. Gasiecki, A. R. Howell, M. A. Russell J. Am. Chem. SOC.1989, 111, 1392 (e) acetal reduction-H. Ohtake, T. Iimori, S. Ikegami Tetrahedron Lett. 1997,38, 3413. 60. H. Waldmann, U. Schmid Chem. Eur. J. 1998,3, 494. 61. C. Schuerch Acc. Chem. Res. 1973, 6, 184. 62. H. Paulsen Angew. Chem. Int. Ed. Engl. 1982, 21, 155. 63. F. Baressi, 0. Hindsgaul Modern Synthetic Methods: Glycosylation Methods in Oligosaccharide Synthesis, B. Ernst, C. Leuman, Ed.; VCH, Basel, 1995.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
18 Intramolecular Glycosidation Reactions Jacob Mudsen and Mikael Bols
18.1 Introduction Carbohydrate derivatives found in nature are almost always linked to each other by glycosidic bonds. There has been increasing interest in the synthesis of biologically active oligo- and polysaccharides and reliable methods for the formation of glycosidic linkages have been the subject of intensive study. Two problems are usually encountered in the synthesis of a glycosidic bondunreactive hydroxyl groups and lack of diastereoselectivity (i.e. formation of a and epimers). To overcome these problems in the last decade chemists have explored a new approach-intramolecular glycosidation reactions. In this review we will define the term intramolecular glycosidation reaction as a type of reaction in which the glycosyl donor (the glycone) is linked to the glycosyl acceptor (the aglycone) by means of a tether producing a 0-glycosidic bond (Scheme 1). The tether can be either a temporary tether which is removed during the glycosidation reaction or it can be a stable tether which is cleaved after the glycosidation procedure. This chapter is divided into two main parts: 1) reactions in which the tether participate in the reaction; and 2) reactions in which the tether does not participate in the reaction.
Non-tethered intramolecular glycosidations which produce spiroketals [ 11 or natural products [2, 31 have been reported; these will not be discussed here.
18 Intramolecular Glycosidation Reactions
450
GIycosyl
X
donor
7d@& Tetherin procedure
L
Glycosyl acceptor
Ho przdure GI sidationb L Tethered disacchari d
RoL*
X
Scheme 1. Outline of tethered intramolecular glycosidation reactions. (T1 = temporary tether, T2 = stable tether)
18.2 Reactions in which the Tether Participates in the Reaction 18.2.1 Tethering to the Glycosyl Donor Carbon Tethers
In 1991 Barresi and Hindsgaul reported the first example of an intramolecular glycosidation procedure and introduced the expression ‘intramolecular aglycone delivery’. The idea was to use this approach in syntheses of 1,2-cis glycosidic linkages in the difficult mannose case [4]. The set-up for this reaction was thioglycoside 1, a glycosyl donor with a 2-0-acetate group (Scheme 2). The acetate was converted to methylene derivative 2 with the Tebbe reagent. Treatment of 2 with the alcohols a, b or c in the presence of a catalytic amount of acid formed the key acetals 3a-3c in moderate yield. Activation of 3a-3c with 5 equiv. N-iodosuccinimide (NIS) resulted in selective intramolecular glycosidation at the p face via intermediates 4a-4c, which on aqueous work-up gave the desired pmannopyranosides 5a-5c. In a later paper an improved method, in which the base 2,6-di-t-butyl-4methylpyridine (DBMP) was added in the activation step, was reported [5].This increased the yields of 5b and 5c significantly. Further investigation, by means of competition experiments, showed that the reaction was intramolecular, but also revealed some limitations of the method. The mixed acetals 3a-3d were relatively unstable and had to be prepared carefully to avoid hydrolysis to the corresponding monosaccharides or dimerization. Furthermore the yield dropped drastically when trisaccharides, e. g. 5d, or tetrasaccharides were synthesized [6]. A similar approach was taken by Ito and Ogawa who introducedp-methoxybenzyl (PMB) as carbon tether [7]. Alcohols protected with PMB, e.g. 6, can be oxidatively deprotected with DDQ [8] or CAN [9] presumably via the oxocarbenium intermediate 7 (Scheme 3 ) .
451
18.2 Reactions in which the Tether Participates in the Reaction
~~7% A
- Bny%FiR
reagent, Tebbe
BnO
ROH
BnO
cat. TsOH
2
SEt
SEt
BnO
4"C,20min
1:z
SEt
NIS (and base)
R- OH
BnO BnO O *"
Mixed
P-glycoside 5 without basel with base
51%
61%
-
45%
42%
17%
r
-I
Brio OMe
a
(OBn HO%
BnO Brio OMe b
OBn BnO BnO
Hosoe C
38%
BnO
NPhthBno d
-
21%
O(CHz)&H3 NF'hth
Scheme 2. Isopropylidene-tethered glycosidation.
r
6
L
+ l
7
J
CHO
Scheme 3. Protection and deprotection of PMB ethers.
OR
5
452
18 Intramolecular Glycosidation Reactions
OMe
I
Q Brio\
OPMB R-OH
BnO BnO
8
F
'
/
BnO
-
L
R- OH
~
P-glycosides
9
F
yield[%]*'
OH
Brio* BnO
OBn OBn
10a
61-14'
BnO BnO
a
OR
10
OBn
OBn
Ho?&& BnO
OBn NPhth
1oc
40
P
* Variations in different solvents(CH,CI,, ** Total yields for two reactions.
Et,O, MeCN).
Scheme 4. PMB-tethered glycosidation in the synthesis of P-mannopyranosides.
The basic idea was to trap intermediate 7 under anhydrous conditions and react it with another alcohol producing the corresponding mixed acetal. The authors showed that treatment of mannosyl donor 8 with DDQ and alcohols a-c gave the mixed acetals 9a-9c (Scheme 4). Without purification, 9a-9c were activated with AgOTf and SnC12 in the presence of DBMP. Aqueous work-up gave the expected P-mannopyranosides lOa-lOc in good overall yields. By use of this strategy the core penta- and hexasaccharides found in asparaginelinked oligosaccharides were synthesized in good yield [ 10- 121. It has recently been reported that the choice of protection groups also plays an mannose derivative important role. Thus using a 4,6-O-cyclohexylidene-protected as glycosyl donor gave the desired P-mannopyranosides in high yields [13]. Ito and Ogawa have also reported the synthesis of P-mannopyranoside linkages by use of the 'intramolecular aglycone delivery' strategy with PMB as tethering group linked to a solid phase (Scheme 5) [ 141. It is notable that the products 13a-13d contain free hydroxyl groups. By this
18.2 Reactions in which the Tether Participates in the Reaction
453
R-OH
DDQ,CH2C12 room temp.
TBSO $3hp
ph3%+oR TBSO
1
12 SMe
11 sMe
MeOTf
%&H2Cl
P-rnannopyranoside
R- OH
Yield[%]
-.aoBn 139
BnO
50
%"P
OBn
P
TBSO
6
BnO BnO
OBn
b
13b
43
13c
37
13d
54
13
OR
OBn
Ho&F BnO NPhth C
QBn HO-,H,
N3 d
Scheme 5. Solid-phase version of PMB-tethered glycosidation.
means the P-mannopyranoside derivatives will be the only compounds not connected to the polymer phase after the glycosidation reaction. Unreacted starting material, hydrolysed mixed acetals, or 1,2 elimination products will stay bound to the polymer phase. This means that the disaccharides should be easily isolated. A variation of the PMB strategy was used by Krog-Jensen and Oscarson to produce (3-D-fructofuranoside linkages (Scheme 6) [ 15, 161. Reaction of the a-Dfructofuranoside derivative 14 with mannose derivatives 15 or 16 and DDQ gave the corresponding mixed acetals 17 which were not characterized. Immediately after work-up the acetals were treated with dimethyl(methylthi0)sulfonium trifluoromethanesulfonate (DMTST) in CHZC12 to give P-D-fructofuranoside derivatives 19, both in 77% yield.
454
18 Intramolecular Glycosidation Reactions
DMTST
Bnow
____)
CHZCIZ, 71%
OBn
OBn
-
1
17
19
Scheme 6. PMB-tethered glycosidation in the synthesis of P-fructofuranosides
Other promoters (MeOTf, NIS, iodonium dicollidine perchlorate (IDCP), and iodonium dicollidine triflate (IDCT)) have also been investigated, but DMTST usually seems to be the right choice [ 161. Silicon Tethers The use of silicon as a temporary tether in intramolecular reactions has found widespread application in organic chemistry [ 171. This concept was first described by Stork et al. in 1992 for intramolecular stereocontrolled glycosidation reactions. With a silyl group connected to the 0-2 position of a P-mannosyl donor it was demonstrated that the glycosyl acceptors could contain primary and secondary hydroxyl groups [18, 191. Treatment of sulfoxide 20 with MezSiClz and alcohols a-f gave the mixed silyl acetals 21a-21f in good yields (Scheme 7). Activation of the sulfoxide with trifluoromethane sulfonic acid anhydride (Tf2O) and base, presumably resulting in the five-membered intermediates 22a-22f responsible for exclusive glycosidation at the [3 face, gave 0-mannopyranosides 23a-23f. Special interest was now devoted to glycoside 23d because it was produced in 12% yield only. It was found that compound 24, containing a silicon bridge, could be isolated as the major product in 82% yield (Scheme 8). A similar observation was made when silyl acetal 21f was activated, producing the expected glycoside 23f in 54% yield and 41% of a debenzylated compound [ 191. A similar approach was simultaneously investigated by Bols et al. Initially thioglycoside 25 was reacted with Me2SiC12 and primary, secondary, or tertiary aliphatic alcohols or phenol in the presence of pyridine to produce the corresponding silyl acetals 26a-26d in good yields (Scheme 9). Activation of 26a-26d with NIS and a catalytic amount of TfOH resulted in exclusive formation of a-glucopyranosides 28a-28d [20].
455
18.2 Reactions in which the Tether Participates in the Reaction
+B ;n
? '
. l
Me$iC12/imidazole
* : nB
BnO
ROWTHF
Hh
BnO
CH2CI2, Et2O
. l
BnO BnO
21 OGs'Ph
20 OGS'Ph
R- OH
DTBP/Tf20
BnO
'x,,
silyl a d s 21
P-mannopyranosides23
89%
92%
84%
65%
88%
82%
Brio OMe a
BnO, Bno*OMe BnO OH b
&:nB HO Brio OMe
BnO BnO
OR
23
98%
12%
60%
48%
d
HBnO O% Brio OMe e
BnO.
54%
+ @-O-debenzylated product.(as 24) 41%
Scheme 7. Dimethylsilyl-tethered glycosidation in the synthesis of fi-mannopyranosides.
It is worthy of note that only small differences were found between the reactivity of primary and tertiary alcohols a-c in the above glycosidation reactions. The study was therefore extended to include carbohydrate derivatives e, f, and g, with one unprotected hydroxyl group as glycosyl acceptor. It was found that no acid catalyst was needed when the solvent was changed from CH2C12 to MeN02. Thus activation of silyl acetals 26e-26g with NIS in MeN02 gave the desired a-glucopyranosides 28e-28g in good yield. No P-glucopyranosides were detected [21, 221. In addition
456
18 Intramolecular Glycosidation Reactions
"*,.sil BnO BnO B+ O \*OBn
OBn
BnO
'... 1 Tf20, DTBP
OMe
BnO
' g o - O B n
Et20, CH2C12,829:'
OMe
Brio BnO
o/'i'ph 24
2ld
Scheme 8. Side-reaction in dimethylsilyl-tethered glycosidation.
+
AcO AcO
2 ROSiMe&I
-
OAc
Aco%sph
Pyridine
THF, 2h, 25'C
AcO
25
..
Activation of 2694: 2.5 eq.NIS cat. TmH, CH2C12, 10 min.
I
Si
silyl acetals 26
a-glumsides28
15%
59%
Cyclohexyl b
14%
62%
terr-Butyl
16%
61%
54%
12%
R
n-Octyl
'
'OR 26
Activation of 26e-g: 2.5 eq. NIS, MeNO,
a
C
Phenyl d
I
(OBn
66%
BnO* BiO
74%
I
OMe AcO% AcO
O @ OfF
12%
85%
Ho OR
28
39%
Scheme 9. Dimethylsilyl-tethered glycosidation in the synthesis of a-glucopyranosides,
18.2 Reactions in which the Tether Participates in the Reaction
451
AcO MezSiCl2,pyridine
AcO “&SPh
alcohole,73%
*
OH
29
5
OMe
30
I) NIS, MeN02 NIS, MeNOz
OAc
31(31%)
+
AcO
OMe
0 OMe
31 R = Bn (32%) 32 R = H (49%)
Scheme 10. Debenzylation observed in dimethylsilyl-tethered glycosidation.
the intramolecular nature of the glycosidation reaction was proven by means of a competition experiment [ 231. Treatment of thiogalactopyranoside 29 with MezSiCl2 and glucose derivative e gave silyl acetal30 in 73% yield (Scheme 10). Activation of 30 with NIS in MeN02 produced the desired a-galactopyranoside 31 in 320/0yield and, highly unexpectedly, another a-galactopyranoside 32 (identified by careful spectroscopic and chemical studies) which had lost a benzyl group, in 49% yield. Compound 33 could, furthermore, be isolated when the reaction mixture was not quenched with acid, so it was proposed that debenzylation occurred via an intramolecular pathway involving silicon [22]. It has recently been demonstrated that treatment of disaccharide 31 with NIS produces the debenzylated compound 32, probably via a benzylidene intermediate and without participation of silicon [24]. In the glycosidation reactions described above the silyl groups are attached to the 0-2 positions of the glycosyl donors. Bols and Hansen investigated the effect of having the silyl group attached to other positions of the donor [25].This could be a useful strategy because a free hydroxyl group, which could be a target for further glycosidation, is obtained in this type of glycosidation reaction. Thioglucopyranoside 34 was silylated to give 35 in 77% yield. Activation of 35 with NIS in MeN02 gave a 1 : 4 mixture of glucopyranosides 36 in 22% yield (Scheme 11). Higher yields and better selectivity were obtained when the silyl group was anchored to the 0-4 position of thioglucoside derivative 37, giving exclusively the a-
458
I8 Intramolecular Glycosidation Reactions OBn
CISiMe20C8HI,, pyridine SMe HO
* C*H,,OMqSiO BnO-&&
SMe
77%
OBn
OBn
35
34
BnO& HO
0 OBn
36 a$
1:4
Scheme 11. Dimethylsilyl-tethered glycosidation with tethering to the 3 position of the donor.
glucopyranoside 38 in 45% yield (Scheme 12). Silylation of 39 at the 0 - 6 position gave P-glucopyranoside 40 and tribenzyl levoglucosan 41 in 2 :7 ratio. Finally, silylated ribose derivative 42 was treated with NIS in MeN02 to give P-riboside 43 in 63% yield. That complete stereocontrol is observed in the glycosidation of 37, 39, and 42 indicates that this approach could lead to new strategies for oligosaccharide synthesis. Kojitriose has been prepared with complete stereocontrol by use of silicon tethers at two points in the synthesis (Figure 1) [26].
6
C,H,,OMqSiO BnO
NIS, MeNO, >
SMe
--.S;i BnO
45%
BnO
OBn
37
38
OBn
arn
39
40
yJal\sph 63% YYo-
OSiMqOC,H,,
.
.
O 42 2
OBn
41 211
OH
NIS, MeNO,
.
.
O 43 X
Scheme 12. Dimethylsilyl-tethered glycosidation with tethering to the 4, 5 and 6 positions of the donor.
18.3 Reuctions in which the Tether does not Participate in the Reaction
459
HO HO
HO
HO
Figure 1. Kojitriose.
HO
Kojitriose
18.2.2 Tethering to the Leaving Group
In contrast with the above strategies, Schmidt and Behrendt investigated another approach in which the glycosyl acceptor was attached directly to the leaving group [27]. Treatment of 44 with LDA and benzaldehyde resulted in 45 as a 1 : 1 mixture of diastereomers in 90% yield (Scheme 13). Without separation of the diastereomers 45 was reacted with triflates a-c, in the presence of NaH and 15-crown-5 as base, to give predominant P-glucopyranosides 48a-48c and p-lactone 49 in good yields. It was not possible to obtain glycopyranosides 48a-48c directly from 44 and no alkylation on the hydroxyl group in 45 was observed when 45 was treated with triflates a-c, apparently because of steric hindrance. In support of this idea some alkylation was found when the a isomer of 45, which has a less hindered hydroxyl group, was treated with triflate c. It was, therefore, proposed that alkylation occurred at the oxide of the carbonyl group via intermediate 46 and orthoester 47 to produce 48a-48c and 49. A similar approach was taken by Hanaoka et al., who used CO~(CO)S as complexing agent in the glycosidation step [28]. A variation of the above strategy has been applied by Ikegami et al. Decarboxylation of a carbonate tether under acid conditions was claimed to produce intramolecular glycosidic bonds, even though anomeric selectivity was not very high [29]. Schmidt et al. have re-investigated this decarboxylative glycosidation reaction and by means of a competition experiment have found evidence that the mechanism is intermolecular rather than intramolecular [30].
18.3 Reactions in which the Tether does not Participate in the Reaction Ziegler et al. have explored another approach in an attempt to produce p-mannoside [31] or a-glucoside [32] linkages. A succinyl linker between the glycosyl donor and the glycosyl acceptor was used to tether the glycosyl donor and acceptor together. It is noteworthy that the glycosyl acceptor bears a free hydroxyl group.
460
18 Intramolecular Glycosidation Reactions
-
,OBn
-
OBn B O nFo* BnO BnO
44
LDA,THF, PhCHO, -80°C &O -nB BnO
R-OTf
Wh(1:I)
NaH, lkmwn-5 45
O
.B BnO n o
OBn
& c q
Ph
R
-
49
R-OTf
BnO
BnO
BnO
,OBn
-0H
H. 'Ph
46
47
Bno*ORBnO Bnb
48a-c
R-OTf
/OTf
B-rrlucoovranosides
Yield
48a
90%
P
48b
7Ph
P:a8:1
a
C,,H,OTf
b (OTf
Scheme 13. Leaving group-tethered glycosidation.
In contrast to the intramolecular aglycone-delivery strategy the glycosidic bond is here formed directly between the free hydroxyl group and the glycosyl donor producing a bridged disaccharide. Finally the tether can be removed. Treatment of thioglycoside 50 with 10 equivalents of succinic anhydride 51 gave mannopyranoside derivative 52 in almost quantitative yield (Scheme 14). Reaction of 52 with glucose derivative 53 then regioselective opening of the benzylidene acetal with NaCNBH3 gave the tethered saccharide 54 in good yield (Scheme 15). Activation of 54 with NIS and a catalytic amount of TMSOTf gave the bridged a-mannopyranoside 55 in 540/0yield (Scheme 16).
18.3 Reactions in which the Tether does not Participate in the Reaction
461
Scheme 14. Succinoylation of a glycosyl donor.
BnO,
I ) DCC, CH2C12,60%
52
+
HO
2) NaCNBH3 THF HCI in ether, 87%
53
SEt
54
Scheme 15. Tethering with a succinic acid tether.
0
BnO
54
0
55
Scheme 16. Succinic acid-tethered glycosidation.
The effect of connecting the succinyl linker to positions other than 0 - 2 and 0-3’ was also investigated and prearranged saccharides 56 and 57 were prepared by use of the same strategy as described above. Activation with NIS and TMSOTf gave mixtures of bridged a- and b-mannopyranosides 57 and 59 (Scheme 17). There has also been much interest in the formation of P-L-rhamnosides (6-deoxy13-L-mannopyranosides) in which the glycosyl donors and acceptors are tethered with succinyl, malonyl, or phthaloyl bridges [33-351. It has been pointed out that not only the configuration of the glycosyl donors but also the configuration of the glycosyl acceptors plays an important role in the anomeric selectivity obtained. Comparison of reaction 57 + 59 with reaction 57a + 59a, in which the glycosyl acceptor has been changed from a D-glucose derivative to an L-glucose derivative, reveals that the anomeric selectivity has changed significantly (Scheme 18) [36].
462
18 Intramolecular Glycosidation Reactions
Bnr+hr&$, 0
~~0OBn NIS, MeCN,-30°C cat. TMSOTft
BnO
BnO SEt
56 0 NIS, cat.TMSOTf MeCN, -30°C BnO BnO
BnO BnO SEt
45%~ 25%p
0 59
51
Scheme 17. Succinic acid-tethered glycosidation
22Ya
NIS,cat. TMSOTf McCN, -30°C
OBz
BnO SPh 57a
0 59a
Scheme 18. Stereodifferentiation in succinic acid-tethered glycosidation.
It has recently been found that anomeric selectivity is sometimes strongly dependent on the nature of the activating agent. Thus treatment of 60 with MeOTf or NIS resulted in the corresponding a- or p-glycopyranosides, respectively (Scheme 19). It was, furthermore, found that use of galactopyranoside derivatives with a free hydroxyl group in the 4 position as glycosyl acceptors gave P-mannopyranosides exclusively [ 371. One of the main drawbacks of all these strategies is the time-consuming task of synthesizing monosaccharides with the desired protection group patterns. To avoid too much protection-group chemistry Valverde et al. investigated the possibility of having more than one unprotected hydroxyl group on the glycosyl acceptor [38]. Tethered saccharides 61 and 62 were prepared by using phthalic anhydride to tether the glycosyl donor and glycosyl acceptor [39]. Activation with NIS-TfOH resulted in the bridged a-mannopyranoside derivatives 63 and 64,with stereo- and regiocontrol, in good yields. The tether was finally removed and the free hydroxyl groups acetylated to give the corresponding a-mannopyranosides 65 and 66 (Scheme 20).
463
18.3 Reactions in which the Tether does not Purticipate in the Reaction
Brio>
-I 00 -
60
I
OBn
Brio\ 5 eqv. NIS, TMSOTflcat.) CH&N, -3OoC, 10 min.,
OBn
/OBn
BnO
z%
Scheme 19. Dependence of anomeric selectivity on activating agent. (OTBDPS
/ 1)NaOMelMeOH
2) A%OPyridine
OAc OMe
Me0 AcO-
OMe
65
MeO. M
nu
62
64.
n,n/
YLV
Ad)
1OMe
66
,
Scheme 20. Phthalic acid-tethered glycosidation.
Interestingly, it was found that different anchoring positions on the glycosyl acceptor resulted in differences between the selectivity of the free hydroxyl groups. Thus anchoring to the 0-2' position gave the 3' a-glycoside 65 whereas anchoring to the 0-6' position gave the 4' a-glycoside 66. Treatment of 67 with CpzHfClz and AgOTf has been found to give the regioselective intramolecular glycosidation product 68 in 37% yield (Scheme 21) [40]. This regioselectivity was in agreement with Monte Carlo calculations.
I OMe
464
18 Intramolecular Glycosidution Reactions
C&HfClz, AgOTf
CH,CI,, 4A-MS, 31%
68
67
Scheme 21. Phthalic acid-tethered synthesis of a trisaccharide.
a, a’-dibromo-rn-xylene has recently been used to produce the tethered compounds 69 and 70 which, upon activation with NIS/TMSOTf, resulted in the macrocyclic derivatives 71 and 72 containing 14- or 15-membered rings respectively (Scheme 22). In both instances p( 1-4)-linkages were formed exclusively [41].
18.4 Conclusion Since the first report of an intramolecular glycosidation reaction in 1991 chemists have shown much creativity within this area. The new and interesting chemistry
1.3 eq. NIS
BnO BnO OBn
x
x = Pentenyioxy
BnO BnO
BnO BnO
OBn OMe
71
69
X = SEt
1.3 eq. NIS
BnO BnO
1 eq. TMSOTf 84%
SEt
OBn
OBn
70
Scheme 22. rn-Xylene-tetheredglycosidation.
BnO BnO
OBn
OBn
72
References
465
which has been developed reveals much about glycoside reactions and also demonstrates the difficulty in stereoselective formation of glycosidic bonds. For everyday use the PMB method seems very attractive; it is the only method extended to solid-phase chemistry. So far, however, interest has been shown only in the formation of P-mannopyranosides and fLfructofuranosides, whereas the use of silyl tethers been very well documented and successfully used in stereoselective formation of a variety of glycosidic linkages. Methods in which the glycosyl acceptor contains more then one unprotected hydroxyl group have occasionally been shown to be stereoselective but the selectivity is relatively unpredictable. Occasionally, however, theoretical calculations have been consistent with the results obtained.
References 1. P. Preuss, K-H. Jung, R. R. Schmidt, Liebigs Ann., 1992, 377-382. 2. M. Nakata, T. Tamai, T. Kamio, M. Kinoshita, K. Tatsuta, Tetrahedron, 1994, 35, 19, 30093102. 3. M. Nakata, T. Tamai, T. Kamio, M. Kinoshita, K. Tatsuta, Bull. Chem. Soc. Jpn., 1994, 67, 3057- 3066. 4. F. Barresi, 0. Hindsgaul, J. Am. Chem. Soc., 1991, 113, 9376-9378. 5. F. Barresi, 0. Hindsgaul, Synlett, 1992, 759-761. 6. F. Barresi, 0. Hindsgaul, Can. J. Chem., 1994, 72, 1447-1465. 7. Y. lto, T. Ogawa, Angew. Chem. Int. Ed. Engl., 1994,33, 17, 1765-1767. 8. Y. Oikawa, T. Yonemitsu, Tetrahedron Lett., 1982, 23, 885-888. 9. R. Johansson, B. Samuelsson, J. Chem. Soc. Chem. Commun., 1984,201-202. 10. A. Dan, Y. Ito, T. Ogawa, J. Org. Chem., 1995,60,4680-4681. 11. A. Dan, Y. Ito, T. Ogawa, Tetrahedron Lett., 1995, 36, 41, 7487-7490. 12. A. Dan, M. Lergenmiiller, M. Amano, Y. Nakahara, T. Ogawa, Y. Ito, Chem. Eur. J., 1998,4, 11, 2182-2190. 13. Y. Ito, Y. Ohnishi, T. Ogawa, Y. Nakahara, Synlett, 1998, 1102-1 104. 14. Y. Ito, T. Ogawa, J. Am. Chem. Soc., 1997, 119, 5562-5566. 15. C. Krog-Jensen, S. Oscarson, J. Org. Chrm., 1996, 61, 4512-4513. 16. C. Krog-Jensen, S. Oscarson, J. Org. Chem., 1998, 63, 1780-1784. 17. M. Bols, T. Skrydstrup, Chem. Rev., 1995, 95, 1253-1277. 18. G. Stork, G. Kim, J. Am. Chem. SOC.,1992,114, 1087-1088. 19. G. Stork, J. J. la Clair, J. Am. Chem. SOL..,1996, 118, 247-248. 20. M. Bols, J. Chem. SOC. Chem. Commun., 1992,913-914. 21. M. Bols, J. Chem. Soc. Chem. Commun., 1993, 791-792. 22. M. Bols, Tetrahedron, 1993, 49. 44, 10049-10060. 23. M. Bols, Acta Chern. Scand., 1993, 47, 829-834. 24. J. Madsen, M. Bols, Angew. Chem., 1998, 110, 22, 3348-3350. 25. M. Bols, H. C. Hansen, Chem. Lett., 1994, 1049-1052. 26. M. Bols, Acta Chem. Scand., 1996, SO, 931-937. 27. M. E. Behrendt, R. R. Schmidt, Tetrahedron Lett., 1993, 34, 42, 6733-6736. 28. C. Mukai, T. Itoh, M. Hanaoka, Tetrahedron Lett., 1997, 38, 26, 4595-4598. 29. T. Iimori, T. Shibazaki, S. Ikegami, Tetrahedron Lett., 1996, 37, 13, 2267-2270. 30. G. Scheffler, R. R. Schmidt, Tetrahedron Lett., 38, 17, 2943-2946. 31. T. Ziegler, G. Lemanski, A. Rakoczy, Tetrahedron Lett., 1995, 36, 49, 8973-8976. 32. T. Ziegler, A. Ritter, J. Huttlen, Tetrahedron Lett., 1997, 38, 21, 3715-3718. 33. T. Ziegler, R. Lau, Tetrahedron Lett., 1995, 36, 9, 1417-1420. 34. R. Lau, G. Schiile, U. Schwaneberg, T. Ziegler, Liebigs Ann., 1995, 1745-1754. 35. G. Schule, T. Ziegler, Liebigs Ann., 1996, 1599-1607.
466
18 Intramolecular Glycosidution Reactions
36. T. Ziegler, G. Lemanski, Eur. J. Chem., 1998, 163-170. 37. T. Ziegler, G. Lemanski, Angew. Chem., 1998,110, 22, 3367-3369. 38. S. Valverde, A. M. Gomez, A. Hernandez, B. Herradon, J. C. Lopez, J. Chem. SOC. Chem. Commun., 1995, 2005-2006. 39. S. Valverde, A. M. Gomez, J. C. Lopez, B. Herradon, Tetrahedron Lett., 1996, 37, 7, 11051108. 40. H. Yamada, K. Imamura, T. Takahashi, Tetrahedron Lett., 1997,38, 3, 391-394. 41. U. Huchel, R. R. Schmidt, Tetrahedron Lett., 1998,39, 7693-7694.
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
19 Classics In Total Synthesis of Oligosaccharides and G1ycoconjugates Jean-Maurice Mullet and Pierre Sinuy
19.1 Introduction Several chapters in this volume illustrate various selected methods which have been developed over the years to achieve efficient glycosylation reactions. It is quite amazing to witness the progress which has been made in the art of oligosaccharide construction, probably the most central challenge in carbohydrate chemistry. Our aim in this brief chapter is to highlight these achievements and illustrate various strategical aspects such as the judicious choice of protecting groups and of the activation of the anomeric centre, while describing a few ‘classic’ syntheses of complex oligosaccharides of potential biological importance. One inherent difficulty associated with such a chapter is the selection of what should be considered as ‘classic’ in the art of synthetic oligosaccharide construction. We are sure that the carbohydrate community will understand that many alternative brilliant accomplishments may have equally been selected.
19.2 Syntheses of Nod factors 19.2.1 Introduction
The two Nod factors 1 and 2 (Scheme 1) are secreted by bacteria fixing atmospheric nitrogen [ l , 21. The synthetic challenge associated with 1 or 2 can be summarized as follows: how to selectively differentiate, as shown in Scheme 2, hydroxyl and amino groups present in a chitin type fragment. More precisely, the strategical problems inherent to the total synthesis of Nod factors are as follows:
468
19 Classics In Total Synthesis of Oligosaccharides and Glycoconjugates OS03Na
H HO O&o&,O& HO O~oH HN
AcHN
HO
HO AcHN
-
O
1 2
AcHN
A
R=H R=Ac
Scheme 1. The structure of two classical Nod factors.
0-acetylation (Nod 2 only)
sulfation
HO HO NH2
NH2
NH2
Scheme 2. What has to be achieved to build up the Nod factors.
1. The four primary hydroxyl groups have to be carefully differentiated; the presence of a sulfate group and, in compound 2, of an additional acetate, forces the introduction of suitable temporary protecting groups on the primary positions. 2. Similarly, one amino group has also to be differentiated from the others. 3. Last, but not least, the presence of double bonds in the final product precludes the classical use of hydrogenation conditions, subsequent to the introduction of the unsaturated chain. This means for example that the very classical benzyl ether cannot be used as a protecting group in the last steps. This kind of problem is similar to the one met in the synthesis of sphingoglycolipids where a double bound is also present in the sphingosine moiety. 19.2.2 The K. C. Nicolaou Synthesis (1992) [3] The key protected tetrasaccharide to be synthesized in this approach is compound 3, the retrosynthetic analysis being presented in Scheme 3. This strategy illustrates the classical elongation process from three monosaccharidic blocks 4, 5 and 6, starting from the protected reducing end, that is compound 4. The three required blocks 4, 5 and 6 are easily prepared from a common glucosamine precursor 7 (Scheme 4). There are two noticeable features in the assembly of the monosaccharidic blocks which have to be commented for the benefit of the practitioner. First the use of the
469
19.2 Syntheses of Nod factors ,OTBDMS
OTBDPS ' MB &$LOMP NPhth
M B & M & M & M & o M p
MB
NPhth
AcHN
AcHN
AcHN
3
,OMP F
,
$ &
- O c A MB NPhth 5
Scheme 3. Retrosynthetic analysis of the Nicolaou's strategy. Abbreviations: MP (p-methoxyphenyl), MB (p-methoxybenzyl), NPhth (N-phtalimido), TBDMS (tertbutyldimethylsilyl), TBDPS (tertbutyldiphenylsilyl).
phtalimido group (see specific comments in Box 1) and second the use of glycosyl fluorides for the construction of the glycosidic linkage. The combination Cp2MC12AgC104 (M = Zr, Hf) is highly efficient for the activation of glycosyl fluorides [4], Zr-system exhibiting the most powerful reactivity. In this piece of work, the silver perchlorate has been replaced by silver triflate (Scheme 5). The blockwise synthesis selected here allows the transformation in due time of three phtalimido groups into three N-acetyl groups (see compound 3) and of the last one into the required N-acyl group: the problem of differentiation of one amino group among four is thus easily solved, as illustated in scheme 6. The rational for the choice of the various protecting groups present in blocks 4, 5 and 6 - that is
A Ac c O A S P h
Aco&oAc Ac
NPhth
a
1
NPhth 7
steps
5 and 6
+1:n
LAcoAc
9
OMe
steps
4
Scheme 4. Reagents and conditions: a) PhSH, SnC14, CH2C12, 25"C, 74%; b) MeOC6€&0H, SnC14, CHzC12, 25 "C, 72%.
470
19 Classics In Total Synthesis of Oligosaccharides and Glycoconjugates
"'AF
OTBDPS
MB
+
OTBDPS
5
H*MB oMp
NPhth
4
5
AcO&O&& MB
NPhth
MB NPhth
NPhthOMP
OTBDPS
MB
NPhth 5
MB
+ MB
MB NPhth
6
A NPhth M
d
O NPhth M
P
11
OMP NPhth
MB NPhth
Id
12
H&ooMP&oMp
MB NPhth
M
Ic
Aco&&e
FS. . .
H
+
MB
MB
AcHN
AcHN
AcHN
13
Ie
M M & BM &M &G MB
OMP NPhth
AcHN
AcHN
AcHN
3
Scheme 5. Reagents and conditions: a) 5, AgOTf, Cp2ZrCl2, 2,6-ditertbutyl-4-methyl pyridine, MS 4A, CHzC12, 0-25"C, 56%; b) K2C03, THF, 25"C, MeOH; c) 5, AgOTf, Cp2HfCl2, 2,6-ditertbutyl-4-methyl pyridine, MS 4A, CH2C12, 0-25"C, 60%; d) N2H4, EtOH, PhH, 100°C then AczO, MeOH, CH2C12, 72%; e) 6, AgOTf, CpzHfCl2, 2,6-ditertbutyl-4-methyl pyridine, MS 4A, CH2C12,0-25 "C, 50%.
in the resulting protected tetrasaccharide 3 - is fully apparent when contemplating the chemical conversion of 3 into the two final targets 1 and 2, as illustrated in Scheme 6 . Final comments: An interesting feature of the Nicolaou's strategy is the judicious choice of three different protecting groups for the primary alcoholic positions, which on one hand withstand the glycosylation conditions, and on the other hand can be sequentially removed. Such a nicely tuned strategy indeed allows the only synthetic access described so far to the acetylated Nod factor 2. Weaker points are the moderate yields resulting from the use of the fluoride glycosylation method and
19.2 Syntheses of Nod factors M B o g M & M & M & o M p
a
3
47 1
MB NHZ
AcHN ,4
AcHN
AcHN
M B o g M & MB&M&M&Mm?DFg -
RCOOH
MB
b
HN 0
-
AcHN
AcHN
-
AcHN
15
C
OMP
MB
H 0
-
AcHN
AcHN
-
AcHN
16
d
M B o & M & M & M & OMP
MB HN - - - - - -O
e --t
2 (Nod-factor 2)
AcHN
f
AcHN
-
AcHN
17 1 (Nod-factor 1)
Scheme 6. Reagents and conditions: a) N2H4, EtOH, lOO"C, 87%; b) RCOOH (3eq), 2-chloro-lmethyl pyridinium iodide, Et3N, CH3CN, 25 "C, 73%; c ) PPTS, EtOH, 25 "C then Ac20, Et3N, DMAP (cat), CH2C12, 2 5 T , 72%; d) TBAF, THF then Me3NSO3, Pyr. 25"C, 85%; e) CAN, CH3CN, H20, 25"C, 30%, f ) MeONa, MeOH, 2 5 T , 75%.
the rather low yield of the simultaneous removal of MB and MP protecting groups on a complex molecule. 19.2.3 The J.-M. Beau Synthesis (1994) 112)
This is another example of synthesis of an oligosaccharide through stepwise addition of monosaccharide blocks. But in sharp contrast to the previously described achievement, it is representative of a nonconventional construction of a linear oligosaccharide starting from the nonreducing end of the oligomer. The protected tetrasaccharide is built up from two precursors as shown in Scheme 7. The detailed synthetic route is shown in Scheme 8.
472
19 Classics In Total Synthesis of Oligosaccharides and Glycoconjugates
19.2 Syntheses of Nod factors ,OPiv
l
,OPiv
,OPiv
473
,OAc
18
,OAc
Scheme 7. Retrosynthetic analysis of the Beau’s strategy. Abbreviations: Z (benzyloxycarbonyl), Bn (benzyl), Piv (Pivaloyl).
The 1,2-trans selectivity of the first glycosylation reaction is insured by a carbamate participating group, which is not so frequently used (see Box 2) compared to the now very classical phthalimido group. It is worth noting that the acetolysis of l16-anhydrosugars is a rather smooth process, compatible with glycosidic linkages present in the molecule. Such a behaviour qualifies the 1,6-anhydro ring for this rather atypical type of left to right elongation. A feature of this strategy is the daring choice of a nonparticipating azido group at C-2 to successfully achieve a 1,2-trans glycosylation through a clean SN2 process. There are now more and more examples of this, so that such an option should not be disregarded in a synthetic strategy (see Box 3). Looking at the final steps of this strategy (Scheme 9), it is apparent that the fatty acid chain is introduced at the very last step, permitting the use of benzyl ethers as temporary blocking groups. Final comments: It has been stated in the previous section (see Box 1) that N-phthalimido group insures a perfect (3-selectivity, but is sometimes difficult to remove. Especially, such a removal is not compatible with acetates. The J.-M. Beau synthesis highlights an attractive option: the use of the nonparticipating azido group, the selectivity being intrinsically insured by a clean S N process ~ under appropriate experimental conditions. 19.2.4 The T. Ogawa Synthesis (1994) 1161
The strategy designed by T. Ogawa et al. uses three blocks, one being a disaccharide donor (Scheme 10). This strategy (Scheme 10) is rather close to that described by K. C. Nicolaou. The central block 37 has been prepared from a chitobiose derivative (171. The option of the N-phthalimido participating group has also been selected (see Box 1). The in-
19 Classics In Total Synthesis of Oligosaccharides and Glycoconjugates
414
B KBn r & & , ZHN
&g a
+
OCNHCC13
OH
19
C
N3
N3
21R=Ac 22R=Piv
NHZ
-
B"&m+
Bn&o+
d
Bn
NHZ
NHZ
N3 OAc
23
N3 OCNHCCI,
24
R
20
0
__t
NHZ
e
bl
Bn
20
Bn
*n&O
25
B Bn n & B * BNHZ 4
N3
f
N,
20 __t
'
26
"&&Bn ~&o
'"OCNHCCI,
9
N3 NHZ
N3
h
1
27R=Ac 28R=Piv
N3
Scheme 8. Reagents and conditions: a) BF3 EtzO, Toluene, -78°C > -2O"C, 91%; b) MeONa, MeOH then PivC1, DMAP, Pyr. 2 0 T , 100%; c) Ac20, TFA, rt; d) BnNH2, Et20, rt, then CCl3CN, DBU, CH2C12, rt, 73% 3 steps; e) 20, BF3 EtzO, CHzC12 -78°C > -2O"C, 85%; f ) Ac20, TFA, rt; 73%; BnNH2, Et20 rt, then CC13CN, DBU, CH2C12, rt, 73% 3 steps, g) 20, BF3 Et20, CH2C12 -78°C > -2O"C, 91%; h) MeONa, MeOH then PivC1, DMAP, Pyr. 20"C, 100%.
troduction of the unsaturated fatty acid is performed at the very last step, a feature which allows the initial protection with benzyl ethers (see also J.-M. Beau). The protected tetrasaccharide is obtained as described in Scheme 11. The deprotected tetrasaccharide 34 is easily obtained as depicted in Scheme 12 and is identical to the one previously obtained by J.-M. Beau. Final comments: As persistently stated in this chapter, the stability of protecting groups during the removal of N-phthalimido has to be carefully considered while selecting a strategy. This work points out the stability of a primary sulfate function during hydrazinolysis of the N-phthalimido group (Scheme 12).
19.2 Syntheses of Nod factors
A
A
C
O
S
S
~+
HJ
COW
NH-CO-Ct+CCI,
EE rt
-r
415
AK Al-Jg&Jcorm
NH-CC-CH&CI~ 85%
19.2.5 The Y. Z. Hui Synthesis (1992) [lS] The strategy used by Hui et al. illustrates the so-called block synthesis, more precisely the synthesis of a protected tetrasaccharide from two appropriate disaccharidic moieties, as shown in Scheme 13. The disaccharidic glycosyl donor 49 was first prepared as described in Scheme 14. As previously pointed out in Box 3 and exemplified in J.-M. Beau et al. ~ approach, it is possible to achieve a 1,2-trans glycosylation through an S N process,
19 Classics In Total Synthesis of Oligosaccharides and Glycoconjugates
476
a
28
OR NHZ
C
N3
bJ
N3
N3
N3
N3
NHAc
OBn NHAc
NHAc
NHAc
NHAc
NHAc
29 R= AC 30 R = OCNHCC13
B n & B & & B &
OBn NHZ
N3 18
d
Bn&B&&Boi& NHZ
-
NHAc
Bn&B&&Boscg
31
.
8
OBn NHZ
NHAc
32
-----t f
Bn&Bn &B&&OBn NHZ
NHAc
33
-
H & & & &
9
OH NH2
AcHN
AcHN
h
AcHN
34
Scheme 9. Reagents and conditions: a) TFA, Ac20, 81%; b) BnNH2, Et20 then CC13CN, DBU, CH2C12;c) BnOH, BF3.Et20,toluene, 79%; d) NaBH4, NiC12, DME-EtOH, then Ac20, Fyr.; e) MeONa, MeOH, 52%; Me3NS03, DMF, 50"C, 98%; f ) MeONa, MeOH, 88% g) H2, Pd/C AcOEt, MeOH, H20; h) N-acylation 60%.
using a nonparticipating azido group at C-2. Another feature of this synthesis is the very classical choice of the ally1 group as a temporary protecting group of an anomeric centre. The disaccharidic acceptor 50 is obtained as described in Scheme 15. A noticeable aspect of this synthetic sequence is the use of a monochloroacetate protecting group, which is easily removed under mild conditions fully compatible with N-phtalimido and pivaloyl groups.
19.2 Syntheses of Nod factors
471
AcO AcO NPht
NHAc
NHAc
fl AcO AcO &F
NHAc
35
A BnO c o & O ~ CBnO N H C C I 3 NPht
NPht 37
36
HBnO o g
NPht
o
B
n
38 NPht
Scheme 10. Retrosynthetic analysis of the Ogawa's strategy.
AcO BnO
CNHCC13
+
-
HO BnO
MP
NPht
BnO NPht
NPht
39
40
41
NPht
PMP
b
I t "
AcO AcO*O*CNHCC13 BnO Rnn BnO &O&CNHCCl3 BnO NPht 37
AcO BnO NPht
NPht
d
OBnAcOAcO +F
BnO NHAc
*
-
A0&.& BnO NHAc
n
C
OBn NPht
NPht
HO
BnO
-
Ho*OBn4 O B Brio Brio 38 NPht
U
NPht
*
NHAc
e
43
35
Scheme 11. Reagents and conditions: a) BF3.Et20, CH2C12, -78 "C?82%; b) CAN, CH3CN, H20, then CC13CN, DBU; c) BF3.Et20, CH2C12, -78"C, 69%; d) N2H4, EtOH, then Ac20, MeOH, 99%; e) Cp2HfCl2, AgOTf, (CH2Cl)2,79%.
I9 Classics In Total Synthesis of Oligosaccharides and Glycoconjugates
478
35
a
AcO AcO NPht
NHAc
NHAc
OBn NHAc
NH2
NHAc 46
NHAc
OBn NHAc
1
45 R= S03Na
C
HO HO
d
HOHO &o
- & o &HOo g o H NH2
HO
AcHN
HO AcHN
AcHN
34
R4-h 47 0
Scheme 12. Reagents and conditions: a) CAN, CH3CN, H20; b) Et3NSO3, DMF, 50"C, 70% two steps, c) N2H4, EtOH; d) Pd-black, Hz, MeOH, H20, 88% two steps; e) MeOH, Hz0, 97%.
BnO BnO
N3
NPhth
BnO
Bn NPhth
NPhth
48
BnO BnO
-C(NH)CC13 N3
HBnO O & A OBnO B n
NPhth 49
Scheme 13. Retrosynthetic analysis of the Hui's strategy.
NPhth
50
NPhth
19.2 Syntheses of Nod factors
BnO% BnO
+
51
b
bC(NH)CC13
BnO BnO - &
a
HBnO O G , , ,
Bno&o&Al~ BnO
BnO
NPhth
N3
NPhth
52
& N3 O BnO 54 a
479
53
H NPhth L
BnO BnO &O&
N3 BnO
NPhth I
55
BnO-*BnO
BnO
NPhthO-C(NH)CCI,
N3 49
Scheme 14. Reagents and conditions: a) BF,.Et20, MS 4p\, CH2C12, -20"C, 61%; b) PdCI2, NaOAc, aq AcOH, 5O"C, 84%; c) (COC1)2,DMF (cat), 88%; d) CC13CN, DBU, CH2C12, 84%.
57
58
BnO
BnO
NPhth
59
O-C(NH)CC13 NPhth
60
DPiv Ho BnO *OBn
'\
61
HO BnO
BnO NPhth 50
NPhth
OBn NPhth
NPhth
62
Scheme 15. Reagents and conditions: a) ClCH2COC1, Pyr. O T , 88%; b) PdC12, NaOAc, aq AcOH, 50"C, 89%; c) (COC1)2,DMF (cat) ; 89%; d) CC13CN, DBU, CHzC12,85%;e) AgOTf, MS 4A, CHzC12, -1O"C, 58%; f ) BF3.Etz0, CHZC12, -15"C, 76%; g) Thiourea, Pyr, EtOH, 7 0 T , 86Yo.
480
19 Classics In Total Synthesis of Oligosaccharides and Glycoconjugates
The final steps of the synthesis are shown in Scheme 16. The imidate 49 appeared as a better glycosylationg agent than the chloride 55. It has to be pointed out that the final amine 34 is the one previously obtained by J.-M. Beau and T. Ogawa. Final comments: A recurrent theme in the synthesis of Nod factors is the differentiation of the terminal amino group. It is achieved here in a very simple way, using a combination of azido and N-phthalimido groups.
19.2.6 Conclusion There are always two questions when launching an oligosaccharide synthesis: 1. which protecting groups have to be used. 2. which glycosylation methods have to be selected. There are many glycosylating agents developed so far. Nevertheless, only two methods have been selected by the four groups: the fluoride method and the trichloroacetimidate method. As far as the protecting groups are concerned, a larger diversity has been exploited. A careful comparison of these four achievements illustrates many facets of the art of oligosaccharide synthesis.
19.3 Synthesis of the Antithrombin-Binding Pentasaccharide Sequence in Heparin (1984) [19, 201 19.3.1 Introduction Heparin is an extraordinary complex sulfated glycosaminoglycan (GAG) with well known anticoagulant and antithrombotic activities. The coagulation of blood is the result of a cascade of events implying a series of proteases called activated bloodcoagulation factors. Antithrombin (AT) is the main physiological inhibitor of these factors and heparin is known to bind to AT and to amplify its action. A breakthrough in heparinology came in the early 1980’s with the recognition [21, 221 of a specific pentasaccharide sequence within the polysaccharide, capable of binding to AT. The total synthesis of this pentasaccharide 67 (Scheme 17) has first been reported in 1984 [ 191, confirming the specific nature of the interaction heparin/AT. We can consider this selected piece of work as a typical classic because:
1. This is the first synthesis of an oligosaccharidic fragment of heparin with a prominent biological activity [23]. 2. It opened the door to a series of syntheses of other biologically active compounds containing variants of this pentasaccharide.
19.3 Synthesis of the Antithrombin-Binding Pentusacchuride Sequence 55
+
48 1
50
49 +50
b
BnO BnO N3
NPhth
NPhth
OBn NPhth
63
C
BnO BnO
N3
BnO
NHAc
NHAc
NHAc
NHAc
OBn NHAc
64 d __t
BnO BnO
N3
e
NHAc
65 HOHO +o
& o &HO o~:H NH2
HO
AcHN
L1
HO AcHN
AcHN
34
Scheme 16. Reagents and conditions: a) AgOTf, MS 4A, CH2C12, -2O"C, 50%; b) BF,.Et20, CH2C12, -15"C, 72%; c) N2H4, H20, EtOH, 9 5 T , then AczO, Pyr., 2 0 T , then KOH, MeOH, THF 20"C, 66% in 3 steps; d) Pyr.SO3, DMF, 50°C; e) Hz, 10% Pd/C, THF, EtOH, H20, 83%; f ) 66, Et3N (cat), EtOH, 45 "C, 45%.
3. This synthetic pentasaccharide and analogues are potential antithrombotic drugs devoid of certain detrimental side effects when compared with natural standard heparin [24].
19.3.2 An Overview of the Synthesis of the Protected Pentasaccharide 73 The synthesis, which is summarized in Scheme 18, is based on the use of three blocks: two disaccharides and one monosaccharide. The hydroxyl groups which are going to be sulfated are protected by acetyl groups, the hydroxyl groups being free in the final pentasaccharide are protected as benzyl ethers.
482
19 Classics In Total Synthesis of Oligosucchurides and Glycoconjugutes
OH
67
oso3-
Scheme 17. The structure of the specific heparin pentasaccharide sequence binding to AT.
19.3.3 Synthesis of the Disaccharidic Bromide Donor 68 The preparation of this bromide is described in Scheme 19. The bromide 75 is easily prepared from the corresponding hemiacetal using the Vilsmeier reagent. This represents a mild and convenient way to achieve this kind of transformation (see Box 4). The selective formation of a P-glucoside from 75 goes through a so called push-pull mechanism, that is an SN?process at the surface of an insoluble silver salt such as silver carbonate. This method complements the combination a-trichloroacetimidate/ BF3.Et20, which has been put into practice in the previous classic of this chapter.
19.3.4 Synthesis of the Disaccharidic Acceptor 69 The glycosylating species which has been used is the orthoester 79 (Scheme 20). This derivative of methyl L-iduronate is an interesting compound. The tertbutyl orthoester fulfills three functions: it insures the protection of positions 1 and 2, a necessary step for the selective synthesis of 81; it reacts with an alcohol under acidic conditions to give a 1,2-trans glycoside, with the concomitant introduction of an acetate function at position 2 (a progenitor of the sulfate group). This is an example of the orthoester procedure nicely developed by the Kotchetkov group in the 1960s [28]. Such a method is rarely used nowadays. The synthesis of 69 is presented in Scheme 20: the orthoester glycosylation is followed by the selective removal of the monochloracetate on position 4.
19.3.5 Synthesis of the Protected Pentasaccharide 73 The two glycosylation reactions 68 & 69 then 71 & 72 (see Scheme 18) illustrate the use of a bromide, in the presence of silver triflate, to synthesize an a-glycoside. It should be noted that the presence of a participating acetyl group at C-6 probably insures an excellent a-selectivity (Scheme 2 1). Without this participation, the re-
19.3 Synthesis of the Antithrombin-Binding Pentasaccharide Sequence
M Bn c A & A 4 BnO
q i
+ 68
N3Br
483
HO
1
69 ZHN OBn
OAc
a
O , Ac
70
I OAc
I b
t
.OAc
,OAc
72
lc 73
I
OAc
I OAc
Scheme 18. Reagents and conditions: a) AgOTf, collidine, CHZC12, -2O"C, 55%; b) thiourea, pyr. EtOH, 100 "C, 82%; c) AgOTf, collidine, CH2C12, -20 "C, 70%.
sulting selectivity is more difficult to predict and relies on subtle factors, such the nature of the solvent and the temperature. 19.3.6 Synthesis of the Active Site of Heparin
The final steps are shown in Scheme 22. This is now a classical sequence of reactions in the field of glycosaminoglycans. Final comments: With eight sulfonato groups on specific oxygen and nitrogen atoms of the molecule, with the presence of the very unusual L-iduronic acid residue, this molecule represents a formidable synthetic challenge. It is amazing to ob-
484
19 Classics In Total Synthesis of Oligosaccharides and Glycoconjugates
Bn
OH
BnO
-
Bn
Bn
---+
M~A&~& Bn
C
BnO
75
74
-
b 77
68
d
BnO 78
N3 O
A ~
Scheme 19. Reagents and conditions: BrCH=N+Me*, Br-, collidine, CH2C12, 0 "C, 90%; b) Ag2C03, MS 4 A , CH2C12, rt,70%; c) TFA, Ac20, rt, 86%; d) TiBr4, CH,Cl,/AcOEt, rt.
Box 4 Preparation of anomeric bromides
,OAc
79
\
80
Scheme 20. Reagents and conditions: a) 2,6-dimethylpyridinium perchlorate, PhCl, 130 "C, 40%; b) thiourea, Pyr. EtOH, lOO"C, 86%.
19.4 Totul Synthesis of VIM-2 Ganglioside
485
Scheme 21. A possible remote acetyl group participation in a glycosylation reaction.
,OAc
Scheme 22. Reagents and conditions: a) NaOH, CHCI3, MeOH, H20, rt then CH2N2 73%; b) Me3N:S03, DMF, 5 0 T , 90%; c) NaOH, MeOH, H20, 90%; d) H2, 10% Pd/C, MeOH,
H20, 93%; e) Me3N:SO3, H20 (pH = 9.5), 36%
serve that closely related analogues of this compound have now been prepared in bulk quantities as drug substances. Following excellent pharmacokinetics and tolerance studies in man [29], clinical investigations are currently underway to confirm the therapeutic potential of such complex carbohydrate derivatives. This is an excellent example of the power of organic synthesis in the field of glycoscience [30].
486
19 Classics In Total Synthesis of Oligosaccharides and Glycoconjugates
H
84
Scheme 23. Structure of a natural human sphingoglycolipid called VIM-2.
OTBDPS
f'
p- glycosylation
HO-p-13H27 N3
c
85
Reduction and fatty acid amidification
Scheme 24. Overall strategy used by Hasegawa et al. to synthesize VIM-2.
19.4 Total Synthesis of VIM-2 Ganglioside [31] 19.4.1 Introduction The so called VIM-2 ganglioside 84 (Scheme 23) is a natural sphingoglycolipid which is regarded as a tumor associated carbohydrate antigen. It has been isolated from human chronic myelogenous leukemia cells. Many compounds of this type have been synthesized. We would like to present in this section the achievement of the late Hasegawa. It is a typical example of the block synthesis approach of a sphingoglycolipid. 19.4.2 The Total Synthesis of VIM-2-a
General Strategy
A key step of the strategy is the glycosylation of compound 85 [32] (Scheme 24) with a complex glycosyl donor which is fully acylated. The presence of any benzyl ether protecting group is precluded at this point of the synthesis, due to the presence of an easily reducible double bond in compound 85; catalytic hydrogenolysis should be avoided. In the case of VIM-2, the fully acylated glycosyl donor is the trichloroacetimidate 87, which has been obtained from the 2- (trimethylsilyl)ethyl glycoside 86 (Scheme25). The 2-(trimethylsilyl) ethyl glycoside, popularized by G. Magnusson 1331 is often used as a temporary protecting group of the anomeric position, and is easily trans-
19.4 Total Synthesis of VIM-2 Ganglioside B
z
W
~
Bzo' OBz
OAcAc
NHAc 86
a, b
~
Ac OAc
487
&
~
dOAc 1 NHAc
OAcAc
0 OAc
OAc OAc
Scheme 25. Reagents and conditions: a) CF3COOH/CH2C12; b) CC13CN, DBU.
formed in a two step sequence here exemplified in an efficient glycosyl donor. It nicely complements in this respect the ally1 group (see the Hui's synthesis of the Nod factor).The main part of the synthesis is thus the preparation of the 2-(trimethylsilyl) ethyl glycoside 86. Let's now analyse the main characteristics of this achievement. 19.4.3 Preparation of the Key Protected Octasaccharide 86
A protected trisaccharidic Lewis X thiophenyl glycoside 92 is prepared as shown in Scheme 26. Several features of this synthesis have to be pointed out. In the diol89, the position 4 is more reactive than position 3, so that there is no need for a temporary protection at position 3, an observation which simplifies the synthetic scheme. This is due to the presence of the bulky neighbouring phthalimido group. This diol is thus an interesting pivotal acceptor for a double glycosylation sequence. The thiophenyl glycoside 89 can be glycosylated with the bromide 88 (so called orthogonality, which has not always been perfectly observed in this case). The bromide 88 is benzylated at the primary position, a feature which will be commented later on. The thiophenyl Lewis X trisaccharide 92 is now condensed without any problem with the easily available alcohol 93 to give the pentasaccharide 94, as shown in Scheme 27. De-0-benzoylation of 94 gave the triol 95. This triol is next glycosylated with the previously reported thiophenyl glycoside 90, as shown in Scheme 28. This critical step of the synthesis demonstrates that the
./
32
488
19 Classics In Total Synthesis of Oligosaccharides and Glycoconjugates
+
H&SPh
Bz
NPhth
Bzo Br
aa
a9
a
BZ 90
Bzs G
b
Bz
h
OBz
?
O
B
M
T
h
n
OBn OBn 92
Scheme 26. Reagents and conditions: a) AgOTf, toluene, -4O"C, 75%; b) NIS, TfOH, toluene, - 15 "C, 93%.
93
W
O
B
n
94
OBn OBn
Scheme 27. Reagents and conditions: a) NIS, TfOH, CH&, 98%.
-6O"C, 86%; b) MeONa, MeOH,
19.4 Total Synthesis of VIM-2 Ganglioside
90+95
-
489
a
BZ OBn
96 b
NHAc OBn
97
Scheme 28. Reagents and conditions: a) NIS, TfOH, CH2C12, -6O"C, 83% b) N2H4, EtOH, H 2 0 , reflux, then AczO, MeOH, 40 "C, 93%.
trio1 95 can be selectively glycosylated at position 3 , a feature which again considerably simplifies the strategy. We now understand why the primary position of the terminal galactosyl residue should be benzylated, and not benzoylated (see compound 88). The more reactive primary hydroxyl group has to be protected by a persistent blocking group. At this stage, the N-phthalimido groups are converted into N-acetyl groups, an operation which is more problematic when the sialyl residue is present. The sialylation of the polyol 97 is finally most easily achieved using the sialyl donor 98, as shown in Scheme 29. It is remarkable that the regioselective sialylation of the required hydroxyl group is achieved in 72% yield, in the presence of several non protected hydroxyl groups. Final steps are now uneventful and are shown in the last Scheme 30. Final comments: Such an overall strategy can be utilized to synthesize a variety of oligosaccharides of the polylactosamine type, with additional possible L-fucose residues. It is thus of general interest and can be considered as a classic. Many considerations in this synthesis have also been delineated by other groups, and this achievement reflects a nice strategical manipulation of many interesting observations done over the years.
19.5 Epilogue The aim of this chapter was to convince the reader that the chemical organic synthesis in solution of oligosaccharides of any complexity is now feasible. More than
490
19 Classics In Total Synthesis of Oligosaccharides and Glycoconjugates
97 +
BzM C02Me
a_
OBz
98
AcH
99
b
86
Scheme 29. Reagents and conditions: NIS, TfOH, CH3CN, -43"C, 72%).b) H2, Pd/C, MeOH/ EtOH/AcOH, then Ac20, Pyr., 63%.
85
+
87
AcH
OAc OAc
100
Scheme 30. Reagents and conditions: a) TMSOTf, CH2C12, 0"C, 47%; b) Ph3P, PhH, H20; c) octadecanoic acid, 2-chloro-1,3-dimethylimidazolium chloride, PhH. 80%; d) NBQF, CH3CN; e) MeONa, MeOH then water, 87%.
enough glycosylation methods are now available, and a great deal is known about the control of the selectivity. Nowadays, organic chemists are more than ever in a position to collaborate with glycobiologists, to contribute to the development of gly coscience. References 1. P. Lerouge, P. Roche, C. Faucher, F. Maillet, G. Truchet, J.-C. Promt, J. Denarie, Nature, 1990,344, 781.
References
49 1
2. P. Roche, P. Lerouge, C. Ponthus, J.-C. Prome, J. Biol. Chem., 1991,266, 10933; G. Truche, P. Roche, P. Lerouge, J. Vasse, S. Camut, F. De Billy, J-C. Prome, J. Denarie, Nature, 1991, 351, 670; M. Schultze, B. Quiclet-Sire, E. Kondorosi, H. Virelizier, J. N. Glushka, G. Endre, S. Gero, A. Kondorosi, Proc. Nat. Acud. Sci. USA, 1992, 89, 192. 3. K. C. Nicolaou, N. J. Bockovich, D. R. Carcanague, C. W. Hummel, L. F. Even. J. Am. Chem. Soc. 1992,114, 8701. 4. T. Matsumoto, H. Maeta, K. Suzuki, G.4. Tsuchihashi, Tetrahedron Lett., 1988, 29, 3567. 5. S. Akira, T. Osawa, Chem. Pharm. Bull., 1960, 8, 583. 6. R. U. Lemieux, T. Takeda, B. Y. Chung, In Synthetic Methods For Carbohydrates, Ed. Hs. El. Khadem, ACS Symposium Series. 39, 1976, 99. 7. P. L. Durette, E. P. Meitzner, T. Y. Shen, Tetruhedron Lett., 1979, 4013. 8. 0. Kanie, S. C. Crawley, M. M. Palcic, 0. Hindsgaul, Curbohydr. Res. 1993, 243, 139. 9. D. R. Bundle, S. Josephson. Can. J. Chem., 1979, 57, 662. 10. J. S. Debeham, R. Madsen, C. Roberts, B. Fraser-Reid, J. Am. Chem. Soc., 1995, 117, 3302. 11. a) J. C. Castro-Palomio, R. R. Schmidt, Tetrahedron Lett., 1995, 36, 5343. b) Zbid, Liebigs Ann., 1996, 1623. 12. a) D. Tailler, J.-C. Jacquinet, J.-M. Beau, XVltlz Intern. Carbohydr. Symp., Paris 1992, A298; b) D. Tailler, J.-C. Jacquinet, J.-M. Beau. J. Chem. Soc, Chem. Commun. 1994, 1827. 13. a) H. Paulsen, B. Helpap, Carbohydr. Res., 1991,216, 289. b) D. Tailler. J.-C. Jacquinet. A-M. Noirot, J. M. Beau, J. Chem. Soc. Perkin. I , 1992, 3163. c) M. Schultz, H. Kunz, Tetrahedron Lett. 1992,33, 5319. d) E. Kaji, F. Lichtenthaler, Y. Osa, S. Zen, Bull. Soc. Jp., 1995,68, 1172. e) A. Vargas-Berengel, M. Meldal, H. Paulsen, K. Bock. J. Chem. Soc. Perkin 1, 1994, 2615. 14. J. Banoub, P. Boullanger, D. Lafont, Chem. Rev., 1992, 92, 1167. 15. W. Kinzy, R. R. Schmidt, Carbohydr. Rex, 1989, 193, 33. 16. S. Ikeshita, A. Sakamoto, Y. Nakahara, Y. Nakahara, T. Ogawa. Tetrahedron Lett., 1994, 3123. 17. H. Kuyama, Y. Nakahara, T. Nukada, Y. Ito, Y. Nakahara, T. Ogawa, Carbohydr. Res. 1993, 243, C1. 18. a) L. X. Wang, Y. Z. Hui, XVIth Intern. Carbohydr. Symp., Paris A203. 1992; b) L. X. Wang, C. Li, Q. W. Wang, Y. Z. Hui, Tetrahedron Lett., 1993, 34, 7763; c) Ihid., J. Chem. Soc. Perkin I, 1994, 621. 19. P. Sinay, J.-C. Jacquinet, M. Petitou, P. Duchaussoy, I. Lederman, J. Choay, Carbohydr. Rex, 1984, 132, C5. 20. M . Petitou, P. Duchaussoy, I. Lederman, J. Choay, P. Sinay, J.-C. Jacquinet, G. Torii, Curbohydr. Res. 1986, 147, 221. 21. J. Choay, J.-C. Lormeau, M. Petitou, J. Fareed, Ann. N. Y. Acad. Sci. 1981, 370, 644. 22. L. Thurnberg, G. Backstrom, U Lindahl, Carbohydr. Res. 1982, 100, 393. 23. see also: C. A. A van Boeckel, T. Beetz, J. N. Vos, A. J. M. de Jong, S. F. van Aelst, R. H. van den Bosch, J. M. R. Mertens, F. A. van der Vlugt, J. Carbohydr. Chem. 1985, 4, 293. 24. M. Petitou, J.-P. Herault, A. Bernat, P.-A. Driguez, P. Duchaussoy, J.-C. Lormeau, J.-M. Herbert Nature, 1999, 398, 417. 25. D. R. Hepburn, H. R. Hudson, J. Chem. Soc. Perkin 1, 1976, 754. 26. H. Paulsen, C. Kolar, W. Stenzel, Chem. Ber. 1978, 111, 2358. 27. F. W. Lichtenhaler, E. Kaji, S. Weprek, J. Org. Chem. 1985, 50, 3505. 28. N. K. Kochetkov, A. J. Khorlin, A. F. Bochkov, Tetrahedron, 1967, 26, 693. 29. B. Boneu, J. Necciari, R. Cariou, A.M. Gabaig, G. Kieffer, J. Dickinson, G. Lamond, H. Moelker, T. Mant, H. Magnani, Thromb. Huemost., 1995, 74, 1468. 30. P. Sinay, Nature, 1999, 398, 377. 31. T. Ehara, A. Kameyama, Y. Yamada, H. Ishida, M. Kiso, A. Hasegawa, Curbohydr. Res., 1996,281; 237. 32. A. Hasegawa, T. Nagahama, H. Ohki, M. Kiso, J. Carbohydr. Chem., 1992, 11, 699. 33. K. Jansson, S. Ahlfors, T. Frejd, J. Kihlberg, G. Magnusson, J. Org. Chem. 1988, 53, 5629.
Part I Volume 1
I1 Synthesis of Oligosaccharide Mimics
Carbohydrates in Chemistry and Biology Beat Ernst, Gerald W. Hart, Pierre Sinay copyrightQ WILEY-VCH Verlag GmbH, 2000
20 Synthesis of C-Oligosaccharides Troels Skrydstrup, Boris Vnuzeilles, and Jean-Marie Beau
20.1 Introduction Cell-surface carbohydrates are well known for participating in key molecular recognition events with protein receptors [l]. Such interactions can be beneficial, as in cell-to-cell recognition and binding, or detrimental, as in bacterial [2] and viral infections 131, and inflammation [4]and metastasis 151, the initial steps of which can involve carbohydrate-protein interactions. To combat such diseases it would be ideal to prepare small, soluble carbohydrate-based pharmaceuticals which inhibit these recognition events by binding preferentially to the protein receptors. The potential instability of such small, soluble oligosaccharides to extra- and intracellular glycosidases has, however, prompted many chemists to develop hydrolytically stable carbohydrate mimics [ 6 ] . C-Glycosides are an important class of such glycomimetics, and are defined as carbohydrates in which the interglycosidic linkage has been substituted by a methylene group. This minor but important modification renders these analogs completely resistant to hydrolysis [7]. Because of their close analogy with the parent 0glycoside, C-disaccharides and C-oligosaccharides have become important compounds for study of carbohydrate-protein interactions in lectins and glycosidases. The incorporation of a methylene group in the glycosidic linkage not only enables binding of the C-oligomer to the receptor without sensitivity to hydrolysis but might also give information about the conformation of the bound sugar around the glycosidic linkage by 'H NMR analysis [8, 91. Realizing that C-glycosides seem to be promising drug candidates against the above detrimental cellular interactions, or are important biological tools for the study of carbohydrate-protein interactions in lectins and glycosidases 19, lo], several groups have developed interesting methodologies for the construction of such compounds. The synthesis of these compounds is not always easy, considering that a C-C bond generation is required not only at the terminal sugar but also at the reducing end.
496
20 Synthesis of C-Oligosuccharides
In this review the different methods for the synthesis of C-disaccharides and higher C-oligomers will be highlighted, covering the literature to the end of 1998. The synthetic approaches to these glycomimetics will be divided into four chapters depending on whether the methodology applied by individual groups was: 1) by direct coupling of intact carbohydrate units via (i) anionic or (ii) radical pathways; 2) by the de novo synthesis of one .or all of the sugar units in the C-disaccharide or C-oligomer; or 3) by applying a cycloaddition reaction to connect the two sugar units. We include only work which represents a true mimic of the sugar, i.e., mimics with the correct number of linking carbons and endocyclic oxygen atoms. A great many series of disaccharide analogs have been synthesized either having no carbon linkage or one carbon more than the correct mimic, or where the ring oxygen has been replaced by nitrogen [ 1 I , 121. For these studies the reader is referred to several other reviews [7, 131.
20.2 The Anionic Approach 20.2.1 C5-Alkynyl Anions An approach to the first synthesis of a true C-disaccharide was reported by Rouzaud and Sinay in 1983 [14]. Having previously worked on the synthesis of C-(alkyny1)-Pglucosides [ 151, the Sinay group took advantage of this chemistry for the synthesis of a Glc(P1-6)Glc C-disaccharide employing a C5-alkynyl anion as shown in Scheme 1. This method is highly effective for the construction of 1,6-linked C-disaccharides owing to the ease of preparation of the starting sugars and the high-yielding coupling steps exemplified below. The glucose derivative 1 was converted in two steps
4
BnO BnO
1. DMSO, (COC1)2,
Br
2. CBq, PPh3 0
BnO OMe 2
1
2+3
-
BuLi BnO 92% B
n BnoOH
BnO O 4
Scheme 1.
OMe
OBn 3
2. 1. EgSiH, H,, PdKBF3.Et20 HHO O L h BnO 52% HO OMe
(
o
i 0
5
l
20.2 The Anionic Approach
BuLi 0
BnO BnO 6
75%
BnO
497
BnO
7
8 1. Et,SiH, BF,.OEt, 2. Hz, Pd/C
74%
OH
I 6, BuLi 67% 1 Me0
9
APBn
n..
Bnow BnO[uB:
OBn
BnO BnOW
\ \O
O
OBn B
*
n
HO
74%
OMe HO
4 steps 29%
BnO B n
0
&
~
&
OBn
BnO BnO BnO BnO
'?
OBn
BnoOH
H
OBn 71% 12
Scheme 2.
to the vinyl dibromide 2 which was transformed to the acetylenic anion with 2 equiv. BuLi at -50 "C. After treatment with the glucopyranolactone 3, the hemiacetal4 was obtained in high (92%) yield. Stereospecific reduction of the hemiacetal with Et3SiH and BF3.Et20 involving hydride attack on the u face of a cyclic oxonium ion intermediate then afforded the b-C-disaccharide 5 after catalytic hydrogenolysis. Applying this methodology, the same group was able to prepare the Ctetrasaccharide 12 in an iterative sense, in addition to the dimer 9 and trimer 11 obtained from intermediates along the synthetic route (Scheme 2) [ 161. Starting this time with the vinyl dibromide 6 and galactopyranolactone 7, a similar reaction sequence as described above was adopted for the preparation of the protected Cdisaccharide 8. After removal of the anomeric hydroxyl group and selective reduc-
OMe HO
498
20 Synthesis of C-Oligosaccharides 1. AllylMgBr
3. oso4, 2. MeZC(OMe)Z, NMO H+
mu-n OBnOG0 B BnO BnO 0 3
BnO% BnO
~
-0BnO
BnO OH 13
84% 84%
6 8 I
0
'
A
+ diastereomer (2:3)
PivCI, Pyr BnO,
B n O S
2, BuLi NaOMe, MeOH HCI C1C02CCI,, Py
86%
OPiv
B n O p un
15
36%
-)-0
14
0
1. C02(CO)* 2. BF,:OEt, 3. CAN
55%
I. TEA, Py, H20 2. PdC, H?
RO- ' "
Ro
17 R=H
OMe
(a$=1:l)
8R
Aczo'Py
R=Ac(9l%)
16
Scheme 3.
tion of the alkyne functionality, a two-step hydrolysis and oxidation sequence gave the corresponding lactone which could be coupled again with 6 to give a Ctrisaccharide intermediate 10. In the same manner, these steps were repeated and followed by a deprotection step to produce the C-tetrasaccharide 12, the parent 0glycoside of which is known as a plant elicitor. Schmidt and coworkers have recently extended this approach for the synthesis of C-disaccharides of ketoses as depicted in Scheme 3 [ 171. In their attempt to prepare a glycosyl transferase inhibitor the Schmidt group reacted the Sinay intermediate 2 with the hemiacetall3, readily prepared in three steps from the glucopyranolactone 3. Although, no coupling was observed in this case, the reaction proceeded when 13 was first transformed to the open-chain compound 14 with pivaloyl chloride and then treated with the alkynyl anion. This led to the formation of two propargylic alcohols, as a 3 :2 diastereomeric mixture in a 61% yield, which were subjected to protecting group manipulations to give 15. Ring closure was best achieved by per-
20.2 The Anionic Approach
499
forming a Nicholas reaction whereby the alkynes were first transformed to their dicobalthexacarbonyl complexes. Thus treatment of either diastereomer with BF3.Et20 readily led to the formation of a stabilized carbocation with immediate cyclization to give a 1 : 1 mixture of a : p anomers 16 (only a anomer shown) after decomplexation with cerium ammonium nitrate. A two-step deprotection protocol of either anomer then gave the corresponding unprotected C-disaccharides 17 (only a anomer shown). Interestingly, with the diastereomer of ketone 14 only one stereoisomer was obtained upon reaction with the vinyl dibromide 2. In addition, ring closure of the coupling product is highly selective, affording an a : p mixture in a 10: 1 ratio. Employing the monosubstituted alkyne 19, easily prepared from vinyl dibromide 2, van Boom recently developed a stereoselective ZnC12-assisted ring-opening of epoxide 18, enabling rapid access to the C-isomaltoside 21 via the alkyne 20 (Scheme 4) [18]. This surprising cis selectivity observed in the epoxide opening is rationalized by the initial formation of a zinc acetylide species (see Scheme 5), followed by an intramolecular delivery of the alkyne moiety in the ion-pair 23 or the p chloride 24. A Nicholas reaction [ 191 involving the triflic acid-induced isomerization [20] of the dicobalthexacarbonyl complex of alkyne 20 can be performed to access the p anomer 22.
4
BnO BnO
+
BuLi, ZnC1, BnO
0
BnoOMe
18
59%
19 O *:B i BnoOMe
H,, Pd/C
80%*
R
E
O
20
X
RoOMe
21 R=H Aczo’Py
R=Ac(92%)
BdOLMe
Scheme 4.
22
500
20 Synthesis of C-Oligosaccharides Sugar-Li
i
B
n
O
G
Sugar
OBn
*ZnCl
BnO 0
18
23
(OBn
20
24
Scheme 5.
20.2.2 C1-Glycal Carbanions An attractive method employing stabilized C1-glycal anions for the synthesis of pC-disaccharides was developed by Schmidt and coworkers (Scheme 6). The Konstanz group had observed earlier that the deprotonation of protected glycals is greatly facilitated by the presence of a phenylsulfinyl group at the C2 position [21]. To obtain a glycal derivative, such as 25, an easy two-step synthesis from tribenzylD-glucal was employed with a 63% overall yield. C1-substituted glycals are then formed by the coupling of the corresponding anion 26 with aldehydes. Reductive removal of the phenyl sulfinyl group and a stereospecific hydroboration/oxidation step affords P-C-glycosides 27 with an equatorial C2-hydroxyl group. This strategy was first reported in 1989 for the preparation of a C-cellobiose derivative as illustrated in Scheme 7 [22]. The electrophilic species was prepared
OBn 1. PhSCI; DBU BnO BnO&
0 2.mCPBA
63%
B g ( 3 0 q
LDA-100°C
Bpo)
Li
PhSO 25
PhSO 26
RCHO R PhSO 21
Scheme 6.
501
20.2 The Anionic Approach
HBnO O
1. DMSO, DCC 2. Ph3P=CH2 3. H3B.Me2S;H202
q
* 26%
BnOOBn 28
30
29
OBn
1. Raney Ni
25,LDA
BnO
___)
3. PdC, H2
74%
OH
OBn 31
32
Scheme 7.
starting from the appropriately protected glucoside 28. DMSO-assisted oxidation of the C4 hydroxyl group then Wittig reaction led to formation of an exocyclic alkene, which upon borane addition, employing the borane dimethylsulfide complex, and subsequent oxidation, afforded in modest yield the C-branched sugar 29 with the gulucto configuration in addition to the reversed borane addition product. Steric control from the C3 and C5 positions is undoubtedly the cause of the high stereoselectivity observed in the hydroboration step. Oxidation of 29 then gave the corresponding aldehyde which could be isomerized by mild basic treatment to the isomer 30 with the gluco configuration. Addition of the C1-anion to aldehyde 30 gave compound 31 in good yield with high diastereoselective control at the new stereogenic center (10 : 1). Transformation of the Cl-substituted glycal to the C-cellobiose derivative 32 then required a threestep sequence involving reductive removal of the phenylsulfoxide with Raney nickel, a stereoselective hydroboration-oxidation step, and finally catalytic hydrogenation. Since this preliminary example, the Schmidt group has adapted this approach for the construction of other p-C-glycosides employing either the 2-(phenylsulfinyl)-~glucal 25 or the galactal counterpart 33 (Scheme 8) [23-261. Many improvements BnO B
n
OBn O
e HO
PkO
HO
HO
35
OH 36
Scheme 8.
OMe
34
OH
HO
Ho I
HO
33
502
20 Synthesis of C-Oligosaccharides 1. BnBr, NaH 2. CS,, NaH; Me1
33 + 30
steps
4.H2. Pd/C
BnO
OH
OBn
Scheme 9.
were made to increase the yields of the synthesis of the electrophilic coupling partner, notably in the alkenylation and hydroboration/oxidation steps. It was found that Tebbe’s reagent gave good yields of the desired alkene where the Wittig reagent failed, whereas 9-BBN proved much more effective than borane, securing consistently higher regioselectivities in the hydroboration step. In one example a cyano group was used for the introduction of a C4-branched formyl glycoside [25], an approach which was also reported by Vasella [27] and Armstrong [28] for onecarbon homologations of uloses. Several hydroxy C-disaccharides, 34-36, have thus been prepared by this approach, as illustrated in Scheme 8. The presence of the hydroxyl group in the carbon linkage might even be an advantage in that it enables the introduction of other functional groups at this position, and provides a conformational preference around the two exocyclic C-C bonds. In one case, the hydroxy C-disaccharide was deoxygenated using the Barton protocol (NaH, CS2, MeI, then Bu3SnH, AIBN) providing an efficient overall entry to C-lactose 37 (Scheme 9) [24]. 20.2.3 Anomeric Samarium Species Application of a Cl-lithiated sugar with a protected C2-hydroxyl group, and its direct coupling with a carbonyl substrate at low temperature, does not lead to the formation of the C-glycoside owing to a rapid, competing p-elimination which leads instead to the corresponding glycal [29]. In contrast, anomeric samarium species have proven to be much more resistant to this process, even at room temperature, and hence have been used for the rapid assembly of C-disaccharides. In 1995, Skrydstrup and Beau reported the synthesis of the C-disaccharide derivative 40 of a-C-mannopyranosyl( 1-2)-~-glucopyranose, via the reductive samariation of the mannosyl pyridyl sulfone 38 in the presence of aldehyde 39 (Scheme 10) [30]. Because this aryl sulfone group has a sufficiently low-lying LUMO energy level, electron transfer from the mild reducing agent, samarium diiodide, is facilitated, resulting in a fragmentation step with concomitant formation of the thermodynamically more stable a-anomeric radical (the anomeric effect), as illustrated in Scheme 11 [31]. A second electron transfer to 49 from an additional equivalent of samarium diiodide then leads, under kinetic control, to the organosamarium species 50 which is configurationally stable because of a fast conformational change, placing the C1 and C2 substituents in a more stable equatorial position, as in 51. Its
20.2 The Anionic Approach
oms
BF%
+
~,g-&+
2.Bu4NF 1. Sm12
503
~
BnO
38
OB n
75%
o'
S02PY
OBn
HO
OBn 40
39
BnO
~$3 ~{3 BnO
S02PY 41
OMe
73%
BzO BnO
42
BnO
1. Im2CS 2. FjPhOH, Ph3SnH
70%
2. I.B MeONa u~NF
OTBS
.,,H BzO BzO BnO
3. H2, Pd/C
+H
*
79% HO HO
.O
-0 I
OMe
OMe
44
Bz% + BnO
B BnO
45
q P
*
oJ S02PY 46
OMe 47
48
OMe
Scheme 10.
B;;oEh BnO
Scheme 11.
syn-elimination to glycal
I
504
20 Synthesis of C-Oligosuccharides
53
Ph
Ph
1. O3 2. Bu$nH, AIBN
,
39
BzO
63% P h d P h
OMe
OMe 42
Scheme 12.
coupling with aldehydes under Barbier conditions therefore produces the a-Cdisaccharides, 52, only [30]. The synthesis of the electrophilic partner 39, reported in 1993 by Jung and Choe, is interesting because of its originality in respect of the introduction of a formyl group into the sugar ring (Scheme 12) [32]. In this work, the authors demonstrated that substrate 53, obtained by iodoglycosylation of tribenzylglucal with 1-phenylprop2-en01 and then ozonolysis, readily underwent a 5-ex0 radical cyclizationfragmentation process resulting in the intramolecular transfer of a formyl group to C2 and generation of a more stable benzyl radical. Not only was the formyl group transferred stereospecifically to give the 1,2-cis relationship in 39, owing to the 5-membered cyclic intermediate in the transfer mechanism, but the original functionality at the anomeric center was then transformed to the well-known benzyl protecting group. An adaptation of this approach has recently been exploited by Skrydstrup and Beau for the preparation of aldehyde 42 required in the synthesis of methyl a-Cmannobioside 45, a C-glycoside analog of the Mycohacterium tuberculosis capping disaccharide (Scheme 10) [33].Rather than a formyl group transfer from C1, the C3 position was selected affording the derivatized sugar 42 with a CZaxial substituent (Scheme 12). Subsequent coupling with the mannosyl pyridyl sulfone 41 under the influence of SmI2 afforded only one C-glycoside, 43, which was transformed to the unprotected C-disaccharide 45 via a deoxygenation step to 44 and subsequent deprotection. Recently, the C-dimer of u-D-Man(1-3)a-~-Man48 has been prepared by a similar approach employing the mannosyl pyridyl sulfone 46 and aldehyde 47 P41. Other extensions of this C-glycosylation methodology have been reported for the preparation of two C-linked 1,6-disaccharides. Reaction of the pyridyl sulfone of N acetylglucosamine 54 with aldehyde 55 produced mainly the a-C-disaccharide 56 (Scheme 13) [35].The stereoselectivity at the anomeric center is explained in this case by internal coordination of the N-acetyl group with the metal ion of the intermediate glycosyl samarium species 57, retarding anomerization to the ther-
20.2 The Anionic Approach
505
Bnr% AcHN SO2Py
BnO
bMe Bnb
54
55
bMe
56 (a$= 4: I )
57
Scheme 13.
modynamically more stable p isomer as observed for the corresponding glucose derivative. Reductive samariation of tetrabenzylmannosyl chloride 58 (X = C1) or phosphate 58 (X = OPO(OPh)2) in the presence of 55 also served as a useful reaction for the synthesis of C-disaccharides as exemplified in the synthesis of C-dimer 59 (Scheme 14) [36, 371. Recently the glycosyl pyridyl sulfones have been applied by the Linhardt group for the synthesis of a C-disaccharide containing N-acetylneuraminic acid (Scheme 15) [38]. The coupling of glycosyl pyridyl sulfone 60 with the previously reported aldehyde 63 gave a high yield of the (1,3)-C-disaccharide64 with complete stereoselectivity both at the anomeric center and at the new exocyclic stereocenter. It is important to point out that these reactions proceed through a samarium enolate intermediate 65 in contrast to 2-deoxy sugars lacking a C1 carboxylate group which involve true anomeric samarium species and which are non-selective at C1. The coupling step can even occur with the glycosyl phenyl sulfone 61 or chloride 62 because the initial homolytic cleavage step after the first electron transfer is favored by the stability of the radical formed (captodative effect). An extension to a KDNcontaining disaccharide derivative 66 has recently been published by the same team with the same aldehyde 63 having identical stereoselectivity [39].
55
+
goq
F o x
Sm12
b
x 58
Scheme 14.
x=c1
X= OPO(OPh),
54% 73%
BnO
BnO 59
BnO
OMe
506
20 Synthesis of C-Oligosaccharides
OAc
OMe
60 X= ~lS02pV 61 X= aS02Ph 62 X= PCl
63
64
63, SmI2 Bn
OAc
X= aS02Ph
66
OAc
a-face
65
x=SCl Scheme 15.
20.2.4 C-Branched Carbanions p-Glycosyl nitromethanes are effectively prepared from the parent sugar in two steps [40]. Because the corresponding nitronate anions do not undergo ring opening, chain elongation at the a carbon is possible and has been exploited by Martin and Lai for the synthesis of both p(1-6)- and p,p(l-l’)-linked disaccharides [41, 421. Treatment of the C-glucoside 67 with KF in the presence of the galactose-derived aldehyde 68 led to the rapid formation of the derivatized C-dimer which was transformed to the unsaturated nitro compound 69 (Scheme 16). Further reduction and denitration involved two steps with eventual deprotection providing the meth1-6)-~-Gal. ylene bridged analog 70 of p-~-Glc(
A AcO c
O
G NO2
~
To%
OAc
67
68 ,OAc
1. N a B Q 2. BqSnH, AlBN 40%
Scheme 16.
1. KF, 18-crown-6 2. Ac20, Py * 47%
20.2 The Anionic Approach
507
1.80% AcOH 2. AclO, BF,.Et,O
1.67, KF, 18-crown-6 2. A c ~ OPy ,
c
36%
44% OPiv 71
OPiv
u I 70
OH 74
73
Scheme 17.
Elaboration of this approach for the synthesis of C-P,P-trehalose was successfully achieved as illustrated in Scheme 17. The nitroaldol condensation of 67 with the aldehyde-glucose derivative 71 provided an epimeric mixture of unstable nitroalditols which were immediately dehydrated to give mainly the E-nitroalkene 72. The sensitive deacetalization and cyclization step is best brought about by heating 72 under reflux in 80% AcOH. In this way, both deprotection and ring formation take place to produce the second p-C-glycoside of the required C-disaccharide via attack of the internal nucleophile on the si face of the double bond. A substantial amount of the C-pyranosyl-C-furanosyl isomer was likewise isolated. Radical denitration and deprotection then afforded the C-p, b-trehalose 74. Reduction of the nitro group in 73 also gave rise to a C-disaccharide containing a bridging methylenamine group; similar compounds were demonstrated by Schmidt to be good glycosidase inhibitors. A similar approach was also published by Witczak and collaborators for the preparation of 3-deoxy-C-cellobiose (Scheme 18) [43]. The starting glycosyl nitro-
-? B
n
O
w +
BnO
Qo
BnO NO,
0
BnoOH 75
76
1. BqSnH, ABCN 2. BF3:OEt2, Et&H 3. L-Selectride 5 . Pd(OH)& 4. EgSiOTf, AczO C,H,,, EtOH
-
-& AAcO
28% OAc 78
Scheme 18.
508
BBnO n
20 Synthesis of C-Oligosaccharides
O
G
+
k0
BnO BnO
BnoOCH3
OBn
KF, 18-crown-6 52%
BnO-- ..BnO
BnO
BnO
80
79 1. BuLi, PhOC(S)Cl
2. Bu$nH, AIBN 3. TsNHNH2, AcONa 4.Na, NH3 10%
81
HO
B"bbCH3
OH
HO
Scheme 19.
methane 75, easily prepared in two steps, underwent stereospecific conjugate addition to the sterically less hindered exo face of the bicyclic enone 76 under mild basic conditions (KF or triethylamine). Tributyltin hydride reduction of the nitro group in 77 followed by removal of the anomeric hydroxyl group with Et$iH/BF3.Etz0 and stereoselective reduction of the ketone subsequently afforded the C-disaccharide 78 after a deprotection-acetolysis sequence. Reversing the roles of the two coupling partners in the nitroaldol condensation is also an option for construction of C-linked (1-6)-disaccharides [44]. In this instance aldehyde 80 was coupled to the C6-nitro derivative 79 to afford adduct 81 (Scheme 19). Tin hydride-induced reduction of the corresponding thionocarbonate then led to the C-dimer 82 after two reduction steps. Schmidt and coworkers, on the other hand, have developed an approach to Clinked (3-disaccharides employing CCbranched carbanions which can be prepared by lithium-halogen exchange as exemplified with the 1,6-anhydro sugar 85 in Scheme 20 [45]. This precursor was prepared via a stereoselective hydroborationoxidation of the exocyclic alkene in 84 obtained in two steps from alcohol 83. Addition of the anion generated from 85 to gluconolactone 3 gave a hemiketal which was stereoselectively reduced to the C-disaccharide 86. Further standard manipulations provided the octaacetate derivative of (3-~-Glc( 1-4)-~-Gal87. The peracetylated derivative of p-D-Gal(1-4)-~-Gal88was prepared in a similar manner. 20.2.5 CdPhosphoranes Wittig olefination is another attractive and rapid route to (1,6)-C-disaccharides. The Dondoni team demonstrated that the readily accessible phosphonium ylides at C6 of sugars coupled well with C-formyl glycosides available through thiazole-based formylation of glyconolactones [46]. In Scheme 21, a representative of this approach is shown for the synthesis of the C-dimer of p-~-Gal(l-6)-~-Gal [47]. Hence, treat-
20.2 The Anionic Approach
1. DMSO,DCC 2. Ph3P=CH2
HO
83
pq
39%
OBn
;:;wy;
H202
+ 67%
OBn
I
1. Ac20, TFA 2. HZ, WIC
3.Ac20,Py
B 0
%no
40%
OBn
OBn
86
+ . &+& o ? i A
OBn 85
84
1. BuLi 2. Et3SiH, BF3.Et20 85 + 3 * 34%
509
OAc
87
OAc
OAc
88
AcO
OAc
OAc
Scheme 20.
BnO
6-step formylation
53%
BnO BnO 80
80 + 89
Scheme 21.
ment of the phosphonium salt 89 with base and then aldehyde 80 led to the formation of the cis olefin 90, which upon Pd-catalyzed hydrogenation subsequently afforded the C-disaccharide 91. A set of ten (1-6)-linked p-C-disaccharides could therefore be prepared by means of this approach starting from C-glycosyl formaldehydes 80 and 93-98 with the phosphonium salts 89 and 92 (Scheme 22) [48]. The basic conditions of the Wittig reaction nevertheless prohibit the formation of the corresponding u-C dimer; e. y. starting with the u-C-glucosyl precursor, the corresponding P-C-disaccharide is obtained by initial isomerization at the anomeric center.
510
20 Synthesis of C-Oligosaccharides 'PPh3 I-
BnO% BnO
BBnO n BnoOMe 92
y
%:nBBnO
' a0
93
96
BnOR R
94 R = H, R = CHO 95R= CHO,R'=H
98
91
Scheme 22.
steps
BnO
c
P
BnO
70%
68
99
BE+ 101
-P
102
Ho
Scheme 23.
The reverse strategy has recently been reported by the same group for the preparation of a similar C-mimic of the p(1-6) tetragalactoside as shown in Scheme 2 [49]. Condensation of the D-galactopyranosyl-derived phosphonium salt 99 and aldehyde 68 led to the unsaturated dimer 100 (Scheme 23). Repetition of the sequence twice with phosphonium salt 99 furnished tetramer 101. These steps are, however, characterized by low coupling efficiencies (36% for the trimer and 11% for the tetramer) mostly because of the base-induced a,p-unsaturation of the aldehyde substrates. The desired tetrasaccharide 102 was then obtained after standard deprotection and double-bond hydrogenation.
70
20.3 The Radical Approach
51 1
20.3 The Radical Approach 20.3.1 Intermolecular Anomeric Radical Addition In principle, intermolecular radical-mediated addition of anomeric radicals to an exocyclic double bond of the C-glycosyl acceptor should provide an efficient route to C-oligosaccharides leading directly to the desired mimic. In practice, however, these reactions with unactivated alkenes are not favorable as they lead solely to products of direct H-abstraction. Reducing the LUMO of the alkene with electronwithdrawing groups, typically a,P-unsaturated carbonyl compounds or similar derivatives, greatly improves the overall efficiency of these coupling reactions. For example, Giese demonstrated that the anomeric radical obtained by reaction of acetobromoglucose with Bu3SnH adds to the a-methylene y-lactone 103 in good yields and with high a-selectivity to give the C-dimer 104 (Scheme 24) [50].In addition, hydrogen abstraction at the new radical center at C2 of the reducing sugar is stereoselective leading to the predominant formation of the product with an equatorial oriented CZsubstituent. Further modification involving reduction of the lactone to lactol and acetylation afforded the protected C-mimic of kojibiose 105. Two other examples 106 and 107 have also been successfully prepared, as shown in Scheme 24. In a similar approach, Vogel described the use of ‘naked sugars’ such as 108 as radical acceptors, as shown in Scheme 25 [51, 521. The coupling product 109 was further elaborated before a true C-disaccharide was reached, with the key step being Baeyer-Villiger oxidation to introduce the fully oxygenated ring system. Several
Ace% OAc
AcO
nBo- \+
Bu$nH, AIBN 70%
AcoB,
OBn 104
103 1. Na[A1(OC2H,0Me)2(OEt)H1 2. AQO
64%
OBn
105
BnO BnO 106
Scheme 24.
AcO
Me0B-n
I OAc AcO
OAc
107
5 12
20 Synthesis of C-Oligosucchurides
1. Bu3SnH, AIBN 2. mCPBA, NaHC03 OAc AcO OAc 1 0 9 ( a : p =5.5:l) OAc
66%
108
C02Me
1. NaHC03, MeOH 2. A c ~ O DMAP, , Py
1.NaBH4 2. HCI OAC 3. A c ~ ODMAP, , Py
*
* 36%
0 0
n
Acd
OAc
Scheme 25.
variations of this approach have now been reported for the synthesis of several Cdimers with different substitution patterns of the radical acceptor in addition to the use of acetobromogalactose (Scheme 26) [ 531. In a different approach, Martin reported the use of an exocyclic radical derivative The as a means of preparing the a(1-6) C-linked disaccharide 113 (Scheme 27) [54]. corresponding radical from the cobaltoxime derivative 111, prepared from iodide
1. Bu3SnH, AIBN + pl,sed\
Atgo% AcO AcoBr
c;
0
2.NaBH4
PhSe, 48%
1. mCPBA 2. Ac7O 3. osb, 4. Ac,O
I\
r5
c
OH
86%
R mCPBA
steps
R
AcO 0
OAc
Scheme 26.
20.3 The Radical Approach
2. 1. A hv,c NaOH, ~O 95% E
t *O
111 + 112 30%
"
"
"
~
Bk'
Bu3SnH
AcO AcO
*
AcO
AcO
AcO
OMe
513
113
AcO
OMe
Scheme 27.
Bu3SnH
Bno%I BnO
+
BnoOH
114
Q 0 76
ABCN 26% 0
BnO BnO
Scheme 28.
110, was generated photochemically and coupled to the nitronate derivative of the C6 functionalized glucoside 112. Tin hydride-promoted denitration then afforded the desired C-dimer. An exocyclic radical approach to C-disaccharides was also employed by Witczak starting with the readily available iodide 114 (Scheme 28) [43]. An exo-facial selective radical addition to levoglucosenone 76 afforded, in low yield, the C-dimer 115, with direct reduction being the major competing pathway. This compound was then converted to the 3-deoxy derivative of C-cellobiose as depicted in Scheme 18. 20.3.3 Intramolecular Anomeric Radical Addition
If the radical addition reaction is performed in an intramolecular fashion, olefin activation with an electron-withdrawing group will not be a prerequisite for Cglycosylation, because the cyclization event competes well with hydrogen abstraction. In such examples, ring size and the stereochemistry of the ring substituents greatly influence the stereoselectivity and efficiency of the cyclization reaction. With the above points in mind, Sinay's group developed a direct approach to Clinked disaccharides employing a temporary silicon tether approach to graft the
5 14
20 Synthesis of C-Oligosaccharides
o& &no
SePh
BuLi, Me$iC12; Imidazole
BnO
OH
116
117
2. 1. HF, BqSnH, THF AIBN *
BnbAMe
.
95%
BnO& BnO
SePh
9,Sic
B&(30% B
40%
n
BnO
118
4 BnoOMe
BnoOMe
Scheme 29.
radical donor and acceptor together [55]. These reactions are surprisingly efficient considering the medium-sized ring intermediates formed, which upon liberation from the silicon linker afford the C-dimer. Illustrative of this approach is the synthesis of a C-linked analog of methyl amaltoside (Scheme 29) [56]. Linkage of the two glucose derivatives 116 and 117 with dimethyldichlorosilane followed by tin hydride-promoted 9-endo-cyclization afforded the C-disaccharide 118 in 40% yield after liberation from the linker. The anomeric effect seems to dominate in this reaction with exclusive formation of the a anomer, whereas H-abstraction by the resulting intermediate radical after cyclization affords the equatorially substituted product. The nature of the attacking sugar radical has a substantial impact on the cyclization yields and stereoselectivites of these reactions-when the method was extended to the galacto series 60% yield of three isomeric Clinked disaccharides was obtained in the ring-closure step [57]. The ring size and tethering positions are equally important, as is observed in the 8-endo cyclizations of the gluco and galacto derivatives 119 and 121, respectively (Scheme 30). In the first case an inseparable mixture of all four isomers 120 was obtained [56],whereas the C4-epimer 121 led to the C-linked disaccharide 122 in 45% yield [ 5 8 ] . Quite remarkably, the anomer was furnished in the latter case,
6 1
OBn Bno& -BnO $S -,ePh
OBn
/
BnO
\
BnO !&&SePh
121
Scheme 30.
O
w
o
B OBn
120
OBn
\
B BnOn
50%
119
\&
OBn
O\Si//
OBn
2. 1. Bu+nH, HF, THF AIBN
OMe
1. BusSnH, AIBN 2. BQNF
45% (tethering incl.)
OBn 122
n
515
20.3 The Radical Approach
~ ~ 0 % SO2R B n g o M C
~1. .SmI2 HF
BnO
\si’o
/
50%, R = Ph 70%, R = Py
\
BnO
118
OMe
Scheme 31.
Br:o&
0
CSA, 4..4 MS
Tebbe’s reagen:
81%
87%
BnO
SePh
iePh
-
117
123 1. BqSnH, AIBN
BnO
Brio\
OH
BnO
BnO
35% 124
125
+
pproduct (3%)
BnOOMe
Scheme 32.
where there was undoubtedly a strong conformational bias induced by the tether because attack of the anomeric radical species occurred on the p face. The standard tin hydride conditions for generating an anomeric radical can be replaced by the divalent samarium-induced reductions of glycosyl aryl sulfones [ 59, 601. In one instance the Sinay group employed the 2-pyridyl sulfone group, introduced by Skrydstrup and Beau in 1994 [30, 311, which was rapidly reduced by SmI2 at room temperature (Scheme 31). In this way, a significant increase in the cyclization yield of 118 was achieved [60]. The ketal connectors which were exploited earlier in 0-glycosylations proved equally efficient in the synthesis of C-linked disaccharides as shown in Scheme 32 [61]. With acetate 123 a two-step sequence via vinyl ether formation with Tebbe’s reagent and acetalization with alcohol 117 produced the linked sugars 124. Upon tin hydride-promoted cyclization and removal of the acetal tether the a-D-Man(1 4 ) - ~ - G l cC-disaccharide 125 was obtained in 35% yield. Similarly, a benzylidene acetal linkage as in 128 could be produced via a DDQmediated oxidation of the p-methoxybenzyl group in 126 in the presence of the fucose derivative 127 (Scheme 33) [62], a strategy introduced in 0-glycosylations by Ito and Ogawa [63]. Subsequent radical cyclization and acetal hydrolysis afforded the a-~-Fuc(lL2)-~-Glc analog 129 in good (60%) yield. A phosporamidic linker also proved viable, as reported in the synthesis of the Cdisaccharide analog of a-~-GalNAc( 1-4)-~-Glc(Scheme 34) [64]. Intermediate 131 prepared in two steps from the galactosamine derivative 130 and the unsaturated -
5 16
20 Synthesis of C-Oligosucchurides
1. Bu3SnH, AIBN 2. DDQ, HZO
60%
129
Scheme 33.
1. PhPCl,, Et3N, THF 2. tBuOOH
BnO
BnO 80%
OMe
SePh
130
117
1. Bu+nH, AIBN 2. HCI, MeOH 3. MeONa, MeOH 4.AQO, Py
BnO OBn
+ p product (4%)
BnOQ+++
16% 132
OMe
Scheme 34.
alcohol 117 underwent 9-endo cyclization to give the C-dimer 132 after a deprotection and acetylation sequence. An approach to P-C-disaccharides was reported by the same group whereby the roles of the two tethered sugars are interconverted as exemplified with silaketal 133 in Scheme 35, in which the radical acceptor has an anomeric exo methylene [65]. Radical addition to the double bond led to formation of an a-oriented anomeric radical which subsequently abstracts a hydrogen leaving the C1 carbon branch in the j3 position as in the C-disaccharide 134. Because of the facile access to these C-dimers by use of this intramolecular approach, multigram quantities of the analogs are available and have been exploited for the synthesis of mixed C/O-glycosides of biologically relevant oligosaccharides. In this way, the analogs of sialyl LewisXand the pentasaccharide re-
517
20.3 The Radical Approach (OBn 1. Bu$nH, AIBN 2. B u ~ N FTHF ,
OMe
37% (tethering incl.) '
OH
-
\
OH 134
133
Scheme 35.
135
136
Scheme 36.
sponsible for the antithrombin I11 binding region of heparin, 135 [66] and 136 [67], respectively, have been prepared (Scheme 36). Predictable stereochemistry at C 1 would be expected when applying h x o cyclizations in silicon-tethered radical reactions for the construction of Cdisaccharides. A sole example has been provided by Skrydstrup and Beau using glycosyl pyridyl sulfones for the generation of the anomeric radical [31]. This approach has been successfully exploited in the synthesis of peracetylated methyl Cisomaltoside 137 (Scheme 37).
1. SmL 2. B u 4 h 3. H2, PdC 4. A c ~ OPy ,
,OAc
a
*
48%
AcO AcO 137
Scheme 37.
Ac'O 1 OMe
5 18
20 Synthesis of C-Oligosaccharides
20.4 The Partial de Novo Approach In the partial de nuuu synthesis of C-oligosaccharides one of the carbohydrate units is constructed from smaller building blocks. The advantage of this approach is that in the designed synthesis of a C-mimic little variation can easily lead to other stereoisomers, which has resulted in the preparation of a large collection of C-di- and trisaccharides. Much of the work in this area has been dominated by Kishi and coworkers, primarily because of their interest in studying the solution conformation of the parent oligosaccharides and their bound conformation on carbohydrate binding proteins [9], an interest which was sparked from their phenomenal synthetic work on the marine toxin, palytoxin [68]. Kishi and coworkers found that the solution conformations of C-disaccharides strongly resemble those of the corresponding O-glycosides and can therefore be exploited to predict the conformational preferences of the natural sugars. Because the C-analogs have two protons on the connecting carbon atom their solution conformations are readily extracted from simple 'H NMR experiments, which is not the case for the parent sugars. As a result of other NMR experiments Jimenez-Barber0 suggested that C-disaccharides are more flexible around the methylene bridge, and it was observed that in certain instances such compounds are bound to lectins in conformations different from that of the normal disaccharide [ 10, 691. In general, the de nuuu approach applied by the Kishi group for the synthesis of C-oligosaccharides relies on acyclic stereocontrol for the construction of the contiguous chiral centers of at least one of the carbohydrate moieties. The first examples of this approach were reported by this group in 1987 and involved the synthesis of four C-disaccharides, including the C-analogs of methyl maltoside and cellobioside, from a common synthetic intermediate [70]. Initial Wittig reaction of the readily available arabinose derivative 138 and the phosphonium salt 139 led to the cis alkene 140 (Scheme 38). Stereoselective dihydroxylation, exploiting the 1,3-allylic strain effect, then monoprotection of the diol, oxidation and cleavage led to the cyclic hemiacetal 142 after protection of the primary alcohol. Reduction of the hemiacetal was best performed with n-Pr$iH, affording exclusively the p anomer 143. Deprotection and acidic methanolysis then led to the C-disaccharide analog 144 of P-~-Man(l-4)-~-Glc. On the other hand, simple oxidation of the C2-hydroxyl group in 143 followed by stereoselective reduction converted the mannu series to the gluco series with the eventual formation of C-cellobioside 145. Preparation of the corresponding a isomers of these C-dimers requires only slight modification of the syntheses, as illustrated in Scheme 39 by simple reversion of the order of the synthetic steps in the preparation of the acyclic precursor. Hence, the isopropylidene in intermediate 141 was hydrolyzed, followed by diol cleavage and preparation of the anomeric p-nitrobenzoate 146. Introduction of the C6' carbon could be achieved by stereoselective allenylation resulting in formation of the manno derivative 147 after chair inversion. Oxidative cleavage then reduction of the allene moiety and subsequent debenzylation and methyl glycoside formation led to the a - ~ - M a n1-4)-~-Glc ( carbon analog 148. Access to methyl C-maltoside 149 was
519
20.4 The Partial de Novo Approach
138
139
140
1. DMSO, (COC1)2, Et3N 2. HC1 3. BzCl, Py
1. Os04 2. PMBBr, NaH
B z * BnO 75% BnO
O
G
Y
n
OH 142
2. 1. DMSO, BH3.Et3N (COCI),, Et3N
nPr$iH, BF3.Et20
B
z
O
G
BnO BnO OH 143
90%
1
1. H2, Pd(OH)2/C 2. MeOH, HC1 90%
1
145
Scheme 38.
1. AcOH, HzO 2. Pb(OAc)4 3. pN02PhCOCl
//SiMe,,
*
141
PNBO
*
I OBn
1. 03; NaBH4 2. Hz, Pd(OH)2/C 3. MeOH, HC1
-
HO
146
H 148
Scheme 39.
BnO BnO
BF,.Et,O
,OH
W
HoOMe
:
,
,
520
20 Synthesis of C-Oligosaccharides 1. 0 3 ; MezS 2. CBr4, PPh? 3. BuLi 4. 12, morpholine 5 . KOzCN=NC02K. AcOH
I
BnO
150 1. Hz, WA1203
DHP, PPTS 3. B u ~ N F
OBn
151
OBn
/OBn
L.
- '\?oI?;1,
1. NaH, Me1 2. Hz, Pd(OH t>/I
5 . DMSO, 4. TsOH (COC1)2, Et3N
BnO% BnO BnO
'
'"'OH
OBn BnO
OTBS
152
- OH
nn
153
" HO :
o
UDll
x
HO 21
154
HoOMe
Scheme 40.
achieved by a sequence of reactions similar to that used for the synthesis of the Ccellobioside 145. The Kishi-Nozaki reaction [71] is the key step in the preparation of methyl Cisomaltoside (Scheme 40) [72]. Coupling of vinyl iodide 150 with the xylose derivative 151, readily available in six steps from L-xylose, afforded the corresponding allylic alcohol with a diastereomeric ratio of 15 : 1 in favor of 152. Finally, selective hydrogenation of the double bond and oxidation of the primary alcohol led to the C-disaccharide 153, which could be converted to its methyl glycoside 21. A useful dideuterated derivative of C-isomaltoside 154 for solution conformation studies was also accessible from this synthetic approach via the cis-deuteration of the alkene intermediate 152 with Pt on A1203 and D2. A similar strategy was applied for efficient synthesis of the C-glycoside analog of sucrose, starting from the vinyl iodide 155 and the aldehyde 156 (Scheme 41) [73]. The wrong diastereomer at the C3' position is obtained in the anti-selective Cr(I1)mediated reaction; this can nevertheless be corrected via a two-step Mitsunobu reaction and hydrolysis sequence. An anti-favored epoxidation and a regioselective acid-catalyzed ring formation by attack of the C5' hydroxyl group at the quaternary center of the epoxide finally completes the C-disaccharide 157 after a debenzylation step. Nicotra and coworkers have reported another synthesis of a C-glycoside analog of sucrose, as shown in Scheme 42, although missing the C2/-hydroxymethyl group [74].Wittig reaction of the stabilized phosphorane 159, prepared in four steps from C-ally1 glucose 158, with the isopropylene derivative of D-glyceraldehyde 160 rep-
521
20.4 The Partial de Novo Approach
1. DEAD, PhCOOH
2. KzCO~,MeOH 3. mCPBA
-OH 1. CSA
+
30% overall from 155
OH OH 157
Scheme 41.
Bi:oT
kc
Hg(OAc)2, NaCl97%t B g 3 0 Y o H i : 3. PPh,, Et,N
Brio
t
36%
HgCl
158
BnO0v
?%
-
160
88%
BnO
\
1. oso4, NMO 2. AczO, Py
-
159
PPh,
,OBn
52%
161 0 162
FeCI,.SiOz
-Bi:o%
1. Florisil 2. K2C03, EtOH 3. H2, Pd/C
HoOAc
78%
OAc
89%
OH
163
OH
164 OH
Scheme 42.
resents the key coupling step in this synthesis. Subsequent stereoselective dihydroxylation of alkene 161 afforded adduct 162 as the major product (4: 1 ratio) after acetylation; this was subjected to acid hydrolysis furnishing the pyranose derivative 163. Florisil treatment and two-step deprotection then gave the C-sucrose analog 164 in w
