Ernest G. Nolen

Research Projects Description

Synthesis of Methylene Isosteres of Glycosyl Amino Acids and Glycosyl Phosphates

Glycosylation of proteins and lipids is a post-translational modification important for numerous physiological processes (1).  These glycoconjugates act in protein folding, altering the physico-chemical properties and activity, and adorn the landscape of the cell surface, influencing cell–cell and cell–matrix interactions.  Our attention has been directed toward isosteric mimics, which entail methylene substitution for the anomeric oxygen, the so-called C-linked glycosyl amino acids, C-linked glycosyl phosphonates, and C-linked glycosyl lipids, Figure 1.  While such an exchange (CH2 for O) does slightly alter the bond angle, bond lengths, and the dihedral angles, there is ample precedent to suggest the conformational change is not insurmountable for binding (2)  The methylene isostere offers a great deal of stability, in that degradation by glycosidases, reaction with glycosyltransferases, acid hydrolysis of the anomeric acetal, and b-elimination from the serine are all prohibited.

Our goal is to provide convenient synthetic routes to C-linked glycosyl amino acids and C-linked glycolipids, as well as their biosynthetic precursors.  Previous synthetic work on C-linkages has explored a wide variety of C-C bond forming reactions (see suggested reading). 

Despite the significant effort, many of the synthetic approaches require expensive starting materials, are limited in scope to the preparation of only one anomeric stereochemistry, and lose the isosteric relationship by excessively lengthening the intervening carbon chain.  While our work does not remedy all these shortcomings, it does offer complementary approaches to these biologically significant materials and is well-suited for serious undergraduate research.  The proposed syntheses are short, are adaptable to a variety of biomimetics, and utilitize chemistry that is well precedented and not experimentally tedious.

Synthesis of C-Glycosyl Serines

Our previous work in this area involved alkylation of iodides 3 with the enolate derived from Williams’ Boc-protected diphenyloxazinone 4, Scheme 1.  The iodides originated from a-C-allyl glycosides, which are readily prepared and well-known in the literature.  Although this reaction was stereoselective and provided access to the isosteric glycosyl serine 5, and to the b-anomers as well, it was not amenable to acetyl glycosides or N-acetyl precursors (3).

A more functional group-tolerant reaction methodology for C-glycosyl amino acid synthesis was utilized in our lab by employing the Grubbs’ second generation catalyst (4) for the olefin cross-metathesis (CM) between C-allyl glycoside 6 and vinyl glycines 8-10, prepared from methionine or glutamic acid (5), Scheme 2.  A explosion of reports have appeared since the catalyst has become commercially available and these have been recently reviewed (6).  This methodology has provided a concise entry into glycosyl amino acids in which the intervening carbon chain contains three or more methylenes.  In our hands and in accordance with others, the linking carbon chain between the glycoside and the amino acid cannot be made shorter than three carbons (e.g., serine mimics require only two methylenes) without seriously diminishing the yield, as evidenced by >70% recovered C-vinyl glycoside 7 in all attempted reactions utilizing this cross-metathesis partner and our vinyl glycines 8-10. 

Students working in my lab have enjoyed recent success, attested to by the five undergraduate co-authors on publications in the past two years.  As a mentor, the PI works alongside the students as they gain confidence learning the techniques involved in running reactions, purifying products on our new Isco Flash Chromatography station, and identifying structures.  The acquisition of a new 400 MHz NMR has greatly simplified training on how to acquire NMR data, and has focused much more attention on the interpretation of data with students.   The recent purchase of an ion-trap GC-MS and the soon to be acquired MALDI-TOF mass spectrometer facilitate molecular weight determination for characterization of products.

Jennifer Potter ('04)  writing up the results of a reaction in her laboratory notebook.  (Spring 2004)	Michael Orlando ('04) loading a sample into Colgate's Bruker 400 MHz NMR spectrometer. (Spring 2004)

References

1. See the entire issue of Science. 2001, 291(9), 2263-2502.
2. (a) J. Jiménez-Barbero, J. F. Espinosa, J. L. Asensio, F. J. Canada, A. Poveda Adv. Carbohydrate Chem. Biochem. 2001, 56, 235-85.  The Conformation of C-Glycosyl Compounds.  (b) L. M. Mikkelsen, M. J. Hernaiz, M. Martin-Pastor, T. Skrydstrup, J. Jiménez-Barbero J. Am. Chem. Soc. 2002, 124, 14940-51.  Conformation of Glycomimetics in the Free and Protein-Bound State: Structural and Binding Features of the C-glycosyl Analogue of the Core Trisaccharide -D-Man-(1>3)-[-D-Man-(1>6)]-D-Man.
3. E. G. Nolen, M. M. Watts, D. J. Fowler Org. Lett. 2002, 4, 2963-65.  Synthesis of C-Linked Glycopyranosyl Serines via a Chiral Glycine Enolate Equivalent.
4. (a) H. E. Blackwell, D. J. O’Leary, A. K. Chatterjee, R. A. Washenfelder, D. A. Bussmann, R. H. Grubbs J. Am. Chem. Soc. 2000, 122, 58-71.  New Approaches to Olefin Cross-Metathesis.  (b) M. Scholl, S. Ding, C. W. Lee, R. H. Grubbs Org. Lett. 1999, 1, 953-56.  Synthesis and Activity of a New Generation of Ruthenium-Based Olefin Metathesis Catalysts Coordinated with 1,3-Dimesityl-4,5-dihydroimidazol-2-ylidene Ligands.
5. E. G. Nolen, A. J. Kurish, K. A. Wong, M. D. Orlando Tetrahedron Lett. 2003, 44, 2449-53. Short, stereoselective synthesis of C-glycosyl asparagines via an olefin cross-metathesis.
6. S. J. Connon, S. Blechert Angew. Chem. Int. Ed. 2003, 42, 1900-23. Recent Developments in Olefin Cross-Metathesis.

Suggested Reading

1. A. Dondoni, A. Marra Chem. Rev. 2000, 100, 4395-4422. Methods for Anomeric Carbon-Linked and Fused Sugar Amino Acid Synthesis: The Gateway to Artificial Glycopeptides.
2. (a) D. E. Paterson, F. K. Griffin, M.-L. Alcaraz, R. J. K. Taylor Eur. J. Org. Chem. 2002, 1323-1336. A Ramberg-Bäcklund Approach to the Synthesis of C-Glycosides, C-Linked Disaccharides, and C-Glycosyl Amino Acids. (b) Y. Ohnishi, Y. Ichikawa Bioorg. & Med. Chem. Lett. 2002, 12, 997-99. Stereoselective synthesis of a C-glycoside analogue of N-Fmoc-serine b-N-acetylglucosamine by Ramberg-Bäcklund rearrangement.
3. (a) Dondoni, A.; Mariotti, G.; Marra, A. J. Org. Chem. 2002, 67, 4475-4486. Synthesis of  a b-Glycosyl Asparagine Ethylene Isosteres via Sugar Acetylenes and Garner Aldehyde Coupling. (b) Dondoni, A.; Mariotti, G.; Marra, A.; Massi, A. Synthesis 2001, 2129-2137. Expeditious Synthesis of b-Linked Glycosyl Serine Methylene Isosteres via Ethynylation of Sugar Lactones.
4. S. P. Vincent, A. Schleyer, C.-H. Wong J. Org. Chem. 2000, 65, 4440-4443. Asymmetric Strecker Synthesis of C-Glycopeptide.
5. For a review see F. Nicotra, Synthesis of Glycosyl Phosphate Mimics. In Carbohydrate Mimics; Y. Chapleur, Ed.; Wiley-VCH:Weinheim, 1998, pp 67-85.
6. H.-D. Junker, W.D. Fessner Tetrhedron Lett. 1998, 39, 269-72. Diastereoselective Synthesis of C-Glycosylphosphonates via Free-Radical Glycosylation.
7. R. B. Meyer, Jr., T. E. Stone, P. K. Jesthi J. Med. Chem. 1984, 27, 1095-98. 2,5-Anhydro-1-deoxy-1-phosphono-D-altriol, an Isosteric Analogue of a-D-Ribofurnaose 1-Phosphate.
8. O. Gaurat, J. Xie, J.-M. Valéry Tetrahedron Lett. 2000, 41, 1187-89. A concise synthesis of C-glycosyl phosphate and phosphonate analogues of N-acetyl-a-D-glucosamine-1-phosphate.
9. (a) F. Orsini, A. Caselli Tetrahedron Lett. 2002, 43, 7259-61.SmI₂-mediated reactions of diethyl iodomethylphosphonate with esters and lactones: a highly stereoselective syntheiss of a precursor of the C-glycosyl analogue of thymidine 5’-b-L-rhamnosyl)diphosphate. (b) A. Dondoni, A. Marra, C. Pasti Tetrahedron: Asymmetry 2000, 11, 305-17. Stereoselective synthesis of C-glycosylphosphonates from their ketols. Reconsideration of an abandoned route.
10. A. Schäfer, J. Thiem J. Org. Chem. 2000, 65, 24-29. Synthesis of Novel Donor Mimetics of UDP-Gal, UDP-GlcNAc, and UDP-GalNAc as Potential Transferase Inhibitors.
11. C. A. Centrone, T. L. Lowary J. Org. Chem. 2002, 67, 8862-70. Synthesis and Antituberculosis Activity of C-Phosphonate Analogues of Decaprenolphosphoarabinose, a Key Intermediate in the Biosynthesis of Mycobacterial Arabinogalactan and Lipoarabinomannan.
12. G. Brooks, P. D. Edwards, J. D. I. Hatto, T. C. Smale, R. Southgate Tetrahedron 1995, 51, 7999-8014. Synthesis of Derivatives of Muramic Acid and C-1 Homologated a-D-Glucose as Potential Inhibitors of Bacterial Transglycosylase.
13. V. S. Borodkin, M. A. J. Ferguson, A. V. Nikolaev Tetrahedron Lett. 2001, 42, 5305-08. Synthesis of b-D-Galp-(1→4)-a-D-Manp methanephosphonate, a substrate analogue for the elongating a-D-mannosyl phosphate transferase in the Leishmania.
14. (a)L. Qiao, J. C. Vederas J. Org. Chem. 1993, 58, 3480-82. Synthesis of a C-Phosphonate Disaccharide as a Potential Inhibitor of Peptidoglycan Polymerization of Transglysosylase. (b) Zahra, J.; Hennig, L.; Findeisen, M.; Giesa, S.; Welzel, P.; Muller, D.; Sheldrick, W. S. Tetrahedron 2001, 57, 9437-9452. Synthesis of a building block for phosphonate analogues of moenomycin A(12) from D-tartaric acid. (c) Abu Ajaj, K.; Hennig, L.; Findeisen, M.; Giesa, S.; Muller, D.; Welzel, P. Tetrahedron 2002, 58, 8439-8451. Synthesis of a complex disaccharide precursor of phosphonate analogues of the antibiotic moenomycin A(12).
15. A. Dondoni, P. P. Giovannini, A. Marra J. Chem. Soc., Perkin Trans. 1, 2001, 2380-88. A concise C-glycosyl amino acid synthesis by alkenyl C-glycoside–vinyloxazolidine cross-metathesis. Synthesis of glycosyl serine, asparagine and hydroxynorvaline isosteres.
16. G. J. McGarvey, T. E. Benedum, F. W. Schmidtmann Org. Lett. 2002, 4, 3591-94. Development of Co- and Post-Translational Synthethic Strategies to C-Neoglycopeptides.
17. (a) M. H. D. Postema, J. L. Piper Org. Lett. 2003, 5, 1721-23. Synthesis of Some Biologically Relevant b-C-Glycoconjugates. b) D. Calimente and M. H. D. Postema J. Org. Chem. 1999, 64, 1770-71. Preparation of C-1 Glycals via Olefin Metathesis. A Convergent and Flexible Approach to C-Glycoside Synthesis. See also: E. A. Voight, C. Rein, S. D. Burke J. Org. Chem. 2002, 67, 8489-99. Synthesis of Sialic Acids via Desymmetrization by Ring-Closing Metathesis

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