Synthesis of a serine-based neuraminic acid C-glycoside

Cell-surface carbohydrates are classified by the nature of their linkages to the protein as either N-linked or O-linked. O- and N-glycans are involved in a number of important biological functions. These activities can be lost on glycoprotein catabolism when these glycan linkages are enzymatically hydrolyzed. The design and synthesis of novel C-linked glycans should provide catabolically stable glycoproteins useful for understanding and regulating important biological processes. Our efforts are currently directed toward the synthesis of C-glycosides of ulosonic acids. This paper describes the first synthesis of a serine-based neuraminic acid C-glycoside. The protecting group chemistry required for both carbohydrate and peptide syntheses complicates this approach. Different protecting group strategies were investigated for use in the samarium diiodide mediated C-glycosylation reaction. The key elements of our synthetic approach involve the following: (i) the substitution of homoserine for serine in the C-glycosylation reaction to introduce a carbon in place of the O-glycosidic oxygen, (ii) the use of benzyloxycarbonyl as a homoserine protecting group, compatible with samarium diiodide mediated C-glycosylation reaction, and (iii) the reduction of the carbonyl group in homoserine early in the synthesis to improve C-glycosylation yield and to avoid lactone formation. Using this combined approach, we prepared 4-O-acetyl-4-[2-C-(1-methyl 5-acetamido 4,7,8,9-tetra-O-acetyl-2,6-anhydro-3,5-dideoxy-d-erythro-l-manno-nononate)]-2S-(benzyloxycarbonyl)amino-1-carboxylic acid (1), which will be used in peptide synthesis to prepare glycopeptides containing catabolically stable C-linked neuraminic acid.