Antibody–oligosaccharide interactions: the synthesis of 2-deoxy-α-L-rhamnose containing oligosaccharide haptens related to Shigella flexneri variant Y antigen

A series of di- and trisaccharide glycosides based on the α-L-Rha(1 → 3)β-D-GlcNAc and α-L-Rha(1 → 3)α-L-Rha(1 → 3)β-D-GlcNAc elements have been synthesized to locate the minimal oligosaccharide determinant of the Shigella flexneri O-polysaccharide, which is built from a tetrasaccharide repeating unit, [ → 2) α-L-Rhap(1 → 2)α-L-Rhap(1 → 3)α-L-Rhap(1 → 3)β-D-GlcNAcp(1-]n. These compounds also serve to identify the carbohydrate surface of the Shigella antigen that interacts with a monoclonal antibody, currently the subject of crystallographic studies. Two strategies utilizing suitably protected glycals 1 and 19 were employed to obtain analogs bearing either terminal or glycosylated 2,6-dideoxy-α-L-arabino-hexopyranosyl (2-deoxy-α-L-rhamnopyranosyl) residues. N-Iodosuccinimide activation of the glycals in the presence of selectively protected mono- and disaccharide alcohols afforded 2-deoxy-2-iodo-α-L-rhamnopyranosides and these were ultimately reduced during deprotection stages to afford the desired functionality. Di-O-acetyl L-rhamnal 1 reacted with monosaccharides 2 and 7, and with disaccharide 11, to yield disaccharides 4 and 8, and trisaccharide 12, each bearing a terminal 2-deoxy-α-L-rhamnopyranosyl residue. The selectively protected 3-O-benzoyl-4-O-benzyl-L-rhamnal 19 was synthesized from L-rhamnal and used to prepared trisaccharide 22, which contained an internal 2-deoxy-2-iodo-α-L-rhamnopyranosyl unit. Removal of protecting groups gave the oligosaccharides 6, 10, 14, and 23. Oligosaccharides that contained a 2-deoxy-α-L-rhamnopyranosyl residue showed enhanced inhibitory power: in the case of trisaccharide 23 a 1.8 kcal mol−1 relative increase in free energy of binding compared to a larger pentasaccharide epitope, α-L-Rhap(1 → 2)α-L-Rhap(1 → 3)α-L-Rhap(1 → 3)β-D-GlcNAcp(1 → 2)α-L-Rhap-1 → OMe. These data suggest that the rhamnose O–2 hydroxyl of residue C points toward and has important interactions with binding site amino acids.