Synthesis of some oligosaccharides containing the O-α-l-fucopyranosyl-(1→2)-d-galactopyranosyl unit

A practical approach for the synthesis of oligosaccharides containing the O-α-l-fucopyranosyl-(1→2)-d-galactopyranosyl unit has been developed by utilizing the readily accessible 3,4,6-tri-O-acetyl-2-O-(2,3,4-tri-O-acetyl-α-l-fucopyranosyl)-α-d-galactopyranosyl bromide (1). Thus, condensation of bromide 1 with 8-(methoxycarbonyl)octanol in benzene, in the presence of mercuric cyanide, afforded, after column-chromatographic separation, the anomers (3 and 4) of 8-(methoxycarbonyl)octyl 3,4,6-tri-O-acetyl-2-O-(2,3,4-tri-O-acetyl-α-l-fucopyranosyl)-d-galactopyranoside in the ratio of 13:4. On the basis of their specific rotations, compounds 3 and 4 were tentatively assigned the α and β configuration, respectively. Similarly, bromide 1 was allowed to react with some other, appropriately protected sugars having a free alcohol group, to afford the corresponding α and β anomers of a galactopyranosyl residue having an α-l-fucopyranosyl substituent at O-2. Column-chromatographic separation of the anomeric mixtures, followed by systematic removal of the protecting groups, then provided the final oligosaccharides desired. In this manner, the synthesis of the following oligosaccharides has been accomplished: α-l-Fuc-(1→2)-α-d-Gal-1→O(CH₂)₈CO₂Me (5), α-l-Fuc-(1→2)-β-d-Gal-1→O(CH₂)₈CO₂Me (6), α-l-Fuc-(1→2)-α-d-Gal-(1→3)-β-d-GlcNAc-1→OC₆H₄NO₂-p (11), α-l-Fuc-(1→2)-β-d-Gal-(1→3)-β-d-GlcNAc-1→OC₆H₄NO₂-p (13), α-l-Fuc-(1→2)-α-d-Gal-(1→3)-α-d-GalNAc-1→OC₆H₄NO₂-o, and α-l-Fuc-(1→2)-β-d-Gal-(1→3)-α-d-GalNAc-1→OC₆H₄NO₂-o (18). The structures assigned to compounds 5, 6, 11, 13, and 18 were all supported by their respective ¹³C-n.m.r. spectra.