Synthesis of 2-acetamido-2-deoxy-6-O-β-d-galactopyranosyl-d-galactopyranose and o-nitrophenyl 2-acetamido-2-deoxy-6-O-β-d-galactopyranosyl-α-d-galactopyranoside

Acetalation of o-nitrophenyl 2-acetamido-2-deoxy-α-d-galactopyranoside with p-methoxybenzaldehyde-zinc chloride complex afforded the 4,6-O-(p-methoxybenzylidene) derivative (1). Acetylation of 1 with pyridine-acetic anhydride gave the crystalline monoacetate (2), the acetal group of which was cleaved by brief treatment with hot, 80% aqueous acetic acid to furnish the diol (3). The ¹H-n.m.r. spectra of both 2 and 3 supported their overall structures. Glycosylation of 3 with 2,3,4,6-tetra-O-acetyl-α-d-galactopyranosyl bromide (5) in 1:1 nitromethane-benzene, in the presence of mercuric cyanide, afforded the disaccharide derivative (6). O-Deacetylation of 6 in methanolic sodium methoxide gave the title disaccharide (8). A similar glycosylation of benzyl 2-acetamido-3-O-acetyl-2-deoxy-α-d-galactopyranoside with bromide 5, followed by O-deacetylation of the resulting intermediate, produced disaccharide 9. The structures of 8 and 9 were confirmed by ¹³C-n.m.r. spectroscopy. Hydrogenolysis of the benzyl aglycon of 9 gave the free disaccharide 2-acetamido-2-deoxy-6-O-β-d-galactopyranosyl-d-galactopyranose (13). The ¹³C-n.m.r. spectrum of 13 is recorded, and discussed in conjunction with those of the β-(1→3)- and the β-(1→4)-linked isomers.