Synthesis of methyl 3-O- and 2-O-(2-acetamido-2-deoxy-β-d-glucopyranosyl)-α-d-galactopyranoside

Condensation of methyl 2-O-benzoyl-4,6-O-benzylidene-α-d-galactopyranoside with 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-β-d-glucopyranosyl bromide (1) in dichloromethane, in the presence of silver trifluoromethansulfonate, 2,4,6-trimethylpyridine, and molecular sieves, afforded methyl 2-O-benzoyl-4,6-O-benzylidene-3-O-(3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-β-d-glucopyranosyl)-α-d-galactopyranoside (4). Deacetalation of 4 in hot, 80% aqueous acetic acid gave methyl 2-O-benzoyl-3-O-(3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-β-d-glucopyranosyl)-α-d-galactopyranoside (5), which, on deacylation, followed by peracetylation, furnished the peracetylated disaccharide derivative (6). The structures of 5 and 6 were established by ¹H-n.m.r. spectroscopy. O-Deacetylation of 6 afforded the title β-(1→3)-linked disaccharide 7. For the synthesis of the β-(1→2)-linked isomer, methyl 3-O-benzoyl-4,6-O-benzylidene-α-d-galactopyranoside was similarly condensed with bromide 1 to give the fully protected disaccharide derivative (8). Cleavage of the benzylidene group of 8 gave methyl 3-O-benzoyl-2-O-(3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-β-d-glucopyranosyl)-α-d-galactopyranoside (9). Deacetylation of 9, followed by peracetylation, afforded the peracetate (10). O-Deacetylation of 10 gave the β-(1→2)-linked disaccharide (11). The structures of the disaccharides 7 and 11 were confirmed by ¹³C-n.m.r. spectroscopy.