Synthesis of the isomeric trisaccharides, methyl O-α-l-fucopyranosyl-(1→3, 4, and 6)-O-(2-acetamido-2-deoxy-β-d-glucopyranosyl)-(1→3)-β-d-galactopyranoside

Benzylation of methyl 3-O-(2-acetamido-4,6-O-benzylidene-2-deoxy-β-d-glucopyranosyl)-2,4,6-tri-O-benzyl-β-d-galactopyranoside with benzyl bromide in N,N-dimethylformamide in the presence of sodium hydride afforded methyl 3-O-(2-acetamido-3-O-benzyl-4,6-O-benzylidene-2-deoxy-β-d-glucopyranosyl)-2,4,6-tri-O-benzyl-β-d-galactopyranoside (3). Reductive ring-opening of the benzylidene group of 3 gave methyl 3-O-(2-acetamido-3,6-di-O-benzyl-2-deoxy-β-d-glucopyranosyl)-2,4,6-tri-O-benzyl-β- d-galactopyranoside (4). Cleavage of the 4,6-acetal group of 3 with hot, 80% aqueous acetic acid afforded the diol (5). Compounds 3, 4, and 5 were each subjected to halide ion-catalyzed glycosylation with 2,3,4-tri-O-benzyl-α-l-fucopyranosyl bromide to produce the corresponding trisaccharide derivatives, which, on catalytic hydrogenation, furnished the title trisaccharides, respectively.