Synthesis of O-α-l-fucopyranosyl-(1→3)-O-β-d-galactopyranosyl-(1→4)-2-acetamido-2-deoxy-d-glucopyranose (N-acetyl-3′-O-α-l-fucopyranosyllactosamine)

Methyl 2-O-benzyl-β-d-galactopyranoside (6) was obtained in five, good yielding steps from methyl β-d-galactopyranoside (1). Treatment of 1 with tert-butylchlorodiphenylsilane in N,N-dimethylformamide in the presence of imidazole afforded a 6-(tert-butyldiphenylsilyl) ether, which was converted into its 3,4-O-isopropylidene derivative (3). Benzylation of 3 with benzyl bromide-silver oxide in N,N-dimethylformamide, and subsequent cleavage of its acetal and ether groups then afforded 6. On similar benzylation, followed by the same sequence of deprotection, benzyl 2-acetamido-3,6-di-O-benzyl-4-O-[6-O-(tert-butyldiphenylsilyl)- 3,4-O-isopropylidene-β-d-galactopyranosyl]-2-deoxy-α-d-glucopyranoside gave the 2-O-benzyl derivative (10). Compound 10 was converted into its 4,6-O-benzylidene acetal (11). Glycosylation (catalyzed by halide-ion) of 11 with 2,3,4-tri-O- benzyl-α-l-fucopyranosyl bromide afforded the fully protected trisaccharide derivative (13). Cleavage of the benzylidene and then the benzyl groups of 13 furnished the title trisaccharide (16). The structure of 16 was established by ¹³C-n.m.r. spectroscopy.