Synthesis of O-α-l-fucopyranosyl-(1→2)-O-β-d-galactopyranosyl-(1→4)-d-glucopyranose (2′-O-α-l-fucopyranosyl-lactose)

Selective benzoylation of 2,3:5,6:3′,4′-tri-O-isopropylidenelactose dimethyl acetal (1) with benzoyl chloride in dichloromethane afforded the 6′-O-benzoyl derivative (2). The constitution of 2 was inferred from its n.m.r. spectrum, and confirmed by methylation and subsequent hydrolysis to 2-O-methyl-d-galactose and d-glucose. Glycosidation of 2 (catalyzed by bromide ion) with 2,3,4-tri-O-benzyl-α-l-fucopyranosyl bromide gave the 2′-O-linked, fully protected trisaccharide (9). O-Debenzoylation of 9, followed by deacetalation, furnished the tri-O-benzyl trisaccharide (11). Subsequent hydrogenolysis of the benzyl groups of 11 afforded the title trisaccharide. The acetal 1 was converted into a monoacetate and a dibenzoate, and a simple synthesis of 2′-O-methyl-lactose, starting from 1, is also described.