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

Abstract

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.