Glycosylation of lactose: synthesis of branched oligosaccharides involved in the biosynthesis of glycolipids having blood-group I activity.

Methyl 4-O-(2-O-benzoyl-4,6-O-benzylidene-beta-D-galactopyranosyl)-2,3, 6-tri-O-benzoyl-beta-D-glucopyranoside (2) was prepared in four steps from methyl beta-lactoside. Crystalline 2 was a convenient substrate for the synthesis of branched oligosaccharides derived from lactose. High-yield glycosylations were performed first at the 3'-position and then, after removal of the benzylidene group, at the 6'-position, using trichloroacetimidates of N-phthaloylamino sugars. 3,4,6-Tri-O-acetyl-2-deoxy-2-phthalimido-beta-D-glucopyranosyl trichloroacetimidate was thus used in two consecutive glycosylations, and also in a simultaneous disubstitution of the triol 9, leading in each sequence to the branched tetrasaccharide, beta-D-GlcpNAc-(1----3)-[beta-D-GlcpNAc-(1----6)]-beta-D-Galp-(1----4)- beta-D-GlcOMe. Similar glycosylations performed with 3,6-di-O-acetyl-2-deoxy-2-phthalimido-4-O-(2,3,4, 6-tetra-O-acetyl-beta-D-galactopyranosyl)-beta-D-glucopyranosyl trichloroacetimidate afforded the branched hexasaccharide beta-D-Galp-(1----4)-beta-D-GlcpNAc-(1----3)-[beta-D-Galp-(1----4)-beta- D- GlcpNAc-(1----6)]-beta-D-Galp-(1----4)-beta-D-GlcOMe, which corresponds to the human-milk oligosaccharide lacto-N-neohexaose and has a strong blood-group I activity.