Stereoselective synthesis of beta-D-GlcNAc-(1->4)-D-Glc disaccharide starting from lactose

The stereoselective preparation of the beta-D-GlcNAc-(1->4)-D-Glc disaccharide starting from known 4-O-[6-O-(1-methoxy-1-methylethyl)-3,4-O-isopropylidene-beta-D-talopyranosyl]-2,3:5,6-di-O-isopropylidene-aldehydo-D-glucose dimethyl acetal (2), in turn easily obtained from lactose, is reported. Key steps of this new procedure, that avoids the glycosylation reaction, are (a) a first epimerization at C-4' through an unusual procedure involving a completely stereospecific hydroboration–oxidation of the enol ether group of the hex-4-enopyranoside 4, obtained from 3 by base promoted acetone elimination, (b) an amination with inversion by SN2 reaction on an imidazylate intermediate, and, finally, (c) N-acetylation followed by complete deprotection.