Stereoselective synthesis of l-vancosamine methyl β-glycoside by addition of an organocerium reagent to O-benzyloxime ethers

The methyl β-glycopyranoside of l-vancosamine (1, 3-amino-2,3,6-trideoxy-3-C-methyl-l-lyxo-hexose), a constituent of vancomycin and related glycopeptide antibiotics, was synthesized stereoselectively from methyl 2,6-dideoxy-β-l-lyxo-hexopyranoside (2) in eight steps. Compound 2 was subjected in sequence to regioselective 3-O-p-methoxybenzylation, 4-O-tert-butyldimethylsilylation, oxidative 3-O-deprotection, and pyridinium dichromate oxidation to give methyl 4-O-tert-butyldimethylsilyl-2,6-dideoxy-β-l-threo-hexopyranosid-3-ulose (8). O-Benzyloximation of 8 and removal of the tert-butyldimethylsilyl protective group furnished methyl 2,6-dideoxy-β-l-threo-hexopyranosid-3-ulose O-benzyloxime (12). Addition of a methylcerium reagent to 12 provided the branched-chain hydroxyamino sugar methyl 3-benzyloxyamino-2,3,6-trideoxy-3-C-methyl-β-l-lyxo-hexopyranoside (14) which was easily converted into the title compound by hydrogenolysis. On the other hand, reaction of the 4-benzyl ether 11 with the organocerium reagent gave only traces of the 3-epi-analogue of 14, methyl 4-O-benzyl-3-benzyloxyamino-2,3,6-trideoxy-3-C-methyl-β-l-xylo-hexopyranoside.