STEREOSELECTIVE SYNTHESIS OF L-VANCOSAMINE METHYL BETA-GLYCOSIDE BY ADDITION OF AN ORGANOCERIUM REAGENT TO O-BENZYLOXIME ETHERS

The methyl beta-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-beta-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-beta-L-threo-hexopyranosid-3-ulose (8) O-Benzyloximation of 8 and removal of the tert-butyldimethylsilyl protective group furnished methyl 2,6-dideoxy-beta-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-beta-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-beta-L-xylo-hexopyranoside.