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

Article abstract of DOI:10.1016/0008-6215(95)00066-3

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 stereoselectivity 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.Keywords: Vancosamine; Branched-chain amino sugars; Organocerium reagents; Oxime ethers; Antibiotics