DIASTEREOSELECTIVITY IN THE TRANSGLYCOSIDATION OF METHYL -O-METHYL-2-(N-METHYLACETAMIDO)-D-GLUCOPYRANOSIDE, -METHYL-2-(N-METHYLACETAMIDO)-D-GALACTOPYRANOSIDE, AND I-O-METHYL-2-(N-METHYLACETAMIDO)-D-MANNOPYRANOSIDE WITH RACEMIC 2-BUTANOL UNDER REDUCTIVE-CLEAVAGE CONDITIONS

The fully methylated derivatives of methyl 2-acetamido-2-deoxy-beta-D- glucopyranoside, methyl 2-acetamido-2-deoxy-beta-D-galactopyranosid an d methyl 2-acetamido-2-deoxy-alpha-D-mannopyranoside were subjected to reductive-cleavage conditions in the presence of various promoters, a nd the rates of formation of the respective oxazolinium ions were esta blished by H-1 NMR spectroscopy. When oxazolinium ion formation was co mplete, racemic 2-butanol was added to each reaction, and the time-cou rse for transglycosidation to form the respective 2-butyl glycosides w as established by gas-liquid chromatography. These studies established the minimum times required for conversion of these acetamido sugars t o their respective oxazolinium ions under various reductive-cleavage c onditions and the minimum times required for their transglycosidation with 2-butanol. In separate experiments, oxazolinium ions having the g luco, galacto, and manno configuration were formed in the presence of trimethylsilyl trifluoromethanesulfonate as the promoter, then reacted with racemic 2-butanol either in the presence or absence of a proton acceptor (2,6-di-tert-butylpyridine), and the time-course for formatio n of the individual diastereomeric glycosides was established by gas-l iquid chromatography. For all three oxazolinium ions, diastereoselecti vity was observed in their conversion to the respective diastereomeric (R)- and (S)-2-butyl glycosides. When the transglycosidations were co nducted in the absence of a proton acceptor, the kinetically preferred diastereomers were, in all three cases, present in the lowest proport ion at equilibrium, whereas when the transglycosidations were conducte d in the presence of a proton acceptor, the ratios of the diastereomer s were constant throughout the reaction and the kinetically preferred diastereomers predominated, These experiments have established the rat ios of the diastereomeric 2-butyl glycosides to be expected when the r eductive-cleavage method is employed for the analysis of acetamido sug ar-containing polysaccharides. (C) 1997 Elsevier Science Ltd.