An improved procedure for the analysis of linkage positions in 2-acetamido-2-deoxy-D-glucopyranosyl residues by the reductive-cleavage method.

The conditions of the reductive-cleavage method were modified to allow simultaneous analysis of 2-acetamido-2-deoxy-D-glucopyranosyl residues and monosaccharides of other classes. Methyl 2-deoxy-3,4,6-tri-O-methyl-2-(N-methylacetamido)-beta-D-glucopyran oside was found to undergo transglycosidation under reductive-cleavage conditions when the reaction was quenched with an alcohol. Transglycosidation proceeded via an oxazolinium-ion intermediate, which then acted as a glycosyl donor to form an anomerically pure product. Time-course studies showed that in the presence of trimethylsilyl trifluoromethanesulfonate (Me3SiOSO2CF3), 4 h were required for complete conversion of the substrate into this intermediate, which was then trapped with methanol-d4. When the reaction was conducted in the presence of a mixture of trimethylsilyl methanesulfonate (Me3SiOSO2Me) and boron trifluoride etherate (BF3.OEt2) or with BF3.OEt2 alone, 24 h and 48 h, respectively, were required for complete conversion. The alpha anomer was unreactive after 24 h under all conditions, confirming earlier results. Reaction with racemic 2-butanol yielded a pair of diastereomers, in a 1:1 ratio, which were distinguishable by their GLC retention times and their 1H NMR spectra. Reaction with (S)-2-butanol gave only one of the diastereomeric products. These experiments demonstrated the feasibility of using the reductive-cleavage method to determine the absolute configuration of 2-acetamido sugars.