Mechanism of anomerization of permethylated methyl d-glycopyranosides by trimethylsilyl trifluoromethanesulfonate

Anomerization of permethylated methyl d-glycopyranosides catalyzed by trimethylsilyl trifluoromethanesulfonate was examined by NMR spectroscopy and gas–liquid chromatography. The initial-rate analysis showed that the rates of anomerization of β to α were faster than those of the opposite at various concentrations of the catalyst. The overall rates of disappearance of the substrates were faster than the rates of appearance of the opposite anomers because of the formation of a pyrylium-type of compound. The structure of the latter was proposed based on 1H and 13C NMR spectra, although it was not isolable. Both cyclic and acyclic oxonium ions appear to be intermediates in the anomerization. The former seems to be the key intermediate for the formation of the pyrylium-type of compound as well as the anomerization of α-glycosidic linkages. β-Glycosidic linkages, on the other hand, appear to undergo anomerization mostly via acyclic oxonium ions.