Gas phase glycosidic cleavage of oxynions from alkyl glycosides

The oxyanion M? H]^? from several methylglycosides were generated by fast atom bombardment and their decomposition was studied by mass-analysed ion kinetic energy spectrometry. The main decomposition pathway is the loss of methanol. The hydroxylic hydrogen arises by proton transfer from the hydroxyl groups of the sugar. In the gluco-series, no anomeric effect is found. The absence of either the hydroxyl groups at C-2 or C-6 does not inhibit the glycosidic cleavage. However, the blocking of both the hydroxyl groups at C-4 and C-6, by a benzylidene group or two methyl groups, inhibit completely the glycosidic cleavage. From these results, it is proposed that the glycosidic cleavage occurs after opening the sugar ring by a vicinal attack of an oxyanion at C-6 or C-4 to the C-5 carbon atom. Then, the ionized hemi-acetals fragment into a methanolate anion and a 5,6- or 4,5-anhydrosugar which exchange another proton before their separation into charged and neutral species.