A Mechanism for the Anomerization of Acetylated Alkyl Glycopyranosides

Unequivocal evidence that the anomerization of acetylated alkyl glycopyranosides can proceed by way of an intramolecular mechanism was obtained through the observation that a racemic mixture of methyl β-glucopyranoside tetraacetate with the D-isomer labelled in the methoxyl group with carbon-14 is anomerized both by titanium tetrachloride and boron trifluoride without transfer of radioactive methoxyl groups to the L-isomer. It is submitted that these intramolecular anomerizations are best rationalized as the result of an unsuccessful attempt by the environment to bring about glycosidic cleavage and proceed by way of an ion-pair intermediate in which the anion is derived from the aglycon group.