The mechanism of the anomerisation of methyl D-glucopyranosides and methyl D-glucofuranosides

Abstract

The acid-catalysed anomerisation of the methyl D-glucopyranosides in [²H₄]methanol proceeds with exchange of the methoxy-group with the solvent. D-Glucose dimethyl acetal is not an intermediate since under the reaction conditions it cyclises to furanosides. A mechanism is proposed in which the D-glucopyranosyl cation is captured by solvent methanol with predominant or exclusive inversion of configuration. Methyl β-D-glucofuranoside also undergoes acid-catalysed anomerisation in [²H₄]methanol with exchange of the methoxy-group. Possible mechanisms, consistent also with the negative entropy of activation, involve a concerted ring-opening to yield D-glucose dimethyl acetal followed by ring-closure and a bimolecular displacement of methanol.