Thermal Degradation of Methyl β-D-Glucoside. A Theoretical Study of Plausible Reaction Mechanisms

Thermal conversion of methyl. beta-D-glucoside to levoglucosan was studied with the MP4//DFT(B3LYP) method. The first step is conformational change of the reactant to C-1(4) from C-4(1). The second step is intramolecular nucleophilic substitution at the anomeric Cl, which occurs via one step without oxacarbenium ion intermediate. The Delta G(0 double dagger) value (52.5 kcal/mol) is smaller than the Cl-Ol bond energy, indicating the direct homolysis mechanism is clearly ruled out. Bimolecular reaction also occurs with smaller activation energy via the similar transition state.