Theoretical study of the mutarotation of erythrose and threose: acid catalysis.

The acid catalysis of the mutarotation mechanism in the two aldotetroses, d-erythrose and d-threose, has been studied at B3LYP/6-311++G(d,p) computational level in gas phase and in solution employing the PCM-water model. The open-chain, the furanose and the connecting TS structures have been characterized. To study the enhancing effect of acid groups on the electrophilicity of the carbonyl carbon atom, four situations have been considered: (i) a classical Lewis acid as BH3; (ii) a classical hard-Pearson acid as Na(+); (iii) classical Brønsted acids as H(+) and H3O(+); and (iv) a combined strategy using H3O(+) and one bridge-H2O molecule as assistant in the proton transfer process. All the acidic groups reduce the activation energy with exception of the Na(+), which can act, in some cases, as inhibitor. It is greatly reduced by the presence of Brønsted acids (iii) and through the combined strategy (iv). For this last mechanism, the electronic activation energies are between 12 and 43 kJ mol(-1) in vacuum and between 13 and 25 kJ mol(-1) in water solution, through the use of the PCM model.