Theoretical studies on the conformation of aldopyranoses

Abstract

The potential energy of eight aldopentopyranoses in their C1(D) and 1C(D) conformations have been calculated. Minimization of the energies have been studied by suitably tilting the axial ---OH groups. A small reduction in the total potential energy is observed when tilts of about 2° are given for the axial -OH groups. Conformational entropies have also been computed for all of the aldohexoses and aldopentoses, and these values have been used to obtain the free energies. The calculated free-energy values agree well with the experimental data on the equilibrium after adding a value of 0.4 kcal.mole-1 for the anomeric effect. The calculated values of the percentage of a-anomers present in the equilibrium mixture agree fairly well with those assigned from n.m.r. studies. The present data also indicate that all the aldohexopyranoses except α -D-altrose and α -D-idose exist only in C1(D) conformations, whereas all of the aldopentopyranoses except xylose exist in a C1(D)^⇆ 1C(D) equilibrium in solution.