Theoretical studies on the conformations of methyl glycopyranosides

Abstract

The σ-charges on various atoms of methyl glycosides have been computed by using the MO-LCAO method of Del Re. The potential and free energies of methyl aldohexopyranosides and methyl aldopentopyranosides in their C1(D) and 1C(D) conformations have been calculated. Minimization of the energies of these conformations has been studied by suitably tilting the axial C-C and C-O bonds. Considerable release of strain is achieved when tilts of 4.5 and 3° are given to the axial hydroxymethyl and hydroxyl groups, respectively, that are involved in Hassel-Ottar effect. A tilt of 3° is also found necessary for the axial OMe group involved in the Hassel-Ottar effect. The calculated free-energy values are in accord with experimental ones, after adding a value of 0.8 kcal.mole^-1 for the anomeric effect of -OMe group. These studies predict that all of the methyl aldohexopyranosides, except methyl α -D- and methyl β -D-idopyranosides, favour the C1 conformation. On the other hand, the energy calculations also predict that, of the eight methyl aldopentopyranosides studied, only methyl α -D- and methyl β -D-xylopyranosides and methyl α -D-ribopyranoside favour the C1(D) conformation; for the other pentopyranosides, considerable amounts of both C1(D) and 1C(D) conformations are present in the equilibrium mixture. The calculated values of the percentage of α -anomer present in the equilibrium mixture agree fairly well with those obtained experimentally.