Modeling of aldopyranosyl ring puckering with MM3 (92)

The molecular mechanics algorithm MM3 was used to compute energy surfaces for aldopyranosyl rings having a full range of shapes. Energies were plotted against the ΦΘ puckering coordinates of Cremer and Pople. The 4C1 conformations of the model pyranosyl rings are dominant for both anomers of d-allose, d-galactose, d-glucose, d-mannose, and d-talose, as are the 4C1 conformations of β-d-altropyranose, β-d-gulopyranose, and β-d-idopyranose. α-d-Altropyranose is predicted to exist as an equilibrium of 1C4 and 4C1, α-d-idopyranose as an equilibrium among OS2, 1C4, and 4C1, and α-d-gulopyranose is predominately 4C1 but has some contribution from 1C4 (18%) and OS2 (9%). The calculated and measured hydrogen-hydrogen coupling constants agree well, although the energies for the β anomers in water are systematically low by an average of 0.4 kcal/mol. Because the errors in the predicted anomeric ratios are small and are similar for the eight hexoses, and because the only concession to the solvent was a dielectric constant of 3.0, specific solvent effects are apparently small.