THEORETICAL STUDIES ON THE CONFORMATION OF SACCHARIDES .1. CONFORMATIONAL FLEXIBILITY OF THE PYRANOSE RING

Conformations of 2-methoxytetrahydropyran as a model for the six-membered ring in aldopyranosides have been calculated by the PCILO method using the algorithm of the conjugated gradient to optimize the geometry. The calculated geometry of the fourteen basic forms of 2-methoxytetrahydropyran was found to be in agreement with the available data obtained by X-ray diffraction of pyranosides. The results indicate differences in the geometry of 2-methoxytetrahydropyran resulting from the change of the axial vs. equatorial position of the methoxyl group. These changes are particularly meaningful in the values of bond angles and they are in agreement with the anomeric and exoanomeric effects. The experimentally found differences in the energies of an axial (4C1) and equatorial (1C4) conformer, ΔG = 2.9-3.7 kJ/mol, and the dipole moment, μ = 1.20 ± 0.05 D (1D = 3.33 10-30mAs) agree well with the calculated values ΔE = 3.18 kJ/mol and <μ> = 1.18 D which, in turn, suggest that the axial conformer is preferred over the equatorial one by a ratio a:e = 78:22. © 1979 Springer-Verlag.