Theoretical studies of the conformation of saccharides. Part II. The conformational properties of the glycosidic linkage

Abstract

Stereochem. properties of the glycosidic linkage have been studied by the quantum-chem. PCILO method, using 2-methoxytetrahydropyran as a model. Calcns. of the two-dimensional, conformational (j,y) maps showed that the rotation around the C-1-O-1 bond is more hindered than that around the O-1-C-6 bond, and that there are differences in the shape of the energy curve for the axial and equatorial forms of 2-methoxytetrahydropyran. The obsd. population of the five stable conformers at equil. is consistent with the prediction of the anomeric and exo-anomeric effects. The calcd. abundance (76.8%) of the axial form of 2-methoxytetrahydropyran is comparable with exptl. results (77-80%) obtained by NMR measurements in non-polar solvents. The energies found for individual conformers made it possible to calc. the magnitude of the anomeric effect (3 kJ/mol) and to det., for the first time, the values of the exo-anomeric effect for axial (6 kJ/mol) and equatorial 2-methoxytetrahydropyran (7 kJ/mol). The calcd. variations of the geometry arising from rotation around the C-1-O-1 bond are consistent with results obtained by statistical anal. of exptl. data for a- and b-glycosides. The results obtained, indicating that the energy, geometry, and electronic structure of glycosides are largely affected by the conformation of the acetal segment, are discussed from the point of view of conformational anal. of oligo- and polysaccharides. [on SciFinder (R)]