Solvation effects on conformational equilibria. Studies related to the conformational properties of 2-methoxytetrahydropyran and related methyl glycopyranosides

Abstract

Further evidence is presented (8) that changes in optical rotation at the D-line of sodium which occur with changes in solvent largely reflect changes in conformational equilibria.Changes in the nuclear magnetic resonance spectrum of 4,4,5,5-tetradeuterio-2-methoxytetra-hydropyran and in the rotation of s(+)-2-methoxytetrahydropyran with changes in solvent are interpreted to show that solvents capable of donating a hydrogen to a hydrogen bond have an effect on the magnitude of the anomeric effect which is greater than that resulting from the change in the dielectric constant of the solvent. In the case of methyl 2-deoxypyranosides with the methoxy group in axial orientation, water appears to have an especially profound influence in stabilizing that orientation of the methyl group which brings it into gauche relationship with both the C2-grouping and the anomeric hydrogen; that is, counter to the anomeric effect.The methyl 2-deoxy-β-L-erythro-pentopyranoside was synthesized from di-O-acetyl-L-arabinal by way of an iodomethoxylation. Deuterolysis of the resulting 1,2-trans-acetylated methyl 2-deoxy-2-iodopentosides is shown to proceed in one case with extensive inversion of configuration and with extensive retention in the other case and offers a route to the preparation of 2-deoxy-2-deuterio-erythro-pentose with a high proportion (>75%) of the diastereoisomer with the arabino-configuration.