Theoretical studies on the conformation of saccharides XII.* The anomeric effect in glycosyl halides

Molecular orbital calculations have been carried out on 2-fluorotetrahydropyran and 2-chlorotetrahydropyran as models for examining the anomeric effect in glycosyl halides. The calculated population of chair conformers depends on the solvent and for the axial form of fluoro derivative decreases from 90.6 % in dioxan to 37.0 % in water. For chloro derivative the dependence is less pronounced; the population of the axial form decreases from 97.8% in dioxan to 78.4% in water. The calculations indicate that the magnitude of the anomeric effect decreases with increasing polarity of the solvent and that the anomeric energies for anomeric substitution decrease in the order of substituents CI, OCH3, F. The optimized bond lengths and bond angles reproduce successfully the differences in molecular geometry obtained by X-ray crystal structure analysis of the glycosyl halides.