AB-INITIO EXAMINATION OF ANOMERIC EFFECTS IN TETRAHYDROPYRANS, 1,3-DIOXANES, AND GLUCOSE

Axial and equatorial structures of 2-methyl-, 2-hydroxy-, 2-methoxy-, 2-amino-, 2-fluoro-, and 2-chlorotetrahydropyran, of the 2-tetrahydropyranylammonium cation, of 2-methyl-, 2-hydroxy, and 2-methoxy-1,3-dioxane, and of the corresponding cyclohexanes have been fully optimized at the HF/6-31G* level. NBO analysis of the Hartree-Fock wave functions confirms that the anomeric effects of hydroxy-, methoxy-, fluoro-, and chloropyrans and of glucose and methyl glucoside are indeed due to hyperconjugation. In cyclohexane, tetrahydropyran, and glucose theoretical Delta E values involving the OH and OMe substituents are nearly identical. The experimentally observed differences of about 1 kcal/mol in the Delta H-0 and Delta G(0) values of the OH and OMe compounds are very likely due to solvent interactions involving the OH group. In the gas phase, glucose orientates its ring hydroxy groups and the CH2OH group differently than in the crystal. The structures observed in the crystal lead to 7.9 kcal/mol higher energies. This might be rationalized by the fact that the isolated molecule tries to maximize the number of intramolecular hydrogen bonds. 2-Hydroxy-1,3-dioxane prefers an equatorial conformation. NBO analysis reveals that the exo-anomeric effect favoring the equatorial form dominates over the endo-anomeric effect in dioxanes. In contrast, 2-methoxy-1,3-dioxane shows an axial preference which is not due to hyperconjugation. The reverse anomeric effect in 2-aminotetrahydropyran is due to steric repulsions because the axial conformation with the largest hyperconjugation contribution requires one of the NH2 hydrogens to point above the ring. The equatorial preference of NH3+ is a result of steric and other (probably electrostatic) contributions, since hyperconjugation strongly favors: the axial conformation. For all cases for which the hyperconjugation contributions are overcompensated by the Lewis energies, steric repulsions are also indicated by significant ring distortions. Dipole moments correlate with relative energies qualitatively in some cases, but a quantitative relationship cannot be ascertained. Single point calculations with Huzinaga basis sets reveal that at the HF/6-31G* level axial-equatorial energy differences of all compounds considered in this investigation are biased toward axial structures by 0.5-1 kcal/mol. Thus, the 6-31G* basis set fails to reproduce the small equatorial preference of 2-fluorohexane at the HF as well as at correlated levels.