Exo-anomeric effects on energies and geometries of different conformations of glucose and related systems in the gas phase and aqueous solution

Ab initio calculations predict that in 2-hydroxy- and 2-methoxytetrahydropyran, hyperconjugative delocalization of lone-pair density on the exocyclic oxygen atom at C-1 into the σ ∗ orbital of the C-1-O-5 bond is maximized when the OR group at C-1 is oriented gauche to C-1-O-5. This exo-anomeric effect lengthens the C-1-O-5 bond, shortens the exocyclic C-1-O bond, and stabilizes the gauche conformers by about 4 kcal/mol over the anti. In the anti orientation, hyperconjugative interaction of the OR group at C-1 with other appropriately oriented σ ∗ orbitals increases, but the geometric and energetic consequences are less marked. Solvation effects reduce the energetic stabilization associated with the exo-anomeric effect in the tetrahydropyrans. This derives from a combination of changes in the overall electrostatics and also from decreased accessibility of the hydrophilic groups in the gauche conformers. For glucose or glucosides, instead of the simple tetrahydropyran model systems, the interactions of the exocyclic OR group at C-1 with the hydroxy group at C-2 can significantly affect these hyperconjugative delocalizations. In the glucose and glucoside systems, solvation effects oppose the formation of intramolecular hydrogen bonds. MM3(94) force field calculations show systematic deviations in the relative energies and structures of the various model systems with respect to the more reliable HF/cc-pVDZ predictions.