EXOANOMERIC 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-methoxytetrahy dropyran, hyperconjugative delocalization of lone-pair density on the exocyclic oxygen atom at C-1 into the sigma orbital of the C-1-O-5 bo nd 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 ex ocyclic C-1-O bond, and stabilizes the gauche conformers by about 4 kc al/mol over the anti. In the anti orientation, hyperconjugative intera ction of the OR group at C-1 with other appropriately oriented sigma orbitals increases, but the geometric and energetic consequences are l ess marked. Solvation effects reduce the energetic stabilization assoc iated with the exo-anomeric effect in the tetrahydropyrans, This deriv es from a combination of changes in the overall electrostatics and als o from decreased accessibility of the hydrophilic groups in the gauche conformers, For glucose or glucosides, instead of the simple tetrahyd ropyran model systems, the interactions of the exocyclic OR group at C -1 with the hydroxy group at C-2 can significantly affect these hyperc onjugative delocalizations. In the glucose and glucoside systems, solv ation effects oppose the formation of intramolecular hydrogen bonds, M M3(94) force field calculations show systematic deviations in the rela tive energies and structures of the various model systems with respect to the more reliable HF/cc-pVDZ predictions, (C) 1997 Elsevier Scienc e Ltd.