Anomeric and Reverse Anomeric Effects in the Gas Phase and Aqueous Solution

Anomeric stabilization within the (hydroxymethyl)oxonium, (hydroxymethyl)ammonium, and 2-tetrahydropyranosylammonium systems has been explored at both the ab initio (MP2/6-31G**//HF/6-31G**) and semiempirical (AM1) levels of theory. Aqueous solvation effects have been explored using a continuum dielectric model including local field and hydrophobic/hydrophilic effects (AM1-SM2). By analysis of C-O bond rotational coordinates in (hydroxymethyl)oxonium and stationary points for the other two molecules, it is apparent that hyperconjugative delocalization is operative in all instances of favorable orbital overlap; i.e., an anomeric effect exists. It is smaller in the ammonium case by comparison to the oxonium. Aqueous solvation tends to reduce anomeric stabilization, especially for the tetrahydropyranosyl system, where steric and dipole-dipole effects are also in opposition. The combination of these effects is sufficient to outweigh anomeric stabilization and leads to an equilibrium dominated by the equatorially substituted isomer, i.e., the reverse anomeric effect. Implications for the relative basicity of axial and equatorial anomeric substituents are also discussed.