Influence of solvent on the magnitude of the anomeric effect

A novel application of 13C nuclear magnetic resonance provided the effects of solvent polarity and hydrogen-bond formation on the conformational equilibria for a range of 2-substituted tetrahydropyrans and the results are interpreted in terms of how solvent affects the competition between the endo- and exo-anomeric effects in determining the magnitude of the anomeric effect. In accord with the generally accepted origin of the endo- and exo-anomeric effects (anti-periplanar n–σ* interaction of the oxygen lone-pair orbital with the antibonding orbital of the adjacent C—O bond), the exo-anomeric effect for the α anomer is expected to be weaker because charge delocalization from the glycosidic oxygen to anomeric center is in competition with delocalization from the ring-oxygen atom. The effects of solvent on the relative magnitudes of the endo- and exo-anomeric effects are then considered to arise from the formation of specific complexes with the solvent, and the exo-anomeric effect of a β-glycoside is more s...