The exo-deoxoanomeric effect in the conformational preferences of C-glycosides

Abstract

Rotational studies of a series of β- d- C-glycopyranosides were carried out by CD and NMR spectroscopy. The populations around the C-glycosidic bond were strongly dependent on the structure of the C-aglycon, the exo– syn rotamer population increasing with the degree of substitution on the C-aglycon. The hydroxymethyl group populations also showed dependence on the aglycon, although to a lesser degree; its gt rotamer smoothly increases with the substitution on the aglycon. These rotational preferences, together with the experimentally observed correlations between 1H and 13C NMR chemical shifts and the structural nature of the C-aglycon, point to a stereoelectronic σ CH – σ CO ∗ effect (hyperconjugation) directly involved in the rotation around the pseudo-glycosidic bond and indirectly around the C5–C6 bond (hydroxymethyl group).