Conformation of monomers and dimers of 2,3-trans and 2,3-cisflavan-3-ols with differing hydroxylation patterns in the B-ring

Abstract

A conformational analysis of derivatives of 2,3-trans and 2,3-cisflavan-3-ols with different numbers of phenolic hydroxyl groups on the B-ring (the aromatic substituent at C(2)) has been performed using the MM2 algorithm. The equatorial-axial equilibrium (defined by the orientation of the B-ring) of the heterocyclic ring is nearly independent of the oxidation pattern in the 2,3-trans derivatives. The axial conformation becomes more competitive with the equatorial conformation upon increasing oxidation of the B-ring in the 2,3-cis series that includes (−)-epicatechin. Detailed analysis has been performed of the C(4)–C(6) and C(4)–C(8) linked prodelphinidin dimers (trihydroxy phenyl substituents at C(2)) in both axial and equatorial orientations with respect to the fused ring system. In dimers, the conformation of the heterocyclic ring and the value of the linkage torsion are not very sensitive to the oxidation pattern of the B-ring, indicating that the unperturbed dimensions of the polymers are probably not sensitive to the oxidation pattern of the aryl substituent.