Synthesis and conformation of procyanidin diastereoisomers

Abstract

The four theoretically possible diastereoisomeric 4 → 8 linked procyanidin dimers with two 2,3-cis-flavanoid units, epicatechin-(4β→ 8)-epicatechin (6), epicatechin-(4β→ 8)-ent-epicatechin (8), ent-epicatechin-(4α→ 8)-epicatechin (7), and ent-epicatechin-(4α→ 8)-ent-epicatechin (2), have been synthesized together with the 2,3-trans diastereoisomers catechin-(4α→ 8)-catechin (14) and catechin-(4α→ 8)-ent-catechin (15). The preferred rotamer conformations of the deca-acetate derivatives of (6), (8), (14), and (15) in chloroform were deduced from their ¹³C n.m.r. and high-field ¹H n.m.r. spectra. The ¹H n.m.r. spectra of the preferred conformers of the pairs of acetate derivatives of (6) and (15), and (8) and (14), were qualitatively similar as the relative configuration about the interflavanoid bond of one pair has a meso-relationship to the other.