Synthesis of condensed tannins. Part 12. Direct access to [4,6]- and [4,8]-all-2,3-cis-procyanidin derivatives from (–)-epicatechin : assessment of bonding positions in oligomeric analogues from Crataegus oxyacantha L.

Abstract

Synthesis of methyl ether acetates of [4,6]- and [4,8]-all-2,3-cis-procyanidin biflavanoids is effected by oxidative functionalization of (–)-epicatechin tetramethyl ether with lead tetra-acetate, and condensation of the resultant 2,3-cis-flavan-3,4-diol derivative with (–)-epicatechin. Their natural phenolic counterparts in Crataegus oxyacantha are associated with exclusively [4,8]-linked 2,3-cis-tri- and tetra-flavanoid procyanidin analogues. Bonding positions on each of the phenolic nuclei are established by chemical shift differences (Δδ_{6-H, 8-H}) based on 6- and 8-bromo-(–)-epicatechin methyl ether acetates as reference compounds, supported by associated ¹H n.m.r. shift parameters.