A-type proanthocyanidins from Prunus spinosa

Abstract

The range of natural dimeric A-type proanthocyanidins is extended by identification of ent-epicatechin-(4? › 8; 2? › O › 7)-catechin and ent-epiafzelechin-(4? › 8;2? › O › 7)-epicatechin. They are accompanied in the flowers of Prunus spinosa by the structurally related metabolites ent-epicatechin-(4? › 8;2? › O-7)- epicatechin, ent-epiafzelechin-(4? › 8;2? › O › 7)-catechin and ent-epiafzelechin-(4a@8;2a@0-7)-epiafzelechin, these being reported from a plant source for the second time. Nuclear Overhauser effect difference spectroscopy facilitated assessment of both 3,4-relative stereochemistry of the heterocyclic ring C and absolute configuration of terminal units, as well as determination of the mode of interflavanyl linkage. Whereas methylation and subsequent acetylation of A-types with 'lower' catechin units produce the expected methyl ether acetates, those species with a 'lower' epicatechin moiety indicate restrictedaccess of diazomethane to 5-OH(A)