A-TYPE PROANTHOCYANIDINS FROM PRUNUS-SPINOSA

The range of natural dimeric A-type proanthocyanidins is extended by identification of ent-epicatechin-(4-alpha-->8; 2-alpha-->O-->7)-catechin and ent-epiafzelechin-(4-alpha-->8;2-alpha-->O-->7)-epicatechin. They are accompanied in the flowers of Prunus spinosa by the structurally related metabolites ent-epicatechin-(4-alpha-->8;2-alpha-->O-->7)-epicatechin, ent-epiafzelechin-(4-alpha-->8; 2-alpha-->O-->7)-catechin and ent-epiafzelechin-(4-alpha-->8;2-alpha-->O-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 restricted access of diazomethane to 5-OH(A).