Synthesis of condensed tannins. Part 7. Angular [4,6 : 4,8]-prorobinetinidin triflavanoids from black wattle (‘Mimosa’) bark extract

Abstract

The triflavanoid fraction from the bark extract of the black wattle (Acacia mearnsii) comprises five angular prorobinetinidins with their constituent robinetinidol units [4,6 : 4,8]-linked to both (+)-catechin and (+)-gallocatechin. Synthetic proof of structure is provided for three bi-[(–)-robinetinidol]-(+)-catechin diastereoisomers. The complete dominance of prorobinetinidins in the higher oligomeric fractions correlates with the faster condensation rate of the parent (+)-leucorobinetinidin with the nucleophilic substrates compared with competing (+)-leucofisetinidin.