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

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.