Condensed tannins: direct synthesis, structure, and absolute configuration of four biflavonoids from black wattle bark (‘mimosa’) extract

Abstract

Optically pure (+)-2,3-trans-3,4-trans-flavan-3,3′,4,4′,7-pentaol [(+)-mollisacacidin] and (+)-2,3-trans-flavan-3,3′,4′,5,7-pentaol [(+)-catechin] condense rapidly in 0·1 M HCl to give good yields of 4,8-linked diastereoisomeric biflavonoid proanthocyanidins of all-trans and 2,3-trans-3,4-cis-2,3-trans configurations, and also the 4,6-linked all-trans isomer; c.d. comparison thus enables confirmation of the absolute configurations of three known 4,8-linked all-trans biflavonoids from black wattle bark as 2R,3S,4S–2R,3S, and also identification of a fourth as 2R,3S,4R–2R,3S.