Synthesis of condensed tannins. Part 5. The first angular [4,6 : 4,8]-triflavanoids and their natural counterparts

Abstract

Angular triflavanoids comprising a group of four diastereoisomeric [4,6 : 4,8]-bi-[(–)-fisetinidol]-(+)-catechins with definable absolute configurations (2R,3S,4S- and 2R,3S,4R-2R,3S-2R,3S,4S; 2R,3S,4S- and 2R,3S,4R,2R,3S-2R,3S,4R) result from flavanyl-4-carbocation mediated condensation of (2R,3S,4R)-flavan-3,3′,4,4′,7-pentaol [(+)-mollisacacidin] at C-6 of the (+)-catechin units of all-trans-(2R,3S,4S-2R,3S)-[4,8]-(–)-fisetinidol-(+)–catechin and its 3,4-cis-(2R,3S,4R-2R,3S) diastereoisomer respectively. The natural coexistence of some of the forementioned species of bi- and tri-flavanoids and their precursors indicate a comparable sequence of 4,8 preceding 4,6 condensation with (+)-catechin as common nucleophile.