Synthesis of condensed tannins. Part 19. Phenol oxidative coupling of (+)-catechin and (+)-mesquitol. Conformation of bis-(+)-catechins

Abstract

Phenol oxidative coupling of (2R,3S)-2,3-trans-3′,4′,5,7-tetrahydroxyflavan-3-ol[(+)-catechin] generates low yields of ether-{[3′O,8]- and [4′O,8]-} and atropisomeric carbon-linked {[2′,8]-,[2′,6]-, and [6′,8]-} bis-(+)-catechins. The isomeric structures were differentiated by nuclear Overhauser effect difference spectroscopy. Extension of condensation of (+)-catechin with (2R,3S)-2,3-trans-3′,4′,7,8-tetrahydroxyflavan-3-ol[(+)-mesquitol] readily affords the four atropisomeric [5,6:5,8]-bis-[(+)-mesquitol]-(+)-catechins previously encountered in the heartwood of Prosopis glandulosa(‘Mesquite’).