Synthesis of condensed tannins. Part 17. Oligomeric (2R,3S)-3,3′,4′,7,8-pentahydroxyflavans: atropisomerism and conformation of biphenyl and m-terphenyl analogues from Prosopis glandulosa(‘mesquite’)

Abstract

(2R,3S)-2,3-trans-3′,4′,7,8-Tetrahydroxyflavan-3-ol [(+)-mesquitol], the predominant metabolite in the heartwood of Prosopis glandulosa, represents a putative precursor of a variety of oligomers, including conventional [4,6]- and [4,8]-biflavan-3-ols, a [1,6]-1,3-diarylpropylflavan-3-ol, [5,6]- and atropisomeric [5,8]-biphenyl-type biflavan-3-ols, and [5,6:5,8]-m-terphenyl-type triflavan-3-ols. Other participants in these condensations are mainly (+)-catechin, and also the flavan-3,4-diol analogue of (+)-mesquitol. Oligomeric structures were confirmed by biomimetic oxidative and acid-induced couplings, and by nuclear Overhauser effect difference spectroscopy. These applications enabled correction of previous structural assignments for atropisomeric [5,8]-(+)-mesquitol-(+)-catechins and [5,6:5,8]-bis-[(+)mesquitol]-(+)-catechins, and determination of their conformations.