Synthesis of condensed tannins. Part 3. Chemical shifts for determining the 6- and 8-bonding positions of ‘terminal of’(+)-catechin units

Abstract

Pairs of 8- and 6-functionalized (Br, OH, OAc, CO₂Me, and CH₂Me groups) 3′,4′,5,7-tetra-O-methyl-(+)-catechins available through selective bromination and debromination reactions and hence via lithio-analogues, provide diagnostic chemical shifts of their residual 6- and 8-protons. These fall within narrow ranges [δ(CDCl₃) 6.07–6.24 and 6.30–6.47 respectively] devoid of overlap, which are useful for differentiating between 4,6- and 4,8-linked biflavanoids. Supplementary ¹H-shift parameters are also available.

The mechanism of a newly observed acid-catalysed migration of bromine from the 6- to the 8-position is examined. Direct carboxylation of (+)-catechin proceeds regiospecifically at the 6-position.