Synthesis of condensed tannins. Part 20. Cycloconformations and conformational stability among derivatives of ‘angular’ tetraflavanoid profisetinidins

Abstract

Conformational analysis of tridecamethyl ether tetra-acetates of two natural and three synthetic ‘tetra-meric’ profisetinidin condensed tannins based on nuclear Overhauser effect difference spectroscopy reveals overall ‘cyclic’ arrangements of flavanyl units in each despite mutual stereochemical and structural differences. The unique thermodynamic stability of their dominant (85–90% abundance) conformers is attributed to the combined effects of the relative configurations of constituent flavanyl units, to steric repulsion by functional groups ortho to interflavanyl bonds, and to steric inhibition of mobility about interflavanyl bonds due to partial overlap of terminal units.