Oligomeric flavanoids. Part 16. Novel prorobinetinidins and the first A-type proanthocyanidin with a 5-deoxy A- and a 3,4-cis C-ring from the maiden investigation of commercial wattle bark extract

Abstract

Structural examination of the phenolic metabolites of commercially used wattle bark extract reveals the presence of a range of novel flavanoids comprising (–)-epirobinetinidol 1, the first C-methyl proanthocyanidin, (–)-fisetinidol-(4α,8)-6-methyl-(+)-catechin 3, the first prorobinetinidins with 3,4-cis C-ring configurations 7 and 9, and the unique A-type prorobinetinidin 11 representing the first entry amongst this class of oligoflavanoids exhibiting a 5-deoxy A- and a 3,4-cis C-ring. They are accompanied by a range of functionalized prorobinetinidin -type tetrahydropyrano[2,3-f]chromenes 20, 23, 25 and 28 and the trimeric ‘isomerization-intermediate’32, all exhibiting the characteristic structural features that are essential for the use of ‘Mimosa’ extract in cold-setting adhesives and leather-tanning applications. In addition, evidence demonstrating that the dynamic A-E conformational equilibrium of flavan-3-ol moieties in condensed tannins may be influenced by external factors is presented.