Oligomeric flavanoids. Part 4. Base-catalysed conversions of (–)-fisetinidol-(+)-catechin profisetinidins with 2,3-trans-3,4-cis-flavan-3-ol constituent units

Abstract

Additional novel members of the class of natural ‘phlobaphene’ condensed tannins, representing the products of C-ring isomerization of 2,3-trans-3,4-cis-(–)-fisetinidol units present in (4β,6)- and (4β,8)-biflavanoid profisetinidins have been characterized. These comprise the functionalized 8,9-trans-9,1 0-trans-3,4,9,10-tetrahydro-2H,8H-pyrano[2,3-h]chromenes (2) and (11), and 8,9-cis-9, 10-trans analogue (8), and a 6,7-cis-7,8-trans-[2,3-f]-regioisomer (27). Analogues (8) and (11) are prototypes of a unique class of phlobatannins in which the resorcinol A- and pyrocatechol B-rings are interchanged relative to their positions in the more common isomers. Their formation represents a novel rearrangement of profisetinidins with 2,3-trans-3,4-cis-flavan-3-ol units, e.g. (1) with concomitant inversion of absolute configuration at 3-C(C), under base catalysis. The proposed structures of the natural products were confirmed by synthesis via base-catalysed conversion of (–)-fisetinidol-(4β,8)- and (4β,6)-(+)-catechin O-methyl ethers (1) and (14).