Oligomeric flavanoids. Part 11. Structure and synthesis of the first phlobatannins related to (4α,6:4α,8)-bis-(–)-fisetinidol-(+)-catechin profisetinidin triflavanoids

Abstract

Several members of the unique class of natural phlobatannins, representing the products of stereospecific pyran rearrangement of 2,3-trans-3,4-trans-flavan-3-ol units present in, e.g., (4α,6 : 4α,8)-bis-(–)-fisetinidol-(+)-catechin triflavanoid profisetinidins, have been characterized. These include the functionalized 6,7-trans-7,8-cis-10,11-trans-11,12-cis-hexahydrodipyrano[2,3-f : 2′,3′-h]chromene (3) and the 10-flavanyl-6,7-trans-7,8-cis-tetrahydropyrano[2,3-f]chromene ‘isomerization-intermediates’(6), (15), and (17). The proposed structures of compounds (3) and (6) were confirmed by synthesis via base-catalysed conversion of the 3-O(E)-methyl ether (1) of its apparent biogenetic precursor, and the relative nucleophilicity of the A- and D-ring in the ‘dimeric’ tetrahydropyrano[2,3-h]chromene (14) assessed by condensation with the flavan -3,4-diol, (+)-mollisacacidin.