Oligomeric flavanoids. Part 8. The first profisetinidins and proguibourtinidins based on C-8 substituted (–)-fisetinidol units and related C-ring isomerized analogues

Abstract

Structural examination of the phenolic metabolites of Colophospermum mopane reveals the presence of the first profisetinidins and proguibourtinidins based on C-8 substituted (–)-fisetinidol units i.e. the (4α,8)-bis-(–)-fisetinidol (1), (+)-epifisetinidol-(4α,8)-(–)-fisetinidol (3), and the (+)-guibourtinidol-(4α,8)-(–)-fisetinidol (5). They are accompanied by the related functionalized tetrahydropyrano[2,3-h]chromenes (9), (11), (13), (15), and (17), and by a 2,4-diaryl-6-(2,-benzopyranyl)chroman (19), the first C-ring isomerized analogue derived from a B-ring coupled profisetinidin. Efforts towards the synthesis of the (4,8)-bis-fisetinidols from 6-bromo-(–)-fisetinidol and the appropriate flavan-3,4-diol, lead to the biaryl type biflavanoids (33) and (35). Their genesis is explained in terms of an oxidative substitution reaction initiated by bromonium ion.