Oligomeric flavanoids. Part 10. Structure and synthesis of the first tetrahydropyrano[3,2-g]chromenes related to (4,6)-bis-(–)-fisetinidol profisetinidins

Abstract

The range of natural bis-fisetinidol profisetinidins is extended by identification of (+)-epifisetinidol-(4α,6)-(–)-fisetinidol (5), (4α,6)-bis-(+)-epifisetinidol (7), (4β,6′)-bis-(–)-fisetinidol (13), and (–)-fisetinidol-(4β,6′)-(+)-epifisetinidol (15). They are accompanied in the heartwood of Colophospermum mopane by the first tetrahydropyrano[3,2-g]chromenes (17), (19), (21), and (23), related to the (4,6)-bis-(–)-fisetinidol profisetinidins (1) and (2). Under mild basic conditions the latter compounds undergo pyran rearrangements affording tetrahydropyrano[3,2-g]chromenes (17), (19), (21), and (23) as well as two additional C-2(F) epimeric pairs (25), (27), and (29), (31). The same intermediate quinone-methide presumably leading to the C-ring-isomerized analogues may feasibly also explain the genesis of the variety of compounds in the mopane displaying a C-2 epimeric relationship to the predominant (–)-fisetinidol monomeric precursor.