Oligomeric flavanoids. Part 7. Novel base-catalysed pyran rearrangements of procyanidins

Procyanidin B-3 (1) is subject to readily occurring C-ring isomerizations in NaHCO3–Na2CO3 buffer solution to form a novel 8,9-cis-9,10-trans-3,4,9,10-tetrahydro-2H,8H-pyrano[2,3-h]chromene (3) and a series of 2,3-cis-3,4-trans-4-aryl-2-flavanylbenzopyrans (6), (9), and (12) in which the C-2 pyrocatechol and C-4 (+)-catechin moieties are interchanged relative to their positions in the biflavanoid (1). These compounds presumably originate via 1,3-aryl migrations in intermediate quinone-methides with concomitant inversion of the absolute configuration at C-3. The lability of the interflavanyl bond at alkaline pH is reflected by the presence of considerable quantities of (+)-catechin as well as high-molecular-mass analogues of precursor (1).