STRUCTURE AND ABSOLUTE CONFIGURATION OF BIFLAVONOIDS WITH BENZOFURANOID CONSTITUENT UNITS

Notwithstanding the somewhat arbitrary differentiation between oligomeric flavonoids and oligomeric flavans (proanthocyanidins), the former, which are considered to originate via phenol oxidation, represent a less common but major group of phenolic natural products.1 Unlike proanthocyanidins, however, assessment of the absolute configuration of oligomeric flavonoids possessing stereocenters other than those originating from atropisomerism of the interflavonoid bond has hitherto met with very little success. In effect, this prevented the stereochemical assignment to zeyherin,2 the first (and thus far sole) entry with a benzofuranoid constituent unit, which was isolated some 27 years ago from the heartwood of “red ivory” (Berchemia zeyheri) Sond.3 The collective utilization of chemical degradation and 1 H NMR-, CD- and computational data now permits estimation of the absolute stereochemistry of such zeyherin analogues as well as related structures from the same source,4,5,6 and for the first time also of 2-benzyl-2-hydroxybenzo[1]furan-3(2H)-one enantiomers, a small but biosynthetically significant group of aurone derivatives.7