Structural Variations of Biflavonoids

Substantial hindrance to substitution at the C-6 position is evident from the restricted occurrence of the robustaflavones (I-3, II-6 biflavonoids) (2). The biapigenin, robustaflavone, and its biflavanone analog have been reported in Araucaria and Juniperus of the gymnosperms (125). Within the angiosperms, robustaflavone is present in Rhus species and a biflavanone has been reported in Semecarpus (125). 

The cupressuflavones (1-8,11-8 biflavonoids) (3) are also a small group of compounds compared to the amentoflavones, and usually co-occur with them (Table 7.6.2). Cupressuflavone and its variously C-4 and C-7 methylated derivatives are prominent constituents of the Araucariaceae and Cupressaceae (125). Curiously they have not yet been found in the Thxodiaceae. All of these compounds carry a 5,7,4 -substitution pattern. One flavanone-flavone dimer, mesuferrone-B (307), and a biflavanone, mesuferrone-A (308), have been described from Mesuaferrea, 

The agathisflavones (1-6,11-8 biflavonoids) (4) are much more rare than their corresponding 1-8,11-8 analogs with only agathisflavone and a few C-4 and C-7 methylated derivatives being present in the Araucariaceae (125, 126). These compounds have not yet been found in the Cupressaceae, Podocarpaceae, or Ihx-odiaceae. As expected from the restricted substitution at the C-6 position, the 1-6,11-6 biflavonoids (5) are not common, and, in fact, succedaneaflavone (43) is the only known natural product. 

The 1-3,11-3 linked chamaejasmins (8) have received much attention recently and their number has grown to at least eight known natural products. All compounds reported to date are biflavanones, and variations in their stereochemistry account for much of (he structural diversity. Chamaejasmin is reported to have the rare 2S,3S configuration (125). Seven biflavanones of this group have been isolated from Stellera chamaejasme in the past three years and all of these compounds are of a IR stereochemistry (48, 231, 269, 270, 390). 

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