Flav-3-ene

Both modes of a-cleavage have been observed for flav-3-ene (10), which eliminates both hydrogen and phenyl radicals from the molecular ion (67JCS(C)1933). A further pathway is operative for 2-(o-hydroxyphenyl)chromene, the spectrum of which shows an ion at m/e 118... 

Of the simpler examples, the formation of a 2-methylchromene and a flav-3-ene from the reaction of 2,4-dihydroxyacetophenone with but-2-enal and cinnamaldehyde, respectively, are worthy of mention (7lJCS(C)81l). 

I. include isoflavones, isoflavanones, isoflavans, iso-flav-3-enes, rotenoids, pterocarpans, coumestans, 3-aryl-4-hydroxycoumarins, 2 -hydroxy-3-arylcoumar-ins, 2-arylbenzofurans, a-methyldeoxybenzoins and individual compounds like lisetin and ambanol (see separate entries). I. have a restricted botanical distribution, occurring chiefty in the subfamily Papilionoi-deae of the Leguminoseae, and sometimes in the subfamily Caesalpinioideae. Their occasional presence in other families has also been reported (Rosaceae, Mor-aceae, Amaranthaceae, Podocarpaceae, Chenopodia-ceae, Cupressaceae, Iridaceae, Myristicaceae, Stemo-naceae). I. have also been reported from microbial cultures (e.g. T.Hazato et al. J.Antibiot. Tokyo 32 (1979) 217-222), but in all cases the culture media contained soybean meal, so the possibility remains that these 1. are of plant origin. 

Rearrangement of propargyl aryl ether 19 smoothly proceeds under thermal conditions to afford chromene derivatives 22 [23, 24]. The mechanism involves the formation of ortho allenyl phenol 20 followed by a 1,5-hydrogen shift and elec-trocyclic ring closure sequence via 21. For example, the Claisen rearrangement of propargyl aryl ether 23 prepared by the Mitsunobu reaction smoothly took place at 180 °C to give a flav-3-ene derivative 24 in excellent yield [25]. 

A new isoflav-3-ene, pallidiflorene (2, 7-dihydroxy-4 -methoxyiso-flav-3-ene), has been isolated from this species 78, 315). A new isoflavone pallidiflorin (5-hydroxy-4 -methoxyisoflavone) was isolated by Liu et al. 196), and a new chalcone glypallichalcone (247) has been isolated from the roots by Zhang et al. 316). 

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