Isolation of amorpholone, a potent rotenoid insecticide from Tephrosia candida

Amorpholone (6-hydroxy-6a,12a-dehydrorotenone, III) has been isolated and identified from the genus Tephrosia for the first time. Its structure has been confirmed by its partial synthesis from rotenone (6a,12a,4',5'-tetrahydro-2,3-dimethoxy-5'-isopropenylfurano [3',2':8,9]- 6H-rotoxen-12-one, I). The petroleum ether extract and the different fractions of the methanolic extract of the stems and leaves of Tephrosia candida along with some pure rotenone derivatives, viz., amorpholone (III), dehydrorotenone (II) (naturally obtained), and rotenonone (IV), a synthetic analogue of amorpholone, were tested for their insecticidal activity against the larvae of Spodoptera litura F. (armyworm), taking pure rotenone as the reference insecticide. Among the different crude extracts tested, petroleum ether showed the highest activity (LD50 = 0.38 micrograms/g of body weight) and rotenone (I) was the most active of the pure compounds (LD50 = 0.16 micrograms/g) followed by amorpholone (III, 0.31 micrograms/g) and rotenonone (IV, 0.68 micrograms/g), while dehydrorotenone (II) was found to be completely inactive.