Biosynthesis of rotenone and amorphigenin. Study of the origins of isopropenyl-substituted dihydrofuran E-rings using isotopically labelled late precursors

Abstract

Whilst epoxidation of rot-2′-enonic acid is the most likely source of dalpanol in Amorpha fruticosa seedlings, administration of (5′R,6′S)-[7′-3H]dalpanol shows that it is not an intermediate on the path to rotenone and amorphigenin. Labelled 4′-hydroxy- or 5′-hydroxy-rot-2′-enonic acid also do not qualify as intermediates in rotenone biosynthesis, but they are each converted into amorphigenin with chemospecific attack on the methyl group. By administration and re-isolation of [8′-14C]amorphigenin from A. fruticosa seedlings, our earlier conclusion that hydroxylation of rotenone to form amorphigenin proceeds with even label scrambling between C-7′ and C-8′, probably via an allylic radical, is confirmed. Competitive double-labelling experiments are employed to support a scheme in which rotenone derives directly from rot-2′-enonic acid by an enzyme-induced radical-type reaction without the intervention of an hydroxylated intermediate, and the two labelled hydroxyrot-2′-enonic acids are similarly cyclised using their methyl groups. The incorporations into amorphigenin of labelled 4- and 5-hydroxyrot-2′-enonic acids, both of which are shown to occur naturally in A. fruticosa, are similar, but only about one sixth that of rotenone.