Biosynthesis of rotenoids. Evidence for 1,2-aryl migration and the isoflavonoid construction of rings A, C, and D in rotenone and amorphigenin

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Abstract
[1-14C]-, [2-14C]-, and uniformly labelled phenylalanine have been administered by wick-feeding to Derris elliptica plants, and the resulting radioactive rotenone has been degraded to locate the label. With [1-14C]-phenylalanine 90% of the label enters the 12-position; [2-14C]phenylalanine labels the 12a-centre (91%). Uniformly labelled phenylalanine causes distribution of label between risic acid and tubaic acid, degradation products of rotenone, in the ratio 8 : 1. These findings show that, during the construction of rotenone in nature, the aromatic ring A undergoes a 1,2-shift to the 12a-position in rotenone (originally the 2-position in phenylalanine). The building process for the A, C, D-system thus follows the isoflavonoid pattern established by the work of Grisebach. An earlier suggestion which involves benzoyl migration is excluded by our results. With a different biological system, germinating seeds of Amorpha fruticosa, the biosynthesis of amorphigenin has also been shown to involve 1,2-aryl migration. In work concerned with degradation of the isoprenoid unit which forms ring E and its side-chain, 6′,7′-dihydroxy-6′,7′-dihydrorotenone and the derived nor-6′-rotenone ketone have been made. The corresponding nor-6′-ketone in (–)- and (±)-forms, has been made from tubaic acid. The (±)-form is different from an intermediate given the same structure in Shamshurin's synthesis of tubaic acid; this supports an earlier comment on the validity of the synthesis.

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