Flavonoids of Tephrosia procumbens—revised structure for praecansone A and conformation of praecansone B

Abstract

Two rotenoids (rotenone and sumatrol), a flavanone (obovatin), a β-diketone (praecansone B), and its isomeric methyl ether (praecansone A) have been isolated from the chloroform extract of roots of Tephrosia procumbens. Furthermore, an unusual flavonol (7-ethoxy-3,3′,4′-trihydroxyflavone; fisetin 7-ethyl ether) and an isoflavone (7,4′-dihydroxy-3′-methoxyisoflavone) have been isolated from an ethyl acetate fraction of the alcoholic extract of the roots. The structures of all the compounds were established spectroscopically.

The structure of praecansone A has been revised as (3) on the basis of ¹³C and ¹H n.m.r. data, and confirmed by an n.O.e. experiment. The possible conformations of praecansone B in CDCl₃ are discussed on the basis of ¹H n.m.r. data and D₂O-exchange studies.