Synthesis of the natural isoflavanones ferreirin, dalbergioidin, and ougenin

Abstract

2′-Benzoyloxy-7-benzyloxy-4′,5-dimethoxy-2-methoxycarbonylisoflavone (12), prepared in nine steps by established methods, has been degraded to the corresponding phenyl benzyl ketone (9), which cyclises on treatment with ethyl formate and sodium to give 7-benzyloxy-2′-hydroxy-4′5-dimethoxyisoflavone (15); the latter has been converted in four steps into 2′,5,7-trihydroxy-4′-methoxyisoflavanone (1), identical with natural ferreirin.

(±)-Dalbergioidin has been prepared by hydrogenation and subsequent saponification of 2′,4′,5,7-tetra-acetoxyisoflavone.

3-Methyl-2,4,6-trihydroxyphenyl 2,4-dimethoxybenzyl ketone (21), when treated with ethoxalyl chloride, gives mainly 2-ethoxycarbonyl-5,7-dihydroxy-2′,4′-dimethoxy-6-methylisoflavone (23), which has been transformed in six steps into (±)-2′,4′,5-trihydroxy-7-methoxy-6-methylisolfavanone, (±)-ougenin (20).