2:5-Dihydroxy-4:6-dimethoxyphenyl benzyl ketone (I) described earlier as a yellow oil, b.p. 230–50°/1 mm., has now been obtained
as yellow prismatic needles, m.p. 108°. The acid-boron trifluoride complex method is of general use for the synthesis of phenolic
ketones; 2-hydroxy-4:5:6-tri-methoxyphenyl 4-methoxybenzyl ketone (II), 2-hydroxy-5-ethoxy-4:6-di-methoxyphenyl 4-ethoxybenzyl
ketone (III) and 2:5-dihydroxy-4:6-di-methoxyphenyl 4-methoxybenzyl ketone (IV) have thus been prepared. The cyclization of
(II) with ethyl orthoformate gave 5:6:7:4′-tetramethoxy-isoflavone, which was then demethylated to 5:6:7:4′-tetrahydroxyisoflavone
(V). Partial demethylation of 5:6:7:4′-tetramethoxyisoflavone to 5-hydroxy-6:7:4′-trimethoxyisoflavone and 5:6:4′-trihydroxy-7-methoxy-isoflavone
(VI) has been effected.
Treatment of (VI) in acetone with benzoyl chloride (2 mol.) and potassium carbonate gave 6 : 4′-dibenzoyloxy-5-hydroxy-7-methoxyisoflavone
(VII), methylation of which yielded 6:4′-dibenzoyloxy-5:7-dimethoxyisoflavone (VIII). The m.p. of (VIII) was not depressed
by mixing with the dibenzoyl derivative prepared from natural muningin by means of benzoyl chloride and potassium carbonate
in acetone. Hydrolysis of (VIII) with 2% methanolic caustic potash at 30° and acidification gave 6:4′-dihydroxy-5:7-dimethoxyisoflavone,
identical with natural muningin. The ditosyl analogues of (VII) and (VIII) were similarly prepared.
Ethyl orthoformate cyclization of the ketone (III) gave muningin diethyl ether, and cyclization of the ketone (IV) yielded
muningin 4′-methyl ether.