Synthesis of isoflavones containing naturally occurring substitution pattern by oxidative rearrangement of respective flavanones using thallium(III) <i>p</i>-tosylate

Abstract

Claisen condensation of substituted 2'-hydroxyacetophenones <b>1a-c</b> with aromatic aldehydes affords respective substituted 2'-hydroxychalcones <b>2a-n</b> which on base catalyzed cyclization in pyridine : methanol : water (1:1:1) give respective flavanones <b>3a-n</b>. The oxidative rearrangement of flavanones with thallium (III) <i>p</i>-tosylate furnishes respective isoflavones <b>4a-n</b> in overall 62-72% yields starting from <b>1</b>. The present methodology has been successfully applied for the synthesis of naturally occurring isoflavones such as di-<i>O</i>-methyldaidzein <b>4a</b>, cabruvin <b>4b</b>, pseudobabtigenin methylether <b>4d</b>, 5,7-dimethoxyisoflavone <b>4f</b>, 5,7,4'-trimethoxyisoflavone <b>4g</b>, derrustone <b>4i</b>, 7,8,3',4'-tetramethoxyisoflavone <b>4l</b>, purpuranin-A <b>4m</b> and 7,8,3',4',5'-pentamethoxyisoflavone <b>4n</b> and thus the first synthesis of <b>4n</b> is reported.