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

2575-2581Claisen condensation of substituted 2'-hydroxyacetophenones 1a-c with aromatic aldehydes affords respective substituted 2'-hydroxychalcones 2a-n which on base catalyzed cyclization in pyridine : methanol : water (1:1:1) give respective flavanones 3a-n. The oxidative rearrangement of flavanones with thallium (III) p-tosylate furnishes respective isoflavones 4a-n in overall 62-72% yields starting from 1. The present methodology has been successfully applied for the synthesis of naturally occurring isoflavones such as di-O-methyldaidzein 4a, cabruvin 4b, pseudobabtigenin methylether 4d, 5,7-dimethoxyisoflavone 4f, 5,7,4'-trimethoxyisoflavone 4g, derrustone 4i, 7,8,3',4'-tetramethoxyisoflavone 4l, purpuranin-A 4m and 7,8,3',4',5'-pentamethoxyisoflavone 4n and thus the first synthesis of 4n is reported