IODINE CATALYZED CASCADE SYNTHESIS OF FLAVONE DERIVATIVES FROM 2'-ALLYLOXY-α, β-DIBROMOCHALCONES

Abstract

Synthesis of flavones from 2'-allyloxy-α, β-dibromochalcones has been described. The iodine induced oxidative cyclization of 2'-allyloxy-α, β-dibromochalcones results into the formation of 3-bromoflavanones which ultimately gives flavones. Dehydrobromination of 3-bromoflavanone to give flavone is the preferred reaction over dehydrogenation. 6 3-Bromoflavones are used in drugs and for synthesis of alkylated and arylated flavones. It is observed that variety of chalcones, flavones and flavanones with bromo substituent exhibits anti-tuberculosis activity. 7 6-Bromoflavone has shown binding affinity to central benzodiazepine receptors and anxiolytic effect in mice. 8 3-Haloflavones are used in the synthesis of biflavonoids. 9 Debromination of 1,2-bis(bromomethyl)arenes by using tetrakis(dimethylamino)ethylene (TDAE) in the presence of olefins and catalytic amount of iodine has been reported. 10 As an extension of previous studies 11 we have been interested in the synthesis of 2'-allyloxy-α, β-dibromochalcones, which leads to the corresponding 3-bromoflavones by using iodine in dimethylsulfoxide reagent. In earlier work 2'-allyloxy chalcones when reacted with DMSO-I 2 reagent, allyl group was smoothly removed followed by cyclization and dehydrogenation to give flavones. We envised that α, β-dibromochalcones expected to eliminate HBr and then dehydrogenation to give 3-bromoflavones. Same study is reported in present work.