Iodine-Catalyzed Cascade Reaction of 2-Styrylbenzaldehydes with Indoles in the Synthesis of 1H-Indenes via 4π-Electrocyclization

A cascade reaction between 2-styrylbenzaldehydes and indoles has been developed for the synthesis of 1H-indenes. An iodine-catalyzed nucleophilic addition of indoles to an aldehyde group of 2-styrylbenzaldehydes leads to the formation of carbocation intermediates that undergo 4π-electrocyclization to give indenes with a trisubstituted double bond. These indenes are transformed into thermodynamically more stable products with a fully substituted double bond under the reaction conditions. Minor quantities of indolylbenzo[b]carbazoles are also produced as by-products.