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.