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

Abstract A cascade reaction between 2‐styrylbenzaldehydes and indoles has been developed for the synthesis of 1 H ‐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.