Synthesis of Fused Indoline‐Cyclobutanone Derivatives via an Intramolecular [2+2] Cycloaddition

A serendipitously‐discovered process for the synthesis of heterocyclic products containing a novel fused indoline‐cyclobutanone ring system is reported. This process is believed to take place through in situ generation of a ketene intermediate, followed by intramolecular [2+2] cycloaddition with a pendant enamide. The formation of a ketene intermediate in this process is significant as the reaction conditions employed are analogous to those commonly used in tertiary amine Lewis base catalysis, where the potential intermediacy of ketenes is an important consideration that is often overlooked. A serendipitously‐discovered intramolecular [2+2] cycloaddition process is reported for the synthesis of novel indoline‐cyclobutanone derivatives. The reaction is proposed to proceed by Brønsted base‐promoted elimination of pivalic acid from a mixed anhydride to generate a ketene in situ.