Triplet Energy Transfer‐Driven Intermolecular [2+2] Photocycloaddition of Acyclic Imines and Indoles: Facile Access to Azetidine‐Fused Indolines

ABSTRACT Currently, successful examples of the aza‐[2+2] photocycloaddition (aza‐Paternò‐Büchi reaction) of indoles primarily rely on intramolecular variants or cyclic imine equivalents. To unlock the full synthetic potential of aza Paternò‐Büchi reaction in indoles, it is essential to extend the reaction to acyclic imine. Here, an intermolecular [2+2] photocycloaddition of indoles with acyclic imines has been developed, enabling visible light‐mediated aza‐Paternò‐Büchi reaction by triplet energy transfer. A range of azetidine‐fused indolines are successfully provided with yields of up to 93% for 30 examples. Given the growing significance of nitrogen‐containing heterocycles in pharmaceutical research and the challenge of their limited accessibility, this study afforded a novel synthetic methodology to construct previously inaccessible structural motifs with high molecular complexity by extending indole intermolecular [2+2] photocycloaddition with acyclic imines.