Photochemical Synthesis of Dihydropyrido[1,2‐ a :3,4‐ b ′]diindole Derivatives

Herein, we report a photochemical method for the intramolecular cyclization of N‐(substituted benzoyl)‐1‐substituted‐1,2,3,4‐tetrahydro‐β‐carbolines. This transformation relies on photoexcitation of the substrate itself in the absence of external photocatalysts, with catalytic p ‐toluenesulfonic acid (PTSA) facilitating the cyclization process. This reaction provides a straightforward and practical approach for the synthesis of dihydro‐pyrido[1,2‐ a :3,4‐ b ′]diindoles. We investigate the substrate scope by varying substituents at the R ² and R ⁹ positions of the starting materials. A variety of dihydro‐pyrido[1,2‐ a :3,4‐ b ′]diindoles were successfully synthesized in moderate to excellent yields (up to 84%). Substrates bearing alkyl, aryl, polysubstituted, and fused‐ring moieties are well tolerated. Furthermore, the synthetic practicability of this protocol is validated by a gram‐scale reaction.