Visible Light-induced Autocatalytic Aza-6π Electrocyclization to Access Polysubstituted Phenanthridines

Autocatalytic reaction represents an appealing approach in organic chemistry and life sciences. The development of a novel autocatalytic mode for aza-6π electrocyclization reactions represents a promising protocol for the efficient construction of aromatic N-heterocycles. Drawing inspiration from natural biosynthetic pathways for aromatic heterocycles, a wide variety of bioactive natural products can be synthesized efficiently using a biomimetic aza-6π electrocyclization strategy. However, conventional thermal catalysis inevitably suffers from a limited substrate scope and diversity. The photochemical protocol is considered a promising approach in elegant organic synthesis, but the application of photocatalysis in 6π electrocyclization has been rarely explored. In this context, we hypothesized that the aza-hexatriene system of phenanthridine precursor might cyclize with the intermediate imine, which could be formed in situ from the reaction of 2-arylaniline and aldehyde source, respectively. Subsequently, the aza-hexatriene is excited by the corresponding phenanthridine via energy transfer under visible light. The final intramolecular cyclization could generate the substituted phenanthridines. Herein, a novel visible light-induced autocatalytic aza-6π electrocyclization method was reported for the synthesis of diverse phenanthridines. A broad spectrum of 2-arylaniline and (hetero)aryl aldehydes could be well tolerated under metal- and oxidant-free conditions, affording the corresponding phenanthridines in moderate to good yields. This newly developed method represents a highly efficient and cost-effective synthetic protocol. The late-stage functionalization of celecoxib and bromopride derivatives also demonstrated the practical value of this method.