Visible‐Light‐Induced Singlet‐Oxygen‐Mediated Synthesis of Substituted Benzo[c]chromeno[4,3,2‐gh]phenanthridines

Abstract

Visible‐light‐promoted environmentally benign synthesis of various 9‐alkylbenzo[c]chromeno[4,3,2‐gh]phenanthridine derivatives in the presence of 5 mol % disodium salt of eosin Y (EY) as photocatalyst is reported from 2‐(7,8,9,10‐tetrahydrobenzo[c]phe‐nanthridin‐6‐yl) phenol derivatives, which were prepared through a three‐component reaction. In the present protocol, EY photosensitized singlet oxygen (¹O₂) promotes intramolecular cyclization through an oxidative hetero cross‐coupling reaction, followed by aromatization to provide the final product in a single step, which has not been reported earlier. The mechanism of the reaction was established through DFT calculation and by trapping of the oxygen radical intermediate A with BHT, which was confirmed through 1H, 13C NMR spectra and HRMS. This process forms the highly fused hexacyclic heteroaromatics without requiring any metal catalyst or dehydrogenating agent. This intramolecular photocyclization, through the triplet energy transfer (EnT) process, is a fascinating example of the oxidative coupling reaction via distal sp3‐C‐H.