Visible Light‐Induced Singlet‐Oxygen‐Mediated Synthesis of Substituted Benzo[<i>c</i>]chromeno[4,3,2‐<i>gh</i>]phenanthridines^†

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 ( 1 O 2 ) promotes intramolecular cyclization through an oxidative hetero‐crosscoupling 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 density functional theory calculation and by trapping of the oxygen radical intermediate A with Butylated Hydroxytoluene (BHT), which was confirmed through 1 H, 13 C 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 process, is a fascinating example of the oxidative coupling reaction via distal sp 3 ‐CH.