Photoinduced Annulation of N‐Phenylbenzamides for the Synthesis of Phenanthridin‐6(5H)‐Ones

Abstract A general concise method for the synthesis phenanthridin‐6(5 H )‐ones via photoinduced intramolecular annulation of N ‐phenylbenzamides was developed. Under argon atmosphere and room temperature, phenanthridin‐6(5 H )‐ones were obtained via irradiation N ‐phenylbenzamides with a 280 nm UV lamp in the presence of methanesulfonic acid in toluene. The mechanism is illustrated and believed to proceed in the order of amides tautomerization, 6 π ‐electric cyclization, [1,5]‐H shift, amide‐imidine tautomerization, keto‐enol tautomerism and evolution hydrogen. magnified image