A Novel Intramolecular Photocyclization of N‐(2‐Bromoalkanoyl) Derivatives of 2‐Acylanilines via 1,8‐Hydrogen Abstraction

Abstract The photochemical reactions of different N ‐(2‐acylphenyl)‐2‐bromo‐2‐methylpropanamides have been investigated. Irradiation of the N ‐unsubstituted anilides 1a – 1c gave the corresponding dehydrobromination, cyclization, and bromo‐migration products 2, 3 , and 4 , respectively ( Table 1 ). Irradiation of the N ‐alkyl anilides 1e – 1g afforded the corresponding deacylation and cyclization products 5 and 6 , respectively, whereas irradiation of the N ‐alkyl anilides 1i – 1k , carrying 2‐benzoyl groups on the aromatic rings, afforded the unexpected tricyclic lactams 7 (besides 2, 5 , and 6 ). The formation of the cyclization products 6 could be rationalized in terms of an electrocyclic ring closure of the 6 π ‐electron‐conjugated enamides 2 produced by dehydrobromination of 1 , followed by thermal 1,5‐acyl migration ( Path B in the Scheme ). The formation of the bridged lactams 7 probably follows a mechanism involving the 1,7‐diradical 8 generated by ζ ‐H‐abstraction (1,8‐H transfer) by an excited acyl O‐atom ( Path A ).