Photochemistry of N ‐(2‐Acylphenyl)‐2‐methylprop‐2‐enamides: Competition between Photocyclization and Long‐Range Hydrogen Abstraction

The photochemical reactions of various ‘ N ‐methacryloyl acylanilides’ (= N ‐(acylphenyl)‐2‐methylprop‐2‐enamides) have been investigated. Under irradiation, the acyl‐substituted anilides 1a – 1c and 1o afforded exclusively the corresponding quinoline‐based cyclization products of type 2 ( Table 1 ). In contrast, irradiation of the benzoyl (Bz)‐substituted anilides 1e – 1h afforded a mixture of the open‐chain amides 4e – 4h and the cyclization products 2e – 2h . Irradiation of the para ‐acyl‐substituted anilides 6a – 6e and 6h afforded the corresponding quinoline‐based cyclization products of type 5 as the sole products ( Table 2 ). The formation of the cyclization products 2a – 2c and 2o can be rationalized in terms of 6 π ‐electron cyclization, followed by thermal [1,5] acyl migration, and that of compounds 3p, 5a – 5e , and 5h can be explained by a 6 π ‐electron cyclization only. The formation of the open‐chain amides 4e – 4h probably follows a mechanism involving a 1,7‐diradical, C and a spirolactam of type D ( Scheme ). Long‐range ζ ‐H abstraction by the excited carbonyl O‐atom of the benzoyl group on the aniline ring is expected to proceed via a nine‐membered cyclic transition state, as proposed on the basis of X‐ray crystallographic analyses ( Fig. 2 ).