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

Abstract

AbstractThe 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 6π‐electron cyclization, followed by thermal [1,5] acyl migration, and that of compounds 3p, 5a–5e, and 5h can be explained by a 6π‐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).