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

✍ Scribed by Takehiko Nishio; Hiroyuki Koyama; Daigo Sasaki; Masami Sakamoto

• Publisher: John Wiley and Sons
• Year: 2005
• Tongue: German
• Weight: 241 KB
• Volume: 88
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.200590095

✦ Synopsis

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).