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 6p-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 z-H-abstraction (1,8-H transfer) by an excited acyl O-atom (Path A).

Complementary Strategies for Directed C(sp3)−H Functionalization: A Comparison of Transition‐Metal‐Catalyzed Activation, Hydrogen Atom Transfer, and Carbene/Nitrene Transfer

“…Another example has been reported by Nishio et al. (Scheme ) for the synthesis of amide 423 from ketone 422 . Irradiation of 422 with UV light results in the formation of diradical 424 .…”

“…[150] Another example has been reported by Nishio et al (Scheme 79) for the synthesis of amide 423 from ketone 422. [175] Irradiation of 422 with UV light results in the formation of diradical 424.T he oxygen radical abstracts ah ydrogen atom from the methyl C À Hb ond through 1,8-HAT, thereby leading to the formation of alkyl 425,w hich cyclizes to give 426.A mide 423 is formed via either intermediate 427 or 428.T he key to the success is the lack of hydrogen atoms for 1,5-, 1,6-, and 1,7-HATasw ell as the geometry constraint imposed by the sp 2 -hybridized atoms which significantly reduces the entropy cost for 1,8-HAT. Breslow et al also investigated long-range HATfor the C À H functionalization of long hydrocarbon chains [176] and complex steroid molecules [177] (Scheme 80).…”

1,n‐Hydrogen‐Atom Transfer (HAT) Reactions in Which n≠5: An Updated Inventory

“…The tricyclic-bridged lactam 59.5 was isolated in 56 % yield (Scheme 59). [111,112] Scheme 56. Formation of eight-membered rhodacycle by 1,7-HAT.…”

The Truce–Smiles rearrangement and related reactions: a review

“…73,[75][76][77] Additionally, examples of silyl ethers, 78 hydroxamate esters, 79 amides, [80][81][82] and ethers 83,84 have been reported. There is also a series of isolated reports of aryl migrations to carbon believed to proceed by photochemical processes.…”