Photochemical synthesis of 1,2,3,4-tetrahydroisoquinolin-3-ones and oxindoles from N-chloroacetyl derivatives of benzylamines and anilines. Role of intramolecular exciplex formation and cis conformation of amide bonds.

Abstract

Although N-chloroacetyl derivatives of benzylamines (2, 8) and anilines (21, 25, 29) disappeared quite rapidly when irradiated with a high pressure mercury lamp, no photocyclization to six- and five-membered lactams occurred. Measurements of fluorescence quenching and disappearance quantum yields of N-chloroacetyl-(3, 4-dimethoxylphenyl)-alkylamines having various lengths of alkyl chain revealed that the shorter the alkyl chain is, the more efficient the exciplex formation is. Therefore, the failure of photocyclization seemed to be due to trans conformation of amide bonds in the benzylamine and aniline derivatives. Introduction of an alkyl group on the amide nitrogen changed the stable conformation of amides from trans to cis, and hence N-alkyl-N-chloroacetylbenzylamines (11, 13, 15, 17, 19) readily gave the corresponding 1, 2, 3, 4-tetrahydroisoquinolin-3-ones (12, 14, 16, 18, 20) on irradiation. Oxindoles (36, 38, 40, 41, 43, 46) were similarly synthesized by photocyclization of N-alkyl-N-chloroacetylanilines (35, 37, 39, 42, 45).