Photo-oxidative aromatization and photo-rearrangement of 2, 4-diaryl-5-oxohexahydroquinolines

Abstract

Various 2,4-bisaryl substituted 5-oxohexahydroquinolines were synthesized and the electronic effects of 2- and 4-substitutions on the photochemical behaviors of these compounds were investigated. UV light irradiation of these compounds resulted in the occurrence of two types of photoreactions; oxidative normal photo-oxidation and photo-rearranged reactions. In both reactions, the aromatization of the heterocyclic 1,4-dihydropyridine ring to the pyridine ring was observed. The photochemical results indicated the involvement of the radical intermediate, responsible for the aryl ring migration and the occurrence of the photo-rearrangement reaction. The partial synergistic effect of the capto-dative radical stabilization of the involved radical intermediate was supported by the results of the experimental works and the DFT-computational studies. X-ray crystal structure analysis of two of 5-oxohexahydroquinoline compounds indicated that i). The C4-aryl ring occupies the pseudo-axial position of the twisted-boat conformation of the heterocyclic dihydropyridine ring and, ii). Owing to the steric ortho-repulsion, the C2-aryl ring has a non-planar orientation concerning the attached CC double bond of the heterocyclic ring. These observations were in good agreement with the data of the DFT-computational studies and the X-ray crystal structure analysis.