Effect of the heterocyclic ring-fusion on the photochemical reactivity of 1,4-dihydropyridines, polyhydroquinolines, and decahydroacridine-1,8-diones

Abstract

Photo-oxidation of a series of 1,4-dihydropyridine, polyhydroquinoline, and decahydroacridine-1,8-dione derivatives was investigated by exposing them to the UV light to elucidate the electronic and steric effects of the C_4/9-aryl substitutions and the extent of the ring-fusion on the completion irradiation times. Based on the proposed photoreaction mechanism, the steric and electronic effects of the C_4/9-aryl substitution affect the rate of reaction, while the increased ring-fusion, enhances the rigidity of the heterocyclic ring core, significantly facilitates the photoreaction rate. Among these three series of compounds, decahydroacridine-1,8-diones, and 1,4-dihydropyridines have respectively the largest and lowest rate of the photo-oxidation. The geometry of the considered heterocyclic compounds and their corresponding photo-oxidation intermediates was optimized using the time-dependent density functional theory, and DFT B3LYP method with 6-311++G(d,p) basis set. The results of the computational study explained that the deviations of the N₁- and C₄-atoms of the dihydropyridine ring core from the ring boat plane can affect the rate of the photoreaction of these three series of compounds. These deviations are dependent on the preferred orientation of the CO group towards the heterocyclic ring, the rigidity of the molecule, the type and position, and the electronic nature of the additional substituent on the C_4/9-aryl ring.