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

Photo-oxidation of 1,4-dihydropyridine, polyhydroquinoline, and decahydroacridine-1,8-dione derivatives was investigated by exposing them to the UV light to elucidate the extent of the ring-fusion and the impact of molecular rigidity on the photo-oxidation completion time. The results indicated that the extent of the ring-fusion affects the time of irradiation, which is mostly due to the rigidity of the heterocyclic ring core regarding the freedom of rotation of the attached CO-groups and the effective delocalization of the nitrogen lone pair towards the attached conjugated part. Furthermore, the results showed the impact of the steric and electronic effects of the C-aryl substitution on the rate of the photo-oxidation. Among these three series of compounds, decahydroacridine-1,8-diones, and 1,4-dihydropyridines have the largest and lowest rate of the photo-oxidation, respectively. The geometry of the considered heterocyclic compounds and their corresponding photo-oxidation intermediates was optimized using the density functional theory (DFT) B3LYP method with 6-311 + + G(d,p) basis set. Computational results explained that the deviations of the N ₁ - and C ₄ -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, and the type, position, and electronic nature of the additional substituent on the aryl ring. The dihedral angle scanning clarified the intramolecular interaction of unbounded atoms in the molecules besides the extent of the electron delocalization on the total energy content of the molecules.