Photoinduced Aromatization of Unsymmetrically Substituted 1,4‐Dihydropyridines

Abstract Irradiation of unsymmetrically substituted 1,4‐dihydropyridines (1b‐1j) while bubbling oxygen or argon through the solution resulted in aromatization to the corresponding pyridine derivatives (3b‐3j). Compound 1a with 2‐nitrophenyl substituent in 4‐position undergoes elimination of water upon irradiation under both oxygen and argon atmospheres and formation of 3a with 2‐nitrosophenyl substituent. On the other hand, irradiation of the compounds 1e, 1k and 1l with 4‐hydroxy‐3‐methoxyphenyl, 5‐methyl‐2‐furyl and 2‐furyl substituent in this position, respectively, resulted in the expulsion of these substituents and formation of a pyridine derivative unsubstituted in position 4, namely compound 2. Chloroform as a solvent causes the photo‐oxidation of these compounds by an electron transfer mechanism which is supported by the formation of dichloromethane according to GC analysis and presence of acid (HCl) in the solution after irradiation.