Photooxidation of 1,4-Dihyropyridines

Abstract

Abstract 1,4-Dihydropyridines were successfully photooxidized to corresponding pyridines under suitable conditions. The substituents of the dihydropyridines, reaction mediums, and atmospheres affect the course of the reactions. When diethyl 2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate (1a), diethyl 1,4-dihydropyridine-3,5-dicar-boxylate (1b), 2,6-dimethyl-3,5-diacetyl-1,4-dihydropyridine (1c), and 3,5-diacetyl-1,4-dihydropyridine (1d) were irradiated by a super high pressure mercury lamp with filter in ethanol under air or oxygen, corresponding pyridines, diethyl 2,6-dimethylpyridine-3,5-dicarboxylate (2a), diethyl pyridine-3,5-dicarboxylate (2b), 2,6-dimethyl-3,5-diacethylpyridine (2c), and 3,5-diacetylpyridine (2d) were formed in good yields. Irradiation of 1a, 1c, and 1d in acetonitrile afforded 2a, 2c, and 2d irrespective of their atomosphere, oxygen or nitrogen. On the other hand, photolysis of 1b by the same lamp and filter in ethanol or acetonitrile in an atmosphere of nitrogen resulted in the formation of photodimer (3) which on irradiation by low pressure mercury lamp in acetonitrile dissociated to 1b followed by photooxidation giving 2b. Diethyl l-acetyl-1,4-dihydropyridine-3,5-dicarbolxyate (4e) and 1,3,5-triacetyl-1,4-dihydropyridine (4f) afforded 2b and 2d in fairly good yields when the photolyses were carried out in an atmosphere of air in ethanol or acetonitrile.