Nifedipine and nitrendipine reactivity toward singlet oxygen

The ability to generate singlet molecular oxygen, O2(1Δg), and the scavenging activity of two well-known 1,4-dihydropyridines (1,4-DHPs) such as nifedipine (1,4-dihydro-2,6-dimethyl-4-(2-nitrophenyl)-3,5-pyridine-dicarboxylic acid dimethyl ester) and nitrendipine (1,4-dihydro-2,6-dimethyl-4-(3-nitrophenyl)-3,5-pyridine-dicarboxylic acid ethyl methyl ester) is assessed. s show that nifedipine does not generate O2(1Δg) under our experimental conditions. In contrast, this 1,4-dihydropyridine behaves as a good scavenger of excited oxygen, mainly via physical deactivation with values of the total rate constant ranging from 20.8 × 105 M−1 s−1 in dioxane to 93.0 × 105 M−1 s−1 in propylencarbonate. The less favored reactive pathway generates a photooxidation product, which has been isolated and identified by GC–MS as the nitropyridine derivative. Voltammetric experiments also confirm the generation of this oxidation product. other hand, nitrendipine yields O2(1Δg), but it is a less efficient scavenger of this species. Rate constants range from 1.88 × 105 M−1 s−1 in ethyl acetate to 15.8 × 105 M−1 s−1 in N,N-dimethylacetamide, the reactive channel being the main O2(Δg) deactivation pathway. ence on solvent microscopic parameters of the total rate constant for the reaction between singlet oxygen and 1,4-DHPs permits us to propose a mechanism involving a perepoxide-like encounter complex in the first step of the reaction path.