Abstract

This work reports the electrochemical oxidation of a new series of 4-methyl-1,4-dihydropyridine derivatives with platelet activation factor antagonistic activity. Differential pulse and cyclic voltammetry studies on a glassy carbon electrode showed an irreversible single peak due to the oxidation of the dihydropyridine ring via two electrons. Rotating disk electrode studies show a linear dependence between the current and the rotating rate, indicating that the oxidation process is diffusion-controlled. Calculated diffusion coefficients did not exhibit significant differences between the derivatives. Voltammetric apparent pKa values for the protonation-deprotonation equilibrium of the N-heterocyclic nitrogen are compared with 4-phenyl substituted 4-aryl-1,4-dihydropyridine (1,4-DHP) derivatives. Based on voltammetric experiments on glassy carbon electrodes, it is seen that the potential peak values are directly related to the electron density on the dihydropyridine ring, wherein C-5 substituent with electron-withdrawing character produces the most oxidizable 1,4-DHP derivative. The oxidation mechanism follows the general pathway: electron, H+, electron, H+, with formation of an unstable cation radical in the first step. A one-electron oxidation intermediate was confirmed with controlled potential electrolysis and electron spin resonance experiments. On applying N-benzilydene-t-butylamine-N-oxide as the spin trap, the unstable radical intermediates from the oxidation of 4-methyl-1,4-dihydropyridine derivatives were intercepted. Comparison of the electron spin resonance spectra of the nitroxide spin adducts revealed no significant differences in the splitting constants of the radicals.