Electroreduction of nifedipine.

At pH greater than 6, the nitro group of nifedipine is reduced in a four-electron step to a hydroxylamino group. At pH less than 6, the hydroxylamino derivative undergoes an acid-catalyzed dehydration, and the rate of this reaction governs the limiting current. Resulting quinonemethide is further reduced, and a six-electron reduction step results. Formation and reducibility of the quinonemethide is favored by conjugation with the 1,4-dihydropyridine ring. Further reduction of the hydroxylamino derivative is confirmed by the absence of the anodic oxidation peak on the reverse scan at pH less than 6.5. At potentials more negative than that of the nitro group, the protonated form of the hydroxylamino derivative is reduced between pH 4 and 8 in direct current (dc) polarography and between pH 6 and 11 in cyclic voltammetry, the difference resulting from the difference in the measurement time window. The yields and fate of the hydroxylamino derivative obtained in controlled potential electrolysis with a hanging mercury drop electrode (HMDE) differ from those in dc polarography, indicating the presence of different competing reactions. For analyses, pH 2 and 11 are most suitable.