Oxidation of 4-(3-Indolyl)- and 4-(5-Indolyl)-1,4-dihydropyridines in Aprotic and Protic Media: Reactivity toward Alkylperoxyl Radicals

Electrochemical oxidation of 4-(3-indolyl)- and 4-(5-indolyl)-1,4-dihydropyridines (DHPs) in aprotic and protic media is reported. Also, the reactivity of compounds toward alkylperoxyl radicals 2,2'-azobis-(2-amidinopropane) dihydrochloride-derived in aqueous media at pH 7.4 is assessed. Derivatives were electrochemically oxidized exhibiting two anodic signals in both electrolytic media. The first signal is due to oxidation of the dihydropyridine ring, and the second one is due to oxidation of the indolyl moiety. Electron spin resonance experiments proved the formation of carbon-centered dihydropyridyl radicals as intermediates in the oxidation of the dihydropyridine moiety in aprotic medium. Pyridine was identified as the final product of the oxidation in both electrolytic media by gas chromatography/mass spectrometry. The 4-substituted 1,4-DHPs were more reactive than tested commercial 1,4-DHPs toward alkylperoxyl radicals