Dihydropyridines anodic oxidation

Dihydropyridines and their /V-alkyl derivatives undergo anodic oxidation in basic medium to the corresponding pyridines (reaction 27). The process may be complicated by the presence of other moieties for example, a nitro group may reductively condense... 

The formation of a double bond during anodic oxidations can result from eliminations of protons, carbon dioxide or acylium cations. The electrooxi dative aromatization of dihydropyridine derivatives and heterocycles containing nitrogen atom (di-hydroquinoxalines, tetrahydrocinnolines) involves an ECE mechanism as previously... 

The anodic oxidation of various dihydropyridine derivatives to the corresponding pyridinium or pyridine derivatives has been the subject of several investigations both in protic and aprotic solvents.229-237. The great interest in this process is because of the biological importance of the pyridinium-dihydropyridine redox system. 

Considering the importance of dihydropyridines as intermediates and as valuable products in their own right, the lack of citations directly concerned with anodic oxidation is interesting. Granted, most of the work with NAD+ and its analogs have been concerned with both the cathodic reduction and the reverse anodic reaction, but there are hardly any other studies of such systems. 

The anodic oxidation of 4,4-disubstituted 1,4-dihydropyridines in MeCN gives a relatively stable cation radical, which catalyzes the oxidation of MeCN some of the cation radicals lose a 4-substituent as a radical, and the nucleus is oxidized further [231]. 

The anodic electrochemical behavior of various 1,4-dihydropyridines (including nimodipine) was studied in acetate-phosphate buffer (pH 8) containing 70% (v/v) ethanol [10]. An anodic peak due to a two-electron oxidation of the dihydropyridine ring to a pyridine derivative was obtained. Recoveries were 91.7-104%, with a RSD being less than 3%. A cathodic polarographic method for nimodipine in tablets was developed. The dependence of peak current on concentration in acetate-phosphate buffer of pH 5 containing 20% ethanol was linear over the range of 1 pM-1 mM, with a coefficient of variation of 0.2% [5]. 

Difunctional 1,4-dihydropyridine derivatives[19] are relatively easily oxidized at platinum electrodes (e.g., rotated platinum). In solutions containing a mixture of acetonitrile and water (1 1) and 0.05 M LiClOi a 1 electron anodic wave results with 2,4,4,6-tetra-methy1-3,5-dicyano-1,4-dihydropyridine (I) and a 2 electron wave with 2,6-dimethyl-3,5-dicyano-l,4-dihydropyridine (II) ... 

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