Electrochemical oxidation of N-substituted 1,4- and 1,2-dihydropyridine derivatives

Expedient access to saturated nitrogen heterocycles by photoredox cyclization of imino-tethered dihydropyridines

A large proportion of medicinally relevant molecules bear nitrogen and sp³-hybridized carbon functionalities. Overwhelmingly, these atoms are found as part of (hetero)cyclic structures. Despite their importance, synthetic approaches to saturated nitrogen heterocycles are limited to several established stoichiometric alkylation techniques, as well as a few methods involving C-H bond activation. The synthetic community remains interested in more general, mild, and sustainable ways to access these motifs. Here we describe a dual-catalyst system composed of an iridium photocatalyst and a lithium phosphate base that is capable of selectively homolyzing the N-H bond of 4-alkyl-1,4-dihydropyridines, presumably by proton-coupled-electron-transfer (PCET), and mediating efficient cyclization of the resultant carbon-centered radicals with tethered imines. The outcome of this transformation is access to a broad range of structurally complex nitrogen heterocycles obtainable from simple aldehyde starting materials in a highly chemoselective manner.

Voltammetric oxidation of 2-oxo-1,2,3,4-tetrahydropyrimidin-5-carboxamides: substituent effects

Electrochemical oxidation of a series of 20 substituted 2-oxo-1,2,3,4-tetrahydropyrimidin-5-carboxamides (THPMs) in acetonitrile has been studied using voltammetric methods at a glassy carbon electrode to investigate the influence of the substituents on the 4- and 5-positions of the heterocyclic ring. Analysis of the results shows that the electronic nature and steric hindrance of the substituents, especially their orientations toward the heterocyclic ring, determine their effects on the oxidation potential. Analysis of the computational results obtained at the DFT-B3LYP/6-31++G** level of theory suggests a mechanism in which the first electron removal occurs from either the N(1) of the heterocyclic ring or N(17) of the amide substitution. This process is followed by a fast proton removal resulting in the formation of stable allylic and/or benzylic radicals which then undergo further oxidation to the 2-oxo-1,2-dihydropyrimidin-5-carboxamides (DHPMs).

Oxidation of C4-hydroxyphenyl 1,4-dihydropyridines in dimethylsulfoxide and its reactivity towards alkylperoxyl radicals in aqueous medium

This work reports the electrochemical oxidation of three newly synthesized C4-hydroxyphenyl-substituted 1,4-dihydropyridine derivatives in dimethylsulfoxide. The reactivity of the compounds with ABAP-derived alkylperoxyl radicals in aqueous buffer pH 7.4, was also studied. The oxidation mechanism involves the formation of the unstable dihydropyridyl radical, which was confirmed by controlled-potential electrolysis (CPE) and ESR experiments. The final product of the CPE, that is, pyridine derivative, was identified by GC-MS technique for the three derivatives. A direct reactivity of the synthesized compounds toward ABAP-derived alkylperoxyl radicals was found. The pyridine derivative was identified by GC-MS as the final product of the reaction. Results reveal that this type of 1,4-DHPs significantly reacts with the radicals, even compared with commercial 1,4-DHP drugs with a well-known antioxidant ability.

An electroanalytical investigation on the redox properties of lacidipine supporting its anti-oxidant effect

The redox properties of lacidipine (PyH2), one of the most pharmacologically active N-unsubstituted 1,4-dihydropyridines, have been studied by cyclic voltammetry and controlled potential electrolysis in acetonitrile, an aprotic solvent that is, at best, a mimic of the lipofilic layer of biological membranes. PyH2 undergoes a two-electron oxidation process involving two consecutive one-electron releases, the latter requiring potentials much less positive than the former. The overall process occurs through a primary one-electron step accompanied by a fast proton release, with the formation of a neutral radical (PyH*), which undergoes a further and quite easier one-electron step, thus providing the main ultimate product (PyH+) consisting in the protonated form of the parent pyridine derivative. This appears relevant for the anti-oxidant effect since the radical intermediate is much more prone to be oxidized than to be reduced, thus preventing the propagation of the oxidative chain reaction. The mentioned release of protons in the primary electrode step causes the overall process to be complicated by a parassite side reaction involving the coupling between one of the electrode products (H+) and the starting species. The protonation of PyH2 subtracts part of the original species from the electrode process because the parent cationic species (PyH3+) is no longer electroactive. This parassite reaction occurs rather slowly in the timescale of electroanalytical measurements (the relevant kinetic constant has been estimated to be 6.4 l mol(-1) s(-1)), thus markedly affecting the process only in the presence of relatively high PyH2 concentrations and progressively decreasing with the starting PyH2 concentration. All the products formed in the oxidation process (PyH+, H+ and PyH3+) have been identified by voltammetric evidences based on deep investigations on their cathodic behaviour. The advantageous anti-oxidant properties displayed by PyH2 with respect to those exhibited by phenolic anti-oxidants such as vitamin E are also discussed.