Intramolecular Electron Transfer in the Photochemistry of Some Nitrophenyldihydropyridines

Abstract

4-Phenyl-1,4-dihydropyridine-3,5-dicarboxylates contain two pi chromophores separated by an sp3 carbon. The lowest singlet is localized on the dihydropyridine moiety (1PyH2-Ph) and emits a blue fluorescence (with close to unitary efficiency in glass at 77 K). In 3-nitrophenyl derivatives (PyH2-PhNO2, some of which are photolabile drugs) the fluorescence is completely quenched. Reasonably, this is due to intramolecular electron transfer between the close-lying donor and acceptor moieties to give the charge-separated species (PyH2*+-PhNO2*-). In EPA glass at 77 K, back-electron transfer gives the dihydropyridine-localized triplet (3PyH2-PhNO2), which emits a yellow phosphorescence. In solution, deprotonation from the radical cation on the dihydropyridine moiety initiates rearomatization, finally giving Py-PhNO2 with low quantum yield (5 x 10(-4) to 5 x 10(-3), increasing up to 0.013 by irradiation at 254 nm, where direct excitation of the nitrophenyl chromophore contributes). In the presence of triethylamine, the reaction changes to neat reduction of the nitro group. When a tethered alkylamino group is present, oxidative degradation of that moiety occurs, again via an electron-transfer intramolecular process. This has been found with the drug nicardipine, where photodegration is more efficient (phi 0.02 to 0.1). Donor-acceptor dyads of this type, easily available through the Hantzsch synthesis, may be useful for building new photoinduced electron-transfer systems.

Photodegradation of Nimodipine and Felodipine in Microheterogeneous Systems

“…15 The zwitterionic biradical intermediate generated by the fast intramolecular electron transfer yields a pyridine derivative as main photoproduct by an intermolecular proton transfer to the solvent. 16 In our preceding study we have analyzed the photophysics and photochemistry of both 4-aryl-1,4-dihydropyridines (NIMO and FELO) in homogeneous media. 15 In the present work, considering their reported lipophilicity 17 and a photostabilizing effect when they are incorporated into organized microenvironments, 18 we were interested in the study of the behavior of these drugs in micro-organized media.…”

“…Thus, when solvent is able to accept a proton from the zwiterionic biradical intermediate involved, the rate constant increases notably. 15,16 In this way, if 4-aryl-1,4-dihydropyridines are incorporated near to the interface of the micelle, the microenvironment sensed would be relatively polar due to the presence of water molecules nearby, however they could not interact easily with the intermediate probably because the preferential interaction involves the surfactant heads and its presence hinders the approach of the solvent. For comparison with other more complex microheterogeneous systems, the compounds were also incorporated into extruded dipalmitoylphosphatidylcholine liposomes (DPPC).…”

Microwave-Assisted Synthesis of Bioactive Six-Membered Heterocycles and Their Fused Analogues

“…This new method provided the expected products 35 in excellent yields (90%-98%) after short reaction times (40-80 s) in comparison to the classical Hantzsch methods (Scheme 7) [44][45][46]. 1,4-Dihydropyridine derivatives have a wide range of biological and pharmaceutical activities and are also involved in hydride transfer from the reduced nicotinamide adenine dinucleotide (NADH and NADPH) coenzymes, and analogues thereof, which mediate hydrogen transfer reactions in biological systems [35][36][37].…”

“…In addition, others 1,4-dihydropyridines have been discovered as potent calcium channel agonists (such as BAY K 8644 (31)) [41,42]. 1,4-Dihydropyridine derivatives have a wide range of biological and pharmaceutical activities and are also involved in hydride transfer from the reduced nicotinamide adenine dinucleotide (NADH and NADPH) coenzymes, and analogues thereof, which mediate hydrogen transfer reactions in biological systems [35][36][37].…”

Novel intramolecular electron transfer in axial bis(terpyridoxy)phosphorus(V) porphyrin studied by time-resolved EPR spectroscopy

“…A wide vafiety of donor-acceptor supramolecular complexes has been studied with the aim of gaining a better understanding of the factors governing light-induced electron transfer [1][2][3][4][5][6][7]. These complexes usually consist of a chromophore that absorbs in the visible region with a number of substituents tethered to it.…”