Electrochemical conversions of 5-oxo-1,4-dihydroindeno[1,2-b]pyridine derivatives

The electrochemical oxidation and reduction potentials of 9 derivatives of 5-oxo-1,4-dihydroindeno[1,2-b]pyridine in acetonitrile on a glassy graphite disk electrode have been established. Indenodihydropyridines are oxidized with considerably greater difficulty and reduced considerably more easily than the corresponding 3,5-diethoxycarbonyl derivatives of 1,4-dihydropyridines which is explained by the presence of the cyclopentane fragment which ensures the coplanarity of the carbonyl groups with the aminovinyl system of the heterocycle. The mechanism of electrooxidation has been studied by the rotating disk electrode with a ring method which revealed the reasons for the process.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, 1992 pp. 1223–1229, September, 1992.     

NAD(P)+-NAD(P)H models. 66. Stereospecific interconversion between different chiralities in the reduction of a quinolinium salt

An axial chirality is converted into a central chirality with a high stereospecificity in the reduction of a chiral quinolinium ion into the corresponding 1,4-dihydroquinoline derivative. Mechanism of the reduction is discussed.     

Free radicals in electrochemical reduction of 3,5-dicarbethoxy-1,4-dihydropyridines with nitrophenyl groups in positions 2, 4, and 6

In the electrochemical reduction of 2,6-bis- and 2,4,6-tris(nitrophenyl) derivatives of 3,5-dicarbethoxy-1,4-dihydropyridine, in the first stage, one of the para-nitrophenyl groups in position 2 or 6 of the heterocycle is reduced. Free radicals have been obtained and identified, the primary species being ion radicals of the nitrophenyl type. The presence of the heterocycle in the molecule of the 1,4-dihydropyridine derivative stabilizes secondary free radicals of the nitrosophenyl type. In the process of electrochemical reduction, no evidence has been found of any intramolecular transfer of electrons or protons from the dihydropyridine part of the molecule to the nitrophenyl groups. Derivatives of 2,6-bis(p-nitrophenyl)-3,5-dicarbethoxy-1,4-dihydropyridine have been synthesized, and the oxidation and methylation of these derivatives have been studied.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 481–487, April, 1991.     

Effect of the substituent position at the heterocycle on proton transfer from alcohols to the excited molecules of 1,2-dihydroquinolines

A novel compound 1,2,2,3-tetramethyl-1,2-dihydroquinoline (1223TMDQ) was synthesized, and the products of its steady-state photolysis in water, MeOH, EtOH, trifluoroethanol (TFE), PrⁿOH, and BuiOH were analyzed by ¹H NMR and mass-spectrometry. The corresponding adducts with water and alcohols were identified. The presence of the adducts for alcohols with a number of carbon atoms n > 1 distinguishes 1223TMDQ from 2,2,4-trimethyl-1,2-dihydroquinolines with methyl-, alkoxy-, and hydroxy-substituents at positions 1, 6, and 8, for which the formation of adducts was observed only in the presence of water and MeOH. The results were interpreted in terms of the effect of steric hindrance caused by substituents in the heterocycle and increasing size of the alkyl group of alcohol on proton transfer from a solvent to the molecule of 1,2-dihydroquinoline, which occurs in the complex between the solvent and the dihydroquinoline molecule in the excited singlet state. It was shown that the main steric hindrance for the photoinduced proton transfer in 2,2,4-substituted 1,2-dihydroquinolines is the substituent at position C(4) of the heterocycle.     

Special Features of the Electrochemical Oxidation of Substituted 4-Carboxy-1,4-dihydropyridines

Electrochemical oxidation potentials of 1,4-dihydropyridines substituted with 4-COOH, 4-COOR, and 4-CONRR' groups have been determined in aprotic acetonitrile by the rotating ring-disk electrode method (RRDE). The electrochemical reduction potentials of the resulting products were also determined at the ring electrode. It was established that protonated pyridines are formed in the oxidation of derivatives with and without a substituent in position 4 of the heterocycle. In the case of 4-alkoxycarbonyl substituted compounds the substituent at position 4 is generally retained. 1,4-Dihydrogenated derivatives of isonicotinic acid as a rule loose the substituent at position 4 on oxidation, and both types of product were recorded for the corresponding 4-carbamoyl derivatives. The substituent at position 9 of the heterocycle was mainly retained on electrochemical oxidation of the 3,3,6,6-tetramethyl-1,8-dioxo-1,2,3,4,5,6,7,8,9,10-decahydroacridine derivatives studi! ed.     

Electrochemical transformations of 4-(nitroaryl)-1,2- and -1,4-dihydropyridines in acetonitrile

The potentials of the electrolytic oxidation and reduction of 3,5-diethoxycarbonyl- 1,2- and -1,4-dihydropyridines with a nitroaryl substituent in the 4 position and their oxidized forms were determined by the method of a rotating disk electrode with a ring. The mechanism of the electrochemical oxidation of the starting dihydropyridines in acetonitrile on a graphite electrode was ascertained. The first step in the electrolytic reduction of the starting dihydropyridines is the addition of an electron to the nitro group to give anion radicals, the fine electronic structures and the hyperfine structure (hfs) constants of which were determined by EPR spectroscopy. The pyridinium fragment in the electrolytic oxidation products is reduced more readily than the nitro group in the aryl substituent.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1099–1105, August, 1985.     

Consideration of molecular arrangements in regio- and enantioselective reduction of an NAD model compound controlled by carbonyl oxygen orientation

The regio- and enantioselectivity of the reduction of an NAD model compound having axial chirality with respect to the C(3)(quinolinium)-C(carbonyl) bond, 3-piperidinylcarbonyl-1,2,4-trimethylquinolinium ion (1), by using several reducing agents is described. Reaction of 1 with sodium hydrosulfite affords the 1,4-reduced product, 3-piperidinylcarbonyl-1,2,4-trimethyl-1,4-dihydroquinoline (), with low enantioselectivity, whereas sodium borohydride promotes 1,2-reduction, affording 3-piperidinylcarbonyl-1,2,4-trimethyl-1,2-dihydroquinoline () as the sole product in a moderate enantioselectivity. When 1 was reduced by the chiral NADH model compound, 2,4-dimethyl-3-(N-alpha-methylbenzylcarbamoyl)-1-propyl-1,4-dihydropyridine (Me(2)PNPH (4)), the regioselectivity and enantioselectivity of the reaction were significantly altered by the stereochemistry of 1 and 4. An achiral NADH model compound, 1-propyl-1,4-dihydronicotinamide (PNAH (5)) exhibited both high regio- and enantioselectivities. The product selectivity reflects the change in molecular arrangement in the transition state of the reaction and reveals the relative importance of the parameters governing the molecular arrangement in the reaction.     

Role of steric hindrance in photoinduced proton transfer from solvent to excited molecules of 1,2-dihydroquinolines

It was shown that the photolysis of 1,2,6-trimethyl-1,2-dihydroquinoline (126TMDHQ) in water, methanol, ethanol, and isopropanol affords the corresponding adducts of water and the alcohols, unlike the case of 2,2,4-trimethyl-1,2-dihydroquinolines bearing the methyl, alkoxyl, and hydroxyl substituents in the 1-, 6-, and 8-positions, which were previously found to form adducts only in the presence of water and MeOH. The quantum yield of the 126TMDQ photolysis (Φ) in this solvent series changes as Φ_MeOH:Φ_EtOH:Φ_PrOH = 10:3:1. The results were rationalized in terms of the effect of steric hindrance caused by substituents on the heterocycle and increasing size of the alcohol alkyl group on proton transfer from the solvent to the 1,2-dihydroquinoline molecule in the excited singlet state. The existence of two adduct isomers was revealed. The preferential formation of one of the isomers was considered from the standpoint of carbocation accessibility to the solvent by nucleophilic attack.     

Synthesis and characterization of p-phenylenediamine derivatives bearing an electron-acceptor unit

[reaction: see text] Two series of aniline oligomers bearing fused heterocycles as an electron-acceptor unit were synthesized. They consist of aniline or its derivatives as an electron donor and benzothiadiazole (BT) or quinoxaline (QX) as an electron-acceptor unit. Benzothiadiazoles 1-3 were synthesized by palladium-catalyzed amination. Quinoxalines 4-6 were prepared by palladium-catalyzed amination or transformation from the benzothiadiazoles. These compounds showed a HOMO-LUMO gap smaller than those of their analogues such as thiophene-substituted BT/QXs. Cyclic voltammetry revealed that the electrochemical behavior is dependent on the position of the acceptor heterocycle. Chemical oxidation with Ag2O afforded the corresponding 1,4-quinonediimine derivatives as an E,E-isomer, stereoselectively. As for the BT pentamer analogues 2 and 3, the first oxidation selectively occurred at the amino group adjacent to the benzothiadiazole unit, giving the regiospecific half-oxidized derivatives. Furthermore, the fully oxidized derivative 24 was isolated and characterized.     

Electrochemical and electron spin resonance studies of selected benzazolo[3,2-a]quinolinium salts

The electrochemical reduction of the chloride or perchlorate salts of benzazolo[3,2-a]quinolinium ion and several of its analogues is reported. The compounds studied are the perchlorate salt of 3-nitrobenzothiazolo-and 3-nitro-9-methoxybenzothiazolo[3,2-a]quinolinium, and the chloride salts of 7-ethyl-, 3-nitro-7-methyl-, 3-nitro-7-ethyl-, 3-nitro-7-isopropyl-, 3-nitro-7-butyl- and 3-nitro-7-benzylbenzimidazolo[3,2-a]-quinolinium, respectively. Cyclic voltammetry of the corresponding 3-nitrobenzothiazolo[3,2-a]quinolinium derivatives in N,N-dimethylformamide shows an irreversible peak potential at -0.6 and a quasi-reversible peak at -(1.2–1.3) volts, respectively, relative to the standard calomel electrode. In contrast, the corresponding 3-nitrobenzimidazolo[3,2-a]quinolinium derivatives show, in general, reversible peaks at near -0.8 and -(1.2–1.4) volts, respectively. Upon electrolytic reduction, only the nitro-substituted derivatives produced observable electron paramagnetic resonance electron spin resonance spectra. This observation is explained in terms of the stabilization of the radicals produced by the nitro group. Theoretical MM+/AM1/UHF calculations support the idea that the larger nitrogen splitting is caused by N-12 and the minor splittings by N-7 in the benzimidazolo[3,2-a]quinolinium ion series.     

Reduction of 2,3,4-substituted quinolines with sodium borohydride

1,2-Dihydroquinolines were obtained by the reduction of 3-substituted 2-methyl-4-phenylquinolines with sodium borohydride in aliphatic carboxylic acids; N-alkyl derivatives are also formed. The corresponding 1,4-dihydroquinoline was obtained in the reaction of 2-methyl-3-nitroquinolinium perchlorate with sodium borohydride.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1680–1686, December, 1991.     

An investigation of the reaction of 2-aminobenzaldehyde derivatives with conjugated nitro-olefins: an easy and efficient synthesis of 3-nitro-1,2-dihydroquinolines and 3-nitroquinolines

2-Aryl-3-nitro-1,2-dihydroquinolines 3 were prepared from the reaction of beta-nitrostyrenes 2 and 2-aminobenzaldehyde 1 in the presence of DABCO. Not only beta-nitrostyrenes but other alkyl nitro olefins also can be used in this reaction as well. When DDQ or silica gel was added to a solution of 3-nitro-1,2-dihydroquinolines 3, 3-nitro-2-substituted-quinolines 4 were obtained. When 2-aminobenzaldehyde derivatives 7 and 12 were reacted with beta-nitrostyrenes 2, unique rearrangement products were produced.     

A novel synthesis of substituted quinolines using ring-closing metathesis (RCM): its application to the synthesis of key intermediates for anti-malarial agents

A method for synthesizing substituted quinolines using ruthenium-catalyzed ring-closing metathesis as a key step has been developed. Substituted 1,2-dihydroquinolines, 4-silyloxy-1,2-dihydroquinoline and 4-methoxy-1,2-dihydroquinoline, were successfully synthesized in excellent yields via ene-ene metathesis and silyl or alkyl enol ether-ene metathesis, respectively. The synthetic intermediates of the antimalarial agents quinine, chloroquine, and PPMP-quinine hybrid were efficiently synthesized by this methodology.     

The preparation and some chemistry of 2,2-dimethyl-1,2-dihydroquinolines

The cyclisation of N-(1,1-dimethylpropargyl) anilines, using cuprous chloride in refluxing toluene, yields 6-substituted-2,2-dimethyl-1,2-dihydroquinolines. The reactivity of the double bond in the heterocyclic ring of these products is exemplified by chlorination, to yield 6-substituted-3,4-cis-dichloro-2,2-dimethyl-1,2,3,4-tetrahydroquinolines which can be selectively dechlorinated to provide 6-substituted-3-chloro-2,2-dimethyl-1,2,3,4-tetrahydroquinolines; epoxidation to yield an epoxide, which can be hydrogenolysed to the corresponding 3-hydroxy product and in turn oxidised to the 3-keto derivative; and oxymercuration to provide a 4-hydroxy product and hence a 4-keto derivative. Dehydrochlorination of a 3,4-dichloro product provides a 3-chloro-1,2-dihydroquinoline which can be hydrolysed to a 3-keto system. The formation of cis 3,4-dichloro products from the chlorination, as well as the formation of a cis chlorohydrin from the chlorination of N-acetyl-2,2,6-trimethyl-1,2-dihydroquinoline in partially aqueous solution, suggests that N-acetyl, or N-trifluoroacetyl groups, participate in the addition process.     

Electrochemical reduction of nitrotetralones

The electrochemical reduction of three nitrotetralone derivatives by tast and differential pulse polarography and cyclic voltammetry in a wide pH range was studied. In aqueous media, the reduction of mono- and dinitrotetralone follows the general pattern of reduction of aromatic nitro compounds: the nitro group is reduced in a four-electron step to a hydroxylamine group. However, in mixed media this reduction differs from that of other nitrobenzenes, due to the fact that the formation of the nitroradical anion was not observed. The reduction of the acetoxy derivative was studied only at alkaline pH, because it suffered acid hydrolysis. The ionization pK of the protonatable groups were polarographically obtained.     

A convenient synthesis of novel pyrido(1′,2′:1,2)imidazo[5,4‐d]‐1,2,3‐triazinones from imidazo[1,2‐a]pyridines

The imidazo[1,2‐a]pyridine system was investigated as a synthon for the building of very attractive fused triazines, a planar, angular tri‐heterocycle with potential biological activity. Thus ethyl 3‐nitroimidazo[1,2‐a]pyridine‐2‐carboxylate was treated with ammonia or with an excess of primary amines to generate the corresponding substituted nitro carboxamidoimidazopyridines. The nitro substituent in the latter products, was reduced to yield 3‐amino‐2‐carboxamidoimidazo[1,2‐a]pyridine derivatives, which in turn were treated with nitrous acid to furnish 1‐oxo‐2‐substituted pyrido(1′,2′:1,2)imidazo[5,4‐d]‐1,2,3‐triazines.     

Spectroelectrochemical Study on the Electrooxidation in Aqueous Medium of some 1,4‐Dihydropyridines: Effects of Substitution in 1‐Position and 4‐Position

Spectroelectrochemical and HPLC characterization of the electrochemical oxidation in aqueous medium of a series of six N‐1 and C‐4 substituted 1,4‐dihydropyridines is presented. Based on the analysis of spectra obtained by in situ spectroscopic measurements it was possible to detect the generation of final oxidation products, which resulted in differences depending of the nature of the substitution on the nitrogen in the dihydropyridine ring. Controlled potential electrolysis (CPE) in aqueous medium was followed by the HPLC technique using EC and PDA detectors. This latter resulted adequately to follow the parent 1,4‐DHP derivatives and their respective oxidation products. Electrochemical oxidation of parent N‐H substituted 1,4‐dihydropyridines generated the corresponding neutral pyridine derivative as final oxidation product. However, the N‐ethyl substituted 1,4‐dihydropyridine derivatives gave rise to the pyridinium salt derivatives.     

Effect of the excitation wavelength and the structure of nitrated 1,2-dyhydroquinolines on dynamics of primary photophysical and photochemical processes

Dynamics of transformations of excited states and active transient species generated in the photolysis of nitrated 1,2-dihydroquinolines (N-DHQ) has been studied by femto- and nanosecond laser pulse photolysis. Spectral and kinetic parameters of primary photophysical and photochemical processes have been determined, and their dependence on the substituent position at the aromatic ring of 1,2-dihydroquinoline (DHQ) and on the wavelength of excitation light has been established. The lifetime of the excited singlet state S₁ in N-DHQ is ca. 100 and 500 fs for 8- and 6-nitro-substituted DHQ, respectively, which is shorter in comparison with DHQ without the nitro group by a factor of 10⁴ and more. The major decay channel of the S₁ state is the successive formation of three transient species with lifetimes of 0.5 to 16 ps. A triplet state is generated only upon excitation of the short-wavelength band by UV light. The quantum yield of the triplet state depends on the structure of N-DHQ.     

Synthesis and study of 3-methyl-6H-indolo[2,3-b]quinoxalines

By the condensation of isatin, N-methyl-, N-propyl-, N-benzoylisatin with 4-methyl-1,2-benzeneldiamine 3-methyl-6H-indolo[2,3-b]quinoxalines were obtained in high yields. The reaction occurs stereoselectively, in the mixture only one of possible isomers is present. By the oxidation of methyl group of the quinoxaline fragment new carbonyl derivatives were obtained, and by the reduction of nitro group, the corresponding indoloquinoxalinamines.