Electrochemical transformations and anti/prooxidant activity of sterically hindered <Emphasis Type="Italic">o</Emphasis>-benzoquinones

The electrochemical properties of a series of sterically hindered o-benzoquinones in aprotic solvents were examined. The reaction of O₂ and KO₂ with o-benzoquinones and electrogenerated o-semiquinone radical anions was studied. The formation of o-semiquinone radical anions in the reactions of KO₂ with the studied o-quinones was detected using UV-visible and ESR spectroscopy. The influence of the test compounds on lipid peroxidation was considered at the molecular and tissue levels in vitro. In the most cases, o-quinones exhibit antioxidant activity, which varies depending on the type of lipid substrate.     

Azonia polycyclic quinones,o-diazo-oxides and related products

4α-Azoniaanthracene-5,6- and 5,8-dione salts have been prepared by nitric acid oxidation of their respective diols, and are shown to function as either an electrophilic dienophile or a diene component in Diels-Alder-type reactions. Several azonia o-quinones have been treated with tosylhydrazine in methanol saturated with hydrogen chloride to give unusually stable o-diazo-oxides. A four-step dehydroxylation procedure is detailed for the conversion of .5,6-dihydroxy-4α-azonia-anthracene salts to new 5-hydroxy-4α-azoniaanthracene salts.     

Effects of electron-withdrawing substituents on DPPH radical scavenging reactions of protocatechuic acid and its analogues in alcoholic solvents

The DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging activity of protocatechuic acid (3,4-dihydroxybenzoic acid) and its related catechols was examined. Compounds possessing strong electron-withdrawing substituents showed high activity. NMR analysis of the reaction mixtures of catechols and DPPH radical in methanol showed the formation of methanol adducts. The results suggest that high radical scavenging activity of catechols in alcohol is due to a nucleophilic addition of an alcohol molecule on o-quinones, which leads to a regeneration of a catechol structure. Furthermore, the radical scavenging activity in alcohols would largely depend on the electron-withdrawing/donating substituents, since they affect the susceptibility toward nucleophilic attacks on o-quinone.     

Highly selective synthesis of triptycene o-quinone derivatives and their optical and electrochemical properties

A convenient and efficient method for the selective synthesis of a series of triptycene o-quinone derivatives is described. The triptycene o-quinones, especially the ones containing the methoxy group(s) (electron donor) and the o-quinone group(s) (electron acceptor) simultaneously, show interesting intramolecular CT interactions and electrochemical properties. Moreover, DFT calculations display that introducing a strong electron-donor group into triptycene o-quinone results in an increasing of the HOMO energy level, which subsequently decreases the HOMO-LUMO energy gap.     

Untersuchungen an reagenzien zur photometrischen zinnbestimmung

The formation of complexes between tin(IV) and o-diphenols, and o-hydroxyquinones, has been studied spectrophotometrically. Simple o-hydroxyquinones do not react with tin(IV), chloranilic acid gives an SnR₂ complex, and o-diphenols give SnR₃ complexes. The reactions of tin(IV) with hydroxyxanthene dyes to give coloured products were considered for their possible analytical utility: the pH range, absorption spectrum, solubility and composition of the complexes were investigated, and the stability constants determined. Analytical procedures have been developed for tin and checked for precision and sensitivity: gallein and hydroxyhydroquinonephthalein (dihydroxyfluorescein) are to be recommended.     

Reaktionen von o-Chinonen mit Aminen und Proteinen. 4. Mitteilung. 7a-Methyl-5,6-dioxo-5,6,7,7a-tetrahydroindol-Derivate aus 4-Methylbrenzcatechin und Enaminen

Reactions of o-Quinones with Amines and Proteins. 7a-Methyl-5,6,7,7a-tetrahysroindole Derivatives from 4-Methylcatechol and Enamines Methyl l-[2′-(methoxycarbonyl)ethyl]-7a-methyl-5,6-dioxo-5.6.7,7a-tetrahydro-indole-3-carboxylate ( 1 ) was isolated after the oxidation of 4-methylcatechol with silver ( 1 ) oxide in the presence of b?-alanine methyl ester in glacial acetic acid. The formation of 1 requires in situ dehydrogenation of the b?-aminocarboxylate and addition of the resulting enamine to 4-methyl-1,2-benzoquinone. Reaction of ethyl 3-(phenylamino)crotonate with 4-methyl-1,2-benzoquinone afforded ethyl 2,7a-dimethyl-5,6-dioxo-1-phenyl-5,6,7,7a-tetrahydroindole-3-carboxylate ( 6 ). Despite the fact that the yields are low, the addition of enamines to o-quinones represents an interesting novel extension of the Nenitzescu-reaction which is well known in the p-quinone series. Compound 1 may be considered as a novel model for the crosslinking of proteins by o-quinones. Formation of 1 was, however, not observed under physiological conditions.     

Regioselective reaction of quinones with Reformatsky reagent: (BrZnCH2CO2Et·THF)2

Reformatsky reactions of p-quinones with crystalline reagent (BrZnCH₂CO₂Et·THF)₂ were investigated and took place successfully, providing β-hydroxy esters in high yield. Notably, in the case of 2,6-disubstituted-p-quinones, regioselective Reformatsky reactions occurred to give corresponding β-hydroxy esters in good yields.     

Synthesis and Structure of Some 1,3,2-Oxazaphospholenes

Abstract

The reaction of trialkyl phosphites and phosphines with o-quinones result in the formation of oxyphosphoranes [1]. In an effort to prepare o-quinone monoimine metal complexes with amine [2] and phosphine ligands it has been found that in the reaction either of o-quinones (1) with phosphines in the presence of ammonia or of o-quinone monoimines (2) with phosphines 1,3,2-oxazaphospholenes (3) are formed in good yields.     

Functionalization of sterically hindered <Emphasis Type="Italic">o</Emphasis>-benzoquinones: amino-substituted 3,6-di(<Emphasis Type="Italic">tert</Emphasis>-butyl)-<Emphasis Type="Italic">o</Emphasis>-benzoquinones

New sterically hindered functionalized o-quinones were synthesized by the 1,4-nucleophilic addition of secondary cyclic amines to 3,6-di(tert-butyl)-o-benzoquinone. The ability of these o-quinones to form o-semiquinone complexes with transition and main-group metals was studied by ESR spectroscopy in solution. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1786–1793, September, 2007.     

The mechanism of photoinduced hydrogen transfer during photoreduction of carbonyl compounds

A model reaction of hydrogen transfer has been proposed that describes well the results of kinetic studies of the photoreduction of o- and p-quinones and fluorenone in the presence of para-substituted N,N-dimethylanilines and polyalkylbenzenes in various solvents.     

Reaction of superoxo Co(III) complex with quinones formation of a semiquinone Co(III) complex

The superoxo complex [Co(CN)₅O₂]^3- was found to act as a reducing agent towards quinones. One-electron reduction took place with $\text{o}$-quinones whereas two-electrons reduction with $\text{p}$-quinones. 3,5-Di-$\text{t}$-butyl-$\text{o}$-benzoquinone gave the corresponding semiquinone Co(III) complex quantitatively.     

Arsenorgano-Verbindungen : XXII. Zur synthese der 1,3-benzazarsoline

o-Aminophenylarsine is prepared by reduction of o-nitrophenylarsonic acid. Metallation and subsequent treatment with alkyl halides give the secondary o-aminophenylarsines. o-Aminophenylarsines react with aldehydes, ketones and ketoesters forming derivates of the 1,3-benzazarsoline. Exceptions are redox reactions between o-aminophenylarsine and aldehydes. The properties of the compounds are described in more detail.     

Efficient access to bisphenol A metabolites: Synthesis of monocatechol,mono-o-quinone,dicatechol, and di-o-quinone of bisphenol A

2-Iodoxybenzoic acid (IBX) oxidation of bisphenol A (BPA) is described. The selective production of either the mono-o-quinone or the di-o-quinone can be controlled by IBX stoichiometry. Isolated yields of quinone were greater than 80%. Previous synthesis of BPA-di-o-quinone using a large excess of Fremy’s salt produced only trace amounts of product. In addition to o-quinone products, both mono- and dicatechols of BPA can synthesize in high yield and isolated without chromatography. The more stable catechols can be quantitatively converted back to o-quinones using silver oxide oxidation in either acetone or DMF. These one-pot reactions provide access to four different BPA metabolites in high yield and significant scale.     

Reversal of the regioselectivity in a cycloaddition of o-quinones by varying the position of alkoxy substituents

We have investigated the regioselective cycloaddition of o-quinones 1b-e with the protected sinapyl alcohol 2. It was found that the position of the alkoxy substituent on the o-quinone ring controlled the regioselectivity of the cycloaddition. In addition, our reported procedure for determining the location of the side chains on 1,4-benzodioxanes has been improved.     

o-Phosphinophenole durch 1,3-carbanionische umlagerungen o-metallierter aroxyphosphine

Phosphinous acid o-bromoaryl esters or phosphorus acid o-haloaryl ester diamides react with sodium under mild conditions to give the corresponding sodium aryls which undergo rapid 1,3-carbanionic rearrangements to give o-phosphinophenolates. These may be worked up to the free phenoles or silylated by treatment with Me₃SiCl.     

An ESR study of photochemical reactions of Co2(CO)6(PBu3)2 with quinones and phenothiazine

The photochemical reactions of Co₂(CO)₆(PBu₃)₂ with various quinones and phenothiazine were studied by ESR. The results indicate that the photolysis of Co₂(CO)₆(PBu₃)₂ in the presence of o-quinones led to the observation of an ESR spectrum, showing a broad 8 or 12 groups of hyperfine lines due to interaction with cobalt (Co, 1 = 7/2). The results show that with o-quinones the cobalt radical adducts are formed via metal chelation to the carbonyl oxygens. However, when Co₂(CO)₆(PBu₃)₂ was irradiated in the presence of p-quinones, only the para-semiquinone radicals were observed. The photolysis of Co₂(CO)₆(PBu₃)₂ with phenothiazine in toluene yields phenothiazine radical. The sixteen and eighteen electrons rule have been used to account for these observations.     

New tri-tert-alkyl substituted o-quinones of tetraline family

New tri-tert-alkyl substituted o-quinones of 3-tert-alkyl-5,5,8,8-tetramethyl-5,6,7,8-tetrahydronaphthalene-1,2-dione chemotype were obtained from phenol and 2,5-dichloro-2,5-dimethylhexane, with the SeO₂ oxidation of the sterically hindered phenol moiety having been performed at the final step. The electrochemical reduction of these quinones proceeds in two stages: the first reduction wave (E_1/2 = –0.60 ÷ –0.62 V) is reversible, while the second stage is irreversible.     

Photoreduction ofortho-benzoquinones in the presence ofpara-substitutedN,N-dimethylanilines

Photoreduction ofo-benzoquinones irradiated at the wavelengths λ_max ≈ 400 and 600 nm corresponding to the S(π → π*) and S(n → π*) electron transitions in the >C=0 groups, respectively, in the presence ofN,N-dimethylaniline and its derivatives was studied. The apparent rate constants of the photoreduction (k _H) ofo-quinones are determined by the free energy of electron transfer from the amine molecule to a photoexcitedo-quinone molecule (ΔG _e.t). The ΔG _e.t. values are calculated as the sums of the energies of the 0→0 transitions of the lowest triplet excited state ofo-quinones, the reduction energies ofo-quinones, and the oxidation energies of amines (the last two terms are numerically equal to the corresponding redox potentials). The maximum rate of photoreduction was found for ΔG _e.t≈0. The reaction mechanism is proposed, in which the reversible formation of a triplet exiplex is the rate-determining stage and hydrogen transfer proceeds in parallel with electron transfer within the exiplex. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1515–1521, September, 2000.     

A one-pot, non-catalytic approach to 1,2,4-benzothiadiazine-1,1-dioxides

Condensations of o-halo-substituted benzenesulfonyl chlorides with 2-aminopyridines and amidines may give the corresponding 1,2,4-benzothiadiazine-1,1-dioxides under mild, non-catalytic conditions in nearly quantitative yields. The successful one-pot cyclization depends on three factors: (i) the nature of the o-halogen, (ii) the electronic character of the benzene ring substituent, and (iii) the steric load around the amidine unit. O-Fluorobenzenesulfonyl chlorides bearing methylcarboxyl- or nitro-group and o-chloro- and o-bromobenzenesulfonyl chlorides bearing nitro-group are reactive enough to give the desired 1,2,4-benzothiadiazine-1,1-dioxides in a one-pot base-promoted reaction. In all other cases, open-chain sulfonylated amidine intermediates are isolated. The latter are converted to the title compounds either in the presence of potassium carbonate or upon the addition of a copper(I) catalyst.     

Catalytic fluorination of 1,3-dicarbonyl compounds using iodoarene catalysts

Catalytic fluorination of 1,3-dicarbonyl compounds with aqueous hydrofluoric acid proceeded efficiently with the aid of iodoarene catalysts in the presence of m-CPBA as a terminal oxidant. o-Iodotoluene, o-iodoanisole, and o-ethyliodobenzene showed a high catalytic efficiency to give 2-fluoro-1,3-dicarbonyl compounds in good yields.