Regioselective (biomimetic) synthesis of a pentasulfane from ortho-benzoquinone

A mechanism is proposed for the formation of cyclic 5,6,7,8,9-pentathiabenzocycloheptene-1,2-diol, 4, from the reaction of o-benzoquinone with reduced elemental sulfur, H2Sx. 1,6-conjugate addition to the quinone is favored over 1,4-conjugate addition. Hydrogen bonding to the quinone oxygen enhances the nucleophilicity of H2Sx by facilitating the removal of the S-H proton. We propose that initially formed 3-polysulfidobenzene-diol intermediates are oxidized to their corresponding quinones and closure of the polysulfur ring subsequently takes place at the C3-C4 bond leading to 4. A possible mechanism for the formation of the pentasulfur linkage in 4 is discussed, which is the key moiety found in a number of natural products.     

Electron ionization mass spectral fragmentation study of sulfation derivatives of polychlorinated biphenyls

Background  

Polychlorinated biphenyls are persistent organic pollutants that can be metabolized via hydroxylated PCBs to PCB sulfate metabolites. The sensitive and selective analysis of PCB sulfate monoesters by gas chromatography-mass spectrometry (GC-MS) requires their derivatization, for example, as PCB 2,2,2-trichloroethyl (TCE) sulfate monoesters. To aid in the identification of unknown PCB sulfate metabolites isolated from biological samples, the electron impact MS fragmentation pathways of selected PCB TCE sulfate diesters were analyzed and compared to the fragmentation pathways of the corresponding methoxylated PCBs.     

3-己炔-2,5-二醇电还原中间产物的ESR研究

3-己炔 - 2 ,5-二醇 ( HD)是半光亮电镀镍的一种添加剂 ,它在电镀镍时也发生电还原反应 .我们采用 GC- MS谱对其电还原产物作过分析 ,表明它与 1 ,4-丁炔二醇电还原类似 [1] ,在 p H值 1～ 2时 ,主产物为 3-己炔、己烷 ;p H值 >3.5时 ,主产物为 3-己烯 - 2 ,5-二醇和 2 ,5-己二醇 .对这类炔二醇电还原过程尚未见深入研究 ,为深入了解 HD在不同 p H值的溶液中的电还原过程 ,我们对其电还原中间产物作了电化学 - ESR研究 .1　实验部分PARC公司电化学测试系统 (美国 ) ,其中包括 Model 1 73恒电位 /恒电流仪 ,Model 30 3A汞电极装置 ;…     

Conformation inversion of an inositol derivative by use of silyl ethers: a modified route to 3,6-di-O-substituted-L-ido-tetrahydroxyazepane derivatives

The trans-diequatorial 3,4-diol of 2,5-di-O-benzyl-D-chiro-inositol cleaved selectively with the periodate ion in the presence of the trans-diaxial 1,6-diol to give a dialdehyde (dialdose) from which 3,6-di-O-benzyl-D-manno-tetrahydroxyazepane (1) was made. The trans-diaxial 1,6-diol of 3,4-di-O-allyl-2,5-di-O-benzyl-D-chiro-inositol was not cleaved satisfactorily by periodate, but replacement of the allyl substituents with tert-butyldimethylsilyl groups caused conformational inversion of the inositol ring, and the resulting trans-diequatorial 1,6-diol cleaved efficiently to give a dialdehyde from which 3,6-di-O-benzyl-L-ido-tetrahydroxyazepane (2) can be prepared.     

Mimicking primary processes in photosynthesis—covalently linked porphyrin quinones

Porphyrin quinones (P-Qs), covalently linked via different aliphatic bridges, have been synthesized and studies in their (porphyrin) cationic and (semiquinone) anionic radical states by EPR, ENDOR and TRIPLE resonance techniques. Electron transfer (ET) from the porphyrin donor to the quinone acceptor could be observed by time-resolved picosecond fluorescence spectroscopy (singlet ET) and by time-resolved EPR spectroscopy (triplet ET) in isotropic fluid solution and in anisotropic media (liquid crystals and reversed micelles). Steady-state in situ photoexcitation of P-Qs in CTAB cationic reversed micelles yielded the corresponding semiquinone radical anions. In TRITON X-100 reversed micelles both the radical cation of the porphyrin and the radical anion of the semiquinone could be detected, which occured in complete emission. In covalently linked porphyrin flavins ET from the photoexcited porphyrin fragment to the flavin and, in addition, energy transfer from the photoexcited flavin to the porphyrin could be observed.     

Synthesis of dioxin-like monofluorinated PCBs: for the use as internal standards for PCB analysis

Monofluorinated polychlorinated biphenyls (fluoro-PCBs) have been prepared using the Suzuki-coupling, for use as analytical standards for PCB measurements. Seven of these fluoro-PCBs are analogues of the dioxin-like PCBs, listed by the WHO as the most toxic PCB congeners. Four highly chlorinated fluoro-PCBs have been prepared by Suzuki-coupling of 2,3,5,6-tetrachloro-bromoaniline with various substituted arylboronic acids. The resulting amino-fluoro-PCBs are chlorinated using the Sandmeyer reaction or deaminated to yield tetra-, penta- and hexa-chlorinated fluoro-PCBs. The fluoro-PCBs elute just before the corresponding PCBs in the GC chromatogram, which strongly indicates their potential as analytical standards.     

Photosensitized reduction and DNA covalent binding of aziridinylquinones

Photolysis of anaerobic aqueous mixtures (at wavelength maxima above 600 nm and at pH 7.4) containing either aluminum phthalocyanine tetrasulfonate (AlPcS₄), chlorin e6 (CHLORIN), pheophorbide-a (PHEO) or a novel tetracationic phthalocyanine derivative (TETCHLORIN) in the presence of the quinones diaziquone (AZQ), carboquone (CARBOQ) or 2,5-dicloro-diaziridinyl-1,4-benzoquinone (AZDClQ) produces the corresponding semiquinones. Photolysis of these mixtures under the conditions stated above, but in the presence of DNA and at pH 5.5 produces quinone–DNA covalent adducts. Absorption bands seen in irradiated solutions suggest binding of these quinones to DNA through the open aziridine ring. In general, the quinone CARBOQ yielded the largest amounts of adducts photosensitized by the dyes studied here. No quinone–DNA adducts were detected if samples were irradiated at pH 7.4.Thus, both photoreduction of these quinones and an acidic environment are needed for these quinones to bind DNA. These results suggest a potential mode of therapy with special applications to hypoxic regions in solid tumors which are characterized by an acidic environment.     

Quantitative determination of toxic mono- and non-ortho polychlorinated biphenyls in cod liver oil after selective liquid chromatographic separation

Summary By application of chromatographic column filled with Supelclean ENVI-Carb/Celite and elution with three solvents of different polarity three PCB fractions were obtained. Fraction A contained poly-ortho PCBs, Fraction B mono-ortho PCBs, and Fraction C non-ortho PCBs. The Supelclean ENVI-Carb/Celite column was used in combination with a sample preparation procedure for pre-cleaning of acid-stable chlorinated hydrocarbons such as DDT and its metabolites, HCH isomers, and regulation-relevant PCB congeners. The method was optimized using standard solutions of 55 PCB congeners, 8 chlorinated pesticides and contaminated cod liver oil samples. The influence of traces of remaining matrix on the elution profile of the organochlorine compounds on Supelclean ENVI-Carb/Celite was observed. Quantitation was carried out by GC-ECD with fused silica capillary columns of different polarity.     

Synthetic studies on mitomycins. 2. A synthesis of 9a-hydroxy-5,8-dideoxomitosane skeleton through a novel retroaldol type of ring-opening reaction.

A synthesis of 9a-hydroxy-5,8-dideoxomitosanes was accomplished by the transannular cyclization of hydro-1-benzazocinone intermediates derived from 3-(2,5-dioxo-1-methylcyclopentyl)-6-methyl-p-phenylenediamine derivatives. These mitosanes were led to the 8-membered ring system by an oxidative ring-opening reaction.     

Disproportionation of 2,5-dichlorosemiquinone radicals in benzene according to unsteady-state kinetic data for the chain reaction between 2,5-dichlorohydroquinone and <Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>′-diphenyl-1,4-benzoquinone diimine

A method for determining the rate constant of disproportionation of 2,5-dichlorosemiquinone radicals (k ₆) from unsteady-state kinetic data for the initiated chain reaction of N,N′-diphenyl-1,4-benzoquinone diimine with 2,5-dichlorohydroquinone have been developed, and two variants of this method are presented. The method is based on the study of the unsteady-state disappearance kinetics of one of the initial reactants (quinone diimine) in its initiated chain reaction with hydroquinone. The unsteadiness of the reaction is due to the presence of semiquinone radicals or initiator radicals accumulated before the start of the reaction at a concentration exceeding the steady-state concentration of semiquinone radicals in the chain reaction. The variants of the method differ in the order of mixing the reactants and initiator, on which the nature and concentration of the radicals accumulated in the system before the reaction depend. In the first variant, a quinone diimine + initiator solution is initially prepared and initiator radicals are accumulated. Hydroquinone is added to this solution (start of the reaction). In the second variant, a hydroquinone + initiator solution is initially prepared and semiquinone radicals from hydroquinone are accumulated. Quinone imine is then added to the solution (start of the reaction). The disproportionation rate constant of semiquinone radicals (k ₆) is derived from the dependence of the decrease in the quinone imine concentration in a certain short time (∼20 s) after the start of the reaction on the initiation rate. The rate constant k ₆ in benzene is (7.3 ± 3.7) × 10⁶ l mol⁻¹ s⁻¹ according to the first variant of the method and (5.0 ± 2.2) × 10⁶ l mol⁻¹ s⁻¹ according to the second one.     

The interaction between polymerization initiators and inhibitors in solutions—VIII. The mechanism of the reaction of azobisisobutyronitrile with p-benzoquinone and chloranil in polar and nonpolar solvents

The reactions of azobisisobutyronitrile (AIBN) with p-benzoquinone (BQ) and chloranil in toluene, chlorobenzene and acetonitrile, have been investigated by isolation and identification of the reaction products. In toluene and chlorobenzene, isobutyroylamino hydroquinone together with tetramethyl dioxazinobenzene are formed in the case of (BQ); the corresponding mono- and dioxazinobenzene derivatives are formed in the case of chloranil. In acetonitrile, however, only polyquinonoid resinous derivatives of the quinones are obtained. The crystalline derivatives as well as the resinous products have no nitrile groups in their structures and the nitrogen atom is directly attached to the parent quinone nucleus. This fact indicates that the radicals from AIBN react with quinones exclusively in the ketenimine form. A mechanism based on the possibility of electron-transfer from the radical to the quinone molecule to form charged species has been suggested. The degree of separation of these species is determined by the polarity of the solvent. Combination of the charged entities produces nuclear-substituted intermediates which may be eventually isolated as hydroquinones or subjected to further radical reactions with the ketenimine substituent to form the oxazinobenzene derivatives. The formation of ether derivatives of hydroquinone and tetrachlorohydroquinone in the reaction of BQ and chloranil with AIBN is accordingly excluded.     

Phylloquinone and related radical anions studied by pulse electron nuclear double resonance spectroscopy at 34 GHz and density functional theory

1H hyperfine (hf) coupling constants of semiquinone radical anions of 1,4-naphthoquinone, 2-methyl-1,4-naphthoquinone, and 2-methyl-3-phytyl-1,4-naphthoquinone in frozen alcoholic solutions were measured using pulse Q-band electron nuclear double resonance spectroscopy. The resolved signals of the quinone protons as well as from hydrogen bond and solvent shell protons were analyzed and assigned. Both in-plane and out-of-plane hydrogen bonding with respect to the pi-plane of the radical is observed. Interactions with nonexchangeable protons from the surrounding matrix are detected and assigned to solvent protons above and below the quinone plane. Density functional theory was used to calculate spin Hamiltonian parameters of the radical anions. Solvent molecules of the first solvent shell that provide hydrogen bonds to the quinones were included in the geometry optimization. The conductor-like screening model was employed to introduce additional effects of the solvent cage. From a comparison of the experimental and calculated hf tensors it is concluded that four solvent molecules are coordinated via hydrogen bonds to the quinone oxygens. For all radicals very good agreement between experimental and calculated data is observed. The influence of different substituents on the spin density distribution and hydrogen bond geometries is discussed.     

Electron transfer from aromatic amino acids to triplet quinones

The photoreduction of 1,4-benzoquinone, 1,4-naphthoquinone, 9,10-anthraquinone (AQ) and several methylated or halogenated derivatives in argon-saturated acetonitrile-water mixtures by indole, N-acetyltryptophan and N-acetyltyrosine was studied by time-resolved UV-vis spectroscopy using 20 ns UV laser pulses. The quinone triplet state is quenched by the aromatic amino acids and the rate constants are (1-5)x10(9)M(-1)s(-1). The semiquinone radical anion Q.(-) is the major observable transient after electron transfer from amino acids to the quinone triplet state. Termination of Q.(-) and amino acid derived radicals takes place in the mus-ms range. The effects of structure and other specific properties of quinones and amino acids are discussed. The radicals are subjects of intercept with oxygen, whereby hydrogen peroxide is eventually formed. The quantum yield of oxygen uptake Phi(-O2) as a measure of formation of hydrogen peroxide increases with increasing amino acid concentration, approaching Phi(-O2) for AQ in air-saturated solution.     

Photoreduction of p-benzoquinones: effects of alcohols and amines on the intermediates and reactivities in solutions

The photochemistry of 1,4-benzoquinone (BQ) and alkyl-, Cl- and related derivatives, e.g. methyl-, 2,6-dimethyl-, chloro-, 2,5-dichloro-1,4-benzoquinone, duroquinone and chloranil, was studied in nonaqueous solvents by UV-vis spectroscopy using nanosecond laser pulses at 308 nm. The reactivity of the triplet state (3Q*) of the quinones with 2-propanol in the absence of water is largest for BQ and depends mainly on the quinone structure, whereas the rate constant of electron transfer from amines, such as triethylamine (TEA) or 1,4-diazabicyclo[2.2.2]octane, is close to the diffusion-controlled limit for BQ and most derivatives. Photoinduced charge separation after electron transfer from amines to 3Q* and the subsequent charge recombination or neutralization are supported by time-resolved conductivity measurements. The half-life of the decay kinetics of the semiquinone radical (*QH/Q*-) depends significantly on the donor and the medium. The photoconversion into the hydroquinones was measured under various conditions, the quantum yield, lambda(irr) = 254 nm, increases with increasing 2-propanol and TEA concentrations. The effects of quenching of 3Q*, the *QH/Q*- radicals and the photoconversion are outlined. The mechanisms of photoreduction of quinones in acetonitrile by 2-propanol are compared with those by TEA in benzene and acetonitrile, and the specific properties of substitution are discussed.     

CHLOROPHYLL-QUINONE PHOTOCHEMISTRY IN LIPOSOMES: MECHANISMS OF RADICAL FORMATION AND DECAY*

Abstract— Laser flash photolysis has been used to investigate the mechanism of formation and decay of the radical species generated by light-induced electron transfer from chlorophyll a (Chi) triplet to various quinones in egg phosphatidyl choline bilayer vesicles. Chlorophyll triplet quenching by quinone is controlled by diffusion occurring within the bilayer membrane (kq～ 10⁶M^?1 s^?1. as compared to ～ 10⁹ M^?1 s^?1 in ethanol) and reflects bilayer viscosity. Radical formation via separation of the intermediate ion pair is also inhibited by increased bilayer viscosity. Cooperativity is observed in the radical formation process due to an enhancement of radical separation by electron transfer from semiquinone anion radical to a neighboring quinone molecule. Two modes of radical decay are observed, a rapid (t_1/2= 150μ) recombination between Chi and quinone radicals occurring within the bilayer and a much slower (t_1/2= 1–100 ms) recombination occurring across the bilayer-water interface. The latter is also cooperative, which accounts for a t_1/2 which is dependent upon quinone concentration. The slow decay is only observed with quinones which are not tightly anchored into the bilayer, and is probably the result of electron transfer from semiquinone anion radical formed within the bilayer to a quinone molecule residing at the bilayer-water interface. Direct evidence for such a process has been obtained from experiments in which both ubiquinone and benzoquinone are present simultaneously. With benzo-quinone, approx. 60% of the radical decay occurs via the slow mode. Triplet to radical conversion efficiencies in the bilayer systems are comparable to those obtained in fluid solution (～ 60%). However, radical recombination, at least for the slow decay mechanism, is considerably retarded.     

Synthesis of chlorinated and non-chlorinated biphenyl-2,3- and 3,4-catechols and their [2H3]-isotopomers

A synthetic scheme is described for chlorinated biphenyl-2,3- and 3,4-catechols to be used as standards for structural assignment of metabolites and protein adducts of 2,2',5,5'-tetrachlorobiphenyl in which both rings retain chlorine substituents. The scheme has general applicability to the synthesis of chlorinated biphenyl catechols. Dimethyl catechol ethers are coupled to dichloroaniline via the Cadogan reaction to give a library of isomers, followed by demethylation of the ethers with BBr3 to yield the target catechols. Separation of pure isomers is accomplished by TLC or HPLC prior to or following demethylation, depending on the isomer mixture. [2H3]-Isotopomers are generated using 2,5-dichloroaniline-d3 as the starting arylamine in the coupling reaction. The dichloroaniline-d3 hydrochloride is obtained as the sole product from nitration of p-dichlorobenzene-d4 followed by Pd/C-catalyzed hydrogenation under strongly acidic conditions. This hydrogenation procedure provides a simple and convenient approach to selective reduction of aryl nitro groups in the presence of halogen ring substituents.     

Mechanistic study of electrochemical oxidation of 2,5-dihydroxybenzoic acid and 3,4-dihydroxybenzaldehyde in the presence of 3-hydroxy-1H-phenalene-1-one

The mechanism of the electrochemical oxidation of 2,5-dihydroxybenzoic acid and 3,4-dihydroxybenzaldehyde in the presence of 3-hydroxy-1H-phenalene-1-one as a nucleophile has been studied in water/acetonitrile (80/20 v/v) solution using cyclic voltammetry and controlled-potential coulometry methods. The results indicate that the quinones derived from oxidation of 2,5-dihydroxybenzoic acid and 3,4-dihydroxybenzaldehyde participate in Michael addition reactions with 3-hydroxy-1H-phenalene-1-one and via ECE and ECEC mechanisms convert to the different products, with good yield under controlled potential conditions, at carbon electrode.     

Features of Sulfonation of Polychlorinated Biphenyl Congeners

Sulfonation of polychlorinated biphenyls (PCBs) and their commercial mixture (Trichlorobiphenyl) with oleum has been studied. General sulfonation patterns have been revealed for PCB congeners having no substituents in the ortho positions. The sulfonation of PCBs with chlorine atoms in only one aromatic ring gives exclusively the corresponding polychlorobiphenylmonosulfonic acids. The sulfonation of PCBs with chlorine atoms in both aromatic rings is accompanied by side formation of polychlorodibenzothiophene S,S-dioxides.     

Theoretical studies of electron and hydrogen transfer reactions between semiquinone radicals and oxygen

 To explore the interactions between ubiquinones and oxygen in living organisms, the thermodynamics of a series of electron and hydrogen transfer reactions between semiquinone radicals, as well as their corresponding protonated forms, and oxygen, singlet or triplet, were studied using the hybrid Hartree–Fock–density functional theory method Becke's three parameter hybrid method with the Lee, Yang, and Parr correlation functional. Effects of the solvent and of the isoprenyl tail on the electron and hydrogen transfer reactions were also investigated. It is found that semiquinone radicals (semiquinone anion radicals or protonated semiquinone radicals) cannot react with triplet oxygen to form the superoxide anion radical O₂ ⁻. In contrast, neutral quinones can scavenge O₂ ⁻ efficiently. In the gas phase, only protonated semiquinone radicals can react spontaneously with singlet oxygen to produce peroxyl radical (HO₂). However, both semiquinone anion radicals and protonated semiquinone radicals can react with singlet oxygen to produce harmful oxygen radicals (O₂ ^{− a l l b u l l} and HO₂, respectively) in aqueous and protein environments. The free-energy changes of the corresponding reactions obtained for different ubiquinone systems are very similar. It clearly shows that the isoprenyl tail does not influence the electron and hydrogen transfer reactions between semiquinone radicals and oxygen significantly. Results of electron affinities, vertical ionization potentials, and proton affinities also show that the isoprenyl tail has no substantial effect on the electronic properties of ubiquinones. Received: 3 July 2000 / Accepted: 6 September 2000 / Published online: 21 December 2000