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Selected AbstractsRole of phenoxy radicals in PCDD/F formationINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 9 2002Sukh Sidhu In this work, the role of phenoxy radicals in polychlorinated dibenzo- p -dioxins and polychlorinated dibenzofurans (PCDD/F) formation was investigated by studying the slow oxidation of 2-chlorophenol (2-CP) and 2-chloroanisole (2-CA) at a gas-phase concentration of 4 ppm (,2.1 × 104 ,g/m3) over a temperature range of 400,800°C. Residence times were maintained at 2.0 ± 0.10 s. PCDD/F reaction products were dibenzofuran, dibenzo- p -dioxin, 4-chlorodibenzofuran, 1-chlorodibenzo- p -dioxin, 4,6-dichlorodibenzofuran, and 1,6-dichlorodibenzo- p -dioxin (1,6-DCDD). Major products observed in these experiments were 2,6-dichlorophenol, 3-phenyl-2-propenal, 1-indanone, 1,3-isobenzofurandione, and 3-phenyl-2-propenoyl chloride. The 2-CP and 2-CA experiments, along with the variable concentration 2-CA experiments, showed that the concentration of radicals present in the oxidation system has a significant effect on the PCDD/F product distribution and ultimately the PCDD/PCDF ratio. Also, the observation of dichlorinated phenoxy phenol and dichlorinated dihydroxybiphenyl, the proposed intermediate species in the radical,radical mechanism, suggests that radical,radical mechanism dominates gas-phase PCDD/F formation. This information will be helpful in constructing a detailed kinetic mechanism of PCDD/F formation/destruction in combustor postcombustion zone. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 531,541, 2002 [source] Oxidative transformation of tetrachlorophenols and trichlorophenols by manganese dioxideENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 6 2009Ling Zhao Abstract This study examined the transformation kinetics of three tetrachlorophenols (TeCPs) and three trichlorophenols (TCPs) in the presence of MnO2 under different solution chemistry conditions. The reaction rate measured for each CP decreased as a function of solution pH, and under the same solution chemistry conditions, the measured rates may depend primarily on both the adsorbability at the MnO2 surfaces and the isomeric structures of the CPs. Isomeric effects indicated that chloro substituent on ortho or para positions exhibited faster rates of transformation than on meta positions. Gas chromatography-mass spectrometry analysis with a derivatization method showed that dimers including polychlorinated phenoxyphenols and chlorinated polyhydroxybi-phenyl were among the major products for all CPs. Monomeric products were among the major products of 2,4,6-TCP, 2,3,4-TCP, and 2,3,4,6-TeCP, whereas trimeric products also were among the major products of 2,3,4-TCP and 2,4,5-TCP. It appeared that hydroxylation of CPs and formation of dimeric or trimeric products via oxidative coupling were the major reaction mechanisms involved in the oxidation of CPs by MnO2. [source] Oxidation kinetics of pentachlorophenol by manganese dioxideENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 11 2006Ling Zhao Abstract This study examined the abiotic transformation kinetics of pentachlorophenol (PCP) by manganese dioxide (MnO2) at different solution chemistry and initial concentrations of PCP and MnO2. The measured PCP transformation rates were found to be on the order of 1.07 with respect to [PCP] and 0.91 and 0.87 with respect to [MnO2] and [H+], respectively. Dissolved Mn2+ and Ca2+ as background electrolytes considerably decreased the reaction rate because of their adsorption and hence blocking of active sites on MnO2 surfaces. The dechlorination number, 0.59 chloride ions per transformed PCP after a 1-h reaction, suggests that a fraction of the transformed PCP was not dechlorinated and may be coupled directly to dimeric products. Gas chromatography/ mass spectrometry and liquid chromatography/mass spectrometry/mass spectrometry techniques were used to identify two isomeric nonachlorohydroxybiphenylethers as major products and 2,3,5,6-tetrachloro-1,4-hydroquinone and tetrachlorocatechol as minor products. Product identification suggested that the reaction may include two parallel reactions to form either dimers or 2,3,5,6-tetrachloro-1,4-hydroquinone and tetrachlorocatechol via simultaneous dehydrochlorination and hydroxylation. [source] Biological activity associated with noncoplanar polychlorinated biphenyls after microbial dechlorination of aroclor 1242® and aroclor 1254®ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 5 2000Patricia E. Ganey Abstract Bioremediation of polychlorinated biphenyls (PCBs) byanaerobic microbial dechlorination occurring naturally in the subsurface and in engineered systems results in mixtures of lower-chlorinated, primarily ortho-substituted biphenyls. The purpose of this study was to determine whether this process of bacterial dechlorination results in a mixture that differs in biological activity from that of the parent PCB mixture. Two biological assays sensitive to the action of ortho-substituted PCBs were employed: insulin release by RINm5F cells, and superoxide anion (O2) production by rat neutrophils. The PCB mixtures Aroclor 1242® and Aroclor 1254® were incubated for nine months with microbes from PCB-contaminated sites (Silver Lake, MA, USA, or River Raisin, MI, USA), and the products of dechlorination were then extracted. Exposure of RINm5F cells to dechlorinated Aroclor 1242 or 1254 product mixtures caused an increase in insulin release similar to the hormone release from cells exposed to non-dechlorinated Aroclors. When tested alone, several of the major products identified in the dechlorination mixture (i.e., 2,2,,4,4,-tetrachlorobiphenyl, 2,2,,4-trichlorobiphenyl [TCB], 2,3,,4-TCB, 2,3,,5-TCB, and 2,2,-dichlorobiphenyl) caused an increase in insulin release. In studies using neutrophils isolated from rat peritoneum, the amount of O2 produced on exposure to product mixtures resulting from dechlorination of Aroclor 1242 was not different from the amount produced in nondechlorinated controls. The product mixture resulting from Aroclor 1254 dechlorination by organisms from River Raisin increased generation of O2, relative to the parent Aroclor. Taken together, these results suggest that anaerobic dechlorination of Aroclor mixtures of PCBs does not reduce the biological activities associated with lightly chlorinated and ortho -substituted PCBs. This observation has implications for the usefulness of PCB bioremediation efforts that involve only anaerobic dechlorination. [source] Dynamic Stereochemical Behaviour of Congested Ruthenium(II) Complexes Containing Asymmetric Thioether Ligands Based on Pyridine and PyrimidineEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 30 2008Giuseppe Tresoldi Abstract The asymmetric thioethers L [L = 2-pyridylmethyl 2,-pyrimidyl sulfide (pps) and 2-(4-methylpyrimidyl) 2,-pyridylmethyl sulfide (mps)] reacted with cis -[RuCl2(N,N -L,)2] [L, = di-2-pyridyl sulfide (dps); 2,2,-bis(4-methylpyridyl) sulfide (4mdps); 2,2,-bis(5-methylpyridyl) sulfide (5mdps)] to give the five-membered-ring chelate complexes [Ru(N,N -L,)2(Npyridine,S -L)]++ as the major products (92,95,%). Because the sulfur and ruthenium atoms are stereogenic centres, with (R) and (S) and , and , configurations, respectively, four isomers, including the enantiomers were obtained. At low temperature and in the methylene region of the 1H NMR spectra, two AB systems due to the enantiomer couples ,S ,R (a) and ,R ,S (b) were observed with abundances of 77,89 and 6,18,%, respectively. Furthermore, NMR spectroscopic investigations showed that the hybrid polydentate ligands L change their coordination mode. Thus, although a and b largely predominate, a mixture of species containing L and the Ru(N,N -L,)2 unit in the ratio 1:1 are present. The four-membered-ring chelate complexes [Ru(N,N -L,)2(Npyrimidine,S -L)]++ (c), as minor species (abundance 1,8,%), are always observed, whereas the dinuclear species [{Ru(N,N -L,)2}2(,-L)2]+4 (d, e) are observed when L, = dps or 5mdps. In these cases, four AB systems are assigned to dinuclear species d and e containing two bridging L that act as Npyridine,S- or Npyridine,Npyrimidine -donor ligands. The 1H NMR spectra are temperature dependent in that at low temperature the complexes undergo inversion of the chiral centre of the coordinated sulfur atom (a [rlhar2] b) and the dimer (d, e) and monomer (c) are in equilibrium; at higher temperatures the complexes undergo a structural dynamic rearrangement, which involves exchange between the coordinated and uncoordinated N atoms (b [rlhar2] c). One-dimensional band-shape analysis of the exchanging methylene and methyl proton signals showed that the energy barriers for inversion of the sulfur centre are in the 50,53 kJ,mol,1 range, whereas those for the higher-temperatures process are in the 62,68 kJ,mol,1 range. The possible mechanisms of the processes are discussed. NMR spectroscopic findings suggest that inversion at the sulfur centre occurs without any bond rupture, whereas the exchange, at higher temperatures (b [rlhar2] c), is a dissociative process involving the breaking of a Ru,Npyridine bond.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] 3(5)-(2-Hydroxyphenyl)-5(3)-styrylpyrazoles: Synthesis and Diels,Alder TransformationsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 21 2004Vera L. M. Silva Abstract Reactions between cinnamoyl(2-hydroxybenzoyl)methanes and hydrazine hydrate in acetic acid gave 3-(2-hydroxyphenyl)-5-styrylpyrazoles, while the corresponding reactions with phenylhydrazine yielded 5-(2-hydroxyphenyl)-1-phenyl-3-styrylpyrazoles as the major products and 3-(2-hydroxyphenyl)-1-phenyl-5-styrylpyrazoles as by-products. The reaction mechanism of this transformation is discussed. The first cycloaddition reactions between ortho -benzoquinodimethane and either 3-(2-hydroxyphenyl)-5-styrylpyrazoles or 5-(2-hydroxyphenyl)-1-phenyl-3-styrylpyrazoles afforded 5-[2-(3-aryl-1,2,3,4-tetrahydronaphthyl)]-3-(2-hydroxyphenyl)pyrazoles or 3-[2-(3-aryl-1,2,3,4-tetrahydronaphthyl)]-1-phenyl-5-(2-hydroxyphenyl)pyrazoles, respectively. These cycloadducts were converted into the corresponding naphthylpyrazoles by oxidation with DDQ in dry 1,4-dioxane. The structures of all new derivatives have been established by NMR spectroscopy. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Competition between Non-Classical Single and Double Epimerizations in Cyclitol ChemistryEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 9 2004Ralf Miethchen Abstract Two competitive regio- and stereoselective epimerization reactions were investigated in four cyclitols characterized by four contiguous OH groups with a cis - trans - trans sequence and by varied substituents (OMe, OBz, F, H) adjacent to this tetrol unit. The starting materials were synthesized from L -quebrachitol (compounds 5,7) and myo -inositol (compound 8). Their acetalization with the chloral/DCC reagent system gave cyclic acetals with one epimerized chiral ring atom and also with two epimerized chiral centres. The single epimerization takes place exclusively at the middle C-atom of the cis - trans triol unit in the tetrol sequence (products 15, 17, 19/20 and 24,27), whereas the double epimerization occurs at both of the "centrally located" C-atoms in the cis - trans - trans tetrol unit (products 16, 18, 21 and 28). The product ratios of singly to doubly inverted compounds change as follows: the lower the electron-withdrawing effect of the substituents adjacent to the tetrol unit, the higher the percentage of the corresponding doubly inverted product. However, the singly inverted products remain the major products in all cases. X-ray analyses are given for the starting material 1-fluoro-2- O -(methyl)cyclohexane-2,3,4,5,6-pentol (5) and for the products 1- O -cyclohexylcarbamoyl-2,3- O -(2,2,2-trichloroethylidene)5- O -(methyl)cyclohexane-1,2,3,4,5-pentol (17), 3- O -acetyl-1- O -benzoyl-6- O -cyclohexylcarbamoyl-2- O -methyl-4,5- O -(2,2,2-trichloroethylidene)- muco -inositol (22) and 2,3-di- O -ethylidene)-(+/-)- chiro -inositol (24). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] A Radical Version of the Bromo- and the Iodocyclization of Bis(homoallylic) Alcohols , The Synthesis of Halogenated Tetrahydrofurans by Stereoselective Alkoxyl Radical Ring ClosuresEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 20 2003Jens Hartung Abstract A new synthesis of bromo- and iodomethyl-substituted tetrahydrofurans has been devised. The sequence starts with the conversion of aryl-functionalized bis(homoallylic) alcohols 1 into N -alkenoxythiazole-2(3H)-thiones 6 or pyridine-2(1H)-thiones 7. When photolyzed in the presence of appropriate trapping reagents, thiones 6 and 7 efficiently liberated substituted 4-penten-1-oxyl radicals 2, which underwent synthetically useful 5- exo -trig cyclizations. Cyclized radicals 3 were trapped with BrCCl3 or an adequate iodine atom donor (either n -C4F9I or diethyl 2-iodo-2-methyl malonate) to provide halocyclization products 4 or 5. This strategy has been applied for the synthesis of 3-, 4-, or 5-phenyl-substituted 2-(1-bromo-1-methylethyl)tetrahydrofurans 4a,c (75,90%, 36,96% de), which were not attainable as major products from polar, for example NBS-mediated, bromocyclizations. Aryl-substituted 2-iodomethyl tetrahydrofurans 5 (46,80%) were prepared in a similar way starting from N -alkenoxypyridine-2(1H)-thiones 7 and a suitable iodine atom donor. Diastereomerically pure iodides cis - 5 and trans - 5 served as starting materials for a stereochemical analysis of disubstituted tetrahydrofurans by NMR spectroscopy and X-ray diffraction analysis. The results of this investigation clarified that all new alkoxyl radical cyclizations followed in terms of regio- and diastereoselectivity the general guidelines which had been established for this type of ring-closure reaction. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Unusual Rearrangements of cis -4,7-Disubstituted 4,7-Dihydro4,7-dihydroxy[2.2]paracyclophanes on Dehydration: Stereoselective Formation of Planar Chiral CyclohexadienonesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 4 2003Natalia Vorontsova Abstract Under acid conditions, cis -4,7-disubstituted 4,7-dihydro-4,7-dihydroxy[2.2]paracyclophanes undergo dehydration accompanied by rearrangement, affording cyclohexadienone derivatives as major products and with polysubstituted phenols being formed as minor products. The formation of either an ortho or a para semiquinoid system, as well as the configuration of the newly formed asymmetric center, depended strictly on the nature of the substituent (alkyl, allyl, or phenyl). The structures of 3,4-dihydro-3,7-dimethyl[2.2]paracyclophane-4-one (10), 3,7-diallyl-3,4-dihydro[2.2]paracyclophane-4-one (12), and 4,7-dihydro-7,8-diphenyl[2.2]paracyclophane-4-one (22) were determined by X-ray structural analysis. [2.2]Paracyclophane-4,7-quinone (1) was obtained in optically active form. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Selenium-Containing Heterocycles from Isoselenocyanates: 4-Methylselenazole Derivatives from the Reaction with Malononitrile and Propargyl ChlorideHELVETICA CHIMICA ACTA, Issue 2 2008Geoffroy Abstract Aryl isoselenocyanates 1 react with malononitrile (6a) and propargyl chloride (8) in DMF in the presence of Et3N to give the corresponding 2-(3-aryl-2,3-dihydro-4-methyl-1,3-selenazol-2-ylidene)malononitriles 12 as major products. The analogous reaction takes place with benzoylacetonitrile (6f) instead of 6a. In some cases, the corresponding noncyclic 2-[(arylamino)(prop-2-ynylselanyl)methylidene]malononitriles 9 were obtained as minor products. The structures of 9e and 11a have been established by X-ray crystallography. [source] Facile and Selective Synthesis of 4-Methyl- and 4-Phenylthiosemicarbazide (=N -Methyl- and N -Phenylhydrazinecarbothioamide) Derivatives of Benzil (=1,2-Diphenylethane-1,2-dione)HELVETICA CHIMICA ACTA, Issue 11 2007David Abstract A selective synthesis of 4-methylthiosemicarbazide (=N -methylhydrazinecarbothioamide; 4a) derivatives by reaction with benzil (=1,2-diphenylethane-1,2-dione; 3) is described. The reaction conditions determined the condensation product formed. The most important factor was the acid used: in the presence of conc. HCl solution, the open-chain 2,:,1 compound 1a was exclusively obtained, whereas in the presence of 2M HCl, the cyclic 1,:,1 condensation product 2a was formed. The alcohol used, the presence of H2O, and the time of heating were additional crucial factors. The new cyclic compound 2a with a MeO group was exclusively formed when working under high-dilution conditions. The reaction with the 4-phenyl derivative 4b gave new cyclic compounds as the major products under all conditions used (Scheme). [source] Atmospheric chemistry of isopropyl formate and tert -butyl formateINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2010Andre Silva Pimentel Formates are produced in the atmosphere as a result of the oxidation of a number of species, notably dialkyl ethers and vinyl ethers. This work describes experiments to define the oxidation mechanisms of isopropyl formate, HC(O)OCH(CH3)2, and tert -butyl formate, HC(O)OC(CH3)3. Product distributions are reported from both Cl- and OH-initiated oxidation, and reaction mechanisms are proposed to account for the observed products. The proposed mechanisms include examples of the ,-ester rearrangement reaction, novel isomerization pathways, and chemically activated intermediates. The atmospheric oxidation of isopropyl formate by OH radicals gives the following products (molar yields): acetic formic anhydride (43%), acetone (43%), and HCOOH (15,20%). The OH radical initiated oxidation of tert -butyl formate gives acetone, formaldehyde, and CO2 as major products. IR absorption cross sections were derived for two acylperoxy nitrates derived from the title compounds. Rate coefficients are derived for the kinetics of the reactions of isopropyl formate with OH (2.4 ± 0.6) × 10,12, and with Cl (1.75 ± 0.35) × 10,11, and for tert -butyl formate with Cl (1.45 ± 0.30) × 10,11 cm3 molecule,1 s,1. Simple group additivity rules fail to explain the observed distribution of sites of H-atom abstraction for simple formates. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 479,498, 2010 [source] Detailed chemical kinetic modeling of pyrolysis of ethylene, acetylene, and propylene at 1073,1373 K with a plug-flow reactor modelINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2008Koyo Norinaga This study examines the predictive capability of our recently proposed reaction mechanism (Norinaga and Deutschmann, Ind Eng Chem Res 2007, 46, 3547) for hydrocarbon pyrolysis at varying temperature. The conventional flow reactor experiments were conducted at 8 kPa, over the temperature range 1073,1373 K, using ethylene, acetylene, and propylene as reactants to validate the mechanism. More than 40 compounds were identified and quantitatively analyzed by on- and off-line gas chromatography. The chemical reaction schemes consisting of 227 species and 827 reactions were coupled with a plug-flow reactor model that incorporated the experimentally measured axial temperature profile of the reactor. Comparisons between the computations and the experiments are presented for more than 30 products including hydrogen and hydrocarbons ranging from methane to coronene as a function of temperature. The model can predict the compositions of major products (mole fractions larger than 10,2) in the pyrolysis of three hydrocarbons with satisfactory accuracies over the whole temperature range considered. Mole fraction profiles of minor compounds including polycyclic aromatic hydrocarbons (PAHs) up to three ring systems, such as phenanthrene, anthracene, and phenylnaphthalene, are also fairly modeled. At temperatures lower than 1273 K, larger PAHs were underpredicted and the deviation became larger with decreasing temperature and increasing molecular mass of PAHs, while better agreements were found at temperatures higher than 1323 K. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 199,208, 2008 [source] Catalytic Asymmetric Intramolecular Cascade Reaction for the Construction of Functionalized Benzobicyclo[4.3.0] Skeletons.ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-12 2010Remote Control of Enantioselectivity Abstract A catalytic asymmetric version of the intramolecular ylide annulation has been developed which affords high ee values and diastereoselectivities and which further shows that spirobiindane-based chiral phosphines can be excellent organocatalysts. Both optically active benzobicyclo[4.3.0] compounds 2 and 2, with three continuous stereogenic centers could be obtained as major products selectively under neutral and mild conditions just by a choice of an additive. [source] Ab initio MO study of potential energy surface of NH2 with CN reactionINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 8 2006Benni Du Abstract An extensive quantum chemical study of the potential energy surfaces (PES) for the association reaction of NH2 with CN and the subsequent isomerization and dissociation reactions has been carried out using density functional theory (DFT)/B3LYP/6-311++G(3df,2p) level of theory on both singlet and triplet states. The reaction mechanism on the triplet surface is more complicated than that on the singlet surface. A total of 19 isomers and 46 transition states have been identified and characterized on the triplet PES. Among them, IM2 (IM2a), IM3 (IM3a, IM3b), and IM10 are the lowest-lying isomers with thermodynamic stability. Twenty available dissociation channels, depending on the different initial isomers, have been identified. On the singlet surface, only 12 isomers and 16 transition states have been found, and among them IM1(S) and IM2(S) are the lowest-lying isomers. The higher isomerization and dissociation barriers on the singlet surface indicate that the addition and the subsequent reactions of NH2+CN are most likely to occur on the triplet PES because of the lower barriers. A prediction can be made for the possible mechanism explaining the production of H+HNCN. Besides HNCN, other major products are NH+HCN and NH+HNC, which are produced by direct dissociation reactions from triplet IM2 and IM3, respectively. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] Stereoselective Chemoenzymatic Preparation of ,-Amino Esters: Molecular Modelling Considerations in Lipase-Mediated Processes and Application to the Synthesis of (S)-DapoxetineADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 2-3 2010María Rodríguez-Mata Abstract A wide range of optically active 3-amino-3-arylpropanoic acid derivatives have been prepared by means of a stereoselective chemoenzymatic route. The key step is the kinetic resolution of the corresponding ,-amino esters. Although the enzymatic acylations of the amino group with ethyl methoxyacetate showed synthetically useful enantioselectivities, the hydrolyses of the ester group catalyzed by lipase from Pseudomonas cepacia have been identified as the optimal processes concerning both activity and enantioselectivity. The enantiopreference of this lipase in these reactions has been explained, at the molecular level, by using a fragment-based approach in which the most favoured binding site for a phenyl ring and the most stable conformation of the 3-aminopropanoate core nicely match the (S)-configuration of the major products. The conversion and enantioselectivity values of the enzymatic reactions have been compared in order to understand the influence of the different substitution patterns present in the phenyl ring. This chemoenzymatic route has been successfully applied to the preparation of a valuable intermediate in the synthesis of (S)-dapoxetine, which has been chemically synthesised in excellent optical purity. [source] Iron-Catalyzed Cross-Coupling Reaction of Vinyl Bromides or Chlorides with Imidazoles in the Absence of Ligands and AdditivesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 9 2009Jincheng Mao Abstract Highly effective coupling of imidazoles with (E)-vinyl halides can be achieved by using readily available iron catalysts under ligand-free, copper-free and palladium-free conditions. Coupling of (E)-vinyl bromides led to (Z)-products predominantly, while the reactions of (E)-vinyl chlorides afforded (E)-isomers as the major products. [source] Simple Methodology for Heck Arylation at C-8 of Adenine NucleosidesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 4 2008Pallavi Lagisetty Abstract A simple method for the arylation of 8-vinyladenine nucleoside derivatives is reported. With a broad set of aryl iodides and bromides, the reaction is catalyzed by the simple combination palladium acetate/tris(o -tolyl)phosphine/triethylamine [Pd(OAc)2/(o -tol)3P/Et3N]. As expected, aryl chlorides are more difficult coupling partners but some undergo reactions with more exotic catalysts. Although trans -olefins are the major products, minor amounts of cis -isomers are detected in some cases, and a post -arylation mechanism for their formation is proposed. Finally, by subtle catalyst modulation chemoselective N -arylation of the nucleoside can be achieved in the presence of the vinyl moiety. [source] Exploring strategic priorities for regional agricultural R&D investments in East and Central AfricaAGRICULTURAL ECONOMICS, Issue 2 2010Liangzhi You O13; O32; O55; Q16 Abstract The 11 countries of East and Central Africa have diverse but overlapping agroclimatic conditions, and could potentially benefit from spillovers of agricultural technology across country borders. This article uses high-resolution spatial data on actual and potential yields for 15 major products across 12 development domains to estimate the total benefits available from the spread of new agricultural technologies around the region. Market responses and welfare gains are estimated using the,Dynamic Research Evaluation for Management,model, taking account of current and future projections of local and international demand. Results suggest which crops, countries, and agroclimatic regions offer the largest total benefits. Downloadable data and program files permit different assumptions and additional information to be considered in the ongoing process of strategic priority setting. [source] Investigations on regio- and stereoselectivities in cycloadditions involving ,- (3-pyridyl)- N -phenylnitrone: Development of an efficient route to novel nicotine analogsJOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 6 2005Gurpinder Singh Thermal reactions of hitherto ,-(3-pyridyl)- N -phenylnitrone (1) with mono-substituted electron-rich and electron-neutral dipolarophiles are regio-, and stereo-selective (exo -selective), controlled by LUMO - dipole - HOMO- dipolarophile interaction, and furnish syn -5-substituted-3-(3-pyridyl)-isoxazolidines (5) in high yields. With electron deficient dipolarophiles such as acrylonitrile there is observed a loss of regioselectivity as well as stereoselectivity and the regioselectivity is reversed in reactions with methyl vinyl ketone and methyl acrylate, due to intervention of HOMO-dipole - LUMO-dipolarophile interaction, affording 4-substi-tuted-3-(3-pyridyl)-isoxazolidines (7) as major products. Reactions of nitrone (1) with disubstituted dipolarophiles such as methyl methacrylate and ethyl coronate furnish methyl syn -5-methy-3-pyridyl-1-phenyl-isoxazolidine-5-carboxylate (8) and ethyl anti -5-methy-3-pyridyl-1-phenyl-isoxazolidine-4-carboxylate (10), respectively, in high yields. Reaction with N -Phenylmaleimide affords novel isoxazolidino-pyrro-lidinediones bearing a 3-pyridyl moiety (11, 12). A mechanistic rationalization of the obtained results in terms of electronic, steric and secondary interactions is proffered. [source] Nitrite anions induce nitrosative deamination of peptides and proteinsRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 24 2006Haiteng Deng In the present study, reactions of sodium nitrite with proteins/peptides were characterized with mass spectrometry. The reaction generates two major products: replacement of the amino group by a hydroxyl group and formation of an alkene derivative by loss of a NH3 group at the N-terminus and the side chain of lysine residues of proteins/peptides. The reaction proceeds rapidly in weak acidic solution and at 37°C in the presence of a millimolar concentration of nitrite, demonstrating that nitrite induces nitrosative deamination in proteins and peptides. The facile nitrite-induced modification of amino groups of protein/peptides changes the chemical nature of proteins and may have various applications in peptide synthesis, analytical chemistry, and protein engineering. It also provides information to enhance our understanding of functions of nitrite ions in biology and food preservation. Copyright © 2006 John Wiley & Sons, Ltd. [source] Extracellular cross-linking of maize arabinoxylans by oxidation of feruloyl esters to form oligoferuloyl esters and ether-like bondsTHE PLANT JOURNAL, Issue 4 2009Sally J. Burr Summary Primary cell walls of grasses and cereals contain arabinoxylans with esterified ferulate side chains, which are proposed to cross-link the polysaccharides during maturation by undergoing oxidative coupling. However, the mechanisms and control of arabinoxylan cross-linking in vivo are unclear. Non-lignifying maize (Zea mays L.) cell cultures were incubated with l- [1- 3H]arabinose or (E)-[U- 14C]cinnamate (radiolabelling the pentosyl and feruloyl groups of endogenous arabinoxylans, respectively), or with exogenous feruloyl-[3H]arabinoxylans. The cross-linking rate of soluble extracellular arabinoxylans, monitored on Sepharose CL-2B, peaked suddenly and transiently, typically at ,9 days after subculture. This peak was not associated with appreciable changes in peroxidase activity, and was probably governed by fluctuations in H2O2 and/or inhibitors. De-esterified arabinoxylans failed to cross-link, supporting a role for the feruloyl ester groups. The cross-links were stable in vivo. Some of them also withstood mild alkaline conditions, indicating that they were not (only) based on ester bonds; however, most were cleaved by 6 m NaOH, which is a property of p- hydroxybenzyl,sugar ether bonds. Cross-linking of [14C]feruloyl-arabinoxylans also occurred in vitro, in the presence of endogenous peroxidases plus exogenous H2O2. During cross-linking, the feruloyl groups were oxidized, as shown by ultraviolet spectra and thin-layer chromatography. Esterified diferulates were minor oxidation products; major products were: (i) esterified oligoferulates, released by treatment with mild alkali; and (ii) phenolic components attached to polysaccharides via relatively alkali-stable (ether-like) bonds. Thus, feruloyl esters participate in polysaccharide cross-linking, but mainly by oligomerization rather than by dimerization. We propose that, after the oxidative coupling, strong p- hydroxybenzyl,polysaccharide ether bonds are formed via quinone-methide intermediates. [source] Synthesis and molecular structures of 1-boracyclopent-2-enesAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 5 2009Ezzat Khan Abstract The reaction of 1-silyl-1-borylalkenes with alkyn-1-yltin compounds affords borol-2-enes, organometallic-substituted allenes, mixtures thereof or even more complex mixtures with buta-1,3-dienes, depending on the third substituent at the CC bond (Bu or Ph), on the number of SiCl functions (two or three) and the nature of the alkyn-1-yltin compound. Six new borol-2-enes were isolated in pure state, and two of them were characterized by X-ray structural analysis. The solution-state structures of all major products were clearly established by multinuclear magnetic resonance methods (1H, 11B, 13C, 29Si, 119Sn NMR). Copyright © 2009 John Wiley & Sons, Ltd. [source] Rapid and simple method for determination of N, -(carboxymethyl)lysine and N, -(carboxyethyl)lysine in urine using gas chromatography[sol ]mass spectrometryBIOMEDICAL CHROMATOGRAPHY, Issue 9 2005Robert Petrovi Abstract A new procedure was developed to determine in urine the concentrations of N, -(carboxymethyl)lysine (CML) and N, -(carboxyethyl)lysine (CEL), the major products of oxidative modification of glycated proteins, to assess levels of oxidative stress in physiological systems. The urine samples were acetonitrile-deproteinized, then derivatized by ethylchloroformate, and N(O,S)-ethoxycarbonyl ethyl esters of amino acids were analysed by isotope dilution gas chromatography[sol ]mass spectrometry. Recovery averaged 89%. Linearity was excellent (r = 0.998,0.999) in the 0.5,25 µmol[sol ]L range for CML and CEL. The limit of detection of this assay was 0.1 µmol[sol ]L (corresponding to 0.20 pmol of CML or CEL on column). Intra-day and inter-day precisions were likewise excellent, with relative standard deviations <4.63 and <6.15%, respectively. Accuracy of CML and CEL determination (15 µmol[sol ]L) was 2.9 and 5.9% of the estimated theoretical value. The time from obtaining the urine sample to determination of the concentration from the chromatographic peak was 80 min or less. This method is sensitive, reproducible, accurate, relatively cheap and very simple. It can be useful for laboratories involved in the diagnosis and monitoring of age-related chronic diseases. Copyright © 2005 John Wiley & Sons, Ltd. [source] Production of Hydrogen from Dimethyl Ether over Supported Rhodium CatalystsCHEMCATCHEM, Issue 2 2009Gyula Halasi Abstract Infrared (IR) spectroscopy revealed that dimethyl ether (DME) undergoes partial dissociation on pure and rhodium-containing CeO2 at 300,K to yield methoxy and methyl species. This process is promoted by the presence of rhodium. By means of thermal desorption measurements (TPD), the adsorption of DME on Rh/CeO2 at 300,K and subsequent decomposition of DME (Tp,370,K), releasing H2, CO, CO2, and CH4, with Tp between 420 and 673,K, were ascertained. Rh/CeO2 is an effective catalyst for the decomposition of DME to give H2 (29,35,%), CO (27,30,%) and CH4 (32,38,%) as major products with complete conversion at 673,723,K. Adding water to DME changed the product distribution and increased the selectivity of H2 formation from 30,35,% to 58,% at 723,K. In,situ IR spectroscopy showed absorption bands of CO at 2034 and 1893,cm,1 during the reaction at 673,773,K. Deactivation of the catalyst did not occur at 773,K during the time measured (approximately 10 h). Rh deposited on carbon Norit also exhibited a high activity towards the decomposition of DME, but the selectivity towards hydrogen was lower. [source] Bifunctional-Thiourea-Catalyzed Diastereo- and Enantioselective Aza-Henry ReactionCHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2006Xuenong Xu Dr. Abstract Bifunctional thiourea 1,a catalyzes aza-Henry reaction of nitroalkanes with N -Boc-imines to give syn -,-nitroamines with good to high diastereo- and enantioselectivity. Apart from the catalyst, the reaction requires no additional reagents such as a Lewis acid or a Lewis base. The N-protecting groups of the imines have a determining effect on the chirality of the products, that is, the reaction of N -Boc-imines gives R adducts as major products, whereas the same reaction of N -phosphonoylimines furnishes the corresponding S adducts. Various types of nitroalkanes bearing aryl, alcohol, ether, and ester groups can be used as nucleophiles, providing access to a wide range of useful chiral building blocks in good yield and high enantiomeric excess. Synthetic versatility of the addition products is demonstrated by the transformation to chiral piperidine derivatives such as CP-99,994. [source] Aldol Additions of Dihydroxyacetone Phosphate to N -Cbz-Amino Aldehydes Catalyzed by L -Fuculose-1-Phosphate Aldolase in Emulsion Systems: Inversion of Stereoselectivity as a Function of the Acceptor AldehydeCHEMISTRY - A EUROPEAN JOURNAL, Issue 5 2005Laia Espelt Dr. Abstract The potential of L -fuculose-1-phosphate aldolase (FucA) as a catalyst for the asymmetric aldol addition of dihydroxyacetone phosphate (DHAP) to N -protected amino aldehydes has been investigated. First, the reaction was studied in both emulsion systems and conventional dimethylformamide (DMF)/H2O (1:4 v/v) mixtures. At 100,mM DHAP, compared with the reactions in the DMF/H2O (1:4) mixture, the use of emulsion systems led to two- to three-fold improvements in the conversions of the FucA-catalyzed reactions. The N -protected aminopolyols thus obtained were converted to iminocyclitols by reductive amination with Pd/C. This reaction was highly diastereoselective with the exception of the reaction of the aldol adduct formed from (S)- N -Cbz-alaninal, which gave a 55:45 mixture of both epimers. From the stereochemical analysis of the resulting iminocyclitols, it was concluded that the stereoselectivity of the FucA-catalyzed reaction depended upon the structure of the N -Cbz-amino aldehyde acceptor. Whereas the enzymatic aldol reaction with both enantiomers of N -Cbz-alaninal exclusively gave the expected 3R,4R configuration, the stereochemistry at the C-4 position of the major aldol adducts produced in the reactions with N -Cbz-glycinal and N -Cbz-3-aminopropanal was inverted to the 3R,4S configuration. The study of the FucA-catalyzed addition of DHAP to phenylacetaldehyde and benzyloxyacetaldehyde revealed that the 4R product was kinetically favored, but rapidly disappeared in favor of the 4S diastereoisomer. Computational models were generated for the situations before and after CC bond formation in the active site of FucA. Moreover, the lowest-energy conformations of each pair of the resulting epimeric adducts were determined. The data show that the products with a 3R,4S configuration were thermodynamically more stable and, therefore, the major products formed, in agreement with the experimental results. [source] Facile Oxidation of Leucomethylene Blue and Dihydroflavins by Artemisinins: Relationship with Flavoenzyme Function and Antimalarial Mechanism of ActionCHEMMEDCHEM, Issue 8 2010Richard Abstract The antimalarial drug methylene blue (MB) affects the redox behaviour of parasite flavin-dependent disulfide reductases such as glutathione reductase (GR) that control oxidative stress in the malaria parasite. The reduced flavin adenine dinucleotide cofactor FADH2 initiates reduction to leucomethylene blue (LMB), which is oxidised by oxygen to generate reactive oxygen species (ROS) and MB. MB then acts as a subversive substrate for NADPH normally required to regenerate FADH2 for enzyme function. The synergism between MB and the peroxidic antimalarial artemisinin derivative artesunate suggests that artemisinins have a complementary mode of action. We find that artemisinins are transformed by LMB generated from MB and ascorbic acid (AA) or N -benzyldihydronicotinamide (BNAH) in,situ in aqueous buffer at physiological pH into single electron transfer (SET) rearrangement products or two-electron reduction products, the latter of which dominates with BNAH. Neither AA nor BNAH alone affects the artemisinins. The AA,MB SET reactions are enhanced under aerobic conditions, and the major products obtained here are structurally closely related to one such product already reported to form in an intracellular medium. A ketyl arising via SET with the artemisinin is invoked to explain their formation. Dihydroflavins generated from riboflavin (RF) and FAD by pretreatment with sodium dithionite are rapidly oxidised by artemisinin to the parent flavins. When catalytic amounts of RF, FAD, and other flavins are reduced in,situ by excess BNAH or NAD(P)H in the presence of the artemisinins in the aqueous buffer, they are rapidly oxidised to the parent flavins with concomitant formation of two-electron reduction products from the artemisinins; regeneration of the reduced flavin by excess reductant maintains a catalytic cycle until the artemisinin is consumed. In preliminary experiments, we show that NADPH consumption in yeast GR with redox behaviour similar to that of parasite GR is enhanced by artemisinins, especially under aerobic conditions. Recombinant human GR is not affected. Artemisinins thus may act as antimalarial drugs by perturbing the redox balance within the malaria parasite, both by oxidising FADH2 in parasite GR or other parasite flavoenzymes, and by initiating autoxidation of the dihydroflavin by oxygen with generation of ROS. Reduction of the artemisinin is proposed to occur via hydride transfer from LMB or the dihydroflavin to O1 of the peroxide. This hitherto unrecorded reactivity profile conforms with known structure,activity relationships of artemisinins, is consistent with their known ability to generate ROS in,vivo, and explains the synergism between artemisinins and redox-active antimalarial drugs such as MB and doxorubicin. As the artemisinins appear to be relatively inert towards human GR, a putative model that accounts for the selective potency of artemisinins towards the malaria parasite also becomes apparent. Decisively, ferrous iron or carbon-centered free radicals cannot be involved, and the reactivity described herein reconciles disparate observations that are incompatible with the ferrous iron,carbon radical hypothesis for antimalarial mechanism of action. Finally, the urgent enquiry into the emerging resistance of the malaria parasite to artemisinins may now in one part address the possibilities either of structural changes taking place in parasite flavoenzymes that render the flavin cofactor less accessible to artemisinins or of an enhancement in the ability to use intra-erythrocytic human disulfide reductases required for maintenance of parasite redox balance. [source] Boron (III) Tribromide or Titanium (IV) Bromide and Lewis Base Promoted Baylis-Hillman ReactionCHINESE JOURNAL OF CHEMISTRY, Issue 3 2002Min Shi Abstract It was found that, when the Baylis-Hillman reaction of arylaldehydes with methyl vinyl ketone was carried out at below -20 °C in the presence of boron (III) tribromide or titanium (IV) bromide using a catalytic amount of Lewis base such as amine, the brominated compounds and the Baylis-Hillman adducts could be obtained as the major products in good yields for various aryl aldehydes. But at room temperature, the elimination products were the major products. In addition, the palladium catalyzed allylic substitution reactions of the elimination products were also examined. [source] Photolysis of oxygen saturated ethers in the presence of Sn(II) or Cu(II) saltsCHINESE JOURNAL OF CHEMISTRY, Issue 6 2000Min Shi Abstract Photolysis of diethyl ether-oxygen charge transfer complex in the presence of Sn(II) or Cu(II) salts gave higher yields of the oxidation products, ethyl acetate, acetaldehyde, ethanol, ethyl formate and methanol compared with those without the salts. In addition, the photolysis of an oxygen saturated tetrahydrofuran (THF) or dibutyl ether solution gave ,-butyrolactone or butanol and butyl butyrate as major products with small amounts of undetermined compounds, respectively. Their yields were also affected by the addition of Cu(II) or Sn(II) salts. [source] | ||||||||||||||||||||||||||||