共查询到20条相似文献,搜索用时 31 毫秒
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2.
Louis A. Carpino Eberhard Krause Calin Dan Sferdean Heinz Fabian Michael Beyermann 《Tetrahedron letters》2004,45(40):7519-7523
Recently a 26-mer peptide 1 incorporating Ser and Thr was described as a ‘difficult’ sequence that could not be synthesized by standard methods. If the first Ser residue was used to incorporate a depsipeptide unit, the resulting hybrid was readily assembled. The 26-mer ester was then converted to the native peptide by an O→N acyl shift. The technique may be general for other systems containing appropriate Ser and Thr units and was demonstrated here for the case of the amyloid peptide Aβ(1-42). 相似文献
3.
N. M. Kolyadina A. T. Soldatenkov S. Soro B. N. Anisimov N. S. Prostakov 《Chemistry of Heterocyclic Compounds》1998,34(9):1066-1069
The 9-(-pyridylmethylene)-4-azafluorene was obtained as the mixture of the Z- and E-isomers. Condensation of 4-azafluorene with methiodides of 3-formylpyridine afforded the methiodides of these isomers which were isolated in discrete form. Their configuration was established. It was established using the example of the reduction of the methiodide of the Z-isomer by sodium borohydride that the -pyridinium portion of this quaternary salt is completely reduced to the piperidyl.Russian University of the Friendship of Peoples, Moscow 117923. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1243–1246, September, 1998. 相似文献
4.
The kinetic regularities of the liquid-phase hydrogenation and isomerization of α- and β-pinenes over the Pd/C, Ru/C, Rh/C,
Pt/C, and Ir/C catalysts were studied at temperatures ranging from 20 to 100 °C and at hydrogen pressures of 1–11 bar using
n-octane as the solvent. The hydrogenation and isomerization of α- and β-pinenes occur simultaneously on the Ru/C, Rh/C, Pt/C,
and Ir/C catalysts, and the reaction mixture contains the products of double bond hydrogenation, viz., cis- and trans-pinanes. The Ru, Rh, and Pd metals have a higher catalytic activity in β-pinene isomerization than Ir and Pt. Among the VIII
Group metals studied, the Pd-based catalyst has the highest catalytic activity in double bond isomerization of α- and β-pinenes.
The general scheme of the mechanism of hydrogenation and isomerization of α- and β-pinenes on the Pd/C catalyst was proposed. 相似文献
5.
Ryoko Yasumura Kumar Dilip Ashtekar Akio Tonouchi Tatsuo Nehira Babak Borhan Masaru Hashimoto 《Tetrahedron》2013
Novel 7-β- and 10-β-hydroxylated congeners of CAF-603 (4 and 5, respectively) were isolated from Trichoderma crassum. Relative configuration of 4 was determined by NOE experiments. Stereochemistry of 7-β-hydroxy group in 5 was established by a combination of vicinal 3JH–H analyses and quantum chemical calculations. Absolute configurations of 4 and 5 were established by observing the Cotton effects of their bis(2-naphthoate) esters. T. crassum also produce known CAF-603 (1), 14-hydroxy and 8,9-β-epoxy derivatives (2 and 3, respectively). Present study first established the absolute configuration of CAF-603 family by taking their biosyntheses into account. Preliminary biological experiments revealed that the molecular target strictly recognizes the β-side of the cycloheptane ring. 相似文献
6.
Khalid M. Tawarah Hakam M. Abu-Shamleh 《Journal of inclusion phenomena and macrocyclic chemistry》1991,11(1):29-40
The color fading caused by the addition of-cyclodextrin or-cyclodextrin to an aqueous solution of a tautomeric mixture of methyl orange or methyl yellow is studied spectrophotometrically at pH 1.1 and 25.0°C. A model involving 1 : 1 stoichiometry has been used to analyze the spectrophotometric data. The addition of a cyclodextrin shifts the tautomeric mixture towards the side of the ammonium tautomer. An expression allowing the calculation of the tautomeric equilibrium constant of the inclusion complexes is derived. The formation constants of the inclusion complexes of the individual tautomers are determined. Both- and-cyclodextrins bind the ammonium tautomer stronger than the azonium tautomer. The inclusion complexes of-cyclodextrin are more stable than the corresponding ones of-cyclodextrin. 相似文献
7.
André Ricard Jean-Philippe Sarrette Soo-Ghee Oh Yu Kwon Kim 《Plasma Chemistry and Plasma Processing》2016,36(6):1559-1570
We report a detailed comparison between RF and microwave (HF) plasmas of N2 and Ar–20 %N2 as well as in the corresponding afterglows by comparing densities of active species at nearly the same discharge conditions of tube diameter (5–6 mm), gas pressure (6–8 Torr), flow rate (0.6–1.0 slm) and applied power (50–150 W). The analysis reveals an interesting difference between the two cases; the length of the RF plasma (~25 cm) is measured to be much longer than that of HF (6 cm). This ensures a much longer residence time (10?2 s) of the active species in the N2 RF plasma [compared to that (10?3 s) of HF], providing a condition for an efficient vibrational excitation of N2(X, v) by (V–V) climbing-up processes, making the RF plasma more vibrationally excited than the HF one. As a result of high V–V plasma excitation in RF, the densities of the vibrationally excited N2(X, v > 13) molecules are higher in the RF afterglow than in the HF afterglow. Destruction of N2(X, v) due to the tube wall is estimated to be very similar between the two system as can be inferred from the γv destruction probability of N2(X, v > 3–13) on the tube wall (2–3 × 10?3 for both cases) obtained from a comparison between the density of N2(X, v > 3–9) in the plasmas to that of the N2(X, v > 13) in the long afterglows. Interestingly enough, densities of N-atoms and N2(A) metastable molecules in the afterglow regions, however, are measured to be very similar with each other. The measured lower density of N2 + ions than expected in the HF afterglow is rationalized from a high oxygen impurity in our HF setup since N2 + ions are very sensitive to oxygen impurity . 相似文献
8.
Galina S. Zakharova Christina Schmidt Alexander Ottmann Ewa Mijowska Rüdiger Klingeler 《Journal of Solid State Electrochemistry》2018,22(12):3651-3661
Two modifications of molybdenum trioxide with orthorhombic (α-MoO3) and hexagonal (h-MoO3) crystal structure have been synthesized by a microwave-assisted hydrothermal method, facilitated by formic acid. Characterization by means of X-ray diffraction, scanning electron microscopy, specific surface analysis, and Fourier-transform infrared, Raman, and UV-Vis spectroscopy reveals phase-pure crystalline powder samples of hexagonal h-MoO3 microrods and of α-MoO3 nanobelt bundles, respectively. The electrochemical properties of the MoO3 compounds, studied by cyclic voltammetry and galvanostatic cycling vs. Li/Li+, strongly depend on the structure and the applied potential range. In the range of 1.5–3.5 V, Li+-ions can be reversibly intercalated into the α-MoO3 nanobelts. Utilizing the material in this way as intercalation cathode material yields an initial discharge capacity of 295 mA h g?1 at 100 mA g?1 and comparably moderate capacity fading of 25% between cycles 20 and 100. Extending the potential range to 0.01–3.0 V induces the conversion reaction to Mo, which for both modifications yields high initial capacities of around 1500 mA h g?1 but is associated with much stronger capacity fading. 相似文献
9.
ZHU Chuan-Zheng YANG Bao-Lin ZHAO Hong GU Wen-Xiu AI Shi-YunDepartment of Chemistry East China Normal University Shanghai China 《中国化学》1997,15(1):68-75
The dissociation constant of each step for TB-chlorosulphophenol has been determined by potentiometric method, and the thermodynamic constants, △G°, △H° and △S°, of the dissociation process have been calculated. The protonation constants were measured by the spectrophotometric method. The pH values of various forms of anions of the chromogenic reagent at their concentrations were also calculated. 相似文献
10.
L. T. Denisova Yu. F. Kargin L. T. Chumilina V. M. Denisov K. S. Gavrichev M. A. Ryumin A. V. Tyurin 《Russian Journal of Inorganic Chemistry》2016,61(1):1-6
The heat capacity of TbVO4 has been measured by the adiabatic calorimetry (5–346 K) and differential scanning calorimetry (344–859 K) methods. The Cp = f(T) plot has an extreme point (32 K). The thermodynamic properties of the oxide compounds have been calculated from the experimental data. A general equation that describes the heat capacity of terbium orthovanadate as a function of temperature in the range 35–859 K has been derived. 相似文献
11.
Xiao-Qing Yang Li-Jun Yang Ka-Ma Huang Wen-Yan Tian Hui Shang 《Journal of solution chemistry》2010,39(6):849-856
This paper focuses on the measurement of the permittivity of dimethyl sulfoxide (DMSO)–water (H2O) mixture solutions, at 2.45 GHz by using a resonant cavity perturbation method. A specific phenomenon was found, in that
the imaginary part of the permittivity for the mixture solution was larger than the imaginary part for each component. Theoretical
calculation indicated that the reason for that phenomenon was that the high frequency friction of the mixture was larger than
that of each component. When comparing the theoretical results with the experimental data, it was found that the classical
Debye equation must be modified in order to calculate the complex permittivity. 相似文献
12.
The crystal structure of SrCeCuS3, a complex sulfide synthesized for the first time, has been solved using X-ray powder diffraction data. The crystals are orthorhombic, space group Pnma. SrCeCuS3 has two polymorphs: the high-temperature phase of Ba2MnS3 structural type (a = 8.1393(3) Å, b = 4.0587(2) Å, c = 15.9661(2) Å) and the low-temperature phase isostructural to BaLaCuS3 (a = 11.1626(2) Å, b = 4.0970(2) Å, c = 11.5307(1) Å). The incongruent melting temperature and enthalpy of SrCeCuS3 are, respectively, 1486 ± 3 K and 13.4 ± 1.5 J/g. 相似文献
13.
Leszek Rycerz Ewa Ingier-Stocka Slobodan Gadzuric Marcelle Gaune-Escard 《Journal of Thermal Analysis and Calorimetry》2010,101(2):455-461
Differential Scanning Calorimetry was used to study phase equilibrium in EuBr2–RbBr binary system. It was established that this system includes two eutectics and three stoichiometric compounds. First
of them, Rb2EuBr4, decomposes peritectically at 778 K. Second one, RbEuBr3, undergoes the solid–solid phase transition at 732 K and melts incongruently at 852 K. Third compound, RbEu2Br5, melts congruently at 888 K. The composition and temperature values of eutectics were determined as x(EuBr2) = 0.316; T
eut = 776 K and x(EuBr2) = 0.797; T
eut = 859 K. Mixing enthalpy was measured by direct calorimetry on the whole composition range. The minimum of the mixing enthalpy
occurs around the composition x(EuBr2) ≈ 0.4. The electrical conductivity of liquid mixtures was also investigated over the whole composition range and measured
down to temperatures below solidification. The specific conductance (liquid phase) plotted against the mole fraction of EuBr2 shows a broad minimum at x(EuBr2) ~ 0.6. The activation energy for conductivity changes with temperature. Results obtained are discussed in terms of possible
complex formation. 相似文献
14.
Yitao Yu Qingsi Zhang Peng Li Leili Liu Meng Zhang Haifeng Sun 《Research on Chemical Intermediates》2009,35(2):209-217
A new crosslinker, N-[(1,1-dimethyl-2-acetyl)ethyl]-β-dihydroxyethylamine (DDP), which contains two hydroxyl groups and carbonyl, was successfully
synthesized by Michael addition reaction between diacetone acrylamide (DAAM) and diethanolamine (DEA) in different solvents.
This crosslinker could crosslink during film formation through a reaction between ketone and hydrazide. The structure of DDP
was characterized by IR and NMR, and effects of solvents, temperature, and reaction time on the yield of DDP were determined.
The reaction kinetics including the order of the reaction and apparent activation energy were also investigated. Under the
experimental conditions chosen (temperature 90 °C, reaction time 12 h), the crosslinker was obtained in 75.28% yield. The
reaction was a second-order reaction and the apparent activation energy was 91.0 kJ mol−1. The reaction rate constants were 0.11 mol−1 dm3 h−1 at 90 °C and 0.03 mol−1 dm3 h−1 at 75 °C. 相似文献
15.
Jianjun ZhangFanzuo Kong 《Tetrahedron》2003,59(9):1429-1441
A general method has been developed for the synthesis of oligosaccharides consisting of (1→2)- and (1→3)-linked rhamnans with GlcNAc side chains. As examples, highly effective and convergent syntheses of two decasaccharides in the O polysaccharide moiety of the lipopolysaccharide of the phytopathogenic bacterium Pseudomonas syringae pv. ribicola NCPPB 1010 were achieved. The two decasaccharides consist of O polysaccharide repeating units I+II and II+I, respectively. Allyl 3-O-acetyl-4-O-benzoyl-α-l-rhamnopyranoside, allyl 2-O-benzoyl-3-O-chloroacetyl-α-l-rhamnopyranoside, 2,4-di-O-benzoyl-3-O-chloroacetyl-α-l-rhamnopyranosyl trichloroacetimidate, and 3-O-acetyl-2,4-di-O-benzoyl-α-l-rhamnopyranosyl trichloroacetimidate, which were obtained by highly regioselective 3-O-acylations, were used as the key synthons to obtain the required α-(1→2)- and α-(1→3)-linked rhamnoocta saccharide acceptors with 33- and 37-free hydroxyl groups. Therefore, several disaccharides were synthesized, from which tetrasaccharides and hexasaccharides were then synthesized. Coupling of the hexasaccharide donors with the disaccharide acceptors gave the octasaccharide acceptors. Finally, the coupling of 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-β-d-glucopyranosyl trichloroacetimidate with the octasaccharide acceptors, followed by deprotection, afforded the two target decasaccharides. A repeating hexasaccharide unit of the cell wall polysaccharide of β-hemolytic Streptococci Group A was also synthesized in a similar way. 相似文献
16.
T.L. Egorova M.V. Kalinina E.P. Simonenko N.P. Simonenko G.P. Kopitsa O.V. Glumov N.A. Mel’nikova I.V. Murin L. Almásy O.A. Shilova 《Russian Journal of Inorganic Chemistry》2017,62(10):1275-1285
Two alternative chemical synthesis methods—cryotechnological coprecipitation of hydroxides and cocrystallization of salts—were used for preparing (CeO2)1–x (Y2O3) x nanopowders (x = 0.10, 0.15, 0.20) with a mean coherent scattering domain size of ~7–11 nm and S sp = 2.1–97.5 m2/g. From these nanopowders, ceramic nanomaterials with mean coherent scattering domain sizes of ~61–85 nm were synthesized. It was studied how the phase composition, microstructure, and electrical transport properties of the produced samples depend on the Y2O3 content of a CeO2-based solid solution and on the synthesis method. It was shown that, in the series (CeO2)1–x (Y2O3) x (x = 0.10, 0.15, 0.20), the solid solution (CeO2)0.90(Y2O3)0.10 has the highest ionic conductivity with the ion transport number t i = 0.73 (600°C). In its physicochemical characteristics, this ceramic can be used as a solid electrolyte of intermediate-temperature fuel cells. 相似文献
17.
N. Mabrouk H. Berriche 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2017,91(8):1474-1485
Calculations of the adiabatic potential energy curves and the transition dipole moments between the ground (A1Σ+) and the first excited (A1Σ+) states have been determined for the LiCs and NaCs molecules. The calculations are performed using an ab initio approach based on non-empirical pseudopotentials for Cs+, Li+ and Na+ cores, parameterized l-dependent polarization potentials and full configuration interaction calculations. The potential energy curves and the transition dipole moment are used to estimate the radiative lifetimes of the vibrational levels of the A+Σ+ state using the Franck–Condon (FC) approximation and the approximate sum rule method. The radiative lifetimes associated with the A+Σ+ state are presented here for the first time. These data can help experimentalists to optimize photoassociative formation of ultracold molecules and their longevity in a trap or in an optical lattice. 相似文献
18.
Russian Chemical Bulletin - 相似文献
19.
The conformational stabilities of the α- and β-substituted enamines and vinyl ethers were predicted by orbital phase theory
and confirmed by ab initio molecular orbital calculations. Cyclic interaction significantly occurs among the nonbonding orbital
n
Y for the lone pair on the hetero atom Y (N in the enamines or O in the ethers), the π and π* orbitals of the CC bond, and the
σC-H or σ*C-X orbitals on the substituent CH2X. The cyclic -n
Y-π-σC-H-π*- interaction is favored by the orbital phase continuity in the α-substituted molecules, while the cyclic -n
Y-π-σ*C-X-π*- interaction is favored in the β-substituted molecules. The most stable conformation was then predicted to be synperiplanar
or (pseudo)equatorial in the α-substituted molecules and anticlinical or (pseudo)axial in the β-substituted molecules.
Received: 8 May 1998 / Accepted: 30 July 1998 / Published online: 16 November 1998 相似文献
20.
A. A. Komlev V. V. Panchuk V. G. Semenov O. V. Almjasheva V. V. Gusarov 《Russian Journal of Applied Chemistry》2016,89(12):1932-1938
Specific features of the process in which oxide nanopowders are formed in a hydrothermal treatment of coprecipitated magnesium and iron oxides were studied. It was shown that the rate at which oxide nanoparticles are formed increases when reagents structurally close to the final product are used. It was found that, with the hydrothermal treatment of coprecipitated magnesium and iron hydroxides at 450°C combined with the subsequent thermal treatment in air at temperatures of 400–600°C, it is possible to obtain a homogeneous mixture of nanocrystalline powders based on an iron-containing spinel phase and magnesium oxide. 相似文献