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排序方式: 共有409条查询结果,搜索用时 140 毫秒
351.
Rami Suleiman 《Tetrahedron letters》2010,51(24):3211-541
The alkoxycarbonylation of α,β-unsaturated amides proceeded efficiently and regioselectivity to give ω-amido esters with complete conversion in the presence of the catalyst system: Pd(PPh3)2Cl2/MeOH/CO/H2O. The reaction was successfully applied to the alkoxycarbonylation of bis-acrylamides yielding, selectively, the corresponding di-ω-amido esters. These mono and di-ω-amido esters have been used as precursors for the synthesis of N-substituted cyclic succinimides in moderate to high yields. 相似文献
352.
Jaisiel Meléndez 《Tetrahedron letters》2006,47(7):1187-1191
Diols 2, easily prepared by a DTBB-catalysed lithiation of the dithioether 1 in the presence of different carbonyl compounds, react with ozone in dichloromethane at −78 °C leading, after treatment with thiourea at 20 °C, to the corresponding substituted 1,6-dioxaspiro[4.4]nonanes 3. 相似文献
353.
Thermal degradation and fire resistance of unsaturated polyester, modified acrylic resins and their composites with natural fibres 总被引:1,自引:0,他引:1
The thermal degradation and fire resistance of different natural fibre composites were studied. Unsaturated polyester (UP) and modified acrylic resins (Modar) were used as matrix composites. The smoke emission of the materials was also analysed, as well as, the performance against the fire of the biocomposites and glass reinforced composites was compared. Thermal degradation indicated that the Modar matrix composites were more resistant to temperature than the composites with UP matrix. Flax fibre, due to their low lignin content, exhibit the best thermal resistance among the natural fibres studied.From the results obtained about the thermal and fire resistance of the composites it is possible to conclude that the flax fibre seems to be the most adequate to be used, due to the long time to ignition and the long period prior to reach the flashover. On the other hand, the jute fibre composites showed a short duration but a quick growing fire with the lowest smoke emission. The low smoke is an important advantage, which reduces one of the main hazards of fire. 相似文献
354.
《Numerical Methods for Partial Differential Equations》2018,34(3):857-880
We consider a degenerate parabolic system modeling the flow of fresh and saltwater in a porous medium in the context of seawater intrusion. We propose and analyze a finite volume scheme based on two‐point flux approximation with upwind mobilities. The scheme preserves at the discrete level the main features of the continuous problem, namely the nonnegativity of the solutions, the decay of the energy and the control of the entropy and its dissipation. Based on these nonlinear stability results, we show that the scheme converges toward a weak solution to the problem. Numerical results are provided to illustrate the behavior of the model and of the scheme. 相似文献
355.
建立二维非饱和水流问题的全离散广义差分格式,讨论了全离散广义差分解的存在唯一性,并给出最优误差估计的证明.最后给出数值算例,验证方法的有效性. 相似文献
356.
对均质土坝在水位下降时坝体的渗流和坝坡稳定性进行了分析,坝体渗流分析时采用有限单元法.坝坡稳定性分析时采用条分法.发挥有限单元法和条分法两个方法的长处,并结合运用,可供工程设计人员参考使用. 相似文献
357.
Marianne Nofz Hans-Georg Bartel Jens Rainer Lochmann Werner Haberditzl 《Monatshefte für Chemie / Chemical Monthly》1982,113(6-7):645-649
The procedure derived in1–4 was used to calculate the magnetic susceptibilities of a series of unsaturated hydrocarbons (propene, isobutene, 1-butene, cyclohexene, butadiene,trans-1,3,5-hexatriene). The difference between the measured and computed susceptibilities of these molecules is discussed. The discussion shows, that there is no correlation existing between the difference and the presumed delocalisation effects.
Die Ergebnisse sind Bestandteil einer der letzten Arbeiten, die von dem von uns hochverehrten, leider zu früh verstorbenen, Prof. Dr.W. Haberditzl betreut wurden. 相似文献
358.
Mohd A. A. Ansari 《Proceedings Mathematical Sciences》1980,89(2):125-132
The generalising dispersion equations of flow through porous media have been investigated. The Laplace transform has been
applied to obtain the solution to dispersion problem as a result of adsorption. The generalised closed form solution for dispersion
has been presented and the different types of variations in concentration have been graphically discussed. When the steady
state occurs, the concentration becomes constant but for small value of time (say 0.5) the concentration tends to zero as
distance increases. 相似文献
359.
Cecilia Gómez 《Tetrahedron》2005,61(39):9325-9330
The reaction of (Z)- or (E)-1,2-bis(phenylsulfanyl)ethene (1) with an excess of lithium and a catalytic amount of 4,4′-di-tert-butylbiphenyl (DTBB, 2.5 mol%) in the presence of a carbonyl compound as electrophile (Barbier conditions) in THF at −78 °C leads, after hydrolysis with water at temperatures ranging between −78 °C and rt, to a mixture of the corresponding (Z/E)-unsaturated 1,4-diols 2, the diastereomers ratio being independent of the stereochemistry of the starting materials. Allylic alcohols 3 are the main by-products, resulting from a lithium-hydrogen exchange on some of the lithiated intermediates along the whole process. A mechanistic explanation for the observed behaviour is given. 相似文献
360.
《Electroanalysis》2003,15(20):1625-1631
This article describes the formation of a SAM with chemisorbed cysteine to a gold surface by the thiol group to obtain a surface electrode with an amino and an acid group free for later reaction and accumulation with other molecules on the electrode surface. We explore the accumulation of unsaturated fatty acid and the electrochemical response of the electrode after modification with cysteine. The electrochemical study confirmed the accumulation of linoleic acid on the modified electrode. The optimum conditions for the determination of linoleic acid by differential pulse voltammetry of linoleic acid are studied a detection limit (3σ) of 0.03 μg mL?1 and a determination limit (10σ) of 0.10 μg mL?1 were obtained and applied to determination in olive oil and ham from Iberian breed hams. 相似文献