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排序方式: 共有473条查询结果,搜索用时 31 毫秒
401.
Jiang H Nielsen JB Nielsen M Jørgensen KA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(32):9068-9075
An easy and affordable route for obtaining chiral beta-aminated- and alpha,beta-diaminated aldehydes, 1,3-aminoalcohols, and related compounds by using organocatalysis is presented. The chiral secondary amine (S)-2-[bis(3,5-bistrifluoromethylphenyl)trimethylsilanyloxymethyl]pyrrolidine is used as the catalyst to activate alpha,beta-unsaturated aldehydes, which allows succinimide to add in a 1,4-regio- and stereoselective fashion thereby forming N-protected 1,3-aminoaldehydes in good yields and enantioselectivities. This is followed by two easy transformations giving rise to optically active 1,3-aminoalcohols, a common motif in many biologically active compounds, for example, fibrinogen receptor antagonists. Furthermore, optically active alpha,beta-syn-diaminated aldehydes were obtained by the addition of diethyl azodicarboxylate in a one-pot reaction. 相似文献
402.
Ying JW Cordova A Ren TY Xu GL Ren T 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(24):6874-6882
Conditions to prepare trans-[Ru2(dmba)4(C[triple chemical bond]CAr)2] from [Ru2(dmba)4(NO(3))2] (DMBA=N,N'-dimethylbenzamidinate) and HC[triple chemical bond]CAr were optimized; Et2NH was found to be the most effective among a number of weak bases in facilitating the product formation. Furthermore, a series of unsymmetric trans-[(ArC[triple chemical bond]C)Ru(2)(dmba)4(C[triple chemical bond]CAr')] compounds were prepared under optimized conditions, in which one or both of Ar and Ar' are donor (NMe2)-/acceptor (NO(2))-substituted phenyls. While the X-ray crystallographic studies revealed a minimal structural effect upon donor/acceptor substitution, voltammetric measurements indicated a significant influence of substituents on the energy level of frontier orbitals. In particular, placing a donor and an acceptor on the opposite ends of trans-[(ArC[triple chemical bond]C)Ru2(dmba)4(C[triple chemical bond]CAr')] moiety results in an energetic alignment of frontier orbitals that favors a directional electron flow, a necessary condition for unimolecular rectification. 相似文献
403.
Javier Lpez-Andarias Krikor Eblighatian Quentin T. L. Pasquer Lea Assies Naomi Sakai Sascha Hoogendoorn Stefan Matile 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2022,134(1):e202113163
Mechanosensitive flipper probes are attracting interest as fluorescent reporters of membrane order and tension in biological systems. We introduce PhotoFlippers, which contain a photocleavable linker and an ultralong tether between mechanophore and various targeting motifs. Upon irradiation, the original probe is released and labels the most ordered membrane that is accessible by intermembrane transfer. Spatiotemporal control from photocleavable flippers is essential to access open, dynamic or elusive membrane motifs without chemical or physical interference. For instance, fast release with light is shown to place the original small-molecule probes into the innermost leaflet of the nuclear envelope to image changes in membrane tension, at specific points in time of membrane trafficking along the secretory pathway, or in the inner leaflet of the plasma membrane to explore membrane asymmetry. These results identify PhotoFlippers as useful chemistry tools to enable research in biology. 相似文献
404.
Qihao Jiang Sohan Seth Theresa Scharl Tim Schroeder Alois Jungbauer Simone Dimartino 《Journal of separation science》2022,45(8):1445-1457
Pre-packed columns have been increasingly used in process development and biomanufacturing thanks to their ease of use and consistency. Traditionally, packing quality is predicted through rate models, which require extensive calibration efforts through independent experiments to determine relevant mass transfer and kinetic rate constants. Here we propose machine learning as a complementary predictive tool for column performance. A machine learning algorithm, extreme gradient boosting, was applied to a large data set of packing quality (plate height and asymmetry) for pre-packed columns as a function of quantitative parameters (column length, column diameter, and particle size) and qualitative attributes (backbone and functional mode). The machine learning model offered excellent predictive capabilities for the plate height and the asymmetry (90 and 93%, respectively), with packing quality strongly influenced by backbone (∼70% relative importance) and functional mode (∼15% relative importance), well above all other quantitative column parameters. The results highlight the ability of machine learning to provide reliable predictions of column performance from simple, generic parameters, including strategic qualitative parameters such as backbone and functionality, usually excluded from quantitative considerations. Our results will guide further efforts in column optimization, for example, by focusing on improvements of backbone and functional mode to obtain optimized packings. 相似文献
405.
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407.
One interesting coordination polymer, [Zn2(1,2,4-BTC)(OH)(H2O)2]2·2H2O 1, has been synthesized from 1,2,4-BTC (1,2,4-BTC = 1,2,4-bentricarboxylate) under hydrothermal conditions and characterized by elemental analyses, IR, TG and single-crystal X-ray diffraction. Complex I crystallizes in triclinic, space group P^-1, with a = 6.5200(13), b = 9,0600(18), c = 10.968(2) A^°, α = 111.55(3), β = 92.07(3),γ= 95.03(3)°, C9H10O10Zn2, Mr = 408.91, V= 598.7(2) A^°^3, Dc = 2.268 g/cm^3, F(000) = 408 and Z = 2. X-ray diffraction analysis reveals that complex 1 is a three-dimensional network built from tetranuclear Zn(Ⅱ) building unit. In this complex, the Zn4 unit is an eight-connected knot, while 1,2,4-BTC a four-connected knot. This results in a CaF2 topology. To the best of our knowledge, such Zn4 unit is the first 8-connected building block built from asymmetry ligand. 相似文献
408.
本文修正了Richaudeau(1999)提出的保障-风险条件相关模型,考虑到索赔次数中的"零膨胀"现象,采用零膨胀Poisson分布拟合索赔次数,以我国汽车商业第三者责任保险作为研究对象,研究了中国车险市场的信息不对称问题。实证结果表明,在控制公开信息的基础上,我国汽车保险市场仍存在显著的信息不对称问题。但是,保险公司可以通过费率厘定、无赔款优待制度、附加险设计等方法分离不同风险的投保人,减轻信息不对称程度对公司经营的影响。 相似文献
409.
论文针对沪深股市牛熊市中所呈现出的波动非对称性的差异,从牛熊市中投资者对信息反应的差异角度予以解释。论文以非预期交易量变化率作为投资者对信息冲击反应的代理变量,研究显示投资者在牛市行情中的过度反应,是造成沪深股市牛市行情波动正向非对称性的重要原因。与此同时论文通过对比美国、香港和沪深股市上牛熊市波动非对称性差异,进一步验证沪深市场上不完善的市场机制加剧了投资者在牛市行情中的过度反应,进而导致牛市行情中的波动正向非对称性。 相似文献
410.
田成诗 《数学的实践与认识》2012,42(9):12-18
利用EGARCH模型对我国部分具有代表性的分类商品零售价格波动的信息效应进行了实证分析.分析结果显示,我国分类商品零售价格波动特征是不同的.在六个具有代表性的分类商品零售价格指数中,有四个指数的方差具有时变性特征.在四个当中,有三个指数有非对称信息效应,即非期望的价格上涨或下降信息对价格波动的影响是非对称的.另外的两个价格指数的方差为常数,价格波动稳定. 相似文献