Synthesis of macrocyclic natural products by catalyst-controlled stereoselective ring-closing metathesis

Many natural products contain a C = C double bond through which various other derivatives can be prepared; the stereochemical identity of the alkene can be critical to the biological activities of such molecules. Catalytic ring-closing metathesis (RCM) is a widely used method for the synthesis of large unsaturated rings; however, cyclizations often proceed without control of alkene stereochemistry. This shortcoming is particularly costly when the cyclization reaction is performed after a long sequence of other chemical transformations. Here we outline a reliable, practical and general approach for the efficient and highly stereoselective synthesis of macrocyclic alkenes by catalytic RCM; transformations deliver up to 97% of the Z isomer owing to control induced by a tungsten-based alkylidene. Utility is demonstrated through the stereoselective preparation of epothilone C (refs 3-5) and nakadomarin A (ref. 6), the previously reported syntheses of which have been marred by late-stage, non-selective RCM. The tungsten alkylidene can be manipulated in air, delivering the products in useful yields with high stereoselectivity. As a result of efficient RCM and re-incorporation of side products into the catalytic cycle with minimal alkene isomerization, desired cyclizations proceed in preference to alternative pathways, even under relatively high substrate concentration.     

Enantiomeric Binaphtyl-brided SALEN Zirconium Complexes as Catalysts for Stereoselective Polymerization of Chiral Olefins

1 Results Chiral Schiff-base complexes have been used as catalysts for several stereoselective organic transformations,but they have never been tested for the polymerization of chiral olefins.The preferential polymerization of a single monomeric enantiomer (stereoselective polymerization) pursues two intriguing goals: i) the development of polymeric materials with peculiar optical or physical properties and ii) the kinetic resolution of racemic alkenes.     

Asymmetric additions to dienes catalysed by a dithiophosphoric acid

Chiral Br?nsted acids (proton donors) have been shown to facilitate a broad range of asymmetric chemical transformations under catalytic conditions without requiring additional toxic or expensive metals. Although the catalysts developed thus far are remarkably effective at activating polarized functional groups, it is not clear whether organic Br?nsted acids can be used to catalyse highly enantioselective transformations of unactivated carbon-carbon multiple bonds. This deficiency persists despite the fact that racemic acid-catalysed 'Markovnikov' additions to alkenes are well known chemical transformations. Here we show that chiral dithiophosphoric acids can catalyse the intramolecular hydroamination and hydroarylation of dienes and allenes to generate heterocyclic products in exceptional yield and enantiomeric excess. We present a mechanistic hypothesis that involves the addition of the acid catalyst to the diene, followed by nucleophilic displacement of the resulting dithiophosphate intermediate; we also report mass spectroscopic and deuterium labelling studies in support of the proposed mechanism. The catalysts and concepts revealed in this study should prove applicable to other asymmetric functionalizations of unsaturated systems.     

Tunable gold catalysts for selective hydrocarbon oxidation under mild conditions

Oxidation is an important method for the synthesis of chemical intermediates in the manufacture of high-tonnage commodities, high-value fine chemicals, agrochemicals and pharmaceuticals: but oxidations are often inefficient. The introduction of catalytic systems using oxygen from air is preferred for 'green' processing. Gold catalysis is now showing potential in selective redox processes, particularly for alcohol oxidation and the direct synthesis of hydrogen peroxide. However, a major challenge that persists is the synthesis of an epoxide by the direct electrophilic addition of oxygen to an alkene. Although ethene is epoxidized efficiently using molecular oxygen with silver catalysts in a large-scale industrial process, this is unique because higher alkenes can only be effectively epoxidized using hydrogen peroxide, hydroperoxides or stoichiometric oxygen donors. Here we show that nanocrystalline gold catalysts can provide tunable active catalysts for the oxidation of alkenes using air, with exceptionally high selectivity to partial oxidation products ( approximately 98%) and significant conversions. Our finding significantly extends the discovery by Haruta that nanocrystalline gold can epoxidize alkenes when hydrogen is used to activate the molecular oxygen; in our case, no sacrificial reductant is needed. We anticipate that our finding will initiate attempts to understand more fully the mechanism of oxygen activation at gold surfaces, which might lead to commercial exploitation of the high redox activity of gold nanocrystals.     

超声波辅助提取决明子中脂溶性成分的GC-MS分析

采用气相色谱-质谱联用技术对超声辅助-石油醚提取决明子中脂肪酸成分进行了分析,研究表明:以石油醚为提取剂、超声功率100 W、提取时间约30 min时,不饱和脂肪酸的提取率高于传统回流方法,且并不改变有效成分的结构.结果共检测出7种脂肪酸,其中亚油酸在脂肪酸中的质量分数为44.45%.结果表明,决明子种子中富含不饱和脂...     

光学活性胺的不对称合成

采用前手性的酮1a-d为原料,R-或S-α-苯乙胺为手性试剂,经3步反应合成光学活性的胺类化合物4a-d.全部合成过程不用昂贵试剂,不用加压,立体选择性在中等以上,总产率39%～50%.亚胺化反应的时间长短对产物的E/Z选择性有明显影响,降低还原温度有助于提高立体选择性.     

过渡金属/磷配体络合物催化不对称氢甲酰化研究进展

手性醛是合成生物活性物质如药物活性分子等的重要中间体,及各种高附加值精细化学品如香料等的起始原料,潜手性烯烃及其衍生物的不对称氢甲酰化是合成光学活性醛的重要方法之一.在烯烃的不对称氢甲酰化中铑与膦形成的催化体系因活性高、选择性好等优点而被广泛应用.主要介绍了近年来几类新型手性磷配体与铑形成的络合物催化剂在不对称氢甲酰化反应中的应用研究结果,如Rh/手性双-3,4-二氮磷杂环戊烷配体(S,S,S)-Bisdiazaphos和(R,S)-NEt-Yanpho络合物催化剂等.     

叶片式流体机械叶片的数学表示

本文用参数B样条来表示任意四边形网格上的曲面,并给出了曲面偏导数的计算公式,数值实验证明了这种表示方法是合理的、可靠的,插值效果很好.对混流式水轮机HL220转轮叶片工作面的实际应用表明,这种表示方法能够用来表示叶片式流体机械叶片形状,其效果令人满意.     

硫酸铁水合物催化下醇脱水成烯反应

研究了在硫酸铁水合物催化下,不同叔醇脱水成烯反应,结果表明该催化剂对叔醇脱水成烯的反应有较好的催化效果,生成烯烃的产率为85～97％。     

混合菌构建及其对硝基苯直接好氧生物降解

选取本实验室分离的3株硝基苯高效降解菌———Rhodotorula mucilaginosa Z1、Streptomyces albidoflavusZ2和Micrococcus luteusZ3,将其混合并进行正交实验,得出Z1、Z2、Z3的最佳配比为1∶3∶3,在此基础上构建了混合菌.与单菌相比,混合菌能够在较为苛刻的环境条件下降解硝基苯,并具有较宽的底物范围.同时结合实际硝基苯工业废水特点,考察了混合菌在高盐度下以及苯酚或苯胺和硝基苯共存时对硝基苯的降解.结果表明:混合菌具备较强的耐盐能力,可在5%的高盐(NaCl)条件下有效降解硝基苯;当苯酚或苯胺和硝基苯(200mg/L)共存,初始浓度分别为100和50mg/L时,混合菌对硝基苯的降解不受影响.     

浅水长波近似方程组的多孤波解、有理分式解

使用王明亮引进的齐次平衡法，求出了浅水长波近似方程组的Backlund变换以及它与热传导方程和二阶线性方程之间的Darboux变换，并借助于这些变换，获得了浅水长波近似方程组的多孤波解、有理分式解。推广了王明亮等人的结果。     

应用基团键贡献法计算烯烃的折光指数

:根据分子中基团的特性和连接性 ,发展了一种计算烯烃折光指数的新方法———基团键贡献法 ,该方法考虑分子中基团的特性 ,又考虑基团间的连接性 (化学键 ) ,同时具有基团贡献法和拓扑方法的特点。对 2 16种烯烃折光指数的计算结果表明 ,计算值与实验值的一致性令人满意 ,平均误差 0 .48%。     

应用基团键贡献法计算烯烃的折光指数

根据分子中基团的特性和连续性,发展了一种计算烯烃折光指数的新方法－－基团键贡献法,该方法既考虑分子中基团的特性,又考虑基团间的连接性（化学键）,同时具有基团贡献献法和拓扑方法的特点,对216种烯烃折光指数的计算结果表明,计算值与实验值的一到致性令人满意,平均误差0．48％。     

Crystal symmetry and the reversibility of martensitic transformations

Martensitic transformations are diffusionless, solid-to-solid phase transitions, and have been observed in metals, alloys, ceramics and proteins. They are characterized by a rapid change of crystal structure, accompanied by the development of a rich microstructure. Martensitic transformations can be irreversible, as seen in steels upon quenching, or they can be reversible, such as those observed in shape-memory alloys. In the latter case, the microstructures formed on cooling are easily manipulated by loads and disappear upon reheating. Here, using mathematical theory and numerical simulation, we explain these sharp differences in behaviour on the basis of the change in crystal symmetry during the transition. We find that a necessary condition for reversibility is that the symmetry groups of the parent and product phases be included in a common finite symmetry group. In these cases, the energy barrier to lattice-invariant shear is generically higher than that pertaining to the phase change and, consequently, transformations of this type can occur with virtually no plasticity. Irreversibility is inevitable in all other martensitic transformations, where the energy barrier to plastic deformation (via lattice-invariant shears, as in twinning or slip) is no higher than the barrier to the phase change itself. Various experimental observations confirm the importance of the symmetry of the stable states in determining the macroscopic reversibility of martensitic transformations.     

Extreme value distributions of mixing two sequences with different MDA＇s

Suppose [Xi, i> or =1] and [Yi, i> or =1] are two independent sequences with distribution functions FX(x) and FY(x), respectively. Z(i,n) is the combination of Xi and Yi with a probability pn for each i with 1< or =i< or =n. The extreme value distribution GZ(x) of this particular triangular array of the i.i.d. random variables Z(1,n), Z(2,n),..., Z(n,n) is discussed. We found a new form of the extreme value distribution LambdaA(Rhox)Lambda(x)(0相似文献   

Her变换的数字水印算法

经对目前数字水印变换域算法的研究,发现常用的变换大多都是正交变换（如DCT和DWT等）。作者通过对Haar函数系、Haar类函数系和Walsh函数系这三大类正交函数系的研究,找到了与之对应的三类性能优良的正交变换,Her类正交变换就是其中的一种。由于Her函数系所对应的Her矩阵不是归一化的正交矩阵,所以不能像DCT等矩阵一样直接应用于数字水印技术,通过对Her连续函数的采样,得出了其对应的离散矩阵,然后通过一系列的实验数据和理论证明,成功的将其应用于数字水印中。最后,提出一种新颖的、鲁棒的Her域盲水印算法。实验结果表明该算法计算简单,且具有良好的不可见性,通过与传统DCT水印的对比表明,该算法在抵抗噪声和和滤波等方面具有较强的鲁棒性。     

应用分子结构信息预测烯烃的临界温度

根据分子结构的特点，探讨了烯烃的临界温度与其分子结构之间的定量关系，提出了一种直接由分子结构信息预测烯烃临界温度的方法，并开发了相应的计算机程序，对１６９种烯烃的预测结果表明，临界温度预测值与实验值的平均误差为０．９７％。     

The mechanism of Z alpha 1-antitrypsin accumulation in the liver.

Most northern Europeans have only the normal M form of the plasma protease inhibitor alpha 1-antitrypsin, but some 4% are heterozygotes for the Z deficiency variant. For reasons that have not been well-understood, the Z mutation results in a blockage in the final stage of processing of antitrypsin in the liver such that in the Z homozygote only 15% of the protein is secreted into the plasma. The 85% of the alpha 1-antitrypsin that is not secreted accumulates in the endoplasmic reticulum of the hepatocyte; much of it is degraded but the remainder aggregates to form insoluble intracellular inclusions. These inclusions are associated with hepatocellular damage, and 10% of newborn Z homozygotes develop liver disease which often leads to a fatal childhood cirrhosis. Here we demonstrate the molecular pathology underlying this accumulation and describe how the Z mutation in antitrypsin results in a unique molecular interaction between the reactive centre loop of one molecule and the gap in the A-sheet of another. This loop-sheet polymerization of Z antitrypsin occurs spontaneously at 37 degrees C and is completely blocked by the insertion of a specific peptide into the A-sheet of the antitrypsin molecule. Z antitrypsin polymerized in vitro has identical properties and ultrastructure to the inclusions isolated from hepatocytes of a Z homozygote. The concentration and temperature dependence of this loop-sheet polymerization has implications for the management of the liver disease of the newborn Z homozygote.     

Z-164D溶液对铝土矿强度的影响及其作用机理

以Z-164D为助磨剂，对广西田阳铝土矿进行了邦德球磨功指数试验和单轴抗压强度试验.结果表明:铝土矿经水和Z-164D溶液浸泡后，邦德球磨功指数分别下降了105%和379%，单轴抗压强度分别下降了2684%和4279%.红外光谱分析及X射线能谱微区元素分析表明，Z-164D只能在铝土矿表面发生物理和化学吸附，并不能够渗入铝土矿内部.建立了Z-164D溶液对铝土矿强度影响的作用机理模型，水分子与Z-164D相互促进，共同导致铝土矿缺陷程度增加、强度降低.     

New refinement relations of Z specifications for multiple viewpoints oriented requirements method

In this paper we develop several new refinement relations of Z for multiple viewpoints oriented requirements method (MVORM). The original motivation is that we found the standard Z refinement relation is not adequate or correct when considering specifications that have temporal relationships of operations. The concept of temporal state variables is introduced into Z. Then new implementation relations are defined and new refinement relations are deduced, mainly for temporal state variables to process temporal relationships of operations. We use state transition systems to abstract the temporal state transitions. A simple example is used to show the procedures of MVORM. Finally some directions of further work are forwarded.