Catalytic enantioselective Strecker reaction of ketoimines using catalytic amount of TMSCN and stoichiometric amount of HCN

Catalyst loading as low as 0.1 mol % was achieved in the enantioselective Strecker reaction of ketoimines. Excellent enantioselectivity was obtained with a combined use of a catalytic amount of TMSCN and a stoichiometric amount of HCN as a reagent, and a chiral gadolinium complex as a catalyst.     

Determination of traces of fluorine or sulfur by x-ray analysis

New X-ray methods have been developed for the determination of as little as 1 μg of fluorine or as little as 0.1 μg of sulfur. Fluorine as fluoride is precipitated as lanthanum fluoride in 75% ethanol-water mixture and determined by measuring the amount of lanthanum present in the precipitate. Sulfur as sulfate is precipitated as barium sulfate from a barium sulfate saturated 50% ethanol-buffer mixture, using selenate as a coprecipitant, and determined by measuring the amount of sulfur present. Sulfur as sulfide is precipitated as cadmium sulfide and determined by measuring the amount of sulfur or cadmium in the precipitate.     

Synthesis of a bicyclic analogue of AZT restricted in an unusual O4'-endo conformation.

The [3.2.0]bicyclic beta-nucleoside analogue 5 has been designed as a conformationally restricted analogue of the anti-HIV drug AZT. The synthesis of 5 as well as its alpha-anomer 29 is hereby described. The synthesis was accomplished from D-arabinose via a modified Corey-Link procedure stereoselectively incorporating the azide moiety as well as a methyl ester function. When the tert-butyldiphenylsilyl group was used as a permanent protecting group, a selective formation of an oxetane ring failed. When using the p-methoxyphenyl group as a permanent protecting group, 5 and 29 were efficiently obtained via a selective reduction of the ester, a nucleobase coupling followed by separation of the anomers and ring-closing procedures. The nucleoside 5 is conformationally restricted in an unusual O4'-endo (East) conformation, which is an intermediate between the North- and South-type conformations. Nevertheless, neither 5 nor 29 displayed any anti-HIV activity.     

Criteria of efficiency of cellulose acetate plasticization

Using cellulose acetate plastics as an example, it was shown that the search for the optimal concentrations of plasticizers should take into account the compatibility of components, as well as the thermophysical and mechanical properties of plasticized polymers. It was suggested that the temperature range of durability of a plastic, i.e., the difference between its glass-transition and brittle temperatures, be used as a plasticization efficiency criterion. Plasticizers that are well compatible with a polymer at processing temperatures but show a limited compatibility at its service temperatures make it possible to manufacture goods with an extended durable temperature range.     

GC-MS Determination of an Unknown Pesticide. II. An Experiment Investigating Quality of Analysis Using Random Sampling and Estimation of Variance

Newly devised components are described which significantly enhance both the scientific and pedagogical components of a previously published, qualitatively oriented experiment for students in an undergraduate instrumental analysis course. To provide a laboratory exercise containing as much real-world experience as possible, the earlier experiment was modified in such a way as to allow its use as a quantitative experiment illustrating such principles as sampling, experimental design, analysis of variance, and the use of internal standards.     

核-壳型赤藓红分子印迹聚合物的制备及吸附性能的评价

以氨基化修饰的SiO_2为内核,人工合成色素赤藓红为模板,甲醇/水为溶剂,4-乙烯基吡啶为功能单体,二甲基丙烯酸乙二醇酯为交联剂,偶氮二异丁腈为引发剂,采用表面印迹技术,制备核-壳型赤藓红分子印迹聚合物。通过红外光谱对其结构进行表征,并通过动力学吸附、等温饱和吸附和实际样品加标实验对其吸附性能进行评价。结果表明,核-壳型赤藓红分子印迹聚合物具有较快的吸附能力,在15min左右达到吸附平衡,有较好的吸附容量,能够从复杂的食品样品中选择性吸附模板,且回收可达85%。     

Influence of architecture on the kinetic stability of molecular assemblies

The strength of a multimolecular system depends on the number of interactions that hold it together. Using dynamic force spectroscopy, we show how the kinetic stability of a system decreases as the number of molecular bonds is increased, as predicted by theory. The data raise important considerations for experimental tests of bond strength and, as a paradigm, suggest both routes to and pitfalls in methods for computational simulation of molecular transitions, such as ligand binding and protein folding.     

Molecular fundamentals of enzyme nanogels

The assembly of a monomer around an enzyme as the essential step in the fabrication of enzyme nanogel by in situ polymerization was illustrated by molecular dynamics simulation and evidenced by a fluorescence resonance energy transfer spectrum, using lipase/acrylamide as a model system. The subsequent polymerization generated a hydrophilic gel network which not only strengthened the protein structural integrity via multipoint linkage but also increased the number of intramolecular H-bonds of the encapsulated protein, as suggested by the blue shift of the fluorescence spectrum of the encapsulated lipase. This greatly enhanced the stability of lipase at high temperature, as experimentally demonstrated. The exclusion of polar solvent molecules from the encapsulated enzyme, in contrast to the enrichment of water molecules, due to the presence of a hydrophilic gel network was displayed. This established a hydrophilic microenvironment for the encapsulated protein and thus gave the encapsulated protein an enhanced tolerance to the organic solvent, as experimentally observed in the present study and reported elsewhere. These results have given a molecular insight into the enzyme nanogel as well as its high potential as a robust enzyme model for an expended application spectrum of enzymatic catalysis.     

Effect of butterfly-shaped sulfur-bridged ligand and counter anions on the catalytic activity and diastereoselectivity of organobismuth complexes

In the direct Mannich reaction and synthesis of α,β-unsaturated ketones, the use of organobismuth complexes as catalysts leads to high diastereoselectivity and products of single trans conformation. In this paper, we illustrate the relationship between structure and catalytic activity as well as diastereoselectivity of organobismuth complexes having a 5,6,7,12-tetrahydrodibenz [c,f][1,5]thiobismocine framework as well as bearing a butterfly-shaped sulfur-bridged ligand and tunable anions. With the exposed bismuth center acting as a Lewis acid site and the uncoordinated lone pair electrons of sulfur as a Lewis base site, the cationic organobismuth complexes work as bifunctional Lewis acid/base catalysts. Due to the steric influence of the butterfly-shaped structure and synergistic effect of Lewis acid and Lewis base centers, the complexes can direct substrate attack in organic synthesis. By adjusting the electron-withdrawing ability of the counter anions, the S-Bi bond strength can be regulated, leading to a significant change in Lewis acidity and Lewis basicity as well as catalytic activity. Through synergistic modulation of the above effects, one can control the diastereoselectivity of the organobismuth complexes for the generation of a single diastereoisomer.     

毛喉萜的合成研究概况

毛喉萜(1),forskolin,具有抗高血压、抗肿瘤转移和抗青光眼等药理活性,以及复杂分子结构所具有的挑战性,引起世界上众多有机合成化学家和药物化学家的研究兴趣。本文主要综述该化合物合成研究的各种策略以及路线设计。同时也简要介绍毛喉萜衍生物的合成及其构效关系。     

醋酸和丙三醇对制备SrTiO3的影响

醋酸和丙三醇对制备ＳｒＴｉＯ_３的影响颜秀茹，霍明亮，王建萍，王菲，曾淑兰（天津大学化学系天津３０００７２）关键词　冰醋酸，溶胶－凝胶过程，丙三醇，ＳｒＴｉＯ_３ＳｒＴｉＯ３是电子材料中使用最广的材料之一，有关它的制备已有不少报道。一般可以通过ＳｒＣＯ...     

Further analysis of delayed luminescence of plants

The delayed luminescence curves of biological tissues, in particular the "hyperbolic" oscillations of the form exp(-iln (1 + lambda t)) with t as the time and lambda as a constant, were subjects of further experimental and theoretical investigations. It turned out that the oscillations are solutions of a Hamiltonian that keeps coherent states coherent. In agreement to the expectations the oscillations disappear as soon as the biological system looses its integrity or its collective structure. The photocount statistics (PCS) confirms these results. The illumination itself influences the biological state under consideration. The temperature dependence displays small deviations from the Arrhenius-law with a mean activation energy of about 0.5 eV in case of plants. The results may be useful for investigating supermolecular interactions as well as for assessing the quality or quality changes of biological tissues.     

Effects of macromolecules on the crystallization of CaCO3 the Formation of Organic/Inorganic Composites

The effects of macromolecules as soluble additives and solid matrices have been examined for the crystallization of CaCO₃. A vaterite form grows on a glass substrate in the presence of poly(glutamic acid) (PGA) containing a carboxylic acid group as a soluble additive. In contrast, no crystal growth has been observed when poly(acrylic acid) (PAA) exists as an additive though it has the same functional group. The conformation or the backbone structure of the polymers may have an influence on the crystal polymorph of CaCO₃. Thin film states of CaCO₃ crystals have been obtained as organic/inorganic composites with chitosan that acts as a solid matrix in the presence of PAA or PGA as a soluble additive.     

Solid-phase synthesis of lipidated peptides

Characteristic partial structures of lipidated proteins embodying different lipid groups as well as additional fluorescent tags or a maleimide for coupling to proteins can be synthesized readily by means of a new solid-phase technique employing the oxidative cleavage of the hydrazide linker as well as on-resin farnesylation and palmitoylation after appropriate deprotection of cysteine thiol groups as the key steps.     

Modeling competitive reaction mechanisms of peroxynitrite oxidation of guanine

5-Guanidino-4-nitroimidazole is a stable product from the peroxynitrite induced one-electron oxidation of guanine. Reaction mechanisms to form the 5-guanidino-4-nitroimidazole as well as 8-nitroguanine, through the combination of the guanine radical cation and nitrogen dioxide radical and through the combination of the deprotonated neutral guanine radical and nitrogen dioxide radical, have been investigated by the use of the B3LYP method of density functional theory. Our calculations suggest that the guanine radical cation mechanism is preferred over the neutral guanine radical mechanism and that a water molecule is involved in the reaction as a catalyst or as a reactant.     

Electron group functions for the analysis of the electronic structures of molecules

The electronic structure of a vast majority of molecular systems can be understood in terms of electron groups and their wave functions. They serve as a natural basis for bringing intuitive chemical and physical concepts into quantum chemical calculations. This article considers the general electron group functions formalism as well as its simple geminal version. We try to characterize the wave function with the group structure and its capabilities in actual calculations. For this purpose we implement a variational method based on the wave function in the form of an antisymmetrized product of strongly orthogonal group functions and perform a series of electronic structure calculations for small molecules and model systems. The most important point studied is the relation between the choice of electron groups and the results obtained. We consider energetic characteristics as well as optimal geometry parameters. In view of practical importance, the structure of variationally optimized local one-electron states is considered in detail as well as intuitive characteristics of chemical bonds.     

Direct Visualization of Electromagnetic Microfields by Superposition of Two Kinds of Electron Holograms

A new electron holographic method is presented to directly visualize pure phase objects such as electromagnetic microfields, which is achieved by superposition of two kinds of electron holograms. The method is very simple and is twice as sensitive as the conventional double-exposure electron holography and the four-electron-wave interference. Using this technique, an electric field around a charged latex sphere and a magnetic field around a barium ferrite particle have been directly observed.     

Design of Nanoscopic Vehicles for Drug Targeting Based on Micellization of Amphiphiuc Block Copolymers

Block copolymers in a selective solvent (a good solvent for one block but a nonsolvent for the other) form micellar structures through the association of the insoluble segments. Micelles formed through the association of amphiphilic block copolymers have several advantageous features as drug carrier systems due to their considerable thermodynamic stability as well as to the formation of a hydrophobic core separated from the outer aqueous milieu by the palisade of hydrophilic segments. The core acts as the microcontainer of hydrophobic drugs. This paper presents a concept and strategy for using a block copolymer micelle as a nanoscopic virus-mimicking carrier of anticancer drugs to treat solid tumors.