Concise, asymmetric total synthesis of spirotryprostatin A

The structurally intriguing cell-cycle inhibitor spirotryprostatin A has been synthesized utilizing an azomethine ylide dipolar cycloaddition reaction as the key step. This pentacyclic alkaloid contains a prenylated tryptophan-derived oxindole moiety that has been created in a regiocontrolled and stereocontrolled manner in a single step.     

恒电流暂态法研究镁在非水体系的电沉积

恒电流暂态法做为研究金属电沉积的有效工具广泛被采用。传统的恒电流暂态法需要用示波器及照相装置,操作较繁。本文利用Model-273恒电势仪和Apple-Ⅱ微机联用完成了恒电流暂态实验,利用该装置可使操作全部自动化,数据的采集、贮存、打印以及超电势与时间之间关系图的绘制全部由计算机完成,不仅操作简便,且精度高。在验证该装置可靠的基础上,本文用其测定了镁在二甲基甲酰胺中电沉积的动力学参数。不仅加深了对镁     

Electrochemical and spectroscopic studies of ethanol oxidation on Pt stepped surfaces modified by tin adatoms

Ethanol oxidation on platinum stepped surfaces vicinal to the (111) pole modified by tin has been studied to determine the role of this adatom in the oxidation mechanism. Tin has been slowly deposited so that the initial stages of the deposition take place on the step, and deposition on the terrace only occurs when the step has been completely decorated. Voltammetric and chronoamperometric experiments demonstrate that tin on the step catalyzes the oxidation. The maximum enhancement is found when the step is completely decorated by tin. FTIR experiments using normal and isotopically labeled ethanol have been used to elucidate the effect of the tin adatoms in the mechanism. The obtained results indicate that the role of tin is double: (i) when the surface has sites capable of breaking the C-C bond of the molecule, that is, when the step sites are not completely covered by tin, it promotes the oxidation of CO formed from the molecular fragments to CO(2) through a bifunctional mechanism and (ii) it catalyzes the oxidation of ethanol to acetic acid.     

Synthesis of furano[2,3-c]pyran-3-one and thieno[2,3-c]pyran-3-one derivatives through the coupling of 3-alkynyl-2-heteroaromatic carboxaldehydes with Fischer carbene complexes: total synthesis of a Baccharis-derived cadinene derivative

The coupling of Fischer carbene complexes with 3-alkynyl-2-heteroaromatic carboxaldehyde derivatives has been examined. The reaction affords pyrones fused to furans or thiophenes in a single step. The compounds are stable enough for isolation. If the carbene complex features a remote alkene substituent, a subsequent Diels-Alder reaction can occur. This reaction has been used as the key step in the synthesis of a naturally occurring cadinene derivative.     

Reinvestigating the role of multiple hydrogen transfers in Baeyer-Villiger reactions

A comparison between a tetramolecular mechanism (J. Org. Chem. 2007, 72, 3031) and our previously proposed mechanism has been performed. The results show that the tetramolecular mechanism is less favored in terms of Gibbs free energy, and therefore, the reaction in acidic media is acid catalyzed. The role of the catalyst in the migration step has been corrected and discussed. A new transition state is proposed for this step. The change in the rate-determining step for the cyclohexanone + performic acid reaction supports our results.     

The Gold(I)‐Catalysed Protodecarboxylation Mechanism

A mechanistic study of the gold‐catalysed protodecarboxylation is described. Each reaction step has been investigated experimentally and computationally. More specifically, the activation parameters for the decarboxylation step have been determined through kinetic studies. Further experimental studies on the hydrolysis of the arylgold intermediate have revealed that the protodeauration can become competitive with the decarboxylation process at high conversions. This switch in rate‐limiting step has been shown to be pK_a‐dependent. These studies have been supported by DFT calculations and permit a better understanding of which prevalent features of the reaction mechanism account for the decarboxylation process.     

A convenient and efficient synthesis of (S)-lysine and (S)-arginine homologues via olefin cross-metathesis

A convenient five step synthesis of (S)-homolysine, incorporating a key olefin cross-metathesis step in the chain extension methodology, has been developed, together with a six step related synthesis of a new homologue of arginine, (S)-bishomoarginine.     

The use of enantiomerically pure ketene dithioacetal bis(sulfoxides) in highly diastereoselective intramolecular nitrone cycloadditions. Application in the total synthesis of the beta-amino acid (-)-cispentacin and the first asymmetric synthesis of cis-(3R,4R)-4-amino-pyrrolidine-3-carboxylic acid

Intramolecular 1,3-dipolar nitrone cycloaddition onto an enantiomerically pure ketene dithioacetal dioxide using a three-carbon tether gave the corresponding 5,5-disubstituted isoxazolidine as a single diastereomer in good yield. This reaction has been used as the key step in an asymmetric synthesis of the naturally occurring antibiotic, (-)-cispentacin. An asymmetric synthesis of 4-amino-pyrrolidine-3-carboxylic acid has also been carried out using the intramolecular nitrone cycloaddition as the stereocontrolling step.     

Preparation,characterisation and thermal decomposition of barium zirconyl oxalate

Barium zirconyl oxalate hydrate (BZO) is prepared and characterised by chemical analysis and IR spectral studies. Thermal decomposition studies have been made using TG and DTA techniques. The decomposition has been found to proceed through four steps. The first step involves a two-stage dehydration (100–190°C, 190–260°C) and the second step the decomposition of oxalate (260–460°C). The third step involves the evolution of carbon monoxide present in the lattice and partial decomposition of carbonate. The fourth step involves the final stage decomposition of carbonate (760–920°C) giving barium zirconate as an end product. The identification of compounds at various stages has been done by IR spectra. The X-ray diffraction pattern of BZO confirms that it is a crystalline compound.     

Mechanism of the oxidation of cycloheptanone by chloramine-T

The oxidation kinetics of cycloheptanone by chloramine-T in alkaline media has been investigated. A mechanism involving the formation of an intermediate by the interaction between the enolic anion of cycloheptanone and chloramine-T in a slow step followed by its interaction with another molecule of chloramine-T in a fast step leading to products has been proposed. The scheme satisfactorily accords with the observed stoichiometry, negligible influence of ionic strength and a positive dielectric effect. Various rate parameters have been computed and 1.2-cycloheptanedione identified as the end product.With 2 Figures     

Improvements in the Synthesis of a Coumaromethacrylate Dye

A new method to prepare a coumaromethacrylate dye has been devised. The older three‐step route was shortened to a one‐pot, solvent‐free step.     

Catalytic stepwise pyrolysis of packaging plastic waste

In this paper the combination of catalytic and stepwise pyrolysis is explored. A mixture of polyethylene (PE), polypropylene (PP), polystyrene (PS), poly(ethylene terephthalate) (PET) and poly(vinyl chloride) (PVC), which resembles real municipal plastic waste, has been pyrolysed in a 3.5 dm³ semi-batch reactor at 440 °C for 30 min using a ZSM-5 zeolite as catalyst. A low temperature (300 °C) dechlorination step has been carried out both with and without catalyst. It has been proved that the application of such dechlorination step gives rise to a 75 wt% reduction of chlorine in the liquid fraction. However, such step has a negative influence on the catalyst, which loses some catalytic activity. The optimum procedure in terms of quality and chlorine content of the products is the combination of first a low temperature step without catalyst, and second the catalytic pyrolysis step.     

Ru(III)-EDTA catalyzed oxidation of ascorbic acid by hydrogen peroxide in aqueous solution

The kinetics of oxidation of ascorbic acid to dehydroascorbic acid by hydrogen peroxide catalyzed by ethylenediaminetetraacetatoruthenate(III) has been studied over the pH range 1.50 – 2.50, at 30°C and μ = 0.1 M KNO₃. The reaction has a first-order dependence on ascorbic acid and Ru(III)-EDTA concentrations, an inverse first-order dependence on hydrogen ion concentration, and is independent of hydrogen peroxide concentration in the pH range studied. A mechanism has been proposed in which ascorbate anion forms a kinetic intermediate with the catalyst in a pre-equilibrium step. Ruthenium(III) is reduced to ruthenium(II) in a rate-determining step and is reoxidized with hydrogen peroxide back to the Ru(III) complex in a fast step.     

苯甲酮不对称还原反应的理论研究

本文用AM1分子轨道方法研究了1,3,2-噁唑硼烷对苯甲酮的不对称催化还原.反应经历了噁唑硼烷-硼烷配合物的形成及其与苯甲酮的结合、氢转移及脱去噁唑硼烷形成手性产物二级醇-硼烷配合物四步过程.获得了各步的反应热、速度控制步骤的过渡态结构和位能曲线及其相应的反应活化能,计算发现反应机理中的第3步氢转移产物有四员环结构特征.     

Studies on the kinetics and mechanism of redox decomposition of 1,2,6-trinitrotriamminecobalt(III) and 1,2,6-trinitrodiethylenetriaminecobalt(III) complexes in aqueous acid

Summary In aqueous acid media the title complexes undergo decomposition ultimately forming cobalt(II). Detailed analysis of the kinetic data reveals that the reactions proceed in two consecutive stages, the initial step being the faster one. Evidence has been presented that the initial faster step involves aquation releasing one NO ₂ ^– ligand and forming an aquodinitro complex, which subsequently undergoes a slower intramolecular redox decomposition into Co²⁺. Each step, on the other hand, has been found to consist of two concurrent paths,viz. spontaneous and acid catalysed, respectively. Activation parameters corresponding to all these specific rate constants have been evaluated and a plausible mechanism for the overall reaction has been proposed.On leave from Agra College, Agra, India.     

Weighted tridiagonal matrix enhanced multivariance products representation (WTMEMPR) for decomposition of multiway arrays: applications on certain chemical system data sets

This work focuses on the utilization of a very recently developed decomposition method, weighted tridiagonal matrix enhanced multivariance products representation (WTMEMPR) which can be equivalently used on continuous functions, and, multiway arrays after appropriate unfoldings. This recursive method has been constructed on the Bivariate EMPR and the remainder term of each step therein has been expanded into EMPR from step to step until no remainder term appears in one of the consecutive steps. The resulting expansion can also be expressed in a three factor product representation whose core factor is a tridiagonal matrix. The basic difference and novelty here is the non-constant weight utilization and the applications on certain chemical system data sets to show the efficiency of the WTMEMPR truncation approximants.     

Synthesis of 1,3-difluoroaromatics in one pot by addition of difluorocarbene from sodium chlorodifluoroacetate to 1,2-substituted cyclobutenes

[formula: see text] The synthesis of 1,3-difluoro-2,4-diphenylbenzene has been accomplished in one step from 1,2-diphenylcyclobutene using the environmentally benign difluorocarbene precursor sodium chlorodifluoroacetate. In addition, the preparation of the previously unknown compound 1,3-difluoro-2,4-di-n-propylbenzene has been accomplished in one step from 1,2-di-n-propylcyclobutene using Seyferth's reagent (Ph-Hg-CF3) and sodium chlorodifluoroacetate.     

Synthesis of (±)-7-hydroxylycopodine

A six step synthesis of (±)-7-hydroxylycopodine has been achieved in 5% overall yield. In the key step, a Prins cyclization of a bicyclic keto alkyne in 60% H(2)SO(4) forms a tricyclic dihydroxy amino ketone.     

MgO活化甲烷碳氢键的密度泛函研究

在B3LYP/6-311 + G（2d, 2p）水平上计算了MgO + CH_4 → Mg+CH_3OH反应的 单态势能曲线。结果发现MgO和CH_4发生相互作用,首先形成两种类型的分子-分子 复合物（MgOCH_4和OMgCH_4）;分子-分子复合物OMgCH_4能发生进一步转化,即 MgO插入到CH_4的C-H键中,产生中间体HOMgCH_3,此中间体在本反应中是能量上最 稳定的构型;它还有可能进一步发生反应,产生原子-分子复合物MgCH_3OH,但其 活化能太高,为299.8kJ·mol~(-1),是整个反应的速率控制步骤;最后一步是 MgCH_3OH放出CH_3OH分子,整个反应放热146.1 kJ·mol~(-1)。     

A one-step four-bond-breaking reaction catalyzed by an electron

Molecular demolition: Long-lived compound states involving a low-energy electron and a molecule generally involve the breaking of only one σ?bond in the first elementary reaction step. Using high-level ab?initio quantum-chemical methods, four-bond breaking in an elementary reaction step has been identified. Upon electron impact, a bicyclic molecule is shown to break four σ?bonds in a concerted and essentially barrierless reaction step, producing three closed-shell neutral molecules.