Total synthesis of a dienynone from Echinacea pallida

The first total synthesis of (8Z,13Z)-pentadeca-8,13-dien-11-yn-2-one is described. This dienynone was recently isolated from the n-hexane extract of Echinacea pallida roots and displayed a selective cytotoxic activity toward cancer cells, thus featuring as a potential anticancer lead. The product was obtained in 11 steps in 25% overall yield.     

Independence for full conditional probabilities: Structure,factorization, non-uniqueness,and Bayesian networks

This paper examines concepts of independence for full conditional probabilities; that is, for set-functions that encode conditional probabilities as primary objects, and that allow conditioning on events of probability zero. Full conditional probabilities have been used in economics, in philosophy, in statistics, in artificial intelligence. This paper characterizes the structure of full conditional probabilities under various concepts of independence; limitations of existing concepts are examined with respect to the theory of Bayesian networks. The concept of layer independence (factorization across layers) is introduced; this seems to be the first concept of independence for full conditional probabilities that satisfies the graphoid properties of Symmetry, Redundancy, Decomposition, Weak Union, and Contraction. A theory of Bayesian networks is proposed where full conditional probabilities are encoded using infinitesimals, with a brief discussion of hyperreal full conditional probabilities.     

The Metallic Zintl Phase Ba3Si4 – Synthesis,Crystal Structure,Chemical Bonding,and Physical Properties

The Zintl phase Ba₃Si₄ has been synthesized from the elements at 1273 K as a single phase. No homogeneity range has been found. The compound decomposes peritectically at 1307(5) K to BaSi₂ and melt. The butterfly‐shaped Si₄⁶⁻ Zintl anion in the crystal structure of Ba₃Si₄ (Pearson symbol tP28, space group P4₂/mnm, a = 8.5233(3) Å, c = 11.8322(6) Å) shows only slightly different Si‐Si bond lengths of d(Si–Si) = 2.4183(6) Å (1×) and 2.4254(3) Å (4×). The compound is diamagnetic with χ ≈ −50 × 10⁻⁶ cm³ mol⁻¹. DC resistivity measurements show a high electrical resistivity (ρ(300 K) ≈ 1.2 × 10⁻³ Ω m) with positive temperature gradient dρ/dT. The temperature dependence of the isotropic signal shift and the spin‐lattice relaxation times in ²⁹Si NMR spectroscopy confirms the metallic behavior. The experimental results are in accordance with the calculated electronic band structure, which indicates a metal with a low density of states at the Fermi level. The electron localization function (ELF) is used for analysis of chemical bonding. The reaction of solid Ba₃Si₄ with gaseous HCl leads to the oxidation of the Si₄⁶⁻ Zintl anion and yields nanoporous silicon.     

Applications of average and projected systems to the study of coherent systems

In this paper, we introduce the concepts of average and projected systems associated to a coherent (parent) system. We analyze several aspects of these notions and show that they can be useful tools in studying the performance of coherent systems with non-exchangeable components. We show that the average and projected systems are especially useful in studying the tail behavior of reliability, hazard rate and mean residual life functions of the parent system and also in obtaining the tail best systems (under different criteria) by permuting the components at the system structure. Moreover, they can be useful in assessing how the asymmetry of the joint distribution of the component lifetimes (with respect to permutations of the components in the system structure) affects the system performance.     

Investigation of fine scale structure of turbulent and molecular diffusion in coaxial jets of He/CO2 in air by LDA and Rayleigh scattering

This paper revisits the important issue of differential diffusion and provides new experimental results and subsequent analysis that attempts to quantify the relationship between molecular diffusion, turbulent diffusion and their mutual interference in non-reacting axisymmetric coaxial jets of variable Reynolds number. The reported investigation has been focused on the analysis of molecular diffusion of a He/CO₂ mixture in air by combining line imaging of Rayleigh scattering and laser Doppler anemometry (LDA) to determine length scales associated with differential diffusion and turbulent transport. Line imaging Rayleigh scattering was performed applying the index-matching method with a mixture of two gaseous species having scattering cross-sections respectively lower and higher of that of air and the cross-section of the mixture identical to that of the co-flowing air. Any measured variation in scattering intensity is therefore due to differential diffusion between the two species. Instantaneous and ensemble averaged line profiles of Rayleigh scattering intensity are presented and a characteristic length scale associated with differential diffusion is deduced. Autocorrelation analysis is applied to obtain the characteristic scale of differential diffusion fluctuations and the integral length scales of velocity fluctuations, as measured by LDA. Theoretical information from the literature is used in relating these scales to the molecular and turbulent diffusion coefficients, assuming homogeneous and isotropic turbulence, and the ratio of molecular to turbulent diffusivity is estimated as a function of the Reynolds number. The results confirm that the average contribution of molecular diffusion to the effective diffusivity into the air stream progressively reduces when the turbulence level increases. They also suggest that, at higher Re, the differential diffusion remains significant down to the scalar dissipation length scale, and could influence mixing at the molecular level and thus chemical reactions.     

Determination of Se using a solid-phase micro-extraction device coupled to a graphite furnace and detection by gas chromatography-mass spectrometry

A solid-phase micro-extraction (SPME) method using an SPME fiber device and graphite furnace (GF) for extracting Se compounds was proposed. Various factors affecting the derivatization and extraction of Se(IV) by SPME-GF were evaluated, including the effect of acid (type and concentration), the concentration of the derivatizing agent, the derivatization temperature, the extraction and derivatization times and the extraction temperature. After optimizing these conditions, the quantification of Se(IV) was performed by Gas Chromatography-Mass Spectrometry (GC-MS). The limit of detection was 0.37 μg L(-1) for Se(IV). The method was successfully applied to the total Se determination in certified reference materials (BCR-414 and SRM 1643e). A recovery of 97% was obtained for water (SRM 1643e). After microwave oven decomposition and the reduction of selenium using a mixture of 2 mol L(-1) HCl and 1% (w/v) KBr, a recovery of 101% and a relative standard deviation of 3.5% were attained for plankton (BCR-414). The SPME-GF method combined with GC-MS was also applied to the determination of the total selenium in a drug sample (selenium chelate).     

Dimethyl Carbonate as a Sacrificial Molecule for the Synthesis of 5‐Memebered N‐ and O‐Heterocycles

In the last twenty years DMC has been employed as an efficient methylating and methoxycarbonylating agent with several monodentate and bidentate nucleophiles, showing great selectivity and unexpected results. In this short review we report on yet another application of DMC chemistry i.e. the synthesis of 5‐membered N‐ and O‐heterocycles. In these reactions DMC acts as a sacrificial molecule since it is not present in the final products, but only in the reaction intermediates as an halogen‐free leaving group. This DMC‐based synthesis of heterocycles resulted of general application, as it is effective for aliphatic and aromatic 1,4‐diols, incorporating several functionalities (primary, secondary, tertiary, allylic, phenolic), as well as, for bifunctional compounds i.e. 4‐amino‐1‐butanol. This synthetic procedure was also employed for industrially relevant compounds such as (‐)‐norlabdane oxide and isosorbide showing to maintain the chiral integrity of the substrate. In one case intramolecular cyclisation of isosorbide was also observed to achieve a strained tricyclic derivative. Comparing this reaction methodology with a chlorine based procedure, the DMC‐mediated pathway is quantitative, occurs in one step, does not require any chlorine‐based chemical or strong acid and does not produce any chlorinated waste material.     

Synthesis and photophysical properties of cyclometalated platinum(II) 1,2-benzenedithiolate complexes and heterometallic derivatives obtained from the addition of [Au(PCy3)]+ units

The cyclometalated compounds [Pt(C^N)(HC^N)Cl] [HC^N = 2-phenylpyridine (Hppy; 1a), 1-(4-tert-butylphenyl)isoquinoline (Htbpiq; 1b)] react with 1,2-benzenedithiol, t-BuOK, and Bu(4)NCl in a 1:1:2:1 molar ratio in CH(2)Cl(2)/MeOH to give the complexes Bu(4)N[Pt(C^N)(bdt)] [bdt = 1,2-benzenedithiolate; C^N = ppy (Bu(4)N2a), tbpiq (Bu(4)N2b)]. In the absence of Bu(4)NCl, the same reactions afford solutions of K2a and K2b, which react with [AuCl(PCy(3))] to give the neutral heterometallic derivatives [Pt(C^N)(bdt){Au(PCy(3))}] [C^N = ppy (3a), tbpiq (3b)]. The cationic derivatives [Pt(C^N)(bdt){Au(PCy(3))}(2)]ClO(4) [C^N = ppy (4a), tbpiq (4b)] are obtained by reacting 3a and 3b with acetone solutions of [Au(OClO(3))(PCy(3))]. The crystal structures of 3b and 4b reveal the formation of short Pt···Au metallophilic contacts in the range 2.929-3.149 ?. Complexes 3b, 4a, and 4b undergo dynamic processes in solution that involve the migration of the [Au(PCy(3))](+) units between the S atoms of the dithiolate. Complexes Bu(4)N2a and 2b display a moderately solvatochromic band in their electronic absorption spectra that can be ascribed to a transition of mixed ML'CT/LL'CT character (M= metal; L = bdt; L' = C^N; CT = charge transfer), while their emissions are assignable to transitions of the same orbital parentage but from triplet excited states. The successive addition of [Au(PCy(3))](+) units to the anions 2a and 2b results in an increase in the absorption and emission energies attributable to lower highest occupied molecular orbital energies. Additionally, the characteristics of the absorption and emission spectra of the heterometallic derivatives indicate a gradual loss of LL'CT character in the involved electronic transitions, with a concomitant increase of the L'C and ML'CT contributions.     

Nanotribology-based novel characterization techniques for the dielectric charging failure mechanism in electrostatically actuated NEMS/MEMS devices using force-distance curve measurements

The work presents a comprehensive package of novel nanoscale characterization techniques to study dielectric charging in electrostatic nano- and microelectromechanical systems (NEMS and MEMS). The proposed assessment methodologies are based on the force-distance curve (FDC) measurements performed using an atomic force microscope (AFM) to measure, for the first time, the induced surface potential and adhesive force over charged dielectric films. They were employed to study plasma enhanced chemical vapor deposition (PECVD) silicon nitride films for application in electrostatic capacitive RF MEMS switches. Three different techniques were introduced including the application of FDC measurements to study charging in bare SiN(x) films, metal-insulator-metal (MIM) capacitors, and MEMS switches. The results from the three methods were correlated and compared with the published data from other characterization techniques, mainly charge/discharge current transient (C/DCT) and Kelvin probe force microscopy (KPFM). The unique advantages of the proposed FDC-based characterization techniques are twofold. First, they can measure the multiphysics coupling between the dielectric charging phenomenon and tribological issues at the interface between the switch bridge and the dielectric surface. Second, the FDC-based techniques can measure larger levels of induced surface potential over charged dielectric films which results from the high electric field normally used to actuate MEMS switches. Based on the proposed FDC techniques, the influence of several parameters on dielectric charging/discharging processes was investigated: the dielectric film thickness, deposition conditions, substrate, and electrical stress conditions.