Intermolecular Pauson-Khand reactions of cyclopropene: a general synthesis of cyclopentanones

[reaction: see text] The Pauson-Khand reaction of cyclopropene with a variety of terminal alkynes has been studied. The best reaction conditions involve NMO activation in CH(2)Cl(2) at -35 degrees C. In this way, 3-substituted-bicyclo[3.1.0]hex-3-en-2-ones have been obtained in good to excellent yields. As a synthetic application, several types of substituted cyclopentenones have been prepared from these cycloadducts by protocols involving conjugate addition and reductive ring opening.     

Exfoliated graphite nanoplatelets and gold nanoparticles based electrochemical sensor for determination of levodopa

This paper describes a rapid, accurate, and sensitive method for the determination of levodopa in a pharmaceutical sample using a glassy carbon electrode modified with a hybrid nanocomposite constituted of exfoliated graphite nanoplatelets dispersed in a suspension of gold nanoparticles in carboxymethylcelullose (AuNP-CMC-xGnP/GCE). The nanocomposite was characterized by scanning electron microscopy, transmission electron microscopy, UV-Vis spectroscopy, and zeta potential. Electrochemical characterization of the proposed sensor by cyclic voltammetry and electrochemical impedance spectroscopy indicated that the nanocomposite used for the electrode modification facilitated electron transfer. Using square-wave voltammetry (SWV) under optimized conditions (0.50% (m/v) of AuNP-CMC-xGnP, 0.1 mol L^?1 sulfuric acid, frequency 30 Hz, pulse amplitude 50 mV, and scan increment 6.0 mV), the calibration curve showed a linear range for levodopa from 5 to 50 μmol L^?1, with a limit of detection of 0.5 μmol L^?1. The sensor demonstrated good repeatability and electrode-to-electrode repeatability, with relative standard deviations of 2 and 4%, respectively. The proposed method was successfully applied to quantify levodopa in a pharmaceutical sample by SWV, showing good accuracy. Recoveries of 98 to 107% demonstrated that the method is suitable for practical applications. Therefore, the proposed sensor represents a useful tool for rapid and accurate determination of levodopa.     

Morphology, flexural, and thermal properties of sepiolite modified epoxy resins with different curing agents

A bisphenol A-based epoxy resin was modified with 5 wt% organically modified sepiolite (Pangel B40) and thermally cured using two different curing agents: an aliphatic diamine (Jeffamine D230, D230) and a cycloaliphatic diamine (3DCM). The morphology of the cured materials was established by scanning and transmission electron microscopy analysis. The thermal stability, thermo-mechanical properties, and flexural behaviour of the sepiolite-modified matrices were evaluated and compared with the corresponding neat matrix. The initial thermal decomposition temperature did not change with the addition of sepiolite. The flexural modulus of the epoxy matrix slightly increases by the incorporation of the organophilic sepiolite. The flexural strength of the sepiolite modified resin cured with D230 increased by a 10% while the sepiolite modified resin cured with 3DCM resulted in a lower flexural strength compared with the unmodified resin. The reduced flexural strength was attributed to the stress concentrations caused by the sepiolite modifier, which rendered the resins more brittle.     

Electroregenerable anion-exchange resin with triiodide carbon paste electrode for the voltammetric determination of adrenaline

An electroregenerable carbon paste electrode modified with triiodide ions immobilized in an anion-exchange resin (Lewatit M500) is proposed for the determination of adrenaline in pharmaceutical products by differential-pulse voltammetry (DPV). Adrenaline was chemically converted into adrenochrome by the I3- ions at the electrode surface. The electrochemical reduction back to adrenaline was obtained at a potential of -0.16 V vs. Ag/AgCl (3 mol l(-1) KCl). A 20% decrease of the initial analytical signal was observed after 350-400 determinations; the carbon paste electrode was 100% electroregenerated at a fixed potential of +0.65 V vs. Ag/AgCl (3 mol l(-1) KCl) in 0.1 mol l(-1) KI solution for 20 min. The differential-pulse voltammograms were obtained by applying a sweep potential between 0.0 and -0.34 V, following the adrenochrome reduction at -0.16 V. Under the optimum conditions established, such as pH 6.0; scan rate 20 mV s(-1) and pulse amplitude 50 mV, the calibration curve was linear from 2.0 x 10(-5) to 3.1 x 10(-4) mol l(-1) adrenaline with a detection limit of 3.9 x 10(-6) mol l(-1). The recovery of adrenaline ranged from 99.8 to 103.1% and the RSD was 2.6% for the solution containing 1.0 x 10(-4) mol l(-1) adrenaline (n = 10). The results obtained for adrenaline in pharmaceutical samples using the proposed carbon paste electrode are in agreement with those obtained using a pharmacopoeial procedure at the 95% confidence level.     

Sensor Modified with Gold Nanoparticles Stabilized in Dialdehyde Starch/DMSO Matrix for Methyldopa Detection

In this study, gold nanoparticles (AuNP) were synthesized using a novel stabilizer based on dialdehyde starch polymer (DAS) and dimethyl sulfoxide (DMSO) and the nanomaterial was applied to develop a sensor for methyldopa detection. The nanoparticles were characterized by transmission electron microscopy and the proposed sensor was electrochemically characterized by electrochemical impedance spectroscopy, chronocoulometry and square-wave voltammetry. Under optimized conditions (0.1 mol L⁻¹ McIlvaine buffer solution at pH 3.5 and 50 μL AuNP/DAS-DMSO), using square-wave voltammetry, the calibration curve presented a linear range of 0.99 to 19.60 μmol L⁻¹, with a detection limit of 0.50 μmol L⁻¹. The sensor showed good accuracy, with coefficients of variation of 7.8 % (inter-day; n=6) and 5.9 % (intra-day; n=3), and recovery values in the range of 95.3 to 105.2 %. The methyldopa content in a pharmaceutical sample was determined and the results show good correlation with the label value and the method recommended by the Pharmacopoeia, representing a fast and accurate alternative for detecting methyldopa in pharmaceutical products.     

Novel Metal‐Polyurethane Complexes with Enhanced Antimicrobial Activity

Summary: The silver coating of polymers has been intensively investigated in the last few decades as an effective non‐resistance‐inducing strategy to prevent medical device‐related infections. We have developed a completely new approach to incorporate silver ions in polymers by the synthesis of a carboxylated polyurethane able to coordinate Ag⁺. The obtained polymers possess mechanical properties suitable for the development of medical devices, without any risk of loss of activity. To minimize the risk of increasing antibiotic resistance, the metal ion‐containing polymers are loaded with ciprofloxacin, which possesses a different mechanism of antimicrobial action, thus a system able to inhibit Staphylococcus epidermidis and Pseudomonas aeruginosa growth for at least one month is developed.

SEM images showing a mature S. epidermidis biofilm on the surface of the carboxylated polyurethane (left) and the surface of the silver ion‐containing polyurethane free from bacterial colonization (right) after 48 h of incubation.     

Development of Quercetin Biosensor Through Immobilizing Laccase in a Modified β‐Cyclodextrin Matrix Containing Ag Nanoparticles in Ionic Liquid

A biosensor containing Ag nanoparticles in ionic liquid (IL) 1‐butyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide (BMI.Tf₂N) and laccase (Lac) immobilized on β‐cyclodextrin modified with epichlorohydrin (β‐CDEpi) was developed for quercetin determination. Lac catalyses the oxidation of quercetin to quinone, which is then reduced on the biosensor surface and the resulting current was investigated by square‐wave voltammetry. The β‐CDEpi support was analyzed by scanning electron microscopy and Ag‐BMI.Tf₂N by transmission electron microscopy and X‐ray diffraction. The linear range (0.499–7.407 μM) and low detection limit (0.037±0.004 μM) show that the proposed biosensor is suitable for quercetin determination in real samples.     

Aromatic nitro substitution reaction between 4‐nitro‐N‐n‐butyl‐1,8‐naphthalimide and n‐heptanethiol in water–methanol binary mixtures

The second‐order rate constants of thiolysis by n‐heptanethiol on 4‐nitro‐N‐n‐butyl‐1,8‐naphthalimide (4NBN) are strongly affected by the water–methanol binary mixture composition reaching its maximum at around 50% mole fraction. In parallel solvent effects on 4NBN absorption molar extinction coefficient also shows a maximum at this composition region. From the spectroscopic study of reactant and product and the known H‐bond capacity of the mixture a rationalization that involves specific solvent H‐donor interaction with the nitro group is proposed to explain the kinetic data. Present findings also show a convenient methodology to obtain strongly fluorescent imides, valuable for peptide and analogs labeling as well as for thio‐naphthalimide derivatives preparations. Copyright © 2008 John Wiley & Sons, Ltd.     

Novel Composite Plastics Containing Silver(I) Acylpyrazolonato Additives Display Potent Antimicrobial Activity by Contact

New silver(I) acylpyrazolonato derivatives displaying a mononuclear, polynuclear, or ionic nature, as a function of the ancillary azole ligands used in the synthesis, have been fully characterized by thermal analysis, solution NMR spectroscopy, solid‐state IR and NMR spectroscopies, and X‐ray diffraction techniques. These derivatives have been embedded in polyethylene (PE) matrix, and the antimicrobial activity of the composite materials has been tested against three bacterial strains (E. coli, P. aeruginosa, and S. aureus): Most of the composites show antimicrobial action comparable to PE embedded with AgNO₃. Tests by contact and release tests for specific migration of silver from PE composites clearly indicate that, at least in the case of the PE, for composites containing polynuclear silver(I) additives, the antimicrobial action is exerted by contact, without release of silver ions. Moreover, PE composites can be re‐used several times, displaying the same antimicrobial activity. Membrane permeabilization studies and induced reactive oxygen species (ROS) generation tests confirm the disorganization of bacterial cell membranes. The cytotoxic effect, evaluated in CD34⁺ cells by MTT (3‐(4,5‐dimethylthiazole‐2‐yl)‐2,5‐diphenyltetrazoliumbromide) and CFU (colony forming units) assays, indicates that the PE composites do not induce cytotoxicity in human cells. Studies of ecotoxicity, based on the test of Daphnia magna, confirm tolerability of the PE composites by higher organisms and exclude the release of Ag⁺ ions in sufficient amounts to affect water environment.