Rhenium(I)-induced cyclization of thiosemicarbazones derived from beta-keto esters

The reactions of methylacetoacetate and ethyl 2-methylacetoacetate thiosemicarbazones (H(2)L(A) and H(2)L(B), respectively) with [ReX(CO)(5)] and [ReX(CO)(3)(CH(3)CN)(2)] (X = Cl, Br) were explored under various experimental conditions. Besides the adducts fac-[ReX(CO)(3)(H(2)L)], in which the rhenium is coordinated to three carbonyl groups, the X anion, and the N,S-bidentate thiosemicarbazone ligand, the following complexes were also isolated: fac-[ReBr(CO)(3)(Hpyz(B))], the tetrameric complexes fac-[Re(pyz(A))(CO)(3)](4) and fac-[Re(pyz(B))(CO)(3)](4), and fac-[Re(pyz(B))(CO)(3)(H(2)O)] (where Hpyz(A) and Hpyz(B) are pyrazolones derived by cyclization of H(2)L(A) and H(2)L(B), respectively). The cyclization reactions were monitored by (1)H NMR spectroscopy and the complexes isolated were identified by elemental analysis, mass spectrometry, IR and (1)H NMR spectroscopy, and in some cases by X-ray diffractometry. The isolation and the full structural identification of the rather unusual fac-[ReBr(CO)(3)(Hpyz(B))], which contains the enol form of the pyrazolone ligand, affords new insight into the cyclization of thiosemicarbazones derived from beta-keto esters.     

Diels-Alder reaction of optically active (E)-γ-keto-α,β-unsaturated p-tolylsulfoxides with cyclopentadiene

The Diels-Alder reaction of enantiomerically pure (E)-γ-keto-α,β-unsaturated p-tolylsulphoxides 3 with cyclopentadiene give four easily separable diastereomers. The effect of several Lewis acids on the reaction was studied, finding a high endo selectivity with respect to the carbonyl group and moderate π-diastereoselectivity using BF₃·Et₂O as catalyst. The reactivity of compounds 3 as well as their endo selectivity are both higher than those observed for the corresponding (E)-3-sulfinylacrylates.     

Analysis of nickel-niobium alloys by inductively coupled plasma optical emission spectrometry

Inductively coupled plasma optical emission spectrometry (ICP-OES) was applied to the analysis of major and minor elements of Ni-Nb alloys obtained by aluminothermic reduction process. Digestion of samples was made using a mixture of HF+HNO₃. Minor and trace elements were determined without matrix separation. The precision for all constituents was <3%. Recoveries for the analyte-spiked samples were 95%.     

Biogenetically inspired enantioselective approach to Indolo[2,3-a]- and benzo[a]quinolizidine alkaloids from a synthetic equivalent of secologanin

[reaction: see text] Racemic oxodiester 1 undergoes stereoselective cyclocondensation with (S)-tryptophanol, (S)-(3,4-dimethoxyphenyl)alaninol, or the corresponding amino acids, in a process involving a tandem dynamic kinetic resolution/desymmetrization of diastereotopic groups, to give bicyclic lactams, which are cyclized to substituted indolo[2,3-a]- and benzo[a]quinolizidines.     

Self-assembly of [2]rotaxane exploiting reversible Pt(II)- pyridine coordinate bonds

A dinuclear self-assembled cationic macrocycle based on Pt(II)-N(pyridine) coordinative bonds and having competitive triflate anions, as metal counterions, is used in the construction of [2]rotaxane and [2]pseudorotaxane architectures assisted by hydrogen bonding. The kinetic lability of the Pt(II)-N(pyridine) coordinative bond controls the dynamics of the [2]rotaxane.     

A new approach to the synthesis of oligomers. Application to the synthesis of p-phenylene thioether wires

Oligothioethers 4-RC₆H₄(SC₆H₄-4)_nX (n = 1-3; X = Br, I; R = NO₂; X = Br; R = MeO. n = 1 and 2; X = I; R = MeO. n = 4; X = Br; R = NO₂) have been prepared through a process involving (i) palladium-catalyzed C-S coupling between 4-RC₆H₄(SC₆H₄-4)_n−1I and 4-BrC₆H₄SH to give 4-RC₆H₄(SC₆H₄-4)_nBr and (ii) copper-catalyzed replacement of Br by I.     

Environmental effects on a prion's helix II domain: copper(II) and membrane interactions with PrP180-193 and its analogues

An abnormal interaction between copper and the prion protein is believed to play a pivotal role in the pathogenesis of prion diseases. Copper binding has been mainly attributed to the N-terminal domain of the prion protein, but this hypothesis has recently been challenged in some papers which suggest that the C-terminal domain might also compete for metal anchoring. In particular, the segment corresponding to the helix II region of the prion protein, namely PrP180-193, has been shown both to bind copper and to exhibit a copper-enhanced cytotoxicity, as well as to interact with artificial membranes. The present work is aimed at extending these results by choosing the most representative model of this domain and by determining its copper affinity. With this aim, the different role played by the electrostatic properties of the C- and N-termini of PrP180-193 (VNITIKQHTVTTTT) in determining its conformational behaviour, copper coordination and ability to perturb model membranes was investigated. Owing to the low solubility of PrP180-193, its copper affinity was evaluated by using the shorter PrPAc184-188NH2 (IKQHT) analogue as a model. ESI-MS, ESR, UV/Vis, and CD measurements were carried out on the copper(II)/PrPAc184-188NH2 and copper(II)/PrP180-193NH2 systems, and showed that PrPAc184-188NH2 is a reliable model for the metal interaction with the helix II domain. The affinity of copper(II) for the helix II fragment is higher than that for the octarepeat and PrP106-126 peptides. Finally, the different ability of PrP180-193 analogues to perturb the DPPC model membrane was assessed by DSC measurements. The possible biological consequences of these findings are also discussed briefly.     

Incorporation conditions guiding the aggregation of a glycosylphosphatidyl inositol (GPI)-anchored protein in Langmuir monolayers

This work investigates the process of incorporation of a glycosylphosphatidyl inositol (GPI)-anchored alkaline phosphatase into Langmuir monolayers of dimyristoyl phosphatidic acid (DMPA). Three different methods of protein incorporation were assayed. When the protein solution was injected below the air–water interface after formation of the lipid monolayer a micro-heterogeneous distribution of alkaline phosphatase throughout the interface was observed. Adsorption kinetics studied by fluorescence microscopy, associated with surface pressure measurements, led to the proposition of a model in which the protein penetration is modulated by the surface packing of the monolayer and intermolecular interactions occurring between the phospholipid and the protein. At initial surface pressures higher than 20 mN m⁻¹, the protein is quickly adsorbed on the interface and the lateral diffusion drives the alkyl chains to turn towards the air phase while the polypeptide moiety faces the aqueous subphase.     

Solid-phase extraction system for Pb (II) ions enrichment based on multiwall carbon nanotubes coupled on-line to flame atomic absorption spectrometry

The present paper proposes the application of multiwall carbon nanotubes (MWCNTs) as a solid sorbent for lead preconcentration using a flow system coupled to flame atomic absorption spectrometry. The method comprises the preconcentration of Pb (II) ions at a buffered solution (pH 4.7) onto 30 mg of MWCNTs previously oxidized with concentrated HNO₃. The elution step is carried out with 1.0 mol L⁻¹ HNO₃. The effect of the experimental parameters, including sample pH, sampling flow rate, buffer and eluent concentrations were investigated by means of a 2⁴ full factorial design, while for the final optimization a Doehlert design was employed. Under the best experimental conditions the preconcentration system provided detection and quantification limits of 2.6 and 8.6 μg L⁻¹, respectively. A wide linear range varying from 8.6 up to 775 μg L⁻¹ (r > 0.999) and the respective precision (relative standard deviation) of 7.7 and 1.4% for the 15 and 200 μg L⁻¹ levels were obtained. The characteristics obtained for the performance of the flow preconcentration system were a preconcentration factor of 44.2, preconcentration efficiency of 11 min⁻¹, consumptive index of 0.45 mL and sampling frequency estimated as 14 h⁻¹. Preconcentration studies of Pb (II) ions in the presence of the majority foreign ions tested did not show interference, attesting the good performance of MWCNTs. The accuracy of the method was assessed from analysis of water samples (tap, mineral, physiological serum and synthetic seawater) and common medicinal herbs submitted to the acid decomposition (garlic and Ginkgo Biloba). The satisfactory recovery values obtained without using analyte addition method confirms the feasibility of this method for Pb (II) ions determination in different type of samples.     

Solar energy conversion from water photolysis by biological and chemical systems

The production of chemicals and fuels, or energy-rich compounds, from water by sunlight is described as a particularly attractive means for the conversion of solar energy to a valuable renewable resource. The redox properties of photoexcited molecules and the operating mechanism of light-driven systems are first considered. The mechanism of water oxidation carried out by higher plants and green algae-which is actually one of the most important biochemical reactions—as well as that of artificial photosystems, up-to-now designed trying to simulate the natural process with higher efficiency and simplicity, are likewise discussed. A number of biological and chemical light-driven systems are presented as practical ways to solar energy conversion.