Conjugate addition of organocuprates to chiral bicyclic delta-lactams. Enantioselective synthesis of cis-3,4-disubstituted and 3,4,5-trisubstituted piperidines

[reaction: see text] Chiral nonracemic bicyclic lactam 3, easily accessible by cyclodehydration of (R)-phenylglycinol and racemic methyl 4-formylhexanoate, was converted to the unsaturated lactams 5, which undergo the stereoselective conjugate addition of lower order cyanocuprates to ultimately lead to enantiopure cis-3,4-disubstituted and 3,4,5-trisubstituted piperidines.     

Determination of hydroxycarbamates in aqueous matrices by direct derivatization and GC-MS analysis in chemical ionization mode

A rapid and effective procedure, developed for the determination of polar compounds in water, has been applied to hydroxycarbamates. In few minutes, it is possible to perform a catalyzed derivatization of such compounds directly in the aqueous medium, using n-hexyl chloroformate. Extraction with n-hexane and injection into a benchtop GC-MS system are the only two further steps to complete the analysis. Chemical ionization proved to be more effective than electron impact to produce valuable mass spectra in terms of selectivity and sensitivity. Linear quantitative responses were demonstrated over a two-order-of-magnitude range. The minimum detectable concentration was 3 ppb for acetohydroxamic acid and 10 ppb for hydroxyurethane, obtained from Po river water samples spiked with the two hydroxycarbamate standards.     

A new insight on the structural changes of linear quadrupole liquids

Molecular-dynamics simulations for linear quadrupole liquids are presented. The study is carried out for two different molecular lengths at constant density and a number of temperatures and quadrupole moments. All the simulated thermodynamic states correspond to the condensed phases and some of them show typical features of a solid structure. Furthermore, a change on the preferred intermolecular orientation in the liquid phase is observed from a shifted parallel molecular arrangement to a perpendicular orientation as the quadrupole raises. This change depends on the quadrupole moment as well as on the molecular length and is put in relation with the solid structure of different "diatomic" molecules such as nitrogen, ethane, and acetylene. The appearance of a plastic solid phase at low quadrupole moment and density is also justified. A thoroughly discussion about the availability of classical perturbation theories for this kind of systems is presented.     

Equations of state of mixtures of hard dumbbells

Two ways to predict pVT behavior of hard-dumbbell mixtures in the fluid region are proposed. These are adaptations of the hard convex body equation devised by Boublik and an extension of the first Barker-Henderson perturbation theory. Calculated compressibility factors show good agreement with the ideal mixing assumption and yield a useful equation of state.     

Synthesis of amberlite XAD-2-PC resin for preconcentration and determination of trace elements in food samples by flame atomic absorption spectrometry

A new chelating sorbent has been developed using Amberlite XAD-2 resin anchored with pyrocatechol through –N=C– group. This sorbent, characterised by elemental analysis and infrared (IR) spectra, was used as packing for the minicolumn in an on-line system preconcentration system for cadmium, cobalt, copper and nickel determination. Metal ions were sorbed in the minicolumn, from which it could be eluted directly to the nebulizer–burner system of the flame atomic absorption spectrometer (FAAS). Elution of all metals from minicolumn can be made with 0.50 mol L^− 1 HCl or HNO₃. The enrichment factors obtained were 16 (Cd), 24 (Co), 15 (Cu) and 19 (Ni), for 60 s preconcentration time, and 39 (Cd), 69 (Co), 36 (Cu) and 41 (Ni), if used 180 s preconcentration time. Under the optimum conditions, the proposed procedure allowed the determination of cadmium, cobalt, copper and nickel with detection limits of 0.31, 0.32, 0.39 and 1.64 μg L^− 1, respectively, when used preconcentration periods of 180 s. The accuracy of the developed procedure was sufficient and evaluated by the analysis of the certified reference materials NIST 1515 apple leaves and NIST 1570a spinach leaves. The method was applied to the analysis of food samples (spinach, black tea and rice flour).     

Determination of Sulfaguanidine in Food and Veterinary Pharmaceuticals by Flow Injection Analysis with Spectrophotometric Detection

This work proposes a flow injection analysis system for sulfaguanidine determination in pharmaceutical and food samples. The method was based on the reaction of sulfaguanidine with nitronium ion to produce a colored complex whose absorbance was measured at 545?nm. The flow injection analysis system’s significant parameters were checked by a fractional factorial design 2^7–2 and optimization by a Doehlert matrix. The flow injection analysis system shows optimum values at 0.28, 2.00, and 0.11% (w/v) for N-naphtil ethylenediamine, ammonium sulfamate, and sodium nitrite concentrations, respectively. The possible interferents present in pharmaceutical and food samples were assessed by a multivariate technique and depicted on probability charts, indicating no significant interferences at the 95% level of confidence interval. The method showed detection and quantification limits of 0.012 and 0.039?mg?L^?1, an analytical frequency of 30 readings h^?1 and precision always lower than 5.0% expressed as the relative standard deviation. The obtained results were in accordance with those obtained by high-performance liquid chromatography.     

Experimental and theoretical investigation of the parabanic acid molecule following VUV excitation and photodissociation

Photodissociation experiments have been performed for the parabanic acid (C₃H₂N₂O₃) molecule in vapor phase using time-of-flight mass spectrometry and synchrotron radiation in the VUV photon energy range. Electron ion coincidence (PEPICO) spectra and partial ion yields have been recorded as a function of the photon energy covering the 11–21 eV valence range region. The resulting photoionization products as well as proposed fragmentation pathways leading to those species are presented and discussed. Electronic structure computations for the neutral and ionic species were also carried out at the B3LYP/aug-cc-pVTZ level of theory.     

Highly sensitive ion preconcentration method based on flow sorbent extraction using multiwall carbon nanotubes

In this work a solid phase on-line uranium ion preconcentration system coupled with spectrophotometry has been developed. The method is based on uranyl ion preconcentration at pH 3.75 onto multiwall carbon nanotubes treated with HNO₃. After preconcentration, the uranyl ions are eluted with 0.32?mol?L^?1HCl followed by reaction with 3,6-bis[(2-arsonophenyl)-azo]-4,5-dihydroxy-2,7-naphthalendisulfonic acid 0.08%[w/v] (Arsenazo III), which had maximum monitored absorbance of 650?nm. Effects of the pertinent experimental parameters on the system were investigated by means of 2^6?2 fractional factorial design, while optimization was carried out using the Doehlert matrix. Under optimized conditions, detection and quantification limits were found to be 0.21 and 0.7?µg?L^?1, respectively. The analytical curve ranged from 5 to 150?µg?L^?1 (r?=?0.998), while the relative standard deviations (RSD) were 3.27 and 2.56% for the respective uranium concentrations of 10 and 100?µg?L^?1 (n?=?10). The features obtained for the on-line preconcentration system were: preconcentration factor of 228, concentration efficiency of 57?min^?1, consumption index of 0.13?mL and sample throughput of 15?h^?1. In order to assess the accuracy of the proposed method, addition and recovery studies were carried out on spring water samples from different sources and synthetic seawater with satisfactory results ranging from 94.85 up to 103.65%.