Synthesis of vinca alkaloids and related compounds. 100. Stereoselective oxidation reactions of compounds with the aspidospermane and quebrachamine ring system. First synthesis of some alkaloids containing the epoxy ring

The first syntheses of the alkaloids (-)-mehranine (3), (+)-voaphylline/conoflorine (4), (+)-N(a)-methylvoaphylline/hecubine (5), and (-)-lochnericine (2) were achieved by stereoselective epoxidation starting from (-)-tabersonine (1), through intermediates with the aspidospermane and quebrachamine skeleton.     

Quantitative analysis of six pesticides in fruits by capillary electrophoresis-electrospray-mass spectrometry

A method to identify and quantify six pesticide residues - dinoseb, pirimicarb, procymidone, pyrifenox, pyrimethanil, and thiabendazole - in peaches and nectarines using capillary electrophoresis-electrospray ionization-quadrupole ion trap-tandem mass spectrometry (CE-ESI-MS/MS) is described. Separation was carried out using a buffer of 0.3 M ammonium acetate at pH 4 with 10% methanol. Pesticide residues present in peach and nectarine samples were preconcentrated by solid-phase extraction using C(18), eluted with CH(2)Cl(2), concentrated to dryness, and redissolved in buffer to obtain lower detection limits. The recoveries of the analytes ranged from 58 to 99% and the relative standard deviations were 9 to 19%. Under optimized CE-MS/MS conditions the minimum detectable levels for the six pesticides in spiked peach samples were between 0.01 mg/kg for pirimicarb and 0.05 mg/kg for procymidone with pressure injection of 50 mbar for 5 s (5 nL) at a signal-to-noise ratio of 3, which constitutes a severalfold increase in sensitivity compared to CE-MS, using a single quadrupole, and to conventional CE-UV. The potential of the method was demonstrated by analyzing different samples taken from regional agricultural cooperatives. The pesticides most often detected were thiabendazole and procymidone.     

Epoxides,cyclic sulfites,and sulfate from natural pentacyclic triterpenoids: theoretical calculations and chemical transformations

Several triterpenic derivatives, with the A-ring functionalized, were semisynthesized from oleanolic and maslinic acids. The reactivities of sulfites, sulfate, and epoxides in these triterpene compounds were investigated under different reaction conditions. Moreover, contracted A-ring triterpenes (five-membered rings) were obtained, by different treatments of the sulfate 7. From the epoxide 8, deoxygenated and halohydrin derivatives were semisynthesized with several nucleophiles. Ozonolysis and Beckmann reactions were used to yield 4-aza compounds, from five-membered ring olanediene triterpenes. The X-ray structure of sulfate 7 is given and compared with density functional theory geometries. Theoretical (13)C and (1)H chemical shifts (gauge-invariant atomic orbital method at the B3LYP/6-31G*//B3LYP/6-31G* level) and (3)J(H,H) coupling constants were calculated for compounds 5-9 and 34-36, identifying the (R)- or (S)-sulfur and alpha- or beta-epoxide configurations together with 4-aza or 3-aza structures.     

Palladium-catalyzed electrophilic substitution of allyl chlorides and acetates via bis-allylpalladium intermediates

Palladium-catalyzed electrophilic allylic substitution of functionalized allyl chlorides and allyl acetates can be achieved in the presence of hexamethylditin under mild and neutral reaction conditions. This efficient one-pot procedure involves palladium-catalyzed formation of transient allylstannanes followed by generation of a bis-allylpalladium intermediate, which subsequently reacts with electrophiles. Using this catalytic transformation, various aldehydes and imines can be allylated providing highly functionalized homoallyl alcohols and amines. Furthermore, tandem bis-allylation reactions could be performed by employing tosyl isocyanate and benzylidenemalonitrile as substrates. A particularly interesting mechanistic feature of this reaction is that palladium catalyzes up to three different transformations in each catalytic cycle. Various allylic functionalities, including COOEt, CONH(2), COCH(3), CN, Ph, and CH(3), are tolerated in the catalytic reactions due to the application of neutral and mild reaction conditions. The substitution reaction occurs with very high regioselectivity at the branched allylic terminus. Moreover, in several reactions, a high stereoselectivity was observed indicating that this new catalytic process has a high potential for stereoselective synthesis. The regioselectivity of the reaction can be explained on the basis of DFT calculations. These studies indicate that the allylic substituent prefers the gamma-position of the eta(1)-allyl moiety of the reaction intermediate.     

Application of principal component regression to luminescence data for the screening of ciprofloxacin and enrofloxacin in animal tissues

A new screening method for the analysis of enrofloxacin and ciprofloxacin in edible animal tissues is described. The method is based on the application of principal component regression to luminescence measurements after reaction of quinolones with terbium(III) in a micellar medium. The method was used, first, to discriminate between quinolone-containing or quinolone-not-containing samples (concentration below the detection limit) and, then, to quantify the sum of both analytes. Standards in a pure-water matrix, using the first three principal components, were used for the determination. RRMSE range from 4 to 10% depending on the analyte. Calibration was successfully applied to the analysis of spiked chicken and trout muscle at concentrations between 10 and 50 micrograms kg-1.     

Determination of macrolide antibiotics by liquid chromatography

The liquid chromatographic separation of seven macrolides used in food producing animals in the European Union has been studied. Separation was performed by using an end-capped high-purity silica-based C₁₈ column and mobile phases consisting of phosphate buffer (pH 2.5)–acetonitrile mixtures. The effect of pH and acetonitrile percentage on the separation was examined. Two UV-based detection systems, wavelength programming and diode array, were assayed. Detection limits were in the range 6–33 μg l⁻¹ for spiramycin, tilmicosin, tylosin, kitasamicin and josamicin and about 400 μg l⁻¹ for erythromycin and oleandomycin. The suitability of the method for multiresidue determination of the five macrolides is demonstrated by the analysis of spiked samples of chicken muscle.     

A new approach for the sequential injection spectrophotometric determination of the total antioxidant activity

A sequential injection system based on the ABTS (2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulphonic-acid) methodology was developed. The proposed method, incorporating a mixing chamber in the side port of the selection valve, was evaluated to measure the total antioxidant activity of several beverages and foods.The ABTS⁺ is generated by oxidation of ABTS with potassium persulfate and is reduced in presence of hydrogen-donating antioxidants converting into a colourless product. The applicability of the developed method was tested by measurement of the antioxidant activity of pure compounds as well as by analysing complex food and beverage samples. The antioxidant activity was presented as l(+) ascorbic acid equivalence. The values obtained by this methodology were not significantly different from the results obtained by the original spectrophotometric ABTS assay. For most of the studied antioxidants, antioxidant activity varied with pH and dilution. The proposed SIA system is suitable for screening direct or diluted total antioxidant activity of pure compounds or food samples.     

Experimental and quantum chemical studies of structure and reaction mechanisms of dioxouranium(VI) complexes in solution

This perspective article describes the combination of experimental data and quantum chemical methods for the determination of structure and reaction mechanisms of uranyl(vi) complexes in aqueous solution. The first part assesses the accuracy of the chemical and thermodynamic properties of solvated uranyl(vi) complexes as obtained by various quantum chemical methods. The second part discusses structure determination, mechanisms for ligand exchange and the lability of coordinated water molecules for various uranyl(vi) complexes using a combination of NMR and quantum chemical data.     

Treating singularities present in the Sutcliffe-Tennyson vibrational Hamiltonian in orthogonal internal coordinates

Two methods are developed, when solving the related time-independent Schrodinger equation (TISE), to cope with the singular terms of the vibrational kinetic energy operator of a triatomic molecule given in orthogonal internal coordinates. The first method provides a mathematically correct treatment of all singular terms. The vibrational eigenfunctions are approximated by linear combinations of functions of a three-dimensional nondirect-product basis, where basis functions are formed by coupling Bessel-DVR functions, where DVR stands for discrete variable representation, depending on distance-type coordinates and Legendre polynomials depending on angle bending. In the second method one of the singular terms related to a distance-type coordinate, deemed to be unimportant for spectroscopic applications, is given no special treatment. Here the basis set is obtained by taking the direct product of a one-dimensional DVR basis with a two-dimensional nondirect-product basis, the latter formed by coupling Bessel-DVR functions and Legendre polynomials. With the basis functions defined, matrix representations of the TISE are set up and solved numerically to obtain the vibrational energy levels of H3+. The numerical calculations show that the first method treating all singularities is computationally inefficient, while the second method treating properly only the singularities having physical importance is quite efficient.     

An algorithm for determining dynamically defined reaction paths (DDRP)

Summary A numerically stable and well-parallelizable curve variational algorithm is described for determining tangent curves of vector fields between two given stationary points. In particular, the method is suitable for finding reaction paths and saddle points on potential energy hypersurfaces (PHS). The stability of the procedure is illustrated by an artificial mathematical function, showing phases of following the reaction on the PHS.Dedicated to Professor Zoltán G. Szabó, the great teacher and scientist in reaction kinetics and in many other fields of physical chemistry, on his 84th birthday.