Reactions of pyrrolidine enamines of cyclic and acyclic 3,4-dioxobutanoic acid Derivatives with dimethyl acetylenedicarboxylate. A new case of atropoisomerism.

Reaction conditions and structure of the starting enamines (cyclic or open-chain) determine greatly the final products of the title reactions. Whereas in benzene and acetonitrile, DMAD and 1 give a mixture of the diastereoisomeric dienamines 5, in methanol they afford pirrolizine 3. Enaminofuranones 2 and 10 furnish the corresponding “Michael adducts” 7a,b,c and 11a,b,c but fail to yield pirrolizines. It has been demonstrated that above b and c adducts differ exclusively on the arrangement of groups around a chiral axis.     

Part I. epoxydes diterpeniques : synthese et reactivite d'epoxydes derives d'acides resiniques

Epoxidation of methyl esters of resin acids with the pimarane skeleton with p-nitroperbenzoic acid has permitted the isolation of epoxides of the intracyclic double bond. Allylic diterpenic alcohols were obtained by acid-catalysed isomerisation of diterpene epoxides, and the action of Collins reagent on these allylic alcohols has been studied.     

Synthesis of [13]-membered macrocyclic stevastelins via a transesterification reaction as the key step: total synthesis of stevastelin C3

A new synthesis of stevastelin C3 (3), a [13]-membered ring component of the stevastelin family, whose structure was recently revised, is reported. Initially, a macrolactonization approach was attempted to generate the [13]-membered macrolactone but this met with failure, so a translactonization reaction was tried to obtain the targeted stevastelin C3 (3) from the corresponding [15]-membered ring counterpart. Unfortunately, this strategy did not prove successful, and, consequently, we opted to undertake a transesterification reaction from 23, as a means to accommodate the requisite aminoacid moiety at the correct position, to obtain 24. From 24, and through intermediates 25-28, the acyclic precursor of the [13]-membered ring macrolactone, compound 30, was efficiently prepared. By utilizing the synthetic course developed by Chida, we took 30 forward and completed the total synthesis of stevastelin C3 (3).     

Additions of enantiopure alpha-sulfinyl carbanions to (S)-N-sulfinimines: asymmetric synthesis of beta-amino sulfoxides and beta-alphamino alcohols

The addition of the lithium anions derived from (R)- and (S)-methyl and -ethyl p-tolyl sulfoxides to (S)-N-benzylidene-p-toluenesulfinamide provides an easy access route to enantiomerically pure beta-(N-sulfinyl)amino sulfoxides. Stereoselectivity can be achieved when the configurations at the sulfur atoms of the two reagents are opposite (matched pair), thus resulting in only one diastereoisomer, even for the case in which two new chiral centers are created. The N-sulfinyl group primarily controls the configuration of the carbon bonded to the nitrogen, whereas the configuration of the alpha-sulfinyl carbanion seems to be responsible for the level of asymmetric induction, as well as for the configuration of the new stereogenic C-SO carbon in the reactions with ethyl p-tolyl sulfoxides. An efficient method for transforming the obtained beta-(N-sulfinyl)amino sulfoxides into optically pure beta-amino alcohols, based on the stereoselective non-oxidative Pummerer reaction, is also reported.     

Determination of cadmium by electrothermal atomic absorption spectrometry after microwave-assisted digestion of animal tissues and sewage sludges

The determination of cadmium in different sample types has been carried out by electrothermal atomization atomic absorption spectrometry with D(2)-background correction using a unpyrocoated graphite tube, after pressurized microwave-assisted digestion. Five chemical modifiers [(NH(4))(2)HPO(4), Pd(NO)(3))(2), Ni(NO(3))(2), thiourea and Triton X-100] have been assayed and nickel nitrate has been found to be most effective for an accurate determination of cadmium in mussel tissue, pig kidney and sewage sludge. The characteristic mass of the method is of the order of 1 pg and the limit of detection is lower than 0.1 ng/ml.     

REDUCTION OF NO BY CO OVER NiWO4, NiO, AND WO3 CATALYSTS

We present a comparative study of NiWO₄, NiO, and WO₃ catalysts for simultaneous conversion of NO and CO. Samples were synthesized by reacting ammonium metatungstate and/or nickel nitrate at high temperature (773 K to 903 K) under an oxygen stream. Catalysts were characterized by X-ray diffraction, surface area measurements, energy dispersive spectroscopy and scanning electron microscopy. The catalytic reduction of NO by CO took place in the temperature range (523 to 973) K under highly reductive conditions (NO:CO= 1:5) over NiWO₄NiO, and WO₃, respectively. The 100 % NO conversion at GHSV of 11460 h^-1 was achieved at 773 K over NiWO₄ and at 848 K over NiO. The WO₃ was deactivated at 898 K. However, in the range (523 to 723) K NiO was more active than NiWO₄ and WO₃ catalysts.     

Conformational dynamics of a bispyridinium cyclophane

A complete study of the conformational behavior of 4,8-diaza-3(1,4),9(4,1)-dipyridina-1,6(1,4)-dibenzenacyclodecaphan-3(1),9(1)-bis(ilium) bishexafluorophosphate is described. This study allows us to conclude that the process observed by which the different chemical shifts of the pyridinium protons show coalescence at a high-temperature 1H NMR is the rotation around the C-N bond, whereas the conformational equilibrium between the four conformers is produced at low temperature.     

Rapid determination of aliphatic amines in water samples by pressure-assisted monolithic octadecylsilica capillary electrochromatography-mass spectrometry

A pressure-assisted capillary chromatography-mass spectrometry method based on the use of a monolithic octadecylsilica (ODS) capillary is proposed for the determination of aliphatic amines. A 25 mM citric acid buffer containing 10% methanol is used as running electrolyte. Separation is achieved by simultaneously applying a capillary electrophoresis (CE) voltage of 13 kV and an overimposed pressure of 8 bar. The use of pressure is required to ensure stable electrospray conditions. Analysis times are reduced by using a capillary column consisting of a 30 cm long monolithic silica capillary column bound with ODS and a fused-silica capillary column also 30 cm long. The proposed method was successfully applied to the determination of low-molecular-weight aliphatic amines in tap and river water. The analysis of real samples requires cleanup and preconcentration, which can be performed automatically by inserting a minicolumn in the replenishment system of the commercial instrument.     

Product distributions, rate constants, and mechanisms of LiH+H reactions

We present a quantum-mechanical investigation of the LiH depletion reaction LiH+H-->Li+H2 and of the H exchange reaction LiH+H'-->LiH'+H. We report product distributions, rate constant, and mechanism of the former, and rate constant and mechanism of the latter reaction. We use the potential-energy surface by Dunne et al. [Chem. Phys. Lett. 336, 1 (2001)], the real-wave-packet method by Gray and Balint-Kurti [J. Chem. Phys. 108, 950 (1998)], and the J-shifting approximation. The 1H2 nuclear-spin statistics and progressions of vib-rotational states (v',j') rule both initial-state-resolved and thermal product distributions, which have saw-toothed shapes with odd j' preferred with respect to even j'. At high collision energies and temperatures, we obtain a regular 3-to-1 intensity alternation of rotational states. At low collision energies and temperatures, the degeneracy and density of many H2 levels can, however, give more irregular distributions. During the collision, the energy flows from the reactant translational mode to the product vibration and recoil ones. The rate constants of both reactions are not Arrhenius type because the reactions are barrier-less. The low-temperature, LiH depletion rate constant is larger than the H exchange one, whereas the contrary holds at high temperature. The real-time mechanisms show the nuclear rearrangements of the nonreactive channel and of the reactive ones, and point out that the LiH depletion is preferred over the H exchange at short times. This confirms the rate-constant results.     

Polarographic determination of pilocarpine using the catalysed pre-wave of nickel(II)

On the basis of a detailed study of the pilocarpine-induced nickel(II) pre-wave using various polarographic techniques, an electrode process mechanism is proposed in which the formation of a catalytic complex between aquo-nickel(II) and veronalate-nickel(II) on the one hand and unprotonated pilocarpine adsorbed on the electrode surface on the other is followed by the reduction of nickel(II) in the complex and the release of the catalytic ligand. The pre-peak recorded by differential-pulse polarography in the system 1 × 10^?3 M Ni(II)-1 × 10^?2 M sodium veronal, nitric acid (pH 8.5) (with ionic strength maintained at 0.2 with sodium nitrate) can be used for quantitative determination of pilocarpine at concentrations in the range 2.5 × 10^?7-8 × 10^?6 M.