Stereochimie de la deprotonation et de la reaction d'aldolisation des dithioesters

Cis lithium thioenolates are preferentially formed by deprotonation of dithiopropanoates with lithium diisopropylamide in tetrahydro-furan at -78°C. The cis selectivity observed, is unproved by increasing the alkylthio group size or by the ability of this group to chelate the lithium cation. When more bulkier bases such as lithium 2,2,6,6-tetramethylpiperi-dide or lithium hexamethyldisilazane are used, the selectivity is lowered. This lowering of selectivity is suppressed when the deprotonation is performed in presence of 12-crown-4. Addition of hexamethyl phosphoramide to the base does not invert the selectivity as it uas reported for the deprotonation of esters ; a rather better cis selectivity is reached in the case of methoxymethyl dithiopropanoate. These original results are well understood in terms of an open transition state model. Preformed lithium thioenolates are reacted uith a variety of aldehydes and afford stereo-specifically syn aldols. The influence of hexamethylphosphoramide and reaction time is also examined.     

Prostaglandin D2 synthase and its post-translational modifications in neurological disorders

Prostaglandin D2 synthase (PGDS) (beta-trace protein) is a highly abundant cerebrospinal fluid (CSF) glycoprotein. A number of studies have been performed to determine the potential value of this protein for the diagnosis of various neurological disorders. The measurement of total PGDS levels in CSF has proved marginally useful for this purpose, but promising results were obtained while investigating changes in the posttranslational modifications (PTM) pattern. Using 2-DE analysis, we previously showed that PGDS is differentially expressed in ante- and post mortem CSF samples. In the present study, we examined whether the PGDS isoforms may help to distinguish stroke and neurodegenerative disease patients from healthy subjects. The pattern of PGDS PTM was analyzed in CSF from patients with various neurological disorders (n = 44) using IEF/immunoblotting techniques. Strong alterations of this pattern were detected in patients with different forms of degenerative dementia. These findings are consistent with PGDS being altered in some neurological diseases and provide new opportunities for clinical applications.     

Sodium-23 Nuclear Magnetic Resonance Spectroscopy

With the availability of Fourier transform spectrometers, ²³Na-NMR spectroscopy has become an important tool. It affords direct insight into solvation and ion pairing phenomena, both in organic and in bio-inorganic systems. Monitoring the chemical shifts and linewidths of ²³Na signals gives access to binding constants, reorientational correlation times and the microdynamics of the sodium coordination shell. The binding of other cations can also be studied by ²³Na-NMR spectroscopy, in competition experiments.     

CHEMILUMINESCENCE AND ENERGY TRANSFER FROM SINGLET OXYGEN PAIRS

Abstract— Chemiluminescence and photochemiluminescence of systems generating excited (singlet) molecular oxygen has been studied at low temperature in fluid media. A possible interpretation is proposed: that dimers of singlet oxygen are directly or indirectly responsible for the observed emission, the species [¹δg +¹σg⁺] predominating. Energy transfer to a fluorescent molecule is possible only when conditions of spectral overlap, required for transfer by inductive resonance, are fulfilled. Evidence is also presented for the formation of a transient complex between oxygen and a sensitiser or acceptor molecule [S…O₂], which is the precursor of singlet oxygen in these systems. However, other possible mechanisms cannot be excluded.     

New film-forming poly(urethane-amide-imide) block copolymers: influence of soft block on membrane properties for the purification of a fuel octane enhancer by pervaporation

A new family of eight poly(urethane-amide-imide) (PUAI) block copolymers with the same hard block and different soft blocks were synthesized in two steps from a dianhydride monomer containing amide functions (4,4′-methylene-bis(trimellitic anhydride-N-phenylamide)) and α,ω-dihydroxy telechelic oligomers which varied in both chemical structure (polyethers: PEG, PTMG, PPG; polyester: PCL) and molar weight (MW ≅ 600 or 1000 g/mol). The PUAI were obtained in high yields (ranging from 81 to 98 wt%) and with reduced viscosities which varied from 0.36 to 0.84 dL/g (for C = 1 mg/mL in DMF at 25 °C). Their characterization by FTIR and ¹H NMR fully confirmed their chemical structure. Their solubility was typically limited to a few wt/vol% even in strong apolar diprotic solvents like DMF and NMP. This particular feature showed the very strong physical cross-linking of their very stiff hard block and enabled to cast membranes capable of withstanding exposure to many common organic solvents. Systematic permeability experiments showed that the PUAI membranes could be used to separate the azeotropic mixture EtOH (20 wt%)/ETBE very easily, with interesting prospects for the purification of ETBE (a fuel octane enhancer used instead of lead derivatives in the European Community). An analysis in terms of structure-property relationships pointed out that the soft block molar weight and polarity were two key parameters for the optimization of selective permeability. The best compromise was obtained with the soft block PEG1000. The corresponding polymer led to performances so far outstanding for polyamideimides with a very high flux of more than 1.1 kg/h m² for a normalized thickness of 5 μm at 50 °C and a selectivity α = 22.7 in the high range for this kind of separation.     

Spontaneous dissolution of a single-wall carbon nanotube salt

Upon reduction with alkali metals, single-wall carbon nanotubes (SWNTS) are shown to form polyelectrolyte salts that are soluble in polar organic solvents without any sonication, use of surfactants, or functionalization whatsoever, thus forming true thermodynamically stable solutions of naked SWNTs.     

Sur le role fondamental du cation alcalin dans les reductions par les hydrures metalliques. Utilisation de coordinats macrocycliques—III : Etude de divers groupements fonctionnels reduits par LiAlH4

The reactions of the following functional groups with LiAlH₄ in Et₂O or DME were studied: ketone, aldehyde, carboxylic acid, amide, ester, nitrile, oxime, tosylate, epoxide, acyl chloride and alkyl bromide. All these reductions are catalysed by Li⁺, as shown by the use of a specific macrocyclic ligand allowing the reaction to be run free of Li⁺. Moreover, in the cases of acyl chloride and alkyl bromide the reaction (free of Li⁺) is catalysed by AlX₃ which is generated in situ. Several new mechanistic features are revealed for reductions by LiAlH₄.     

Direct Clay-Catalyzed Friedel-Crafts Arylation and Chlorination of the Hydrocarbon Adamantane

Multiple chlorinations and arylations at the tertiary positions of adamantane are promoted by FeCl₃-doped K10 montmorillonite in CCl₄ or in aromatic solvents. The process, remarkably easy to implement, can be tailored to selective formation of monosubstituted 1-adamantyl derivatives or 1,3-disubstituted adamantanes. The process achieves alkylation at the meta- and para-positions of toluene leading to a purely statistical distribution and displays no substrate specificity in competition experiments between toluene and benzene.     

Kinetics of thermal decomposition of Co3O4 powder and single crystals

Kinetics of thermal decomposition in vacuum of Co₃O₄ powder as well as single crystals has been investigated. Discrepancies with the results of previous authors have been discussed. Decomposition of Co₃O₄ proceeds through formation of a compact layer of CoO and hence diffusion is the rate-limiting factor. The experimental curves α(t) be described for 0.05 < α < 0.85 using a modified Ginstling-Brounshtein equation: 1 ? 2α/3 ? (1 ? α)^2/3 = ktⁿ where the activation energy varies with the degree of decomposition.     

Fragmentation sous Impact Electronique du Phényl-2 Oxétanne

The mass spectral fragmentation processes of 2-phenyloxetane have been studied by means of isotopic labelling. The origin of the benzoyl ion, which is the base peak, is discused