Investigation of the diastereoselective cyclization of bis-sulfonyl esters

[reaction: see text] The diastereoselective cyclization of bissulfonyl esters was investigated by varying both the size and the placement of the substituent on the tether adjoining the reacting centers. Substitution at either the alpha or beta position relative to the ester moiety gave diastereomeric ratios of (1-3):1, while gamma substitution dramatically increased the diastereomeric ratios to (6-20):1.     

Comparison of different capillary isoelectric focusing methods--use of "narrow pH cuts" of carrier ampholytes as original tools to improve resolution

Two capillary isoelectric focusing (CIEF) systems have first been optimized: one uses a bare silica capillary and 30% (v/v) of glycerol in the separation medium while the other uses a coated capillary and an aqueous background electrolyte. To perform permanent capillary coating, two neutral polymers have been compared: hydroxypropylcellulose (HPC) and polyvinylalcohol (PVA). HPC coating gave best results for electroosmotic flow (EOF) limitation on a wide pH range: as compared to a bare silica capillary, it allowed to decrease EOF by 96% at pH 7.2 after acidic and basic treatments, whereas PVA coating lead only to a 76% decrease. The glycerol CIEF system was more satisfying for the separation of model proteins classically used as pI markers. Finally, the use of "narrow pH cuts" of carrier ampholytes added to commercial ampholyte mixtures allowed increasing resolution up to a factor 2.4 at a chosen pH for the separation of pI markers and milk proteins.     

New benchmark for basic and neutral nitrogen compounds speciation in middle distillates using comprehensive two-dimensional gas chromatography

This paper reports an analytical method for the comprehensive two-dimensional gas chromatography (GC x GC) separation and identification of nitrogen compounds (N-compounds) in middle distillates according to their types (basicity). For the evaluation of the best chromatographic conditions, a non-polar x polar approach was chosen. The impact of the second dimension (stationary phase and column length) on the separation of basic and neutral N-compounds was evaluated by mean of two-dimensional resolution. This study revealed that the implementation of polar secondary column having free electron pairs improves drastically the separation of N-compounds. Indeed, the presence of permanent dipole-permanent dipole interactions between neutral N-compounds and the stationary phase was enlightened. The comparison of two different nitrogen chemiluminescence detectors (NCD) was also evaluated for GC x GC selective monitoring of N-compounds. Owing to higher resolution power and enhanced sensitivity achieved using developed chromatographic and detection conditions, it was possible to identify univocally and to quantitate N-compounds (i) by class of compounds and (ii), within a class, by carbon number. Finally, quantitative comparison of GC x GC-NCD with conventional gas chromatography illustrates the benefits of GC x GC leading to an excellent correlation with results obtained by American Society for Testing Materials (ASTM) methods for the determination of basic/neutral nitrogen ratio in diesel samples.     

Palladium-mediated N-arylation of heterocyclic diamines: insights into the origin of an unusual chemoselectivity

The chemoselectivity of the palladium-mediated reaction of bromobenzene with various heterocyclic diamines was studied. Whatever the ligand used, 3-aminopyrrolidine underwent arylation of the secondary amine function (>82%), whereas the more flexible 3-aminoazepinine was arylated on its primary function (>70%). The ratio "arylation of primary amine versus arylation of secondary amine" of 3-aminopiperidine with bromobenzene varied from 90:10 (BINAP, electron-enriched and hindered biphenyls L2 or L3) to 32:68 with the Josiphos-type ligand L10. The same trend was observed when 4-aminopiperidine was used (82:18 with L2 and 17:83 with L10). This selectivity can be tuned by the choice of aryl halide partners having different steric and electronic properties. A cooperative effect of both nitrogens of diamines during the reaction was deduced from competitive experiments. Finally, 13C and 31P NMR experiments, carried out with 3-aminopyrrolidine at room temperature, support a fast coordination of the primary amine to the metal. Indeed, a palladium complex resulting from the unusual displacement of one phosphane group of the intermediate ArPdX(BINAP) by the primary amino group was characterized.     

Highly enantioselective decarboxylative protonation of alpha-aminomalonates mediated by thiourea cinchona alkaloid derivatives: access to both enantiomers of cyclic and acyclic alpha-aminoacids

Thiourea derived cinchona alkaloids promote the asymmetric decarboxylative protonation of cyclic, acyclic, or bicyclic alpha-aminomalonate hemiesters under mild and metal-free conditions to afford enantioenriched aminoesters in high yields and enantioselectivities up to 93%. Both enantiomers of the aminoesters have been synthesized with the same selectivity when using organic base 3 and its pseudoenantiomer 6 derived from quinine.     

Online preconcentration using monoliths in electrochromatography capillary format and microchips

Online preconcentration and separation of analytes using an in situ photopolymerized hexyl acrylate-based monolith stationary phase was evaluated using electrochromatography in capillary format and microchip. The band broadening occurring during the preconcentration process by frontal electrochromatography and during the desorption process by elution electrochromatography was studied. The hexyl acrylate-based monolith provides high retention for neutral analytes allowing the handling of large sample volumes and its structure allows rapid mass transfer, thus reducing the band broadening. For moderately polar analytes such as mono-chlorophenols that are slightly retained in water, it was shown that enrichment factors up to 3500 can be obtained by a hydrodynamic injection of several bed volumes for 120 min under 0.8 MPa with a decrease in efficiency of 50% and a decrease of 30% for the resolution between 2- and 3-chlorophenol. An 8 min preconcentration time allows enrichment factors above 100 for polyaromatic hydrocarbons. The interest of these monoliths when synthesized in microchip is also demonstrated. A 200-fold enrichment was easily obtained for PAHs with only 1 min as preconcentration time, without decrease in efficiency.