A gas electron diffraction and ab initio quantum-mechanical investigation into the molecular structure of tricyclo-(3.1.002,4)-hexane

The structure of tricyclo-(3.1.00^2,4)exane has been determined by gas phase electron diffraction. The molecule has an inversion centre. The mean carbon—carbon bond length, averaged over both three- and four-membered rings is 1.508 A. A model with equal C-C bond lengths fits to the measured diffraction intensities. The four-membered ring is planar with valency angles of 90°, while the carbon atoms of the three-membered rings form isosceles triangles. Ab initio quantum mechanical calculations at the STO-3G level support this geometry. The valency angle CCC (between three- and four-membered rings) for the equilateral four-membered ring model has been found experimentally to be 109.9°. The average C-H bond distance (1.080 Å) is small as a result of increased s-character in these bonds in agreement with reported INDO—LMO calculations.     

Obtention de selenopysultones lors de l'oxydation de thio-1 chromenes par le dioxyde de selenium

Selenium dioxide oxidation of 1-thiochromene leads to 3-formyl (5,6)benzo-1,2 thiaseleninne-1,1-dioxide, which is a representative of a hitherto unknown class of heterocyclic compounds. Pyrolysis of this compound leads to 2-formyl benzo(b)selenophen, by loss of sulphur dioxide. The reaction can be applied to other 1 thiochromenes but no similar behaviour has been observed for their selenium isologues.     

Practical asymmetric enzymatic reduction through discovery of a dehydrogenase-compatible biphasic reaction media

[reaction: see text] An enzyme-compatible biphasic reaction media for the asymmetric biocatalytic reduction of ketones with in situ cofactor regeneration has been developed. In this biphasic reaction media, which is advantageous for reactions at higher substrate concentrations, both enzymes (alcohol dehydrogenase and FDH from Candida boidinii) remain stable. The reductions with poorly water-soluble ketones were carried out at substrate concentrations of 10-200 mM, and the optically active (S)-alcohols were formed with moderate to good conversions and with up to >99% ee.     

Optimization of solid-phase clean-up prior to liquid chromatographic analysis of ochratoxin A in roasted coffee

An extraction and clean-up method for ochratoxin A (OA) in roasted coffee has been developed and the HPLC method optimized. An interfering compound with a similar retention time as OA was adsorbed by the aminopropyl (NH2) material at < or = 5% NaHCO3. Residual OA on the column was recovered by washing with the extraction solution followed with methanol. Fractions were mixed together for further clean-up with Ochratest immunoaffinity columns (IACs). Analysis by HPLC resulted in a well resolved OA peak and reduction in matrix interferences. Recoveries ranged from 72 to 84% and the detection limit was 1 ng/g.     

Multimonomer partitioning in latex systems with moderately water-soluble monomers

Simple equations describing monomer partitioning in latices during intervals 2 and 3 in emulsion polymerization with any number of low to moderately water soluble monomers were derived from the extended Morton equation by making various assumptions. It appears that it is mainly the combinatorial entropy of mixing that governs the partitioning behavior, and that other contributions to the free energy of the monomers in the polymer particles are marginal. Experimental results with styrene, methyl methacrylate, and methyl acrylate confirm the validity of the assumptions. In interval 3 of emulsion polymerization the sum of all contributions to the free energy of the monomers in the particles other than the combinatorial entropy of mixing can be taken as a constant that is dependent only on the monomer composition in the particles and independent of the degree of swelling of the particles. The only parameters one needs to know to calculate the monomer concentrations in all phases with help of the derived equations, are the saturation concentrations of each monomer in the polymer particles, and the saturation concentrations of each monomer in the aqueous phase. © 1994 John Wiley & Sons, Inc.     

Routine clinical determination of carotene, vitamin E, vitamin A, 25-hydroxy vitamin D3 and trans-vitamin K1 in human serum by straight phase HPLC

A universal extraction procedure is described for fat-soluble vitamins in human serum. Methods are presented for routine quantitative analysis by isocratic straight phase HPLC with UV-detection of (alpha + beta)-carotene, vitamin E (alpha-tocopherol) and vitamin A (all-trans-retinol) in one single run, and of vitamin K1 (trans-phylloquinone) and 25-hydroxy vitamin D3 after sample clean-up using disposable reversed-phase cartridges. The limits of detection, precisions and selectivities of the developed assays are shown to be satisfactory after more than three years' experience. The routine clinical determination of fat-soluble vitamins can be performed in less than 5 mL of serum. Analyses of external quality control and randomly taken outpatient samples are shown to be of great value in assessing laboratory performance.     

Oxidative addition of some geminal halonitroalkanes to iridium(I) and platinum(0) compounds

Oxidative addition of 1-chloro-1-nitroethane to trans-IrCl(CO)-[P(CH₃)₂C₆H₅]₂ followed by treatment of the initial product with pyridine yields a new iridium(III) complex IrCl(py)[COC(NO₂)CH₃][P(CH₃)₂C₆H₅]₂, whose structure has been confirmed by X-rays crystallography. Two intermediate products have been observed by NMR spectroscopy; their structures have been tentatively assigned. The reaction of the corresponding bromine derivatives yields two isomers of the composition IrBr₂(CO)[CH(NO₂)CH₃][P(CH₃)₂C₆H₅]₂, and these are not affected by pyridine. The reaction of 1-chloro-1-nitroethane with Pt[P(C₆H₅)₃]₄ takes a completely different course in that yields nitrorethane and cis-PtCl₂[P(C₆H₅)₃]₂ as the main products, with no detectable formation of the products of oxidation addition. A brief mechanistic investigation points towards the participation of radicals and radical anions as transient intermediates and a mechanism is proposed which explains most of the experimental results.     

The effect of electronegative atoms on the structures of hydrocarbons. ab initio calculations on molecules containing fluorine or (carbonyl) oxygen

A series of ab initio calculations have been carried out, using the 4-21G basis set. Ethane and propane were first studied to obtain reference points. The effect of adding an electronegative atom (fluorine, or carbonyl oxygen) onto the framework was then studied as a function of the torsional angle about the single bond. Some pronounced trends in structural changes were observed, and these can in part be correlated with hyperconjugative effects. For example, fluoroethane has bond lengths which are shorter than those in ethane itself, by 0.024 Åin the C C bond, and 0.003 Åin the α C H bonds. These changes are essentially torsionally independent. On the other hand, in propionaldehyde, the C C bond length of the methyl group and the C H bond lengths of the hydrogens attached to the alpha carbon vary as a function of the torsion angle. If the methyl C C bond in the carbonyl plane is taken as a reference, the bond stretches .016 Åwhen the torsion angle is increased to 90°, an α C H bond similarly stretches up to .007 Å. Many of these geometric changes are large, well beyond the experimental errors in modern measurements.     

Optimization and validation of a capillary zone electrophoretic method for the analysis of several angiotensin-II-receptor antagonists

We optimized a capillary zone electrophoretic method for separation of six angiotensin-II-receptor antagonists (ARA-IIs): candesartan, eprosartan, irbesartan, losartan potassium, telmisartan, and valsartan. A three-level, full-factorial design was applied to study the effect of the pH and molarity of the running buffer on separation. Combination of the studied parameters permitted the separation of the six ARA-IIs, which was best carried out using 60 mM sodium phosphate buffer (pH 2.5). The same system can also be applied for the quantitative determination of these compounds, but only for the more soluble ones. Some parameters (linearity, precision and accuracy) were validated.     

Synthesis of 1,1-diarylethylenes from an α-stannyl β-silylstyrene

The synthesis of a family of 1,1-diarylethylenes from an α-stannyl β-silylstyrene through a combination of a Stille coupling and a protodesilylation reaction is described. This approach avoids the problematic cine-substitution, which is a well documented side reaction during the palladium-assisted elaboration of α-substituted vinylstannanes to 1,1-disubstituted ethylenes.