Hyperconjugative Control by Remote Substituents of Diastereoselectivity in the Oxygenation of Hydrocarbons

The oxidation of 2-substituted adamantanes (2) with TFDO (1) is reported. The data show a stereodifferentiation of the chemical environments induced by remote electron-withdrawing substituents which produces remarkable Z/E diastereoselectivity in the oxidation of the tertiary C(5)-H and C(7)-H bonds. The results show a bell-shaped correlation between the Z/E stereoselectivity and the substituent constant sigma(I), which is interpreted in terms of hyperconjugative stabilization of the diastereomeric transition states.     

Dynamics of the Electric Double Layer: Analysis in the Frequency and Time Domains

No rigorous theory of electrokinetic phenomena is conceivable without properly accounting for double layer polarization under the action of external fields. Since processes leading to such polarization need a finite time to develop, an analysis of the behavior of the quantities of interest (potential and ion concentration profiles, particle or fluid velocity, and so on) as a function of time should be extremely illustrative. In this work, we analyze how those quantities evolve in the nanosecond to microsecond time range after the application of an electric field. The network method is proposed (in which, essentially, an electric circuit simulator program is used to solve the differential equations involved, after their proper interpretation in terms of fluxes and forces) to gain information about the evolution with time of the potential, counterion, and co-ion perturbations, the particle velocity, and the fluid velocity profile. The performance of the method is first ckecked in the frequency domain, for which rigorous solutions exist, and then the procedure is used in the time domain. Reasons are discussed for the observed time dependencies of the analyzed quantities. Copyright 2000 Academic Press.     

Correction of predicted concentration in the use of solvent-based calibration lines for determining carbendazim, fuberidazole and thiabendazole in water after a SPE step

This paper addresses an attempt to overcome the deviation that results from the use of a solid phase extraction (SPE) procedure for extracting trace levels of three benzimidazole pesticides (carbendazim, fuberidazole and thiabendazole) from water samples, for their subsequent quantitative determination by spectrofluorimetry, using univariate calibration. The deviation is due to an attenuation effect originating in the C(18) cartridge used in the SPE step. The approach developed is based on the calculation of a correction factor (fc) that is dependent on the signal measured after the SPE step. In order to calculate fc a study of the intermediate precision of two calibration graphs (with and without SPE) was performed. The fc was added to the predicted concentrations for the analytes using a calibration graph for pure solvent, built every time that the analysis is done. In addition, predictions were made using both average calibration graphs obtained from the intermediate precision study. In this study, the first of these three options was shown to improve the accuracy of predictions in the presence of matrix effects.     

Change of sign in the pyroelectric coefficient of KDP at 15.3 K

Low temperature measurements of the charge released (or absorbed) by prepolarized crystalline samples of ferroelectric potasium dihydrogen phosphate clearly indicate that just above 3 K the samples absorb charge, (i.e. the spontaneous polarization increases slightly). After passing through a maximum, the samples begin to release charge at 15.3 K, implying a change of sign in the pyroelectric coefficient, a behavior which had been only observed previously in a few non-ferroelectric or polycrystalline pyroelectric materials.     

Structure of 18-hydroxy-14-O-methylgadesine a new diterpene alkaloid from consolida orientalis

The structure of 18-hydroxy-14,O-methylgadesine, isolated from Consolida orientalis, was determined by spectroscopic methods and X-ray crystallography.     

Ring cleavage reactions of 3- and 5-non-substituted isoxazoles

5-Methylisoxazoles with electron-accepting groups at C-4 (1a-c) and 2,3-dimethylisoxazolium iodide (II) undergo ring cleavage when treated with organic bases. The nature of the open chain products which were obtained (stable enolates, β-diketones, esters) depends on the group at C-4 and the strength of the base. In some of these processes aromatic aldehydes were used in order to determine the competition between the condensation and the cleavage reaction. The mechanism of the nucleophilic ring cleavage of II is also shown.     

Total synthesis of obtusenol

The total synthesis of obtusenol ($\text{9}$), a dibrominated sesquiterpene isolated from the red alga $\text{Laupencia obtusa}$, has been achieved by a three-step synthesis starting from commercial farnesol.     

The stereoselective reduction of α-aminodeoxybenzoin derivatives with sodium borohydride

Total stereoselectivity is observed in the sodium borohydride reduction of α-aminodeoxybenzoins and their hydrochlorides in various hydroxylic solvents. RS - SR isomer (erythro) was the only aminoalcohol obtained.     

Indirect determination of chloride as chromyl chloride by atomic-absorption spectrophotometry

Summary A method for the determination of chloride in aqueous solution by atomic-absorption Spectrophotometry is described. The chloride is converted into chromyl chloride, which is extracted into carbon tetrachloride and subsequently back-extracted and determined in a solution buffered with NH₄H₂PO₄/(NH₄)₂HPO₄. Calibration plots of the atomic absorption of the aqueous back-extracted phase at the chromium 425.4 nm resonance line are linear in the range 5–270g of chloride. The calibration graph can be obtained directly with aqueous solutions of Cr(VI). The proposed method can be applied directly to mineral and medicinal water samples.

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Indirekte AAS-Bestimmung von Chlorid als Chromylchlorid

Zusammenfassung Eine Methode zur Bestimmung von Chlorid durch Atom-Absorptions-Spektroskopie wurde entwickelt, deren Basis die Bildung von Chromylchlorid, dessen Extraktion mit Cl₄C und die anschließende Bestimmung des rückextrahierten Chroms in einer Lösung von NH₄H₂PO₄/(NH₄)₂HPO₄ ist. Das lineare Verhältnis zwischen Absorbanz und Chromkonzentration ermöglicht die Bestimmung des Chlorids im Bereich 5–270g; zur direkten Eichung dienten wäßrige Lösungen von Chrom(VI). Die vorgeschlagene Methode wurde für die Chlorid-Bestimmung in Mineral-und Medizinalwasser benutzt.

     

On the Laplace transforms of retarded,Lorentz-invariant functions

Let ø(t) (t ∈ $\text{R}$ⁿ) be a retarded, Lorentz-invariant function which satisfies, in addition, condition (c). We call “R” the family of such functions. Let f(z) be the Laplace transform of ø(t) ∈ $\text{R}$. We prove (Theorem 1) that f(z) can be expressed as a K-transform (formula (I, 2; 1)). We apply this formula to evaluate several Laplace transforms. We show that it affords simple proofs of important known results. Formula (I, 2; 1) is an effective complement to L. Schwartz' method of evaluating Fourier transforms via Laplace transforms (“Théorie des distributions,” p. 264, Hermann, Paris, 1966). We think this is the most useful application of our formula.