Stereochemical Aspects in the Asymmetric Michael Addition of Chiral Imines to Substituted Electrophilic Alkenes

The Michael-type addition of chiral imines, derived from racemic alpha-substituted cyclanones and optically active 1-phenylethylamine, to electrophilic alkenes, in neutral conditions, constitutes one of the most efficient methods for the stereocontrolled construction of quaternary carbon centers. In order to create an additional stereogenic center at the alpha- or beta-position to the quaternary one, the behavior of a variety of alpha- and beta-substituted alkenyl acceptors was examined. In general, these additions are highly regioselective, the alkylation taking place predominantly, if not exclusively, at the more substituted alpha-side of the imine function; however, in some cases (electrophilic alkenes 28 and 49), significant amounts (10-15%) of regioisomeric adducts were obtained. With the exception of methyl propiolate 52, a remarkable control of the absolute configuration of the adducts were always observed with these Michael acceptors. According to the general rule we have previously proposed, the alkylation process takes place preferentially on the less hindered pi-face of the more substituted secondary enamine, in tautomeric equilibrium with the starting imine. An excellent diastereocontrol was always obtained by using the present alpha- and beta-substituted alkenes. These stereochemical outcomes can be interpreted by invoking that the reaction proceeds through a compact approach of the reactants, the hydrogen atom at the nitrogen center of the enamine being transferred to the alpha-vinylic carbon atom of the acceptor, concertedly with the creation of the C-C bond. In this respect the "endo-approach" 58, in which the electron-withdrawing group of the acceptor faced to the nitrogen atom of the enamine (case of acceptors 10, methyl methacrylate, 24, 28, 43, 47, and 49) largely prevails over the "exo-approach" 59 (case of acceptor 38). This predominant "endo-preference" can be reasonably interpreted in terms of a cooperative effect between steric and stereoelectronic factors.     

Stability study of simvastatin under hydrolytic conditions assessed by liquid chromatography

In this work, a liquid chromatography stability-indicating method was developed and applied to study the hydrolytic behavior of simvastatin in different pH values and temperatures. The selected chromatographic conditions were a C18 column; acetonitrile-28 mM phosphate buffer solution, pH 4 (65 + 35) as the mobile phase; 251 degrees C column temperature; and flow rate 1 mL/min. The developed method exhibited an adequate repeatability and reproducibility (coefficient of variation 0.54 and 0.74%, respectively) and a recovery higher than 98%. Furthermore, the detection and quantification limits were 9.1 x 10(-7) and 2.8 x 10(-6) M, respectively. The degradation of simvastatin fitted to pseudo-first order kinetics. The degradation was pH dependent, being much higher at alkaline pH than at acid pH. Activation energy, kinetic rate constants (k) at different temperatures, the half life (t1/2) and the time for 10% degradation to occur (t90) values are also reported.     

Bis(o-nitrophenyl)ethanediol: A practical photolabile protecting group for ketones and aldehydes

The ketals of bis(o-nitrophenyl)ethanediol and ketones or aldehydes are smoothly deprotected in neutral conditions by irradiation with 350 nm light. The chemical stability in basic, acidic, and oxidizing media makes this form of protection orthogonal to classical protecting groups. Both racemic and enantiopure forms are readily available in two steps from inexpensive starting materials.     

Mimicking the two-dimensional spectrochemical series using density functional computations

With tetragonal distortions of tetrahedral d2 complexes as examples, nonadditive and additive ligand fields are compared computationally, using Kohn-Sham density functional theory (KS-DFT) and ligand-field theory to obtain 45 linear, parametrical equations. For each complex, a "data" reduction from three nonadditive-field parameters to two parameters of the additive field occurs. The complexes V and CrX4- (where X=F, Cl, Br, I) provide the two-dimensional spectrochemical series of the sigma and pi AOM parameters, which are known semiempirically for the halide ligands. The same parametrical results can be obtained from the Kohn-Sham orbital energies of the "average of configuration" computation.     

Préparation,configuration et réduction électrochimique d'α-cyano β-nitrostyrènes. Synthèse d'amino-5 isoxazoles

Z α-Cyano-β-nitrostyrenes were prepared by nitration with dinitrogentetroxide of the corresponding α-cyanostyrenes. Elsomers were obtained by photoirradiation of Z isomers. The electrochemical reduction of these cyanonitro compounds generates the α-cyanooximes which lead, according to the experimental conditions(ring closure or hydrolysis), either to 5-aminoisoxazoles or to β-ketonitriles.     

Influence of amphiphilic block copolymers on lyotropic liquid crystals in water-oil-surfactant systems

In ternary water-oil-nonionic alkyl polyglycol ether (C(i)E(j)) microemulsions, an increase in efficiency is always accompanied by the formation of a lamellar (L(alpha)) phase. The addition of an amphiphilic block copolymer to the ternary base system increases the efficiency of the microemulsion drastically while suppressing--at least partly--the formation of the L(alpha) phase. However, amphiphilic block copolymers can be used not only to suppress the formation of lyotropic liquid crystals but also for the opposite effect, namely, to induce their formation. To understand to what extent the increase in efficiency is accompanied by the formation of lyotropic liquid crystals, we studied phase diagrams of water-n-alkane-n-alkyl polyglycol ethers (C(i)E(j))-PEPX-PEOY at a constant volume fraction of oil in the water/oil mixture. Using polymers of the poly(ethylene propylene)-copoly(ethylene oxide) type, with M(PEP) = X kg mol(-1) and M(PEO) = Y kg mol(-1), we determined phase diagrams as a function of the polymer concentration, size, and symmetry. Moreover, the influence of a particular polymer mixture was studied, which turned out to be the best system if both a high efficiency and a low tendency to form an L(alpha) phase are needed.     

On-line SFE-GC for determination of PCBs in human milk and blood serum

On-line coupled supercritical fluid extraction and gas chromatography (SFE-GC) has been utilized for the determination of PCBs and other organochlorine compounds in human milk and blood serum. The procedure involved preconcentration of the sample on C₁₈-silica sorbent in an extraction cell: after precipitation of the proteins up to 20 ml of human milk was concentrated on 0.5 g of sorbent. Serum (up to 5 ml) was applied to the C₁₈ material without pretreatment. Basic alumina was utilized as a selective adsorbent for lipids in the on-line SFE-GC system. The method was used to analyze milk and serum spiked with 0.5 and 10 ng of Aroclor 1260 and the results compared with those obtained by liquid–liquid extraction of serum.     

Bicyclic aziridines as intermediates in the photolysis of polycyclic aromatic azides.

According to the subsequent treatment, azepines $\text{16}$, $\text{17}$, $\text{19}$ or 1,2-dihydro derivatives $\text{18}$ and $\text{20}$ are obtained in the photolysis in basic medium of β-anthracenic azides $\text{7b}$ and $\text{7c}$ and provide strong evidence for the intermediate formation of bicyclic aziridines.     

Investigating the interaction of crystal violet probe molecules on sodium dodecyl sulfate micelles with hyper-Rayleigh scattering

We report the use of the nonlinear optical technique of hyper-Rayleigh scattering to investigate the interaction of the cationic probe molecule crystal violet with micelles of sodium dodecyl sulfate. An absolute value of (847 +/- 80) x 10(-30) esu is measured at the fundamental wavelength of 870 nm for the molecular hyperpolarizability of crystal violet free in pure aqueous solutions. In aqueous solutions of sodium dodecyl sulfate, above and below the critical micelle concentration, the measured hyperpolarizability of crystal violet is weaker than in the solution free of sodium dodecyl sulfate. From the comparison with linear optical photoabsorption spectroscopy data, this difference is attributed to electrostatic interactions between the cationic crystal violet molecules and the negatively charged sodium dodecyl sulfate surfactant molecules present in excess. Polarization resolved hyper-Rayleigh scattering measurements are then performed to show that, below and above the critical micelle concentration, crystal violet molecules also undergo symmetry changes upon interaction with sodium dodecyl sulfate. Above the critical micelle concentration, the minimum fraction of micelles interacting with at least one CV molecule is estimated. For instance, for a crystal violet aqueous concentration of 150 microM, this fraction is larger than 7%.     

From a biogenetic scenario to a synthesis of the ABC ring of manzamine A

On the basis of a biogenetic proposal for explaining the biogenesis of manzamine A, the cycloaddition of dihydropyridinium salt 26 with diene derivative 5 leads to adducts 27. These adducts, as well as their related and previously described analogues 9, are now shown to be precursors of diene derivatives such as 10, 13, and 28. Treatment of diene 32 with sodium azide resulted in a one-step formation of the tricyclic imino derivative 34. This key intermediate was further transformed into tricyclic derivative 40, which possesses the essential features of the ABC ring of manzamine A.