Can the presence of chemically inert species modify the face selectivity in asymmetric induction by cyclodextrins?

Reduction of-cyclodextrin (-CD) aromatic ketone (acetophenone and acetonaphthones) inclusion compounds were carried out in the presence of a large number of chemically inert species as potential co-guests. In several cases, it was observed that stoichiometric molar ratios of these compounds to ketone significantly modify the chiral induction yielding the inverted alcohol enantiomer and increasing the face selectivity. The results were found to depend strongly on the respective structure and shape of both the ketone and the additive, and on the molar ratio of-CD:ketone:third compound. These observations suggest the formation of a three-component inclusion complex in which the geometry of binding of the substrate and its mobility are changed with respect to the binary system.     

Development of an HPLC method with electrochemical detection of femtomoles of 8-oxo-7,8-dihydroguanine and 8-oxo-7,8-dihydro-2'-deoxyguanosine in the presence of uric acid

A selective method based on high performance liquid chromatography with electrochemical detection (HPLC-ECD) was developed to enable simultaneous detection of 8-oxo-7,8-dihydroguanine (8-oxoGua) and 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodGuo), products of DNA oxidative damage, in the presence of uric acid (UA), a strong interferent in their electrochemical detection. The method developed consists of HPLC isocratic elution with amperometric detection on a glassy carbon electrode, enabling a detection limit for 8-oxoGua and 8-oxodGuo lower than 1 nM in standard mixtures. Detection of low concentrations up to 25 nM of 8-oxoGua and 8-oxodGuo in the presence of UA in a 10⁴-fold higher concentration was achieved after one-step solid phase extraction (SPE). The method was tested with urine samples and it was possible to detect and quantify the presence of 8-oxoGua, and to confirm that UA was eliminated after uricase degradation and SPE. The LOD found in urine samples was about 80 nM, a value higher than in standard mixtures, due to the increase of background current in the urine matrix. The results presented here contribute to the development of a methodological approach to simultaneous determination of 8-oxoGua and 8-oxodGuo in urine samples.     

Note sur le mécanisme réactionnel de la transposition de Dimroth. Une étude ab initio

An ab initio study of the Dimorth rerarrangement of the -amino-1,2,3,4-tetrazole leads to the conclusion teht in vapour phase the rate determining step is not the ring-chain isomerism, but is either the Z-E iomerism around the C?N double bond or the 1,3-sigmatropic shift of the proton.     

Molecular models of lipopeptide detergents: large coiled-coils with hydrocarbon interiors

We have constructed molecular models of octameric micelles formed by a recently developed lipopeptide detergent consisting of a single amphipathic alpha-helix coupled to two acyl chains at either end of the helix. The models explain the experimentally observed aggregation behavior of peptides with different acyl chain lengths. The octameric micelles form a unique coiled-coil structure, with the acyl chains in a nearly frozen conformation inside the cylindrical assemblies. Two extreme models with helices either all parallel or in an alternating orientation suggest that the alternating orientation is energetically more favorable. The models suggest several new directions for further diversifying this new class of detergents for the structural studies of membrane proteins.     

Photoinduced metalation of nonactivated C-Cl bonds with samarium diiodide: synthesis of alkenes with high (Z)-selectivity through beta-elimination reactions

[reaction: see text] The photoinduced metalation of nonactivated C-Cl bonds of O-acetyl chlorohydrins is promoted by samarium diiodide. As a result of this, beta-elimination of O-acetyl chlorohydrins is achieved, affording the corresponding (Z)-alkenes with total or high stereoselectivity.     

A very efficient,copper-free palladium catalyst for the Sonogashira reaction with aryl halides

The new complex [Pd[t-Bu2PCH2N(CH2Ph)CH2P t-Bu2](OAc)2] is a very efficient catalyst for the Sonogashira cross-coupling reaction of aryl halides with acetylenes at room temperature, without co-catalyst.     

Dynamic contact angle and spreading rate measurements for the characterization of the effect of dentin surface treatments

A new methodology capable of providing reliable and reproducible contact angle (theta) data has been employed to study the effect of clinical treatments grinding, acid etching, and deproteinization on medial dentin tissue. It is based on the application of the ADSA-CD algorithm to the determination of low-rate dynamic contact angles, obtained from slowly growing drops, and on contact angle measurement, as well as spreading behavior analysis, during the relaxation of the system (water on treated dentin) after initial drop growth. The theta data obtained were substantially more reproducible than those obtained with classical methods. A net effect of the treatment on theta was found, increasing dentin wettability: theta (polished) >theta (etched) >theta (deproteinized). The spreading rates correlate with the angles and are adequate for the dentin surface characterization. ANOVA and SNK tests show that for advancing contact angles the means corresponding to all treatments are significantly different. In the relaxing phase, mean angle and spreading rates on polished dentin differ significantly from those on etched and deproteinized dentin, but the latter do not differ significantly from each other.     

Extraction of environmental information from large aerosol data sets through combined application of cluster and factor analyses

To reveal useful environmental information which is contained in large analytical data sets, an approach, based on the successive application of hierarchical cluster analysis and factor analysis, is proposed. Estimation criteria to determine the most suitable number of clusters and/or factors, are discussed and the interpretation of the cluster and factor analyses results is performed using visual techniques. The data sets were obtained by scanning electron microscope-energy-dispersive X-ray analysis of individual North Sea aerosol particles.     

One-pot enantioselective formation of eight-membered rings from alkenyl Fischer carbene complexes and ketone enolates

Eight-membered carbocycles with up to five new stereogenic centers are enantioselectively obtained following a one-pot procedure that involves the coupling of three components: an alkenyl Fischer carbene complex, a ketone enolate, and allyl lithium.