Intramolecular sulfur transfer in N-enoyl oxazolidine-2-thiones promoted by Brønsted acids. Practical asymmetric synthesis of beta-mercapto carboxylic acids and mechanistic insights

The ability of Br?nsted acids alone to efficiently promote the sulfur transfer process in N-enoyl oxazolidine-2-thiones to give beta-mercapto carbonyl derivatives is demonstrated. The reactions proceed with essentially perfect diastereocontrol for a range of alkyl-substituted N-enoyl oxazolidine-2-thiones (d.r. regularly above 98:2) and high selectivity for most aryl-substituted counterparts (d.r. typically above 92:8). Importantly, the reaction works remarkably well in beta,beta-disubstituted N-enoyl oxazolidine-2-thiones as well, giving rise to quaternary C-S stereocenters in selectivities usually above 95:5. The relative efficiency of a range of acids (trifluoroacetic, difluoroacetic, acetic, triflic) is assessed showing TFA and TfOH as the most efficient and acetic acid as a totally inefficient reaction promoter. The new procedure complements the Lewis acid promoted reaction previously described by our group in two aspects: First, stereodivergent results are obtained for the Lewis acid or Br?nsted acid promoted reactions of beta,beta-disubstituted enoyl compounds. Second, while the Br?nsted acid promoted reactions are stereospecific, providing a good correlation between the substrate E/Z configuration and products stereochemistry, the reactions mediated by Lewis acids (BF3/OEt2) provide invariant d.r. values regardless of the E/Z composition of the starting olefin. The synthetic value of the method is illustrated by (a) removal of the oxazolidinone moiety from the rearranged products under reducing conditions (NaBH4, H2O-THF) which yields beta-mercapto alcohols and (b) treatment with Sm(OTf)3 in MeOH which affords the corresponding beta-mercapto carboxylic esters, both categories of compounds being isolated in up to 97% ee. Remarkably, the method constitutes the first general approach to highly enantioenriched building blocks bearing a quaternary C-S stereocenter. On the other hand, spectroscopic and inhibition experiments are carried out that demonstrate the participation of protons also in the Lewis acid promoted reactions. Finally, the computational studies carried out at the B3LYP/6-31G* level give support for an activation of the substrate enoyl by complexation with two molecules of either the Br?nsted or Lewis acid and serve to explain the stereochemical outcome of the reactions.     

Comparison of supercritical fluid extraction and liquid-liquid extraction for isolation of selected pesticides stored in freeze-dried water samples

Summary The stability of freeze-dried water samples spiked with eight agrochemicals (atrazine, simazine, linuron, carbaryl, propanil, fenitrothion, parathion and fenamiphos) were examined to evaluate their suitability as candidate reference materials for their determination in water samples. In addition, two different extraction procedures, liquid-liquid and supercritical fluid extraction, were compared for the isolation and trace enrichment of target analytes from freeze-dried water samples. Final analytical determinations were by gas chromatography-nitrogen phosphorus detection and electronic impact mass spectrometry, and by liquid chromatography-diode array detection. The whole methodology developed in this paper permitted the determination of pesticides spiked in water at levels varying from 0.03 to 6.9 g L^–1.     

The aldol addition reaction: an old transformation at constant rebirth

The main recent conceptual advances in asymmetric aldol reactions are presented. Methods ranging from stoichiometric chiral auxiliary-mediated to direct, catalytic reactions are covered, including the Mukaiyama aldol reactions which use stoichiometric base and silylating reagents, but catalytic (substoichiometric) amounts of the chiral inductor. The salient features of each new development are noted, paying special attention to practical concerns and to the potential implementation for large scale production. After examination of pros and cons of each strategy, gaps and limitations that deserve further investigation are highlighted.     

Classification of extended chaotic inflation potentials

Brans-Dicke gravity is remarkable not only in that General Relativity and Mach's Principle find a common enlarged scenario where they are mutually consistent, but also in that it provides a very interesting quantum cosmological model within the inflationary paradigm. The interplay between the Brans-Dicke scalar Φ and the inflaton field σ plays an important rôle during the course of inflation, and although the dynamics as such is governed by the potential, the onset and the end of inflation are determined by the values of both fields jointly. The relative position of the beginning – and end-of-inflation curves (BoI and EoI respectively) is the most relevant factor in determining the resulting quantum cosmological scenario. The classification of potentials that is given in this paper is based on the criterion of whether the BoI and EoI boundaries enclose a finite or infinite area in the (σ,Φ) plane where inflation takes place. It is shown that this qualitative classification distinguishes two classes of potentials that yield very different cosmologies and it is argued that only those theories in which BoI and EoI enclose a finite area in the (σ,Φ) plane are compatible with our observable universe.     

New approach on the alkylthiol determination in water by in situ derivatization SPME followed by GC-ECD/NPD analysis

Alkylthiols are very reactive and highly volatile compounds, and thus it is difficult to determine these in the water phase. In the present work, an in situ derivatization step prior to solid-phase microextraction (SPME) has been developed for their determination in water samples. The dinitrobenzylation reaction was selected because the high chemical stability of the corresponding thioethers formed provides a significant increase in the distribution coefficient between the SPME fibre and the aqueous phase, and a potential increase in the selectivity and sensitivity. Therefore, different derivatization reaction conditions (i.e. pH, temperature, reaction time and derivatizating reagent concentration) have been studied. Then, the main parameters affecting to the SPME process, that is coating selection, extraction time profile, extraction and desorption temperatures, have been optimized. Finally, a method based on a simple 2,4-dinitrophenylation reaction at pH 8–10, in 60?min at 75°C, coupled to direct SPME using PDMS-DVB fibres at 30°C for 45?min is proposed. The performance of the method provided a good linearity and precision data, and the detection limits were in the low ng?L^?1 level.     

Catalytic Enantioselective Quick Route to Aldol‐Tethered 1,6‐ and 1,7‐Enynes from ω‐Unsaturated Aldehydes

An effective asymmetric route to functionalized 1,6‐ and 1,7‐enynes has been developed based on a direct cross‐aldol reaction between ω‐unsaturated aldehydes and propargylic aldehydes (α,β‐ynals) promoted by combined α,α‐dialkylprolinol ether/Brønsted acid catalysis. This synergistic activation strategy is key to accessing the corresponding aldol adducts with high stereoselectivity, both enantio‐ and diastereoselectivity. The aldol reaction also proceeds well with propargylic ketones (α,β‐ynones) thus enabling a stereocontrolled access to the corresponding tertiary alcohols. The utility of these adducts, which are difficult to prepare through standard methodology, is demonstrated by their transformation into trisubstituted bicyclic enones using standard Pauson–Khand conditions.     

Femtosecond pulsed laser deposition of nanostructured TiO2 films

Nanostructured deposits of TiO₂ were grown on Si (1 0 0) substrates by laser ablating a TiO₂ sintered target in vacuum or in oxygen using a Ti:sapphire laser delivering 80 fs pulses. The effect of the laser irradiation wavelength on the obtained nanostructures, was investigated using 800, 400 and 266 nm at different substrate temperatures and pressures of oxygen. The composition of the deposits was characterized using X-ray photoelectron spectroscopy (XPS) and the surface morphology was studied by environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM). Deposits are absent of microscopic droplets in all conditions explored. The best deposits, constituted by nanoparticles of an average diameter of 30 nm with a narrow size distribution, were obtained at the shorter laser wavelength of 266 nm under vacuum at substrate room temperature.