The reactivity, as electrogenerated bases, of chiral and achiral phenazine radical-anions, including application in asymmetric deprotonation

Radical-anions, electrochemically generated in aprotic solvent from C(2) symmetric homochiral phenazine derivatives, act as chiral electrogenerated bases (EGBs) in the desymmetrisation by selective deprotonation of a prochiral epoxide (3,4-epoxy-2,3,4,5-tetrahydrothiophene-1,1-dioxide); the anion produced is trapped by mesitoic anhydride. The phenazines may be recovered in high yield by air oxidation. Enantiomeric excesses are modest (8-34%) but this is to our knowledge the first demonstration of such stereoselective electrochemically-initiated deprotonation. The reactivity of phenazine radical-anions as EGBs has also been explored by measurements of the rates of proton transfer; the prochiral epoxide was found to have a kinetic acidity similar to that of the methyltriphenylphosphonium cation.     

Evolution of the structural stability of large Cu,Ni, Pd,and Ag clusters with size: An analysis within the embedded atom method

A study of the structural stability of clusters made up of a single component has been carried out within the Embedded Atom Method. Perfect icosahedral and cuboctahedral Cu, Ni, Pd, and Ag clusters with up to 5083 atoms have been compared. The icosahedron is found to be the stable structure for small clusters, and a change of structure from icosahedral to cuboctahedral is found as the cluster size increases. A contraction of the interatomic distances results when the cluster size decreases.     

Useful dual Diels-Alder behavior of 2-azetidinone-tethered aryl imines as azadienophiles or azadienes: a beta-lactam-based stereocontrolled access to optically pure highly functionalized indolizidine systems

Imines derived from 4-oxoazetidine-2-carbaldehydes have been found to be versatile Diels-Alder reagents in that they exhibit two reactivity patterns. 2-Azetidinone-tethered imines undergo diastereoselective reaction with Danishefsky's diene in the presence of different Lewis acids. The effect of the amount of catalyst on the conversion rate as well as on the product ratio has been studied. Under standard reaction conditions, indium(III) chloride and zinc(II) iodide provided the best yields, and indium(III) triflate the highest diastereoselectivity in the Lewis acid promoted aza-Diels-Alder cycloaddition. Treatment of the aforementioned imines with cyclopentadiene, 2,3-dimethyl-1,3-butadiene or 3,4-dihydro-2 H-pyran led to cycloadducts arising from inverse electron-demand condensation involving the beta-lactam-tethered aryl imine as the heterodiene component. In addition, the first methodology for preparing indolizidines from beta-lactams has been developed. This process involves amide bond cleavage of the beta-lactam ring in the aza-Diels-Alder cycloadducts with concomitant cyclization. Full chirality transfer occurs when the reaction is performed with an enantiomerically pure substrate.     

Acid catalyzed hydrolysis of brotizolam,a thienotriazolodiazepine: Spectroscopic study

The acid catalyzed hydrolysis of the thienotriazolodiazepine, Brotizolam, 2-bromo-4-(2-chlorophenyl)-9-methyl-6H-thieno[3,2-f][1,2,4]triazolo[4,3-a][1,4]diazepine ( 1 ), has been studied spectrometrically (ir, pmr, cmr, and ms). The cleavage reaction of the azomethine bond is reversible and the open-ring compound is in equilibrium with the ring closed compound (protonated form of the parent drug).     

Chiral separation of some triazole pesticides by supercritical fluid chromatography

The enantiomeric separation of six triazole pesticides: cyproconazole, propiconazole, diniconazole, hexaconazole, tebuconazole and tetraconazole, using supercritical fluid chromatography and the Chiralpak AD column, is presented in this work. The effect of different organic modifiers such as methanol, ethanol and 2-propanol on the retention and resolution was studied as well as the presence of additives in the mobile phase. The results obtained were highly successful, all the compounds were enantiomerically separated and in most of the cases the analysis time was close to 10 min. The type of organic modifier that provided the best results depended on the compound.     

Dilithiated synthons: synthesis of cyclic ethers

Recent advances in the generation of dilithiated synthons by arene-catalyzed lithiation of the corresponding dichloro compounds in the presence of carbonyl compounds (Barbier-type reaction conditions) as the key step are described. Further cyclization of the generated diols under different reaction conditions affords a variety of mono-, bi-, and spirocyclic ethers.     

Comparison of different derivatization approaches for mercury speciation in biological tissues by gas chromatography/inductively coupled plasma mass spectrometry

A novel interface design for coupling gas chromatography and inductively coupled plasma mass spectrometry (GC/ICP-MS) was used to perform mercury speciation in biological tissues. Three derivatization approaches were optimized and compared for this purpose: anhydrous butylation using a Grignard reagent, aqueous ethylation by means of NaEt(4)B and aqueous propylation with NaPr(4)B. The last reagent was synthesized in the laboratory as it is not commercially available. Detection limits obtained by GC/ICP-MS ranged between 100 and 200 fg (as absolute mass) for methylmercury and between 500 and 600 fg for inorganic mercury using a 1 microl injection. Quantification of methyl- and inorganic mercury was carried out by resorting to aqueous calibration, using ethylmercury as internal standard for both propylation and butylation derivatization techniques. For ethylation procedures, a methylpropylmercury solution was used as internal standard. The absence of transmethylation during sample preparation was checked using a 97% enriched (202)Hg inorganic standard. The accuracy of the three derivatization approaches was evaluated by the analysis of the certified reference material DOLT-2 (dogfish liver) from the National Research Council of Canada and certified for methylmercury, with satisfactory results.     

Extractive fluorimetric determination of ultratraces of lead with cryptand 2.2.2 and eosin

A new speetrofluorimetric method for determination of ultratraces of lead is based on solvent extraction into chloroform of the ion-pair formed between the positively-charged cryptate of lead with cryptand 2.2.2 and the eosinate anion. The detection limit for lead is 1 ng ml , and the linear working range is from the detection limit to 250 ng ml . The relative standard deviation is 3.7% at the 100 ng ml level. The method is highly selective for the extraction and determination of lead in the presence of other cations, and has been tested for direct determination of lead contamination in soft drinks. Aggregation of the extracted ion-pair in the organic phase has been demonstrated in fundamental extraction studies.     

Kinetic modelling of cadmium removal from phosphoric acid by non-dispersive solvent extraction

In this work it is reported that the kinetic modelling of the separation of cadmium from phosphoric acid by non-dispersive solvent extraction. Using Cyanex 302 as selective extractant, the extraction step was carried out in a hollow fibre module containing polypropylene fibres, whereas the concentration step required a ceramic module with tubular channels due to the high acidity of the backextraction agent. Application of the methodology previously reported by the authors led to the development of a kinetic model with three design parameters, i.e., equilibrium constant of the extraction reaction (K'_e = 6 × 10³ mol⁻²/l⁻²), membrane mass transport coefficient of the extraction module K_me=8.33×10⁻⁸ m/s) and of the backextraction module (K_ms=3.33×10⁻⁸ m/s), that described satisfactorily the behaviour of the separation-concentration system. Thus, in this work a new application of the non-dispersive solvent extraction technology is presented, characterising at the same time the behaviour and parameters of a new type of contactor, i.e., a tubular ceramic module.