The rôle of countercurrent chromatography in the fractionation of complex mixtures

The biologically active pricipals in nature are frequently present as only a few parts per million of complex mixtures of non-volatile components and often have limited stability. Their isolation often requires the application of all available techniques, such as adsorption chromatography, ion exchange procedures, size exclusion techniques, and solvent partition methods consistent with their physical properties and stability. The process of countercurret chromatography is essentially liquid-liquid chromatography in which the stationary liquid bed is retained in the column by a force field rather than by a solid supporting matrix. Adsorption effects are thereby eliminated. The technique is particularly advantageous in the preparative separation of milligram to gram quantities of polar and labile organic compounds and bio-particulate materials such as cells and cell fragments. Virtually any twophase solvent system, either aqueous or non-aqueous may be employed. Countercurrent chromatography (CCC) provides a convenlent alternative to adsorption chromatography for fractionation of natural products or other complex mixtures. In some cases, this high resolution method offers advantages with regard to the avoidance of contamination from solid adsorbents, versatility, and relatively inexpensive operation. The article covers some of the applications, selection of solvents, and advantages of CCC.     

Fe3O4@SiO2-polymer-imid-Pd magnetic porous nanosphere as magnetically separable catalyst for Mizoroki–Heck and Suzuki–Miyaura coupling reactions

The activity of palladium nanoparticles supported on poly (N-vinylpyrrolidone) grafted Fe₃O₄@SiO₂ was investigated in the cross-coupling reactions. We have applied this catalyst under low loading of the supported palladium nanoparticles for the coupling of aryl halides with alkenes (Mizoroki–Heck reaction) and organoboronic acids (Suzuki–Miyaura reaction) in the absence of phosphorous ligands. Short reaction times and excellent yields of the products express the effectiveness of this catalyst. The nanocatalyst can be separated from the reaction mixture by applying a permanent magnet externally and can be reused for six times without appreciable change in catalytic activity. Also, the amount of leaching of Pd nanoparticles has been determined by ICP analysis and results showed low leaching of the metal into solution from the supported catalyst.     

Novel polyesters and polyester/Cloisite 30B nanocomposites based on a new rhodanine-based monomer

A novel bis-diol monomer, 3,3'-bis-(2-hydroxyethyl)-2,2'-dithioxo-[5,5']bithiazolidinylidene-4,4'-dione, was prepared in a one-pot three component reaction and used in a polyesterification reaction with various aromatic dicarboxylic acids using tosyl chloride/dimethylformamide/pyridine system as a condensing agent in order to generate a series of new polyesters. The prepared polyesters were characterized by FTIR spectroscopy and elemental analysis. In the next part, a series of polyester/clay nanocomposites were synthesized through a polyesterification reaction of the synthesized monomer with iso-phthalic acid. Cloisite 30B was used as the filler at different concentrations. Wide-angle X-ray diffraction and transmission electron microscopy studies showed that the organoclay layers are mainly exfoliated in the nanocomposites containing 1 wt % of Cloisite 30B.     

Synthesis of (-)-centrolobine by Prins cyclizations that avoid racemization

[formula: see text] The segment-coupling Prins cyclization avoids two of the problems common to other Prins cyclization protocols: side-chain exchange and partial racemization by reversible 2-oxonia Cope rearrangement. Model studies demonstrate the stereochemical fidelity of Prins cyclizations using alpha-acetoxy ethers compared with direct aldehyde-alcohol Prins reactions. Furthermore, we propose a mechanism for the racemization observed in some intermolecular Prins cyclizations. Two straightforward syntheses of optically pure (-)-centrolobine highlight the utility of Prins cyclizations.     

Reactivity of homoallylic substituted adamantylideneadamantanes with bromine. Substituent effects on the stability of the ionic and nonionic intermediates

Sterically congested adamantylideneadamantanes (1b-g) (X = Br, Cl, F, OH, OEt, OCOCH(3)), homoallylically substituted with equatorial groups (X), react with bromine in 1,2-dichloroethane to give a stable bromonium ion intermediate or a substitution product depending on the nature of the substituent and on the bromine concentration. The nature of the substituent markedly affects the formation constant of the 1:1 pi-complexes, as well as of the formation constant and reactivity of bromonium ion intermediates. The different reactivity of the ionic intermediates, which depends on the nature of substituents, is attributed to bromonium or bromocarbenium character of the intermediate, with the support of theoretical investigations. Ab initio calculations on 1:1 adamatylideneadamantane-Br(2) complexes (2a-f) show that the substituent affects the stability of these species through electrostatic and dispersion effects. Solvent effects may also contribute to modulate the relative stability of these species.     

The order bidual of <Emphasis Type=Italic>f</Emphasis> -algebras revisited

It is shown that the order bidual X ^~~ of an Archimedean semiprime f -algebra X has a unit element for the Arens multiplication if and only if every positive linear functional on X extends to a positive linear functional on the f -algebra Orth (X) of all orthomorphisms on X.     

Learning effects and the phenomenon of moving bottlenecks in a two-stage production system

This paper studies the effects of learning and forgetting on a two-stage production system and the position of a potential bottleneck in the system. We start with developing a model for a two-stage serial production system where semi-finished items are fed by the first stage to the second stage, which, in turn, processes the items to their final state. The finished items are transferred either to a subsequent stage or to customers. The paper assumes that both stages of the production system considered are subject to learning and forgetting effects. Learning quickens the production rate as more experience is gained (i.e., when the number of repetitions increases), while forgetting has the opposite effect when production is intermittent (i.e., experience is lost over production breaks). The paper studies how different values of the learning and forgetting parameters influence the ratio of the production rates of both stages and the flow of material in the system. The results of the paper indicate that learning may cause a bottleneck to shift its position in a production system. This happens when an initially slower stage overtakes a previously faster stage over time due to a higher learning rate. The paper thus contributes to the literature on moving bottlenecks and provides practitioners with a model that helps predicting where bottlenecks may arise in the production system, and which enables the system to flexibly react to moving bottlenecks.