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31.
32.
A new approach for the evaluation of chiral purity of serine esterification products bearing long-chain alkyl substituents was developed. The compounds were simply converted to aryl-substituted oxazolines which: (i) facilitates effective chromatographic enantioseparation and (ii) enables direct detection using ultraviolet absorption. The method employs a polysaccharide-based chiral stationary phase and allows enantioseparation of highly stable oxazoline products in less than 6 min using a simple binary mobile phase. As opposed to the previously used normal phase method the developed method was performed in the reversed-phase mode. Aside from the benefits of switching to less hazardous solvents with regard to the principles of Green Chemistry, this has also led to a reduction in the analysis time. In comparison with known serine chromophores, the best enantioseparation of aryloxazoline rigid structure may be achieved only based on non-polar interactions with the chiral stationary phase. In contrast, the substitution of the chromophore moiety with hydroxyl substituent affected intra and intermolecular interactions that caused enantioseparation differences. Concurrently, we found high chirality retention of (R)- and (S)-configuration oxazoline standards (≥99% enantiomeric excess) during the introduction of the ultraviolet label. The method is suitable for rapid injection of the mixture containing the ultraviolet absorption marker without prior purification. 相似文献
33.
Summary The addition of dienes to the system [(Cp2TiCl)2] LiAlH4toluene changes the system so that the complex [Cp2TiAlH4] is quantitatively formed instead of a titanocene hydride — aluminium hydride cluster. The complex [Cp2TiAlH4] is further converted into 3-allyltitanocene derivatives ([Cp2TiA]) if the diene structure is suitable for formation of stable [Cp2TiA] compounds and if the equilibrium [Cp2TiAlH4]+diene[Cp2TiA]+A1H3 is shifted towards the formation of [Cp2TiA] by the excess of diene. All the compounds [Cp2TiA] exhibit high-resolution e.s.r. spectra at g=1.993, showing interaction of the unpaired electron with the cyclopentadienyl and 3-allyl protons. The e.s.r. spectra clearly reveal the presence of alkyl substituents atsyn-1,3-positions of 3-allyl ligand, and show a triplet of multiplets for (3-allyl)titanocene, doublets of multiplets for (1-alkyl-3-allyl)titanocenes and single multiplets for (1,3-dialkyl-3-allyl)-titanocenes. thermal isomerization of (1,3-dimethyl-3-allyl)-titanocene and (1-methyl-3-ethyl-3-allyl)titanocene, hitherto considered as the stable Cp2TiA compounds, into (1-alkyl-3-allyl)titanocenes was confirmed by e.s.r. and electronic absorption spectroscopy as well as by chemical means. 相似文献
34.
Enzymatic reactions in supercritical gases 总被引:10,自引:0,他引:10
D. A. Hammond M. Karel A. M. Klibanov V. J. Krukonis 《Applied biochemistry and biotechnology》1985,11(5):393-400
The enzyme polyphenol oxidase has been found to be catalytically active in supercritical carbon dioxide and fluoroform: it
readily oxidizesp-cresol andp-chlorophenol to their correspondingo-benzoquinones. 相似文献
35.
The dynamics of gel isoelectric focusing were studied by using amphoteric low-molecular-mass colored substances (isoelectric point markers). The polyacrylamide gel in slab format was in direct contact with the electrodes. In addition to isoelectric focusing with a pH gradient composed of synthetic carrier ampholytes, pH gradients created by simple buffers of acetic acid, 2-(N-morpholino)ethanesulfonic acid, histidine and N,N,N',N'-tetramethylethylenediamine were applied. The progress of the electrofocusing process was monitored by a charge-coupled device camera and video recording. The gradient profile and dynamics were approximated from the positions of isoelectric point markers, which were focused both on boundaries between individual zones of simple buffers and within the zones themselves. The obtained animated records enabled the observation of the entire real focusing run within fractions of a minute, which is useful both for the understanding and optimization of the focusing. 相似文献
36.
Changes in solvent activity and thermodynamic stability of a system undergoing crosslinking in the presence of a solvent are modeled by combination of branching theory with thermodynamics of polymer solutions and swelling of a crosslinked polymer. The system is considered as a quasiternary one in which the solvent, polydisperse soluble polymer (sol) and crosslinked network, respectively, represent the components. Crosslinking brings about growth of polymer structures but also changes in polymer-solvent interactions. The results are compared with onset of phase separation and changes in evaporation rates from films formed from a hydroxy-functional star oligomer and triisocyanate. 相似文献
37.
38.
39.
Karel Jelínek Zuzana Limpouchov Karel Prochzka 《Macromolecular theory and simulations》2000,9(9):703-714
Conformations of chains in swollen middle layers of onion‐skin micelles were studied by Monte Carlo simulations on a tetrahedral lattice under conditions that mimic real systems of swollen onion‐skin micelles. Polymer blocks are modeled as tethered self‐avoiding chains, enclosed in a narrow spherical layer. Average density of segments, 〈gS〉 ca. 0.6, corresponds to swollen micellar systems. Only the excluded volume effect was taken into account since it plays the most important role in dense polymer systems. Individual chains are described by equivalent ellipsoids of gyration. Distributions of the ellipsoid half‐axes were calculated during simulations. Results based on a large series of simulations indicate that the middle layer‐forming blocks may be described as prolonged ellipsoids oriented preferentially perpendicular to the radial direction. Analysis of the data concerning the orientations of end‐to‐end vectors and distributions of segments within one chain indicates that individual chains are strongly interpenetrated and the multi‐chain system is fairly disordered. 相似文献
40.
Jeremy P. Scott Deborah C. Hammond Elizabeth M. Beck Karel M.J. Brands Antony J. Davies Ulf-H. Dolling Derek J. Kennedy 《Tetrahedron letters》2004,45(17):3345-3348
The efficient preparation of 4-aryl-4-phenylsulfonyl cyclohexanones, containing a quaternary sulfone-bearing carbon centre, is described. Their synthesis proceeds in 38-78% overall yield by way of three steps: (i) sulfinate alkylation; (ii) methylenation; and (iii) regioselective Diels-Alder condensation with 2-trimethylsiloxybutadiene. The scope and limitations of the one-pot Mannich-type methylenation described were examined. 相似文献