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排序方式: 共有117条查询结果,搜索用时 31 毫秒
21.
Chirasil-β-Dex containing an undecamethylene spacer (C11-Chirasil-Dex) was used as chiral stationary phase (CSP) in enantioselective gas chromatography (GC). The versatility of the new stationary phase in the simultaneous enantiomeric separation of a set of cyclopropane derivatives is demonstrated. The GC method provides information about the chemical yields of the cyclopropane products, enantioselectivity, substrate specifity, and catalytic activity of the chiral catalysts used in the intermolecular cyclopropanation of olefins and avoids time-consuming work-up procedures. 相似文献
22.
A. Ghanem H. Y. Aboul-Enein F. Lacrampe S. Grass Z. Jiang P. Müller 《Chromatographia》2005,61(3-4):103-111
Chirasil--Dex was used as chiral stationary phase for the enantioselective gas chromatographic analysis of several new chiral cyclopropane derivatives. The GC method provides information about the chemical yields of the cyclopropane products, enantioselectivity, substrate specifity, and catalytic activity of the chiral catalysts used in the inter- and intra-molecular cyclopropanation reactions and avoids time-consuming work-up procedures. 相似文献
23.
In this work, a direct enantioselective HPLC monitoring system is developed. The system consists of an HPLC equipped with an immobilized cellulose-based chiral stationary phase Chiralpak IB, UV and a polarimetric detector in series. The developed system is used in the direct monitoring of the enantioselectivity of lipase-catalyzed enantioselective esterification of 2-phenoxy propionic acid of which the (R)-enantiomer is a precursor of the (R)-2-(p-hydroxyphenoxy)propionic acid known as an important intermediate in synthesizing aryloxyphenoxypropionic acid based agrochemicals. The versatility of the biocatalyzed reaction in chiral separation to access to both enantiomers of racemic 2-phenoxy propionic acid is demonstrated. The solvent versatility of the immobilized chiral stationary phase used in the direct monitoring of such reaction in non-standard HPLC solvent is shown. 相似文献
24.
The reaction between benzoyl peroxide and p-benzoquinone in concentrated solutions in a wide variety of solvents has been investigated by isolation and identification of the reaction products. Despite the high efficiency of p-benzoquinone as a trap for benzoyloxy radicals, partial decarboxylation to phenyl radicals usually occurs. Complete suppression of decarboxylation is achieved only when p-benzoquinone is present at such a high concentration that it is effectively the solvent for the reaction.The benzoyloxy- and phenyl semiquinones show marked differences in reactivity, the former tend to combine to form dibenzoyloxy dibenzoquinone while disproportionation is favoured by the latter to form quinhydrone of monophenylbenzoquinone.At lower quinone ratio, the peroxide undergoes induced decomposition by phenyl radicals both in “reactive” and “unreactive” solvents. The induced decomposition involves the formation of radical intermediates which undergo disproportionation, but not intramolecular rearrangement, to form p-phenylbenzoyloxy radicals. The latter can be captured, before undergoing decarboxylation, by the benzoyloxysemiquinones formed in the reaction.A correlation between the electron donating property of a radical and its capability to induce the decomposition of the peroxide was developed. 相似文献
25.
26.
Nada A. Imam N. El Aassy Ibrahim E. Ghanem A. 《Journal of Radioanalytical and Nuclear Chemistry》2019,322(2):347-359
Journal of Radioanalytical and Nuclear Chemistry - This research focused on studying the effect of sulfuric acid concentrations on the leaching of different radionuclides (238U and 235U series) in... 相似文献
27.
Radouen Ghanem & Yazid Dendani 《计算数学(英文版)》2023,41(4):717-740
In this paper we deal with the convergence analysis of the finite element method for an elliptic penalized unilateral obstacle optimal control problem where the control and the obstacle coincide. Error estimates are established for both state and control variables. We apply a fixed point type iteration method to solve the discretized problem.To corroborate our error estimations and the efficiency of our algorithms, the convergence results and numerical experiments are illustrated by concrete examples. 相似文献
28.
Ashraf Ghanem Mohammed Nabil Aboul-Enein Aida El-Azzouny Mohammed F. El-Behairy 《Journal of chromatography. A》2010,1217(7):1063-1074
The enantioselective resolution of a set of racemic acidic compounds such as non-steroidal anti-inflammatory drugs (NSAIDs) of the group arylpropionic acid derivatives is demonstrated. Thus, a set of lipases were screened and manipulated in either the esterification or hydrolysis mode for the enantioselective kinetic resolution of these racemates in non-standard organic solvents. The accurate determination of the enantiomeric excesses of both substrate and product during such reaction is demonstrated. This was based on the development of a direct and reliable enantioselective high performance liquid chromatography (HPLC) procedure for the simultaneous baseline separation of both substrate and product in one run without derivatization. This was achieved using the immobilized chiral stationary phase namely Chiralpak IB, a 3,5-dimethylphenylcarbamate derivative of cellulose (the immobilized version of Chiralcel OD) which proved to be versatile for the monitoring of the lipase-catalyzed kinetic resolution of racemates in non-standard organic solvents. 相似文献
29.
Hermann JC Ghanem E Li Y Raushel FM Irwin JJ Shoichet BK 《Journal of the American Chemical Society》2006,128(49):15882-15891
With the emergence of sequences and even structures for proteins of unknown function, structure-based prediction of enzyme activity has become a pragmatic as well as an interesting question. Here we investigate a method to predict substrates for enzymes of known structure by docking high-energy intermediate forms of the potential substrates. A database of such high-energy transition-state analogues was created from the KEGG metabolites. To reduce the number of possible reactions to consider, we restricted ourselves to enzymes of the amidohydrolase superfamily. We docked each metabolite into seven different amidohydrolases in both the ground-state and the high-energy intermediate forms. Docking the high-energy intermediates improved the discrimination between decoys and substrates significantly over the corresponding standard ground-state database, both by enrichment of the true substrates and by geometric fidelity. To test this method prospectively, we attempted to predict the enantioselectivity of a set of chiral substrates for phosphotriesterase, for both wild-type and mutant forms of this enzyme. The stereoselectivity ratios of the six enzymes considered for those four substrate enantiomer pairs differed over a range of 10- to 10,000-fold and underwent 20 switches in stereoselectivities for favored enantiomers, compared to the wild type. The docking of the high-energy intermediates correctly predicted the stereoselectivities for 18 of the 20 substrate/enzyme combinations when compared to subsequent experimental synthesis and testing. The possible applications of this approach to other enzymes are considered. 相似文献
30.
Raed Ghanem Hutaf Baker Mobeen Abu Seif Raed A. Al-Qawasmeh Abdl-Azizi Mataneh Samer Ibrahim Al-Gharabli 《Journal of solution chemistry》2010,39(4):441-456
Spectroscopic methods and theoretical calculations were used to study the photophysical properties of colchicine (1). The characteristic bands of colchicine (1) transitions occurring at ∼245 nm and 350 nm arise from n–π
* and π–π
* transitions, and were found to be blue shifted with increasing polarity index of the solvent and red shifted (bathocrhomic)
with increasing acidity of the media. Solvent type and polarity were found to affect both the rate constant and photo conversion
yield. Excitation of colchicine (1) at 366 nm populates the S1 orbital, from which the photoreaction of colchicine (1) initially produces lumicolchicine directly in its ground state by a concerted disrotatory electrocyclization (i.e., 4π electrocyclization involving the tropolone ring). The acidity of the media affects both rate constants and conversion yield;
both of these were inversely proportional to the acid concentration. The presence of acid decreases the photo conversion yield
of colchicine (1) from 90% to 40%. The ionic strength affects both the rate constants and the photo conversion yield; rate constants are inversely
related to the ionic strength. The photochemistry of colchicine (1) is explained on the basis of the initial formation of a solvation complex in the ground state. 相似文献