排序方式: 共有123条查询结果,搜索用时 0 毫秒
11.
The reactivity of 4-aryl-1-(2-chloroethyl)azetidin-2-ones and 4-aryl-1-(3-bromopropyl)azetidin-2-ones with regard to lithium aluminium hydride has been evaluated for the first time. 4-Aryl-1-(2-chloroethyl)azetidin-2-ones were transformed into novel 1-(1-aryl-3-hydroxypropyl)aziridines through an unprecedented conversion of beta-lactams into 2,3-unsubstituted aziridine derivatives. Unexpectedly, 4-aryl-1-(3-bromopropyl)azetidin-2-ones underwent dehalogenation towards 3-aryl-3-(N-propylamino)propan-1-ols upon treatment with LiAlH(4). 1-(1-Aryl-3-hydroxypropyl)aziridines were further elaborated by means of ring opening reactions using benzyl bromide in acetonitrile towards 3-aryl-3-[N-benzyl-N-(2-bromoethyl)amino]propan-1-ols and using aluminium(iii) chloride in diethyl ether, affording 3-aryl-3-[N-(2-chloroethyl)amino]propan-1-ols. 相似文献
12.
Jasper Vanlerberghe Tom Maertens Joris Walraevens Stijn De Vuyst Herwig Bruneel 《4OR: A Quarterly Journal of Operations Research》2016,14(3):281-308
Numerous scheduling policies are designed to differentiate quality of service for different applications. Service differentiation can in fact be formulated as a generalized resource allocation optimization towards the minimization of some important system characteristics. For complex scheduling policies, however, optimization can be a demanding task, due to the difficult analytical analysis of the system at hand. In this paper, we study the optimization problem in a queueing system with two traffic classes, a work-conserving parameterized scheduling policy, and an objective function that is a convex combination of either linear, convex or concave increasing functions of given performance measures of both classes. In case of linear and concave functions, we show that the optimum is always in an extreme value of the parameter. Furthermore, we prove that this is not necessarily the case for convex functions; in this case, a unique local minimum exists. This information greatly simplifies the optimization problem. We apply the framework to some interesting scheduling policies, such as Generalized Processor Sharing and semi-preemptive priority scheduling. We also show that the well-documented \(c\mu \)-rule is a special case of our framework. 相似文献
13.
The Woodward-Hoffmann rules for pericyclic reactions, a fundamental set of reactivity rules in organic chemistry, are formulated in the language of conceptual density functional theory (DFT). DFT provides an elegant framework to introduce chemical concepts and principles in a quantitative manner, partly because it is formulated without explicit reference to a wave function, on whose symmetry properties the Woodward-Hoffmann [J. Am. Chem. Soc. 87, 395 (1965)] rules are based. We have studied the initial chemical hardness response using a model reaction profile for two prototypical pericyclic reactions, the Diels-Alder cycloaddition of 1,3-butadiene to ethylene and the addition of ethylene to ethylene, both in the singlet ground state and in the first triplet excited state. For the reaction that is thermally allowed but photochemically forbidden, the initial hardness response is positive along the singlet reaction profile. (By contrast, for the triplet reaction profile, a negative hardness response is observed.) For the photochemically allowed, thermally forbidden reaction, the behavior of the chemical hardness along the initial stages of the singlet and triplet reaction profiles is reversed. This constitutes a first step in showing that chemical concepts from DFT can be invoked to explain results that would otherwise require invoking the phase of the wave function. 相似文献
14.
The local aromaticity of benzenoid rings is examined by means of the Polansky index (P) and generalized population analysis (GPA). The results are found to agree very well with recently published circuit-condensed ring currents and magnetic-energetic aromaticity indices, but no correlation is found with nucleus independent chemical shifts (NICS). This is usually seen as a manifestation of the more general multidimensional nature of aromaticity. This paper examines the sources for the observed correlations, showing that some indices give conflicting results because they inherently reflect different phenomena. 相似文献
15.
Brooks NR Schaltin S Van Hecke K Van Meervelt L Fransaer J Binnemans K 《Dalton transactions (Cambridge, England : 2003)》2012,41(23):6902-6905
The first examples of structurally characterised mixed-ligand metal-containing ionic liquids (ILs) are presented, synthesised by the use of different N-alkylimidazoles. The cations consist of two-coordinate silver(i) centres ligated by two different N-alkylimidazole ligands. It is shown that the resulting ionic liquids have lower melting points than the single ligand ILs. 相似文献
16.
17.
Stijn Dekeukeleire Matthias D’hooghe Matthieu Vanwalleghem Willem Van Brabandt Norbert De Kimpe 《Tetrahedron》2012,68(52):10827-10834
Chiral piperazine and 1,4-diazepane annulated β-lactams, prepared from the corresponding (3R,4S)-4-imidoyl-1-(ω-haloalkyl)azetidin-2-ones through reduction with sodium borohydride in ethanol, were transformed into novel methyl (R)-alkoxy-[(S)-piperazin-2-yl]acetates and methyl (R)-alkoxy-[(S)-1,4-diazepan-2-yl]acetates upon treatment with hydrogen chloride in methanol. On the other hand, bromination of (3R,4R)-1-allyl-4-formyl-β-lactams and (3R,4S)-1-allyl-4-imidoyl-β-lactams in dichloromethane, followed by sodium borohydride reduction of the resulting dibrominated azetidin-2-ones in ethanol, did not afford the envisaged bicyclic β-lactams but unexpectedly furnished (3R,4S)-1-(2-bromo-2-propenyl)azetidin-2-ones instead. 相似文献
18.
Dieter Fiems Stijn De Vuyst Sabine Wittevrongel Herwig Bruneel 《Annals of Operations Research》2009,170(1):113-131
In this contribution we investigate higher-order loss characteristics for M/G/1/N queueing systems. We focus on the lengths of the loss and non-loss periods as well as on the number of arrivals during these
periods. For the analysis, we extend the Markovian state of the queueing system with the time and number of admitted arrivals
since the instant where the last loss occurred. By combining transform and matrix techniques, expressions for the various
moments of these loss characteristics are found. The approach also yields expressions for the loss probability and the conditional
loss probability. Some numerical examples then illustrate our results. 相似文献
19.
Novel [2n]thiacalixarenepyrazine and [2n]thiacalixarenetriazine systems were synthesised by one-pot SNAr reactions. A screening of the metal-complexing ability of [26]hexathiacalix[3]arene[3]pyrazine revealed its affinity for CuI, CuII and AgI metal salts. 相似文献
20.
Van Dyck S Vissers S Van Schepdael A Hoogmartens J 《Journal of chromatography. A》2003,986(2):303-311
The in-capillary reaction of angiotensin converting enzyme (ACE) with the tripeptide substrate hippuryl-L-histidyl-L-leucine was studied. ACE activity was determined by the quantitation of the product, hippuric acid, at 230 nm. Reaction occurred at the capillary inlet during a predetermined waiting period, followed by the electrophoretic separation of the compounds. When the set-up was reversed, i.e. reaction at the opposite side after short-end injection of enzyme and substrate, separation was achieved in less than 5 min. Using the Lineweaver-Burk equation, an average Michaelis constant for ACE from rabbit lung was calculated to be 1.16 +/- 0.12 mM, a value consistent with previously reported data. 相似文献