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181.
Mauricio Gomes Constantino Valdemar Lacerda Júnior Paulo Roberto Invernize Luiz Carlos da Silva Filho Gil Valdo José da Silva 《合成通讯》2013,43(20):3529-3539
Abstract The behavior of several epoxides when treated with NbCl5 was studied. In general, the studied epoxides reacted rapidly with NbCl5, giving, in most cases, more than one product (chlorohydrins, products containing solvent residues, as well as rearrangement products). A detailed study was performed to verify the effects of the temperature (rt, 0°C, or ?78°C) and of the NbCl5 molar concentration on the composition of the products, yield, and time required for the reactions. 相似文献
182.
The Bakkenolide group 1 3 of sesquiterpenes present two interesting synthetic problems. The cisdimethyl cis-hydrindane portion 2, because of its structural and stereochemical relationship with the eremophilane sesqui terpenes,4 has synthetic solutions available.5 The spiro-β-methylene-γ-butyrolactone unit 3,6 which is presently unique to this class of natural products, has been synthesised by four different routes.7–10 相似文献
183.
Daiane Cristina Sass Vladimir Constantino Gomes Heleno Jader da Silva Barbosa Gustavo Oliveira Morais Fernando Batista Da Costa Mauricio Gomes Constantino 《Tetrahedron letters》2013,54(7):625-627
The conversion of a germacranolide structure (tagitinin C) into a furanoheliangolide one (diversifolin) was achieved by hydride conjugate addition using Stryker’s reagent. 相似文献
184.
Mauricio Retamal Christian Costa Juan M. Suárez 《International journal of environmental analytical chemistry》2013,93(1):93-107
A multi-determination method has been developed for the determination and confirmation of 68 organic pollutants in water samples by gas chromatography coupled to triple quadrupole mass spectrometry (GC-MS/MS). The following chemical families were determined in a chromatographic run of less than 26?min: polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), polybrominated diphenylethers (PBDEs), and pesticides (organochlorine, organophosphorus, triazine and others). The sample preparation involved a liquid-liquid extraction (LLE) procedure, obtaining recoveries ranging from 70 to 130% when dichloromethane was used as the extracting solvent. The detection limits of the proposed method were between 0.75 and 19.8?ng?L?1. Samples from the Maipo River in central Chile were taken from 29 different points. Seven pesticides and two PAHs were detected in field collected samples with concentrations ranging from 10 to 95?ngL?1. Concentrations of benzo[a]pyrene in environmental samples ranged from 25 to 33?ngL?1 and were near the maximum levels established by the European Union Directives (50?ng?L?1). 相似文献
185.
Random-key genetic algorithms were introduced by Bean (ORSA J. Comput. 6:154–160, 1994) for solving sequencing problems in combinatorial optimization. Since then, they have been extended to handle a wide class
of combinatorial optimization problems. This paper presents a tutorial on the implementation and use of biased random-key
genetic algorithms for solving combinatorial optimization problems. Biased random-key genetic algorithms are a variant of
random-key genetic algorithms, where one of the parents used for mating is biased to be of higher fitness than the other parent.
After introducing the basics of biased random-key genetic algorithms, the paper discusses in some detail implementation issues,
illustrating the ease in which sequential and parallel heuristics based on biased random-key genetic algorithms can be developed.
A survey of applications that have recently appeared in the literature is also given. 相似文献
186.
The set k-covering problem (SC
k
P) is a variant of the classical set covering problem, in which each object is required to be covered at least k times. We describe a hybrid Lagrangean heuristic, named LAGRASP, which combines subgradient optimization and GRASP with path-relinking
to solve the SC
k
P. Computational experiments carried out on 135 test instances show experimentally that by properly tuning the parameters
of LAGRASP, it is possible to obtain a good trade-off between solution quality and running times. Furthermore, LAGRASP makes
better use of the dual information provided by subgradient optimization and is able to discover better solutions and to escape
from locally optimal solutions even after the stabilization of the lower bounds, whereas other strategies fail to find new
improving solutions. 相似文献
187.
We address the question of whether superfluidity can survive in the case of fermion pairing between different species with mismatched Fermi surfaces using as an example a population-imbalanced mixture of 6Li atomic Fermi gas loaded in a two-dimensional optical lattice at nonzero temperatures. The collective mode is calculated from the Bethe-Salpeter equations in the general random phase approximation assuming a Fulde-Ferrell order parameter. The numerical solution shows that, in addition to low-energy (Goldstone) mode, two rotonlike minima exist, and therefore, the superfluidity can survive in this imbalanced system. 相似文献
188.
Mauricio Gomes Constantino Paulo Marcos Donate Daniel Frederico Tecia Vieira Carvalho Luiz Eduardo Cardoso Julio Zukerman-Schpector 《合成通讯》2013,43(18):3327-3340
An eight-step synthesis of the cyclopentane keto-alcohol 2, which has previously been converted in one step into the carotenoid pigment capsorubin (1), is described. The key step in our synthesis is a stereospecific epoxide rearrangement with ring contraction, thus producing the cyclopentane ring from an epoxide of a cyclohexene. 相似文献
189.
The reactions of secondary alicyclic amines with the title substrate (PDTC) are subjected to a kinetic study in 44 wt.% aqueous ethanol, 25.0°C, ionic strength 0.2 M (KCl). Pseudo-first-order rate coefficients (kobs) are found under amine excess. Linear plots of [N]/kobs against 1/[N], where N is the free amine, are obtained for the reactions with piperidine, piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine. The reaction with 1-formylpiperazine exhibits a linear plot of kobs against [N]2. These results are interpreted through a mechanism consisting of two tetrahedral intermediates: a zwitterionic ( T ±) and an anionic ( T ?), where the amine catalyzed proton transfer from T ± to T ? is partially rate determining for the four former reactions and is fully rate determining for the reaction of 1-formylpiperazine. The rate microcoefficients involved in the reaction scheme are either determined experimentally or estimated. Comparison with the corresponding microcoefficients reported for the same reactions in water reveals that the rate coefficient for formation of T ± from reactants (k1) is smaller and that for the reversal of this (k?1) is larger in aqueous ethanol compared to water, in agreement with the expected structure of the corresponding transition state. Bronsted-type plots are obtained for k1, k?1, and K1 (=k1/k?1) with slopes ca. 0.4, ?0.6, and 1.0, respectively. Comparison of the present stepwise reactions with the concerted ones found in the same aminolysis of O-ethyl 2,4,6,-(trinitrophenyl) dithiocarbonate indicates that T ± is so destabilized by the change of PhS by the 2,4,6-trinitrobenzenethio group that T ± no longer exists and becomes a transition state. © 1995 John Wiley & Sons, Inc. 相似文献
190.
Stefano Passerini William H. Smyrl Mario Berrettoni Roberto Tossici Mauricio Rosolen Roberto Marassi Franco Decker 《Solid State Ionics》1996,90(1-4):5-14
The use of sol-gel processes in the preparation of cathode materials is of growing interest because of their ease and flexibility. The electrochemical properties, e.g. the rate of lithium intercalation, appear to depend on the morphology of the thin-film vanadium oxide xerogels that can be changed by modifying the preparation. In this context, in order to extend the study to bulk materials, xerogel powder samples with surface areas in the range 2–5 m2/g have been prepared from pure vanadium pentoxide hydrogels, or in the form of composites, from carbon powder added to hydrogels. The electrochemical properties have been correlated with the morphological and structural changes induced by the presence of carbon using X-ray and XAS spectroscopy. 相似文献