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151.
152.
The new solvent-free technique called headspace sorptive extraction (HSSE) was used to determine 2,4,6-trichlorophenol, 2,3,4,6-tetrachlorophenol, 2,4,6-trichloroanisole, 2,3,4,6-tetrachloroanisole, 2,4,6-tribromoanisole, pentachloroanisole in cork stoppers, without grinding them, as these may be responsible for the cork taint off-flavour in wine. The best HSSE sorption kinetics for the target analytes were obtained after submitting the spiked corks to 100 degrees C for 1h, followed by a 30 min stabilization time at room temperature. The stir bar was desorbed in a thermal desorption system coupled to a gas chromatograph-mass spectrometer. The method proposed showed good linearity over the concentration range tested, 1-70 ngg(-1), and correlation coefficients ranged from 0.90 to 0.99 for all the analytes. The reproducibility and repeatability of the method were estimated between 4.91 and 12.67%. The effect of the different cork matrixes on the extraction recovery of the target compounds was studied, with the natural corks showing the higher recovery percentage in relation to agglomerate ones. 相似文献
153.
Blay G Fernandez I Marco-Aleixandre A Pedro JR 《The Journal of organic chemistry》2006,71(17):6674-6677
The (S,S)-mandelamide III catalyzes the additions of both aryl- and alkylalkynylzinc reagents to heteroaromatic aldehydes with good yields and enantioselectivities up to 92%. This catalyst is easily prepared in a one-step procedure, and both enantiomers are available. Unlike most other described methods, using this catalyst does not require the addition of Ti(O(i)Pr)4. 相似文献
154.
Jiménez-Osés G Corzana F Busto JH Pérez-Fernandez M Peregrina JM Avenoza A 《The Journal of organic chemistry》2006,71(5):1869-1878
An extensive conformational study of different 2-substituted cyclobutane-alpha-amino acid derivatives in the solid state, in the gas phase, and in solution has been carried out. The study combines experimental techniques, such as X-ray diffraction and NMR spectroscopy, and computational methods, such as DFT calculations and molecular dynamics (MD) simulations, in a set solvent. The study reveals that the substituent at C2 in the cyclobutane ring, when fixed in an equatorial position, modulates the conformational preference of the ring-puckering. 相似文献
155.
Avenoza A Busto JH Jiménez-Osés G Peregrina JM 《The Journal of organic chemistry》2006,71(4):1692-1695
The work described here demonstrates that the five-membered cyclic alpha-methylisoserine-derived sulfamidate, (R)-1, behaves as an excellent chiral building block for the ring-opening reaction by S(N)2 attack with sulfur nucleophiles at the quaternary carbon. As a synthetic application of this methodology, and to show that this sulfamidate is a valuable starting material, the synthesis of two new alpha-methylisocysteine derivatives has been carried out to cover the lack of alpha- and beta-methylated amino acids that incorporate the cysteine or isocysteine skeleton. These compounds are two new alpha,alpha-disubstituted beta-amino acids (beta(2,2)-amino acids), and the synthetic routes involve nucleophilic ring opening followed by acid hydrolysis. 相似文献
156.
[reaction: see text] A strategy based on the control of the electron-donating capabilities of the coordinating groups of the ligand has been applied in the catalytic asymmetric addition of organometallic reagents to ketoesters. Mandelamides having deprotonated alcohol and carboxyamido groups catalyzed the addition of dimethylzinc to alpha-ketoesters with good yields and ee (up to 90%). 相似文献
157.
The Maximal Graded Left Quotient Algebra of a Graded Algebra1) 总被引:1,自引:0,他引:1
Gonzalo ARANDA PINO Mercedes SILES MOLINA 《数学学报(英文版)》2006,22(1):261-270
We construct the maximal graded left quotient algebra of every graded algebra A without homogeneous total right zero divisors as the direct limit of graded homomorphisms (of left A-modules) from graded dense left ideals of A into a graded left quotient algebra of A. In the case of a superalgebra, and with some extra hypothesis, we prove that the component in the neutral element of the group of the maximal graded left quotient algebra coincides with the maximal left quotient algebra of the component in the neutral element of the group of the superalgebra. 相似文献
158.
Using a derivation of black hole radiance in terms of two-point functions one can provide a quantitative estimate of the contribution of short distances to the spectrum. Thermality is preserved for black holes with kappalp<1. However, deviations from the Planckian spectrum can be found for mini black holes in TeV gravity scenarios, even before reaching the Planck phase. 相似文献
159.
This paper introduces a new Petri Net based approach for resource allocation and scheduling. The goals are (i) minimize the
number of required resources given a set of jobs, (ii) find both an assignment for all jobs in the span of a predefined shift
and (iii) the sequence in which such jobs are executed. The studied problem was inspired from a complex real life manufacturing
shop as described in this document. The modeling of the processes and jobs is carried out with Petri Nets due to their capability
of representing dynamic, concurrent discrete-event dynamic systems. The resource assignment starts with an initial feasible
solution (initial number of resources) and then follows with a re-optimization process aimed to further reduce the resource
requirements. The algorithm is based on a modified Heuristic Search method previously presented. The algorithm was tested
first on a number of instances from the literature and then on the aforementioned system (a car seat cover manufacturer).
The proposed approach shows not only good results in terms of performance but also shows the potential of Petri Nets for modeling
and optimizing real-life systems. An implementation phase at the first stages of the process is underway at the time of writing. 相似文献
160.
Gonzalo Alduncin 《Nonlinear Analysis: Theory, Methods & Applications》2010,72(5):2639-2644
Variational formulations of nonlinear constrained boundary value problems in reflexive Banach spaces are discussed from a compositional duality approach. The mixed variational compatibility conditions of the theory correspond to the surjectivity of the primal coupling boundary and interior operators. 相似文献