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61.
The enantioselective addition of organozinc reagents to some 2-alkynyl benzaldehydes and the subsequent regioselective cyclization step was performed in one pot to form chiral 1,3-dihydroisobenzofurans with good product yields and excellent regio- and enantioselectivities. In the case of 2-alkynylcycloalkene aldehydes, tetrasubstituted furans were obtained in good product yields through a 1, 5-hydride shift of the preformed cyclization product. 相似文献
62.
63.
Assessment of Joule heating and its effects on electroosmotic flow and electrophoretic transport of solutes in microfluidic channels 总被引:4,自引:0,他引:4
Joule heating is inevitable when an electric field is applied across a conducting medium. It would impose limitations on the performance of electrokinetic microfluidic devices. This article presents a 3-D mathematical model for Joule heating and its effects on the EOF and electrophoretic transport of solutes in microfluidic channels. The governing equations were numerically solved using the finite-volume method. Experiments were carried out to investigate the Joule heating associated phenomena and to verify the numerical models. A rhodamine B-based thermometry technique was employed to measure the solution temperature distributions in microfluidic channels. The microparticle image velocimetry technique was used to measure the velocity profiles of EOF under the influence of Joule heating. The numerical solutions were compared with experimental results, and reasonable agreement was found. It is found that with the presence of Joule heating, the EOF velocity deviates from its normal "plug-like" profile. The numerical simulations show that Joule heating not only accelerates the sample transport but also distorts the shape of the sample band. 相似文献
64.
Yin YingZhuo Chai Hai-Feng WangPeng Li Chang-Wu ZhengGang Zhao Yue-Peng Cai 《Tetrahedron》2011,67(19):3337-3342
A highly enantioselective [4+2] cycloaddition reaction of β,γ-unsaturated α-keto esters with oxazolones was realized with readily available cinchona alkaloids as the catalysts. Using this reaction, a series of highly functionalized δ-lactones with adjacent α-quaternary-β-tertiary stereocenters were obtained in high yields (up to 97%) and with good-to-excellent enantioselectivities (up to 97% ee). 相似文献
65.
A combination of fluorobenziodoxole (FBX) and BF3 ? OEt2 in cyclopentyl methyl ether promotes regio‐ and stereoselective addition of benziodoxole and methoxy groups to alkynes. This difunctionalization reaction tolerates a variety of functionalized internal and terminal alkynes to afford trans‐β‐alkoxyvinylbenziodoxoles, which represent versatile precursors to stereochemically well‐defined multisubstituted vinyl ethers. The reaction is proposed to involve cleavage of the I?F bond of FBX by BF3, followed by electrophilic activation of the alkyne by the resulting cationic IIII species that triggers the nucleophilic addition of the ethereal oxygen. 相似文献
66.
Distribution of samarium and ytterbium in rats measured by enriched stable isotope tracer technique and INAA 总被引:2,自引:0,他引:2
Fuliang Li Yuqi Wang Zhiyong Zhang Jingxin Sun Haiqing Xiao Zhifang Chai 《Journal of Radioanalytical and Nuclear Chemistry》2002,251(3):437-441
In the present study a method using enriched stable isotope tracer and instrumental neutron activation analysis (INAA) was developed to study the dynamic distribution of rare earth elements (REEs) in a variety of organs and tissues of Wistar rats. Stable isotopes 152Sm and 168Yb were selected as tracers for the experiment. Intravenously injected 152Sm and 168Yb in chloride form could be quickly absorbed and distributed in almost all the organs and tissues of interest, including liver, skeleton, kidney, spleen, heart, lung, testicle, and blood serum. Liver and skeleton had high ability to take up 152Sm and 168Yb under the experimental conditions, whereas the contents of the elements in other organs were generally lower than 2% of the given dose during the whole experimental period. The difference in distribution of 152Sm and 168Yb in the body was also discussed. 相似文献
67.
Chai W Piskarev V Lawson AM 《Journal of the American Society for Mass Spectrometry》2002,13(6):670-679
We previously reported that sequence and partial linkage information, including chain and blood-group types, of reducing oligosaccharides can be obtained from negative-ion electrospray CID MS/MS on a quadrupole-orthogonal time-of-flight instrument with high sensitivity and without derivatization (Chai, W.; Piskarev, V.; Lawson, A. M. Anal. Chem. 2001, 73, 651-657). In contrast to oligonucleotides and peptides, oligosaccharides can form branched structures that result in a greater degree of structural complexity. In the present work we apply negative-ion electrospray CID MS/MS to core-branching pattern analysis using nine 3,6-branched and variously fucosylated oligosaccharides based on hexasaccharide backbones LNH/LNnH as examples. The important features of the method are the combined use of CID MS/MS of singly and doubly charged molecular ions of underivatized oligosaccharides to deduce the branching pattern and to assign the structural details of each of the 3- and 6-branches. These spectra give complimentary structural information. In the spectra of [M - H]-, fragment ions from the 6-linked branch are dominant and those from the 3-linked branch are absent, while fragment ions from both branches occur in the spectra of [M - 2H]2-. This allows the distinction of fragment ions derived from either the 3- or 6-branches. In addition, a unique D2beta-3 ion, arising from double D-type cleavage at the 3-linked glycosidic bond of the branched Gal core residue, provides direct evidence of the branching pattern with sequence and partial linkage information being derived from C- and A-type fragmentations, respectively. 相似文献
68.
Mulder FA Hon B Mittermaier A Dahlquist FW Kay LE 《Journal of the American Chemical Society》2002,124(7):1443-1451
Recently developed carbon transverse relaxation dispersion experiments (Skrynnikov, N. R.; et al. J. Am. Chem. Soc. 2001, 123, 4556-4566) were applied to the study of millisecond to microsecond time scale motions in a cavity mutant of T4 lysozyme (L99A) using methyl groups as probes of dynamics. Protein expressed in E. coli cells with (13)CH(3)-pyruvate as the sole carbon source contained high levels of (13)C enrichment at a total of 80 Val gamma, Leu delta, Ile gamma (2), Ala beta, and Met epsilon methyl positions with little extraneous incorporation. Data for 72 methyl groups were available for analysis. Dispersion profiles with large amplitudes were measured for many of these residues and were well fit to a two-state exchange model. The interconversion rates and populations of the states, obtained from fitting relaxation dispersion profiles of each individual probe, were remarkably homogeneous and data for nearly all methyl groups in the protein could be collectively fit to a single cooperative conformational transition. The present study demonstrates the general applicability of methyl relaxation dispersion measurements for the investigation of millisecond time scale protein motions at a large number of side-chain positions. Potential artifacts associated with the experiments are described and methods to minimize their effects presented. These experiments should be particularly well suited for probing dynamics in high molecular weight systems due to the favorable NMR spectroscopic properties of methyl groups. 相似文献
69.
Phoebe Y. T. Hon Man-ying Wan Della W. M. Sin Yiu-chung Wong 《Analytical and bioanalytical chemistry》2013,405(23):7241-7250
This paper describes a metrological approach to evaluate the measurement capability of laboratories participating in two proficiency testing (PT) programmes involving the analysis of five polycyclic aromatic hydrocarbons (PAHs) in sediment samples. Reference values of PAHs in the programmes for performance assessment were obtained from an accurate isotope dilution gas chromatography mass spectrometry (ID-GCMS) method which was thoroughly validated and verified. Isotope dilution mass spectrometry (IDMS) technique usually has a well-defined measurement uncertainty budget and a traceability link to an International System of Units. Provision of the metrological reference values in PT enables the establishment of a technical platform to assess the actual competence of the participating laboratories in sediment PAHs analysis. Results of the PT programmes showed that about 80 % of the laboratories employed gas chromatography in their analyses and the remaining used liquid chromatography. Irrespective of the techniques being used, however, the majority of the participating laboratories were observed to underestimate values in which the mean values of the five reported PAHs were less than those of the ID-GCMS-derived reference values by 13–20 %. Only 41–44 % of the participating laboratories were able to achieve satisfactory z-scores. The present study revealed that the reinforcement of the capability for accurate measurement of PAHs in sediment samples in laboratories worldwide should be addressed. 相似文献
70.
Preparation of trimethylchlorosilane-modified acid vermiculites for removing diethyl phthalate from water 总被引:2,自引:0,他引:2
A hybrid organic-inorganic material based on vermiculite was prepared to remove diethyl phthalate (DEP) from aqueous solution. Natural vermiculite was activated with HCl to improve the specific surface area and was then modified by silanization using trimethylchlorosilane. Organovermiculite prepared by ion exchange with hexadecyl trimethylammonium bromide (HDTMAB) was also tested for comparison. The leaching of 2 mol L(-1) HCl at 80°C increased the specific surface area of vermiculite from 14.4 to 500.0m(2)g(-1), and the average pore-diameter was decreased from 7.90 nm to 2.75 nm. Fourier transform infrared spectroscopy (FTIR) spectra indicated that trimethysilyl groups were grafted covalently on the surface of acid vermiculites. The specific surface area of trimethylchlorosilane-modified acid vermiculites (TMAVs) (355.4 m(2) g(-1)) was much larger than that of organovermiculite (6.0 m(2) g(-1)). The isotherm adsorption experiments of DEP showed that TMAVs exhibited linear isotherms, suggesting that the uptake of DEP was controlled by partitioning mechanism. The maximal partition coefficient (K(d)) of TMAVs was 3.1 times higher than that of organovermiculite, implying that TMAVs had stronger organic affinity than organovermiculite. The results demonstrate that the adsorption capacity and mechanism of organoclays were controlled by the specific surface area and organic loading, whereas the length of alkyl chain of organic modifier was not the key factor. 相似文献