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Facile Access to cis‐2,6‐Disubstituted Tetrahydropyrans by Palladium‐Catalyzed Decarboxylative Allylation: Total Syntheses of (±)‐Centrolobine and (+)‐Decytospolides A and B 下载免费PDF全文
Jing Zeng Yu Jia Tan Dr. Jimei Ma Min Li Leow Davin Tirtorahardjo Prof. Dr. Xue‐Wei Liu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(2):405-409
cis‐2,6‐Tetrahydropyran is an important structural skeleton of bioactive natural products. A facile synthesis of cis‐2,6‐disubstituted‐3,6‐dihydropyrans as cis‐2,6‐tetrahydropyran precursors has been achieved in high regio‐ and stereoselectivity with high yields. This reaction involves a palladium‐catalyzed decarboxylative allylation of various 3,4‐dihydro‐2H‐pyran substrates. Extending this reaction to 1,2‐unsaturated carbohydrates allowed the achievement of challenging β‐C‐glycosylation. Based on this methodology, the total syntheses of (±)‐centrolobine and (+)‐decytospolides A and B were achieved in concise steps and overall high yields. 相似文献
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A Sedimentological Approach to Upscaling 总被引:1,自引:0,他引:1
Optimised upscaling in reservoir simulations requires the construction of realistic petrophysical properties that are representative of the heterogeneity in the sedimentary deposits. Reservoir heterogeneities are controlled by the arrangement of various hierarchies of sedimentary facies and their internal bounding surfaces. The conventional sedimentological approach to reservoir upscaling involves subdivision and ranking of various hierarchies of architectural units and associated bounding surfaces of the reservoir sequence according to their geological significance. This global upscaling approach produces realistic scaled up models that retain both the structural and non-structural heterogeneities of the original sedimentological models. Analyses of sedimentary sequences from various depositional environments indicate that the fractional Levy model can adequately describe the heterogeneity and scaling characteristics of individual genetic sediment sequences in the clastic sedimentary system without further subdividing and ranking of the heterogeneous sequences. The heterogeneous nature of each sedimentary system can be quantified by the Levy index parameter, whereas the maximum upscaling magnitude (or upscaling index) for a particular sequence can be determined from the Levy width parameter plot. Depositional modelling mimics the sedimentary processes in a range of scales and honours hierarchies of sedimentary facies and their bounding surfaces. It can be used effectively for upgridding and upscaling in accordance with the stratigraphic framework and sedimentological models. Both the fractional Levy model and the depositional modelling provide quantitative alternatives to the conventional global sedimentological upscaling approach. 相似文献
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Cheong KW Leow TC Rahman RN Basri M Rahman MB Salleh AB;Enzyme Microbial Technology Research Group 《Applied biochemistry and biotechnology》2011,164(3):362-375
A thermostable lipase from Geobacillus zalihae strain T1 was chemically modified using propionaldehyde via reductive alkylation. The targeted alkylation sites were lysines,
in which T1 lipase possessed 11 residues. Far-UV circular dichroism (CD) spectra of both native and alkylated enzyme showed
a similar broad minimum between 208 and 222 nm, thus suggesting a substantial amount of secondary structures in modified enzyme,
as compared with the corresponding native enzyme. The hydrolytic activity of the modified enzymes dropped drastically by nearly
15-fold upon chemical modification, despite both the native and modified form showed distinctive α-helical bands at 208 and
222 nm in CD spectra, leading us to the hypothesis of formation of a molten globule (MG)-like structure. As cooperative unfolding
transitions were observed, the modified lipase was distinguished from the native state, in which the former possessed a denaturation
temperature (T
m) in lower temperature range at 61 °C while the latter at 68 °C. This was further supported by 8-anilino-1-naphthalenesulfonic
acid (ANS) probed fluorescence which indicated higher exposure of hydrophobic residues, consequential of chemical modification.
Based on matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) analysis, a small number
of lysine residues were confirmed to be alkylated. 相似文献
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The absorption and fluorescence spectroscopic responses of three pyridinium hemicyanine dyes to anions F−, Cl−, Br−, I−, H2PO4−, HSO4− and OAc− were investigated. At lower concentrations of OAc− (less than 1 equiv.), both the absorption and the fluorescence intensities of 1 – 3 were more effectively changed than F− at identical concentrations. At higher concentrations of OAc− (more than 1 equiv.), the interaction was opposite for each compound. 1H NMR results indicated the interaction between 1 , 2 or 3 and F− proceeded through hydrogen bonding. The results showed that these dyes are promising to develop dual fluorescence and chromogenic chemosensors toward F− and OAc− according to the subtle difference in the affinity of F− and OAc−. 相似文献
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用原子吸收光谱法测定了70例健康老年人、30例健康成年人和30例老年高血压病人血清中Cu、Zn、Fe、Mn、Ca、Mg6种元素的含量.结果表明,健康老年人组与健康成年人组(对照组)相比,除Fe以外,Cu、Zn、Mn、Ca、Mg均有显著性差异;与老年高血压病人组相比,除Mg以外,Cu、Zn、Fe、Mn、Ca均有显著性差异.我们认为稳定的Cu/Zn比值及较高的血Mg水平是健康老年人未发生心血管疾病及糖尿病的重要原因之一,而高Cu、高Ca及显著偏高的Cu/Zn比值,则是发生高血压的一个重要原因。 相似文献
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Ning Ling Jiguang Zhang Meng Wang Zhen Wang Ziyu Mi Surani Bin Dolmanan Mingsheng Zhang Bingqing Wang Wan Ru Leow Jia Zhang Yanwei Lum 《Angewandte Chemie (International ed. in English)》2023,62(36):e202308782
Electrochemical CO2 reduction (CO2R) in acidic media with Cu-based catalysts tends to suffer from lowered selectivity towards multicarbon products. This could in principle be mitigated using tandem catalysis, whereby the *CO coverage on Cu is increased by introducing a CO generating catalyst (e.g. Ag) in close proximity. Although this has seen significant success in neutral/alkaline media, here we report that such a strategy becomes impeded in acidic electrolyte. This was investigated through the co-reduction of 13CO2/12CO mixtures using a series of Cu and CuAg catalysts. These experiments provide strong evidence for the occurrence of tandem catalysis in neutral media and its curtailment under acidic conditions. Density functional theory simulations suggest that the presence of H3O+ weakens the *CO binding energy of Cu, preventing effective utilization of tandem-supplied CO. Our findings also provide other unanticipated insights into the tandem catalysis reaction pathway and important design considerations for effective CO2R in acidic media. 相似文献
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Ultra-performance liquid chromatography/time-of-flight mass spectrometry based metabolomics of raw and steamed Panax notoginseng 总被引:4,自引:0,他引:4
Chan EC Yap SL Lau AJ Leow PC Toh DF Koh HL 《Rapid communications in mass spectrometry : RCM》2007,21(4):519-528
At present, metabolite profiling is of growing importance in herbal medicine fields such as breeding, formulation, quality control and clinical trials. This preliminary study indicated that ultra-performance liquid chromatography/time-of-flight mass spectrometry (UPLC/TOFMS)-based metabolomics allows direct detection of down-stream derivatives of metabolites, arising from the herbal formulation process. This analytical approach allows the discrimination and tentative authentication of unique biomarkers related to different herbal extracts using unsupervised multivariate principal component analysis (PCA). The tentative identification of biomarkers is complemented significantly by the accurate mass measurement of TOFMS and the high resolution and high retention time reproducibility rendered by UPLC. The application of this approach in herbal extract discrimination and ginsenoside biomarker discovery of raw and steamed Panax notoginseng (Burk.) F.H. Chen is demonstrated and discussed. 相似文献