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排序方式: 共有4911条查询结果,搜索用时 15 毫秒
71.
Laura Gosalbo Mireia Barrot Gemma Fabriás Gemma Arsequell Francisco Camps 《Lipids》1993,28(12):1125-1130
To develop a synthesis of tritiated cyclopropene fatty acids (CPFA), compounds that should prove useful for affinity labeling
of desaturases in insect pheromone biosynthetic studies, a series of novel, selectively deuterated CPFA analogues was prepared
and characterized. In methyl [16-2H]12,13-methylene-12-hexadecenoate, the incorporation of deuterium was achieved by treatment of the corresponding ω-chloro
derivative with sodium borodeuteride in dimethylsulfoxide at 70°C for 24 h (67% yield) following conventional procedures.
Alkylation of the tetrahydropyranyl derivative of 13-tridecynol in the presence of lithium diisopropylamide in tetrahydrofuran
at −20°C with 1-chloro-3-iodopropane in hexamethylphosphoramide, followed by Jones oxidation of the crude product, yielded
16-chloro-12-hexadecynoic acid (54%), which was esterified to the corresponding methyl ester by treatment with potassium carbonate
and methyl iodide in dimethylformamide. Treatment of this acetylenic ester with ethyldiazoacetate in the presence of activated
copper-bronze as catalyst followed by hydrolysis in KOH solution at room temperature yielded 16-chloro-12,13-(carboxymethylene)-12-hexadecenoic
acid. This diacid was treated with excess oxalyl chloride to give the corresponding diacyl chloride, which was decarbonylated
in a diethyl ether solution with zinc chloride, and the cyclopropenium ions thus formed were added at −40°C to a methanolic
sodium hydroxide solution of sodium borohydride to give methyl 16-chloro-12,13-methylene-12-hexadecenoate. Analogous procedures
were followed to prepare methyl [17-2H]10,11-methylene-10-hexadecenoate, methyl [17-2H]11,12-methylene-11-hexadecenoate and methyl [17-2H]12,13-methylene-12-hexadecenoate from the corresponding diacids using sodium borodeuteride in the reduction of the cyclopropenium
ions. Alternatively, methyl [2,2,3,3-2H4]hexadecynoate, prepared by reaction of methyl 2,11-hexadecadiynoate with magnesium in deuterated methanol at room temperature,
was submitted to the above cyclopropenylation and reductive decarbonylation sequence to give methyl [2,2,3,3,17-2H5]-11,12-methylene-11-hexadecenoate. In summary, complementary methods for the selective incorporation of one to five deuterium
atoms into cyclopropene fatty acids, at different sites, in moderate to high yields have been developed. The methods should
easily be applicable to the preparation of the corresponding tritiated analogues. 相似文献
72.
Samuel J.A. Guieu Anna Maria Manotti Lanfredi Chiara Massera Laura Durn Pachn Patrick Gamez Jan Reedijk 《Catalysis Today》2004,96(4):259-264
Poly(2,6-dimethyl-1,4-phenylene ether) (PPE), which is widely used in high-performance engineering plastics, is obtained by the copper-catalyzed oxidative coupling of 2,6-dimethylphenol. The oxidative polymerizations have been carried out in acetonitrile with structurally related [copper-(N,O-containing ligand)] complexes as the catalyst precursor compounds, which appeared to be of great interest for a better understanding of the factors influencing the catalytic activities. Steric effects (influence of a methyl group close to the metal center; ligands 4–7) or electronic effects (imino versus amino group; ligands 4, 5, 8 and 6, 7, 9, respectively) on the polymerization rates have been demonstrated. The use of mono- or dinucleating ligands has strengthened the proposed mechanism of the reaction involving dinuclear active species. 相似文献
73.
Jean-Marc Tulliani Laura MontanaroAlfredo Negro Mario Collepardi 《Cement and Concrete Research》2002,32(6):843-849
A case history of a severe degradation of concrete foundation plinths and piers of an about-35-year-old building located in Northern Italy is described. Significant amounts of gypsum, near ettringite and/or thaumasite were detected by X-ray diffraction analyses performed on ground concrete samples. Large gypsum crystals were mainly located at the interface between the cement paste and aggregates, as observed by scanning electron microscopy coupled to energy-dispersive spectroscopy microanalysis. The degradation effects increased with decreasing the distance of concrete structures from an absorbing well located in the courtyard of the building. The well was recognized as the sulfate source due to the microorganism metabolism of sulfur compounds present in the sewage. Consequences of this attack were a very poor bond strength between cement paste and aggregates and a severe cracking of the concrete cover of the steel reinforcement. 相似文献
74.
The objective of this research was to evaluate the effectiveness of several different methods for controlling the pore size and pore size distribution in activated carbon fibers. Variables studied included fiber shape, activation time, and the addition of small amounts of silver nitrate. Pure isotropic pitch and the same isotropic pitch containing 1 wt.% silver were melt spun to form fibers with round and trilobal cross sections. These fibers were then stabilized, carbonized, and activated in carbon dioxide. Field emission scanning electron microscopy (FE SEM), electron dispersive spectra (EDS), and wavelength dispersive spectra (WDS) were used to monitor the size and distribution of the silver particles in the fibers before and after activation. Each of these analyses showed that the distribution of silver particles was extremely uniform before and after activation. The fibers were also weighed before and after activation to determine the percent burn-off. The BET specific surface areas of the activated fibers were determined from N2 adsorption isotherms measured at −196 °C. The results showed that round and trilobal fibers with equivalent cross-sectional areas yielded similar burn-off values and specific surface areas after activation. Also, activation rates were found to be independent of CO2 flow rate. The porosity of the activated fibers depended on the total time of activation and the cross-sectional area of fibers. The N2 adsorption measurements showed that the activated fibers had extremely high specific surface areas (greater than 3000 m2/g) and high degrees of meso- and macro-porosity. FE SEM was also used to investigate surface texture and size of pore openings on the surfaces of the activated fibers. The photos showed that silver particles generated surface macro- and mesopores, in agreement with the inferences from N2 adsorption measurements. 相似文献
75.
Phytotoxic properties of cnicin,a sesquiterpene lactone fromcentaurea maculosa (spotted knapweed) 总被引:4,自引:0,他引:4
Water and solvent extracts from the aerial tissues ofCentaurea maculosa, spotted knapweed, inhibited the root growth of lettuce. Column chromatography and lettuce bioassay of a chloroform extract led to the isolation of cnicin, a sesquiterpene lactone. Pure cnicin was bioassayed at 0, 1, 2, 4, 6, 8, and 10 mg/5 ml water with lettuce, created wheatgrass, bluebunch wheatgrass, rough fescue, western larch, lodgepole pine, and spotted knapweed. Germination was inhibited at one or more concentrations for all species except lodgepole pine and spotted knapweed. Growth, particularly of the roots, was retarded between 1 and 4 mg of cnicin. Lettuce, bluebunch wheatgrass, and spotted knapweed were inhibited significantly at all concentrations tested. 相似文献
76.
77.
Nikolaos Dimitratos Jose Antonio Lopez-Sanchez David Morgan Albert Carley Laura Prati Graham J. Hutchings 《Catalysis Today》2007,122(3-4):317-324
Solvent free oxidation of benzyl alcohol was investigated in the absence of a base using Au catalysts prepared by sol immobilization on titania and carbon supports. Comparison between the Au supported catalysts revealed that activity and distribution of products was dependent on the nature of support and heat treatment. Specifically, heat pre-treatment of the Au catalysts has a beneficial effect in terms of activity, but is detrimental in terms of selectivity to the benzaldehyde. We conclude that sol immobilization is a suitable technique for preparing gold catalysts with small particle size and narrow particle size distributions and very high activity and selectivity for benzyl alcohol oxidation. 相似文献
78.
Laura Pascual 《Electrochimica acta》2006,51(16):3193-3201
Lithium-deficient LiYMn2O4 spinels (LD-LiYMn2O4) with nominal composition (0.9 ≤ Y < 1) have been synthesized by melt impregnation from Mn2O3 and LiNO3 at temperatures ranging from 700 °C to 850 °C. X-ray diffraction data show that LD-LiYMn2O4 spinels are obtained as single phases in the range Y = 0.975-1 at 700 °C and 750 °C. Morphological characterization by transmission electron microscopy shows that the particle size of LD-LiYMn2O4 spinels increases on decreasing the Li-content. The influence of the Li-content and the synthesis temperature on the thermal and electrochemical behaviours has been systematically studied. Thermal analysis studies indicate that the temperature of the first thermal effect in the differential thermal analysis (DTA)/thermogravimetric (TG) curves, TC1, linearly increases on decreasing the Li-content. The electrochemical properties of LD-LiYMn2O4 spinels, determined by galvanostatic cycling, notably change with the synthesis conditions. So, the first discharge capacity, Qdisch., at C rate increases on rising the Li-content and the synthesis temperature. The sample Li0.975Mn2O4 synthesized at 700 °C has a Qdisch. = 123 mAh g−1 and a capacity retention of 99.77% per cycle. This LD-LiYMn2O4 sample had the best electrochemical characteristics of the series. 相似文献
79.
Estrada H Gamboa Mdel M Arias ML Chaves C 《Archivos latinoamericanos de nutrición》2005,55(2):167-171
The evaluation of the microbiological charge present in Costa Rican samples as the evaluation of its antimicrobial activity over different microorganisms, including those associated to wound infections, will allow to emit criteria referred to its use in therapeutic treatments, specially as alternative therapy for cases involving antibiotic resistant bacteria. The microbiological charge of 25 honey samples, acquired in Costa Rican markets was evaluated through several indicators including total plate aerobic count, total plate anaerobic count, total aerobic spore count, total anaerobic spore count and molds and yeast count. Also, samples were inoculated in tubes with chopped meat media and plated in egg yolk agar in order to determine the presence of Clostridium botulinum. For the antimicrobial activity evaluation, the diffusion method in Muller Hinton agar was performed, testing different honey concentrations (100, 75, 50, 25, 12,5 and 6,25 % v/v) against Staphylococcus aureus (ATCC 25923), Staphylococcus epidermidis (UCR 2902), Pseudomonas aeruginosa (ATCC 9027), Escherichia coli (ATCC25922), Salmonella enteritidis (ATCC 13076), Listeria monocytogenes (ATCC 19116) and Aspergillus niger. The results obtained for the microbiological characterization of honey show that 91% of samples had counts equal or lower than 1,0 x 10(1) CFU/g. No positive result was obtained for the isolation of C. botulinum. 24 of the samples analyzed inhibited the growth of S. aureus even in a 25% v/v concentration, nevertheless, A. niger was no inhibited by any of the samples tested. 相似文献
80.