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991.
Justicia J Rosales A Buñuel E Oller-López JL Valdivia M Haïdour A Oltra JE Barrero AF Cárdenas DJ Cuerva JM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(7):1778-1788
The titanocene-catalyzed cascade cyclization of epoxypolyenes, which are easily prepared from commercially available polyprenoids, has proven to be a useful procedure for the synthesis of C(10), C(15), C(20), and C(30) terpenoids, including monocyclic, bicyclic, and tricyclic natural products. Both theoretical and experimental evidence suggests that this cyclization takes place in a nonconcerted fashion via discrete carbon-centered radicals. Nevertheless, the termination step of the process seems to be subjected to a kind of water-dependent control, which is unusual in free-radical chemistry. The catalytic cycle is based on the use of the novel combination Me(3)SiCl/2,4,6-collidine to regenerate the titanocene catalyst. In practice this procedure has several advantages: it takes place at room temperature under mild conditions compatible with different functional groups, uses inexpensive reagents, and its end step can easily be controlled to give exocyclic double bonds by simply excluding water from the medium. 相似文献
992.
R. Mathur-De-Vré 《Accreditation and quality assurance》1997,2(2):63-68
There is growing interest in developing a general strategy and quality standards for possible accreditation or certification
of R&D laboratories. This article discusses the scope and limitations of Quality Systems in R&D activities. The extension
of QA to R&D centres in general requires emphasis on project management and scientific competence in addition to quality management
and technical competence.
Received: 11 September 1996 Accepted: 13 November 1996 相似文献
993.
P. Espeau B. Nicolaï R. Céolin M. -A. Perrin L. Zaske J. Giovannini F. Leveiller 《Journal of Thermal Analysis and Calorimetry》2007,90(2):341-342
Investigation into the thermal behavior of orthorhombic Forms I and II of spironolactone, by means of differential scanning
calorimetry and high-resolution X-ray powder diffraction, showed that Form I melts then recrystallizes into Form II at 373–393
K, i.e. in the temperature range within which high resolution X-ray powder diffration showed that Form I transforms into Form II.
Refinements of the lattice parameters of the two forms indicated that Form I is denser than Form II in the range from 298
K up to the temperature at which it melts. 相似文献
994.
Barluenga J Diéguez A Rodríguez F Flórez J Fañanás FJ 《Journal of the American Chemical Society》2002,124(31):9056-9057
Eight-membered carbocycles with up to five new stereogenic centers are enantioselectively obtained following a one-pot procedure that involves the coupling of three components: an alkenyl Fischer carbene complex, a ketone enolate, and allyl lithium. 相似文献
995.
The Through Oven Transfer Adsorption Desorption (TOTAD) interface is used to directly introduce large volumes of water (1 mL or more) into a capillary gas chromatograph. The TOTAD interface is a greatly modified programmed temperature vaporizer injector incorporating changes that affect the pneumatics, sample introduction, solvent elimination, and operation mode. The system can easily be automated. The technique is applied to the analysis of pesticide residue in standard solutions and real water samples from the Ebro River (northeastern Spain). The speed of sample introduction was 1 mL/min, and the solvent elimination was almost complete. A nitrogen phosphorous detector is used, and the relative standard deviation varied from 5.7% to 11.7% for the absolute peak areas. The sensitivity achieved by introducing 1 mL of the sample is sufficient for most pesticide-residue analyses in water. The limits of detection ranged from 0.5 to 8.1 ng/L. 相似文献
996.
C. André L. Ismaili M. Thomassin T. T. Truong B. Refouvelet Y. C. Guillaume 《Chromatographia》2003,58(3-4):165-170
Summary Lithium chloride (LiCl) effect on the retention process of a phenol derivative series was investigated on two types of fluorinated stationary phase (i.e. a silica grafted with fluorinated linear alkyl chain (L-FSP) and a silica grafted with fluorinated aromatic ring stationary phase (A-FSP)). The results showed that the solute retention is enhanced when the A-FSP was used instead of the L-FSP due to additional – interactions. For the two fluorinated stationary phases (FSPs), the phenol-FSP association process can be divided into two LiCl concentration domains demonstrated that it was important to take into account the adsorbtion of Cl– anion on the FSPs. As well, enthalpy-entropy compensation revealed that the solute retention mechanism was independent of the solute molecular structure and confirmed a change on the solute retention mechanism at a critical LiCl concentration value around 0.02M. 相似文献
997.
Assignment of Saharan dust sources to episodes in Hungarian atmosphere by PIXE and TOMS observations
E. Koltay I. Borbély-Kiss Zs. Kertész Á. Z. Kiss Gy. Szabó 《Journal of Radioanalytical and Nuclear Chemistry》2006,267(2):449-459
Summary Thirty-seven episodes of dust intrusion were observed between February 12, 1991 and August 31, 2000 in the Hungarian atmosphere
and found to be of Saharan origin. They have been assigned to typical source areas in Northern Africa selected by systematic
investigations related to the Mediterranean Basin. Elemental concentrations and regional signatures deduced from PIXE analysis,
total suspended particle mass, and black carbon mass have been measured on dust samples collected in Debrecen, Hungary. These
data combined with Aerosol Index data, and dust plume axes obtained from TOMS data from NASA satellites were used for assigning
the most probable source areas of the dust transported to the sampling site. Our data and conclusions are in accordance with
other published investigations on the predominant role of Saharan dust emission in building up the aerosol load of the global
atmosphere. 相似文献
998.
Fluorocarbons containing higher halogens have the potency of breaking certain hydrogen bonds in solutions. The relative strength of this potency varies in the series F < Cl < Br < I and the presence of hydrogen atoms in the fluorocarbon increases it. There is a striking parallelism between the hydrogen bond breaking and the anaesthetic potency of these molecules. It is suggested that the breaking of hydrogen bonds is an important step in the mechanism of anaesthesia. 相似文献
999.
The degradation behaviour of methylmercury (MeHg) under microwave irradiation is investigated, as is the (different) degradation
behaviour of ethylmercury (EtHg) under similar irradiation. A simple and highly sensitive SPME-GC-pyrolysis-AFS system was
used to analyse the aqueous MeHg and EtHg standard solutions after derivatization with sodium tetraphenylborate (NaBPh4). Samples were irradiated in a microwave digester at microwave powers ranging from 20 to 160 W for durations of 2 to 10 min.
The different tolerances towards microwave treatment of the two organomercury species were evident. Practically no degradation
was experienced for MeHg for up to 8 minutes of irradiation at 120 W or for up to 4 minutes at 160 W. Significant analyte
loss was observed for EtHg after 2 minutes at 40 W of microwave power.
Awarded a Poster Prize on the occasion of the European Winter Conference on Plasma Spectrochemistry, February 2005, Budapest,
Hungary 相似文献
1000.
Jakubowicz K Wong YS Chiaroni A Bénéchie M Marazano C 《The Journal of organic chemistry》2005,70(19):7780-7783
[reaction: see text] 3-Alkyl-1,4-dihydropyridines dimerize in acidic medium, at low temperature, to give polycyclic imminium salts derivatives that were reduced to afford new polycyclic diamine scaffolds. The reaction can be extended to enantiopure series starting from R-(+)- or S-(-)-1-phenylethylamine. Long exposure of the polycyclic imminium salt intermediates to air moisture at 20 degrees C resulted in formation of new amide derivatives. This is probably due to the addition of water followed by an intramolecular oxido-reduction process. 相似文献