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991.
Frank Jäger Günter Wagner Harro A. J. Meijer Erik R. T. Kerstel 《Isotopes in environmental and health studies》2013,49(4):373-378
The potential use of non-dispersive infrared spectroscopy for measuring δ13C in air is demonstrated. This technique has already been successfully established for breath test analyses in medical diagnostics, where the CO2 concentration ranges from 1 to 5 vol.% in the exhaled breath of vertebrates. For breath tests, the sensitivity and accuracy has been improved to reach a standard deviation of 0.2 ‰ (delta-value). Further adjustments were necessary to improve the sensitivity of the instrument at concentration levels typical of atmospheric air. The long-term stability is given by a standard deviation of 0.35 ‰ for CO2 concentrations of about 400 ppm with signal averaging over 60 s. 相似文献
992.
A tandem Prins/Friedel-Crafts reaction useful for the construction of the indeno-tetrahydropyridine core of the haouamine alkaloids and a formal synthesis of (-)-haouamine A are described. 相似文献
993.
Poul Erik Hansen Alexsander S. Peregudov Dimitrii N. Kravtsov Antonina I. Krylova Galina M. Babakhina Ludmila S. Golovchenko Valentina M. Pachevskaya 《Central European Journal of Chemistry》2011,9(2):253-264
Tautomerism of benzaurins and hydration are studied. 1H and 19F chemical shifts have been determined for a number of substituted 4-hydroxyphenyl-diphenyl carbinols containing fluorine
in a 3-, 3*- or 4*-position, and for similar compounds containing additional methyl groups in a position of 3, 3** or 4**.
The same data have been obtained for the fuchsones prepared by dehydration of the above carbinols. On this basis chemical
shifts of fluorine in different positions have been evaluated as a monitor of the transformation of 4-hydroxyphenyl group
to the semiquinone moiety. The 19F NMR can be used to monitor the transformation of 4**-fluorobenzaurin and the related 3,3*-disubstituted and 3,3*,5,5*-tetramethylsubstituted
compounds to the corresponding carbinols due to the addition of a water molecule and to study the tautomerism of the two latter
benzaurins as well as that of 3,3*,4**trifluorobenzaurin. Furthermore, fluorine and methyl group chemical shifts are sensitive
to syn-anti-isomerism in substituted fuchsones. 相似文献
994.
Ortet R Regalado EL Thomas OP Pino JA Fernández MD 《Natural product communications》2011,6(8):1179-1182
The chemical composition of Tornabenea bischoffii J. A. Schmidt essential oil from Cape Verde was studied by GC and GC/MS. Sixty volatile compounds were identified of which myristicin was the major constituent (33.6%). Using the 2,2-diphenyl-2-picrylhydrazyl free-radical scavenging method and the in vitro assay for prevention of lipid peroxidation by thiobarbituric reactive species, significant antioxidant activities were evidenced. 相似文献
995.
Peshkov VA Pereshivko OP Sharma S Meganathan T Parmar VS Ermolat'ev DS Van der Eycken EV 《The Journal of organic chemistry》2011,76(14):5867-5872
A one-pot protocol based on a Ag(I)-catalyzed cycloisomerization of propargylic ureas, derived from secondary propargylamines and isocyanates, was developed for the generation of the 2-imidazolone core. 相似文献
996.
Feng H Ermolat'ev DS Song G Van der Eycken EV 《The Journal of organic chemistry》2011,76(18):7608-7613
A novel and efficient microwave-assisted decarboxylative three-component coupling of a 2-oxoacetic acid, an amine, and an alkyne (OA(2)-coulpling) has been developed. This new multicomponent coupling constitutes an efficient approach for the synthesis of polysubstituted propargylamines in the presence of a catalytic amount of copper(I) catalyst. 相似文献
997.
Modha SG Trivedi JC Mehta VP Ermolat'ev DS Van der Eycken EV 《The Journal of organic chemistry》2011,76(3):846-856
An improved and convenient methodology for the synthesis of asymmetrically substituted pyrazines starting from 3,5-dichloropyrazin-2(1H)-ones has been elaborated. Several nucleoside analogues have been synthesized containing the pyrazine core as the organic base coupled with the sugar via a triazole linkage. The beneficial effect of microwave irradiation throughout the sequence has been demonstrated. 相似文献
998.
Sigrid Volden SlungårdTor-Arne Krakeli Thor Håkon Krane Thvedt Erik FuglsethEirik Sundby Bård Helge Hoff 《Tetrahedron》2011,67(31):5642-5650
The effects of both steric and electronic properties of ketones on the selectivity in asymmetric transfer hydrogenation have been studied with aryl alkyl/fluoroalkyl ketones using four ruthenium based catalysts and two different media. The 1-arylethanones, 1-aryl-2-fluoroethanones and 2,2-difluoroacetophenones could be reduced with medium to high ee (86-99%), while the 1-aryl-2,2,2-trifluoroethanones were reduced with low selectivity in most systems. The change in enantioselectivity upon structural variation has been rationalised aided by regression analysis with substituent constants and the partial charge of the carbonyl carbon as predictors. The steric bulk of the alkyl/fluoroalkyl chain was found to be the major factor in determining selectivity in formic acid/triethylamine, while for reduction of a series of substituted 1-arylethanones and 1-aryl-2-fluoroethanones, the selectivity was found to depend on the electronic properties of the aromatic ring, supporting previous evidence that π-π interaction between the substrate and catalyst is important in determining the selectivity. For reductions in water using sodium formate as the hydrogen donor, altered and more complex selectivity mechanisms were observed. Experiments and regression focused on the variation of the alkyl/fluoroalkyl group of phenyl and 1-naphthyl ketones, showed that the selectivity correlated with the size of the substituent, but also the partial charge of the carbonyl carbon. 相似文献
999.
Comfort M. Nkambule Nomfundo W. KweziHenok H. Kinfe Mbulelo G. NokwequDavid W. Gammon Stefan Oscarson Erik Karlsson 《Tetrahedron》2011,67(3):618-623
A cis-1,2-cyclohexanediol, 1,4,5,6-tetra-O-benzyl-myo-inositol, was selectively protected at the axial C2-hydroxyl via acid-mediated rearrangement of the corresponding 1,2-orthoacetate, or via the base-induced migration of a protecting group that had previously been easily installed with complete regioselectivity at the adjacent equatorial hydroxyl. Esters 4a-6a were synthesized in high yields (75-82%) while sulfonate 7a and silyl ether 8a were obtained in 85 and 31% yields, respectively. The migration of the esters induced by DBU results in equilibrium between regioisomers favouring the C2 protected isomer, but NaH induced migration of sulfonyl and silyl groups results in complete migration from equatorial to axial hydroxyl groups. 相似文献
1000.