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971.
Braunstein P Siri O Taquet JP Rohmer MM Bénard M Welter R 《Journal of the American Chemical Society》2003,125(40):12246-12256
A straightforward synthesis of the zwitterionic benzoquinonemonoimine 8 is reported. This molecule is a rare example of a zwitterion being more stable than its canonical forms. It is shown that 8 is best described as constituted of two chemically connected but electronically not conjugated 6 pi electron subunits. Its reactivity with electrophiles such as H(+), CH(3)(+), and metal salts leads to the synthesis of new 12 pi electron molecules 12 (H(+)), 14 (CH(3)(+)), and 20 (Pd(2+)), respectively, in which one or both 6 pi electron subsystems localize into an alternation of single and double bonds, as established by X-ray diffraction. The acidity of the N[bond]H protons of 8 can be modulated by an external reagent. Dependent on the electrophile used, the control of the pi system delocalization becomes possible. When the electrophile simply adds to the zwitterion as in 12, 14, or 15, there is no more negative charge to be delocalized and only the positive charge remains delocalized between the nitrogen atoms. Furthermore, when a reaction with the electrophilic reagent results in deprotonation, as in 17-21, there remains no charge in the system to be delocalized. DFT calculations were performed on models of 8, 12, 14, 20, and on other related zwitterions 9 and 10 in order to examine the influence of the fused cycles on the charge separation and on the singlet-triplet energy gap. An effect of the nitrogen substituents in 8 is to significantly stabilize the singlet state. The dipole moment of 8 was measured to be 9.7 D in dichloromethane, in agreement with calculated values. The new ligands and complexes described in this article constitute new classes of compounds relevant to many areas of chemistry. 相似文献
972.
Manjira Datta Leonard J. Mirman Olivier F. Morand Kevin L. Reffett 《Annals of Operations Research》2002,114(1-4):117-144
In this paper, we provide an overview of an emerging class of monotone map methods in analyzing distorted equilibrium in dynamic economies. In particular, we focus on proving the existence and characterization of competitive equilibrium in non-optimal versions of the optimal growth models. We suggest two alternative methods: an Euler equation method for a smooth, strongly concave environment, and a value function method for a non-smooth supermodular environment. We are able to extend this analysis to study models that allow for unbounded growth or a labor–leisure choice. 相似文献
973.
We use the algorithmic computation of exact sequences of Abelian groups to compute the complete structure of for an ideal of a number field , as well as ray class groups of number fields, and conductors and discriminants of the corresponding Abelian extensions. As an application we give several number fields with discriminants less than previously known ones.
974.
Appelqvist LA Addis P Björkhem I Bosset JO Caboni MF Dutta P Grandgirard A Guardiola F Hau LB Nielsen JH Hugget A Hwang LS Kumpulainen J McCluskey S Ohshima T Przbylski R Sevanian A Yan P 《Journal of AOAC International》2004,87(2):511-519
A compilation of literature data on the content of cholesterol oxidation products (COP) in various food products and in blood demonstrates a large variation in content in products or tissues of very similar nature when analyzed in different laboratories according to a large number of methods. The lack of validated, internationally recognized methodology with published accuracy and precision has so far hindered such assessments. Hence an interlaboratory comparision of methodologies of COP analysis was undertaken on egg yolk powders (EYP), whole milk powders (WMP), skim milk powders (SMP), and lard (L). Each product type had one fresh sample (low) and one aged (high) in COP contents. A total of 17 sets of results on WMP, 15 on SMP and EYP, and 13 on L were compared. Overall results (mg/kg sample) varied extensively: Fresh EYP 0.72-265, aged EYP 2.51-361; fresh WMP 0.02-18.1, aged WMP 0.02-26.9; fresh SMP 0.02-6.51, aged SMP <0.01-6.51; fresh L 0.18-97, aged L 4.15-452. Some results were questioned, viz., those from laboratories not indicating substantial differences between samples "low" and "high" in total COP. Others were excluded because of lack of verification of identity of gas chromatographic peaks by mass spectrometry. Then a more narrow range of core results (mg/kg sample) was observed: Fresh EYP 5.69-29.5 sample, aged EYP 11.8-79.0; fresh WMP 0.12-1.76, aged WMP 1.17-13.7; fresh SMP <0.30-<1.21, aged SMP 0.30-2.26; fresh L 0.18-5.07, aged L 94.4-231. At a workshop discussing the results, numerous recommendations were made toward more reliable methodology for determination of COP in foods. 相似文献
975.
Olivier Mentre Anne-Claire Dhaussy Francis Abraham Hugo Steinfink 《Journal of solid state chemistry》1997,130(2):223-233
Single-phase material of the solid solution PbFexV6−xO11was prepared by solid-state reaction over the range 1≤x≤1.75. Powder preparations of PbV6O11, however, were always accompanied by a small amount of impurity. The noncentrosymmetric space groupP63mcof PbV6O11, which has anR-block structure, remains unchanged with iron substitution. The crystal structure of PbFe1.75V4.25O11was refined from single-crystal X-ray data toR=0.049. The iron atoms are located preferentially in specific crystallographic sites. The triangular–bipyramidal site is completely occupied by Fe3+while the interstices of the central octahedral layer are solely occupied by the vanadium V(1) atoms. The other two crystallographic sites have mixed Fe/V occupancies. Magnetic measurements performed on nearly single-phase PbV6O11and the iron substituted phases reveal a spin-glass-type behavior with freezing temperaturesTfof 50 and 65 K, respectively.Tfis independent of the iron ratio. This spin-disordered system is attributed to strong frustration occuring in thea–bplane because of the existence of V(1) trimers providing alternatively long and short V–V distances along theaaxis. Resistance and Seebeck coefficient measurements indicatep-type conduction aboveTfwhile the system becomes an insulator belowTf. Fe3+ions do not participate in the hopping process, and they block conduction parallel to thecaxis because of their location in the bridging dimeric octahedral and bipyramidal sites. 相似文献
976.
The development of enhanced conditions for Lewis acid catalysed Leimgruber-Batcho indole synthesis using microwave acceleration is described. This approach has permitted the preparation of a variety of heteroaromatic enamine intermediates in good yield and high purities. Subsequent catalytic hydrogenation reactions, under various conditions including the use of a solid-phase encapsulated catalyst, furnish the corresponding indole derivatives in good yields. 相似文献
977.
Alain Gueiffier Henri Viols Yves Blache Jean Pierre Chapat Olivier Chavignon Jean Claude Teulade Florence Fauvelle Grard Grassy Grard Dauphin 《Journal of heterocyclic chemistry》1997,34(3):765-771
The synthesis and reactivity of imidazo[1,2-a][1,8]naphthyridines are reported. Electrophilic substitution reactions were studied and the site of the reaction was established with the aid of high-field 1H and 13C nmr spectra. The experimental C-1 position reaction was correlated with a CNDO/2 calculations. 相似文献
978.
A number of oligomeric alkynes underwent [2 + 2 + 2] intramolecular trimerization to afford arenes under metal-free conditions using focussed microwave heating. 相似文献
979.
[reaction: see text] Stereo-controlled syntheses of two possible C1-C25 diastereomers of spirastrellolide A containing the cis-disubstituted tetrahydropyran and [6,6]-spiroacetal are reported, exploiting boron-mediated asymmetric aldol and allylation methodology. 相似文献
980.
Merkwirth C Mauser H Schulz-Gasch T Roche O Stahl M Lengauer T 《Journal of chemical information and computer sciences》2004,44(6):1971-1978
We describe the application of ensemble methods to binary classification problems on two pharmaceutical compound data sets. Several variants of single and ensembles models of k-nearest neighbors classifiers, support vector machines (SVMs), and single ridge regression models are compared. All methods exhibit robust classification even when more features are given than observations. On two data sets dealing with specific properties of drug-like substances (cytochrome P450 inhibition and "Frequent Hitters", i.e., unspecific protein inhibition), we achieve classification rates above 90%. We are able to reduce the cross-validated misclassification rate for the Frequent Hitters problem by a factor of 2 compared to previous results obtained for the same data set with different modeling techniques. 相似文献