排序方式: 共有31条查询结果,搜索用时 15 毫秒
21.
D. Bisello G. Busetto A. Castro M. Nigro L. Pescara M. Posocco P. Sartori L. Stanco A. Antonelli M. E. Biagini R. Baldini A. Calcaterra M. Schioppa J. E. Augustin G. Cosme F. Couchot F. Fulda G. Grosdidier B. Jean-Marie V. Lepeltier G. Szklarz DM Collaboration 《Zeitschrift fur Physik C Particles and Fields》1990,48(1):23-28
Search for baryon pairs production ine + e ? annihilation at \(\sqrt s = 2386 MeV\) is reported. The data relate to a luminosity of 161 nb?1 collected by the DM2 experiment at DCI, the Orsay colliding ring. First measurements of directe + e ? annihilation into \(\Lambda \bar \Lambda \) and ofe + e ?→ \(p\bar p\) at this energy are presented. First observation of a goode + e ?→ \(n\bar n\) candidate is reported and upper limits are given fore + e ?→ \(n\bar n, \Lambda \bar \sum ^0 + c.c.\) and \(\Sigma ^0 \bar \Sigma ^0 \) . 相似文献
22.
Herein we report that simple α-chloro thioesters undergo soft enolization and direct aldol addition to aldehydes in the presence of MgBr2·OEt2 and i-Pr2NEt. At ?78 °C the reaction proceeds in a kinetically controlled manner giving good diastereoselectivity. Significantly, the transformation is applicable to both enolizable and nonenolizable aldehydes. Moreover, excellent stereoselectivity results when a chiral nonracemic α-hydroxy aldehyde derivative is used. To our knowledge, this is the first report of a kinetically controlled soft enolization-based aldol addition. 相似文献
23.
Krenske EH Houk KN Lim D Wengryniuk SE Coltart DM 《The Journal of organic chemistry》2010,75(24):8578-8584
Density functional theory calculations and experiment reveal the origin of stereoselectivity in the deprotonation-alkylation of chiral N-amino cyclic carbamate (ACC) hydrazones. When the ACC is a rigid, camphor-derived carbamate, the two conformations of the azaenolate intermediate differ in energy due to conformational effects within the oxazolidinone ring and steric interactions between the ACC and the azaenolate. An electrophile adds selectively to the less-hindered π-face of the azaenolate. Although it was earlier reported that use of ACC auxiliaries led to α-alkylated ketones with er values of 82:18 to 98:2, B3LYP calculations predict higher stereoselectivity. Direct measurement of the dr of an alkylated hydrazone prior to removal of the auxiliary confirms this prediction; the removal of the auxiliary under the reported conditions can compromise the overall stereoselectivity of the process. 相似文献
24.
This review covers the total asymmetric synthesis and biological evaluation of derivatives of the marine natural products known as the apratoxins. 相似文献
25.
Marcelo M. M. Pelisson Gil Valdo Jos Da Silva Derrick L. J. Clive Don M. Coltart Fraser A. Hof 《Journal of heterocyclic chemistry》1999,36(3):653-658
The synthesis of 2-methoxy-4-methylpyridine-3-carbonitrile (3) and its conversion, by way of alkylation of the C(4) methyl group, into the pyrodyl acetic acid ester 6 is described. 相似文献
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27.
Coltart DM Royyuru AK Williams LJ Glunz PW Sames D Kuduk SD Schwarz JB Chen XT Danishefsky SJ Live DH 《Journal of the American Chemical Society》2002,124(33):9833-9844
The structural characteristics of a mucin glycopeptide motif derived from the N-terminal fragment STTAV of the cell surface glycoprotein CD43 have been investigated by NMR. In this study, a series of molecules prepared by total synthesis were examined, consisting of the peptide itself, three glycopeptides having clustered sites of alpha-O-glycosylation on the serine and threonine side chains with the Tn, TF, and STF carbohydrate antigens, respectively, and one with the beta-O-linked TF antigen. Additionally, a glycopeptide having the sequence SSSAVAV, triglycosylated with the Le(y) epitope, was investigated. NMR data for the tri-STF-STTAV glycopeptide were used to solve the structure of this construct through restrained molecular dynamics calculations. The calculations revealed a defined conformation for the glycopeptide core rooted in the interaction of the peptide and the first N-acetylgalactosamine residue. The similarity of the NMR data for each of the alpha-O-linked glycopeptides demonstrates that this structure persists for each construct and that the mode of attachment of the first sugar and the peptide is paramount in establishing the organization of the core. The core provides a common framework on which a variety of glycans may be displayed. Remarkably, while there is a profound organizational effect on the peptide backbone with the alpha-linked glycans, attachment via a beta-linkage has little apparent consequence. 相似文献
28.
[reaction: see text] An efficient and highly stereocontrolled approach to the 8,10-dimethyl anti-syn-anti-perhydrophenanthrene carbon skeleton starting with the Wieland-Miescher ketone is described. The approach centers on a Diels-Alder-angular methylation-deoxygenation sequence. 相似文献
29.
Bernard?SonnenscheinEmail author Thomas K. DM.?Peron Francisco A.?Rodrigues Jürgen?Kurths Lutz?Schimansky-Geier 《The European Physical Journal B - Condensed Matter and Complex Systems》2014,87(8):182
We study the collective dynamics of noise-driven excitable elements, so-called active rotators. Crucially here, the natural frequencies and the individual coupling strengths are drawn from some joint probability distribution. Combining a mean-field treatment with a Gaussian approximation allows us to find examples where the infinite-dimensional system is reduced to a few ordinary differential equations. Our focus lies in the cooperative behavior in a population consisting of two parts, where one is composed of excitable elements, while the other one contains only self-oscillatory units. Surprisingly, excitable behavior in the whole system sets in only if the excitable elements have a smaller coupling strength than the self-oscillating units. In this way positive local correlations between natural frequencies and couplings shape the global behavior of mixed populations of excitable and oscillatory elements. 相似文献
30.
Acryloyl chlorides, aldehydes, and PhSLi undergo a direct aldol cascade sequence in the presence of MgBr2 x OEt2 via in situ derived thioester enolates, which is followed by oxidative elimination to give alpha-alkenyl-beta'-hydroxy thioesters. Overall, the procedure is rapid, efficient, and generally applicable, even to beta-substituted acryloyl chlorides, thus providing an alternative to the Morita-Baylis-Hillman reaction with substantially greater synthetic scope and utility. 相似文献