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81.
Aoyagi Y Saitoh Y Ueno T Horiguchi M Takeya K Williams RM 《The Journal of organic chemistry》2003,68(18):6899-6904
Lipase TL-mediated kinetic resolution of (+/-)-5-benzyloxy-1-tert-butyldimethylsilyloxy-2-pentanol (5) at low temperature proceeded to give the corresponding (S)-alcohol 5 and (R)-acetate 6 in quantitative yields with high enantiomeric purity. The addition of bases such as pyridine, DMAP, 2,4- and 2,6-lutidines, or triethylamine considerably enhanced the rate of kinetic resolution. The alcohol (S)-5 and the acetate (R)-6 were converted to piperazic acid derivatives (R)- and (S)-3, respectively, via the intramolecular Mitsunobu reaction as a key step. 相似文献
82.
Filippi A Roselli G Renzi G Grandinetti F Speranza M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(9):2072-2078
The kinetics and the stereochemistry of the protonation-induced unimolecular isomerization of (S)-(+)-1-D(1)-3-(p-tolyl)butane have been investigated in the gas phase in the 100-160 degrees C range. The process leads to the almost exclusive formation of the relevant meta isomer with complete racemization and partial 1,2-H shift in the migrating sec-butyl group. These results, together with the relevant activation parameters, point to the occurrence of low-energy, tightly bound isomeric sec-butyl cation/toluene complexes of defined structure and stability along the isomerization coordinate. The existence and the eta(1)-type structure of these low-energy intermediate species are confirmed by ab initio calculations on closely related systems at the MP2(full)/6-311++G**//HF/6-31+G** level of theory. Their role in the relevant energy surface clearly emerges from the comparison of the present results with those concerning sec-butylation of toluene carried out under comparable experimental conditions. 相似文献
83.
A family of enantiomerically pure oxonium ions, that is O-protonated 1-aryl-1-methoxyethanes, has been generated in the gas phase by the (CH(3))(2)Cl(+) methylation of the corresponding 1-arylethanols. Some information on their reaction dynamics was obtained from a detailed kinetic study of their inversion of configuration and dissociation. The activation parameters of the inversion reaction are found to obey two different isokinetic relationships depending upon the nature and the position of the substituents in the oxonium ions. In contrast, the activation parameters of the dissociation reaction obey a single isokinetic relationship. The inversion and dissociation rate constants do not follow simple linear free-energy relationships. This complicated kinetic picture has been rationalized in terms of different activation dynamics in gaseous CH(3)Cl, which, in turn, determine the reaction dynamics of the oxonium ion. When the predominant activation of the oxonium ion involves resonant energy exchange from the 1015 cm(-1) CH(3) rocking mode of unperturbed CH(3)Cl, the inversion reaction proceeds through the dynamically most favored TS, characterized by the unassisted C(alpha)bond;O bond elongation. When, instead, the activation of the oxonium ions requires the formation of an intimate encounter complex with CH(3)Cl, the inversion reaction takes place via the energetically most favored TS, characterized by multiple coordination of the CH(3)OH moiety with the H(alpha) and H(ortho) atoms of the benzylic residue. The activation dynamics operating in the intimate encounter complex with CH(3)Cl is also responsible for the dissociation of most selected oxonium ions. 相似文献
84.
85.
Synthesis of a new insoluble polymer-supported chiral alcohol auxiliary and its first application to nucleophilic addition to ketenes 总被引:2,自引:0,他引:2
Akkari R Calmes M Mai N Rolland M Martinez J 《The Journal of organic chemistry》2001,66(17):5859-5865
The preparation of a new optically active alcohol with a carboxylic function that allowed its attachment to an amine-functionalized insoluble polymer is described. Its first use as a polymer supported chiral auxiliary is demonstrated by asymmetric transformation of two racemic aryl propionic acids via ketene formation (95-96% ee). 相似文献
86.
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88.
B. Ittermann M. Füllgrabe M. Heemeier F. Kroll F. Mai K. Marbach P. Meier D. Peters G. Welker W. Geithner S. Kappertz S. Wilbert R. Neugart P. Lievens U. Georg M. Keim 《Hyperfine Interactions》2000,129(1-4):423-441
β-active probe nuclei are implanted in nominally undoped ZnSe crystals. β-radiation detected nuclear magnetic resonance (β-NMR)
studies are described for two different probe nuclei, 8Li and 12B. This way, the implantation behavior of two “opposite”dopants, one acceptor (Li) and one donor (B) can be characterized
by the same microscopic technique. Such characterizations are attempted in terms of the structure of intermediate or final
lattice sites, defect charge states, or the kinetics of defect reactions and site changes.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
89.
90.