排序方式: 共有74条查询结果,搜索用时 15 毫秒
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Barrett AG Braddock DC de Koning PD White AJ Williams DJ 《The Journal of organic chemistry》2000,65(2):375-380
The double allylboration of aldehydes using 1, 3-bis(diisopinocampheylboryl)-2-methylenepropanes (R,R)-3 and (S, S)-3 under Brown's salt-free conditions provides C(2)-symmetric 3-methylenepentane-1,5-diols 1 in excellent enantiomeric excess. The absolute stereochemistry of the products was confirmed by a single-crystal X-ray study of bis-Mosher ester 6g. Desymmetrization and further functionalization of diol 1a were achieved by treatment of the bis-BOC carbonate 13 with IBr in toluene at -80 degrees C to give cyclic iodocarbonate 14 as a single diastereomer. This methodology is also applicable in natural product synthesis; enantiomerically pure spiroketals 1,7-dioxaspiro[5.5]undecanes 18 and 25, the latter representing an expedient synthesis of the AB ring system of the spongistatins 20, were easily accessed from simple starting materials in excellent yields and selectivities. 相似文献
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The tellurium(II) dithiolates Te[SCH2CH2C(O)OCH3]2, ( 1 ), Te[SCH2CH2CH2SC(O)CH3]2, ( 2 ), and Te[SCH2CH2CH2CH2SC(O)CH3]2, ( 3 ) were synthesized from Te(StBu)2 and the corresponding thiol. All compounds are sensitive toward higher temperatures and light and decompose to elemental tellurium and the disulfide. In the solid state, the Te atom of 1 exhibits the novel Te(S2Te2) coordination mode. Additionally to the two Te—S bonds, each Te atom forms two long Te···Te contacts to neighboring molecules, leading to a coordination number of four and a distorted sawhorse configuration. No intramolecular Te···O interactions are present in the solid state, in accordance with ab initio calculations (MP2/ecp‐basis) for the isolated molecule. 125Te NMR shifts of all compounds lay within a narrow range and close to the respective shift of other Te(SCH2R)2 compounds. VT 125Te NMR spectra gave no hint to donor acceptor interactions in solution for any of the compounds and thus corroborate results from IR‐spectroscopy, ab initio geometry optimizations, and thermochemical calculations. 相似文献
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Stephan Berner Andreas B. Schmidt Mirko Zimmermann Dr. Andrey N. Pravdivtsev Dr. Stefan Glöggler Prof. Dr. Dr. h.c. Jürgen Hennig PD Dr. Dominik von Elverfeldt Prof. Dr. Jan-Bernd Hövener 《ChemistryOpen》2019,8(6):728-736
The signal enhancement provided by the hyperpolarization of nuclear spins of biological molecules is a highly promising technique for diagnostic imaging. To date, most 13C-contrast agents had to be polarized in an extra, complex or cost intensive polarizer. Recently, the in situ hyperpolarization of a 13C contrast agent to >20 % was demonstrated without a polarizer but within the bore of an MRI system. This approach addresses some of the challenges of MRI with hyperpolarized tracers, i. e. elevated cost, long production times, and loss of polarization during transfer to the detection site. Here, we demonstrate the first hyperpolarization of a biomolecule in aqueous solution in the bore of an MRI at field strength of 7 T within seconds. The 13C nucleus of 1-13C, 2,3-2H2-succinate was polarized to 11 % corresponding to a signal enhancement of approximately 18.000. Interesting effects during the process of the hydrogenation reaction which lead to a significant loss of polarization have been observed. 相似文献
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PD Dr. Holger Kohlmann 《无机化学与普通化学杂志》2010,636(11):2046-2046
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Roger T Büscher J Bastin B Kirsebom OS Raabe R Alcorta M Äystö J Borge MJ Carmona-Gallardo M Cocolios TE Cruz J Dendooven P Fraile LM Fynbo HO Galaviz D Gasques LR Giri GS Huyse M Hyldegaard S Jungmann K Kruithof WL Lantz M Perea A Riisager K Saastamoinen A Santra B Shidling PD Sohani M Sørensen AJ Tengblad O Traykov E van der Hoek DJ Van Duppen P Versolato OO Wilschut HW 《Physical review letters》2012,108(16):162502
A measurement of the final state distribution of the (8)B β decay, obtained by implanting a (8)B beam in a double-sided silicon strip detector, is reported here. The present spectrum is consistent with a recent independent precise measurement performed by our collaboration at the IGISOL facility, Jyv?skyl? [O. S. Kirsebom et al., Phys. Rev. C 83, 065802 (2011)]. It shows discrepancies with previously measured spectra, leading to differences in the derived neutrino spectrum. Thanks to a low detection threshold, the neutrino spectrum is for the first time directly extracted from the measured final state distribution, thus avoiding the uncertainties related to the extrapolation of R-matrix fits. Combined with the IGISOL data, this leads to an improvement of the overall errors and the extension of the neutrino spectrum at high energy. The new unperturbed neutrino spectrum represents a benchmark for future measurements of the solar neutrino flux as a function of energy. 相似文献
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Johann Sjuts PD Dr 《ZDM》2005,37(5):424-430
Can the describable complexity of test problems concerning mathematical thinking and the empirical results of their dealing with be put into a relation? Can graded test problems be constructed which lead to results which can basically be predicted? Empirical studies give interesting and helpful answers which lead to didactically important consequences, just like the evaluation of the PISA results. 相似文献
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Daunne Christa PD Dr. 《ZDM》2003,35(3):102-109
It is reported about a longitudinal study run at the Institute of Cognitive Mathematics of the University of Osnabrueck, in which pupils’ verbal and text productions from mathematics lessons at a grammar school are analysed by means of cognitive theoretical methods. First of all, a teaching scene from an instruction to probability calculus and further text productions from an introductory lesson about exponential functions are analysed, in which five pupils take part whose cognitive structures have been assessed and classified in individual examinations. The characteristics brought out according to these teaching scenes indicate different ideas and thinking processes of the pupils. The second part shows that the pupils’ behaviour described is not only to be regarded in isolation but it can also be found in longitudinal examinations and can therefore be considered as a stable, typical characteristic. These results lead to consequences for the planning and design of mathematics lessons based on a well-founded theory of cognition. 相似文献