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981.
Nataša Mršić Thomas Jerphagnon Adriaan J. Minnaard Ben L. Feringa Johannes G. de Vries 《Tetrahedron: Asymmetry》2010,21(1):7-10
The rhodium-catalyzed asymmetric hydrogenation of 2-substituted N-protected-indoles using monodentate phosphoramidites as ligands was examined. Full conversion and 74% ee, were obtained with a catalyst based on PipPhos. The use of a catalytic amount of base is necessary for activity; best results were obtained with Cs2CO3. 相似文献
982.
Ben Ford 《Transactions of the American Mathematical Society》1999,351(10):3869-3913
Determining the subgroup structure of algebraic groups (over an algebraically closed field of arbitrary characteristic) often requires an understanding of those instances when a group and a closed subgroup both act irreducibly on some module , which is rational for and . In this paper and in Overgroups of irreducible linear groups, I (J. Algebra 181 (1996), 26-69), we give a classification of all such triples when is a non-connected algebraic group with simple identity component , is an irreducible -module with restricted -high weight(s), and is a simple algebraic group of classical type over sitting strictly between and .
983.
The distributed roughness theory of the origins of spectral periodicity in stimulus frequency otoacoustic emissions (SFOAEs) predicts that the spectral period will be altered by suppression of the traveling wave (TW) [Zweig and Shera, J. Acoust. Soc. Am. 98, 2018-2047 (1995)]. In order to investigate this effect in more detail, simulations of the variation of the spectral period under conditions of self-suppression and two-tone suppression are obtained from nonlinear cochlear models based on this theory. The results show that during self-suppression the spectral period is increased, while during high-side two-tone suppression, the period is reduced, indicating that the detailed pattern of disruption of the cochlear amplifier must be examined if the nonlinear behavior of SFOAEs is to be understood. The model results suggest that the SFOAE spectral period may be sensitive to changes in the state of the cochlear amplifier. A companion paper [Lineton and Lutman, J. Acoust. Soc. Am. 114, 871-882 (2003)] presents experimental data which are compared with the results of the above models with a view to testing the underlying theory of Zweig and Shera. 相似文献
984.
A model for estimating the spectral period of stimulus frequency otoacoustic emissions (SFOAEs) is presented. The model characterizes the frequency spectrum of an SFOAE in terms of four parameters which can be directly related to cochlear mechanical quantities featuring in the theory of SFOAE generation proposed by Zweig and Shera [J. Acoust. Soc. Am. 98, 2018-2047 (1995)]. The results of applying the parametric model to SFOAEs generated by cochlear models suggest that it gives a sensitive measure of spectral period. It is concluded that the parametric model may be a useful tool for detecting small changes in cochlear function using SFOAE measurements. 相似文献
985.
Peña D Minnaard AJ Boogers JA de Vries AH de Vries JG Feringa BL 《Organic & biomolecular chemistry》2003,1(7):1087-1089
The combination of monodentate ligands in the rhodium-catalysed enantioselective hydrogenation enables a new approach when searching for the optimal activity and enantioselectivity in catalysis. 相似文献
986.
Díez D Benéitez MT Marcos IS Garrido NM Basabe P Sanz F Broughton HB Urones JG 《Organic letters》2003,5(23):4361-4364
[reaction: see text] Several chiral building blocks have been obtained easily in large quantities from an epoxysulfone (9) that could be obtained in both enantiomeric forms from accessible starting materials. 相似文献
987.
Jiang XB Minnaard AJ Hessen B Feringa BL Duchateau AL Andrien JG Boogers JA de Vries JG 《Organic letters》2003,5(9):1503-1506
Secondary phosphine oxides were prepared from R(1)PCl(2) and R(2)MgBr, followed by hydrolysis. They were obtained in an enantiopure form by preparative chiral HPLC. These new monodentate ligands were tested in the iridium-catalyzed hydrogenation of imines at 25 bar. Enantioselectivities up to 76% were obtained at L/Ir = 2. Addition of pyridine (Pyr/Ir = 1:2) raised the ee to 83%. Using pyridine as an additive allowed reduction of the L/Ir ratio to 1 without reduction of ee. [reaction: see text] 相似文献
988.
[reaction: see text] Bidentate chiral phosphines are no longer essential for achieving a fast and highly enantioselective hydrogenation of alpha- or beta-dehydroamino acid derivatives. In particular, a readily accessible and stable monodentate phosphoramidite can be highly effective in these asymmetric hydrogenations. 相似文献
989.
van Delden RA Hurenkamp JH Feringa BL 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(12):2845-2853
A new type of chiroptical molecular switch is presented where irradiation employing different wavelengths of light induces a reversible helix inversion of a sterically overcrowded alkene bearing a second chiral entity in the form of a stereogenic center present in a pyrrolidine unit. The additional stereogenic center in the chiral auxiliary group has a distinct influence on the switching selectivity of this system and greatly facilitates the resolution of the different diastereoisomers, which is a considerable improvement compared with previously reported systems. In addition, the pyrrolidine stereogenic center causes small energetic differences between the various states of the switch system resulting in a small but significant directional preference in the helix inversion steps. 相似文献
990.
L. BarchiniS.Ben Sa?&#x;d 《Journal of Functional Analysis》2002,194(1):213-230
In this paper, we study a family of hypergeometric functions associated to cones of convex type in a Euclidean Jordan algebra. We obtain upper and lower bounds and give information about their singularities. The family under consideration was introduced by Sahi (Invent. Math.110 (1992), 409-418) and it plays an important role in the construction of some small representations. 相似文献