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691.
Stereoselective synthesis of pyrrolidines from N-allyl oxazolidines via hydrozirconation-cyclization
[reaction: see text] A new diastereoselective synthesis of pyrrolidines from readily available chiral N-allyl oxazolidines is presented. The construction of the pyrrolidine ring is achieved via a tandem hydrozirconation-stereoselective Lewis acid mediated cyclization sequence. 相似文献
692.
A. Alan Pinkerton Dieter Schwarzenbach Jean-Luc Birbaum Pierre-Alain Carrupt Luis Schwager Pierre Vogel 《Helvetica chimica acta》1984,67(4):1136-1153
The crystal structures of (1R,4R,5S,8S)-9,10-dimethylidentricyclo[6.2.1.02,7]undec2(7)-ene-4,5-dicarboxylic anhydride ( 3 ), (1R,4R,5S,8S)11-isopropylidene-9,10-dimethylidenetricyclo[6.2.1.m2,7]undec-2(7)-ene-4,5-dicarboxylic anhydride ( 6 ), (1R,4R,5S8S)-9,10-dimethylidenetricyclo[6.2.2.02,7]dodec-2(7)-ene-4,5-dicarboxylic anhydride ( 9 ), (1R4R5S8S)-TRICYCLO[6.2.2.02,7]dodeca-2(7), 9-diene-4,5-dicarboxylic anhydride ( 12 ) and (4R,5S)-tricyclo[6.1.1.02.7]dec-2(7)-ene-4,5-dicarboxylic acid ( 16 ) were established by X-ray diffraction. The alkyl substituents onto the endocyclic bicyclo[2.2.1]hept-2-ene double bond deviate from the C(1), C(2), C(3), C(4), plane by 13.5°4 in 3 and by 13.9° in 6 , leaning toward the endo-face. No such out-of-plane deformations were observed with the bicyclo[2.2.2]oct-2-ene derivatives 9 and 12 . The exocyclic s-cis-butadiene moieties in 3, 6 and 9 do not deviate significantly from planarity. The deviation from planarity of the double bond n bicyclo[2.2.1]hept-2-ene derivatives and planarity in bicyclo[2.2.2]oct-2-ene analogues is shown to be general by analysis of all known structures in the Cambridge Crystallographic Data File. The non-planarity of the bicyclo[2.2.1]hept-2-ene double bond cannot be attributed only to bond-angle deformations which would favour rehybridizatoin of the olefinic C-atoms since the double bond in the more strained bicyclo[2.1.1]hex-2-ene drivative 16 deviates from planarity by less than 4°. 相似文献
693.
Karin Briner Bruno Bernet Jean-Luc Maloisel Andrea Vasella 《Helvetica chimica acta》1994,77(7):1969-1984
The regio- and stereoselectivity of the glycosidation of the partially protected mono-alcohols 3 and 7 , the diols 2 and 8 , and the triol 4 by the diazirine 1 have been investigated. Glycosidation of the α-D -diol 2 (Scheme 2) gave regioselectively the 1,3-linked disaccharides 11 and 12 (80%, α-D /β-D 9:1), whereas the analogous reaction with the βD -anomer 8 led to a mixture of the anomeric 1,3- and 1,4-linked disaccharides 13 (12.5%), 14 (16%), 15 (13%), and 16 (20.5%; Table 2). Protonation of the carbene by OH–C(4) of 2 is evidenced by the observation that the α-D -mono-alcohol 3 did not react with 1 under otherwise identical conditions, and that the β-D -alcohol 7 yielded predominantly the β-D -glucoside 18 (52%) besides 14% of 17 . Similarly as for the glycosidation of the diol 2 , the influence of the H-bond of HO? C(4) on the direction of approach of the carbene, the role of HO? C(4) in protonating the carbene, and the stereoelectronic control in the interception of the ensuring oxycarbenium cation are evidenced by the reaction of the triol 4 with 1 (Scheme 3), leading mostly to the α-D -configurated 1,3-linked disaccharide 19 (41%), besides its anomer 20 (16%), and some 4-substituted β-D -glucoside 21 (9%). No 1,6-linked disaccharides could be detected. In agreement with the observed reactivity, the 1H-NMR and IR spectra reveal a strong H-bond between HO? C(3) and the phthalimido group in the α-D -, but not in the β-D -allosides. The different H-bonds in the anomeric phthalimides are in keeping with the results of molecular-mechanics calculations. 相似文献
694.
The mass spectra of the title compounds show peaks corresponding to the primary fragmentation of H, CH3 and (H2 + C2H2). Kinetic study suggests that the molecular ion of 1-phenylcyclobutene and 2-phenyl-1,3-butadiene loses H and CH3 through formation of a common intermediate. This is confirmed by appearance potential and ionisation potential measurements, fragmentation of deuterium labelled compounds and the corrected Z values. 相似文献
695.
Aggarwal VK Hynd G Picoul W Vasse JL 《Journal of the American Chemical Society》2002,124(34):9964-9965
The reaction of an amide-stabilized sulfonium ylide bearing chiral groups on sulfur has been investigated. We have discovered that the camphor-derived amide-stabilized ylide reacts with aldehydes at -50 degrees C in ethanol to give glycidic amides in one step with up to 99% ee and complete diastereoselectivity. From analyzing reactions of different ratios of diastereomers at sulfur it was found that the major diastereomer gave very high enantioselectivity, while the minor one gave much lower selectivity (54% ee). Further mechanistic studies have revealed that enantioselectivity is controlled not in the betaine-forming step (C-C bond formation is reversible) but in the different barriers to bond rotation around the newly formed C-C of the two diastereomeric betaines. Further transformations of epoxyamides were investigated. It was found that epoxyamides could be converted into epoxyketones by reaction with organolithium reagents and that they could be ring-opened by nucleophiles with complete regioselectivity using Yb(OTf)3. The practicality of the process has been exemplified in the synthesis of SK&F 104353, a leukotriene D4 antagonist in the potential treatment of bronchial asthma. 相似文献
696.
Realistic Cold Expansion Finite Element Model and Experimental Validations for Aluminium Alloys 总被引:1,自引:0,他引:1
Maxime Lapalme Majid Hoseini Philippe Bocher Alain R. Colle Martin Lévesque 《Experimental Mechanics》2014,54(5):841-855
Accurate understanding of the residual stresses produced by cold expansion (CX) of rivet holes is required for fatigue considerations, as well as for better addressing the loading problems in aerospace parts. This paper presents a physically based finite element (FE) model for predicting the residual stresses in aluminium parts submitted to CX. The model relies on experimental measurements for both construction and validation purposes. The model predictions were compared against the experimental results delivered by three different complementary techniques: Digital Image Correlation (DIC) (provided strain fields on the surface of the workpiece); X-Ray Diffraction (XRD) (provided residual stresses measurements on the surfaces); Microscopy measurements (measured the out-of-plane displacement on the hole bore). The FE model proves to be in good agreement with the experimental data. Further considerations on the simulation method regarding geometries, boundary conditions and friction effect are also discussed. 相似文献
697.
We consider the investor choice among standardized portfolios, which are based on cash, bond and stock indexes. We present
the intertemporal optimization problem with commonly used utility functions. We provide a method to determine the optimal
investor’s choice, based on the knowledge of investor’s type (risk aversion and time horizon) and on market performances.
For the utility functions envisaged, we compute the losses from not having access to a customized portfolio and show these
losses may be severe.
We would like to acknowledge Andrée Arth and Barbara Wirth (UBS), Moshe Ben-Akiva (MIT), Yurii Nesterov (CORE), Alain Fonck
and Yves Wouters (FORTIS), Fidelity Investments, Laura Lindsey (Royal Bank of Canada) as well as from Nicolas Chapon and Nathalie
Picard for numerous discussions, comments and suggestions. We also benefitted from the remarks of the participants of the
European Investment conference HEC Geneva (2003) and ESEM (2004), especially from Luc Leruth (IMF), Emeric Challier (FORTIS)
and Lionel Martellini (University of Southern California) and of the seminar participants at the University of Virginia (Dept.
of Economics and School of Business). Finally, the helpful comments and suggestions of Hercules Vladimirou greatly improved
this paper.
A. de Palma is senior Member of the Institut Universitaire de France. 相似文献
698.
We give the distribution functions, the expected values, and the moments of linear combinations of lattice polynomials from the uniform distribution. Linear combinations of lattice polynomials, which include weighted sums, linear combinations of order statistics, and lattice polynomials, are actually those continuous functions that reduce to linear functions on each simplex of the standard triangulation of the unit cube. They are mainly used in aggregation theory, combinatorial optimization, and game theory, where they are known as discrete Choquet integrals and Lovász extensions. 相似文献
699.
Evaluation of Residual Stresses Induced by Robotized Hammer Peening by the Contour Method 总被引:1,自引:0,他引:1
Welded components suffer from high tensile residual stresses close to the weld beads. These stresses seem to be the origin
of premature cracking which could result in a catastrophic rupture during operation and a reduction of the lifespan of these
components. In this context, the Hydro-Québec’s Research Institute (IREQ) developed a technique of residual stresses relaxation
by robotized hammer peening which makes it possible to release stresses close to surface and preserve the mechanical and dimensional
properties of manufactured components. Robotized hammer peening was used to induce compressive residual stresses on initially
stress free samples of austenitic stainless steel 304L. Hammer peening layers from one to nine were performed and the resulting
residual stresses were evaluated thanks to the contour technique. Complete 2D residual stress fields on samples cross sections
were obtained. The ability of hammer peening to relax residual stresses within welded plates was then quantified on austenitic
stainless steel 304L plates welded with a 308 steel and hammer peened. These tests show the efficiency of hammer peening as
a method to relax tensile residual stresses and induce compressive ones to a depth of a few millimetres. Process parameters
were optimized such as the number of hammer peening layers to be applied to reduce processing time and maximization of the
intensity and spatial distribution of the compressive residual stresses. 相似文献
700.
Using a general approximation setting having the generic properties of finite-elements, we prove uniform boundedness and stability
estimates on the discrete Stokes operator in Sobolev spaces with fractional exponents. As an application, we construct approximations
for the time-dependent Stokes equations with a source term in L
p
(0, T; L
q
(Ω)) and prove uniform estimates on the time derivative and discrete Laplacian of the discrete velocity that are similar to
those in Sohr and von Wahl [20].
On long leave from LIMSI (CNRS-UPR 3251), BP 133, 91403, Orsay, France. 相似文献