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81.
Organocatalytic Cascade Reactions: Towards the Diversification of Hydroisochromenes and Chromenes through Two Different Activation Modes 下载免费PDF全文
Dr. David Cruz Cruz Rasmus Mose Dr. Clarisa Villegas Gómez Stine V. Torbensen Martin S. Larsen Prof. Dr. Karl Anker Jørgensen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(36):11331-11335
The organocatalytic enantioselective syntheses of functionalized hydroisochromenes and chromenes by trienamine‐mediated [4+2]‐cycloaddition/nucleophilic ring‐closing and iminium‐ion/aminal‐mediated oxa‐Michael/Michael/nucleophilic ring‐closing with 2‐nitroallylic alcohols are presented. The corresponding cycloadducts, with up to five stereocenters, are formed in good yield and excellent enantioselectivities. The synthetic applications of the obtained products have been demonstrated. 相似文献
82.
JPC – Journal of Planar Chromatography – Modern TLC - Starch is widely distributed in diverse plant organs as a reserve carbohydrate; it is also a major source of carbohydrates in human... 相似文献
83.
Luis M. Villegas‐Silva 《Mathematical Logic Quarterly》2006,52(4):340-350
We extend the gap 1 cardinal transfer theorem (κ +, κ ) → (λ +, λ ) to any language of cardinality ≤λ, where λ is a regular cardinal. This transfer theorem has been proved by Chang under GCH for countable languages and by Silver in some cases for bigger languages (also under GCH). We assume the existence of a coarse (λ, 1)‐morass instead of GCH. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
84.
Nb films grown on top of an array of asymmetric pinning centers show a vortex ratchet effect. A net flow of vortices is induced
when the vortex lattice is driven by fluctuating forces on an array of pinning centers without reflection symmetry. This effect
occurs in the adiabatic regime and it could be mimiced only by reversible DC driven forces. 相似文献
85.
Rolando Cardenas Tame Gonzalez Osmel Martin Israel Quiros Diosdado Villegas 《General Relativity and Gravitation》2002,34(11):1877-1885
We briefly comment on a paper by Rubano and Scudellaro [Gen. Rel. Grav. 34 (2002) 307, astro-ph/0103335] where they found general exact solutions for two classes of exponential potentials in a scalar field model for quintessence. In that paper the authors were led to some interesting conclusions after a proper choice of the integration constants. By using dimensionless variables we show that the integration constants can be found explicitly without additional assumptions. In consequence we revise some results and conclusions in that paper. We also reproduce observations for Type-Ia supernovae with good accuracy. 相似文献
86.
Gálvez JA Díaz de Villegas MD Badorrey R López-Ram-de-Víu P 《Organic & biomolecular chemistry》2011,9(23):8155-8162
The regio- and stereoselective ring-opening of a 2-(2'-oxiranyl)-1,2,3,6-tetrahydropyridine using organometallic reagents is reported. The choice of the organometallic reagent determines the formation of either 2-[(R)-1-hydroxyalkyl]- or 2-[(S)-2-hydroxy-1-alkyl]-1,2,3,6-tetrahydropyridines. The formation of 2-[(S)-2-hydroxy-1-alkyl]-1,2,3,6-tetrahydropyridines is a rare example of epoxide ring-opening with retention of configuration. The process has been applied to the asymmetric synthesis of β-(+)-conhydrine and to the formal synthesis of (2S,2'R)-erythro-methylphenidate from a common precursor. Extension of the structural diversity of the process has allowed the synthesis of several β-(+)-conhydrine analogs. 相似文献
87.
Javier Vallejo-Montesinos Antonio Villegas Jorge Cervantes Elías Pérez 《Journal of Macromolecular Science: Physics》2013,52(9):624-644
The mesoscopic modeling of three polysiloxanes in solution is reported in this work, with the purpose of predicting their physicochemical properties as functions of the quality of the solvent, so that a judicious choice of polymer/solvent can be made for various applications. The polymers studied were the following polysiloxanes: polydimethylsiloxane (PDMS), polysiloxane with a bulky alkyl side group (PMHS) and a siloxane copolymer with a hydrophilic polar side group (P2DMPAS). The model used and solved through numerical simulations is the one known as dissipative particle dynamics. Density profiles and radial distribution functions were calculated for each system. We analyzed how the polymers behave in the presence of solvents of varying quality and compared their behavior with experimental data. We observed that we could replicate the behavior in good solvents for PDMS and PMHS. We also observed in the simulation box the formation of pseudo-micelles for P2DMPAS. 相似文献
88.
F. Velázquez Villegas S.D. Santillán Gutiérrez 《Revista Internacional de Métodos Numéricos para Cálculo y Dise?o en Ingeniería》2012,28(3):135-141
This paper presents a method for shape optimization of two-dimensional models subjected to simple or multiple load cases. The optimization is performed iteratively using evolutionary rules, based on the stress level. These rules determine regions on the boundary where the material is underused or overused; the objective is that the model evolves to a minimum weight model with a high and homogeneous stress level. The evolution is performed by modifying the boundary of the model slowly. Since the boundary is defined by parametric B-spline curves, generated changes result in a smooth boundary. The main proposal of the method is that boundary modifications are given by a set of displacements that, its magnitude and direction, take into account the geometrical information of the neighborhood, unlike other methods that simply generate displacement perpendicular to the boundary. Finally, the paper presents the design of a frame bike in order to show the good performance of the method. 相似文献
89.
E. López-Luna M.A. Vidal A.G. Rodríguez H. Navarro-Contreras Y. Kudriavtsev R. Asomoza A. Villegas 《Journal of Non》2009,355(16-17):976-981
Low energy shifted photoluminescence from isolated erbium ions incorporated into a-SiGe:H thin films is reported. The Er3+ are thermally diffused from an a-SiGe:H:Er layer to a-SiGe:H subsequently grown, both by magnetron sputtering. The photoluminescence observed is associated with transitions produced by erbium emission centers activated by the oxidation in a 1 h annealing process in air at 250 °C. The resultant Er3+ concentration observed from the a-SiGe:H is affected by the hydrogen concentration already present in the layer. It is observed that at higher hydrogen concentrations in a-SiGe:H the resultant amount of diffused Er3+ decreases. As a consequence of the resultant smaller density of erbium ions, the probability of having isolated Er3+ ions increases. In this last regime, a correlation with stronger photoluminescence is observed. 相似文献
90.
M.A. García S.E. Paje M.A. Villegas J. Llopis 《Applied Physics A: Materials Science & Processing》2002,74(1):83-88
Silica sol-gel coatings doped with calcein (CA) were prepared and optically charecterised by means of UV-VIS spectroscopies
(optical absorption and luminescence). In addition, the stability of samples against thermal treatment, UV exposure and alkali
attack (chemical durability) were studied. Microstructure was verified using transmission electron microscopy and specific
surface area was measured using the BET method. Results highlighted the ability of sol-gel coatings to react reversibly to
pH changes when immersed in liquid media at different pH values. The limits for maximum optical absorption sensitivity were
determined to be 5<pH<9, while the 8.5<pH<14.0 and 0<pH<2ranges could be detected using photoluminescence spectroscopy. Stability
studies showed good behaviour against UV (maximum loss of 50% in peak intensity after 30 h of continuous exposure) and good
chemical durability under alkaline conditions (80 h attack time was necessary to reduce the percentage absorption to 50%).
Thermal stability of doped films was good up to 130 °C (in this range a reduction of about 25% in the intensity of the initial
peak absorption is lost).
Received: 26 September 2000 / Accepted: 29 January 2001 / Published online: 30 August 2001 相似文献