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41.
Andrea‐Nekane R. Alba Xavier Companyó Guillem Valero Albert Moyano Prof. Dr. Ramon Rios Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(18):5354-5361
A new, easy, and highly enantioselective method for the synthesis of quaternary α‐alkyl‐α‐amino acids based on organocatalysis is reported. The addition of oxazolones to 1,1‐bis(phenylsulfonyl)ethylene is efficiently catalyzed by simple chiral bases or thioureas. The reaction affords α,α‐disubstituted α‐amino acid derivatives with complete C4 regioselectivity and with excellent yields and enantioselectivities. This methodology is complementary to previously reported enantioselective approaches to quaternary α‐amino acids and allows the synthesis of α‐phenyl‐α‐alkyl‐α‐amino acids and α‐tert‐butyl‐α‐alkyl‐α‐amino acids. It has distinct advantages in terms of operational simplicity, enviromentally friendly conditions, and suitability for large‐scale reactions. 相似文献
42.
Nelson M. Larocca Edson N. Ito Carlos Triveño Rios Luiz A. Pessan Rosario E. S. Bretas Elias Hage Jr. 《Journal of Polymer Science.Polymer Physics》2010,48(21):2274-2287
Poly(butylene terephthalate) (PBT)/styrene‐acrylonitrile copolymer (SAN) blends were investigated with respect to their phase morphology. The SAN component was kept as dispersed phase and PBT as matrix phase and the PBT/SAN viscosity ratio was changed by using different PBT molecular weights. PBT/SAN blends were also compatibilized by adding methyl methacrylate‐co‐glycidyl methacrylate‐co‐ethyl acrylate terpolymer, MGE, which is an in situ reactive compatibilizer for melt blending. In noncompatibilized blends, the dispersed phase particle size increased with SAN concentration due to coalescence effects. Static coalescence experiments showed evidence of greater coalescence in blends with higher viscosity ratios. For noncompatibilized PBT/SAN/MGE blends with high molecular weight PBT as matrix phase, the average particle size of SAN phase does not depend on the SAN concentration in the blends. However noncompatibilized blends with low molecular weight PBT showed a significant increase in SAN particle size with the SAN concentration. The effect of MGE epoxy content and MGE molecular weight on the morphology of the PBT/SAN blend was also investigated. As the MGE epoxy content increased, the average particle size of SAN initially decreased with both high and low molecular weight PBT phase, thereafter leveling off with a critical content of epoxy groups in the blend. This critical content was higher in the blends containing low molecular weight PBT than in those with high molecular weight PBT. At a fixed MGE epoxy content, a decrease in MGE molecular weight yielded PBT/SAN blends with dispersed nanoparticles with an average size of about 40 nm. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010 相似文献
43.
Photochemical reactions between 1,4-benzo- and 1,4-naphthoquinone and several heteroaromatic carbaldehydes were investigated under solar irradiation conditions. These reactions gave the corresponding heteroacylated hydroquinones in the range 71%-92% yield. 相似文献
44.
Kim J. B. Kim M. J. Kim S. J. Hwang W.-Y. Miller D. L. Das M. B. Rios J. M. M. Lunardi L. M. 《Optical and Quantum Electronics》1997,29(10):953-959
An MBE grown InGaAs metal semiconductor metal (MSM) photodiode (PD) with an InAlAs barrier enhancement layer is reported that
has very low dark current and high speed characteristics. The detector using Cr/Au Schottky metal fingers with 4m spacing
on a large active area of 300×300m2 shows a low dark current of 38nA at 10V. This corresponds to a dark current density of
0.42pA/m2 and is, to our knowledge, the best dark current ever obtained from a large area InGaAs MSM PD. The device also shows
a low capacitance of 0.8pF and a high 3dB bandwidth of 2.4GHz. By fitting the measured frequency response to a model consisting
of both RC time and transit time limited responses, we show that the device has an RC time and a transit time limited 3dB
bandwidth of 3.0 and 4.9GHz, respectively.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
45.
R. Pérez-Olmos A. Rios R. A. S. Lapa J. L. F. C. Lima 《Analytical and bioanalytical chemistry》1998,360(6):659-663
Electrodes selective to potassium and calcium with improved senstivity were constructed like conventional electrode (ISEs), but used an operational amplifier to sum the potentials supplied by the four membranes (ESOAs). The results obtained during the evaluation of their working characteristics were compared to those obtained by conventional ISEs and were found to be similar, but with higher precision. The electrodes were used in the potentiometric determination of potassium, calcium and magnesium in wines. The results given by ESOAs were more precise than those from ISEs. The results obtained by potentiometry were in good agreement with those from AES for potassium and AAS for calcium and magnesium. 相似文献
46.
The properties of the negative parity states of55Fe and57Fe are investigated in the framework of the intermediate coupling model. In the model, a neutron or a quasineutron is coupled to anharmonic vibrations of the core. Anharmonicities of the vibrations are estimated through the observed properties of the core. Energy levels, spectroscopic factors and electromagnetic properties have been calculated. The results of the present calculations are also compared with available experimental results and other theoretical results. The model reasonably accounts for many of the properties of the low-lying states. 相似文献
47.
Diego Cortes José Luis Rios Angel Villar Serafin Valverde 《Tetrahedron letters》1984,25(30):3199-3202
The structures of Cherimolin and Dihydrocherimolin, two new antimicrobial γ-lactone-bis-tetrahydrofuranes from Mill., Annonaceae, have been elucidated by spectroscopic analysis. 相似文献
48.
This paper considers an airline crew allocation and scheduling problem faced by certain divisions of the United States Air Force. Two variants of the problem under consideration were posed to us by the U.S. Air Force. This paper reports our experience with two heuristic methods developed, each applicable to either variant of the problem. Although the problem described herein is peculiar to this situation, the heuristic scheduling and dispatching rules developed have been found to be very effective and are generally applicable in other related contexts of routeing and crew and vehicle scheduling problems as well. The two algorithms developed have been applied to a coded set of real world data. The results indicate that each one of the two methods is preferable to the other for one of the two variants of the problem. This suggests an overall effective composite technique. 相似文献
49.
Single electronic transitions in 1,4-naphthoquinone and seven of its derivatives substituted in the quinoid system have been theoretically studied semi-empirical SCF MO CI calculations, in the frame of π and all valence electron (AVE) approximations. Dipole electrostatic contributions to the shifts originating from the solvent have been calculated and the local nature of the most significant transitions has been analyzed. The results agree satisfactorily with experiment, showing the effects of the substituents, which in some cases are underestimated. 相似文献
50.