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81.
Boeckman RK Del Rosario Ferreira MR Mitchell LH Shao P Neeb MJ Fang Y 《Tetrahedron》2011,67(51):9787-9808
A full account of studies that culminated in the total synthesis of both antipodes and the assignment of its absolute configuration of Saudin, a hypoglycemic natural product. Two approaches are described, the first proceeding though bicyclic lactone intermediates and related second monocyclic esters. The former was obtained via asymmetric Diels-Alder cycloaddition and the latter by an asymmetric annulation protocol. Both approaches employ a Lewis acid promoted Claisen rearrangement, with the successful approach taking advantage of bidentate chelation to control the facial selectivity of the key Claisen rearrangement. 相似文献
82.
Rafael López Guillermo Ramírez Ignacio Ema Jaime Fernández Rico 《Journal of computational chemistry》2013,34(21):1800-1809
The calculation of the electronic structure of large systems is facilitated by the substitution of the two‐center distributions by their projections on auxiliary basis sets of one‐center functions. An alternative is the partition–expansion method in which one first decides what part of the distribution is assigned to each center, and next expands each part in spherical harmonics times radial factors. The method is exact, requires neither auxiliary basis sets nor projections, and can be applied to Gaussian and Slater basis sets. Two improvements in the partition–expansion method for Slater functions are reported: general expressions valid for arbitrary quantum numbers are derived and the efficiency of the procedure is increased giving analytical solutions to integrals previously computed by numerical quadrature. The efficiency of the new version is assessed in several molecules and the advantages over the projection methods are pointed out. © 2013 Wiley Periodicals, Inc. 相似文献
83.
F. Loussayre A. de Savignac I. Rico N. Hajjaji-Srhiri A. Lattes 《Journal of Dispersion Science and Technology》2013,34(2):181-197
Anionic and cationic surfactants have formed the subject of numerous physicochemical investigations, but few studies have been devoted to amphoteric amphiphilic agents. In a series of investigations on new synthetic routes to novel surfactants, we devised a simple method for the preparation of N-alkylbetaïnes with different chain lengths: In addition, we investigated the adsorption of these compounds onto wool fibers in aqueous media. The effects of pH, temperature and chain length on the adsorption isotherms were investigated. A relationship between the behavior of the surfactant at the solid/liquid interface and its behavior in solution was detected. Maximum adsorption was observed for values close to the critical micellar concentration. 相似文献
84.
Guillermo Ramírez Rafael López Ignacio Ema Jaime Fernández Rico 《International journal of quantum chemistry》2013,113(1):52-55
In atomic systems, electron density has a simple finite expansion in spherical harmonics times radial factors. The difficulties in the calculation of some radial factors are illustrated in the low‐lying states of the carbon atom. Single‐particle methods such as Hartree–Fock and approximate density functional theory cannot ensure the correct expansion of the density in spherical harmonics. Wave‐function methods are appropriate but, as some expansion terms are entirely due to correlation, these methods only will give correct results for high‐quality variational functions. Using full‐configuration integration (CI), all the terms predicted by the theory appear and are not negligible but the convergence of the term due to correlation toward its correct value is uncertain even for very large CI spaces. © 2012 Wiley Periodicals, Inc. 相似文献
85.
Cyclization of biarylacetamides to their phenanthrene derivatives is promoted by oxalyl chloride/stannyl chloride. The reaction proceeds with a second cyclization in which the oxalyl fragment acts as an -dicarbonyl transfer agent to give 4,5-dioxoaporphine alkaloids in a single step. This double cyclization was also applied to aryloxyphenyl acetamides to give the corresponding 3,4-dioxocularine alkaloids. Decarbonylated aristocularine alkaloids were also formed in this case. 相似文献
86.
The E ? Z photoisomerization of the O-methyl ether of the oxime of 2-acetylnaphthalene has been studied in various microemulsions. The results obtained with variations of surfactant concentrations or with addition of sodium chloride show the importance of interfacial processes in colloïdal systems. 相似文献
87.
Del Sesto RE McCleskey TM Burrell AK Baker GA Thompson JD Scott BL Wilkes JS Williams P 《Chemical communications (Cambridge, England)》2008,(4):447-449
A series of ionic liquids containing different paramagnetic anions have been prepared and all show paramagnetic behavior with potential applications for magnetic and electrochromic switching as well as novel magnetic transport; also, the tetraalkylphosphonium-based ionic liquids reveal anomalous magnetic behavior. 相似文献
88.
C. Ramírez M. Rico A. Torres L. Barral J. López B. Montero 《European Polymer Journal》2008,44(10):3035-3045
Organic-inorganic hybrid nanocomposites were prepared by reaction of an octaepoxy-silsesquioxane, OECh, with an epoxy-amine system. OECh was used to partially replace the thermosetting resin, diglycidyl ether of bisphenol A, DGEBA, in its reaction with an aromatic diamine, 4,4′-(1,3-phenylenediisopropylidene) bisaniline, BSA. The OECh was characterized by different techniques. The curing kinetics of ternary systems formed by DGEBA, OECh and BSA, was followed by Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy, ATR-FTIR. All the mixtures were prepared with a stoichiometric ratio between epoxy and amine groups. The degree of reaction of glycidyl epoxy ring along the curing cycle selected was obtained from the infrared spectra. A peak-height method based on the ratio of the height of the characteristic to reference absorbance peak was used. The curing kinetic of different blends was obtained by differential scanning calorimetry, DSC. Three different methods, the differential of Kissinger, the integral of Flynn-Wall-Ozawa and the phenomenological model of Kamal, were used in order to obtain the kinetic parameters of the cure reaction. It is observed that the presence of POSS accelerates the rate of opening of glycidyl epoxy rings from DGEBA. The behaviour of the mixture during the curing process can be explained with an autocatalytical model, corrected with the contribution of the diffusion of the molecules during the course of the reaction. 相似文献
89.
Rico Packheiser Björn Bräuer Claude Lapinte Heinrich Lang 《Journal of organometallic chemistry》2008,693(17):2898-2902
The electrochemical behavior of two series of homo- and heterometallic 1,3,5-triethynylbenzene-based transition metal complexes containing [(η2-dppf)(η5-C5H5)Ru], [(PPh3)2(η5-C5H5)Os], [(tBu2bpy)(CO)3Re], and [(bpy)(CO)3ClRe] (dppf = 1,1′-bis(diphenylphosphino)ferrocene; tBu2bpy = 4,4′-di-tert-butyl-2,2′-bipyridyl; bpy = 2,2′-bipyridyl-5-yl) building blocks have been studied, showing that there is electronic interaction between the appropriate metal atoms. The electronic absorption spectra reveal high energy bands corresponding to intraligand π → π∗ transitions (bpy, alkynyl) and low energy absorptions which are attributed to MLCT transitions; replacement of ruthenium by osmium results in a blue-shift of the MLCT bands. The associated radical cations of three complexes were in situ generated by chemical oxidation and characterized by continuous wave electron paramagnetic resonance (EPR) investigations in X-band performed at low temperatures. 相似文献
90.
Rico Walter 《Central European Journal of Operations Research》2017,25(1):139-144
In this short note, we address the coherence between minimizing the sum of squares of machine completion times and minimizing makespan on two identical parallel machines. We show equivalence of the two objectives and identify interesting and useful relations which allow us to transfer worst-case ratios of approximation algorithms from one problem to the other. 相似文献