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61.
Ibrahem I Rios R Vesely J Zhao GL Córdova A 《Chemical communications (Cambridge, England)》2007,(8):849-851
The highly chemo- and enantioselective organocatalytic tandem reaction between N-protected hydroxyl amines and alpha,beta-unsaturated aldehydes is presented; the reaction provides access to 5-hydroxyisoxazolidines and beta-amino acids in high yields and with 90-99% ee. 相似文献
62.
Guerrero-Alvarez JA Moncayo-Bautista A Ariza-Castolo A 《Magnetic resonance in chemistry : MRC》2004,42(6):524-533
The relative configuration of 11 1,4-diazaspiro[4.5]decanes (1a-1j and 1m), 15 1,4-oxazaspiro[4.5]decanes (2a-2o) and 10 1,4-dioxaspiro[4.5]decanes (3a-3n) substituted at the 2-, 6-, 7- or 8-position by a methyl group or using the tert-butyl group as a model for the ananchomeric structure is reported. The relative stereochemistry was analyzed by 1H, 13C, 15N and 17O NMR and all isomers present were characterized spectroscopically. Compounds with a methyl group in the six-membered ring show a chair conformation preference with the methyl group in the equatorial position. Compounds with one or two nitrogens exhibit a tautomeric equilibrium between the imine-diazolidine forms, as demonstrated by IR and 13C NMR. 相似文献
63.
Frontispiece: Biomass Oxidation: Formyl CH Bond Activation by the Surface Lattice Oxygen of Regenerative CuO Nanoleaves 下载免费PDF全文
64.
Lara Bianchi Giovanni Petrillo Egon Rizzato Elda Severi Marco Stenta Cinzia Tavani 《Tetrahedron》2009,65(1):336-757
(1E,3Z)-1-Aryl-4-methanesulfonyl-2-nitro-1,3-butadienes (8), derived from the initial ring-opening of 3-nitrothiophene (5), have been found to undergo a facile base-induced cyclization leading to thiopyran S,S-dioxides (9), thus furnishing a further example of effective ring-enlargement from 5- to 6-membered sulfur heterocycles. Compounds 9 are obtained as single racemic mixtures in satisfactory yields; they still contain a nitrovinylic moiety, which can be exploited for further modifications targeted to new derivatives endowed with either synthetic or pharmacological potentialities e.g., in the field of L-type Ca2+-channel blockers. 相似文献
65.
The two-photon absorption of a class of [2.2]paracyclophane derivatives has been studied using quadratic response and density functional theories. For the molecules investigated, several effects influencing the two-photon absorption spectra have been investigated, such as side-chain elongation, hydrogen bonding, the use of ionic species, and solvent effects, the latter described by the polarizable continuum model. The calculations have been carried out using a recent parallel implementation of the polarizable continuum model in the DALTON code. Special attention is given to those aspects that could explain the large solvent effect on the two-photon absorption cross sections observed experimentally for this class of compounds. 相似文献
66.
Andreina Reyes Rosa M. Dominguez Maria Tosta Armando Herize Gabriel Chuchani 《国际化学动力学杂志》2007,39(5):268-275
The rates of elimination of several ethyl esters of 2‐oxo‐carboxylic acid were determined in a seasoned static reaction vessel over the temperature range 350–430°C and pressure range 33–240 Torr. The reactions, in the presence of a free‐radical inhibitor, are homogeneous, unimolecular, and follow a first‐order rate law. The overall and partial rate coefficients are expressed by the Arrhenius equation. Ethyl glyoxalate Ethyl 2‐oxo‐propionate Ethyl 3‐methyl‐2‐oxo‐butyrate The mechanisms of these elimination reactions are described in terms of concerted cyclic transition state structures. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 268–275, 2007 相似文献
67.
Martí I Ferrer A Escorihuela J Burguete MI Luis SV 《Dalton transactions (Cambridge, England : 2003)》2012,41(22):6764-6776
A family of ligands derived from bis(amino amides) containing aliphatic spacers has been prepared, and their protonation and stability constants for the formation of Cu(2+) complexes have been determined potentiometrically. Important differences are associated to both the length of the aliphatic spacer and the nature of the side chains derived from the amino acid. In general, ligands containing aliphatic side chains display higher basicities as well as stability constants with Cu(2+). In the same way, basicities and stability constants tend to increase when decreasing the steric hindrance caused by the corresponding side-chain. FT-IR, UV-vis and ESI-MS were used for analyzing the complex species detected in the speciation diagram. UV-vis studies showed the presence of different coordination environments for the copper(II) complexes. Complexes with different stoichiometries can be formed in some instances. This was clearly highlighted with the help of ESI-MS experiments. 相似文献
68.
Valdemar I. Esteves Susana S.F. Lima Diana L.D. Lima Armando C. Duarte 《Analytica chimica acta》2004,513(1):163-167
The simultaneous separation and determination of organic acids in several samples of white and red Port wines was performed by capillary zone electrophoresis using indirect UV detection with 2,6-pyridinedicarboxylic acid as a background electrolyte buffer. Operational parameters like migration time, temperature, voltage and capillary length were optimized. Sixteen samples of red wine and four samples of white wine were used to analyze for tartaric, malic, lactic, succinic and acetic acids using glyoxylic acid as the internal standard. The method is rapid, sensitive and quantitative, and time-consuming sample preparation, such as solid-phase extraction or liquid-liquid extraction procedure, is not required. 相似文献
69.
Patricia Lara Margarita Paneque Prof. Dr. Manuel L. Poveda Prof. Dr. Laura L. Santos Dr. José E. V. Valpuesta Verónica Salazar Dr. Ernesto Carmona Prof. Dr. Salvador Moncho Gregori Ujaque Dr. Agustí Lledós Prof. Dr. Celia Maya Dr. Kurt Mereiter Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(36):9046-9057
Experimental and theoretical studies on equilibria between iridium hydride alkylidene structures, [(TpMe2)Ir(H){?C(CH2R)ArO }] (TpMe2=hydrotris(3,5‐dimethylpyrazolyl)borate; R=H, Me; Ar=substituted C6H4 group), and their corresponding hydride olefin isomers, [(TpMe2)Ir(H){R(H)C? C(H)OAr}], have been carried out. Compounds of these types are obtained either by reaction of the unsaturated fragment [(TpMe2)Ir(C6H5)2] with o‐C6H4(OH)CH2R, or with the substituted anisoles 2,6‐Me2C6H3OMe, 2,4,6‐Me3C6H2OMe, and 4‐Br‐2,6‐Me2C6H2OMe. The reactions with the substituted anisoles require not only multiple C? H bond activation but also cleavage of the Me? OAr bond and the reversible formation of a C? C bond (as revealed by 13C labeling studies). Equilibria between the two tautomeric structures of these complexes were achieved by prolonged heating at temperatures between 100 and 140 °C, with interconversion of isomeric complexes requiring inversion of the metal configuration, as well as the expected migratory insertion and hydrogen‐elimination reactions. This proposal is supported by a detailed computational exploration of the mechanism at the quantum mechanics (QM) level in the real system. For all compounds investigated, the equilibria favor the alkylidene structure over the olefinic isomer by a factor of between approximately 1 and 25. Calculations demonstrate that the main reason for this preference is the strong Ir–carbene interactions in the carbene isomers, rather than steric destabilization of the olefinic tautomers. 相似文献
70.
Lee CY Canavan HE Gamble LJ Castner DG 《Langmuir : the ACS journal of surfaces and colloids》2005,21(11):5134-5141
The diversity of techniques used in the synthesis, treatment, and purification of the single-stranded DNA oligomers containing a thiol anchor group (SH-ssDNA) has led to a significant variation in the purity of commercially available SH-ssDNA. In this work, we use X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to study how the impurities present in commercially synthesized SH-ssDNA oligomers affected the structure of the resulting DNA films on Au. XPS results indicate that two of the purchased SH-ssDNA oligomers contain excess carbon and sulfur. The molecular fragmentation patterns obtained with ToF-SIMS were used to determine the identity of several contaminants in the DNA films, including poly(dimethylsiloxane) (PDMS), lipid molecules, and sulfur-containing molecules. In particular, the ToF-SIMS results determined that the excess sulfur detected by XPS was due to the presence of dithiothreitol, a reductant often used to cleave disulfide precursors. Furthermore, we found that the SH-ssDNA self-assembly process is affected by the presence of these contaminants. When relatively pure SH-ssDNA is used to prepare the DNA films, the P, N, O, and C atomic percentages were observed by XPS to increase over a 24-h time period. In contrast, surfaces prepared using SH-ssDNA containing higher levels of contaminants did not follow this trend. XPS result indicates that, after the initial SH-ssDNA adsorption, the remaining material incorporated into these films was due to contamination. 相似文献