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101.
Bringmann G Mühlbacher J Reichert M Dreyer M Kolz J Speicher A 《Journal of the American Chemical Society》2004,126(30):9283-9290
Cyclic bisbibenzyls, like isoplagiochins C (1) and D (2), are stereochemically intriguing molecules: Although not equipped with any of the traditional stereogenic elements that render molecules conformationally stable per se, they are sometimes isolated in an optically active form and are thus chiral at room temperature. The paper describes quantum chemical calculations, in particular investigations of the conformational space and molecular dynamics simulations, showing that the helicity is a property of the entire molecule, whose ring strain makes the molecule configurationally stable overall, with (formally) three stereogenic elements (two biaryl axes and one helical stilbene unit). Only one of the biaryl axes (the 'upper' one, joining C-12' and C-14) has a stable configuration, leading to a population of four interconverting diastereomers, yet without racemization at room temperature. On the basis of these conformational and dynamic calculations, the circular dichroism spectrum of isoplagiochin C (1) was calculated, leading to the first assignment of the absolute configuration of a cyclic bisbibenzyl. Accordingly, 1 has the P-configuration at the stereochemically stable biaryl axis and constitutes a mixture of diastereomers with respect to the other biaryl axis and the helical stilbene unit. From the temperature dependence of the racemization rates, an enantiomerization barrier of 101.6 kJ/mol was determined. Likewise, for the first time for cyclic bisbibenzyls, the enantiomeric ratio of this natural product was determined, by chromatography on a chiral phase with CD-coupling. Accordingly, 1 from Plagiochila deflexa is not enantiomerically pure, but occurs in a 85:15 ratio in favor of the enantiomer that has the P-configuration at the stereochemically stable axis. 相似文献
102.
Michael Karas Hanno Ehring Eckhard Nordhoff Bernd Stahl Kerstin Strupat Franz Hillenkamp Matthias Grehl Bernt Krebs 《Journal of mass spectrometry : JMS》1993,28(12):1476-1481
Selected benzoic acid derivatives and related substances were used as additives to 2,5-dihydroxybenzoic acid (2,5DHB) and the performance of the mixtures in matrix-assisted laser desorption/ionization mass spectrometry was investigated. Using benzoic acid derivatives substituted at position 2 and/or 5 or related substances as a co-matrix in the 1–10% range with 2,5DHB results in improved ion yields and signal-to-noise ratio of analyte molecules, especially for the high-mass range. The enhanced performance is prominent for 2-hydroxy-5-methoxybenzoic acid and exists for both proteins and oligosaccharides. It is suggested that the improvement is caused by a disorder in the 2,5DHB crystal lattice allowing ‘softer’ desorption. Charge transfer from matrix ions to additive molecules at the expense of analyte ionization gives a simple explanation for the deteriorating effects of some tested additives. 相似文献
103.
A fluorimetric assay for cortisol 总被引:2,自引:0,他引:2
Appel D Schmid RD Dragan CA Bureik M Urlacher VB 《Analytical and bioanalytical chemistry》2005,383(2):182-186
A simple, rapid and sensitive fluorimetric assay for the quantitative determination of cortisol is reported. The assay is
based on the formation of a fluorescent dye when cortisol is incubated with a mixture of sulfuric acid and acetic acid. The
fluorescence spectrum recorded for the resulting dye shows a maximum extinction at 475 nm and a maximum emission at 525 nm.
The solvent 2-methyl-4-pentanone was used for extraction and was found to act as a fluorescence amplifier. A limit of detection
of 2.7 μM was achieved, making it possible to forego solvent evaporation. The assay suffers minor interference from 11-deoxycortisol
which exhibits low fluorescence at λ
ex: 460 nm; λ
em: 505 nm. Typical standard deviations were below 4%. We validated the assay using a biotransformation with recombinant Schizosaccharomyces pombe which regioselectively hydroxylates 11-deoxycortisol to cortisol. The method described herein is suitable for preliminary
screening of microorganisms capable of steroid hydroxylation. 相似文献
104.
Pascal Vermeeren Michael T. Doppert F. Matthias Bickelhaupt Trevor A. Hamlin 《Chemical science》2021,12(12):4526
We have studied the activation of dihydrogen by metallylenes using relativistic density functional theory (DFT). Our detailed activation strain and Kohn–Sham molecular orbital analyses have quantified the physical factors behind the decreased reactivity of the metallylene on going down Group 14, from carbenes to stannylenes. Along this series, the reactivity decreases due to a worsening of the back-donation interaction between the filled lone-pair orbital of the metallylene and the σ*-orbital of H2, which, therefore, reduces the metallylene–substrate interaction and increases the reaction barrier. As the metallylene ligand is varied from nitrogen to phosphorus to arsenic a significant rate enhancement is observed for the activation of H2 due to (i) a reduced steric (Pauli) repulsion between the metallylene and the substrate; and (ii) less activation strain, as the metallylene becomes increasingly more predistorted. Using a rationally designed metallylene with an optimal Group 14 atom and ligand combination, we show that a number of small molecules (i.e. HCN, CO2, H2, NH3) may also be readily activated. For the first time, we show the ability of our H2 activated designer metallylenes to hydrogenate unsaturated hydrocarbons. The results presented herein will serve as a guide for the rational design of metallylenes toward the activation of small molecules and subsequent reactions.Quantum chemical analyses reveal how model metallylene catalysts activate H2. This is the first step towards the rational design of metallylenes for the activation of small molecules and subsequent reactions. 相似文献
105.
Matthias Freytag Frank T. Edelmann Ludger Ernst Peter G. Jones Reinhard Schmutzler 《无机化学与普通化学杂志》2004,630(3):377-383
The reaction of the symmetric diphosphene 2, 4, 6‐(CF3)3‐C6H2‐P=P‐C6H2‐2, 4, 6‐(CF3)3 4 with Ru3(CO)12 led to the 50‐electron Ru3P2 nido‐cluster Ru3(CO)9[μ‐P‐C6H2‐2, 4, 6‐(CF3)3]2 5 , which in solution at room temperature displays hindered rotation of the aromatic rings about the C(aryl)—P bonds. The structure of 5 was determined by X‐ray crystal structure analysis; its Ru3P2 centre forms a distorted square pyramid with one ruthenium atom at the apex. One of the two C6H2(CF3)3 groups is also appreciably distorted. Temperature‐dependent 19F NMR studies of the [A3M3X]2 spin system (A = M = CF3, X = 31P) of 5 indicated a rotational barrier ΔG≠ of 82.3 kJ mol‐1 at 141 °C. The same Ru3P2 core was obtained by the reaction of the unsymmetric diphosphene Mes*‐P=P‐Mes 11 with Ru3(CO)12; hindered rotation about the C(aryl)—P bonds was also observed, in this case. 相似文献
106.
107.
Progressive reaction networks as frequently arise in chemistry are naturally identifiable as "partially ordered sets" (or posets). Here the direction of the reaction identifies the partial ordering of the set of molecular species. The possibility that different properties are similarly ordered is a further natural consideration and is here investigated for a suite of over 30 properties for (methyl and chloro) substituted benzenes. Such a posetic correlation is favorably demonstrated for these substituted benzenes, and it is illustrated how suitable properties may be simply predicted in an interpolative parameter-free (albeit not model-free) fashion through the use of the reaction poset. Some numerical model-quality indicators are identified, and the simple approach is deemed quite reasonable. 相似文献
108.
Pre-resonance Raman spectra have been obtained for TCNQ and LiTCNQ in acetonitrile solution using an Ar+—Kr+ laser and a tunable rhodamine 6G dye laser. Using the theory of Albrecht and Hutley, we have calculated frequency factors for the intensity variations for several symmetric vibrational modes of each molecule. The observed spectra for TCNQ and LiTCNQ with violet, blue, and green excitation give evidence for B-type resonance enhancement due to vibronic mixing between at least two violet and ultraviolet transitions. The Raman spectra for LiTCNQ with yellow, orange, and red excitation show A-type enhancement due to a single electronic excitation in the near infrared. 相似文献
109.
Energized molecules are the essential actors in chemical transformations in solution. As the rearrangement of bonds requires a movement of nuclei, vibrational energy is often the driving force for a reaction. Vibrational energy can be redistributed within the "hot" molecule, or relaxation can occur when molecules interact. Both processes govern the rates, pathways, and quantum yields of chemical transformations in solution. Unfortunately, energy transfer and the breaking, formation, and rearrangement of bonds take place on ultrafast timescales. This Review highlights experimental approaches for the direct, ultrafast measurement of photoinduced femtochemistry and energy flow in solution. In the first part of this Review, we summarize recent experiments on intra- and intermolecular energy transfer. The second part discusses photoinduced decomposition of large organic peroxides, which are used as initiators in free radical polymerization. The mechanisms and timescales of their decarboxylation determine the initial steps of polymerization and the microstructure of the polymer product. 相似文献
110.