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41.
Pentacarbonyltetrafluoroboratomanganese(I), (OC)5MnFBF3, reacts with 2-butyne at 15°C in dichloromethane to give tetracarbonyl(η4-tetramethylcyclobutadiene)manganese(I) tetrafluoroborate, [(OC)4Mn(C4Me4)]+ BF4? 相似文献
42.
[reaction: see text] A highly stereoselective synthesis of (-)-erythrodiene starting from 4-isopropylcyclohexanone is described. The key reactions are an asymmetric methoxycarbonylation of the starting ketone and a highly diastereoselective radical cascade involving addition of a phenylthiyl radical to a terminal alkyne followed by a 1,5-hydrogen transfer and a 5-exo-cyclization. 相似文献
43.
Regulus P Spessotto S Gateau M Cadet J Favier A Ravanat JL 《Rapid communications in mass spectrometry : RCM》2004,18(19):2223-2228
High-performance liquid chromatography coupled to electrospray ionization tandem mass spectrometry (HPLC/ESI-MS/MS) has been used to search for the formation of as yet unidentified radiation-induced DNA lesions. For that purpose, the characteristic fragmentation of most of 2'-deoxyribonucleosides that corresponds to the loss of the 2-deoxyribose moiety (loss of 116 mass units) has been utilized to specifically detect modified nucleosides. Aerated aqueous solutions of DNA were exposed to ionizing radiation, and subsequently DNA was digested to nucleosides with a cocktail of endo- and exonucleases. HPLC/ESI-MS/MS analysis of the resulting 2'-deoxyribonucleoside mixture allowed us to detect four novel DNA modifications. In a subsequent step, the sensitivity of the tandem mass spectrometer was used to search for the formation of the newly detected lesions in the DNA of gamma-irradiated cells. Thus, one of the four newly detected lesions was found to be significantly generated in cellular DNA upon exposure to ionizing radiation. In addition, the latter lesion was also shown to be present in untreated cells, indicating that the modified nucleoside could be formed endogenously. 相似文献
44.
Monperrus M Rodriguez Gonzalez P Amouroux D Garcia Alonso JI Donard OF 《Analytical and bioanalytical chemistry》2008,390(2):655-666
A new double-spiking approach, based on a multiple-spiking numerical methodology, has been developed and applied for the accurate
quantification of inorganic mercury (IHg) and methylmercury (MeHg) by GC–ICPMS in different environmental matrices such as
water, sediments and a wide range of biological tissues. For this purpose, two enriched mercury species (201MeHg and 199IHg) were added to the samples before sample preparation in order to quantify the extents of the methylation and demethylation
processes, and thereby correct the final species concentrations. A critical evaluation of the applicability of this methodology
was performed for each type of matrix, highlighting its main advantages and limitations when correcting for the conversion
reactions of the species throughout the whole sample preparation procedure. The double-spike isotope dilution (DSIDA) methodology
was evaluated by comparing it with conventional species specific isotope dilution (IDA) when analysing both certified reference
materials and environmental samples (water, biotissues and sediment). The results demonstrate that this methodology is able
to provide both accurate and precise results for IHg and MeHg when their relative concentrations are not too different (ratio
MeHg/IHg > 0.05), a condition that holds for most natural waters and biotissues. Significant limitations on the accurate and
precise determination of the demethylation factor are however observed, especially for real sediment samples in which the
relative concentrations of the species are substantially different (ratio MeHg/IHg < 0.05). A determination of the sources
of uncertainty in the methylation/demethylation factors has demonstrated that the accurate and precise measurement of the
isotope ratios in the species involved in the transformations is crucial when quantifying the extents of these reactions.
Although the double-spike methodology is established as a reference approach that permits the correction of most analytical
biases and the accurate quantification of Hg species, some limitations have been identified for the first time in this work. 相似文献
45.
Mathilde Lachia François Richard Raphael Bigler Amandine Kolleth-Krieger Michael Dieckmann Alexandre Lumbroso Ulfet Karadeniz Saron Catak Alain De Mesmaeker 《Tetrahedron letters》2018,59(20):1896-1901
γ-Lactams are important building blocks for the synthesis of biologically active molecules and can easily be accessed via Beckmann rearrangement of cyclobutanones. However, Beckmann fragmentation is often a competing reaction for these strained ketones. We found that performing the Beckmann rearrangement with Tamura’s reagent in the presence of aqueous HCl suppresses the undesired fragmentation reaction. This improved procedure was applied to a broad scope of substrates affording monocyclic, bicyclic, tricyclic or spirocyclic lactams.Our experimental results and DFT calculations suggest that the mechanism of the rearrangement probably involves a tetrahedral intermediate and doesn’t proceed via oxime fragmentation as in a classical Beckmann rearrangement. 相似文献
46.
47.
Aliev AE Mia ZA Busson MJ Fitzmaurice RJ Caddick S 《The Journal of organic chemistry》2012,77(14):6290-6295
The potential of the approach combining nuclear magnetic resonance (NMR) spectroscopy, relaxed grid search (RGS), molecular dynamics (MD) simulations, and quantum mechanical (QM) calculations for the determination of diastereomer configurations is demonstrated using four diastereomers of a trisubstituted epoxide. Since the change in configuration of the chiral center is expected to change the distribution of conformer populations (including those of side-chain rotamers), changes in NMR parameters [chemical shifts, J couplings, and nuclear Overhauser effects (NOEs)] are expected. The method therefore relies on (1) identification of possible conformations in each diastereomer using relaxed grid search analysis and MD simulations; (2) geometry optimizations of conformers selected from step (1), followed by calculations of their relative energies (populations) using QM methods; (3) calculations of averaged NMR parameters using QM methods; (4) matching calculated and experimental values of NMR parameters of diastereomers. The diastereomer configurations are considered resolved, if three NMR parameters different in nature, chemical shifts, J couplings, and NOEs, are in agreement. A further advantage of this method is that full structural and dynamics characterization of each of the diastereomers is achieved based on the joint analysis of experimental and computational data. 相似文献
48.
We study permutations that are sorted by operators of the form S ° α ° S, where S is the usual stack sorting operator introduced by Knuth and α is any D 8-symmetry obtained by combining the classical reverse, complement, and inverse operations. Such permutations can be characterized by excluded (generalized) patterns. Some conjectures about the enumeration of these permutations, refined with numerous classical statistics, have been proposed by Claesson, Dukes, and Steingrímsson. We prove these conjectures, and enrich one of them with a few more statistics. The proofs mostly rely on generating trees techniques, and on a recent bijection of Giraudo between Baxter and twisted Baxter permutations. 相似文献
49.
50.
Paul Berceau Rémy Battesti Mathilde Fouché Paul Frings Marc Nardone Oliver Portugall Geert L. J. A. Rikken Carlo Rizzo 《Hyperfine Interactions》2012,207(1-3):7-12
From a series of M?ssbauer measurements performed on a single crystal of FePSe3, we have observed the effect of electromagnetically induced transparency (EIT) in a second nuclear system, after a first experiment using the mineral siderite FeCO3. When the condition of nuclear level crossing was fulfilled, an obvious deficit in absorption was observed at the point of level crossing. This reduced absorption cannot be ascribed to saturation effects because of the specific geometry of the measurement. We conclude that the observed deficit in absorption is due to coherence and interference effects. 相似文献