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951.
Estelle Durantie Dr. Christoph Bucher Prof. Dr. Ryan Gilmour 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(26):8208-8215
Validation of the 2‐fluoro substituent as an inert steering group to control chemical glycosylation is presented. A molecular editing study has revealed that the exceptional levels of diastereocontrol in glycosylation processes by using 2‐fluoro‐3,4,6‐tri‐O‐benzyl glucopyranosyl trichloroacetimidate (TCA) scaffolds are a consequence of the 2R,3S,4S stereotriad. This study has also revealed that epimerization at C4, results in a substantial enhancement in β‐selectivity (up to β/α 300:1). 相似文献
952.
Eva‐Maria Tanzer Dr. W. Bernd Schweizer Dr. Marc‐Olivier Ebert Prof. Dr. Ryan Gilmour 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(7):2006-2013
The C9 position of cinchona alkaloids functions as a molecular hinge, with internal rotations around the C8? C9 (τ1) and C9? C4′ (τ2) bonds giving rise to four low energy conformers ( 1 ; anti‐closed, anti‐open, syn‐closed, and syn‐open). By substituting the C9 carbinol centre by a configurationally defined fluorine substituent, a fluorine‐ammonium ion gauche effect (σC?H→σC?F*; Fδ????N+) encodes for two out of the four possible conformers ( 2 ). This constitutes a partial solution to the long‐standing problem of governing internal rotations in cinchonium‐based catalysts relying solely on a fluorine conformational effect. 相似文献
953.
Kieran A. Murray James E. Kennedy Brian McEvoy Olivier Vrain Damien Ryan Clement L. Higginbotham 《Radiation Physics and Chemistry》2012,81(8):962-966
Radiation is currently being exploited to modify polyethylene in order to improve properties for various applications such as hip replacements. This paper thoroughly examines the effects of high energy electron beam irradiation (10 MeV) on low density polyethylene (LDPE) material. ASTM (American Society for Testing and Materials) testing specimens were manufactured from LDPE and subjected to a broad range of doses ranging between 25 and 400 kGy at room temperature in an air atmosphere. Extensive characterisation techniques such as modulated differential scanning calorimetry (MDSC) and the Fourier transform infrared spectroscopy (FTIR) were conducted on the non-irradiated and irradiated samples. While considering the semicrystalline nature of LDPE during the MDSC experiment, the melting temperature (Tm) and the temperature crystallinity (Tc) were calculated. This revealed that the Tm and the Tc decreased in temperature as the irradiation dose increased. The FTIR analysis was implemented to evaluate the presence of polar species such as carbonyl groups and trans-vinylene double bond groups. The IR spectra illustrated that the concentration of characteristic bands for trans-vinylene bonds increased with increasing radiation dose indicating the formation of carbonyl bond groups. Furthermore, the results demonstrated an occurrence of oxidative degradation due to the formation of carbonyl groups at 1718 cm?1. 相似文献
954.
Whitney R. GritherJames Korang Jacob P. SauerMatthew P. Sherman Pamela L. VanegasMiao Zhang Ryan D. McCulla 《Journal of photochemistry and photobiology. A, Chemistry》2012,227(1):1-10
The redox congener of the important signaling agent nitric oxide (NO), nitroxyl or nitrosyl hydride (HNO) has also been demonstrated to induce distinct physiological effects. The aim of this study was to determine if benzohydroxamic acid, which was selected as a stable model compound of HNO donors, could be released by the o-nitrobenzyl photolabile protecting group (PPG) in a wavelength-dependent manner. It was expected that selective irradiation of the o-nitrobenzyl chromophore would favor the release of benzohydroxamic acid over undesired products associated with N-O bond cleavage. Quantum yields for the release of benzohydroxamic acid protected by the o-nitrobenzyl PPG increased at longer wavelengths, with a concomitant decrease in the yield of minor products. Through the use of triplet photosensitizers, triplet quenchers, computational methods, and the position of the nitro substituent, insights into the nature of the mechanism were suggested. 相似文献
955.
Andrew E. Howery Shelley Elvington Sherwin J. Abraham Kee-Hyun Choi Sierra Dworschak-Simpson Sabrina Phillips Christopher M. Ryan R. Lea Sanford Jonas Almqvist Kevin Tran Thomas A. Chew Ulrich Zachariae Olaf S. Andersen Julian Whitelegge Kimberly Matulef Justin Du Bois Merritt C. Maduke 《Chemistry & biology》2012,19(11):1460-1470
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956.
Freneil B. Jariwala Ryan E. Wood Upul Nishshanka Athula B. Attygalle 《Journal of mass spectrometry : JMS》2012,47(4):529-538
In the negative‐ion collision‐induced dissociation mass spectra of most organic sulfonates, the base peak is observed at m/z 80 for the sulfur trioxide radical anion (SO3–·). In contrast, the product‐ion spectra of a few sulfonates, such as cysteic acid, aminomethanesulfonate, and 2‐phenylethanesulfonate, show the base peak at m/z 81 for the bisulfite anion (HSO3–). An investigation with an extensive variety of sulfonates revealed that the presence of a hydrogen atom at the β‐position relative to the sulfur atom is a prerequisite for the formation of the bisulfite anion. The formation of HSO3– is highly favored when the atom at the β‐position is nitrogen, or the leaving neutral species is a highly conjugated molecule such as styrene or acrylic acid. Deuterium‐exchange experiments with aminomethanesulfonate demonstrated that the hydrogen for HSO3– formation is transferred from the β‐position. The presence of a peak at m/z 80 in the spectrum of 2‐sulfoacetic acid, in contrast to a peak at m/z 81 in that of 3‐sulfopropanoic acid, corroborated the proposed hydrogen transfer mechanism. For diacidic compounds, such as 4‐sulfobutanoic acid and cysteic acid, the m/z 81 ion can be formed by an alternative mechanism, in which the negative charge of the carboxylate moiety attacks the α‐carbon relative to the sulfur atom. Experiments conducted with deuterium‐exchanged and deuterium‐labeled analogs of sulfocarboxylic acids demonstrated that the formation of the bisulfite anion resulted either from a hydrogen transfer from the β‐carbon, or from a direct attack by the carboxylate moiety on the α‐carbon. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
957.
Christopher M. Ryan Craig M. Marianno William S. Charlton William D. James 《Journal of Radioanalytical and Nuclear Chemistry》2012,291(1):267-272
The dissolution of the Soviet Union coupled with the growing sophistication of international terror organizations has brought
about a desire to ensure that a sound infrastructure exists to interdict smuggled nuclear material prior to leaving its country
of origin. To combat the threat of nuclear trafficking, radiation portal monitors (RPMs) are deployed around the world to
intercept illicit material while in transit by passively detecting gamma and neutron radiation. Portal monitors in some locations
have reported abnormally high background counts. The higher background data has been attributed, in part, to the naturally
occurring radioactive materials (NORM) in the concrete surrounding the portal monitors. Higher background increases the minimum
detectable activity (MDA) and can ultimately lead to more material passing through the RPMs undetected. This work employed
two different neutron activation analysis (NAA) methods for the purpose of developing a process to characterize the concrete
surrounding the RPMs. Thermal neutron instrumental NAA (INAA) and fast NAA (FNAA) were conducted on six samples from three
different composition concrete slabs. Comparator standards and quality control materials were used to help ensure that the
methods were both precise and accurate. The combination of INAA and FNAA accounted for 84–100% of the total elemental composition
of the samples. Knowing the composition of the concrete will allow RPM customers to choose suitable materials prior to installation,
thereby increasing the ability of the monitors to detect radiological and nuclear materials. 相似文献
958.
The N-heterocyclic carbene catalyzed (4 + 2) cycloaddition between α,β-unsaturated acid fluorides and TMS dienol ethers provides cyclohexene fused β-lactone intermediates stable below -20 °C. These can be intercepted reductively or with organolithium reagents to produce diastereomerically pure cyclohexenes (>20:1 dr) with up to four contiguous stereocenters. The mechanism has been investigated using theoretical calculations and by examining secondary kinetic isotope effects. Together these studies implicate the formation of a diastereomerically pure β-lactone intermediate by a stepwise (4 + 2) cycloaddition involving Michael addition, aldol cyclization, and lactonization. 相似文献
959.
960.