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101.
The past decade has seen the explosive emergence of "dihydrogen bonds", interactions between the electrons of M-H sigma-bonds, where M is less electronegative than H (M = Al, B, Ga, Ir, Mo, Mn, Os, Re, Ru, W) and traditional proton donors. But 70 years ago, such an interaction was proposed by Zachariasen and Mooney [J. Chem. Phys. 1934, 2, 34-37] on the basis of their single-crystal X-ray diffraction structure (heavy atoms positions only) of ammonium hypophosphite (NH(4)H(2)PO(2)). We redetermined this structure at high resolution with a focus on the hydrogen atoms, using a modern diffractometer equipped with a CCD detector. Molecular orbital calculations were performed to investigate the charge density and the bond polarity of the P-H bonds and to assess their potential for participation in dihydrogen bonds. Neither the theory nor the X-ray structure supports the original claim of H...H interactions in this salt. 相似文献
102.
103.
Graham E. O'Mahony Kevin S. Eccles Robin E. Morrison Alan Ford Simon E. Lawrence Anita R. Maguire 《Tetrahedron》2013
Steric and electronic effects in the copper-catalysed asymmetric oxidation of aryl benzyl, aryl alkyl and alkyl benzyl sulfides have been investigated. The presence of an aryl group directly attached to the sulfur is essential to afford sulfoxides with high enantioselectivities, with up to 97% ee for 2-naphthyl benzyl sulfoxide, the highest enantioselectivity achieved to date for copper-catalysed asymmetric sulfoxidation. In contrast, the benzyl substituent can be replaced by sterically comparable groups with no effect on enantioselectivity. Copper-mediated oxidation of substituted aryl benzyl sulfides display modest steric and electronic effects resulting in comparable or lower enantioselectivities to those obtained with the unsubstituted benzyl phenyl sulfide. 相似文献
104.
Dr. Sylvain Achelle Dr. Jean‐Pierre Malval Dr. Stéphane Aloïse Dr. Alberto Barsella Dr. Arnaud Spangenberg Dr. Loic Mager Dr. Huriye Akdas‐Kilig Dr. Jean‐Luc Fillaut Prof. Bertrand Caro Prof. Françoise Robin‐le Guen 《Chemphyschem》2013,14(12):2725-2736
The nonlinear properties and the photophysical behavior of two π‐conjugated chromophores that incorporate an electron‐deficient pyrimidine core (A) and γ‐methylenepyrans as terminal donor (D) groups have been thoroughly investigated. Both dipolar and quadrupolar branching strategies are explored and rationalized on the basis of the Frenkel exciton model. Even though a cooperative effect is clearly observed if the dimensionality is increased, the nonlinear optical (NLO) response of this series is moderate if one considers the nature of the D/A couple and the size of the chromophores (as measured by the number of π electrons). This effect was attributed to a disruption in the electronic conjugation within the dyes’ scaffold for which the geometry deviates from planarity owing to a noticeable twisting of the pyranylidene end‐groups. This latter structural parameter also has a strong influence on the excited‐state dynamics, which leads to a very efficient fluorescence quenching. 相似文献
105.
Adult tapeworms of the genus Echinococcus (family Taeniidae) occur in the small intestines of carnivorous definitive hosts and are transmitted to particular intermediate mammalian hosts, in which they develop as fluid‐filled larvae (cysts) in internal organs (usually lung and liver), causing the disease echinococcosis. Echinococcus species are of major medical importance and also cause losses to the meat and livestock industries, mainly due to the condemnation of infected offal. Decisions regarding the treatment and control of echinococcosis rely on the accurate identification of species and population variants (strains). Conventional, phenetic methods for specific identification have some significant limitations. Despite advances in the development of molecular tools, there has been limited application of mutation scanning methods to species of Echinococcus. Here, we briefly review key genetic markers used for the identification of Echinococcus species and techniques for the analysis of genetic variation within and among populations, and the diagnosis of echinococcosis. We also discuss the benefits of utilizing mutation scanning approaches to elucidate the population genetics and epidemiology of Echinococcus species. These benefits are likely to become more evident following the complete characterization of the genomes of E. granulosus and E. multilocularis. 相似文献
106.
Harshanie Abeywardena Aaron R. Jex Simon M. Firestone Sandra McPhee Nicole Driessen Anson V. Koehler Shane R. Haydon Georg von Samson‐Himmelstjerna Melita A. Stevens Robin B. Gasser 《Electrophoresis》2013,34(15):2259-2267
In the present study, we undertook a molecular epidemiological survey of Cryptosporidium and Giardia in calves on three dairy and two beef farms within an open drinking water catchment area (Melbourne, Australia). Faecal samples (n = 474) were collected from calves at two time points (5 months apart) and tested using a PCR‐based mutation scanning‐targeted sequencing phylogenetic approach, employing regions within the genes of small subunit (SSU) of ribosomal RNA (designated partial SSU), 60 kDa glycoprotein (pgp60) and triose phosphate isomerase (ptpi) as genetic markers. Using partial SSU, the C. bovis, C. parvum, C. ryanae and a new genotype of Cryptosporidium were characterised from totals of 74 (15.6%), 35 (7.3%), 37 (7.8%) and 9 (1.9%) samples, respectively. Using pgp60, C. parvum genotype IIa subgenotype A18G3R1 was detected in 29 samples. Using ptpi, G. duodenalis assemblages A and E were detected in totals of 10 (2.1%) and 130 (27.4%) samples, respectively. The present study showed that a considerable proportion of dairy and beef calves in this open water catchment region excreted Cryptosporidium (i.e. subgenotype IIaA18G3R1) and Giardia (e.g. assemblage A) that are consistent with those infecting humans, inferring that they are of zoonotic importance. Future work should focus on exploring, in a temporal and spatial way, whether these parasites occur in the environment and water of the catchment reservoir. 相似文献
107.
108.
In connection with our synthetic approach to pentacyclic triterpenes,1 we have required substantial amounts of 2,5,5-trimethyl-1,3-cyclohexanedione (methyldimedone, 2), which is available by methylation of dimedone (1).2 However, methylation may also afford the 2,2-dimethyl product 3, the methyl ether 4, and the C, O-dimethyl product 5. 相似文献
109.
In connection with research at present being pursued in these laboratories we required quantities of thioacetal (1) for study. Reaction of Wieland-Miescher ketone (2) with one equivalent of 1,2-ethanedithiol gave only (3) resulting from reaction at the α,β-unsaturated carbonyl group. The structure for (3) was deduced primarily from C n. m. r. which showed a saturated carbonyl absorption (δ212.6)1. The preferential reaction of 1,2-ethanedithiol with α,β-unsaturated ketone 相似文献
110.
Dr. Sheng Cao Robin Christiansen Prof. Xiaohua Peng 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(27):9050-9058
A series of arylboronic esters containing different aromatic substituents and various benzylic leaving groups (Br or N+Me3Br?) have been synthesized. The substituent effects on their reactivity with H2O2 and formation of quinone methide (QM) have been investigated. NMR spectroscopy and ethyl vinyl ether (EVE) trapping experiments were used to determine the reaction mechanism and QM formation, respectively. QMs were not generated during oxidative cleavage of the boronic esters but by subsequent transformation of the phenol products under physiological conditions. The oxidative deboronation is facilitated by electron‐withdrawing substituents, such as aromatic F, NO2, or benzylic N+Me3Br?, whereas electron‐donating substituents or a better leaving group favor QM generation. Compounds containing an aromatic CH3 or OMe group, or a good leaving group (Br), efficiently generate QMs under physiological conditions. Finally, a quantitative relationship between the structure and activity has been established for the arylboronic esters by using a Hammett plot. The reactivity of the arylboronic acids/esters and the inhibition or facilitation of QM formation can now be predictably adjusted. This adjustment is important as some applications may benefit and others may be limited by QM generation. 相似文献