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991.
Atropisomeric N-methyl-N,N′-diaryl ureas may be obtained in enantiomerically enriched form by oxidative kinetic resolution of their sulfide derivatives. The atropisomeric sulfides may be obtained in up to 97:3 er and display high stability to racemisation (half-lives at 25 °C of up to 500 years). Unlike related fully alkylated ureas, the product sulfoxides exhibit relatively weak thermodynamic conformational selectivity. 相似文献
992.
Effective exposure temperatures (Teff) in Arizona were calculated from hourly or 10-min parsed irradiation data along with ambient, black panel, and sample temperatures. The Teff represents a constant temperature that creates the same amount of photodegradation as the naturally varying temperature and provides a benchmark temperature for making lifetime predictions from accelerated laboratory exposures. The annual ambient and black panel Teff at a Wittmann, Arizona site were 30 °C and 42 °C, respectively, assuming that the photodegradation has an activation energy (Ea) of 21 kJ/mol (5 kcal/mol). Teff was only weakly dependent on Ea over the range of 10-40 kJ/mol (3-10 kcal/mol). Samples exposed as van sunroofs were found to have Teff that were offset from the black panel temperatures by a constant amount for the entire year. Thus, measurements of sample and black panel need to be made for only a few weeks to determine the offset and give the annual sample Teff if the annual black panel Teff is known. Light-colored samples probably are better compared with the ambient temperatures. Sample temperatures in xenon arc exposures usually are higher than the outdoor Teff, so Arrhenius temperature corrections need to be carried out to relate accelerated to outdoor exposures. Temperatures in xenon arc exposure tests often correspond more closely to maximum temperatures that samples might encounter for only a few hours per year. 相似文献
993.
Mohammed K. Abdel-Hamid Jonathan Coates Celia Miländer Brian W. Skelton Anthony C. Willis 《Tetrahedron letters》2009,50(50):6947-8550
The allylation of indigo results in the one-step synthesis of two unique complex heterocyclic systems: a spiroindoline-pyridoindolone arising from the addition of three allyl moieties and a fused pyridoindolo-azepinoindolone generated from the addition and subsequent cyclisation of two allyl moieties. The structures of these novel heterocycles are assigned unambiguously using extensive NMR experiments and by X-ray crystallographic analysis. The distribution of the products is influenced by the use of thermal versus microwave heating. 相似文献
994.
Jinhua J. Song Zhulin Tan Jonathan T. Reeves Daniel R. Fandrick Heewon Lee Nathan K. Yee Chris H. Senanayake 《Tetrahedron letters》2009,50(27):3952-1877
Treatment of unprotected 5-amino-4-methylpyrimidine (2) with n-BuLi gave dianion 3. Direct condensation of the dianion with various carboxylic acid derivatives furnished a range of 2-substituted-4,6-diazaindoles in good yields in one step without the need for protecting groups or oxidation-state adjustment. 相似文献
995.
Jonathan Clayden Stephen J.M. Rowbottom Warren J. Ebenezer 《Tetrahedron letters》2009,50(27):3923-1226
Addition of cyanuric chloride 2 (2,4,6-trichlorotriazine) to 4,6-diaminobenzene-1,3-disulfonic acid 1 gives a bis-triazine 3 which may be cyclised with diaminoarenes to yield water-soluble azacalix[4]arenes 5a-d. Alternatively, double substitution of chloride from the bis-triazine 3 yields new bis-triazine derivatives which may likewise be cyclised to functionalised azacalixarenes bearing functionalised side chains. 相似文献
996.
Álvaro Álvarez Ramón Macías Dr. Jonathan Bould Dr. Carmen Cunchillos Fernando J. Lahoz Prof. Dr. Luis A. Oro Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(22):5428-5431
A sensitive cluster : The labile rhodathiaborane [(PPh3)2(H)‐nido‐RhSB9H9(NC5H5)] combines the redox and coordinative flexibility of the {(PPh3)2(H)Rh} fragment with the capability of the 11‐vertex rhodathiaborane cluster to undergo oxidative nido‐to‐closo transformations induced by coordination of alkynes to the metal centre, which leads to hydrogenation of the triple bond, dehydrogenation of the cluster and oxidative addition of sp C? H bonds.
997.
Victoria E. Campbell Xavier de Hatten Dr. Nicolas Delsuc Dr. Brice Kauffmann Dr. Ivan Huc Dr. Jonathan R. Nitschke Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(25):6138-6142
Systemic change : A system of transformations between helical structures was observed to be governed by interactions mediated by the electronic effects of substituents, entropic effects, the conformational preferences of organic building blocks, and the coordinative preferences of the metal ion. All of these effects were important, but all must be considered together to allow the prediction of the product observed (see scheme).
998.
Michael T. Crimmins Prof. J. Michael Ellis Dr. Kyle A. Emmitte Dr. Pamela A. Haile Dr. Patrick J. McDougall Dr. Jonathan D. Parrish Dr. J. Lucas Zuccarello Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(36):9223-9234
Brevetoxin A is a decacyclic ladder toxin that possesses 5‐, 6‐, 7‐, 8‐, and 9‐membered oxacycles, as well as 22 tetrahedral stereocenters. Herein, we describe a unified approach to the B, E, G, and J rings based upon a ring‐closing metathesis strategy from the corresponding dienes. The enolate technologies developed in our laboratory allowed access to the precursor acyclic dienes for the B, E, and G medium‐ring ethers. The strategies developed for the syntheses of these four monocycles ultimately provided multigram quantities of each of the rings, supporting our efforts toward the completion of a convergent synthesis of brevetoxin A. 相似文献
999.
Kent A. Nielsen Dr. Eric Levillain Dr. Vincent M. Lynch Dr. Jonathan L. Sessler Prof. Jan O. Jeppesen Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(2):506-516
Four tetrathiafulvalene (TTF)‐annulated porphyrins 1 – 4 were synthesized and characterized. All contain a tetraphenylporphyrin (TPP) core onto which four, two, or one TTF subunits were annulated. Absorption and fluorescence spectroscopic studies together with electrochemical investigations reveal that interactions between the porphyrin system and the annulated TTF units take place in solution. The annulation of one or more TTF units to the porphyrin core has a profound effect on the reduction potentials associated with this latter framework, with positive shifts in the range of 0.105 to 0.355 V and 0.200 to 0.370 V for the first and second reduction potential, respectively, compared to the corresponding processes in the model compound TPP, 18 . The redox potentials for the first oxidation of the TTF units are considerably shifted in 4 (ΔEox1=+0.285 V) and 2 (ΔEox1=?0.140 V), whereas for 1 and 3 these potentials remain within the region expected for a normal TTF unit. Considerable changes in the second oxidation potential associated with the TTF subunits were seen for 2 (ΔEox1=?0.085) and 3 (ΔEox1=?0.175). The emission spectra of 1 – 4 revealed that the porphyrin fluorescence is almost quenched in the neutral state of the TTF‐annulated porphyrins, a finding that is consistent with substantial electron transfer taking place from the TTF subunits to the porphyrin core. Oxidation of the TTF unit(s) (TTF→TTF.+) present in 1 – 4 leads to the emission intensity being restored. 相似文献
1000.
Jonathan R.E. Thabano Michael C. BreadmoreJoseph P. Hutchinson Cameron JohnsPaul R. Haddad 《Journal of chromatography. A》2009,1216(25):4933-4940
Polymeric ion-exchange monoliths typically exhibit low capacities due to the limited surface area on the globules of the monoliths. The ion-exchange binding of protonated weakly basic analytes on deprotonated carboxylate sites on methacrylate polymer monoliths has been increased by templating the monoliths with silica nanoparticles. The templating method is achieved by adding the nanoparticles as a suspension to the polymerisation mixture. After polymerisation, the nanoparticles are removed by washing the monolith with strong base. Monolithic columns prepared using this procedure have exhibited a 33-fold increase in ion-exchange capacity when compared to untemplated monoliths prepared and treated under similar conditions. The templating procedure does not alter the macroporous properties of the polymer monolith, confirmed through scanning electron microscopy and BET surface area analysis, but provides increased capacity predominantly through the re-orientation of more carboxylic acid groups. The resulting increase in ion-exchange capacity has proven to be useful for the preconcentration and separation of neurotransmitters by in-line solid-phase extraction–capillary electrophoresis. The increased capacity of the templated monolith allowed the injection time to be increased 10 times over that of an untemplated monolith, allowing 10 times more sample to be injected with the efficiencies and recoveries remaining unaffected. The enhancement in sensitivity for the test mixture of neurotransmitter (dopamine, norepinephrine and metanephrine) ranged 1500–1900 compared to a normal hydrodynamic injection in capillary electrophoresis. Efficiencies obtained for the neurotransmitters were 100 000–260 000 plates, typical of those obtained in capillary zone electrophoresis. The applicability of the increased capacity silica nano-templated polymer monolith was demonstrated by analysing trace levels of caffeine in biological, food and environmental samples. 相似文献