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191.
The first examples of manipulating stereocontrol solely by reaction topography in radical cyclizations starting from acyclic precursors are reported. The kinetic model for acyclic compound stereoselection is verified experimentally by conducting a series of radical cyclizations of 1,3-dihalo-2-(1-phenyl-3-butynyl)propanes with triphenyltin hydride and measuring the ratios of the products. Monohalide intermediates are observed for the first time, and evidence that bromide- and iodide-substituted radicals have different cyclization rate constants is provided. 相似文献
192.
Dennis Stanton 《The Ramanujan Journal》2009,19(1):107-113
Two q-analogues of Euler’s theorem on integer partitions with odd or distinct parts are given. A q-lecture hall theorem is given.
Supported by NSF grant DMS-0503660. 相似文献
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[structure: see text] A fluorescent deoxyuridine analogue is sensitive to the polarity of its environment and exhibits a distinct emission profile in single- vs double-stranded oligonucleotides. Emission-monitored denaturation curves of internally modified dU(phen) duplexes are characteristic of the base opposite dU(phen) and distinguish between perfect and mismatched complementary oligonucleotides. 相似文献
197.
Shestopalov AM Emeliyanova YM Shestopalov AA Rodinovskaya LA Niazimbetova ZI Evans DH 《Organic letters》2002,4(3):423-425
Three-component condensation of 4-piperidinones (7), 5-pyrazolones (8), and malononitrile (4) proceeds chemically and electrochemically and is a convenient one-step means of synthesis of substituted 6-amino-5-cyanospiro-4-(piperidine-4')-2H,4H-dihydropyrazolo[3,4-b]pyrans (12). The electrochemical reactions proceed under milder conditions and with yields 12-15% greater than those of the reactions catalyzed by chemical bases. 相似文献
198.
Petersson EJ Choi A Dahan DS Lester HA Dougherty DA 《Journal of the American Chemical Society》2002,124(43):12662-12663
A series of tethered quaternary ammonium derivatives of Tyr have been incorporated into the binding site of the nicotinic acetylcholine receptor (nAChR) using the in vivo nonsense suppression method, producing constitutively active (self-gating) receptors. We have incorporated primary, secondary, and tertiary amine tethered agonists to give receptors whose constitutive activity can be modulated by pH. Lowering the pH protonates the tethered amine, giving it a positive charge and allowing it to reversibly activate the receptor. Tertiary and secondary tethered amines, TyrO3T and TyrO3S, have been successfully incorporated at alpha149 in the nAChR. Constitutive currents at pH 5.5 are 6 times those at pH 9.0. The pKa of TyrO3T in the binding site appears to be 6 or lower, differing substantially from its pKa in solution ( approximately 9.3). This local pKa perturbation has substantial implications for pharmacological research on the nAChR: of the tertiary agonists considered, noracetylcholine experiences this pKa perturbation, while nicotine does not. 相似文献
199.
De Vita R Anghinolfi M Burkert VD Dodge GE Minehart R Taiuti M Weller H Adams G Amaryan MJ Anciant E Armstrong DS Asavapibhop B Asryan G Audit G Auger T Avakian H Bagdasaryan H Ball JP Barrow S Battaglieri M Beard K Bektasoglu M Bianchi N Biselli AS Boiarinov S Bonner BE Bosted P Bouchigny S Branford D Brooks WK Bueltmann S Calarco JR Capitani GP Carman DS Carnahan B Cazes A Ciciani L Cole PL Coleman A Connelly J Cords D Corvisiero P Crabb D Crannell H Cummings JP De Sanctis E Degtyarenko PV 《Physical review letters》2002,88(8):082001
The double spin asymmetry in the (-->)e(-->)p --> e(prime)pi(+)n reaction has been measured for the first time in the resonance region for four-momentum transfer Q2 = 0.35-1.5 GeV(2). Data were taken at Jefferson Lab with the CLAS detector using a 2.6 GeV polarized electron beam incident on a polarized solid NH3 target. Comparison with predictions of phenomenological models shows strong sensitivity to resonance contributions. Helicity-1/2 transitions are found to be dominant in the second and third resonance regions. The measured asymmetry is consistent with a faster rise with Q(2) of the helicity asymmetry A1 for the F(15)(1680) resonance than expected from the analysis of the unpolarized data. 相似文献
200.
Koga T Seo YS Zhang Y Shin K Kusano K Nishikawa K Rafailovich MH Sokolov JC Chu B Peiffer D Occhiogrosso R Satija SK 《Physical review letters》2002,89(12):125506
We report an anomalous swelling of polymer thin films in carbon dioxide (CO(2)) which is associated (in both locus and form) with the density fluctuation ridge that forms along the extension of the coexistence curve of gas and liquid in the P-T phase diagram. Neutron reflectivity results showed that CO(2) could be sorbed to a large extent ( approximately 60%) in thin polymer films even when the bulk miscibility of the polymer with CO(2) is very poor. The anomalous swelling is found to scale with the polymer radius of gyration (R(g)) and extends to a distance approximately 10 R(g). 相似文献