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61.
Ibrahem I Rios R Vesely J Zhao GL Córdova A 《Chemical communications (Cambridge, England)》2007,(8):849-851
The highly chemo- and enantioselective organocatalytic tandem reaction between N-protected hydroxyl amines and alpha,beta-unsaturated aldehydes is presented; the reaction provides access to 5-hydroxyisoxazolidines and beta-amino acids in high yields and with 90-99% ee. 相似文献
62.
MJ Singh DO Kataria N Madhavan P Sugathan JJ Das DK Awasthi AK Sinha R Shanker 《Pramana》1999,53(4):743-764
A projectile ion-recoil ion coincidence technique has been employed to study the multiple ionization and the charge transfer
processes in collisions of 60–120 MeV Si
q+ (q = 4−14) ions with neutral argon atoms. The relative contribution of different ionization channels, namely; direct ionization,
electron capture and electron loss leading to the production of slow moving multiply charged argon recoil ions have been investigated.
The data reported on the present collision system result from a direct measurement in the considered impact energy for the
first time. The total ionization cross-sections for the recoil ions are shown to scale as q
1.7/E
p
0.5
, where E
p is the energy in MeV of the projectile and q its charge state. The recoil fractions for the cases of total- and direct ionizations are found to decrease with increasing
recoil charge state j. The total ionization fractions of the recoils are seen to depend on q and to show the presence of a ‘shell-effect’ of the target. Further, the fractions are found to vary as 1/j
2 upto j = 8+. The average recoil charge state 〈j〉 increases slowly with q and with the number of lost or captured electrons from or into the projectile respectively. The projectile charge changing
cross-sections σ
qq′ are found to decrease with increasing q for loss ionization and to increase with q for direct-and capture ionization processes respectively. The physics behind various scaling rules that are found to follow
our data for different ionization processes is reviewed and discussed. 相似文献
63.
KM Varier AM Vinodkumar NVSV Prasad PV Madhusudhana Rao DL Sastry Lagy T Baby MC Radhakrishna NG Puttaswamy JJ Das P Sugathan N Madhavan AK Sinha DO Kataria 《Pramana》1999,53(3):529-533
Large enhancements have been observed in the sub-barrier fusion cross sections for Ti+Ni systems in our previous studies.
Coupled channel calculations incorporating couplings to 2+ and 3− states failed to explain these enhancements completely. A possibilty of transfer channels contributing to the residual enhancements
had been suggested. In order to investigate the role of relevant transfer channels, measurements of one- and two-nucleon transfer
were carried out for 46,48Ti+61Ni systems. The present paper gives the results of these studies. 相似文献
64.
Meriem Belhachemi Rachel V.R.A. Rios Fatima Addoun Joaquín Silvestre-Albero Antonio Sepúlveda-Escribano Francisco Rodríguez-Reinoso 《Journal of Analytical and Applied Pyrolysis》2009,86(1):168-172
Two series of activated carbons have been prepared from date pits; series C, using carbon dioxide as activating agent, and series S, prepared by activation with steam under the same experimental conditions. The obtained samples were oxidized with nitric acid in order to introduce more oxygen surface groups. The surface area and porosity of the parent and oxidized activated carbons were studied by N2 adsorption at 77 K and CO2 adsorption at 273 K. The oxygen surface complexes were characterized by temperature-programmed decomposition (TPD). The results show that carbon dioxide and steam activations produce microporous carbons with an increasing amount of CO evolving groups when increasing the burn-off. On the other hand, oxidation with nitric acid increases the amount of CO and CO2 evolved by the decomposition of surface oxygen groups, this increase being related to the development of porosity in the carbon with the degree of activation and to the activating agent used (CO2 versus steam). 相似文献
65.
Gamero-Pasadas A Viera Alcaide I Rios JJ Graciani Constante E Vicario IM León-Camacho M 《Journal of chromatography. A》2006,1123(1):82-91
Hydrocarbons in fresh subcutaneous fat of Iberian pig have been analyzed by GC-MS after fractionation of the unsaponifiable fraction with a new off-line combination of HPLC and GC method. The new method proposed improves the recovery and simultaneous quantification of terpenic hydrocarbons in comparison to the traditional LC method. When necessary and for identification purposes, selective ion monitoring (SIM) was used as acquisition mode in GC-MS. To determine the position of the double bonds in the unsaturated hydrocarbon chain the dimethyl disulfide derivatives (DMDS) were obtained. To elucidate the structure of the branched 1-alkenes the hydrocarbon fraction was submitted to hydrogenation. Thirty-five compounds have been identified, including n-alkanes, n-alkenes, branched (n-1,n-2-dimethyl-1-alkenes) and terpenic hydrocarbons, being the most abundant n-alkenes and n-alkanes of even chain of n-C12-n-C26. Besides the hydrocarbons already described in bibliography, a new diterpenic hydrocarbon, ent-kaurene, have been identified for the first time. The compound reported as Neophytadiene by other authors, has been identified as a 20 atoms hydrocarbon with two double bonds, the 7,11,15-trimethyl-heptadeca-1,4-diene. 相似文献
66.
Rios R 《Chemical Society reviews》2012,41(3):1060-1074
The enantioselective synthesis of spirocycles has been a long time pursued dream for organic chemists. Since the first pioneering efforts of Tamao and coworkers in the enantioselective construction of spirosilanes, many efforts have been devoted to the development of new and promising asymmetric methodologies. Remarkably, with the advent of organocatalysis the number of methodologies has been highly increased. The aim of this tutorial review is to summarize the last trends and developments reported in the literature in the enantioselective synthesis of spirocompounds. 相似文献
67.
Companyó X Valero G Pineda O Calvet T Font-Bardía M Moyano A Rios R 《Organic & biomolecular chemistry》2012,10(2):431-439
An enantioselective α-oxyamination of unprotected 3-substituted oxindoles with nitrosobenzene catalyzed by tertiary amine-thiourea bifunctional organocatalysts has been developed and affords the corresponding 3-amino-2-oxindole derivatives in good yields and with moderate to excellent enantioselectivities (up to > 99.9?:?0.1 er when the product is isolated by direct filtration from the reaction mixture). The absolute configuration of the major enantiomers of the products has been established both by chemical correlation and by comparison between the theoretically calculated and the experimental ECD. 相似文献
68.
Varughese T Riosa N Higginbotham S Arnold AE Coley PD Kursar TA Gerwick WH Cubilla Rios L 《Tetrahedron letters》2012,53(13):1624-1626
Among thirty four endophytic fungal strains screened for in vitro antagonism, the endophytic fungus Cordyceps dipterigena was found to strongly inhibit mycelial growth of the plant pathogenic fungus Gibberella fujikuroi. Two new depsidone metabolites, cordycepsidone A (1) and cordycepsidone B (2), were isolated from the PDA culture extract of C. dipterigena and identified as being responsible for the antifungal activity. Elucidation of their chemical structures was carried out using 1D and 2D NMR spectroscopy in combination with IR and MS spectroscopic data. Cordycepsidone A displayed strong and dose-dependent antifungal activity against the plant pathogenic fungus Gibberella fujikuroi. The isolates were inactive in bioassays for malaria (Plasmodium falciparum), leishmaniasis (Leishmania donovani), Chagas's disease (Trypanosoma cruzi), and cytotoxicity at 10 μg/mL. The compounds were also found to be inactive against several bacterial strains at 50 μg/mL. 相似文献
69.
Carlos A. Rios Perez Justin D. Lowrey Steven Biegalski Mark R. Deinert 《Journal of Radioanalytical and Nuclear Chemistry》2012,291(1):261-265
Developing a better understanding of xenon transport through porous systems is critical to predicting how this gas will enter
the atmosphere after a below ground nuclear weapons test. Radioxenon monitoring is a vital part of the Comprehensive Nuclear-Test-Ban
Treaty (CTBT) International Monitoring System. This work details the development of prompt gamma activation analysis for measuring
the diffusion rates of xenon and argon gases through a porous medium. The University of Texas at Austin maintains a prompt
gamma activation analysis facility with a peak neutron flux of ~1.5 × 107 cm−2 s−1 and a beam diameter of 1 cm. Due to the relatively large prompt gamma cross sections of many stable xenon isotopes at thermal
and sub-thermal neutron energies, prompt gamma activation analysis is a suitable technique for in situ non-destructive analysis
of natural xenon. A test chamber has been designed and constructed to utilize prompt gamma activation analysis to measure
xenon and argon diffusion through geological materials (e.g., sand, soil, etc.). Initial experiments have been conducted to determine the detection limits for stable gas measurements.
The results from these experiments will be utilized to benchmark parts of a xenon transport model that is being used to determine
diffusion coefficients for xenon and argon. 相似文献
70.
This paper describes Bayesian inference and prediction for some M/G/1 queueing models. Cases when the service distribution is Erlang, hyperexponential and hyperexponential with a random number
of components are considered. Monte Carlo and Markov chain Monte Carlo methods are used for estimation of quantities of interest
assuming the queue is in equilibrium.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献