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481.
An efficient approach for the synthesis of a model of the ABC ring system of Hemibrevetoxin B is described. Key features include a ring expansion to yield the ring C oxepane, the reduction of a 2-furyl ketone with high levels of 1,3-stereocontrol, and an Achmatowicz oxidative ring expansion to yield the ring A tetrahydropyran. All seven stereogenic centers present in the model compound were controlled with high levels (>98:<2) of diastereoselectivity. 相似文献
482.
Thomas RB Smith PA Jaleel A Vogel P Crawford C Assefa Z Sykora RE 《Inorganic chemistry》2012,51(6):3399-3408
The highly luminescent bimetallic cyanide materials, Gd(terpy)(H(2)O)(NO(3))(2)M(CN)(2) (M = Au, Ag; GdAu and GdAg, respectively) are quick and easy to synthesize under ambient conditions. A characteristic feature exhibited by both solid-state compounds is an intense red emission when excited with UV light. Additionally, GdAu exhibits a broad-band green emission upon excitation in the near UV region. A combination of structural and spectroscopic results for the compounds helps explain the underlying conditions responsible for their unique properties. Single-crystal X-ray diffraction experiments expose their structural features, including the fact that they are isostructural. Crystallographic data for the representative GdAu compound (Mo K(α), λ = 0.71073 ?, T = 290 K): triclinic, space group P ?1, a = 7.5707(3) ?, b = 10.0671(4) ?, c = 15.1260(4) ?, α = 74.923(3)°, β = 78.151(3)°, γ = 88.401(3)°, V = 1089.04(7) ?(3), and Z = 2. Although the compounds crystallize as dimers containing M···M distances smaller than the sum of their van der Waals radii, the Au···Au (3.5054(4) ?) and/or the Ag···Ag (3.6553(5) ?) interactions are relatively weak and are not responsible for the low energy red emission. Rather, the green emission in GdAu presumably originates from the [Au(CN)(2)(-)](2) dimeric excimer, while the [Ag(CN)(2)(-)](2) dimers in GdAg do not display visible emission at either 290 or 77 K. The unusual red emission exhibited by both compounds likely originates from the formation of an excited state exciplex that involves intermolecular π-stacking of 2,2':6',2"-terpyridine ligands. The room-temperature and low-temperature steady-state photoluminescent properties, along with detailed time-dependent, lifetime, and quantum yield spectroscopic data provide evidence regarding the sources of the multiple visible emissions exhibited by these complexes. 相似文献
483.
AE Pasqua FD Ferrari C Hamman Y Liu JJ Crawford R Marquez 《The Journal of organic chemistry》2012,77(16):6989-6997
The step-economic total synthesis of (+)-crocacin C has been achieved in 20% yield from commercially available starting materials. This approach requires the isolation of only 8 intermediates and can provide a reliable supply of (+)-crocacin C for the development of new antifungal and crop protection agents. 相似文献
484.
Adele E. Pasqua Frank D. Ferrari James J. Crawford Rodolfo Marquez 《Tetrahedron letters》2012,53(16):2114-2116
The formal synthesis of (+)-crocacin C is reported. The approach described takes advantage of a highly regioselective epoxide cuprate addition and a diastereoselective Overman rearrangement. The synthesis is practical and amenable to scale up. 相似文献
485.
In this paper, we derive upper bounds for the heat kernel of the simple random walk on the infinite cluster of a supercritical long range percolation process. For any d ?? 1 and for any exponent ${s \in (d, (d+2) \wedge 2d)}$ giving the rate of decay of the percolation process, we show that the return probability decays like ${t^{-{d}/_{s-d}}}$ up to logarithmic corrections, where t denotes the time the walk is run. Our methods also yield generalized bounds on the spectral gap of the dynamics and on the diameter of the largest component in a box. The bounds and accompanying understanding of the geometry of the cluster play a crucial role in the companion paper (Crawford and Sly in Simple randomwalk on long range percolation clusters II: scaling limit, 2010) where we establish the scaling limit of the random walk to be ??-stable Lévy motion. 相似文献
486.
487.
W.F. Lewis D. Benson R.K. Crawford W.B. Daniels 《Journal of Physics and Chemistry of Solids》1974,35(3):383-391
Isochoric P-T traces have been measured for solid argon at eight different molar volumes by using a capacitance technique which records the radial expansion of the vessel due to pressure. These measurements extend from melting curve temperatures down to 4 °K at pressures ranging up to 7 kbar.Analysis of the data indicates that, for the high temperature region (above 80 °K) the thermodynamic derivative ?P/?Tr is independent of both temperature and density to within experimental errors. Using the fact that ?P/?Tr is constant, it is possible to generate PVT and isothermal bulk modulus data at high temperatures. These data are compared with X-ray diffraction measurements and piston displacement data and are found to be in good agreement with both. The data also agree quite well with current theoretical predictions. The high-temperature bulk modulus data were found to have a simple exponential dependence on the molar volume, prividing another convenient representation of the high-temperature equation-of-state data. 相似文献
488.
We show that the de Sitter space can be foliated into spacelike hypersurfaces with locally rotationally symmetric Bianchi type III properties. This proves as well that a particular Bianchi type III solution is isotropic. In addition, we apply a criterion of isotropy, based on the vanishing of the Weyl tensor, which is particularly suited for cosmological models where there is not a preferred timelike vector field. 相似文献
489.
W.F. Lewis R.K. Crawford W.B. Daniels 《Journal of Physics and Chemistry of Solids》1974,35(9):1201-1204
High temperature isochoric pressure-temperature traces in solid methane and nitrogen are used to evaluate .A corresponding states analysis is presented; deviations from corresponding states are discussed in terms of molecular rotations. 相似文献
490.
M. Asghar J.P. Gautheron G. Bailleul J.P. Bocquet J. Greif H. Schrader G. Siegert C. Ristori J. Crancon G.I. Crawford 《Nuclear Physics A》1975,247(2):359-376
The first results are reported on the Pn values obtained with the recoil focussing parabolatype mass separator for unslowed fission products Lohengrin installed at the Grenoble high flux reactor. The mass chains studied were 90, 91, 93, 94, 95, 99, 134, 137, 138 and 139. Both the neutron and the β activities were measured simultaneously. The technique used to measure the neutron and the β activities and the method of analyzing the experimental data are discussed in detail. The present work led to: (i) three new periods corresponding to the new isotopes of selenium (), strontium (99Sr, 0.6±0.2 sec) and telurium (138Te, 1.3±0.3 sec); (ii) accurate periods of ; (iii) four new delayed neutron precursors consisting of 91Se, 94Kr, 99Sr and 138Te; (iv) six new Pn values corresponding to the precursors ; (v) a precise Pn value of the precursor 137Te ((2.5±0.5)%); (vi) a redetermination of the Pn values of the precursors 90, 91Br, 93Kr, 93, 94, 95Rb and 137, 138, 139I. The results of this work are discussed and compared with the existing data. The low level sensitivity of the present detection system is determined to be (where Yq(m) is the cumulative yield for the mass m and the ionic charge q). 相似文献