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991.
The first fluorous protecting group for phosphate is reported. This group can be used as a facile tag for purification and be removed under mild reducing conditions using zinc and ammonium formate. Synthesis of a disaccharide from Leishmania using this fluorous protecting group demonstrated the group's stability to the acidic conditions necessary for glycosylation as well as its orthogonality to several other common protecting groups. 相似文献
992.
Hobbs C Yang YC Ling J Nicola S Su HL Bazzi HS Bergbreiter DE 《Organic letters》2011,13(15):3904-3907
The preparation of polyethylene-oligomer (PE(olig))-supported N-heterocyclic carbene ligands (NHCs) and their Ru complexes is described. These complexes are structurally analogous to their low molecular weight counterparts and can serve as thermomorphic, recoverable/recyclable ring-closing metathesis (RCM) catalysts. Because of the insolubility of PE(olig)-supported species at 25 °C, such complexes can perform homogeneous RCM reactions at 65 °C and, upon cooling, precipitate as solids. This allows for their quantitative separation from solutions of products. 相似文献
993.
Di Nicola C Garau F Marchetti F Monari M Pandolfo L Pettinari C Venzo A 《Dalton transactions (Cambridge, England : 2003)》2011,40(18):4941-4953
Four new potentially polytopic nitrogen donor ligands based on the 1,3,5-triazine fragment, L(1)-L(4) (L(1) = 2-chloro-4,6-di(1H-pyrazol-1-yl)-1,3,5-triazine, L(2) = N,N'-bis(4,6-di(1H-pyrazol-1-yl)-1,3,5-triazin-2-yl)ethane-1,2-diamine, L(3) = 2,4,6-tris(tri(1H-pyrazol-1-yl)methyl)-1,3,5-triazine, and L(4) = 2,4,6-tris(2,2,2-tri(1H-pyrazol-1-yl)ethoxy)-1,3,5-triazine) have been synthesized and characterized. The X-ray crystal structure of L(3) confirms that its molecular nature consists of a 1,3,5-triazine ring bearing three tripodal tris(pyrazolyl) arms. L(1), L(2), and L(4) react with Cu(I), Cu(II), Pd(II) and Ag(I) salts yielding mono-, di-, and oligonuclear derivatives: [Cu(L(1))(Cy(3)P)]ClO(4), [{Ag(2)(L(2))}(CF(3)SO(3))(2)]·H(2)O, [Cu(2)(L(2))(NO(3))(2)](NO(3))(2)·H(2)O, [Cu(2)(L(2))(CH(3)COO)(2)](CH(3)COO)(2)·3H(2)O, [Pd(2)(L(2))(Cl)(4)]·2H(2)O, [Ru(L(2))(Cl)(OH)]·CH(3)OH, [Ag(3)(L(4))(2)](CF(3)SO(3))(3) and [Ag(3)(L(4))(2)](BF(4))(3). The interaction of L(3) with Ag(I), Cu(II), Zn(II) and Ru(II) complexes unexpectedly produced the hydrolysis of the ligand with formation, in all cases, of tris(pyrazolyl)methane (TPM) derivatives. In detail, the already known [Ag(TPM)(2)](CF(3)SO(3)) and [Cu(TPM)(2)](NO(3))(2), as well as the new [Zn(TPM)(2)](CF(3)SO(3))(2) and [Ru(TMP)(p-cymene)]Cl(OH)·2H(2)O complexes have been isolated. Single-crystal XRD determinations on the latter derivatives confirm their formulation, evidencing, for the Ru(II) complex, an interesting supramolecular arrangement of the anions and crystallization water molecules. 相似文献
994.
Parallel mechanisms have been exploited for the kinematic modelling of the passive motion, i.e. the motion under virtually
unloaded conditions, of the patella-femur-tibia human joint. In particular, a new mechanism is devised in this paper: a 3D
model of the patella-femur relative motion is presented which, combined with a previous simplified model of the femur-tibia
relative motion, provides a suitable tool for the design of knee prostheses. Although less accurate than a previously presented
model of the patella-femur-tibia joint, the new mechanism still replicates passive knee motion quite well and is simpler from
a mechanical point of view. Experimental results validate the efficiency of the proposed model. 相似文献
995.
Recently Frank and Seiringer have shown an isoperimetric inequality for nonlocal perimeter functionals arising from Sobolev seminorms of fractional order. This isoperimetric inequality is improved here in a quantitative form. 相似文献
996.
Boundary behavior of <Emphasis Type="Italic">p</Emphasis>-harmonic functions in the Heisenberg group
In this paper we study the boundary behavior of nonnegative p-harmonic functions in a bounded domain in the Heisenberg group \({\mathbb H^n}\). Under suitable geometric assumptions on the ground domain \({\Omega\subset \mathbb H^n}\) our main contributions can be summarized as follows: (1) In Theorem 1.1 we obtain an estimate from above stating that any such function should vanish at most linearly like the sub-Riemannian distance from the boundary:where \({A_r(g_0)\in \Omega}\) is a non-tangential point relative to \({g_0\in \partial \Omega}\). (2) In Theorem 1.2 we establish an estimate from below which states that, away from the characteristic set of Ω, the order of vanishing is exactly linear, i.e.:
相似文献
$\frac{u(g)}{u(A_r(g_0))}\leq C^{-1}\frac{d(g,\partial \Omega)}{r},$
$\frac{u(g)}{u(A_r(g_0))}\ge C \frac{d(g,\partial \Omega)}{r}.$
997.
Dellus N Kato T Saffon-Merceron N Branchadell V Baceiredo A 《Inorganic chemistry》2011,50(17):7949-7951
The mixed P,S-bisylide 1 reacts with phosphenium salts, affording phosphoniophosphinocarbenes, with elimination of diphenyl sulfide. Using a more compact cyclic phosphenium cation, the reaction leads to the formation of an original three-membered heterocycle featuring a gem-bisylide function. This molecule represents the first isolable analogue of the phosphirenyl anion derivative. 相似文献
998.
Bryan Roberts David LiptrotTim Luker Michael J. StocksCatherine Barber Nicola WebbRobert Dods Barrie Martin 《Tetrahedron letters》2011,52(29):3793-3796
A new, efficient and practical synthesis of quinazolin-4(3H)-ones is reported via molybdenum-mediated cyclocarbonylation using microwave irradiation. These methods allow access to a wide range of quinazolin-4(3H)-ones in reasonable yields without the need for gaseous carbon monoxide and palladium catalysts. A range of reactions illustrating the wide scope of this chemistry was carried out and all proceeded in reasonable yields. 相似文献
999.
The base hydrolysis reaction of Co(NH(3))(5)Cl(2+) was investigated using density functional theory and molecular orbital methods. Geometries and energies of conjugate bases, intermediates, transition states, and minimum energy crossing points were computed. For the base hydrolysis of Co(NH(3))(5)Cl(2+), three pathways might operate: the mechanism proposed by Basolo and Pearson, the mechanism via a hexacoordinated intermediate exhibiting a triplet ground state, and a fully stereomobile I(d) mechanism. The hexacoordinated intermediate can lose the leaving ligand readily to form a square pyramidal pentacoordinated intermediate with a triplet state, which interconverts rapidly and reversibly into the Basolo-Pearson trigonal bipyramid with a singlet state. Due to its high activation energy, a stereochemical rearrangement via a Berry pseudorotation does not take place. The intermediates are not protonated because their pK(a) values are ~5 for the hexacoordinated intermediate and ~-6 for the trigonal bipyramidal pentacoordinated intermediate. The Basolo-Pearson mechanism proceeds with 50% stereoretention and 50% stereomobility. In the case that the hexacoordinated intermediate converts into the trigonal bipyramid, products with the same stereochemistry would be obtained. If, however, for the square pyramidal intermediate with a triplet state the entering ligand competes efficiently with the rearrangement into the trigonal bipyramid or if the substitution takes place at the hexacoordinated intermediate via, e.g., the I(d) mechanism, the reaction would proceed with retention of the configuration. Direct substitution via the I(d) mechanism, operating for azide and to a small extent also for water, is fully stereomobile. Computations on the Basolo-Pearson mechanism have also been performed for the chloro pentaammine complexes of chromium(III), ruthenium(III), and rhodium(III). This pathway might operate for chromium(III) but not for ruthenium(III) and rhodium(III). 相似文献
1000.
Solvent systems for use with LC-MS often result in a compromise between chromatographic performance and mass spectrometric detection, exemplified here by a LC-MS/MS method development for the analysis of ephedrines in doping control. Ephedrines, frequently found in therapeutic and nutritional preparations, are among the most commonly administered doping agents in competitive sport. Improved separation of these hydrophilic, basic compounds, some of which are diastereoisomers, is achieved in reversed-phase LC by the use of a high pH mobile phase in order to suppress analyte ionisation, and thus alter their polarity, resulting in reduced peak tailing and enhanced retention. However, when coupled to an ESI-MS detector, this eluent composition generated a non-linear and poorly reproducible signal. APCI yielded greater stability and reproducibility and is here presented as an ion source for the analysis of basic compounds under conditions that suppress their ionisation. Errors as large as 49.3% were observed with ESI, compared with 15.4% generated using APCI, for pseudoephedrine over the calibration range (25-400 μg/mL) in urine with a simple dilution and injection of samples. These data highlight the importance of suitable MS conditions for stable performance, necessary for accurate quantification, without undue compromise to the LC separation. 相似文献