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排序方式: 共有1165条查询结果,搜索用时 31 毫秒
91.
Pedersen CM Figueroa-Perez I Boruwa J Lindner B Ulmer AJ Zähringer U Schmidt RR 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(42):12627-12641
Streptococcus pneumoniae LTA is a highly complex glycophospholipid that consists of nine carbohydrate residues: three glucose, two galactosamine and two 2‐acetamino‐4‐amino‐2,4,6‐trideoxygalactose (AATDgal) residues that are each differently linked, one ribitol and one diacylated glycerol (DAG) residue. Suitable building blocks for the glucose and the AATDgal residues were designed and their synthesis is described in this paper. These building blocks permitted the successful synthesis of the core structure Glcβ(1‐3)AATDgalβ(1‐3)Glcα(1‐O)DAG in a suitably protected form for further chain extension ( 1 b , 1 c ) and as unprotected glycolipid ( 1 a ) that was employed in biological studies. These studies revealed that 1 a as well as 1 lead to interleukin‐8 release, however not via TLR2 or TLR4 as receptor. 相似文献
92.
The Newman-Kwart thermal rearrangement of two 2,4-disubstituted O-arylthiocarbamates, prepared from the corresponding phenols, is reported. Clean conversion to the S-arylthiocarbamates in high yields was observed. The rearrangement appears to be facilitated by the presence of electron-withdrawing substituents in the 2- and 4-positions of the aromatic ring. 相似文献
93.
Javier A. Cabeza Prof. Ignacio del Río Prof. Enrique Pérez‐Carreño Prof. Vanessa Pruneda 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(18):5425-5436
The cationic cluster complexes [Ru3(μ‐H)(μ‐κ2N,C‐L1 Me)(CO)10]+ ( 1 +; HL1 Me=N‐methylpyrazinium), [Ru3(μ‐H)(μ‐κ2N,C‐L2 Me)(CO)10]+ ( 2 +; HL2 Me=N‐methylquinoxalinium), and [Ru3(μ‐H)(μ‐κ2‐N,C‐L3 Me)(CO)10]+ ( 3 +; HL3 Me=N‐methyl‐1,5‐naphthyridinium), which contain cationic N‐heterocyclic ligands, undergo one‐electron reduction processes to become short lived, ligand‐centered, trinuclear, radical species ( 1 – 3 ) that end in the formation of an intermolecular C? C bond between the ligands of two such radicals, thus leading to neutral hexanuclear derivatives. These dimerization processes are selective, in the sense that they only occur through the exo face of the bridging ligands of trinuclear enantiomers of the same configuration, as they only afford hexanuclear dimers with rac structures (C2 symmetry). The following are the dimeric products that have been isolated by using cobaltocene as reducing agent: [Ru6(μ‐H)2{μ6‐κ4N2,C2‐(L1 Me)2}(CO)18] ( 5 ; from 1 +), [Ru6(μ‐H)2{μ6‐κ4N2,C2‐(L2 Me)2}(CO)18] ( 6 ; from 2 +), and [Ru6(μ‐H)2{μ4‐κ8N2,C6‐(L3 Me)2}(CO)18] ( 7 ; from 3 +). The structures of the final hexanuclear products depend on the N‐heterocyclic ligand attached to the starting materials. Thus, although both trinuclear subunits of 5 and 6 are face‐capped by their bridging ligands, the coordination mode of the ligand of 5 is different from that of the ligand of 6 . The trinuclear subunits of 7 are edge‐bridged by its bridging ligand. In the presence of moisture, the reduction of 3 + with cobaltocene also affords a trinuclear derivative, [Ru3(μ‐H)(μ‐κ2N,C‐L3′ Me)(CO)10] ( 8 ), whose bridging ligand (L3′ Me) results from the formal substitution of an oxygen atom for the hydrogen atom (as a proton) that in 3 + is attached to the C6 carbon atom of its heterocyclic ligand. The results have been rationalized with the help of electrochemical measurements and DFT calculations, which have also shed light on the nature of the odd‐electron species, 1 – 3 , and on the regioselectivity of their dimerization processes. It seems that the sort of coupling reactions described herein requires cationic complexes with ligand‐based LUMOs. 相似文献
94.
Krauter H Muschik CA Jensen K Wasilewski W Petersen JM Cirac JI Polzik ES 《Physical review letters》2011,107(8):080503
Entanglement is a striking feature of quantum mechanics and an essential ingredient in most applications in quantum information. Typically, coupling of a system to an environment inhibits entanglement, particularly in macroscopic systems. Here we report on an experiment where dissipation continuously generates entanglement between two macroscopic objects. This is achieved by engineering the dissipation using laser and magnetic fields, and leads to robust event-ready entanglement maintained for 0.04 s at room temperature. Our system consists of two ensembles containing about 10(12) atoms and separated by 0.5 m coupled to the environment composed of the vacuum modes of the electromagnetic field. By combining the dissipative mechanism with a continuous measurement, steady state entanglement is continuously generated and observed for up to 1 h. 相似文献
95.
96.
97.
A problem of decision making under uncertainty in which the choice must be made between two sets of alternatives instead of two single ones is considered. A number of choice rules are proposed and their main properties are investigated, focusing particularly on the generalizations of stochastic dominance and statistical preference. The particular cases where imprecision is present in the utilities or in the beliefs associated to two alternatives are considered. 相似文献
98.
Marvin J. Núñez María L. Kennedy Ignacio A. JiménezIsabel L. Bazzocchi 《Tetrahedron》2011,67(17):3030-3033
Uragogin and blepharodin were isolated from Crossopetalum uragoga and Maytenus magellanica, respectively. They represent the first examples of a triterpene-neolignan ester and a heptacyclic arylpropanoid-nor-triterpenephenol, hetero-Diels-Alder adducts built with dioxane bridges. Their proposed biosynthetic route is discussed. 相似文献
99.
We have studied the magnetic and electronic properties of double perovskites A2FeReO6 with A=Ba and Ca using ab initio methods based on density functional theory. It has been shown that when changing Ba by Ca the system undergoes a metal–insulator transition coincidentally to a structural phase transition, even if conduction band fillings are not significantly changed. Our results suggest that only considering a quite large correlation term on the Re site could explain an insulating character for Ca2FeReO6. Instead, we found that small correlations could induce a bad metallic behavior as suggested in more recent experiments. 相似文献
100.
We have studied the effect of advection on reaction–diffusion equations by using toroidal velocity fields. Turing patterns formation in diffusion–advection–reaction problems was studied specifically, considering the Schnackenberg and glycolysis reaction kinetics models. Four cases were analyzed and solved numerically using finite elements. For glycolysis models, the advective effect modified the form of Turing patterns obtained with diffusion–reaction; whereas for Schnackenberg problems, the original patterns distorted themselves slightly, making them rotate in direction of the velocity field. We have also determined that the advective effect surpassed the diffusive one for high values of velocity and instability driven by diffusion was eliminated. On the other hand the advective effect is not considerable for very low values in the velocity field, and there was no modification in the original Turing pattern. 相似文献