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951.
952.
Four new aromatic constituents, rhododendroketoside, (-)-sakuraresinoside, acernikol, and nikoenoside, were isolated from a Japanese folk medicine, the stem bark of Acer nikoense MAXIM. The structures of the new constituents were determined on the basis of chemical and physicochemical evidence. The principle cyclic diarylheptanoids were found to show inhibitory effects on the release of beta-hexosaminidase in RBL-2H3 cells. 相似文献
953.
All seven possible geometric isomers (four mer and three facial) for the [Co(dien)(ibn)Cl](2+) system have been synthesized (dien = diethylenetriamine, ibn = 1,2-diamino-2-methylpropane). Their structures in dimethyl sulfoxide solution have been uniquely determined by two-dimensional NMR spectroscopy (DQCOSY and NOESY). 相似文献
954.
The electromechanical properties of a single molecule covalently attached to two gold electrodes are studied by simultaneously measuring the conductance and the force during the stretching of the molecule. The conductance, the spring constant of the molecular junction, and the dependence of the conductance on the stretching force are determined. Like the conductance, the spring constant of a molecule depends also on the molecule-electrode contacts. The forces required to break the molecule-gold contacts are 1.5 nN for alkanedithiols and 0.8 nN for 4,4' bipyridine, indicating that the breakdowns take place at the Au-Au bond and at the N-Au bond, respectively. 相似文献
955.
[structure: see text] A chemoenzymatic synthesis was described to prepare proposed oxidation-cyclization-methylation intermediates of the coumarin antibiotic biosynthetic pathway. The successful synthesis of these fragile molecules relies heavily on mild enzymatic deprotection and efficient enzymatic kinetic resolution to minimize epimerization, decomposition, multiple orthogonal protections, and retro aldol reactions often encountered in their chemical synthesis. 相似文献
956.
[reaction: see text] A series of planar chiral diphosphine-oxazoline ferrocene ligands were effectively applied in the asymmetric arylation of N-methoxycarbonyl-2-pyrroline, and up to 99% ee was observed for the product 3. The regioselectivity of this reaction was strongly affected by the different precursors of the palladium species and the polarity of the solvent. 相似文献
957.
958.
959.
We report gas phase ion/molecule reactions between dialkoxyborinium cations (RO-B+-OR) and small organic amides, such as N,N-dimethylformamide and N,N-diethylpropionamide. Besides direct addition at boron, the results show efficient SN2 methyl transfer from the borinium ion to the amide. Isotopic labeling and collision-induced dissociation (CID) of the methyl transfer products demonstrate O-methylation of the amide. Methyl substitution at the alpha-carbon of the amide affects the degree of alkylation and adduct formation. Direct proton abstraction via beta-elimination is a major competitive reaction for substituents other than methyl. Ab initio calculations at the B3LYP/6-31G(d) level indicate that SN2 transmethylation is highly exothermic with O-methylation favored over N-methylation by 14.8 kcal/mol. 相似文献
960.
Drevensek Olenik I Kocevar K Musevic I Rasing T 《The European physical journal. E, Soft matter》2003,11(2):169-175
Brewster-angle reflection ellipsometry and surface optical
second harmonic generation were used to study the growth of
4'-n-octyl-4-cyanobiphenyl
(8CB) films evaporated in air onto polymeric and quartz glass
substrates. The layer-by-layer growth of the films terminates
after formation of two distinctive interfacial layers. Both of
these two layers are polar and tilted. In the first layer the
molecules lie nearly flat on the surface, while in the second
layer they point on average about 50° toward the surface normal.
The dipole moment of the second layer has a lower magnitude and
an opposite direction with respect to the dipole moment of the
first layer. 相似文献