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Epimeric carbohydrate alkyl cobaloximes 4:5, 9:10, and 12:13 can be equilibrated thermally or photochemically. In each case, one isomer is strongly favored: exo-3-deoxy-3-pyridyldimethylglyoximatocobalt-1,2:5,6-di-O-isopropylidene-alpha-D-glucofuranose 4 for the 4:5 epimer pair, exo-3-deoxy-3-pyridyldimethylglyoximatocobalt-5-O-carboxymethyl-1,2-O-isopropylidene-alpha-D-xylofuranose 9 for the 9:10 epimer pair, and equatorial 1-deoxy-1-pyridyldimethylglyoximatocobalt-2,3,4,6-tetra-O-benzyl-beta-D-glucopyranose 12 for the 12:13 epimer pair. These data indicate that there is a strong facial preference for the coupling of py(dmgH)(2)Co(*) radicals with alkyl R(*) free radicals, with the preferred kinetic path leading to the more stable product. 相似文献
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A bifunctional biaryl lactone has been synthesized that should be capable of iterative unidirectional aryl-aryl bond rotation via: (1) a diastereoselective lactone ring opening, (S)-1 to (P,S)-2 or (M,S)-2; (2) a chemoselective lactonization, (P,S)-2 or (M,S)-2 to (S)-3; and (3) a chemoselective hydrolysis, (S)-3 to (S)-1. Preliminary results of a racemic sample have indicated unidirectional 180 degrees rotation with very high directional selectivity per individual artificial molecular motor molecule through the first two steps of this sequence. [reaction: see text] 相似文献
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The properties of the negative parity states of55Fe and57Fe are investigated in the framework of the intermediate coupling model. In the model, a neutron or a quasineutron is coupled to anharmonic vibrations of the core. Anharmonicities of the vibrations are estimated through the observed properties of the core. Energy levels, spectroscopic factors and electromagnetic properties have been calculated. The results of the present calculations are also compared with available experimental results and other theoretical results. The model reasonably accounts for many of the properties of the low-lying states. 相似文献
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The effect of the radiative cooling of electrons on the gravitational collapse of cold dust grains with fluctuating electric
charge is investigated. We find that the radiative cooling as well as the charge fluctuations, both, enhance the growth rate
of the Jeans instability. However, the Jeans length, which is zero for cold grains and nonradiative plasma, becomes finite
in the presence of radiative cooling of electrons and is further enhanced due to charge fluctuations of grains resulting in
an increased threshold of the spatial scale for the Jeans instability. 相似文献
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The LSD (Logic for Structure Determination) program gener-ates organic molecular structures from 1D and 2D NMR data without resorting to chemical shift databases. Its use in the res-olution of natural product structure determination problems has been already reported in the literature. This paper describes how data and structures are internally represented and pro-cessed by LSD to build solution structures. 相似文献
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Matthias Ahrens Peter Macheroux Anatol Eberhard † Sandro Ghisla ‡ Bruce P. Branchaud J. Woodland Hastings 《Photochemistry and photobiology》1991,54(2):295-299
Bacterial luciferase uses long chain aldehydes as substrates. Alkylboronic acid analogs of these substrates with chain lengths of C7 and C9 have been synthesized, characterized, and used as mechanistic probes for the light emitting reaction. They behave as inhibitors in the in vitro luminescence reaction. Contrary to an earlier report (Macheroux and Ghisla, 1985, Nachr. Chem. Tech. Lab. 33,785–790) they are not substrates for bacterial luciferase, in that they do not lead to light emission and are not oxidized by the flavin-4a-hydroperoxide to the products boric acid and the corresponding alcohol, as would be expected from a Baeyer-Villiger reaction. However, the particular conformation of a putative boronic acid hydroperoxide at the active center might be such that it would preclude a Baeyer-Villiger fragmentation. Thus, while the results do not support the postulate that luciferase proceeds via a Baeyer-Villiger mechanism, they also do not exclude it. A further observation was that the endogenous light emission (no added aldehyde) decays more rapidly than does the luciferase bound flavin-4a-hydroperoxide. This suggests that the endogenous light is not caused by the decomposition of the flavin-4a-hydroperoxide. 相似文献