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排序方式: 共有354条查询结果,搜索用时 218 毫秒
321.
Marianne Lankelma Astrid M. Olivares Prof. Dr. Bas de Bruin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(22):5658-5663
Radical cyclization via cobalt(III)-carbene radical intermediates is a powerful method for the synthesis of (hetero)cyclic structures. Building on the recently reported synthesis of five-membered N-heterocyclic pyrrolidines catalyzed by CoII porphyrins, the [Co(TPP)]-catalyzed formation of useful six-membered N-heterocyclic piperidines directly from linear aldehydes is presented herein. The piperidines were obtained in overall high yields, with linear alkenes being formed as side products in small amounts. A DFT study was performed to gain a deeper mechanistic understanding of the cobalt(II)-porphyrin-catalyzed formation of pyrrolidines, piperidines, and linear alkenes. The calculations showed that the alkenes are unlikely to be formed through an expected 1,2-hydrogen-atom transfer to the carbene carbon. Instead, the calculations were consistent with a pathway involving benzyl-radical formation followed by radical-rebound ring closure to form the piperidines. Competitive 1,5-hydrogen-atom transfer from the β-position to the benzyl radical explained the formation of linear alkenes as side products. 相似文献
322.
Review 总被引:1,自引:0,他引:1
Martin Malmsten Astrid Sigel Helmut Sigel Zory V. Todres 《Reaction Kinetics and Catalysis Letters》2003,79(1):205-209
Surfactants and Polymers in Drug Delivery, Drugs and the Pharmaceutical Sciences, Vol. 122, Edited by Martin Malmsten Molybdenum
and Tungsten: Their Role in Biological Processes, Metal Ions in Biological Systems, Vol. 39, Edited by Astrid Sigel and Helmut
Sigel Organic Ion Radicals, Chemistry and Applications, Zory V. Todres
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
323.
Bentley DJ Fairhurst J Gallagher PT Manteuffel AK Moody CJ Pinder JL 《Organic & biomolecular chemistry》2004,2(5):701-708
Two routes to the protected 4-hydroxymethyl-3-methylindole-2-carboxylate fragment 17 of the thiopeptide antibiotic nosiheptide are described starting from methyl 4-methylindole-2-carboxylate 11, itself prepared in two steps, or from 3-amino-4-chlorobenzoic acid 26. The first route can be adapted to the synthesis of a fragment of the related antibiotic glycothiohexide-alpha, the 3,4-bis(hydroxymethyl)indole-2-carboxylate in which the two hydroxymethyl groups are differentiated as in indole 19 or the lactone 20. 相似文献
324.
Iridoids, belonging to a group of cyclopentanoid monoterpenoids, are secreted by many species of leaf beetles as a defense against predators. Using chemically modified precursors of iridoid biosynthesis, it has been shown that some leaf beetle larvae can synthesize these iridoids de novo as well as sequester plant-produced molecules. Stable isotope techniques can provide useful methods for studying terpenoid biosynthesis without disturbing the natural conditions much. Two terpenoid biosynthesis pathways (mevalonic acid (MVA) pathway and methylerythritol-4-phosphate (MEP) pathway) may lead to different delta13C signatures of the products. Our results from natural abundance 13C and 13C-labelled iridoid precursors in Gastrophysa viridula and Phaedon cochleariae suggested that the two leaf beetle species use only de novo synthesis of their defensive iridoids. We observed that the isotope signature of the leaf-beetle-produced iridoids (via the MVA pathway) resembled that of the MEP-derived monoterpenoids from plants. Owing to this close similarity in the natural 13C abundances in the plant and insect compounds, a determination of iridoid-origin in leaf beetle secretion may only be possible by use of isotopically labelled compounds. 相似文献
325.
We show how the sign of a permutation can be deduced from the tableaux induced by
the permutation under the Robinson-Schensted correspondence. The result yields a simple proof
of a conjecture on the squares of imbalances raised recently by Stanley.AMS Subject Classification: 05E10, 05A15. 相似文献
326.
327.
Astrid?Gutierrez-Zepeda Ross?Santell Zhixin?Wu Marishka?Brown YanJue?Wu Ikhlas?Khan Christopher?D?Link Baolu?Zhao Yuan?LuoEmail author 《BMC neuroscience》2005,6(1):54
Background
Epidemiological studies have associated estrogen replacement therapy with a lower risk of developing Alzheimer's disease, but a higher risk of developing breast cancer and certain cardiovascular disorders. The neuroprotective effect of estrogen prompted us to determine potential therapeutic impact of soy-derived estrogenic compounds. Transgenic C. elegans, that express human beta amyloid (Aβ), were fed with soy derived isoflavones genistein, daidzein and glycitein (100 μg/ml) and then examined for Aβ-induced paralysis and the levels of reactive oxygen species. 相似文献328.
TEM stereo-imaging of mesoporous zeolite single crystals 总被引:1,自引:0,他引:1
Boisen A Schmidt I Carlsson A Dahl S Brorson M Jacobsen CJ 《Chemical communications (Cambridge, England)》2003,(8):958-959
Mesoporous zeolite single crystals with intracrystalline mesopores and metal oxide particles located in the zeolite mesopore are characterised by direct TEM stereo-imaging. 相似文献
329.
Beata G. Vertesse Salvatore Magaz Alfonso Mangione Federica Migliardo Astrid Brandt 《Macromolecular bioscience》2003,3(9):477-481
The present study investigates shape properties of the enzyme dUTPase from Escherichia coli in the solution phase. In this work small angle neutron scattering (SANS) findings on dUTPase/D2O solutions for temperature values of T = 8 °C and T = 37 °C are presented. The analysis of SANS data, carried out by using a prolate ellipsoid core/shell model fitting and the well‐known Guinier and Zimm analysis procedures allows the characterization of the shape of the protein in solution. By means of the comparison with experimental and theoretical data existing in literature on dUTPase in the crystalline state, we find that the protein in solution maintains its dimensions before the denaturation process. Furthermore, by analyzing the SANS spectra of dUTPase/D2O/trehalose solutions, we emphasize the bioprotective effects of trehalose on the protein.
330.
The Kumjian–Pask algebra KP(Λ) is a graded algebra associated to a higher-rank graph Λ and is a generalization of the Leavitt path algebra of a directed graph. We analyze the minimal left ideals of KP(Λ), and identify its socle as a graded ideal by describing its generators in terms of a subset of vertices of the graph. We characterize when KP(Λ) is semisimple, and obtain a complete structure theorem for a semisimple Kumjian–Pask algebra. As a consequence of this structure theorem, every semisimple Kumjian–Pask algebra can be obtained as a Leavitt path algebra of a directed graph. 相似文献