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The tetracyclic core of the lundurine family of alkaloids has been synthesized by a novel approach that features a double ring-closing olefin metathesis to form the five-and eight-membered rings. 相似文献
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Vera J. Meyer Prof. Dr. Meike Niggemann 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(15):4687-4691
A calcium‐catalyzed direct reduction of propargylic alcohols and ethers has been accomplished by using triethylsilane as a nucleophilic hydride source. At room temperature a variety of secondary propargylic alcohols was deoxygenated to the corresponding hydrocarbons in excellent yields. Furthermore, for the first time, a catalytic deoxygenation of tertiary propargylic alcohols was generally applicable. The same protocol was suitable for an efficient reduction of secondary as well as tertiary propargylic methyl, benzyl and allyl ethers. Substrates containing an additional keto‐, ester or secondary hydroxyl function were reduced with exceptional chemoselectivity at the propargylic position. 相似文献
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Leslie‐Anne Fendt Meike Stöhr Dr. Nikolai Wintjes Dr. Mihaela Enache Thomas A. Jung Dr. François Diederich Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(42):11139-11150
The self‐assembly properties of two ZnII porphyrin isomers on Cu(111) are studied at different coverage by means of scanning tunneling microscopy (STM). Both isomers are substituted in their meso‐positions by two voluminous 3,5‐di(tert‐butyl)phenyl and two rod‐like 4′‐cyanobiphenyl groups, respectively. In the trans‐isomer, the two 4′‐cyanobiphenyl groups are opposite to each other, whereas they are located at right angle in the cis‐isomer. For coverage up to one monolayer, the cis‐substituted porphyrins self‐assemble to form oligomeric macrocycles held together by antiparallel CN???CN dipolar interactions and CN???H‐C(sp2) hydrogen bonding. Cyclic trimers and tetramers occur most frequently but everything from cyclic dimers to hexamers can be observed. Upon annealing of the samples at temperatures >150 °C, dimeric macrocyclic structures are observed, in which the two porphyrins are bridged by Cu atoms, originating from the surface, under formation of two CN???Cu???NC coordination bonds. The trans‐isomer builds up linear chains on Cu(111) at low coverage, whereas for higher coverage the molecules assemble in a periodic, densely packed structure. Both cis‐ and trans‐bis(4′‐cyanobiphenyl)‐substituted ZnII porphyrins behave very differently on Cu(111) compared to similar porphyrins in literature on less reactive surfaces such as Au(111) and Ag(111). On the latter surfaces, there is no signal visible between molecular orientation and the crystal directions of the substrate, whereas on Cu(111), very strong adsorbate–substrate interactions have a dominating influence on all observed structures. This strong porphyrin–substrate interaction enables a much broader variety of structures, including also less favorable intermolecular bonding motifs and geometries. 相似文献
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An in‐tournament is an oriented graph such that the negative neighborhood of every vertex induces a tournament. The topic of this paper is to investigate vertex k‐pancyclicity of in‐tournaments of order n, where for some 3 ≤ k ≤ n, every vertex belongs to a cycle of length p for every k ≤ p ≤ n. We give sharp lower bounds for the minimum degree such that a strong in‐tournament is vertex k‐pancyclic for k ≤ 5 and k ≥ n − 3. In the latter case, we even show that the in‐tournaments in consideration are fully (n − 3)‐extendable which means that every vertex belongs to a cycle of length n − 3 and that the vertex set of every cycle of length at least n − 3 is contained in a cycle of length one greater. In accordance with these results, we state the conjecture that every strong in‐tournament of order n with minimum degree greater than is vertex k‐pancyclic for 5 < k < n − 3, and we present a family of examples showing that this bound would be best possible. © 2001 John Wiley & Sons, Inc. J Graph Theory 36: 84–104, 2001 相似文献
45.
Wittke S Fliser D Haubitz M Bartel S Krebs R Hausadel F Hillmann M Golovko I Koester P Haller H Kaiser T Mischak H Weissinger EM 《Journal of chromatography. A》2003,1013(1-2):173-181
The on-line coupling of capillary electrophoresis (CE) with electrospray-time-of-flight mass spectrometry (MS) has been used to obtain patterns of peptides and proteins present in the urine of healthy human individuals. This led to the establishment of a "normal urine polypeptide pattern", consisting of 247 polypeptides, each of which was found in more than 50% of healthy individuals. Applying CE-MS to the analysis of urine of patients with kidney disease revealed differences in polypeptide pattern. Twenty-seven polypeptides were exclusively found in samples of patients. Another 13, present in controls, were missing. These data indicate that CE-MS can be applied as powerful tool in clinical diagnostics. 相似文献
46.
Capillary electrophoresis coupled to mass spectrometry to establish polypeptide patterns in dialysis fluids 总被引:1,自引:0,他引:1
Kaiser T Hermann A Kielstein JT Wittke S Bartel S Krebs R Hausadel F Hillmann M Golovko I Koester P Haller H Weissinger EM Fliser D Mischak H 《Journal of chromatography. A》2003,1013(1-2):157-171
Combination of capillary electrophoresis with mass spectrometry (CE-MS) allows generation of polypeptide patterns of body fluids. In a single CE-MS (45 min) run more than 600 polypeptides were analyzed in hemodialysis fluids obtained with different membranes (high-flux/low-flux). Larger polypeptides (M(r) > 10 000) were almost exclusively present in high-flux dialysates only, while in low-flux dialysates additional small polypeptides were detected. Comparison to the normal urine pattern yielded a surprisingly low consensus: a number of polypeptides present in urine were missing. We established a fast and sensitive technique, easily applicable to the monitoring of different modalities of dialyzers. 相似文献
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Iris Koch Markus Langner Dennis Holzinger Maximilian Merkel Meike Reginka Rico Huhnstock Andreea Tomiţa Claudia Jauregui Caballero Andreas Greiner Arno Ehresmann 《Particle & Particle Systems Characterization》2021,38(12):2100072
The possibility to arrange and embed magnetic micro- and nanoparticles in thin polymer film systems using flat magnetically patterned substrate templates is investigated. In contrast to self-organized particle rows forming by applying a homogeneous magnetic field, particles adapt to the magnetic field landscape of the substrate's magnetic pattern prior to polymer crosslinking. Crosslinking then fixes the particle positions in the polymer. The process is tested for composites of hydrophobic polydimethylsiloxane (PDMS) and maghemite nanoparticles as well as for hydrophilic polyvinyl alcohol (PVOH) and hydrophilic functionalized, superparamagnetic core–shell microspheres. The substrate template is an exchange bias layer system magnetically patterned into parallel-stripe domains with in-plane magnetizations and head-to-head/tail-to-tail remanent magnetization orientation in adjacent magnetic domains. A high occupancy percentage of magnetic beads on a domain wall as well as anisotropic actuation of the composite is achieved. 相似文献
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Titanium‐Catalyzed Vinylic and Allylic C?F Bond Activation—Scope,Limitations and Mechanistic Insight
Dr. Moritz F. Kuehnel Philipp Holstein Meike Kliche Juliane Krüger Stefan Matthies Dominik Nitsch Joseph Schutt Michael Sparenberg Prof. Dr. Dieter Lentz 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(34):10701-10714
The hydrodefluorination (HDF) of fluoroalkenes in the presence of a variety of titanium catalysts was studied with respect to scope, selectivity, and mechanism. Optimization revealed that the catalyst requires low steric bulk and high electron density; secondary silanes serve as the preferred hydride source. A broad range of substrates yield partially fluorinated alkenes, such as previously unknown (Z)‐1,2‐(difluorovinyl)ferrocene. Mechanistic studies indicate a titanium(III) hydride as the active species, which forms a titanium(III) fluoride by H/F exchange with the substrate. The HDF step can follow both an insertion/elimination and a σ‐bond metathesis mechanism; the E/Z selectivity is controlled by the substrate. The catalysts’ ineffieciency towards fluoroallenes was rationalized by studying their reactivity towards Group 6 hydride complexes. 相似文献