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211.
The marine triquinane sesquiterpene Δ9(12)-capnellene () was synthesised from 2,2,5-trimethyl-5-hexenal () by a sequence of all synthetic operations in 5% overall yield. The key steps 3 → 4 → 5 → 6 involve two intramolecular type-I-“Mg-ene” processes. 相似文献
212.
Wolfgang A. Herrmann Martina Flöel Cornelia Weber John L. Hubbard Adolf Schäfer 《Journal of organometallic chemistry》1985,286(3):369-397
The eighteen new μ-alkylidene ruthenium complexes 5a–r and 5t are very easily and cleanly obtained along the diazoalkane or the hydrazone routes that involve treatment of the dinuclear, metal-metal doubly bonded precursor compound [(η5-C5H5)Ru(μ-NO)]2 (3) either with the diazoalkanes oxidizing agent (e.g., MnO2), with the respective hydrazones. Similarly, sulfur dioxide adds cleanly to the RuRu double bond of 3, thus giving the complex (μ-SO2)[(η5-C5H5)Ru(NO)]2 (5s). Regardless of the nature of the carbene bridge ligands, the dimetallacyclopropanes exhibit, in contrast to their iron analogues, exclusively terminal nitrosyl ligands. cis/trans-Isomerism with predominating amounts of the trans-isomers is observed for the derivatives that display unsymmetrically substituted carbene bridges.Treatment of the μ-methylene- and μ-ethylidene complexes (μ-CH2)[(η5-C5H5)Ru(NO)]2 (5a) and (μ-CHCH3)[(η5-C tetrafluoroboric acid or trifluoromethanesulfonic acid in diethyl ether yields, at ambient temperature, quantitatively the ionic complexes 6a,b and 7a,b, respectively, which were shown by 1H NMR spectroscopy to contain metal-metal bridging hydrogen functionalities. The reaction of hydrogen bromide with 5a under the same conditions gives the neutral bromo(methyl) complex 6d. This latter compound results from the isolable ionic intermediate of composition [(μ-CH2)(μ-H){(η5C5H5)Ru(NO)}2]+Br? (6c), which reaction stems from the nucleophilicity of the halide ion present in 6c. 相似文献
213.
The reaction of peri-substituted anthracenediones with sodium dithionite in dimethylformamide and water has been investigated. The system selectively reduces the carbonyl group flanked by the peri substituents of the anthracenediones to give the corresponding 4,5-disubstituted 9(10H)-anthracenones and thus provides a route to anthracenones which are otherwise difficult to obtain. Many functional groups can be tolerated, the reaction is compatible with the presence of peri alkoxy groups and unsaturated side chains of the starting anthracenediones, and the reduction does not go beyond the anthracenone stage. However, the formation of anthracenones depends on the nature of the peri substituents. No products were obtained from the 1,8-dimethyl-substituted anthracenedione and the parent compound with no substituents. 相似文献
214.
Fry BG Wickramaratna JC Hodgson WC Alewood PF Kini RM Ho H Wüster W 《Rapid communications in mass spectrometry : RCM》2002,16(6):600-608
Death adders (genus Acanthophis) are unique among elapid snakes in both morphology and venom composition. Despite this genus being among the most divergent of all elapids, the venom has been historically regarded as relatively quite simple. In this study, liquid chromatography/mass spectrometry (LC/MS) analysis has revealed a much greater diversity in venom composition, including the presence of molecules of novel molecular weights that may represent a new class of venom component. Furthermore, significant variation exists between species and populations, which allow for the LC/MS fingerprinting of each species. Mass profiling of Acanthophis venoms clearly demonstrates the effectiveness of this technique which underpins fundamental studies ranging from chemotaxonomy to drug design. 相似文献
215.
216.
Wolfgang Benetka Manfred Koranda Frank Eisenhaber 《Monatshefte für Chemie / Chemical Monthly》2006,137(10):1241-1281
Summary. Since 1979, when prenylation has been first discovered as chemical oddity of a yeast mating factor, the two forms of this
posttranslational modification of proteins (farnesylation and geranylgeranylation) have been found as wide spread among proteins
from Eukarya and their viruses. This review attempts to summarize as comprehensively as possible the enzymological processes
of prenylation and the various aspects of their biological significance.
The substrate proteins of prenyltransferases are known to carry a sequence signal composed of a cysteine-containing 4–5 residue
stretch at the utmost C-terminal end that is N-terminally preceded by a flexible and polar linker region of ca. 10 residues. Postprenylation processing of substrate proteins can involve C-terminal proteolysis, C-terminal carboxyl methylation,
and other steps of maturation. The prenyl anchor functions as module for membrane attachment or for protein–protein interaction.
Prenyl anchor carrying proteins fulfill a large array of functions in signaling and regulation of cellular processes. Therefore,
they are involved in the pathogenesis of a variety of human diseases, the most prominent one being cancer. Farnesyltransferase
inhibitors show surprisingly high efficiency in controlling tumor growth in model systems but, so far, clinical trials with
human patients have remained without the desired success. Interference into prenylation pathways appears also a promising
treatment principle in a variety of parasitic diseases. 相似文献
217.
Holzwarth AR Müller MG Niklas J Lubitz W 《The journal of physical chemistry. B》2005,109(12):5903-5911
The fluorescence kinetics of photosystem I core particles from Chlamydomonas reinhardtii have been measured with picosecond resolution in order to test a previous hypothesis suggesting a charge recombination mechanism for the early electron-transfer steps and the fluorescence kinetics (Müller et al. Biophys. J. 2003, 85, 3899-3922). Performing global target analyses for various kinetic models on the original fluorescence data confirms the "charge recombination" model as the only acceptable one of the models tested while all of the other models can be excluded. The analysis allowed a precise determination of (i) the effective charge separation rate constant from the equilibrated reaction center excited state (438 ns(-1)) confirming our previous assignment based on transient absorption data (Müller et al. Biophys. J. 2003, 85, 3899-3922), (ii) the effective charge recombination rate constant back to the excited state (52 ns(-1)), and (iii) the intrinsic secondary electron-transfer rate constant (80 ns(-1)). The average energy equilibration lifetime core antenna/RC is about 1 ps in the "charge recombination" model, in agreement with previous transient absorption data, vs the 18-20 ps energy transfer lifetime from antenna to RC within "transfer-to-the-trap-limited" models. The apparent charge separation lifetime in the recombination model is about three times faster than in the "transfer-to-the-trap-limited" model. We conclude that the charge separation kinetics is trap-limited in PS I cores devoid of red antenna states such as in C. reinhardtii. 相似文献
218.
Sarkar B Kaim W Klein A Schwederski B Fiedler J Duboc-Toia C Lahiri GK 《Inorganic chemistry》2003,42(20):6172-6174
The Ru(2)(III,II) mixed-valent state is strongly stabilized in [(bpy)(2)Ru(mu-bttz)Ru(bpy)(2)](5+) (3(5+), bttz = 3,6-bis(2-thienyl)-1,2,4,5-tetrazine, as evident from lowered oxidation potentials and isolability, a strongly increased comproportionation constant K(c) = 10(16.6), and a high-energy intervalence charge transfer band at 10100 cm(-1). Curiously, no such effects were observed for the diosmium(III,II) analogue, whereas the related systems [(bpy)(2)M(mu-bmptz)M(bpy)(2)](5+), bmptz = 3,6-bis(4-methyl-2-pyridyl)-1,2,4,5-tetrazine, exhibit conventional behavior, i.e., a slightly higher K(c) value of the Os(2)(III,II) analogue. EPR signals were observed at 4 K for 3(5+) but not for the other mixed-valent species, and high-frequency (285 GHz) EPR was employed to study the diruthenium(II) radical complexes 2(3+) and 3(3+). 相似文献
219.
Wolfgang Oppolzer 《Angewandte Chemie (International ed. in English)》1989,28(1):38-52
Metallo-ene reactions, hardly recognized until very recently, have experienced a breathtaking development when applied in an intramolecular sense. Efficient regio- and stereoselective magnesium-ene cyclizations have served as a cornerstone for numerous syntheses of structurally diverse natural products (e.g., sesquiterpenes of marine or plant origin, alkaloids, fragrances, insect defense compounds, and a fungitoxin). A brilliant example is the synthesis of the elusive odorant (+)-khusimone which outshines 20 years of work in the field of tricyclovetivane synthesis. Palladium-, platinum-, and nickel-catalyzed versions of the metallo-ene reaction are in a comparatively early stage of exploration, but, nevertheless, reveal intriguing potential. Hence an almost 100% stereospecific C? O→C? ;Pd-→ C? C chirality transfer permits simple and selective, cis- or trans-annelation processes. The mild cyclization conditions are compatible with various functional groups, such as nitrogen moieties, which offer interesting perspectives for the preparation of heterocycles (e.g., alkaloids) difficult to obtain by other methods. Carbon monoxide insertion reactions of the cyclized σ-metal intermediates were shown to afford annelated cyclopentanones and cyclopentenones with concomitant stereocontrolled formation of four carbon–carbon bonds. These and other observations, highlighted in this article, provide a platform for further extensions and applications of this powerful method in organic synthesis. 相似文献
220.