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排序方式: 共有1371条查询结果,搜索用时 281 毫秒
31.
Erich Widmer Milan Soukup Reinhard Zell Emil Broger Hans Peter Wagner Marquard Imfeld 《Helvetica chimica acta》1990,73(4):861-867
Starting from the readily available, optically active (4R)-4-hydroxy-2,2,6-trimethylcyclohexanone ( 1 ), a new technical synthesis of (3R,3′R)-zeaxanthin is described. According to a 2(C9 + C6) + C10 = C40 construction scheme, the ketone 1 was first transformed with (E)-3-methylpent-2-en-4-yn-1-ol ( 5 ) into a C15-intermediate which, by a three-step sequence, could be converted into the known olefinic C15-Wittig salt 4 . Optimized conditions for the final Wittig reaction of 4 with the C10-dialdehyde 3 are discussed. Based on 1 , the overall yield of the entire technical process is ca. 40%. 相似文献
32.
F M Demotes-Mainaird G A Vin?on C H Jarry H C Albin 《Journal of chromatography. A》1989,490(1):115-123
A simple and sensitive high-performance liquid chromatographic micro-method for the determination of roxithromycin in human plasma and urine is described. A dichloromethane extract of the sample was chromatographed on a C18 reversed-phase column with acetonitrile-83 mM ammonium acetate-methanol (55:23:22, v/v) adjusted to pH 7.5 with acetic acid as the mobile phase. Roxithromycin and the internal standard, erythromycin, were detected by dual coulometric electrodes operated in the oxidative screen mode. The applied cell potential of the screen electrode was set at +0.7 V and the sample electrode at +0.9 V. The intra- and inter-assay coefficients of variation were less than or equal to 7.0%. The detection limit (signal-to-noise ratio = 3) was 0.1 microgram/ml for both plasma and urine. A study of drug stability during sample storage at 4, 20 and 37 degrees C showed no degradation of roxithromycin. The method is convenient for clinical monitoring and pharmacokinetic studies. 相似文献
33.
Thale Z Johnson T Tenney K Wenzel PJ Lobkovsky E Clardy J Media J Pietraszkiewicz H Valeriote FA Crews P 《The Journal of organic chemistry》2002,67(26):9384-9391
A reinvestigation of sponge natural products from additional Indo-Pacific collections of Xestospongiacf. carbonaria and X. cf. exigua has provided further insights on the structures, biological activities, and biosynthetic origin of bisannulated acridines. These alkaloids include one known pyridoacridine, neoamphimedine (2), and three new analogues, 5-methoxyneoamphimedine (4), neoamphimedine Y (5), and neoamphimedine Z (6). A completely new acridine, alpkinidine (7), was also isolated. A disk diffusion soft agar assay, using a panel of five cancer cell lines (solid tumors and leukemias) and two normal cells, was used to evaluate the differential cytotoxicity (solid tumor selectivity) of the sponge semipure extracts and selected compounds including amphimedine (1), 2, 4, and 7. While all four compounds were solid tumor selective, 1 and 2 were the most potent and 4 was the most selective. The rationale used to characterize the new structures is outlined along with the related biosynthetic pathways envisioned to generate 2 and 7. 相似文献
34.
Bernskoetter WH Lobkovsky E Chirik PJ 《Journal of the American Chemical Society》2005,127(40):14051-14061
The rates of hydrogenation of the N2 ligand in the side-on bound dinitrogen compounds, [(eta(5)-C5Me4H)2Zr]2(mu2,eta(2),eta(2)-N2) and [(eta(5)-C5Me5)(eta(5)-C5H2-1,2-Me2-4-R)Zr]2(mu2,eta(2),eta(2)-N2) (R = Me, Ph), to afford the corresponding hydrido zirconocene diazenido complexes have been measured by electronic spectroscopy. Determination of the rate law for the hydrogenation of [(eta(5)-C5Me5)(eta(5)-C5H2-1,2,4-Me3)Zr]2(mu2,eta(2),eta(2)-N2) establishes an overall second-order reaction, first order with respect to each reagent. These data, in combination with a normal, primary kinetic isotope effect of 2.2(1) for H2 versus D2 addition, establish the first H2 addition as the rate-determining step in N2 hydrogenation. Kinetic isotope effects of similar direction and magnitude have also been measured for hydrogenation (deuteration) of the two other zirconocene dinitrogen complexes. Measuring the rate constants for the hydrogenation of [(eta(5)-C5Me5)(eta(5)-C5H2-1,2,4-Me3)Zr]2(mu2,eta(2),eta(2)-N2) over a 40 degrees C temperature range provided activation parameters of deltaH(double dagger) = 8.4(8) kcal/mol and deltaS(double dagger) = -33(4) eu. The entropy of activation is consistent with an ordered four-centered transition structure, where H2 undergoes formal 1,2-addition to a zirconium-nitrogen bond with considerable multiple bond character. Support for this hypothesis stems from the observation of N2 functionalization by C-H activation of a cyclopentadienyl methyl substituent in the mixed ring dinitrogen complexes, [(eta(5)-C5Me5)(eta(5)-C5H2-1,2-Me2-4-R)Zr]2(mu2,eta(2),eta(2)-N2) (R = Me, Ph), to afford cyclometalated zirconocene diazenido derivatives. 相似文献
35.
Present proteomics and biomedicine require sensitive analytical methods for all proteins. Recent progress in electrochemical analysis of peptides and proteins based on their intrinsic electroactivity is reviewed. Tyrosine and/or tryptophan‐containing proteins are oxidizable at carbon electrodes. At mercury electrodes all peptides and proteins (about 13 peptides and >25 proteins were tested) produce chronopotentiometric peak H at nanomolar concentrations. This peak is sensitive to changes in protein structure. Microliter sample volumes are sufficient for the analysis. Electrochemical methods can be used in studies of nucleic acid‐protein interactions and can be applied in biomedicine. Examples of such applications in neurogenerative diseases and cancer are presented. 相似文献
36.
Kurt Burdeska Hermann Fuhrer Guglielmo Kabas Adolf Emil Siegrist 《Helvetica chimica acta》1981,64(1):113-152
Preparation of Styryl and Stilbenyl Derivatives of Pyrimidines 2- and 4-(p-Tolyl)-substituted pyrimidines react with anils of hetero-aromatic aldehydes in the presence of dimethylformamide and potassium hydroxide or potassium t-butoxide to yield the corresponding 2- and 4-[4″-(heteroaryl)stilben-4′-yl]pyrimidines or the 2- and 4-[a-(heteroaryl)-4′-styryl]pyrimidines respectively (‘Anil synthesis’). Furthermore, the Schiff′s bases derived from p-chloroaniline and 4-(pyrimidine-2-yl and 4-yl)benzaldehydes give, with methyl- and with p-tolyl-substituted heterocycles, the corresponding heterocyclic substituted styryl and stilbenyl derivatives. Alkyl-, alkoxy- or phenyl-substituted pyrimidines undergo also the ‘Anil synthesis’. 相似文献
37.
Wolfgang Oppolzer Roland Achini Emil Pfenninger Hans Peter Weber 《Helvetica chimica acta》1976,59(4):1186-1202
The N-allyl-N-cinnamyl amide 10 undergoes thermal cyclization to a 2:1-mixture of the trans- and cis-benz(f)isoindolines 11a and 12a . By comparison, the thermolysis of the corresponding bis-cinnamylamide 14 proceeds in a highly stereoselective manner to give the cis-fused[4+2]-adduct 16a . Similarly, the trans-fused stereoisomeric adducts 30a and 31a were obtained with high stereochemical control on heating the N-allyl-N-diphenylallyl amide 28 . The thermal transformations 4 → 5 + 6a and 17 → 18a + 20a show the competitive formation of [2+2]-adducts. An alternative approach to (substituted) benz[f]isoindolines 16 via the all-cis-isomer 24a has been developed. The described structures have been assigned on the basis of spectral evidence, chemical correlations and by X-ray-diffraction study of the isomer 16b . These results illustrate the utility of substituent interactions in order to direct intramolecular cyclo-additions at will towards either endo- or exo-products. 相似文献
38.
39.
Emil W. Kiss 《Algebra Universalis》1992,29(4):455-476
First a problem of Ralph McKenzie is answered by proving that in a finitely directly representable variety every directly indecomposable algebra must be finite. Then we show that there is no local proof of the fundamental theorem of Abelian algebras nor of H. P. Gumm's permutability results. This part may also be of interest for those investigating non-modular Abelian algebras. Finally we provide a Gumm-type characterization of the situation when twonot necessarily comparable congruences centralize each other. In doing this, we introduce a four variable version of the difference term in every modular variety. A two-terms condition is also investigated.Dedicated to the memory of András HuhnPresented by R. W. Quackenbush. 相似文献
40.
Emil Wolff 《Fresenius' Journal of Analytical Chemistry》1864,3(1):85-115
Ohne ZusammenfassungBegutachtet von den Commissionsmitgliedern: Dr. Bretschneider, Dr. Grouven, Professor Dr. Knop, Dr. Peters, Dr. Stohmann und Dr. Zöller.Bei der ersten Wanderversammlung deutscher Agriculturchemiker, welche im Mai 1863 zu Leipzig gehalten wurde, sind zum Behufe der Ausarbeitung gemeinsam zu befolgender Methoden und Untersuchungssysteme Commissionen ernannt worden. Als erste Frucht von deren Arbeiten ist in den Landw. Versuchs-Stationen, dem Organ für wissenschaftliche Forschungen auf dem Gebiete der Landwirthschaft, herausgegeben von Dr. Fr. Nobbe, Band VI, der Entwurf zurBodenanalyse von Dr. Emil Wolff, begutachtet von den andern Commissionsmitgliedern, veröffentlicht worden. Ich theile denselben im Einverständniss mit dem Herrn Verfasser auch an dieser Stelle mit, einerseits um den löblichen Zweck, welchen die Wanderversammlung der deutschen Agriculturchemiker im Auge hat, nach Kräften zu fördern, andererseits die mancherlei neuen Methoden, welche bei agriculturchemischen Untersuchungen in Gebrauch gekommen sind, auch in den Kreisen zu verbreiten, welchen die speciell agriculturchemischen Schriften nicht zu Gesicht kommen. Ich füge hinzu, dass die kleine Modification auf S. 94, sowie die Anmerkungen ebendaselbst in Folge brieflicher Mittheilung des Herrn Prof. Wolff aufgenommen worden sind. Dem Wunsche desselben, dass ich meinerseits Bemerkungen zufügen möchte, werde ich gerne entsprechen, so bald ich Zeit finde, betreffende Experimentaluntersuchungen vorzunehmen. R. F. 相似文献