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61.
Determination of valepotriates 总被引:2,自引:0,他引:2
In this paper an overview is given of qualitative and quantitative methods of analysis used for valepotriates. Methods like spectophotometry, titrimetry, TLC, GC, HPLC, MS, CE as well as p-SFC have been applied. Today HPLC is the method of choice. The usefulness of the individual methods are discussed. 相似文献
62.
Summary The paper deals with mixed boundary value problems and transmission problems for elliptic differential and pseudo-differential operators. These problems are considered as elements of an algebra (where is the considered domain, x=, YX a submanifold of codimension 1, where the conditions have a jump, and x is a certain compactification of X/Y), the elements of which are described on a symbolic level. By means of additional conditions (of trace and potential type, with respect to Y) a given mixed problem is extended to an operator, and the Fredholm property of is established under an ellipticity condition, the parametrix is constructed on symbolic level and ind is expressed. In the second part the authors also study classes of special cases, namely differential operators of second and higher order with mixed boundary conditions for dim 2, where explicit index expressions are derived. 相似文献
63.
Humphrey JM Liao Y Ali A Rein T Wong YL Chen HJ Courtney AK Martin SF 《Journal of the American Chemical Society》2002,124(29):8584-8592
As a prelude to undertaking the total syntheses of the complex manzamine alkaloids, a series of model studies were conducted to establish the scope and limitations of intramolecular [4 + 2] cycloadditions of N-acylated vinylogous ureas with the trienic substrates 17a,b, 28a,b, and 34. These experiments clearly demonstrated that the geometry of the internal double bond and the presence of an electron-withdrawing group on the diene moiety were essential for the facile and stereoselective formation of the desired cycloadducts. The enantioselective syntheses of the manzamine alkaloids ircinol A (75), ircinal A (5), and manzamine A (1) were then completed by employing a convergent strategy that featured a novel domino Stille/Diels-Alder reaction to construct the tricyclic ABC ring core embodied in these alkaloids. Thus, the readily accessible chiral dihydropyrrole 58 was first converted in a single chemical operation into the key tricyclic intermediate 60. Two ring-closing metathesis reactions were then used to form the 13- and 8-membered rings leading to Z-72 and 74, the latter of which was quickly elaborated into ircinal A (5) via ircinol A (75). The synthetic 5 thus obtained was converted into manzamine A (1) following literature precedent. This concise synthesis of ircinal A required a total of 24 operations from commercially available starting materials with the longest linear sequence being 21 steps. 相似文献
64.
Cooperative effects in binuclear zirconocenes: Their synthesis and use as catalyst in propene polymerization 总被引:3,自引:0,他引:3
Stephan Jüngling Rolf MüllhauptHerbert Plenio 《Journal of organometallic chemistry》1993,460(2):191-195
The mono- and bis-cyclopentadienyl compounds 1-(Cp″)-4-(CH3)C6H4 (1) and 1, 4-(Cp″)2C6H4 (2) (Cp″ = 3,4-dimethylcyclopenta-1,3-diene-1-yl) have been synthesized. The reactions of the lithium salts of 1 and 2 with CpZrCl3 · dme (dme = dimethoxyethane) and Cp*ZrCl3(CP* = C5(CH3)5) yielded the mono- and bi-nuclear bridged zirconocenes 1-(Cp″ZrCpCl2)-4-(CH3)C6H4 (3), 1,4-(Cp″ZrCpCl2)2C6H4 (4) and 1,4-(Cp″ZrCp*Cl2)2C6H4 (5). When activated with methylaluminoxane (MAO), the mono- and bi-nuclear zirconocenes 3 and 4 catalyse the polymerization of propene. The influence of the catalyst composition on the polymerization kinetics and molecular weight is discussed. 相似文献
65.
66.
Stephan Jüngling Rolf Mülhaupt Udo Stehling Hans-Herbert Brintzinger David Fischer Franz Langhauser 《Journal of polymer science. Part A, Polymer chemistry》1995,33(8):1305-1317
Propene was polymerized at 40°C and 2-bar propene in toluene using methylalumoxane (MAO) activated rac-Me2Si(Benz[e]Indenyl)2ZrCl2 ( BI ) and rac-Me2Si(2-Me-Benz[e]Indenyl)2ZrCl2 ( MBI ). Catalyst BI /MAO polymerizes propene with high activity to afford low molecular weight polypropylene, whereas MBI /MAO is less active and produces high molecular weight polypropylene. Variation of reaction conditions such as propene concentration, temperature, concentration of catalyst components, and addition of hydrogen reveals that the lower molecular weight polypropylene produced with BI /MAO results from chain transfer to propene monomer following a 2,1-insertion. A large fraction of both metallocene catalyst systems is deactivated upon 2,1-insertion. Such dormant sites can be reactivated by H2-addition, which affords active metallocene hydrides. This effect of H2-addition is reflected by a decreasing content of head-to-head enchainment and the formation of polypropylene with n-butyl end groups. Both catalysts show a strong dependence of activity on propene concentration that indicates a formal reaction order of 1.7 with respect to propene. MBI /MAO shows a much higher dependence of the activity on temperature than BI /MAO. At elevated temperatures, MBI /MAO polymerizes propene faster than BI /MAO. © 1995 John Wiley & Sons, Inc. 相似文献
67.
This study uses infrared (IR) spectroscopic, point detection, mapping procedures to examine tissue samples from normal brain specimens and from astrocytic gliomas, the most frequent human brain tumors. Model systems were derived from cultured glioma cell lines. IR spectra of normal tissue sections distinguished white matter from gray matter by increased spectral contributions from lipids and cholesterol. Qualitatively the same differences were found in IR spectra of low and high grade glioma tissue sections pointing to a significant reduction of brain lipids with increasing malignancy. Whereas spectral contributions of proteins and lipids were similar in IR spectra of glioma cells and tissues, nucleic acid bands were more intense for cells suggesting higher proliferative activities. For statistical analyses of IR spectroscopic maps from 71 samples, a parameter for the lipid to protein ratio was introduced involving the CH(2) symmetric stretch band with lipids as main contributors and the amide I band of proteins. As this parameter correlated with the grade of gliomas obtained from standard histopathological examination, it was applied to classify brain tissue sections based on IR spectroscopic mapping. 相似文献
68.
This study assessed the diagnostic potential of Raman spectroscopic mapping by evaluating its ability to distinguish between normal brain tissue and the human intracranial tumors gliomas and meningeomas. Seven Raman maps of native specimens were collected ex vivo by a Raman spectrometer with 785 nm excitation coupled to a microscope with a motorized stage. Variations within each Raman map were analyzed by cluster analysis. The dependence of tissue composition on the tissue type in cluster averaged Raman spectra was shown by linear combinations of reference spectra. Normal brain tissue was found to contain higher levels of lipids, intracranial tumors have more hemoglobin and lower lipid to protein ratios, meningeomas contain more collagen with maximum collagen content in normal meninges. One sample was studied without freezing. Whereas tumor regions did not change significantly, spectral changes were observed in the hemoglobin component after snap freezing and thawing to room temperature. The results constitute a basis for subsequent Raman studies to develop classification models for diagnosis of brain tissue. 相似文献
69.
Hans-Georg Schmarr Birgit Maas Armin Mosandl Stephan Bihler Hans-Peter Neukom Konrad Grob 《Journal of separation science》1991,14(5):317-321
An unusual peak defocusing effect influencing chromatographic performance over a limited range of elution temperatures is described for hexakis(2,6-di-O-pentyl-3-O-acetyl)-α-cyclodextrin stationary phase. Since this phenomenon is likely to be dependent on minor details of the cyclodextrin molecule, full assignment of the 1H- and 13C-NMR-spectra are given. 相似文献
70.
André L. Beauchamp Stephan Latour Marc J. Olivier James D. Wuest 《Journal of organometallic chemistry》1983,254(3):283-291
The title compound, C58H52Sn3, belongs to the triclinic space group P, with a 10.165, b 13.365, c 18.670 Å, α 96.28, β 93.88, γ 103.15°, V = 2443.8 Å3, fw = 1105.1, Z = 2, Dcalc 1.501 g cm?3, m.p. 206.5–208°C, λ(Mo-K) 0.71069 Å. The structure was refined on 2684 nonzero reflections to an R factor of 0.044. The crystal contains molecules in which the (SnCH2)3CH core possesses an approximate C3 symmetry. The three SnC(H2) bonds are gauche to the C(4)-H bond. Repulsive interactions involving the bulky Ph3Sn substituents lead to large SnC(H2)C(H) angles (av. 117.3°), whereas the C(H2)C(H)C(H2) angles at the tertiary carbon average 111.3°. Little distortion of the Ph3Sn groups themselves is present, since the PhSnPh angles (av. 109.8°) are almost equal to the C(H2)SnPh angles (av. 109.9°). The molecule as a whole has no symmetry because the aromatic rings in the three Ph3Sn groups have different orientations. The phenyl groups create a pocket in the middle of the molecule which encloses and shields the tertiary hydrogen atom. The resulting inaccessibility of this hydrogen accounts in part for the low reactivity of the title compound in redox reactions. 相似文献