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31.
This work deals with the synthesis of piperidine and hydroxypiperidine analogs of nucleosides. Starting from commercially available 3-hydroxypiperidine, proline or 4-hydroxyproline, a series of piperidine derivatives of both purine and pyrimidine nucleobases was prepared. Various methods of nucleobase attachment were evaluated. The prepared compounds were tested for cytostatic, antibacterial, and antiviral properties but no significant activity was found. 相似文献
32.
Dörr FA Kovačevi B Maksi ZB Pinto E Volmer DA 《Journal of the American Society for Mass Spectrometry》2011,22(11):2011-2020
The aim of this study was to investigate the unusual gas-phase dissociation behavior of two epimer pairs of protonated gonyautoxins
(GTX) following electrospray ionization in comparison to their deprotonated counterparts. The chemical structures of the investigated
GTX1-4 variants vary in their substitution pattern at N-1 and the stereochemical orientation of the hydroxysulfate group at C-11
(11α for GTX1/2 versus 11β for GTX3/4). The direct comparison of mass spectra in positive and negative ion modes illustrated two distinct features: first, an intriguing
difference between protonated 11α and 11β species, where 11α conformations exhibited almost complete dissociation of [M +
H]+ ions via facile SO3 elimination, while 11β species remained mostly intact as [M + H]+; and second, the lack of such differences for the deprotonated counterparts. In this study, we propose an acid-catalyzed
elimination mechanism from density functional theory calculations, initiated by a proton transfer of a guanidinium proton
to the hydroxysulfate group with simultaneous SO3 release, which is only possible for the 11α conformation based on intramolecular distances. The same mechanism explains the
lack of a comparable SO3 loss in the negative ion mode. CID experiments supported this proposed mechanism for GTX1 and GTX2. Computational modeling of product ions seen in the CID spectra of GTX3 and GTX4 established that the lowest energy dissociation pathway for the 11β epimers is elimination of water with the possibility
for further SO3 release from the intermediate product. Experimental data for structurally analogous decarbamoyl gonyautoxins confirmed the
evidence for the GTX compounds as well as the proposed elimination mechanisms. 相似文献
33.
B. Kojić-Prodić M. Tonković O. Hadžija V. Kovač 《Analytical and bioanalytical chemistry》1976,279(5):359-362
For the cyanide determination in waste waters containing more than 2.5×10?6 Mol of CN?/l a conductometric method has been proposed. The sulphuric acid containing samples were distilled and the obtained HCN was passed into a 5×10?4 Mol/l AgNO3 solution, of which the conductivity was measured. The method offers the possibility of a precise observation of the completion of the HCN distillation and also the direct measurement of the cyanide concentration by the change of conductivity. The analysis time is about 40 min. 相似文献
34.
Gregor Kovačič Thomas A. Wettergren 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》1996,47(2):221-264
We present a plethora of homoclinic and heteroclinic orbits that exist in the phase space of a weakly nonlinear model describing small oscillations of two resonantly driven coupled pendula. These orbits connect equilibria in a resonance band, which is contained in an invariant plane, and is born under perturbation out of a circle of equilibria. Their existence is shown by using a combination of the Melnikov method and singular perturbation techniques. 相似文献
35.
36.
On a quadrature formula of Micchelli and Rivlin 总被引:4,自引:0,他引:4
Borislav Bojanov 《Journal of Computational and Applied Mathematics》1996,70(2):349-356
Micchelli and Rivlin (1972) obtained a quadrature formula of highest algebraic degree of precision for the Fourier-Chebyshev coefficients An(f), which is based on the divided differences of f′ at the zeros of the Chebyshev polynomial Tn(x). We give here a simple approach to questions of this type, which applies to the coefficients in arbitrary orthogonal expansion of f. As an auxiliary result we obtain a new interpolation formula and a new representation of the Turán quadrature formula. 相似文献
37.
38.
Derivatives of pyrimido(1,2-b)indazole and s-triazino(1,2-b)indazole were prepared by several different routes and some transformations are described. 相似文献
39.
40.