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51.
In this communication we report a stereoselective total synthesis of N-Boc-dolaproine (Dap), an amino acid residue of the antineoplastic pentapeptide Dolastatin 10. Our strategy is based on a Baylis-Hillman reaction between N-Boc-prolinal and methyl acrylate, followed by a diastereoselective double bond hydrogenation and hydrolysis of the ester function. 相似文献
52.
AntnioF.DeC. Alcntara MariaG.F. Vaz HumbertoO. Stumpf Dorila Pil‐Veloso WagnerB. DeAlmeida 《Helvetica chimica acta》2004,87(2):425-438
Conformational studies of 1,3‐dihydroxy‐4,4,5,5‐tetramethyl‐2‐(pyridin‐1‐yl)imidazolidine ( 1a ) and 1,3‐dihydroxy‐4,4,5,5‐tetramethyl‐2‐(pyridin‐3‐yl)imidazolidine ( 1b ), carried out by using 1D 1H‐ and 13C‐NMR and 2D HMQC, HMBC, and NOESY experiments and with the aid of theoretical calculations, indicate that the OH groups are trans to the pyridinyl substituent. Because the two 1H‐NMR signals of the Me groups are distinguishable and do not change between 290 and 380 K, it is proposed that 1a and 1b have each only one conformation in this temperature range. This behavior was not found with 1,3‐dihydroxy‐4,4,5,5‐tetramethyl‐2‐(pyridin‐2‐yl)imidazolidine ( 1c ) because its Me 1H‐NMR signals cross over at 300 K. Hence, more than one conformation must be present, beyond those produced by simple inversions. Theoretical calculations including temperature and solvent effects were performed to provide further information on the conformational analysis and to help to assign the NMR data. The combination of NMR measurements and quantum‐chemical calculations is shown to be a very promising strategy for conformational analysis studies in solution. 相似文献
53.
This work was aimed at understanding the structure of SiO2–MO2 (M = Ti, Zr, Hf) and SiO2–HfO2–MO2 (M = Ti, Zr) materials, used as mixed oxide glass hosts for Er3+ ions in the fabrication of optical planar waveguides by sol-gel processing. This structural study was performed by Waveguide
Raman Spectroscopy (WRS), complemented with X-ray diffraction (XRD). The admixture of TiO2 to HfO2, SiO2–HfO2 and HfO2–ZrO2 compositions was found to cause precipitation of nanocrystals of tetragonal HfO2 or ZrO2, or the formation of hafnia-titania mixed crystals, depending on the HfO2/TiO2 molar ratio. 相似文献
54.
Rui M. Almeida Ana C. Marques Maurizio Ferrari 《Journal of Sol-Gel Science and Technology》2003,26(1-3):891-896
The present paper is focused on multilayer Er3+-doped silica-titania planar waveguides, co-doped with silver, which were prepared by spin-coating on silica glass, or buffered single crystal silicon substrates. The single layer thickness (0.4 m) and refractive index (1.60–1.63) were measured by spectroscopic ellipsometry at 715 nm. The thickness of the waveguides (measured by mechanical profilometry) was 1 m and their optical propagation losses were measured at different laser wavelengths (488 nm, 514 nm and 633 nm), exhibiting an approximately Rayleigh-like behavior. The thermal precipitation of silver nanocrystallites was achieved, both in air and under a controlled atmosphere (dry nitrogen) and these were characterized by visible absorption spectroscopy, which clearly showed the development of a plasmon absorption band near 415 nm, by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). The Er3+ metastable level lifetimes for the emission at ca. 1.54 m were found to be ca. 4–6 ms, for Er3+ concentrations varying between 0.2–2.0 mol% (or (0.4–4.4) × 1020 ions/cm3), but no significant variation was observed with the Ag concentration added (up to 2.5 mol%). 相似文献
55.
L. C. R. Andrade J. A. Paixo M. J. M. de Almeida C. Siquet F. Borges 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(2):o95-o97
The molecule of the title compound, C23H40O4Si2, features an approximate non‐crystallographic C2 symmetry axis. The aldehyde group is disordered over two positions with similar occupancies. The geometry of the isolated molecule was studied by ab initio quantum mechanical calculations employing a molecular orbital Hartree–Fock method. The calculations reproduce well the equilibrium geometry but slightly overestimate the value of the Si—O bond lengths of the trioxadisilepine ring. 相似文献
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The use of a laboratory-made iodide ion-selective electrode with tubular configuration and based on a crystalline membrane (AgI/Ag2S) as the detector for ion chromatographic determination of iodide in urine and serum is described. A CIS reversed-phase column was coated withN-cetylpyridinium chloride to prepare a low-exchange-capacity analytical column and with hexadecyltrimethylammonium bromide to prepare a concentrator pre-column. A 2.0 ml min–1 flow rate of deionized water and 0.1 mol 1–1 KNO3 solution was used for the pre-concentration and for the chromatographic separation, respectively. For optimum performance of the detector a background level of iodide was added into the column effluent. A linear relationship (r = 0.9997) between tubular electrode potential (as peak height) and iodide concentration in the range 5–400 g 1–1 and a detection limit of 1.47 g 1–1 were obtained. The method shows good reproducibility for both peak height (2.2% RSD) and retention time (1.3% RSD). Recoveries on its application to the samples were 93.0–100.9% for urine and 91.4–106.0% for serum. 相似文献
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