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101.
Francisco J. Sayago Manuel Angulo Consolación Gasch M. Ángeles Pradera 《Tetrahedron》2007,63(22):4695-4702
Nor-tropane related bicyclic (6+5) iminocyclitols with an ether bridge and different substituents (hydroxymethyl, aminomethyl, and aminoethyl) on C-1 are prepared starting from a β-d-psicofuranosyl cyanide. The method involves an internal nucleophilic attack of a stabilized amide ion on a 5-mesyloxy derivative. The intermediate N-acetyl O-protected iminocyclitols present atropoisomerism due to restricted rotation of the N-CO amido bond. Conformational aspects of the prepared compounds are discussed. 相似文献
102.
Emilia Iglesias-Martínez Isabel Brandariz Francisco Penedo 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):573-576
The UV-vis absorption and the fluorescence emission spectra of novocaine were analysed in aqueous cyclodextrin (CD) solutions.
Either the absorbance read at λmax 290 nm or the fluorescence emission intensity at λems 346 nm increase in the presence of both α- and β-CD due to the formation of 1:1 inclusion complexes. The lower polarity of
the CD-cavity sensed by the included drug enhances the emitted fluorescence; in fact, the same effect was observed in aqueous
mixtures of acetonitrile, dioxane, or dimethylsulfoxide. The inclusion complex formation between the monocation of novocaine
and CDs diminishes the electrical conductance of aqueous solutions of novocaine hydrochloride (NoHCl). Both the nitrosation
reaction in aqueous acid medium and the ester hydrolysis in alkaline medium are retarded in the presence of CDs. The strongest
effect was observed with β-CD as a consequence of the higher stability inclusion complex. 相似文献
103.
Begoña Hernández Modesto Orozco Francisco J. Luque 《Journal of computer-aided molecular design》1997,11(2):153-162
The tautomerism of 2-azaadenine and 2-hypoxanthine has been examined in the gas phase and in aqueous solution. The tautomerism in the gas phase has been studied by means of semiempirical and ab initio quantum-mechanical computations, as well as density-functional calculations. The influence of the aqueous solvent on the relative stability between tautomers has been estimated from self-consistent reaction field calculations performed with different high-level continuum models. The results provide a detailed picture of the tautomeric preference for these purine bases. The importance of tautomerism in the substrate recognition by xanthine oxidase is discussed. Finally, the rate of oxidation of 2-azaadenine and 2- hypoxanthine by xanthine oxidase is discussed in terms of the recognition model at the enzyme active site. 相似文献
104.
Susana Cabredo Pinillos Jesú s Sanz Asensio Javier Galb n Bernal 《Analytica chimica acta》1995,300(1-3):321-327
This paper describes a method for the simultaneous determination of As(III), Sb(III) and Se(IV) by combining hydride generation and gas phase molecular absorption spectrometry. A system for continuous hydride generation has been designed and developed, based on the use of a double process of gas-liquid separation, and optimal compromise operation conditions for the three compounds have been found. After generation, the hydrides are collected in a liquid nitrogen cryogenic trap, and then evaporated and driven to the flow cell of a diode array spectrophotometer, in which the transient signals over the 190–250 nm wavelength interval are measured. Under the recommended conditions (sample flow: 35 ml min−1, 0.5 M HCl; reductor flow: 4 ml min−1 of 4% NaBH4, solution) linear response ranges above 50 μg 1−1 for As(III), 30 μg 1−1 for Sb(III) and 200 μg 1−1 for Se(IV) are obtained with detection limits of 22 μg 1−1, 15 μg 1−1 and 65 μg 1−1, respectively. Multiwavelength linear regression equations were used for the simultaneous determination of the three elements in different synthetic samples, with good precision and accuracy and to study simultaneously the interference from different chemical species for the three compounds. Results were similar to those obtained by other techniques using hydride generation. 相似文献
105.
Francisco Sarabia 《Tetrahedron letters》2003,44(41):7671-7675
A synthetic approach to the [15]-membered stevastelins, a novel class of immunosuppressant agents, is reported based on a macrolactamization route to the 2,3-epoxy derivative 6. The synthesis of this compound was achieved via a stereoselective epoxidation of the allylic alcohol 13, followed by a coupling reaction with a variety of peptide derivatives to give the epoxy peptides 7-10. After an extensive study of cyclizations with these precursors, the best result was achieved with the macrolactamization of 8 in the presence of DEPC, to obtain the epoxy cyclic depsipeptide 6 in 42% yield. From this product, an epoxy analogue of stevastelin B, compound 27, was prepared. Finally, the synthesis of the natural product was attempted through the opening of the oxirane ring contained in 6 and 26, with a variety of methyl cuprate reagents, but without success. 相似文献
106.
Francisco Faria M. Victoria Martín M. Carmen Paredes Amelia Tito 《Journal of heterocyclic chemistry》1987,24(5):1269-1274
Cycloaddition of diazomethane to pyrrolinones 1a,b,d,e affords only one regioisomer as a mixture of the epimeric pyrrolopyrazolines 2 and 2 ′,4-Halo derivatives 1f,g react with diazomethane to give the two possible regioisomers 2 and 3. The regio- and stereochemistry of the adducts is evidenced by the 1H-nmr data. The primary adducts originated from the halopyrrolinones suffer dehydrohalogenation to give aromatized products, which by further methylation give derivatives of type 7, 8, 10 and 11. 相似文献
107.
Assessing the non-specific hydroxyl radical scavenging properties of melanoidins in a Fenton-type reaction system 总被引:1,自引:0,他引:1
Francisco J. Morales 《Analytica chimica acta》2005,534(1):171-176
Soluble high molecular weight fraction (>10 kDa, named melanoidins) was isolated from Maillard reaction model systems, coffee, beer and sweet wine by ultrafiltration. Deoxyribose method was adjusted for measuring the hydroxyl radical scavenging properties of melanoidins. The presence of competitive melanoidins with deoxyribose for OH decrease the rate of deoxyribose degradation. Possible interferences to the deoxyribose method have been evaluated. Most of isolated melanoidins exhibited a variable and measurable non-site-specific hydroxyl scavenging activity in a Fenton-type reaction system. The iron reducing properties of melanoidins at the reaction conditions were evaluated with ferrozine. It has established a kinetic approach to assess the second rate constants of hydroxyl radical scavenging reactions of melanoidins. This approach may be a valuable tool for addressing the structure-activity relationships of melanoidins in a future. There is no correlation between browning (absorbance at 420 nm) and efficiency for scavenging hydroxyl radicals in solution. 相似文献
108.
From an accurate ground state intermolecular potential energy surface we evaluate the rovibrational spectrum of the Ne-HCl van der Waals complex. The intermolecular potential is obtained by fitting a considerable number of interaction energies obtained at the coupled cluster singles and doubles including connected triple excitations level and with the augmented correlation consistent polarized valence quintuple zeta basis set extended with a set of 3s3p2d1f1g midbond functions. This basis set is selected after a systematic basis set study carried out at geometries close to those of the three main surface stationary points. The surface is characterized by two linear minima, i.e. Ne-ClH and Ne-HCl, with distances from the Ne atom to the HCl center of mass of 3.398 and 3.833 angstroms, respectively; and binding energies of -65.10 and -66.85 cm(-1), respectively. These results agree well with the experimental data available in contrast to previous theoretical results. The rovibrational spectra calculated for the different isotopic species are also compared to the experiments. 相似文献
109.
Lars G.J. HammarströmMichael E. Meyer David B. Smith Francisco X. Talamás 《Tetrahedron letters》2003,44(46):8361-8363
A simple synthesis of tetrasubstituted purines is disclosed based on the solution phase elaboration of 4,6-dichloro-2-methylthio-5-nitropyrimidine. One-pot sequential C4 and C6 chloride substitution by secondary and primary amines yields 4,6-diamino-2-methylthio-5-nitropyrimidines. mCPBA-mediated oxidation of the methylthio moiety to the corresponding sulfone allows facile substitution at the 2-position. CrCl2 assisted reduction of the nitro group, followed by acid catalyzed orthoester cyclization, then provides novel tetrasubstituted purines not accessible by other methods. 相似文献
110.
A tungsten-rhodium treatment on the integrated platform of a transversely heated graphite atomiser was used as a permanent chemical modifier for the determination of copper in biological materials by using digested samples as well as slurry sampling in electrothermal atomic absorption spectrometry. The W-Rh permanent modifier was as efficient as Pd+Mg(NO(3))(2) conventional modifier for obtaining good Cu thermal stabilisation in the digested and slurry samples. The permanent W-Rh modifier remained stable by approximately 300 and 250 firings when 20 mul of digested sample and 20 mul of slurry were delivered into the atomiser, respectively. In addition, the permanent modifier increased the tube lifetime up to 1370 and 744 analytical measurements in the digested and slurry samples, respectively. Also, when the W-Rh permanent modifier was employed, there was less variation of the slope of the analytical curves during the total atomiser lifetime, resulting in a decreased need of re-calibration during routine analysis, increasing the sample throughput, and consequently diminishing the variable analytical costs. Detection limits obtained with W-Rh permanent modifier were 0.64 and 0.33 mug g(-1) Cu for digested (dilution factor 100 ml g(-1)) and 1.0% m/v slurries of biological materials, respectively. Results for the determination of copper in the samples were in agreement with those obtained with decomposed sample solutions by using Pd+Mg(NO(3)), since no statistical differences were found after applying the paired t-test at the 95% level. 相似文献