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251.
An efficient synthesis of N,N'-substituted guanidine derivatives was developed via an aromatic sulfonyl-activated thiourea intermediate. The use of certain aromatic sulfonamides, such as PbfNH(2), as the key reagent to incorporate a TFA-labile guanidine protection group greatly facilitates solid-phase synthesis of N,N'-substituted guanidine compounds. 相似文献
252.
Jun Yue Bai Liang Wang Hong Jing Wang Peng Fei Huang Yu Qing Zhao Sheng Di Fan 《Mikrochimica acta》2006,156(3-4):321-326
The electrochemical behavior of epinephrine (EP) at a mercaptoacetic acid (MAA) self-assembled monolayer modified gold electrode
was studied. The MAA/Au electrode is demonstrated to promote the electrochemical response of epinephrine by cyclic voltammetry.
The possible reaction mechanism is also discussed. The diffusion coefficient D of EP is 6.85 × 10−6 cm2 s−1. In 0.1 mol L−1 phosphate buffer (pH 7.20), a sensitive oxidation peak was observed at 0.177 V, and the peak current is proportional to the
concentration of EP in the range of 1.0 × 10−5–2.0 × 10−4 mol L−1 and 1.0 × 10−7–1.0 × 10−6 mol L−1. The detection limit is 5 × 10−8 mol L−1. The modified electrode is highly stable and can be applied to the determination of EP in practical injection samples. The
method is simple, quick, sensitive and accurate. 相似文献
253.
The reaction of cyclohexyl isocyanide with various aldehydes and 1,3-dicarbonyl compounds catalyzed by piperidine is described. The protocol offers facile and efficient synthesis of 5-hydroxy-2H-pyrrol-2-one derivatives from readily available starting materials in high yields. 相似文献
254.
Dazhong Fan 《Tetrahedron》2005,61(43):10291-10302
A synthesis of 5,15-disubstituted zinc-porphyrins has been developed that employs condensation of a 1,9-bis(N,N-dimethylaminomethyl)dipyrromethane+a dipyrromethane in refluxing ethanol containing zinc acetate followed by oxidation with DDQ. The N,N-dimethylaminomethylation of the dipyrromethane was achieved via Eschenmoser's reagent (N,N-dimethylmethyleneammonium iodide) in CH2Cl2 at room temperature. The synthesis is compatible with diverse substituents (e.g., alkyl, aryl, ester, acetal) and enables rapid synthesis of trans-AB-, A2-, and A-porphyrins. The synthesis of >40 zinc porphyrins has been surveyed; 13 zinc porphyrins were isolated in yields of 5-20% without detectable scrambling. 相似文献
255.
Hydrous amorphous Al (HAO), Fe (HFO), and Mn (HMO) oxides are ubiquitous in the subsurface as both discrete particles and coatings and exhibit a high affinity for heavy metal contaminants. To assess risks associated with heavy metals, such as Pb, to the surrounding environment and manage remedial activities requires accurate mechanistic models with well-defined transport parameters that represent sorption processes. Experiments were conducted to evaluate Pb sorption to microporous Al, Fe, and Mn oxides, as well as to montmorillonite and HAO-coated montmorillonite. Intraparticle diffusion, a natural attenuating process, was observed to be the rate-limiting mechanism in the sorption process, where best-fit surface diffusivities ranged from 10(-18) to 10(-15) cm(2) s(-1). Specifically, diffusivities of Pb sorption to discrete aluminum oxide, aluminum oxide-coated montmorillonite, and montmorillonite indicated substrate surface characteristics influence metal mobility where diffusivity increased as affinity decreased. Furthermore, the diffusivity for aluminum oxide-coated montmorillonite was consistent with the concentrations of the individual minerals present and their associated particle size distributions. These results suggest that diffusivities for other coated systems can be predicted, and that oxide coatings and montmorillonite are effective sinks for heavy metal ions. 相似文献
256.
Three flavonoid glycosides including orientin, vitexin, quercetin-3-O-neohesperidoside and one unknown compound were isolated and purified by high-speed counter-current chromatography (HSCCC) and semi-preparative HPLC from Trollius ledebouri Reichb., a traditional Chinese medicine. Preparative HSCCC with a two-phase solvent system composed of ethyl acetate-n-butanol-water (2:1:3, v/v/v) was successfully performed by increasing the flow-rate of the mobile phase from 1.5 to 2.5 ml/min after 190 min. Consequently, 95.8 mg orientin, 11.6 mg vitexin, 9.3 mg unknown compound with purities of over 97% and one partially purified peak fraction (contained quercetin-3-O-neohesperidoside at 85.1% purity) were obtained from 500 mg of the crude extract. Then the partially purified fraction was further purified by reversed-phase semi-preparative high-performance liquid chromatography. The structure identification of all pure fractions was carried out by UV, MS, 1H NMR and 13C NMR. 相似文献
257.
Non-isothermal and isothermal crystallization behaviors of polystyrene-b-poly(ethylene-co-butene) (PSt-b-PEB) block copolymers with different compositions and chain lengths were investigated by differential scanning calorimetry (DSC). The results show that crystallization of PEB block is strongly dependent on the composition. Crystallization temperature (Tc), melting temperature (Tm) and fusion enthalpy (ΔHf) increase rapidly with PEB volume fraction (VE) for block copolymers with VE below 50%, but there is little change when PEB block becomes the major component. Glass transition temperature (Tg) of the PSt block and order-disorder transition temperature (TODT) of block copolymers also have a weak effect. The isothermal crystallization kinetics results show that Avrami exponent (n) was strongly dependent on the composition and crystallization temperature. For the block copolymers with VE below 38.7 vol%, the values of n vary between 0.9 and 1.3, indicating that crystallization is confined. For the PSt-b-PEB block copolymers with VE higher than 50%, fractionated crystallization behavior is usually observed. A two-step isothermal crystallization procedure is applied to these block copolymers. It is found that breakout crystallization occurs at higher Tc, but confined at lower Tc. Two overlapped melting peaks are observed for the block copolymers with fractionated crystallization behavior after two-step crystallization, and only the higher melting peak corresponding to breakout crystallization can be used to derive equilibrium melting temperature. 相似文献
258.
氨基酸柱前衍生化的3种新荧光试剂的光谱特性及高效液相色谱研究 总被引:1,自引:2,他引:1
合成了3种新的荧光标记试剂:吖啶-N-乙酰氯,咔唑-9-乙酰氯和咔唑-9-丙酰氯。它们的最大发射降激发波长分别为430nm,368nm,和365nm。3种衍生化试剂与氨基酸形成的衍生物在pH6.5的条件下结合梯度洗脱程序在C18反相柱上对色谱条件进行了优化。 相似文献
259.
260.
Faqiong Zhao Liqin Liu Fei Xiao Jiangwen Li Rui Yan Shuangshuang Fan Baizhao Zeng 《Electroanalysis》2007,19(13):1387-1393
An ionic liquid (i.e., 1‐butyl‐3‐methylimidazolium hexafluorophosphate, BMIMPF6)‐single‐walled carbon nanotube (SWNT) gel modified glassy carbon electrode (BMIMPF6‐SWNT/GCE) is fabricated. At it the voltammetric behavior and determination of p‐nitroaniline (PNA) is explored. PNA can exhibit a sensitive cathodic peak at ?0.70 V (vs. SCE) in pH 7.0 phosphate buffer solution on the electrode, resulting from the irreversible reduction of PNA. Under the optimized conditions, the peak current is linear to PNA concentration over the range of 1.0×10?8–7.0×10?6 M, and the detection limit is 8.0×10?9 M. The electrode can be regenerated by successive potential scan in a blank solution for about 5 times and exhibits good reproducibility. Meanwhile, the feasibility to determine other nitroaromatic compounds (NACs) with the modified electrode is also tested. It is found that the NACs studied (i.e., p‐nitroaniline, p‐nitrophenol, o‐nitrophenol, m‐nitrophenol, p‐nitrobenzoic acid, and nitrobenzene) can all cause sensitive cathodic peaks under the conditions, but their peak potentials and peak currents are different to some extent. Their peak currents and concentrations show linear relationships in concentration ranges with about 3 orders of magnitude. The detection limits are 8.0×10?9 M for p‐nitroaniline, 2.0×10?9 M for p‐nitrophenol, 5.0×10?9 M for o‐nitrophenol, 5.0×10?9 M for m‐nitrophenol, 2.0×10?8 M for p‐nitrobenzoic acid and 8.0×10?9 M for nitrobenzene respectively. The BMIMPF6‐SWNT/GCE is applied to the determination of NACs in lake water. 相似文献