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排序方式: 共有441条查询结果,搜索用时 31 毫秒
421.
422.
Chemical investigation of an endophytic fungus, Pestalotiopsis theae, isolated from the leaves of Turraeanthus longipes (Meliaceae) collected in Cameroon, resulted in the isolation of six new epoxyquinols, cytosporins F–K (2–7), together with the known cytosporin D (1). The structures of the new compounds were unambiguously determined by analysis of the 1D, 2D NMR, and HRMS spectra. Cytosporins G–K (3–7) are the first cytosporins with a hydroxyl substituted C7 side chain, while cytosporins F–I (2–5) contain a 13-acetoxyl group that was not reported previously. A plausible biosynthetic pathway for the cytosporin derivatives is proposed. 相似文献
423.
Tang Pei Ling Osman Hassan Khairiah Badri Mohammad Yusof Maskat Wan Aida Wan Mustapha 《Cellulose (London, England)》2013,20(6):3191-3203
An investigation was conducted to study the effect of solvent composition and temperature on the efficiency of pretreatment prior to enzymatic hydrolysis. The aim was to improve the sugar recovery of oil palm empty fruit bunch fiber (EFBF) through enzymatic hydrolysis. Two types of pretreatments, namely, acidified-glycerol (AC-g) pretreatment and alkaline-glycerol (AL-g) pretreatment were conducted. The study proved that AL-g pretreatment promoted higher delignification and enzymatic hydrolyzed sugar yield compared to AC-g pretreatment. Total sugar recovery of 81.44 and 96.55 % was achieved from AL-g pretreatment at 80 and 120 °C respectively, following the enzymatic hydrolysis. However, downstream industrial processes, involving enzyme treatment along the processing line have the preference of acidic condition. Thus, AC-g pretreatment was favorable. Approximately 51.74 % total sugar had been recovered successfully from enzymatic hydrolysis of EFBF after 3 h of pretreatment by using solvent comprising of 50 % acetic acid and 80 % aqueous glycerol at a ratio of 97:3 at 120 °C. 相似文献
424.
Ghenia Bentabed Mustapha Rahmouni Florence Mongin Aicha Derdour Jack Hamelin 《合成通讯》2013,43(17):2935-2948
Abstract Cycloadditions of aldehydes with carbonyl ylides to give dioxolanes have been carried out without solvent under microwave irradiation. The reactions proceeded in similar yields and stereoselectivities, but in shorter reaction times, than those obtained in toluene at reflux using an oil bath. Cycloadditions conducted between imines and carbonyl ylides using the same protocol were less efficient because the oxazolidines formed proved unstable under the reaction conditions. 相似文献
425.
Abdellatif El Kihel Mustapha Ahbala Abdelkrim Mouzdahir Naima Snader El Mokhtar Essassi Patrick Bauchat 《合成通讯》2013,43(2):200-204
Imidazobenzodiazepine derivatives were obtained by cyclization of the condensation products 7‐amino‐2,5,6‐trimethylbenzimidazole with acetylacetone and ethyl acetoacetate. By careful examination of spectroscopic data and the open‐chain intermediate isolation, we have identified the formed compound structure. 相似文献
426.
A new procedure for the ring opening of 4,6-O-benzylidene acetals of carbohydrates with N-bromosuccinimide using calcium carbonate in stoechiometric amount instead of excess barium carbonate is described. This procedure is successfully applied to some highly functionnalized substrates without side reactions. 相似文献
427.
Valter Castelvetro Mustapha RaihaneSabrina Bianchi Salima AtlasIlaria Bonaduce 《Polymer Degradation and Stability》2011,96(2):204-211
The thermal decomposition under non-oxidative conditions of a copolymer of vinylidene cyanide (VCN) and 2,2,2-trifluoroethyl methacrylate (MATRIF) was investigated by thermogravimetry (TG) and Pyrolysis-GC-MS. The type and composition of the pyrolytic products and the shape of the TG curve indicate that both the main thermal degradation process, with onset at 368 °C, and a minor weight loss at around 222 °C are mainly associated with random main-chain scission. The kinetic parameters were determined by means of dynamic and, in the case of the main degradation stage, also isothermal methods. The results obtained from the dynamic methods (Friedman, Flynn-Wall-Ozawa, and Kissinger, respectively) are in good agreement with those obtained from isothermal TG data. The activation energy was in the 177-213 kJ/mol range for the first stage, and 224-295 kJ/mol for the second stage, the highest respective values being determined from the kinetic analysis according to the Kissinger method. 相似文献
428.
We demonstrate the possibility to use near field optics to perform two-dimensional dopant profiling on silicon surface, with deep submicron spatial resolution. The sample surface is contacted by an aqueous electrolyte giving a reverse biased junction that is illuminated by a subwavelength optical source, in near filed conditions. A staircase calibration structure was used with several boron-doped layers with either 4 μm or 0.4 μm thickness and doping between 1017 and 1020 at/cm3. Measurements were performed on the sample cross section. It is shown that photocurrent surface mapping shows up the doped areas with a lateral resolution better than 100 nm. 相似文献
429.
Jason A. Pates Mustapha R. Hatab Donald D. McIntire F. Gary Cunningham Diane M. Twickler 《Magnetic resonance imaging》2010
Objective
The purpose of this study is to determine the feasibility of measuring total uterine blood flow in pregnancy using magnetic resonance imaging (MRI) technique.Methods
Uterine blood flow was determined in pregnant women in whom MRI was being carried out to assess a fetal anomaly. A two-dimensional time-of-flight magnetic resonance (MR) angiogram sequence was performed. Scout images and a peripherally gated phase contrast MR sequence were planned to study simultaneous blood flow in the uterine and ovarian arteries.Results
The MR pelvic angiogram sequence was completed in 13 women. The uterine arteries were visualized and their cross-sectional area determined. The complexity of the pelvic blood supply prevented the calculation of blood flow velocity and, thus, total uterine blood flow.Conclusion
The measurement of total uterine blood flow during pregnancy was not possible using our MR technique. The ovarian vessels were not consistently visualized. Doppler ultrasonography remains the best modality by which to estimate total uterine blood flow in pregnancy. 相似文献430.