全文获取类型
收费全文 | 259篇 |
免费 | 13篇 |
国内免费 | 3篇 |
专业分类
化学 | 208篇 |
晶体学 | 3篇 |
力学 | 12篇 |
数学 | 19篇 |
物理学 | 33篇 |
出版年
2023年 | 2篇 |
2022年 | 3篇 |
2021年 | 11篇 |
2020年 | 9篇 |
2019年 | 10篇 |
2018年 | 13篇 |
2017年 | 11篇 |
2016年 | 16篇 |
2015年 | 15篇 |
2014年 | 16篇 |
2013年 | 28篇 |
2012年 | 25篇 |
2011年 | 23篇 |
2010年 | 32篇 |
2009年 | 7篇 |
2008年 | 20篇 |
2007年 | 9篇 |
2006年 | 9篇 |
2004年 | 5篇 |
2003年 | 1篇 |
2002年 | 3篇 |
2001年 | 2篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1984年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有275条查询结果,搜索用时 31 毫秒
71.
Cytidine triphosphate synthetase (CTPS) catalyzes the formation of cytidine triphosphate from glutamine, uridine-5'-triphosphate (UTP), and adenosine-5'-triphosphate. Inhibitors of CTPS are of interest because of their potential as therapeutic agents. One approach to potent enzyme inhibitors is to use analogues of high energy intermediates formed during the reaction. The CTPS reaction proceeds via the high energy intermediate UTP-4-phosphate (UTP-4-P). Four novel analogues of uridine-4-phosphate (U-4-P) and 3-deazauridine-4-phosphate (3-deazaU-4-P) were synthesized in which the labile phosphate ester oxygen was replaced with a methylene and difluoromethylene group. The methylene analogue of U-4-P, compound 1, was prepared by a reaction of the sodium salt of tert-butyl diethylphosphonoacetate with protected, 4-O-activated uridine followed by acetate deprotection and decarboxylation. It was found that this compound undergoes relatively facile dephosphonylation presumably via a metaphosphate intermediate. The difluoromethylene derivative, compound 2, was prepared by electrophilic fluorination of protected 1. This compound was stable and did not undergo dephosphonylation. Synthesis of the methylene analogue of 3-deazaU-4-P, compound 3, was achieved by ribosylation of protected 4-(phosphonomethyl)-2-hydroxypyridine. Electrophilic fluorination was also employed in the preparation of protected 4-(phosphonodifluoromethyl)-2-hydroxypyridine which was used as the key building block in the synthesis of difluoro derivative 4. These compounds represent the first examples of a nucleoside in which the base has been chemically modified with a methylene or difluormethylenephosphonate group. 相似文献
72.
A protected, unsymmetrical bismethylene triphosphate analogue was prepared by sequential Michaelis-Arbuzov reactions on ethyl bis(halomethyl)phosphinates. This species was monodeprotected at one of the terminal phosphonate groups in high yield. The resulting monodeprotected compound was used to achieve the first syntheses of the bismethylene triphosphate analogues of UTP and CTP. 相似文献
73.
Ali Sharifi Mehdi Barazandeh M. Saeed Abaee Mojtaba Mirzaei 《Tetrahedron letters》2010,51(14):1852-1192
An efficient procedure for the one-pot chemoselective synthesis of 2H-benzo[b][1,4]oxazin-3(4H)-one derivatives from their corresponding o-aminophenols is developed using DBU in the ionic liquid [omim][BF4]. Upon completion of the reaction and separation of the product, the ionic liquid is recovered and successfully reused over nine recycles without any noticeable loss of performance. 相似文献
74.
Density functional theory (DFT) calculations have been performed to investigate the electronic and structural properties of sulfur (S) terminated models of zigzag boron nitride (BN) nanotube. Four models including pristine, boron (B) tip terminated by S, nitrogen (N) tip terminated by S, and both of B and N tips terminated by S have been considered for optimizations and chemical shielding (CS) parameters calculations. The results indicate that the B–N bond lengths do not detect any changes due to the S-termination but the band gaps and dipole moments detect notable changes especially for the model of the N-tip terminated by the S atoms. The CS parameters also indicate that the atoms of the models are divided into layers with similar parameters in each layer. In the model of the B-tip terminated by the S atoms, the CS parameters indicate strong chemical bonding of N- and S-layers; however, only some attractions between the B- and S-layers of the model of the N-tip terminated by the S atoms have been detected. In the model of B and N tips terminated by the S atoms, the most significant changes among the models are detected. 相似文献
75.
Issa Yavari Anvar Mirzaei Loghman Moradi Gholamhossein Khalili 《Tetrahedron letters》2010,51(2):396-6808
An effective route to spiro-functionalized fused polycyclic derivatives of isoquinoline is described via tandem reaction of isoquinoline, dialkyl acetylenedicarboxylates, and indane-1,3-dione. 相似文献
76.
A. R. Jalilian M. Sheikhha M. Mirzaei G. R. Aslani A. Shafiee 《Journal of Radioanalytical and Nuclear Chemistry》2004,260(2):373-377
5-[2-(2-Chlorophenoxy)phenyl]-1,3,4-oxadiazole-2-yl-4-fluorobenzoate 6a, the non-classic benzodiazepine ligand, has been shown to elicit a significant anticonvulsant activity against pentylenetetrazole-induced
convulsion. In order to perform biological studies, we decided to prepare the [18F]-labeled compound. This compound was prepared in no-carrier-added (n.c.a) form from 5-[2-(2-chlorophenoxy)phenyl]-1,3,4-oxadiazole-2-yl-4-N,N,N-trimethylanilinium
triflate 5 in one step at 125 °C in Kryptofix 2.2.2/[18F] and DMSO as the solvent followed by column chromatography. The synthesis took 20 minutes with an overall radiochemical
yield of 70-75% (EOS) and a specific activity about 74 GBq/mmole and chemical-radiochemical purity more than 95%.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
77.
The compounds 5‐ethoxycarbonyl‐1,6‐dimethyl‐4‐(3‐nitrophenyl)‐3,4‐dihydropyrimidin‐2(lH)‐one (5) and 5‐ethoxycarbonyl‐1‐phenyl‐6‐methyl‐4‐(3‐nitrophenyl)‐3,4‐dihydropyrimidin‐2(lH)‐one (1) were prepared by the Biginelli condensation method and they converted to eight N‐3 substituted dihydropyrimidines using NaH and various electrophiles (ClCO2Et, TsCl, Ac2O, AcCl and PhCOCl). Compound (1) was mono‐brominated at the C6‐methyl group using bromine solution. Reaction of the bromo derivative with amino nucleophiles such as methyl amine and cyclohexyl amine produced two pyrrolo‐pyrimidine derivatives. All the compounds except 5‐ethoxycarbonyl‐1‐phenyl‐6‐methyl‐4‐(3‐nitrophenyl)‐3,4‐dihydropyrimidin‐2(lH)‐one ( 4 ) were purified by recrystallization methods. The structure of all the new compounds was confirmed using FT‐ir,1H nmr, 13C nmr spectral and elemental analyses methods. 相似文献
78.
Summary: Modeling of fluidized bed reactor for polyethylene production in the condensed mode operation is proposed in this paper. A two-phase model including the emulsion and bubble phases with the constant bubble size is employed to describe hydrodynamic behavior of the reactor. The kinetics of Ziegler-Natta polymerization is also modeled with a two active site model. The phase behavior and solubility of low molecular weight components in polyethylene are estimated with the Sanchez-Lacombe equation of state. The validation of the model is carried out with industrial data for an ethylene and 1-butene copolymerization with the isopentane as a condensable component. The simulation results are in good agreement with industrial data. The model is also used to study quantitatively the effect of the inlet stream temperature to the reactor and isopentane concentration in the reactor on the production rate. For instance, in a typical process, when the inlet stream temperature changes 10 °C, the production rate will alter about 40%. Furthermore, the change of the isopentane concentration around 1 mole percentage leads to a variation of production rate about 14%. 相似文献
79.
Chromatographia - A novel polyphenol-coated CoFe2O4 system with sorption capacity of lawsone was synthesized as a magnetic sorbent by chemical oxidative polymerization process for magnetic... 相似文献
80.
Davood Mirzaei Hashem Miri-Hakimabad Laleh Rafat-Motavalli 《Journal of Radioanalytical and Nuclear Chemistry》2014,302(3):1095-1101
Considering the advantages of boron neutron capture therapy (BNCT) in treating prostate cancer, the amount of dose delivered to tumour and healthy tissues must be determined. Therefore, in this study, Monte Carlo analyses were performed to evaluate physical doses of adjacent healthy tissues and prostate tumours located at different depths of reference phantom developed by International Commission on Radiological Protection. According to the results, when prostate depth decreases, the amount of physical dose in tumour increases but the physical doses of healthy organs around the tumour remain constant. As expected, lithium filter decreases the damages to normal tissues. The estimations of physical dose, dose uniformity and dose distribution suggested that using BNCT with a filtered neutron beam could be applicable for prostates locate at the depths less than 7 cm from the body surface, which occurs in 5 % of all subjects. 相似文献