全文获取类型
收费全文 | 4401篇 |
免费 | 629篇 |
国内免费 | 164篇 |
专业分类
化学 | 4285篇 |
晶体学 | 3篇 |
力学 | 24篇 |
综合类 | 18篇 |
数学 | 625篇 |
物理学 | 239篇 |
出版年
2024年 | 2篇 |
2023年 | 52篇 |
2022年 | 99篇 |
2021年 | 203篇 |
2020年 | 208篇 |
2019年 | 152篇 |
2018年 | 170篇 |
2017年 | 131篇 |
2016年 | 214篇 |
2015年 | 240篇 |
2014年 | 277篇 |
2013年 | 387篇 |
2012年 | 245篇 |
2011年 | 309篇 |
2010年 | 235篇 |
2009年 | 279篇 |
2008年 | 256篇 |
2007年 | 267篇 |
2006年 | 227篇 |
2005年 | 204篇 |
2004年 | 164篇 |
2003年 | 151篇 |
2002年 | 202篇 |
2001年 | 58篇 |
2000年 | 64篇 |
1999年 | 62篇 |
1998年 | 54篇 |
1997年 | 43篇 |
1996年 | 35篇 |
1995年 | 27篇 |
1994年 | 29篇 |
1993年 | 23篇 |
1992年 | 9篇 |
1991年 | 12篇 |
1990年 | 8篇 |
1989年 | 7篇 |
1988年 | 9篇 |
1987年 | 12篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1976年 | 2篇 |
1973年 | 2篇 |
1969年 | 2篇 |
1968年 | 2篇 |
排序方式: 共有5194条查询结果,搜索用时 453 毫秒
61.
Marine organisms produce a fascinating range of structurally diverse secondary metabolites, which often possess unusual and sometimes unexpected biological activities. This structural diversity makes these marine natural products excellent molecular probes for the investigation of biochemical pathways. Recently, a number of novel and stereochemically complex macrolides, having a large macrolactone (22- to 44-membered) ring, that interact with the actin cycloskeleton have been isolated from different marine sources. Actin, like tubulin, is a major component of the cytoskeleton and has important cellular functions. Although the details of these interactions are still under investigation, these marine macrolides are becoming increasingly important as novel molecular probes to help elucidate the cellular functions of actin. Owing to their potent antitumor activities, these compounds, for example the aplyronines, also have potential for preclinical development in cancer chemotherapy. Their appealing molecular structures, with an abundance of stereochemistry, and biological significance, coupled with the extremely limited availability from the marine sources, have stimulated enormous interest in the synthesis of these compounds. This review summarizes the biological properties of these unusual marine natural products and features the recently completed total syntheses of swinholide A, scytophycin C, aplyronine A, mycalolide A--all of these being potent cytotoxic agents that target actin--and a diastereoisomer of ulapualide A. Rather than detailing each individual step of these multistep total syntheses, the different synthetic strategies, key reactions, and methods adopted for controlling the stereochemistry are compared. 相似文献
62.
Marta Teijeira 《Tetrahedron letters》2005,46(35):5889-5892
We describe an efficient new approach for the synthesis of a chiral butenolide that is based on the oxidation of a chiral furan ring with singlet oxygen in the presence of Hünig’s base, followed by Luche reduction and in situ lactonization. 相似文献
63.
Two published procedures for glass capillary column production are applied to produce capillaries for chromatography of polar natural compounds. Soda lime glass capillaries, after leaching with aqueous HCL, are either treated with colloidal silicic acid or with barium carbonate and coated with either Silar 7CP or SE 52. A test mixture as well as polar carbohydrate and peptide mixtures are chromatographed on the different columns and their chromatographic properties are evaluated. Combined capillary gas chromatography-electron impact and chemical ionization mass spectrometry are used for identification of the peptide sequence. 相似文献
64.
Barontini F. Marsanich K. Cozzani V. 《Journal of Thermal Analysis and Calorimetry》2004,78(2):599-619
The TG-FTIR technique was used in the present study to investigate the thermal degradation behaviour of materials containing
brominated flame retardants under fire conditions. Time-temperature profiles and oxygen concentrations typical of selected
fire scenarios were reproduced in the thermogravimetric analyzer, while the characterization of the gaseous products generated
was performed by the simultaneous FTIR analysis. FTIR analysis combined with the use of specific calibration procedures allowed
the quantitative estimation of the gaseous products evolved as a function of experimental conditions. The results obtained
allowed the straightforward assessment and the comparison of the quantities of hydrogen bromide formed in the oxidation and
thermal degradation of pure brominated flame retardants and of flame retarded materials of industrial interest. Hydrogen bromide
yields resulted dependent on the experimental conditions used, such as oxygen concentration and heating rate. Although TG-FTIR
experiments only provide a representation of the actual heterogeneous combustion products in real fire conditions, the coupled
TG-FTIR technique proved to be a straightforward experimental methodology allowing one to obtain reference data on the nature
and quantities of the macropollutants generated in a fire.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
65.
海洋天然产物研究新进展 总被引:12,自引:0,他引:12
海洋天然产物化学是目前天然产物化学中最活跃的研究领域之一。近年来,从海洋生物中分离到非常多的化学结构和生理活性上令人注目的新化合物,引起了有机化学家和药物化学家的关注。本文按萜类、太环内酯、聚醚类、生物碱,环肽、含氰化合物、甾醇、聚丙酸酯类化合物等化学结构来概述海洋天然产物研究近年来的进展。 相似文献
66.
Patrizia Ciminiello Carmela Dell'Aversano Martino Forino Federico Umberto Santelia Emmanuel N. Pitsinos 《Tetrahedron》2006,62(33):7738-7743
The analysis of a batch of toxic mussels (Mytilus galloprovincialis) from the Northern Adriatic Sea led to the isolation of a novel oxazinin, oxazinin-4. Its structure including the relative stereochemistry has been elucidated through extensive NMR analysis. A synthetic route to oxazinins has been crucial in establishing the absolute stereochemistry of oxazinin-4 and for reassigning the absolute C-2 configuration of oxazinin-1 and -2 previously isolated from toxic shellfish and stereostructurally characterized. 相似文献
67.
Photoreduction of nitro compounds is accompanied by formation of various radical products that can react with the starting
nitro compound, thus causing deviation of the decomposition kinetics from the first-order kinetics with respect to the nitro
compound. The results of quantum chemical modeling of the reactions of nitro compounds with radicals and the pathways of further
transformations of radical adducts formed in the reactions are presented.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 202–206, February, 2006. 相似文献
68.
近红外快速水份检测技术的应用 总被引:1,自引:0,他引:1
近红外检测技术是目前发展最快和最具有前景的分析技术之一,简要介绍了近红外检测技术的发展、优点和原理.利用该技术设计了豆类产品的水份检测仪器.并对近红外技术在水果、蔬菜、奶制品、制药和化工等方面的应用前景做了展望. 相似文献
69.
本文按合成产物的结构类型,综述分子内 Diels-Alder 反应在有机合成方面的新进展。 相似文献
70.
Paul-Henri Ducrot Albert Kollmann Adil E. Bala Amel Majira Lucien Kerhoas Robert Delorme Jacques Einhorn 《Tetrahedron letters》1998,39(52):7282-9658
Two new antifungal resveratrol dimers, cyphostemmins A-B (1–2), have isolated from the roots of Cyphostemma crotalarioides planch (Vitaceae) together with resveratrol 3 and previously known reseveratrol dimers (4–7). Structures of these new compounds have been established on the basis of their MS and 1H and 13C NMR spectroscopic data. 相似文献