全文获取类型
收费全文 | 3384篇 |
免费 | 548篇 |
国内免费 | 213篇 |
专业分类
化学 | 4067篇 |
晶体学 | 7篇 |
数学 | 1篇 |
物理学 | 70篇 |
出版年
2024年 | 4篇 |
2023年 | 54篇 |
2022年 | 72篇 |
2021年 | 100篇 |
2020年 | 173篇 |
2019年 | 128篇 |
2018年 | 137篇 |
2017年 | 109篇 |
2016年 | 199篇 |
2015年 | 182篇 |
2014年 | 282篇 |
2013年 | 415篇 |
2012年 | 194篇 |
2011年 | 240篇 |
2010年 | 208篇 |
2009年 | 204篇 |
2008年 | 183篇 |
2007年 | 187篇 |
2006年 | 175篇 |
2005年 | 156篇 |
2004年 | 136篇 |
2003年 | 164篇 |
2002年 | 194篇 |
2001年 | 49篇 |
2000年 | 46篇 |
1999年 | 26篇 |
1998年 | 19篇 |
1997年 | 14篇 |
1996年 | 12篇 |
1995年 | 16篇 |
1994年 | 16篇 |
1993年 | 11篇 |
1992年 | 7篇 |
1991年 | 7篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 6篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1973年 | 1篇 |
1967年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有4145条查询结果,搜索用时 15 毫秒
91.
Natural (+)-iridomyrmecin and its unnatural enantiomer (−)-iridomyrmecin were synthesized by intramolecular [3+2] dipolar
cycloaddition of silyl nitronates that had been generated from the nitro derivatives of (+)- and (−)-citronellenes, respectively.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1683–1687, September, 1997. 相似文献
92.
以色酮-氧化吲哚合成子与3-烯基苯并呋喃酮为原料,在催化剂1,8-二氮杂二环十一碳-7-烯作用下,在二氯甲烷中发生Michael/Michael加成关环反应,合成了6个未见文献报道的螺环六氢山酮素-氧化吲哚-苯并呋喃酮类拼接物(3a^3f),产率75%~87%,dr值3/1~5/1,其结构经1H NMR,13C NMR和HR-MS(ESI-TOF)表征。采用MTT法研究了3a^3f对人白血病细胞(K562)的体外抗增殖活性。结果表明:化合物3c,3e和3f对K562增殖有一定的抑制活性。 相似文献
93.
综述了亚甲胺叶立德的制备、它参与的[3+2]环加成反应的反应机理、反应选择性及在天然产物全合成中的应用等方面的最新进展. 相似文献
94.
Skorić I Basarić N Marinić Z Visnjevac A Kojić-Prodić B Sindler-Kulyk M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(2):543-551
New heteroaryl-substituted o-divinylbenzenes, 2,2'-(1,2-phenylenedivinylene)difuran (9), 2,2'-(1,2-phenylenedivinylene)bisbenzo[b]furan (10), and 2,2'-(1,2-phenylenedivinylene)bisnaphtho[2,1-b]furan (11), were prepared and irradiated at various concentrations; intramolecular photocycloaddition and intermolecular [2+2] twofold photoaddition reactions took place to give bicyclo[3.2.1]octadiene derivatives 12-14 and cyclophane derivatives 15-17, respectively. Compound 11 was the most selective of these o-divinylbenzenes, which, owing to pi-pi intra- or intermolecular complexation, gave only the exo-bicyclo[3.2.1]octadiene derivative 14 at low concentrations, and only the cyclophane derivative 17 at high concentrations. 相似文献
95.
96.
A new class of dihydropyrazolo[3′,4′:3,4]pyrrolo[1,2-a]indoles and spiro[3H-indole-3,3′-[Δ2-1,2,4]-triazoline]-2-ones were synthesised via intra and intermolecular 1,3-dipolar cycloaddition reactions in good yields. 相似文献
97.
We report a simple synthetic protocol for the 1,3-dipolar cycloaddition of azides with electron-deficient alkynes. Alkyne with at least one neighboring electron-withdrawing group proceeds with the cycloaddition successfully without any catalysts at room temperature in water. Under this simple condition, we evaluated a series of small molecule model reactions and then coupled an azido-DNA molecule with electron-deficient alkynes for the formation of [1,2,3]-triazole heterocycle, providing a potential method for introducing functional groups to DNA under biological conditions. 相似文献
98.
The reactions of vinyl sulfides with -sulfonylvinyl trifluoromethyl ketones afforded CF3-containing 3,4-dihydro-2H-pyrans. An attempt to synthesize 1,1,1-trifluoro-3-(methylthio)but-3-en-2-one resulted in its dimerization into a CF3-containing 3,4-dihydro-2H-pyran. 相似文献
99.
A one-step procedure was developed for the synthesis of new 6-fluoro-7-(isoxazolidin-2-yl)-4-oxo-1,4-dihydroquinolines. The procedure is based on the 1,3-dipolar cycloaddition of the azomethine oxide and 1,1-disubstituted alkenes, which are generated in situ from 6-fluoro-7-hydroxylamino-4-oxo-1,4-dihydroquinoline-3-carboxylic acid and CH-active compounds (dialkyl malonates, ethyl acetoacetate), respectively, in the presence of formaldehyde at 100—120 °C. 相似文献
100.
The π-π stacking interactions between maleimide's and nitrone's aromatic rings during the 1,3-dipolar cycloaddition were assumed to control the exo-endo selectivity of the reaction. The exo-endo ratios change during the reactions until they reach a constant value, which depends on the substituent. Electron-withdrawing groups favour the exo adduct while electron-donating groups favour the endo adduct. The nitrone ZnBr2 complexes react much more slowly than the free nitrone and the cycloaddition is exo selective in all cases independent of the substituents on the maleimide's aromatic ring. Thermal retrocycloaddition of the cycloadducts produce the corresponding nitrones. The ring opening in the presence of secondary amines did not induce imine formation. endo Adducts were shown for the first time to be the stable paramagnetic compounds. 相似文献