全文获取类型
收费全文 | 67557篇 |
免费 | 6368篇 |
国内免费 | 13052篇 |
专业分类
化学 | 63166篇 |
晶体学 | 2281篇 |
力学 | 771篇 |
综合类 | 523篇 |
数学 | 4089篇 |
物理学 | 16147篇 |
出版年
2024年 | 73篇 |
2023年 | 1049篇 |
2022年 | 1082篇 |
2021年 | 1901篇 |
2020年 | 2216篇 |
2019年 | 2966篇 |
2018年 | 1879篇 |
2017年 | 3022篇 |
2016年 | 2544篇 |
2015年 | 2274篇 |
2014年 | 2963篇 |
2013年 | 6264篇 |
2012年 | 4474篇 |
2011年 | 4782篇 |
2010年 | 3905篇 |
2009年 | 4600篇 |
2008年 | 4633篇 |
2007年 | 4776篇 |
2006年 | 4475篇 |
2005年 | 3956篇 |
2004年 | 3807篇 |
2003年 | 3096篇 |
2002年 | 2270篇 |
2001年 | 1868篇 |
2000年 | 1696篇 |
1999年 | 1291篇 |
1998年 | 1139篇 |
1997年 | 1135篇 |
1996年 | 970篇 |
1995年 | 1012篇 |
1994年 | 844篇 |
1993年 | 684篇 |
1992年 | 641篇 |
1991年 | 496篇 |
1990年 | 336篇 |
1989年 | 285篇 |
1988年 | 248篇 |
1987年 | 165篇 |
1986年 | 134篇 |
1985年 | 149篇 |
1984年 | 119篇 |
1983年 | 57篇 |
1982年 | 95篇 |
1981年 | 126篇 |
1980年 | 83篇 |
1979年 | 110篇 |
1978年 | 63篇 |
1977年 | 65篇 |
1976年 | 45篇 |
1973年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
The preparation of some novel 3-(arylideneamino)-3a,8a-dihydroxy-1,3,3a,8a-tetrahydroindeno[1,2-d]imidazole-2,8-diones 8(i–xiv) and 3-(arylideneamino)-3a,8a-dihydroxy-2-thioxo-1,3,3a,8a-tetrahydroindeno[1,2-d]imidazol-8(2H)-ones 9(i–xiv) have been reported through one-pot catalyst-free reaction of aldehydes, semicarbazide hydrochloride/thiosemicarbazide with ninhydrin. All the synthesized compounds have been screened for antimicrobial activity and some of them were observed to possess broad spectrum antibacterial potential as well as significant antagonistic potential against fungal pathogens. 相似文献
992.
A practical copper-catalyzed I-substitutions of alkyl-2-iodobenzoates with alkynes have been developed using Cu powder as a catalyst under solvent, cocatalyst, and base-free conditions. This reaction system is new, facile, efficient, and economical that gives Sonogashira coupling products in excellent yields (up to 97%). The coupled products (A–J) were characterized by CHNS, 1H NMR, and 13C NMR and are found soluble in ethyl acetate and dichloromethane. In addition, simulation studies of A–J were performed with aspulvinone dimethylallyltransferase enzyme and observe good binding affinity. The reported compounds may act as anti-cizmatics, anti-fobic in future and also have the inhibition of aspulvinone dimethylallyltransferase properties to control Alzheimer’s, Schizophrenia, etc. diseases. 相似文献
993.
A new method for the synthesis of marine alkaloids leucettamines B and C from Leucetta sp. sponges is described. The key step is the base-promoted rearrangement of β-lactam into imidazolone ring. Leucettamines B and C as well as their N-benzoyl derivatives were obtained in high yields. Single-crystal structures of both leucettamines B and C were determined by X-ray diffraction confirming Z-configuration of double bond at 4-position of imidazolone. 相似文献
994.
Elham Kanaani 《合成通讯》2017,47(14):1326-1331
A series of novel dihydroquinoline derivatives were synthesized using malononitrile, 2-aminobenzoic acid, and benzaldehydes in the presence of a catalytic amount of acetic acid, without the use of any additional cocatalyst, under solvent-free conditions. The reaction is characterized by high efficiency, easy workup, simple purification of the products, and availability of catalyst. 相似文献
995.
Garbapu Suresh Ratnakaram Venkata Nadh Navuluri Srinivasu Durgaprasad Yennity 《合成通讯》2017,47(17):1610-1621
A new and efficient synthetic route for dual-Src/Abl kinase inhibitor dasatinib (Sprycel®), an anticancer drug, is described. This commercially viable process yields dasatinib monohydrate free of potential impurities with consistent yield of 68% in route A and 61% in route B with HPLC purity >99.80% over four stages. 相似文献
996.
A convenient method has been developed for the preparation of a variety of 2-mercaptobenzothiazoles from 2-haloanilines and CS2 mediated by metal sulfide. In this reaction, 2-haloanilines reacted with CS2 in the presence of Na2S?·?9H2O to form 2-mercaptobenzothiazoles. Na2S?·?9H2O functioned both as an activator of CS2 and as a base. Furthermore, NMR analysis was used to identify the different reaction mechanisms of 2-haloanilines and CS2 mediated by Na2S or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), which demonstrated that Na2S interacted only with CS2, while DBU reacted with both 2-iodoaniline and CS2. 相似文献
997.
Mostafa Kiamehr Batoul Alipour Leyla Mohammadkhani Behzad Jafari Peter Langer 《Tetrahedron》2017,73(21):3040-3047
Various novel polycyclic thiopyranoindol annulated [3,4-c]quinolone derivatives were synthesized via domino Knoevenagel-hetero-Diels–Alder reactions of indoline-2-thions and novel N-acrylated anthranilaldehydes in refluxing ethanol as a solvent in the presence of 20 mol% ZnBr2 as a Lewis acid catalyst. All reactions proceed with high yields with excellent regio- and stereoselectivity. 相似文献
998.
The results of several approaches to make the 4,4,8-trimethyl-12-oxatricyclo[7.2.1.02,5]dodecane motif present in rumphellatins A–C are reported. The strategy adopted involved (9E)-12,12-dimethyl-5-oxatricyclo[8.2.0.04,6]dodec-9-en-11-ol as an intermediate, but when attempts were made to connect the third methyl group to the carbon skeleton by reacting the epoxide with MeLi under boron-trifluoride catalysis, an unprecedented transannular reaction occurred. The desired product was not formed; instead, compounds containing a tricyclo[6.3.0.02,5]undecane moiety were obtained, in one case in better than 70% yield. 相似文献
999.
Wei-Jing Gao Chiu Marco Lam Bao-Guo Sun R. Daniel Little Cheng-Chu Zeng 《Tetrahedron》2017,73(17):2447-2454
The electrochemically oxidative cleavage of lignin β-O-4 model compounds mediated by iodide ion has been studied. The results indicate that electrolytic conditions play a predominant role in determining the distribution of cleavage products. The preparative-scale electrolysis proceeds in a simple undivided cell, employing a catalytic amount of NaI as the redox mediator and supporting electrolyte in methanol. Under these conditions, the CβO bond is selectively cleaved with 2,2-dimethoxy-2-arylacetaldehyde being the main product. In some cases, the reaction gives a good yield of cleavaged products. The results further demonstrate that the indirect electrolysis mediated by halide is a versatile approach for chemical transformation. 相似文献
1000.
A Pd(II)-catalyzed cascade Heck/intramolecular C(sp2)–H amidation reaction is described for the synthesis of 4-aryl-2-quinolinone derivatives. Substituted cinnamamide containing 2-(pyridin-2-yl)ethanamine unit reacts with aryl iodide to form intermediate by Heck reaction. Then, the intermediate takes place intramolecular amidation via C(sp2)–H activated process promoted by orientation group. 相似文献