全文获取类型
收费全文 | 15419篇 |
免费 | 799篇 |
国内免费 | 2498篇 |
专业分类
化学 | 12808篇 |
晶体学 | 426篇 |
力学 | 220篇 |
综合类 | 149篇 |
数学 | 2684篇 |
物理学 | 2429篇 |
出版年
2023年 | 179篇 |
2022年 | 417篇 |
2021年 | 348篇 |
2020年 | 341篇 |
2019年 | 366篇 |
2018年 | 316篇 |
2017年 | 386篇 |
2016年 | 414篇 |
2015年 | 365篇 |
2014年 | 464篇 |
2013年 | 1170篇 |
2012年 | 738篇 |
2011年 | 912篇 |
2010年 | 799篇 |
2009年 | 1031篇 |
2008年 | 1031篇 |
2007年 | 1014篇 |
2006年 | 925篇 |
2005年 | 863篇 |
2004年 | 852篇 |
2003年 | 662篇 |
2002年 | 764篇 |
2001年 | 460篇 |
2000年 | 476篇 |
1999年 | 387篇 |
1998年 | 325篇 |
1997年 | 311篇 |
1996年 | 300篇 |
1995年 | 306篇 |
1994年 | 270篇 |
1993年 | 217篇 |
1992年 | 204篇 |
1991年 | 142篇 |
1990年 | 104篇 |
1989年 | 98篇 |
1988年 | 67篇 |
1987年 | 37篇 |
1986年 | 50篇 |
1985年 | 60篇 |
1984年 | 46篇 |
1983年 | 33篇 |
1982年 | 43篇 |
1981年 | 73篇 |
1980年 | 48篇 |
1979年 | 33篇 |
1978年 | 42篇 |
1977年 | 37篇 |
1976年 | 37篇 |
1975年 | 27篇 |
1974年 | 34篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Georgia Tsolomiti 《合成通讯》2013,43(4):501-507
A simple and efficient method is described for the synthesis of new functionalized 2‐aminothiazoles, the 2‐aminothiazole‐4‐phenyl‐5‐acetamides 5, in 67–96% yields based on an application of the Hantzsch synthesis. The method involves the reaction of thiourea with 3‐benzoyl‐3‐bromo‐propionamides 4 prepared from the corresponding 3‐benzoylpropionamides 3. The tautomeric structure of the γ‐keto amides 3 and 6 is directly related to the present study, because 2‐aminothiazoles 5 are readily obtained from the corresponding open chain γ‐keto amides 3. 相似文献
982.
Huwaida M. E. Hassaneen 《合成通讯》2013,43(20):3579-3588
3‐Oxo‐3‐(pyrrol‐2‐yl)‐propanenitrile 1 coupled with aromatic diazonium salts to yield the corresponding 2‐arylhydrazones 2a–c. The latter products reacted with chloroacetonitrile and ethyl chloroacetate to yield 4‐aminopyrazole derivatives 5a–f. Reaction of 2 with hydrazine hydrate led to formation of 5‐amino‐4‐arylazopyrazole 6a–c. Compound 1 reacted also with trichloroacetonitrile to yield enamine 7, which in turn reacted with hydrazine hydrate to yield 5‐amino‐3‐(pyrrol‐2‐yl)‐pyrazole‐4‐carbonitrile 8. 相似文献
983.
1,3-Dithiolanes are rapidly cleaved by nickel boride, generating corresponding hydrocarbons in excellent yields. The hydrogenolysis is rapid at room temperature and does not require protection from the atmosphere. Mild reaction conditions, simple workup, and good yields of pure products are some of the major advantages of the procedure. 相似文献
984.
Aminosulfonic acid is an environmentally friendly catalyst. Grinding a mixture of aromatic aldehydes, barbituric acid, and H2NSO3H at room temperature (without any solvent) gave 5‐arylidene barbituric acid in high yields, providing a simple and efficient route to synthesis of these compounds. 相似文献
985.
A novel and efficient method for the synthesis of mercaptothiophenes using cyanodithioic acids and their corresponding mono‐ and dithiolate salts as starting components is described. 相似文献
986.
An atom-economic route to densely functionalized thiophenes has been developed via base-catalyzed rearrangement of 5-propargyl-2H-thiopyran-4(3H)-ones involving tandem retro-thio Michael addition, propargyl isomerization, and intramolecular thio-addition to allenes. 相似文献
987.
A simple, efficient, and diastereoselective synthesis of 5-methylene-dihydropyran-2,6-diones and 3-methylene-3,4-dihydropyran-2-ones from substrates afforded by the SN2 reaction between the acetyl derivatives of the Baylis–Hillman adducts and methylacetoacetate or acetylacetone or benzoyl acetone via saponification followed by P2O5-mediated cyclization is described. 相似文献
988.
989.
A compact three-step synthesis of a new fused bisindole system isomeric with the heterocyclic skeleton present in the marine natural product iheyamine A has been achieved. The structure of the synthetic product was confirmed by a single-crystal X-ray structure. 相似文献
990.
Novel ketene N,S‐acetals 3 were readily prepared by the reaction of cyanoacetamide or cyanothioacetamide with phenylisothiocyanate in the presence of potassium hydroxide, followed by alkylation of the produced salts with methyl iodide. The reaction of compounds 3 with hydrazines afforded different substituted pyrazoles 6. 相似文献