全文获取类型
收费全文 | 3592篇 |
免费 | 133篇 |
国内免费 | 293篇 |
专业分类
化学 | 1718篇 |
晶体学 | 22篇 |
力学 | 135篇 |
综合类 | 28篇 |
数学 | 1257篇 |
物理学 | 858篇 |
出版年
2023年 | 39篇 |
2022年 | 47篇 |
2021年 | 51篇 |
2020年 | 45篇 |
2019年 | 89篇 |
2018年 | 71篇 |
2017年 | 89篇 |
2016年 | 82篇 |
2015年 | 71篇 |
2014年 | 213篇 |
2013年 | 272篇 |
2012年 | 133篇 |
2011年 | 256篇 |
2010年 | 246篇 |
2009年 | 271篇 |
2008年 | 253篇 |
2007年 | 244篇 |
2006年 | 235篇 |
2005年 | 246篇 |
2004年 | 210篇 |
2003年 | 159篇 |
2002年 | 95篇 |
2001年 | 55篇 |
2000年 | 49篇 |
1999年 | 45篇 |
1998年 | 45篇 |
1997年 | 48篇 |
1996年 | 31篇 |
1995年 | 26篇 |
1994年 | 26篇 |
1993年 | 18篇 |
1992年 | 18篇 |
1991年 | 7篇 |
1990年 | 16篇 |
1989年 | 17篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1985年 | 15篇 |
1984年 | 10篇 |
1983年 | 8篇 |
1982年 | 19篇 |
1981年 | 29篇 |
1980年 | 23篇 |
1979年 | 16篇 |
1978年 | 11篇 |
1977年 | 13篇 |
1976年 | 16篇 |
1975年 | 7篇 |
1974年 | 4篇 |
1973年 | 7篇 |
排序方式: 共有4018条查询结果,搜索用时 343 毫秒
191.
Conditions are described for one-pot Brønsted acid and organocatalysed enantioselective α-amination of acetals and associated functionalities. Of the organocatalysts screened, proline tetrazole gave the highest ee, while aqueous monochloroacetic acid proved to be the best Brønsted acid activator regarding minimizing racemization and maximizing product yield. The reaction opens up the way for using masked carbonyl functionalities in organocatalysis. 相似文献
192.
The asymmetric Michael addition of 2-alkoxyphenyl α-substituted nitroacetates to non-prochiral α,β-unsaturated carbonyl compounds catalyzed by 10 mol % of sodium 2′-[2-(2-methoxyethoxy)ethoxy]-1,1′-binaphthalen-2-oxide gave the desired adducts in high yields with up to 95% ee. 相似文献
193.
The organocatalytic sulfenylation of α-nitroesters mediated by diaryl-l-prolinols was developed. A range of α-sulfenylated α-nitroesters were obtained in good yields with moderate to good enantioselectivities. 相似文献
194.
An efficient one-pot synthesis of 2-substituted quinoxalines from 1,2-diamines and phenacyl bromides is developed using K10-montmorillonite (K10 clay) as a catalyst at 50 °C in acetonitrile medium. This method offers an easy route for the synthesis of substituted quinoxalines in high yields. A plausible mechanism is proposed in which quinoxalines are formed via dehydration–dehydrohalogenation–cyclization sequence. Further, the K10 clay catalyst is recovered by simple filtration and reused six times without any loss in its catalytic activity. 相似文献
195.
The synthesis of 2,4,6-tris[(4-iodo)phenoxy)]-1,3,5-triazine 6, as a new recyclable nonpolymeric analogue of iodobenzene is achieved using the reaction of 2,4,6-trichloro-1,3,5-triazine with 4-iodophenol in the presence of KOH. The application of 6 as a recyclable ‘iodoarene’ is demonstrated for α-tosyloxylation of enolizable ketones via in situ generation of hypervalent iodine(III) species using PTSA and MCPBA as the terminal oxidant. The recyclability of the reagent 6 was possible owing to the facile recovery and reuse from the reaction mixture due to its near insolubility in methanol. 相似文献
196.
Ahmed M. Metwaly Frank R. Fronczek Guoyi Ma Hazem A. Kadry Atef A. El-Hela Abd-Elsalam I. Mohammad Stephen J. Cutler Samir A. Ross 《Tetrahedron letters》2014
Four new (1–4) and two known (5 and 6) α-pyrone derivatives have been isolated from Alternaria phragmospora, an endophytic fungus from Vinca rosea, leaves. The isolated compounds were chemically identified to be 5-butyl-4-methoxy-6-methyl-2H-pyran-2-one (1), 5-butyl-6-(hydroxymethyl)-4-methoxy-2H-pyran-2-one (2), 5-(1-hydroxybutyl)-4-methoxy-6-methyl-2H-pyran-2-one (3), 4-methoxy-6-methyl-5-(3-oxobutyl)-2H-pyran-2-one (4), 5-(2-hydroxyethyl)-4-methoxy-6-methyl-2H-pyran-2-one (5), and 5-[(2E)-but-2-en-1-yl]-4-methoxy-6-methyl-2H-pyran-2-one (6). Compounds 2 and 4 showed moderate antileukemic activities against HL60 cells with IC50 values of 2.2 and 0.9 μM and against K562 cells with IC50 values of 4.5 and 1.5 μM, respectively. 相似文献
197.
An efficient and practical method for the synthesis of cyano, sulfonyl and phosphoryl substituted naphthalene derivatives via the rearrangement aromatization of benzo[c]oxepine has been developed. The system holds the advantages of metal catalysts free, and mild reaction conditions. 相似文献
198.
A novel synthesis of 1,2,4-oxadiazoles and isoxazoles is described by utilizing the reactions between amidoximes and α,β-alkynic aldehydes and/or ketones. Conjugate addition products, obtained from amidoximes and α,β-alkynic aldehydes and/or ketones, afford 1,2,4-oxadiazoles and isoxazoles when treated with bases and acids, respectively. 1,2,4-Oxadiazoles can also be synthesized directly from amidoximes and α,β-alkynic aldehydes in a one-pot manner under basic conditions. The reactions are general for a variety of starting compounds and tolerate the presence of aryl, heteroaryl and alkyl groups. 相似文献
199.
Readily available off-the-shelf KBrO3 and KHSO4 have been used to selectively oxidize 1,2-diols in water as a solvent. Various cyclic 1,2-diols have been tolerated affording their corresponding α-hydroxy ketones in good yields. 相似文献
200.
Nickel(I) complexes were generated in situ from Ni (PPh3)2Cl2 using activated iron and the complexes combined with N,N′-bis(4-fluorobenzylidene) ethane-1,2-diamine (BFBED) were then used as a catalyst for the 1,4-addition reaction of arylboronic acids to α,β-unsaturated substrates. The reaction proceeded to completion and did not require the addition of a base but the addition of potassium iodide is crucial to this cross-coupling reaction. Moreover, experimental observations suggested a possible Ni(I)–Ni(III) catalytic cycle mechanism. 相似文献