全文获取类型
收费全文 | 109469篇 |
免费 | 11750篇 |
国内免费 | 8079篇 |
专业分类
化学 | 64397篇 |
晶体学 | 1008篇 |
力学 | 6774篇 |
综合类 | 603篇 |
数学 | 18757篇 |
物理学 | 37759篇 |
出版年
2024年 | 267篇 |
2023年 | 1622篇 |
2022年 | 2669篇 |
2021年 | 2894篇 |
2020年 | 2971篇 |
2019年 | 2796篇 |
2018年 | 3692篇 |
2017年 | 3649篇 |
2016年 | 4099篇 |
2015年 | 4115篇 |
2014年 | 4679篇 |
2013年 | 6270篇 |
2012年 | 9862篇 |
2011年 | 9218篇 |
2010年 | 6524篇 |
2009年 | 6170篇 |
2008年 | 5414篇 |
2007年 | 5024篇 |
2006年 | 4723篇 |
2005年 | 7898篇 |
2004年 | 6652篇 |
2003年 | 4432篇 |
2002年 | 2424篇 |
2001年 | 2051篇 |
2000年 | 1728篇 |
1999年 | 1970篇 |
1998年 | 1657篇 |
1997年 | 1402篇 |
1996年 | 1454篇 |
1995年 | 1298篇 |
1994年 | 1197篇 |
1993年 | 1029篇 |
1992年 | 1046篇 |
1991年 | 943篇 |
1990年 | 805篇 |
1989年 | 676篇 |
1988年 | 545篇 |
1987年 | 475篇 |
1986年 | 389篇 |
1985年 | 393篇 |
1984年 | 256篇 |
1983年 | 233篇 |
1982年 | 183篇 |
1981年 | 150篇 |
1980年 | 105篇 |
1978年 | 88篇 |
1976年 | 135篇 |
1975年 | 98篇 |
1974年 | 95篇 |
1973年 | 114篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Pei Tang Qi-Feng Chen Ling Wang Qiao-Hong Chen Xi-Xian Jian Feng-Peng Wang 《Tetrahedron》2012,68(27-28):5668-5676
O-Demethylation at C-1 in the C19-diterpenoid alkaloids is very challenging. In this paper, it was firstly observed that 10-OH group in deltaline (1) is a determining factor for the O-demethylation reaction. After removal of this hydroxyl group, 1-O-methyl group in the corresponding deltaline analogs can be readily removed by treatment with HBr–HOAc. Meanwhile, the C-14 atom in bromides 18 or 20 can be extruded under basic condition probably via a sequence, including Grob fragmentation, aerobic oxidation, deformylation, and SN2 nucleophilic substitution, to give enone 21 (70%) and oxetane 22 (14%). The structure of compound 22 was confirmed by X-ray crystallographic analysis of its derivative 21. 相似文献
992.
δ-Glyconolactams were first synthesized by the intramolecular Schmidt–Boyer reaction using corresponding δ-azidosugars as starting material. The reaction could be efficiently performed in good yields of 61–69% under microwave radiation in acid condition, providing an alternative protocol to iminosugar δ-lactam. 相似文献
993.
A convenient procedure for the preparation of various substituted (thio)hydantoins is described. The method is based on Wittig and aza-Wittig reactions of parabanic acids with phosphonium ylides. The reactions occurred both regio- and stereo-selectively. 相似文献
994.
Polystyrene-bound diethanolamine based ionic liquids (PS-DHEEAB and PS-THEAB) were synthesized and applied for the chemical fixation of CO2 into cyclic carbonates without any additional co-catalyst and solvent. The effect of the catalysts with different number of hydroxyl group in the cation of the IL on the reaction was systematically investigated. Highest activity and selectivity were achieved in the presence of polystyrene supported diethanolamine ethyl bromide (PS-DHEEAB) in comparison with other catalysts employed. The catalyst was tough in stability and also found to be extended to a variety of terminal epoxides and aziridines. The relationship between high catalytic reactivity and the –OH functional groups was proposed. 相似文献
995.
A benzothiazoline bearing 4-carboxyphenyl group at 2-position was developed as an efficient hydrogen donor for the transfer hydrogenation reaction. Introduction of the carboxyl group significantly facilitated the removal of the residual benzothiazoline and benzothiazole by washing with aqueous basic solution. The present approach provides a convenient and straightforward access to various amines with broad substrate scope and in good yields. 相似文献
996.
997.
998.
999.
Shichang Liu Weiqi Chen Kai Fang Xiangning Jiang Ying Gai 《Rapid communications in mass spectrometry : RCM》2012,26(17):2075-2082
1000.
A bicyclo[2.2.2]octane C/D ring system,with a lactonic ring at C-8 and C-9,of the atisine-type C20-diterpenoid alkaloids,was successfully synthesized,using an oxidative dearomatization/intramolecular Diels-Alder reaction. 相似文献