全文获取类型
收费全文 | 2064篇 |
免费 | 187篇 |
国内免费 | 178篇 |
专业分类
化学 | 2277篇 |
晶体学 | 4篇 |
力学 | 4篇 |
综合类 | 3篇 |
数学 | 95篇 |
物理学 | 46篇 |
出版年
2023年 | 19篇 |
2022年 | 24篇 |
2021年 | 38篇 |
2020年 | 57篇 |
2019年 | 61篇 |
2018年 | 65篇 |
2017年 | 69篇 |
2016年 | 59篇 |
2015年 | 73篇 |
2014年 | 130篇 |
2013年 | 246篇 |
2012年 | 96篇 |
2011年 | 132篇 |
2010年 | 115篇 |
2009年 | 111篇 |
2008年 | 120篇 |
2007年 | 123篇 |
2006年 | 131篇 |
2005年 | 154篇 |
2004年 | 150篇 |
2003年 | 129篇 |
2002年 | 54篇 |
2001年 | 26篇 |
2000年 | 54篇 |
1999年 | 28篇 |
1998年 | 17篇 |
1997年 | 20篇 |
1996年 | 12篇 |
1995年 | 11篇 |
1994年 | 24篇 |
1993年 | 13篇 |
1992年 | 23篇 |
1991年 | 5篇 |
1990年 | 11篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1975年 | 1篇 |
1970年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有2429条查询结果,搜索用时 15 毫秒
101.
Ouro-Sama Adetchessi 《Tetrahedron》2005,61(18):4453-4460
2-Substituted-4a-hydroxy-9H-cycloalkyl[1,2-e]oxazolo[3,2-a]pyrimidin-9-ones 2a-c were synthesized by an one-step cyclocondensation from the 5-substituted-2-amino-2-oxazolines 1a-c with ethyl 2-oxocyclohexanecarboxylate in ethanol at room temperature, and easily dehydrated to provide 2-substituted-9H-cycloalkyl[1,2-e]oxazolo[3,2-a]pyrimidin-9-ones 3. In refluxing xylene, the reaction conducted with various ethyl 2-oxocycloalkanecarboxylates led to the two isomeric 2-substituted-8/9H-cycloalkyl[1,2-e]oxazolo[3,2-a]pyrimidin-8/9-ones 3 and 2-substituted-5H-cycloalkyl[1,2-d]oxazolo[3,2-a]pyrimidin-5-ones 4. The structure of some compounds was unambiguously established using X-ray crystallography. According to results from the DSC analysis of compound 2a, formation of the thermodynamically stable pyrimidinones 4 could be related to an intramolecular rearrangement of kinetically controlled pyrimidinones 3. 相似文献
102.
An enantioselective synthesis of (+)‐β‐himachalene ( 2 ) was accomplished starting from (1S,2R)‐1,2‐epoxy‐p‐menth‐8‐ene ( 3 ) in 15 or 16 steps with an overall yield of ca. 6% (Schemes 3, 5, and 6). Key transformations include an Ireland–Claisen rearrangement, a Corey oxidative cyclization, and a ring expansion. 相似文献
103.
A novel method for the synthesis of chiral α-amino acids has been developed where the acid functionality was constructed by oxidizing a hydroxymethyl group introduced by Evans’ method in the α-position of an appropriate acid substrate and the amino part came from the amide of the original carboxyl group following a modified Hofmann rearrangement reaction. 相似文献
104.
A new synthetic method for the preparation of allyl amines has been developed. The key steps of this method are enantioselective addition of diethylzinc and [1,3]-chirality transfer through the [3.3] sigmatropic rearrangement of allyl cyanates. Stereocontrolled syntheses of lentiginosine (1) and polyoxamic acid derivative 2 from a common intermediate 7 derived from D-tartaric acid (8), have been accomplished. 相似文献
105.
Takumi Furuta 《Tetrahedron》2004,60(42):9375-9379
The total synthesis of anti-inflammatory active flavone C-glycoside isolated from oolong tea extract is achieved. Introducing a C-glucosyl moiety to an aryl system and constructing a fused tetracyclic ring characteristic to this natural product were conducted based on the O-to-C rearrangement of sugar moiety and the successive intramolecular Mitsunobu reaction, respectively. This concise and efficient synthetic pathway is applicable to the large-scale synthesis of target flavone and for constructing a large library of related compounds. 相似文献
106.
107.
108.
《中国化学》2017,35(11):1673-1677
An efficient Beckmann rearrangement of ketoximes was developed using carbon tetrabromide/triphenylphosphine as an organocatalyst without addition of any acids or metals. The reaction showed good functional group tolerance and gave various amides in moderate to good yields. 相似文献
109.