全文获取类型
收费全文 | 2040篇 |
免费 | 186篇 |
国内免费 | 178篇 |
专业分类
化学 | 2253篇 |
晶体学 | 4篇 |
力学 | 4篇 |
综合类 | 3篇 |
数学 | 95篇 |
物理学 | 45篇 |
出版年
2023年 | 19篇 |
2022年 | 16篇 |
2021年 | 38篇 |
2020年 | 57篇 |
2019年 | 60篇 |
2018年 | 63篇 |
2017年 | 68篇 |
2016年 | 59篇 |
2015年 | 73篇 |
2014年 | 125篇 |
2013年 | 243篇 |
2012年 | 96篇 |
2011年 | 131篇 |
2010年 | 115篇 |
2009年 | 111篇 |
2008年 | 119篇 |
2007年 | 122篇 |
2006年 | 131篇 |
2005年 | 153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 | 11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2404条查询结果,搜索用时 15 毫秒
71.
Ana Montero 《Tetrahedron letters》2005,46(3):401-405
A stereocontrolled synthesis of an unsaturated sugar bearing two amino groups (one of them masked as an azide), using an Overman rearrangement as key step, is described. This scaffold is used to prepare two peptides having aromatic fragments, which have shown activity as calpain inhibitors. 相似文献
72.
73.
Akira Ando 《Journal of fluorine chemistry》2005,126(5):765-770
Treatment of 2-chloro-3,3-difluoroprop-2-en-1-ol derivatives (2) with methanesulfonyl chloride in the presence of a base did not give the expected esters but 2-chloro-1,1-difluoroprop-2-enyl methanesulfonates (4) through a novel [3,3] sigmatropic rearrangement. Reaction of 4 with diethylzinc in the presence of tetrakis(triphenylphosphine) palladium gave 1-alkyl- or 1-aryl-2-chloro-3-fluoropenta-1,3-dienes in moderate to good yields through a CC bond formation followed by dehydrofluorination. 相似文献
74.
Halland N Alstrup Lie M Kjaersgaard A Marigo M Schiøtt B Jørgensen KA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(23):7083-7090
The mechanism for the 2,5-diphenylpyrrolidine-catalyzed enantioselective alpha-chlorination of aldehydes with electrophilic halogenation reagents has been investigated by using experimental and computational methods. These studies have led us to propose a mechanism for the reaction that proceeds through an initial N-chlorination of the chiral catalyst-substrate complex, followed by a 1,3-sigmatropic shift of the chlorine atom to the enamine carbon atom. The suggested reaction course is different from previously proposed mechanisms for organocatalytic enamine reactions, in which the carbon-electrophile bond is formed directly. Furthermore, the rate-determining step in the overall reaction was determined and the presence of nonlinear effects was probed. 相似文献
75.
76.
A. V. Vasilyev N. P. Solov'eva N. Ya. Podhaluzina A. Yu. Tolmachev V. F. Traven 《Chemistry of Heterocyclic Compounds》2003,39(4):534-538
Fries rearrangement of 7-acetoxy-4-methylquinolin-2-one and subsequent condensation of the 6-acetyl-7-hydroxy-4-methylquinolin-2-one obtained with -chloro ketones gives a series of furo[3,2-g]quinolin-7-ones. 相似文献
77.
A highly convergent and efficient synthesis of (−)-microcarpalide, a 10-membered lactone displaying remarkable microfilament disrupting activity is described. Ring-closing metathesis and Sharpless asymmetric dihydroxylations are the key steps. Our strategy highlights the application of novel hydroxy lactone precursors for the stereoselective synthesis of (−)-microcarpalide. 相似文献
78.
Synthesis of 6α-methyl-16α,17α-cyclohexano-19-norprogesterone from a 19-methyl-6-desmethyl precursor
After prolonged refluxing of 19-tosyloxy-16α,17α-cyclohexanopregn-5-en-3β-ol-20-one (3) with NaI in 2-propanol, the initially formed 19-iodo derivative (4) undergoes supraface migration of the CH2I group from the C(10) atom to the C(6) atom, probably through involvement of a homoallyl cation. The resulting 6β-iodomethyl-16α,17α-cyclohexano-19-norpregn-5(10)-en-3β-ol
(5) was transformed in three steps into 6α-methyl-16α,17α-cyclohexano-19-norprogesterone (6α-methyl-19-nor-D′
6-pentarane,8). The transformation of compound5 into the target product8 also gave a side product, a pentarane with aromatic ringA (10), which was isolated and characterized by spectroscopic methods.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1688–1691, September, 1997. 相似文献
79.
Dongmei Li 《Tetrahedron letters》2005,46(4):671-674
Under mild conditions, Beckmann rearrangement of a variety of ketoximes could proceed in the presence of chlorosulfonic acid using toluene as a solvent with excellent conversion and selectivity. This procedure could also be applied in the dehydration of aldoximes for obtaining the corresponding nitriles. 相似文献
80.
Toshikatsu Takanami 《Tetrahedron letters》2005,46(16):2893-2896
The catalytic Z-selective Claisen rearrangement of simple aliphatic allyl vinyl ethers can be achieved using a chromium(III) porphyrin complex, Cr(TPP)Cl, as a catalyst: Cr(TPP)Cl significantly enhances reversal of E-Z selectivity in the thermal Claisen rearrangement of allyl vinyl ethers, especially, 4,5- and 4,6-disubstituted derivatives, at low catalyst loading. 相似文献