首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   399篇
  免费   11篇
化学   386篇
数学   5篇
物理学   19篇
  2021年   2篇
  2020年   8篇
  2019年   4篇
  2018年   4篇
  2016年   2篇
  2015年   9篇
  2014年   3篇
  2013年   7篇
  2012年   26篇
  2011年   26篇
  2010年   7篇
  2009年   3篇
  2008年   14篇
  2007年   23篇
  2006年   22篇
  2005年   26篇
  2004年   20篇
  2003年   28篇
  2002年   42篇
  2001年   25篇
  2000年   8篇
  1999年   5篇
  1997年   8篇
  1996年   4篇
  1995年   5篇
  1994年   2篇
  1993年   2篇
  1992年   5篇
  1990年   2篇
  1989年   8篇
  1986年   5篇
  1985年   6篇
  1984年   2篇
  1982年   4篇
  1981年   10篇
  1980年   3篇
  1979年   1篇
  1978年   2篇
  1977年   4篇
  1976年   3篇
  1975年   4篇
  1974年   2篇
  1973年   3篇
  1972年   1篇
  1967年   1篇
  1941年   1篇
  1940年   1篇
  1937年   1篇
  1936年   2篇
  1933年   1篇
排序方式: 共有410条查询结果,搜索用时 15 毫秒
141.
Trost BM  Ball ZT  Kang EJ 《Organic letters》2005,7(22):4911-4913
[reaction: see text] Treatment of hydroxy-substituted silyl epoxides with Grignard reagents induces a 1,2-carbon shift to reveal alpha-silyl aldehydes, which are trapped by highly diastereoselective addition reactions of the Grignard reagent. The starting epoxides are readily accessible from propargylic alcohols by regio- and diastereoselective hydrosilylation and epoxidation reactions. In addition to providing functionalized tertiary silane products, the method is shown to offer a tertiary olefin synthesis through chemo- and diastereoselective Peterson elimination of the product tertiary silane diols.  相似文献   
142.
The Pd-catalyzed reorganization of enol allyl carbonates to allylated ketones occurs asymmetrically in the presence of chiral ligands previously developed in this group. With 2-methylcyclohexanone, asymmetric regioselective alkylation occurs at the more substituted carbon without complications of polyalkylation. Alkylation to create quaternary centers in indanones and benzonabenone occurs in much higher ee than using tin or lithium enolates. The sense of enantioinduction in tetralones is opposite from the tin and lithium enolate examples. For the first time, asymmetric creation of tertiary centers occurs with high ee (78-99%). The different results between this reaction and the use of lithium or tin enolates suggest different mechanisms may be involved.  相似文献   
143.
We report a catalytic asymmetric total synthesis of the ascidian natural product perophoramidine. The synthesis employs a molybdenum-catalyzed asymmetric allylic alkylation of an oxindole nucleophile and a monosubstituted allylic electrophile as a key asymmetric step. The enantioenriched oxindole product from this transformation contains vicinal quaternary and tertiary stereocenters, and is obtained in high yield along with high levels of regio-, diastereo-, and enantioselectivity. To install the second quaternary stereocenter in the target, the route utilizes a novel regio- and diastereoselective allylation of a cyclic imino ether to deliver an allylated imino ether product in near quantitative yield and with complete regio- and diastereocontrol. Oxidative cleavage and reductive amination are used as final steps to access the natural product.  相似文献   
144.
Racemic butadiene and isoprene monoepoxide react with unsaturated alcohols in the presence of a chiral palladium catalyst and a boron co-catalyst to give 3-alkoxy-4-hydroxy-1-butene and 3-alkoxy-4-hydroxy-3-methyl-1-butene, respectively, with excellent regio- and enantioselectivity in a dynamic kinetic asymmetric transformation whereby both enantiomers of the starting epoxides provide the same enantiomeric product. In the case of 2-phenylbutadiene monoepoxide, easily available from phenacyl chloride and vinylmagnesium bromide, the reaction proceeds by kinetic resolution. A model to rationalize the result is presented. The bis-olefin products are ideal substrates for the Ru catalyzed ring closing metathesis. In this way, five-, six-, and seven-membered oxygen heterocycles are readily available enantiomerically pure. The value of this very simple two step process is demonstrated by the use of the five-membered ring heterocycles to form unnatural and unusual nucleosides that cannot be easily accessed by other means. The sequence involves a Ru catalyzed isomerization of the initial 2,5-dihydrofuran to a 2,3-dihydrofuran followed by a selenium promoted addition of a pyrimidine or purine base. One advantage of this strategy is the easy access to either enantiomeric series, both of which have important biological applications.  相似文献   
145.
146.
The source of a rate retardation of the trimethylsilyl substituent in an alkylation reaction is probed.  相似文献   
147.
148.
149.
150.
The asymmetric acylation of meso-2-substituted-1,3-propanediols by using an amphoteric chiral dinuclear zinc catalyst is described. It is has been demonstrated that both 2-alkyl- and 2-aryl-1,3-propanediols can be desymmetrized in high yields and enantioselectivities by using the same family of ligands. Given that both antipodes of the chiral catalyst are available, both enantiomers of the desymmetrized product can be obtained from the same starting material. The synthetic utility of the desymmetrized products has been demonstrated by the synthesis of several chiral building blocks with high enantiomeric purities.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号