全文获取类型
收费全文 | 2089篇 |
免费 | 73篇 |
国内免费 | 13篇 |
专业分类
化学 | 1594篇 |
晶体学 | 28篇 |
力学 | 31篇 |
数学 | 158篇 |
物理学 | 364篇 |
出版年
2023年 | 11篇 |
2022年 | 17篇 |
2021年 | 26篇 |
2020年 | 26篇 |
2019年 | 40篇 |
2018年 | 30篇 |
2017年 | 21篇 |
2016年 | 56篇 |
2015年 | 41篇 |
2014年 | 54篇 |
2013年 | 137篇 |
2012年 | 114篇 |
2011年 | 131篇 |
2010年 | 90篇 |
2009年 | 81篇 |
2008年 | 145篇 |
2007年 | 114篇 |
2006年 | 112篇 |
2005年 | 115篇 |
2004年 | 96篇 |
2003年 | 94篇 |
2002年 | 73篇 |
2001年 | 34篇 |
2000年 | 34篇 |
1999年 | 29篇 |
1998年 | 18篇 |
1997年 | 27篇 |
1996年 | 18篇 |
1995年 | 20篇 |
1994年 | 23篇 |
1993年 | 25篇 |
1992年 | 21篇 |
1991年 | 23篇 |
1990年 | 14篇 |
1989年 | 17篇 |
1988年 | 9篇 |
1987年 | 12篇 |
1986年 | 11篇 |
1985年 | 16篇 |
1984年 | 30篇 |
1983年 | 13篇 |
1982年 | 22篇 |
1981年 | 13篇 |
1980年 | 13篇 |
1979年 | 11篇 |
1978年 | 9篇 |
1976年 | 14篇 |
1974年 | 9篇 |
1973年 | 9篇 |
1969年 | 11篇 |
排序方式: 共有2175条查询结果,搜索用时 0 毫秒
21.
22.
23.
24.
The proposed structures of patellamides B and C, cytotoxic lipophilic cyclic peptides from a marine tunicate, have been revised on the basis of the spectral data of their partial hydrolysates, and their revised structures have been synthetically confirmed by the use of diphenyl phosphorazidate(DPPA) and diethyl phosphorocyanidate(DEPC). 相似文献
25.
Diethyl phosphorocyanidate (DEPC) can be efficiently used for the synthesis of α-amino nitriles from carbonyl compounds and amines. The reaction may be called a modified Strecker synthesis. 相似文献
26.
H Miyazaki N Ohkawa N Nakamura T Ito T Sada T Oshima H Koike 《Chemical & pharmaceutical bulletin》1989,37(9):2379-2390
Six-membered lactone and tetrahydropyran analogues of platelet-activating factor (PAF), 4-11, and related antagonistic derivatives 41-46 were synthesized. None of the delta-lactones 4-7 showed PAF-like activities, while the corresponding cyclic ethers 8, 9 and 11 were slightly active. Some of the cyclic antagonists showed more potent inhibitory activities than the open chain antagonist CV-3988 against platelet aggregation (rabbit platelet-rich plasma, IC50) and hypotension (rat, ID50) induced by C16-PAF: e.g. dl-3-[6-[O-(trans-3-heptadecylcarbamoyloxytetrahydropyran-2- yl)methyl]phosphonoxy]hexylthiazolium (inner salt) (41d) (IC50 5.5 x 10(-7) M, ID50 0.046 mg/kg, i.v.); dl-3-[5-[O-(cis-3-heptadecylcarbamoylthiotetrahydropyran-2- yl)methyl]phosphonoxy]pentylthiazolium (inner salt) (43c) (IC50 5.7 x 10(-7) M, ID50 0.076 mg/kg, i.v.). 相似文献
27.
Photoswitching of the coordination number of silicon between four and five in allyldifluoro[2-(phenylazo)phenyl]silane, which was confirmed by X-ray analysis and multinuclear NMR spectroscopy, caused multistep reactions to proceed or stop, yielding tetrafluoro[2-(1-allyl-2-phenylhydrazino)phenyl]silicate without altering other reaction conditions. 相似文献
28.
Tamura S Fukamiya N Okano M Koyama J Koike K Tokuda H Aoi W Takayasu J Kuchide M Nishino H 《Chemical & pharmaceutical bulletin》2003,51(4):385-389
Three new quassinoids, ailantinol E (1), ailantinol F (2), and ailantinol G (3), and related compounds were isolated from Ailanthus altissima grown in Taiwan. Their structures were elucidated from spectral evidence. Each new quassinoid was evaluated for its antitumor promoting effects against Epstein-Barr virus early antigen activation introduced by 12-O-tetradecanoylphorbol-13-acetate in Raji cells. The new quassinoids were found to show potent activity without showing any cytotoxicity. The screening for inhibitors against nitric oxide donor action was also conducted using the new quassinoids and some standard samples. 相似文献
29.
Intramolecular tandem Michael-type addition/aldol cyclization induced by TiCl(4)-R(4)NX combinations
[reaction: see text] Treatment of formyl alpha,beta-enones with a TiCl(4)-R(4)NX combination induces an intramolecular aldol cyclization to furnish 2-acyl-3-halocyclohexanol with three controlled consecutive stereogenic centers. The reaction of bis-alpha,beta-enones with the combination provides cyclic diketones with high stereoselectivity via an intramolecular Michael addition reaction. 相似文献
30.
Makoto OzawaTakaaki Taguchi Takayuki ItohYutaka Ebizuka Kevin I Booker-MilburnG.Richard Stephenson Koji Ichinose 《Tetrahedron》2003,59(44):8793-8798
A novel shunt product was isolated from a disruptant of the actVI-ORFA gene involved in the biosynthesis of actinorhodin (ACT) in Streptomyces coelicolor A3(2). Its structure was elucidated as 1,4-naphthoquinone-8-hydroxy-3-[3(S)-acetoxy-butyric acid], (S)-NHAB, based on NMR, MS, and CD spectroscopic data as well as a single crystal X-ray crystallographic analysis. The formation of (S)-NHAB involves a retro-Claisen type C-C bond cleavage of an ACT biosynthetic intermediate. Feeding experiments with [1-13C] and [2-13C] acetates indicated its biosynthetic origin as a single octaketide chain. The relevant gene product, Act-ORFA, which is a functionally unknown protein, is proposed to play a regulatory role related to the multi-enzymatic steps to ACT production, based on the metabolic profile of its disruptant and the wide distribution of actVI-ORFA homologues in the gene clusters for Streptomyces aromatic polyketides. 相似文献