全文获取类型
收费全文 | 44275篇 |
免费 | 8348篇 |
国内免费 | 1449篇 |
专业分类
化学 | 45911篇 |
晶体学 | 351篇 |
力学 | 738篇 |
综合类 | 1篇 |
数学 | 3697篇 |
物理学 | 3374篇 |
出版年
2022年 | 67篇 |
2021年 | 237篇 |
2020年 | 1298篇 |
2019年 | 2663篇 |
2018年 | 1097篇 |
2017年 | 728篇 |
2016年 | 3496篇 |
2015年 | 3636篇 |
2014年 | 3575篇 |
2013年 | 4378篇 |
2012年 | 3260篇 |
2011年 | 2555篇 |
2010年 | 3075篇 |
2009年 | 3022篇 |
2008年 | 2729篇 |
2007年 | 2084篇 |
2006年 | 1820篇 |
2005年 | 1966篇 |
2004年 | 1777篇 |
2003年 | 1603篇 |
2002年 | 2304篇 |
2001年 | 1450篇 |
2000年 | 1378篇 |
1999年 | 442篇 |
1998年 | 142篇 |
1997年 | 154篇 |
1996年 | 157篇 |
1995年 | 109篇 |
1994年 | 129篇 |
1993年 | 114篇 |
1992年 | 114篇 |
1991年 | 93篇 |
1990年 | 88篇 |
1989年 | 89篇 |
1988年 | 80篇 |
1987年 | 83篇 |
1986年 | 74篇 |
1985年 | 128篇 |
1984年 | 140篇 |
1983年 | 100篇 |
1982年 | 149篇 |
1981年 | 123篇 |
1980年 | 146篇 |
1979年 | 133篇 |
1978年 | 144篇 |
1977年 | 124篇 |
1976年 | 127篇 |
1975年 | 107篇 |
1974年 | 115篇 |
1973年 | 84篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
221.
The synthesis of racemic analogues of phosphatidylinositol (PI) and phosphatidylinositol 4,5-bisphosphate (PIP2) starting from myo-inositol is described. Inositol derivatives with and without homologalion at C(l) and with and without ionic groups (phosphate or sulfate) at C(4) and C(5) were prepared as well as homologated derivatives with deoxy composition at C(2) and/or C(6). In all these compounds, palmitate ester groups were introduced in place of the diacylglyceryl group of PI or PIP2. 相似文献
222.
Contribution to the study of the behaviour of catechin in alkaline medium When catechin ( 1 ) is warmed with aqueous alkali it first undergoes rapid epimerisation. After some time a novel bridged carbocyclic enol, catechinic acid ( 3 ) separates out and may be isolated in excellent yield. Eventually catechinic acid isomerises to 2-(3′,4′-dihydroxyphenyl)-3,9-epoxy-6-oxo-bicyclo[3,3,1]-non-7-en-8,9-diol ( 4 ) via an open chain anion ( 5 ) which is also involved in the epimerisation of catechin. Assignments of structure are given and the equilibria observed are discussed. 相似文献
223.
Synthesis of Novel Types of C-Glycosyl-derivatives from Acetylenic Sugars or Partial Synthetic Equivalents thereof The concept of synthetic equivalent is not unequivocal and, for example, α-fluoroenamines may behave in some cases as synthetic equivalents of ynamines and, in other situations, quite differently. Some uses of these α-fluoroenamines, ‘partial synthetic equivalents’ of ynamines, for the synthesis of C-glycosyl derivatives of coumarins, chromones, quinolines, quinolones, thietenes and isoxazoles are described. We also report the preparation of a series of mono- and diglycosylthiophenes by reacting HS? with mono- and diglycosylbutadiynes. Acetylenic sugar derivatives have also been used for the synthesis of a diglycosylazaphosphorine and a spirooxolenone. 相似文献
224.
Various reactions of ring D of dipterocarpol (hydroxy-dammarenone-II) show this triterpene to have the configuration 13β, 17H, in accordance with biogenetic speculations. 相似文献
225.
Broussy S Bernardes-Génisson V Gornitzka H Bernadou J Meunier B 《Organic & biomolecular chemistry》2005,3(4):666-669
An ortho-metallation-electrophilic substitution sequence was employed as a key step to build the 4-benzoylpyridine framework. It was found that 4-benzoylpyridine-3-carboxamide and an N-pyridyl alkylated derivative both exist in a unique cyclized hemiamidal structure, not in the usually expected keto-amide open form. These structures represent fragment models of the Isoniazid-NAD adducts involved in the mechanism of action of the antituberculous drug Isoniazid. 相似文献
226.
Natural Occurrence of C(15)-Epimeric Coleons C and D and its Significance to the Stereochemistry of the Formation of a Spirocyclopropanring In a previous paper some evidence was presented for a predominant occurrence of epimeric coleons ((15S)-coleon C ((15 S)- 1a ), and (15S)-coleon D ((15S)- 2a )) in Plectranthus lanuginosus [1], The proposed structures have now been proven by careful comparison of their physical data with those of the already described C(15)-epimers whose structures were deduced by X-ray analysis of a derived cis-diketone 3 [5], Examination of the epimeric purity of coleons possessing a 2-hydroxy-1-methylethyl side-chain (coleons C , D , H , I , I ′ and derivatives) isolated from seven different species (Coleus, Plectranthus and Solenostemon) has shown that these coleons always occur as a mixture of C(15)-epimers, although in most cases with one predominant epimer. The consequences for the stereochemistry of the postulated in vivo formation of the methyl-substituted spirocyclopropane substructure is discussed. 相似文献
227.
Jacqueline Ficini Jean dAngelo Arnaud Eman Anne Marie Touzin 《Tetrahedron letters》1976,17(9):683-686
228.
229.
Derrida's random energy model is considered. Almost sure andL
P
convergence of the free energy at any inverse temperature are proven. Rigorous upper and lower bounds to the finite size corrections to the free energy are given. 相似文献
230.
Ai‐Hua Zhao Yan Zhang Zhao‐Hui Xu Jian‐Wen Liu Wei Jia 《Helvetica chimica acta》2004,87(12):3160-3166
Three new enmein‐type ent‐kaurenoids, i.e., the two pairs 1 and 2 of 20‐epimers and the (20R)‐isomer 3 , besides the seven known diterpenoids 4 – 10 , were isolated from the aerial parts of Isodon serra. Their structures were elucidated by spectroscopic techniques and X‐ray diffraction. The immunosuppressive effect for T‐lymphocytes proliferation induced by Con A in BALB/c mouse was evaluated for the isolates 1 – 10 . They all displayed a remarkable inhibitory effect, with multi‐glycosides of Tripterygium wilfordii as positive reference substance (Table 3). 相似文献