全文获取类型
收费全文 | 20434篇 |
免费 | 3771篇 |
国内免费 | 3354篇 |
专业分类
化学 | 15713篇 |
晶体学 | 477篇 |
力学 | 1083篇 |
综合类 | 297篇 |
数学 | 2184篇 |
物理学 | 7805篇 |
出版年
2024年 | 33篇 |
2023年 | 301篇 |
2022年 | 469篇 |
2021年 | 602篇 |
2020年 | 724篇 |
2019年 | 845篇 |
2018年 | 646篇 |
2017年 | 676篇 |
2016年 | 911篇 |
2015年 | 997篇 |
2014年 | 1120篇 |
2013年 | 1506篇 |
2012年 | 1935篇 |
2011年 | 1910篇 |
2010年 | 1536篇 |
2009年 | 1508篇 |
2008年 | 1688篇 |
2007年 | 1419篇 |
2006年 | 1434篇 |
2005年 | 1291篇 |
2004年 | 1044篇 |
2003年 | 810篇 |
2002年 | 860篇 |
2001年 | 672篇 |
2000年 | 544篇 |
1999年 | 425篇 |
1998年 | 267篇 |
1997年 | 198篇 |
1996年 | 181篇 |
1995年 | 160篇 |
1994年 | 167篇 |
1993年 | 131篇 |
1992年 | 77篇 |
1991年 | 98篇 |
1990年 | 61篇 |
1989年 | 47篇 |
1988年 | 44篇 |
1987年 | 19篇 |
1986年 | 32篇 |
1985年 | 34篇 |
1984年 | 18篇 |
1983年 | 18篇 |
1982年 | 13篇 |
1981年 | 18篇 |
1980年 | 10篇 |
1979年 | 13篇 |
1978年 | 7篇 |
1977年 | 10篇 |
1957年 | 4篇 |
1936年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
291.
292.
[reaction: see text] OSW saponins, featuring a 16beta,17alpha-dihydroxycholest-22-one aglycon and an acylated beta-D-xylopyranosyl-(1-->3)-alpha-L-arabinopyranosyl residue attached to the 16-hydroxyl group, have recently been discovered from a group of lily plants, which show potent antitumor activities with a novel mechanism of action. This paper describes an aldol approach to the stereoselective construction of the 16alpha,17alpha-dihydroxycholest-22-one structure from 16alpha-hydroxy-5-androsten-17-ones and propionates. Elaboration of the aldol adducts toward OSW-1, involving installation of the isoamyl ketone side chain, inversion of the 16-hydroxyl configuration, and selective protection of the C22-oxy function, has been explored and accomplished. In particular, the present route was found convenient for the synthesis of OSW saponin analogues with a C22-ester side chain. Thus, the 23-oxa-analogue of OSW-1 (40) was prepared starting from the industrial dehydroisoandrosterone (1) in a linear eight-step sequence and in 26% overall yield. Analogues with a variety of modified side chains were prepared, via aldol condensation with propionates of varying length, thiopropionate, and acetate (for preparation of 68-75) or via aminolysis of the 22,16-lactone 26 (for preparation of the 23-N-analogues). Cross metathesis (CM) reaction was also found feasible for modification at the final stage from C22-allyl ester 70. Valuable structure-activity relationships (SAR), together with the practical synthetic approach, have thus been provided to set a new stage for further studies on this new type of antitumor structures. 相似文献
293.
294.
295.
JunJieKANG ShiBiFANG 《中国化学快报》2004,15(1):87-89
Network polymer electrolytes with free oligo(oxyethylene) chains as internal plasticizers were prepared by cross-linking poly(ethylene glycol) acrylates. The effects of salt concentration and properties of internal plasticizers on ionic conductivity were studied. 相似文献
296.
Gao J Fu J Lin C Lin J Han Y Yu X Pan C 《Langmuir : the ACS journal of surfaces and colloids》2004,20(22):9775-9779
Silver nanoparticles were synthesized by the use of a two-armed polymer with a crown ether core [poly(styrene)]-dibenzo-18-crown-6-[poly(styrene)] based on the flexibility of the polymer chains and the complex effect of crown ether with Ag(+) and Ag. The size of silver nanoparticles could be tailored by controlling the initial concentrations of the polymer and Ag(+), and the molecular weight of the polymer. The emission of silver nanoparticles was blue-shifted, and the intensity of the photoluminescence of silver nanoparticles stabilized by the polymer was significantly increased due to the complex effect between the crown ether embedded in the polymer and the silver nanoparticles. 相似文献
297.
Nakamura Y O-kawa K Nishimura T Yashima E Nishimura J 《The Journal of organic chemistry》2003,68(8):3251-3257
The optical resolution of trans-2 and trans-3 [60]fullerene bisadducts with an inherent chiral addition pattern, modified by Bingel reaction, cycloaddition by benzyne, Prato reaction, and cycloaddition by o-quinodimethane, was systematically investigated by using chiral HPLC columns (Chiralcel OD and Chiralpak AD). The chiroptical properties of enantiomers separated were also examined. 相似文献
298.
Thermal treatment of Zn(NH3)(4)2+ precursor in ethanol solvent led to the formation of the tubular ZnO which exhibited strong ultraviolet photoluminescence around 385 nm at room temperature; TEM images showed the hollow tubules with approximately 450 nm in diameter and approximately 4 microns in length were built up by ZnO polycrystals. 相似文献
299.
Morikawa T Sun B Matsuda H Wu LJ Harima S Yoshikawa M 《Chemical & pharmaceutical bulletin》2004,52(10):1194-1199
A new beta-carboline-type alkaloidal glycoside, glucodichotomine B, four new neolignan glycosides, dichotomosides A, B, C, and D, and a new phenylpropanoid glycoside, dichotomoside E, were isolated from a Chinese natural medicine, the roots of Stellaria dichotoma L. var. lanceolata. The structures of the new glycosides were determined on the basis of chemical and physicochemical evidence. Among them, dichotomoside D inhibited the release of beta-hexosaminidase (IC(50)=64 microM) as well as tumor necrosis factor-alpha and interleukin-4 (IC(50)=16, 34 microM) in RBL-2H3 cells. These findings suggest that dichotomoside D is more effective against the late-phase reactions in type I allergy than in the immediate phase. 相似文献
300.
Alkyl halides underwent unique cross-coupling reaction with vinylmagnesium chloride in the presence of Ni catalyst to give 2-alkyl-3-butenyl Grignard reagent (1) in high yields. This reaction proceeded efficiently at 25 degrees C in THF using primary and secondary alkyl fluorides. On the other hand, PhCH=CHMgBr gave double alkylative vinyl coupling product 4 in good yield as the sole coupling product. Alkyl fluorides react as the most suitable alkylating reagent in comparison to the corresponding chlorides, bromides, and iodides. 相似文献