全文获取类型
收费全文 | 63122篇 |
免费 | 10210篇 |
国内免费 | 7824篇 |
专业分类
化学 | 45435篇 |
晶体学 | 828篇 |
力学 | 3622篇 |
综合类 | 502篇 |
数学 | 7351篇 |
物理学 | 23418篇 |
出版年
2024年 | 193篇 |
2023年 | 1196篇 |
2022年 | 2334篇 |
2021年 | 2436篇 |
2020年 | 2519篇 |
2019年 | 2503篇 |
2018年 | 2114篇 |
2017年 | 2117篇 |
2016年 | 3011篇 |
2015年 | 3148篇 |
2014年 | 3713篇 |
2013年 | 4739篇 |
2012年 | 5765篇 |
2011年 | 5690篇 |
2010年 | 4047篇 |
2009年 | 3834篇 |
2008年 | 4186篇 |
2007年 | 3697篇 |
2006年 | 3290篇 |
2005年 | 2921篇 |
2004年 | 2306篇 |
2003年 | 1799篇 |
2002年 | 1747篇 |
2001年 | 1437篇 |
2000年 | 1235篇 |
1999年 | 1245篇 |
1998年 | 1019篇 |
1997年 | 943篇 |
1996年 | 907篇 |
1995年 | 812篇 |
1994年 | 717篇 |
1993年 | 583篇 |
1992年 | 490篇 |
1991年 | 450篇 |
1990年 | 391篇 |
1989年 | 281篇 |
1988年 | 233篇 |
1987年 | 194篇 |
1986年 | 189篇 |
1985年 | 157篇 |
1984年 | 97篇 |
1983年 | 99篇 |
1982年 | 62篇 |
1981年 | 45篇 |
1980年 | 33篇 |
1979年 | 20篇 |
1977年 | 22篇 |
1976年 | 21篇 |
1975年 | 23篇 |
1972年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
991.
Jian-Jun Tang Li Zhang Li-Ping Jiang Lei Di Yong-Ming Yan Zheng-Chao Tu Cui-Ping Yang Zhi-Li Zuo Bo Hou Hou-Lin Xia Yong-Bin Chen Yong-Xian Cheng 《Tetrahedron》2014
Polyrhachis dives is consumed as an insect food in some regions of China. In this study, new dopamine derivatives, (+)-polyrhadopamine A (1a) and (−)-polyrhadopamine A (1b), (+)-polyrhadopamine B (2a) and (−)-polyrhadopamine B (2b), and polyrhadopamines C–E (3–5), were isolated from this species. The structures and stereochemistry of these substances were assigned by using spectroscopic and computational methods. Compounds 1a, 1b, 2a, and 2b are dimeric N-acetyldopamine derivatives, 3 is a dopamine analog containing an unusual sulfone group, and 4 and 5 possess a rare benzo[d]thiazole moiety. The functions of these substances as ROCK1/2 inhibitors, neural stem cell (NSCs) proliferation stimulators, immunosuppressive, and anti-inflammatory agents were determined. 相似文献
992.
A novel one-pot two-step protocol has been developed to synthesize various spiro[benzofuran-2,2′-naphthalen]-1′-one derivatives from the three-component reaction of tetralones, 2-hydroxyphenyl functionalized α,β-unsaturated ketones, and iodine. One C–C bond and one C–O bond have formed during this process. The notable features of this protocol are simple and mild reaction conditions, applicable to a wide range of readily available starting materials, good yields (up to 91%), and excellent stereoselectivities (up to 97:3). 相似文献
993.
Six new limonoids, ivorenoids A–F (1–6), along with ten known analogues, were isolated from an ethanolic extract of the stems of Khaya ivorensis. Their structures were elucidated on the basis of spectroscopic analyses. Compounds 1 and 2 possessed a rare rearranged skeleton of khayanolides and a unique γ-lactone (C-16/C-8) replacing the common and characteristic δ-lactone D-ring (C-16/C-17) of limonoids. A mechanism of the interesting deuteration of H-2 and H-15β of 1, and H-15β of the solvent CD3OD due to the keto–enol tautomerism was demonstrated. Compounds 3 and 6 showed moderate activity against HL-60 cell line with IC50 values of 15.3 and 17.5 μM, respectively. 相似文献
994.
The first total syntheses of fuzinoside (1b) were achieved from d-galactose through two strategies (BCA and ABC) in 11 (total yield: 5.0%) and 15 (total yield: 3.7%) steps, respectively. Comparison of NMR data of synthetic compound 1b and those of the fuzinoside isolated from the lateral roots of Aconitum carmicaelii suggests that the structure reported in the literature 1 and 2 might not be accurate. The synthetic fuzinoside (1b) exhibited moderate cardiac activity in the isolated bullfrog heart assay. 相似文献
995.
Four cholate-based pyridinium compounds were synthesized and their binding abilities toward unmodified amino acids were investigated by UV spectroscopy and fluorescence emission spectroscopy. Studies revealed that the recognition process involved hydrogen bonding, electrostatic force, and π–π interaction. The receptor 4a was found to recognize l-tryptophan specifically, and the complex was studied by 1H NMR spectroscopy. The receptors 4b and 4c showed very little recognition ability toward l-tryptophan, indicating the important role of the benzyl group at pyridinium ring. 相似文献
996.
Qun Dang Zhibo Zhang Bingyu Tang Yang Song Ling Wu Tongqian Chen Stephane Bogen Vinay Girijavallabhan David B. Olsen Peter T. Meinke 《Tetrahedron letters》2014
To discover novel nucleosides as potential antiviral agents, 2′-spirolactam and 2′-spiropyrrolidine-containing nucleoside analogs were envisioned. Efficient synthetic routes were developed with an epoxide opening as the key step to establish the quaternary center at the 2′ position, leading to the design and synthesis of uridine analogs 8 and 21, prodrugs 13–16, and cytidine analog 11. 相似文献
997.
Design,synthesis and anti-influenza virus activities of terminal modified antisense oligonucleotides
Four novel terminal modified antisense oligonucleotides (ODNs) were designed, synthesized and tested for their anti-influenza virus activity. Initial biological studies indicated that lipophilic and rimantadin emodificated Flutide exhibited more potent anti-H1N1 activity than Flutide. Among them, lipophilic modificated ODN (Flutide-I) showed the most antiviral activity. The EC50 value of Flutide-I for inhibiting H1N1 induced cytopathic effect (CPE) and H1N1 RNA were respectively (0.26 ± 0.16) μM and (0.11 ± 0.03) μM. The cytotoxicity of these compounds has also been assessed. No significant cytotoxicities were found for any of these compounds with the concentrations up to 20 μM. 相似文献
998.
Hucheng Zhu Lijun Huan Chunmei Chen Jing Yang Jiangbo He Yong Chen Guangmin Yao Zengwei Luo Yongbo Xue Yonghui Zhang 《Tetrahedron letters》2014
A pair of unprecedented enantiomers (1a/1b) of cyclohexylethanoid bearing an unusual trioxabicyclo[4.2.1]nonane ring, along with two known structurally related cyclohexylethanoids, (+)-rengyolone (2) and cleroindicin E (3), were isolated from the aerial parts of Clerodendrum bungei. The structures and absolute configurations of the enantiomers were determined by comprehensive spectroscopic analysis, single-crystal X-ray diffraction, and quantum mechanical calculation of the electronic circular dichroic (ECD) spectra. The postulated biogenetic pathway of 1a/1b was also discussed. 相似文献
999.
An efficient one-step method has been developed to construct furans via a Suzuki–Miyaura cross-coupling reaction of 1,2-oxaborol-2(5H)-ols with carboxylic anhydrides. In the presence of Pd(OAc)2/PCy3, the multi-substituted alkenylboron compounds could couple with anhydrides to obtain furans in moderate-to-good yields. The addition of bases promoted the coupling reaction, and the plausible reaction mechanism was proposed. 相似文献
1000.
Xing-Wei Yang Xu-Jie Qin Yun-Li Zhao Paul K. Lunga Xiao-Nian Li Shi-Zhi Jiang Gui-Guang Cheng Ya-Ping Liu Xiao-Dong Luo 《Tetrahedron letters》2014
Three novel ring-fused monoterpenoid indole alkaloids, alstolactines A–C (1–3), were isolated from the long-term stored leaves of Alstonia scholaris. Their structures were identified on the basis of extensive spectroscopic data and X-ray diffractions. Moreover, the absolute configurations of these related structures were indicated by the crystal X-ray diffractions (Mo Kα) of 1a, a chlorinated derivative of 1. 相似文献