全文获取类型
收费全文 | 27695篇 |
免费 | 1763篇 |
国内免费 | 4295篇 |
专业分类
化学 | 26708篇 |
晶体学 | 586篇 |
力学 | 171篇 |
综合类 | 152篇 |
数学 | 1107篇 |
物理学 | 5029篇 |
出版年
2024年 | 36篇 |
2023年 | 227篇 |
2022年 | 534篇 |
2021年 | 514篇 |
2020年 | 592篇 |
2019年 | 1523篇 |
2018年 | 687篇 |
2017年 | 1506篇 |
2016年 | 945篇 |
2015年 | 845篇 |
2014年 | 1139篇 |
2013年 | 2372篇 |
2012年 | 1679篇 |
2011年 | 1828篇 |
2010年 | 1320篇 |
2009年 | 1560篇 |
2008年 | 1764篇 |
2007年 | 1826篇 |
2006年 | 1688篇 |
2005年 | 1517篇 |
2004年 | 1503篇 |
2003年 | 1214篇 |
2002年 | 1023篇 |
2001年 | 854篇 |
2000年 | 838篇 |
1999年 | 623篇 |
1998年 | 568篇 |
1997年 | 490篇 |
1996年 | 404篇 |
1995年 | 406篇 |
1994年 | 289篇 |
1993年 | 262篇 |
1992年 | 257篇 |
1991年 | 183篇 |
1990年 | 115篇 |
1989年 | 103篇 |
1988年 | 81篇 |
1987年 | 49篇 |
1986年 | 44篇 |
1985年 | 50篇 |
1984年 | 37篇 |
1983年 | 20篇 |
1982年 | 41篇 |
1981年 | 42篇 |
1980年 | 24篇 |
1979年 | 45篇 |
1978年 | 19篇 |
1977年 | 18篇 |
1976年 | 15篇 |
1973年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Sarah A. Hackenmueller 《合成通讯》2013,43(10):1439-1446
Thyroid hormones undergo extensive metabolism to regulate hormone activity. A labeled thyroid hormone would be useful to track hormone metabolism through various pathways. While radiolabeled thyroid hormones have been synthesized and used for in vivo studies, a stable isotope labeled form of thyroid hormone is required for studying thyroid hormone metabolism by LC-MS/MS, an analytical technique that has certain advantages without the complications of radioactivity. Here we report the synthesis of 13C9-15N-T2 and 13C9-15N-T4, two labeled thyroid hormone derivatives suitable for in vivo LC-MS/MS studies. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications® to view the free supplemental file. 相似文献
992.
Controlling selectivity of the reactions of aromatic aldehydes and 2-aminobenzamide in ionic liquid catalyzed by iodine at either room temperature or at 80 °C under nitrogen resulted in the synthesis of (E)-Schiff bases, 2,3-dihydro-2-arylquinazolin-4(1H)-one and 2-arylquinazolin-4(3H)-one derivatives, in excellent yields. 相似文献
993.
Under solid-liquid PTC conditions, treatment of 1-nitro-4-(4-nitrophenylsulfonyl-methyl)benzene(1) with α,β -unsaturated esters (2a-h) at 35–75°C gave tandem addition-rearrangement products(3a-h). While at higher temperature(75°C), addition-rearrangement-addition products (4a-b) were obtained when acrylates were used. Finally a possible mechanism is proposed. 相似文献
994.
A variety of aldehyde N,N-dimethylhydrazones are rapidly converted into the corresponding nitriles using oxone supported on wet Al2O3 under microwave irradiation in dry media. 相似文献
995.
A simple synthesis of novel (E)-3-arylidene-2,3-dihydrobenzo[b][1,4]oxazepin-4(5H)-ones from bromides of the Baylis--Hillman adducts using Fe/AcOH for the in situ reduction of nitro group, into an amino group, followed by the cyclization as a key step, has been described. 相似文献
996.
A one‐pot and convenient synthesis of multisubstituted pyrazolo[3,4‐b]pyridines in moderate to high yields has been achieved by a two‐step reaction: diazo‐transfer of 5‐azido‐1‐phenylpyrazole‐4‐carboxaldehydes to ketones in ethanolic KOH to give 5‐amino‐1‐phenylpyrazole‐4‐carboxaldehyde and subsequent Friedlaender reaction of 5‐amino‐1‐phenylpyrazole‐4‐carboxaldehyde with ketones. 相似文献
997.
The first efficient transformation of 4-bromomethylcoumarins and 4-bromomethylcarbostyrils to 4-formylcoumarins and 4-formylcarbostyril under aqueous conditions has been achieved by modifying the Kornblum method, resulting in excellent yield. The experimental method is very simple and economical; no further purification is required and the experimental conditions have been optimized. All the isolated compounds were characterized by infrared, NMR, and mass spectroscopy, and some of the compounds have been analyzed by single-crystal x-ray analysis. 相似文献
998.
《合成通讯》2013,43(4):679-686
Abstract 2-(Cyclohex-2′-enyl)-5,5-dimethyl-3-hydroxycyclohex-2-enone undergoes regioselective heterocyclization to give bridged heterocycles 5a,b and 7 on treatment with pyridine hydrotribromide or hexamethylenetetramine hydrotribromide (1 h); or elemental bromine (7 h); N-iodosuccinimide in acetonitrile at 0–5°C (1 h) and conc. H2SO4 at 0–5°C (2 h), respectively. 相似文献
999.
We have obtained dihydrofurans 3a–j in the radical cyclization of 4‐hydroxycoumarin 1a and 2‐hydroxy‐1,4‐naphtoquinone 1b with electron rich alkenes 2a–i by manganese(III) acetate. Methods A and B, which have different molar ratios were studied comparatively in these reactions, and we observed that method B (molar ratio 2:1:3) gave the best results. Treatment of 4‐hydroxycoumarin 1a and electron rich alkenes 2a–e gave 2,3‐dihydro‐4H‐furo[3,2‐c]chromen‐4‐ones 3a–e in 36–86% yields by the method B. Under the same conditions, the reactions of 2‐hydroxy‐1,4‐naphtaquinone 1b with conjugated alkenes 2b and 2f–i afforded 2,3‐dihydronaphtho[2,3‐b]furan‐4,9‐diones 3f–j in an excellent yields. 相似文献
1000.
Asymmetric total synthesis of the sex pheromones of Japanese beetle and currant stem girdler, (R)‐japonilure (1) and (4R,9Z)‐9‐octadecen‐4‐olide (2), has been achieved. 相似文献