全文获取类型
收费全文 | 16869篇 |
免费 | 946篇 |
国内免费 | 1745篇 |
专业分类
化学 | 8313篇 |
晶体学 | 70篇 |
力学 | 39篇 |
综合类 | 165篇 |
数学 | 5335篇 |
物理学 | 648篇 |
综合类 | 4990篇 |
出版年
2024年 | 42篇 |
2023年 | 256篇 |
2022年 | 341篇 |
2021年 | 391篇 |
2020年 | 546篇 |
2019年 | 516篇 |
2018年 | 466篇 |
2017年 | 523篇 |
2016年 | 396篇 |
2015年 | 396篇 |
2014年 | 669篇 |
2013年 | 1583篇 |
2012年 | 900篇 |
2011年 | 872篇 |
2010年 | 796篇 |
2009年 | 984篇 |
2008年 | 1001篇 |
2007年 | 1010篇 |
2006年 | 1045篇 |
2005年 | 929篇 |
2004年 | 820篇 |
2003年 | 673篇 |
2002年 | 500篇 |
2001年 | 474篇 |
2000年 | 478篇 |
1999年 | 459篇 |
1998年 | 397篇 |
1997年 | 302篇 |
1996年 | 256篇 |
1995年 | 234篇 |
1994年 | 244篇 |
1993年 | 185篇 |
1992年 | 191篇 |
1991年 | 145篇 |
1990年 | 95篇 |
1989年 | 111篇 |
1988年 | 91篇 |
1987年 | 64篇 |
1986年 | 35篇 |
1985年 | 32篇 |
1984年 | 30篇 |
1983年 | 7篇 |
1982年 | 26篇 |
1981年 | 16篇 |
1980年 | 7篇 |
1979年 | 11篇 |
1977年 | 2篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1973年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
831.
A series of novel imidazo[2,1‐b][1,3,4]thiadiazole fused pyridazinones have been synthesized in moderate yields by the reaction of 2‐alkyl/arylimidazo[2,1‐b] [1,3,4]thiadiazole‐6‐carbohydrazides under Vilsmeier–Haack reaction conditions. This simple methodology has utility for the synthesis of various fused heterocyclic systems. 相似文献
832.
The present review highlights the synthetic procedures reported for the preparation of oxazolo[3,2-a]pyridines as a class of 5-6 bicyclic heterocycles with a nitrogen atom at the ring junction. The different sections included the synthesis of the investigated analogs through the reactions of (1) β-ketoesters with α,β-unsaturated ketones (2) δ-oxoacids or esters or unsaturated carboxylic acids with glycinol derivatives (3) unsaturated ketoesters with enamines (4) ethoxymethylenes with amidoglycinol derivatives (5) pyridinium salts with phenylglycinol (6) Multicomponent reactions, and (7) Synthesis of tetracyclic systems. The aim of the present study is to demonstrate a synopsis of the synthesis of compounds containing oxazolo[3,2-a]pyridine skeleton with high yields using readily and accessible starting materials, and efficient synthetic routes until now. The stereochemistry of the obtained enantiopure heterocycles, the isolation of the isomers and the reaction mechanisms of the unexpected products are discussed. 相似文献
833.
Herein, synthesis of a series of naphtho[2,3-f]quinolin-13-one and naphtho[2,3-a]acridin-1(2H)-one derivatives directly by one-pot multi-component reaction of 1,3-dicarbonyl compounds (1,3-indanedione/1,3-cyclohexanedione), 2-aminoantharacene/2-naphthylamine and various substituted aldehydes under solvent-free conditions using heteropoly-11-molybdo-1-vanadophosphoric acid supported on montmorillonite K-10 clay catalyst (10% PVMoK-10) is reported. The successful formation of naphtho[2,3-f]quinolin-13-one and naphtho[2,3-a]acridin-1(2H)-one derivatives was confirmed by various spectroscopic techniques. This study offers a green approach for the synthesis of novel quinolinone derivatives. 相似文献
834.
A versatile class of 2-aminopyrroles containing various electron-withdrawing substituents at the 3-position have been N-arylated on the amine using a palladium-catalyzed cross-coupling reaction. Using this methodology, a pyrimidone-based tricyclic system has been prepared in just one step from a starting 2-aminopyrrole.
835.
836.
Benzo[b][1,4]oxazines have been synthesized in good to excellent yields in the presence of the ionic liquid 1-butyl-3-methylimidazolium bromide [bmim]Br under relatively mild conditions without any added catalyst, The method offers the advantages of good yields and short reaction times, and the ionic liquid can be easily separated from the product and reused. 相似文献
837.
A series of furo[2′,1′:5,6]pyrido[2,3-d]pyrimidine derivatives were synthesized via the three-component reaction of an aldehyde, tetronic acid, and 6-amino-1,3-dimethyl-pyrimidine-2,4-dione in aqueous media without the use of catalyst. This protocol has the advantages of better yields, less cost, reduced environmental impact, and convenient procedure. 相似文献
838.
Nadia Hanafy Metwally 《合成通讯》2013,43(3):398-405
2-Keto-3-mercaptocinchoninic acid derivatives 1a and b react with Schiff's bases 2a–d in toluene at refluxing temperature to give thiazino[6,5-c]quinoline derivatives 4a–h. Also, refluxing of 1a and b with arylazomalononitriles 5a–d in acetic acid afforded the thiazolo[6,5-c]quinoline derivatives 7a–d. The structure of all the newly synthesized products was confirmed based on elemental and spectral data. 相似文献
839.
Abstract A novel 1,1,10,10,19,19-hexamethyl-5,14,23-trimethoxy[3.3.3]metacyclophane (2) was prepared in 25% yield by Friedel–Crafts cyclization of 4-(2-methoxyphenyl)-2-methylbutan-2-ol (1) at ? 78 °C using TiCl4 as Lewis acid catalyst in anhydrous dichloromethane. The structure of cyclophane 2 was determined by single-crystal X-ray diffraction, and the impact of substrate concentration on the yield of macrocycle 2 was also examined. The study on the effect of substituents at the phenyl ring showed that the methoxy group in 1 is crucial for its trimerization to give the hexamethyl[3.3.3]metacyclophane derivative. Demethylation of 2 with BBr3 gave 1,1,10,10,19,19-hexamethyl-5,14,23-trihydroxy[3.3.3]metacyclophane (4) in 96% yield, and its three hydroxyl groups provide the possible modification sites for further construction of supramolecular assemblies. 相似文献
840.
Bismuth chloride was used to catalyze the [2 + 3] cycloaddition between sodium azide with aryl nitriles, aliphatic nitriles, and vinyl nitriles. A number of 5-substituted 1H-tetrazoles were synthesized in water or isopropanol/water mixtures using microwave heating. Good yields were obtained for these reactions when heated for 1 h at 120–160 °C in a 3:1 isopropanol/water mixture. A few of the less reactive nitriles required longer reaction times for good yields. 相似文献