全文获取类型
收费全文 | 229829篇 |
免费 | 3478篇 |
国内免费 | 1359篇 |
专业分类
化学 | 129559篇 |
晶体学 | 3257篇 |
力学 | 9029篇 |
综合类 | 61篇 |
数学 | 25838篇 |
物理学 | 66922篇 |
出版年
2020年 | 1970篇 |
2019年 | 2077篇 |
2018年 | 2605篇 |
2017年 | 2645篇 |
2016年 | 4050篇 |
2015年 | 2771篇 |
2014年 | 3925篇 |
2013年 | 9324篇 |
2012年 | 8310篇 |
2011年 | 10072篇 |
2010年 | 7123篇 |
2009年 | 6844篇 |
2008年 | 9401篇 |
2007年 | 9442篇 |
2006年 | 8890篇 |
2005年 | 8216篇 |
2004年 | 7303篇 |
2003年 | 6377篇 |
2002年 | 6281篇 |
2001年 | 6854篇 |
2000年 | 5193篇 |
1999年 | 3816篇 |
1998年 | 3149篇 |
1997年 | 3159篇 |
1996年 | 3115篇 |
1995年 | 2722篇 |
1994年 | 2821篇 |
1993年 | 2671篇 |
1992年 | 2916篇 |
1991年 | 2952篇 |
1990年 | 2744篇 |
1989年 | 2682篇 |
1988年 | 2610篇 |
1987年 | 2528篇 |
1986年 | 2551篇 |
1985年 | 3345篇 |
1984年 | 3395篇 |
1983年 | 2825篇 |
1982年 | 3090篇 |
1981年 | 2840篇 |
1980年 | 2630篇 |
1979年 | 2811篇 |
1978年 | 3015篇 |
1977年 | 3050篇 |
1976年 | 3071篇 |
1975年 | 2815篇 |
1974年 | 2918篇 |
1973年 | 2972篇 |
1972年 | 2324篇 |
1971年 | 1854篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
Experimental and Theoretical Studies on the Rearrangement of 2‐Oxoazepane α,α‐Amino Acids into 2′‐Oxopiperidine β2,3,3‐Amino Acids: An Example of Intramolecular Catalysis
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Diego Núñez‐Villanueva Dr. M. Ángeles Bonache Laura Lozano Dr. Lourdes Infantes Prof. José Elguero Prof. Ibon Alkorta Prof. M. Teresa García‐López Dr. Rosario González‐Muñiz Dr. Mercedes Martín‐Martínez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(6):2489-2500
Enantiopure β‐amino acids represent interesting scaffolds for peptidomimetics, foldamers and bioactive compounds. However, the synthesis of highly substituted analogues is still a major challenge. Herein, we describe the spontaneous rearrangement of 4‐carboxy‐2‐oxoazepane α,α‐amino acids to lead to 2′‐oxopiperidine‐containing β2,3,3‐amino acids, upon basic or acid hydrolysis of the 2‐oxoazepane α,α‐amino acid ester. Under acidic conditions, a totally stereoselective synthetic route has been developed. The reordering process involved the spontaneous breakdown of an amide bond, which typically requires strong conditions, and the formation of a new bond leading to the six‐membered heterocycle. A quantum mechanical study was carried out to obtain insight into the remarkable ease of this rearrangement, which occurs at room temperature, either in solution or upon storage of the 4‐carboxylic acid substituted 2‐oxoazepane derivatives. This theoretical study suggests that the rearrangement process occurs through a concerted mechanism, in which the energy of the transition states can be lowered by the participation of a catalytic water molecule. Interestingly, it also suggested a role for the carboxylic acid at position 4 of the 2‐oxoazepane ring, which facilitates this rearrangement, participating directly in the intramolecular catalysis. 相似文献
33.
Su X. F. Zhang Y. L. Maximov A. L. Zhang K. Xin Q. Feng C. Q. Bai X. F. Wu W. 《Russian Journal of Applied Chemistry》2020,93(1):127-136
Russian Journal of Applied Chemistry - Nanoscale ZSM-5 (AlZ5-D) and Ga-substituted ZSM-5 zeolites ((Al,Ga)Z5-D and GaZ5-D) were synthesized by a green dry-gel conversion strategy. For comparison,... 相似文献
34.
We consider conformal gravity as a gauge natural theory. We study its conservation laws and superpotentials. We also consider the Mannheim and Kazanas spherically symmetric vacuum solution and discuss conserved quantities associated to conformal and diffeomorphism symmetries. 相似文献
35.
Journal of Thermal Analysis and Calorimetry - In the present work, pyrolysis kinetic mechanism was studied for regenerated cellulosic fiber (RCF) and composite RCF containing silicon/nitrogen flame... 相似文献
36.
Henry Dring David Kreutzer Christoph Ritter Andreas Hilgeroth 《Molecules (Basel, Switzerland)》2021,26(1)
Despite the development of targeted therapies in cancer, the problem of multidrug resistance (MDR) is still unsolved. Most patients with metastatic cancer die from MDR. Transmembrane efflux pumps as the main cause of MDR have been addressed by developed inhibitors, but early inhibitors of the most prominent and longest known efflux pump P-glycoprotein (P-gp) were disappointing. Those inhibitors have been used without knowledge about the expression of P-gp by the treated tumor. Therefore the use of inhibitors of transmembrane efflux pumps in clinical settings is reconsidered as a promising strategy in the case of the respective efflux pump expression. We discovered novel symmetric inhibitors of the symmetric efflux pump MRP4 encoded by the ABCC4 gene. MRP4 is involved in many kinds of cancer with resistance to anticancer drugs. All compounds showed better activities than the best known MRP4 inhibitor MK571 in an MRP4-overexpressing cell line assay, and the activities could be related to the various substitution patterns of aromatic residues within the symmetric molecular framework. One of the best compounds was demonstrated to overcome the MRP4-mediated resistance in the cell line model to restore the anticancer drug sensitivity as a proof of concept. 相似文献
37.
38.
39.
40.