全文获取类型
收费全文 | 3505篇 |
免费 | 649篇 |
国内免费 | 290篇 |
专业分类
化学 | 2666篇 |
晶体学 | 4篇 |
力学 | 68篇 |
综合类 | 50篇 |
数学 | 951篇 |
物理学 | 705篇 |
出版年
2024年 | 7篇 |
2023年 | 40篇 |
2022年 | 106篇 |
2021年 | 105篇 |
2020年 | 166篇 |
2019年 | 107篇 |
2018年 | 114篇 |
2017年 | 115篇 |
2016年 | 202篇 |
2015年 | 196篇 |
2014年 | 253篇 |
2013年 | 331篇 |
2012年 | 244篇 |
2011年 | 243篇 |
2010年 | 183篇 |
2009年 | 224篇 |
2008年 | 214篇 |
2007年 | 265篇 |
2006年 | 192篇 |
2005年 | 195篇 |
2004年 | 171篇 |
2003年 | 153篇 |
2002年 | 93篇 |
2001年 | 90篇 |
2000年 | 79篇 |
1999年 | 58篇 |
1998年 | 51篇 |
1997年 | 63篇 |
1996年 | 39篇 |
1995年 | 30篇 |
1994年 | 17篇 |
1993年 | 11篇 |
1992年 | 6篇 |
1991年 | 15篇 |
1990年 | 5篇 |
1989年 | 5篇 |
1988年 | 10篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1982年 | 5篇 |
1981年 | 1篇 |
1980年 | 6篇 |
1979年 | 7篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1973年 | 3篇 |
排序方式: 共有4444条查询结果,搜索用时 11 毫秒
71.
Gram‐Scale Enantioselective Formal Synthesis of Morphine through an ortho–para Oxidative Phenolic Coupling Strategy 下载免费PDF全文
Dr. Matthieu Tissot Dr. Robert J. Phipps Dr. Catherine Lucas Dr. Rafael M. Leon Dr. Robert D. M. Pace Dr. Tifelle Ngouansavanh Prof. Matthew J. Gaunt 《Angewandte Chemie (International ed. in English)》2014,53(49):13498-13501
A gram‐scale catalytic enantioselective formal synthesis of morphine is described. The key steps of the synthesis involve an ortho–para oxidative phenolic coupling and a highly diastereoselective “desymmetrization” of the resulting cyclohexadienone that generates three of the four morphinan ring junction stereocenters in one step. The stereochemistry is controlled from a single carbinol center installed through catalytic enantioselective hydrogenation. These transformations enabled the preparation of large quantities of key intermediates and could support a practical and scalable synthesis of morphine and related derivatives. 相似文献
72.
Organocatalytic and Scalable Synthesis of the Anti‐Influenza Drugs Zanamivir,Laninamivir, and CS‐8958 下载免费PDF全文
Yunsheng Li Prof. Dr. Dawei Ma 《Angewandte Chemie (International ed. in English)》2014,53(50):13885-13888
Zanamivir, laninamivir, and CS‐8958 are three neuraminidase inhibitors that have been clinically used to combat influenza. We report herein a novel organocatalytic route for preparing these agents. Only 13 steps are needed for the assembly of zanamivir and laninamivir from inexpensive D ‐araboascorbic acid by this synthetic route, which relies heavily on a thiourea‐catalyzed enantioselective Michael addition of acetone to tert‐butyl (2‐nitrovinyl)carbamate and an anti‐selective Henry reaction of the resulting Michael adduct with an aldehyde prepared from D ‐araboascorbic acid. The synthetic procedures are scalable, as evident from the preparation of more than 3.5 g of zanamivir. 相似文献
73.
Frontispiece: NMR Fingerprints of the Drug‐like Natural‐Product Space Identify Iotrochotazine A: A Chemical Probe to Study Parkinson’s Disease 下载免费PDF全文
74.
应用衰减全反射傅里叶变换红外光谱法测定橙子油中柠檬烯的含量。柠檬烯的特征峰(1 645.1±1)cm-1为定量测量峰,测定样品在15min内完成。柠檬烯的体积分数在9.60%~76.8%范围内与测量峰峰高呈线性关系。以橙子油样品为基体进行加标回收试验,所得回收率在99.7%~101%之间,测定值的相对标准偏差(n=5)为1.1%。 相似文献
75.
Dr. Szilárd Varga Péter Angyal Gábor Martin Dr. Orsolya Egyed Dr. Tamás Holczbauer Dr. Tibor Soós 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(32):13649-13653
We report 8-step syntheses of (−)-minovincine and (−)-aspidofractinine using easily available and inexpensive reagents and catalyst. A key element of the strategy was the utilization of a sequence of cascade reactions to rapidly construct the penta- and hexacyclic frameworks. These cascade transformations included organocatalytic Michael-aldol condensation, a multistep anionic Michael-SN2 cascade reaction, and Mannich reaction interrupted Fischer indolization. To streamline the synthetic routes, we also investigated the deliberate use of steric effect to secure various chemo- and regioselective transformations. 相似文献
76.
Bastien Delayre Dr. Cyril Piemontesi Dr. Qian Wang Prof. Dr. Jieping Zhu 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(33):14094-14101
2,3,3-Trisubstituted indolenine constitutes an integral part of many biologically important monoterpene indole alkaloids. We report herein an unprecedented access to this skeleton by a TiCl3-mediated reductive cyclization of tetrasubstituted alkenes bearing a 2-nitrophenyl substituent. The proof of concept is demonstrated firstly by accomplishing a concise total synthesis of (+)-1,2-dehydroaspidospermidine featuring a late-stage application of this key transformation. A sequence of reduction of nitroarene to nitrosoarene followed by 6π-electron-5-atom electrocyclization and a 1,2-alkyl shift of the resulting nitrone intermediate was proposed to account for the reaction outcome. A subsequent total synthesis of (+)-condyfoline not only illustrates the generality of the reaction, but also provides a mechanistic insight into the nature of the 1,2-alkyl shift. The exclusive formation of (+)-condyfoline indicates that the 1,2-alkyl migration follows a concerted Wagner–Meerwein pathway, rather than a stepwise retro-Mannich/Mannich reaction sequence. Conditions for almost quantitative conversion of (+)-condyfoline to (−)-tubifoline by way of a retro-Mannich/1,3-prototropy/transannular cyclization cascade are also documented. 相似文献
77.
Dr. Ze-Jun Xu Yan Zong Ya-Nan Qiao Jiao-Zhen Zhang Xuyuan Liu Ming-Zhu Zhu Yuliang Xu Hongbo Zheng Liyuan Fang Dr. Xiao-ning Wang Prof. Hong-Xiang Lou 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(45):20091-20095
A divergent synthetic approach to biogenetically related diterpenoids such as ent-kauranes, ent-trachylobanes, ent-beyerane, and ent-atisane has been developed. The unified synthetic route involves the De Mayo reaction to rapidly generate the bicyclo[3.2.1]-octane moiety of ent-kaurane. The key reactions also include bioinspired nucleophilic cyclopropanation generating the [3.2.1.02,7]-tricyclic core of ent-trachylobane and regioselective cyclopropane fragmentation furnishing ent-beyerane and ent-atisane through the nucleophilic attack and protonation of the cyclopropane ring. This strategy enables the asymmetric total syntheses of six diterpenoids from the commercially available geraniol. 相似文献
78.
Hui Shao Xiaonan Gao Zhong‐Tian Wang Ziwei Gao Yu‐Ming Zhao 《Angewandte Chemie (International ed. in English)》2020,59(19):7419-7424
A divergent synthetic approach to six Ganoderma meroterpenoids, namely ganocins A–C, ganocochlearins C and D, and cochlearol T, has been developed for the first time. This synthetic route features a two‐phase strategy which includes early‐stage rapid construction of a common planar tricyclic intermediate followed by highly selective late‐stage transformations into various Ganoderma meroterpenoids. Key to the strategy are a bioinspired intramolecular hetero‐Diels–Alder reaction and Stahl‐type oxidative aromatization, allowing efficient formation of the common tricyclic phenol intermediate. A nucleophilic dearomatization of the phenol unit, combined with a regioselective 1,4‐reduction of the resulting dienone, enabled rapid access to ganocins B and C. Additionally, site‐selective Mukaiyama hydration, followed by an intramolecular oxa‐Michael addition/triflation cascade, served as a key strategic element in the chemical synthesis of ganocin A. 相似文献
79.
Ascorbic acid (AA) is one of the essential nutrients in bee pollen, however, it is unstable and likely to be oxidized. Generally, the oxidation form (dehydroascorbic acid (DHA)) is considered to have equivalent biological activity as the reduction form. Thus, determination of the total content of AA and DHA would be more accurate for the nutritional analysis of bee pollen. Here we present a simple, sensitive, and reliable method for the determination of AA, total ascorbic acids (TAA), and DHA in rape (Brassica campestris), lotus (Nelumbo nucifera), and camellia (Camellia japonica) bee pollen, which is based on ultrasonic extraction in metaphosphoric acid solution, and analysis using hydrophilic interaction liquid chromatography (HILIC)-ultraviolet detection. Analytical performance of the method was evaluated and validated, then the proposed method was successfully applied in twenty-one bee pollen samples. Results indicated that contents of AA were in the range of 17.54 to 94.01 µg/g, 66.01 to 111.66 µg/g, and 90.04 to 313.02 µg/g for rape, lotus, and camellia bee pollen, respectively. In addition, percentages of DHA in TAA showed good intra-species consistency, with values of 13.7%, 16.5%, and 7.6% in rape, lotus, and camellia bee pollen, respectively. This is the first report on the discriminative determination between AA and DHA in bee pollen matrices. The proposed method would be valuable for the nutritional analysis of bee pollen. 相似文献
80.
Juyeol Lee Jihun Kang Sukhyun Lee Young Ho Rhee 《Angewandte Chemie (International ed. in English)》2020,59(6):2349-2353
A de novo first collective total synthesis of 11‐deoxylandomycins is reported. A signature step is featured by the Pd‐catalyzed asymmetric addition of alcohol to ene‐alkoxyallenes that assembles oligomeric 2,3,6‐trideoxyoligosaccharides. The unique feature of the protocol is illustrated by a flexible access to various natural 11‐deoxylandomycins as well as non‐natural analogues. 相似文献