全文获取类型
收费全文 | 4242篇 |
免费 | 626篇 |
国内免费 | 165篇 |
专业分类
化学 | 4115篇 |
晶体学 | 3篇 |
力学 | 23篇 |
综合类 | 18篇 |
数学 | 604篇 |
物理学 | 270篇 |
出版年
2023年 | 53篇 |
2022年 | 104篇 |
2021年 | 171篇 |
2020年 | 212篇 |
2019年 | 158篇 |
2018年 | 172篇 |
2017年 | 138篇 |
2016年 | 217篇 |
2015年 | 246篇 |
2014年 | 283篇 |
2013年 | 399篇 |
2012年 | 254篇 |
2011年 | 308篇 |
2010年 | 237篇 |
2009年 | 285篇 |
2008年 | 258篇 |
2007年 | 267篇 |
2006年 | 226篇 |
2005年 | 206篇 |
2004年 | 164篇 |
2003年 | 155篇 |
2002年 | 71篇 |
2001年 | 36篇 |
2000年 | 52篇 |
1999年 | 39篇 |
1998年 | 33篇 |
1997年 | 44篇 |
1996年 | 38篇 |
1995年 | 29篇 |
1994年 | 29篇 |
1993年 | 23篇 |
1992年 | 10篇 |
1991年 | 13篇 |
1990年 | 8篇 |
1989年 | 7篇 |
1988年 | 9篇 |
1987年 | 11篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1976年 | 2篇 |
1973年 | 2篇 |
1969年 | 2篇 |
1968年 | 2篇 |
1959年 | 1篇 |
排序方式: 共有5033条查询结果,搜索用时 15 毫秒
1.
2.
Kento Nishikibe Keisuke Nishikawa Momochika Kumagai Matsumi Doe Yoshiki Morimoto 《化学:亚洲杂志》2022,17(1):e202101137
There are marine cytotoxic bromotriterpenoids, named the thyrsiferol family that are structurally characterized by some tetrahydropyran (THP) and tetrahydrofuran (THF) rings. The thyrsiferol family belongs to natural products that are often difficult to determine their stereostructures even by the current, highly advanced spectroscopic methods, especially in acyclic systems including stereogenic tetrasubstituted carbon centers. In such cases, it is effective to predict and synthesize the possible stereostructures. Herein, to elucidate ambiguous stereostructures and unassigned absolute configurations of aplysiol B, laurenmariannol, and saiyacenol A, members of the thyrsiferol family, we carried out their asymmetric chemical syntheses featuring 6-exo and 5-exo oxacyclizations of epoxy alcohol precursors and 6-endo bromoetherification of a bishomoallylic alcohol. In this paper, we report total assignments of their stereostructures through their asymmetric chemical syntheses and also their preliminary cytotoxic activities against some tumor cells. These results could not have been achieved without depending on asymmetric total synthesis. 相似文献
3.
Bernhard Wölfl Dr. Guillaume Mata Prof. Alois Fürstner 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(1):255-259
The macrocyclic core of the cytotoxic marine natural product callyspongiolide ( 1 ) was forged by ring-closing alkyne metathesis (RCAM) of an ynoate precursor using a molybdenum alkylidyne complex endowed with triarylsilanolate ligands as the catalyst. This result is remarkable in view of the failed attempts documented in the literature at converting electron deficient alkynes with the aid of more classical catalysts. The subsequent Z-selective semi-reduction of the resulting cycloalkyne by hydrogenation over nickel boride required careful optimization in order to minimize overreduction and competing dehalogenation of the compound's alkenyl iodide terminus as needed for final attachment of the side chain of 1 by Sonogashira coupling. The required cyclization precursor itself was prepared via Kocienski olefination. 相似文献
4.
Formulated products represent a particular class of complex chemical products, and their design is typically based on experience and extensive experimentation. Although still at an early stage, and despite that their potential is not fully accessed and not fully used by the industry, computer-aided design (CAD) methods and tools offer many possibilities in the design of formulated products. The CAD methodology based on computerized models enables the formulation chemists to speed up the design process, without completely replacing experiments.In this work, we summarize previous studies in the field and present important elements of the CAD framework, emphasizing estimation methods for key target properties, link to specifications, and finally, some case studies will illustrate how the CAD framework can be used in practice for formulated products. 相似文献
5.
Wei Wang Dr. Parthasarathi Subramanian Oscar Martinazzoli Jun Wu Prof. Dr. Lutz Ackermann 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(45):10585-10589
Late-stage C−H glycosylations of structurally complex amino acids and peptides were accomplished by means of racemization-free manganese(I)-catalyzed C−H activation. Thus, glycosylative modifications proved to be viable by a linch-pin approach, featuring chemo- and site-selective C−H transformations. The peptide–saccharide conjugation provided modular access to structurally complex glycopeptides, likewise enabling the assembly of fluorescent-labelled glycopeptides. 相似文献
6.
Martín Soto Dr. Raquel G. Soengas Prof. Artur M. S. Silva Prof. Vicente Gotor-Fernández Prof. Humberto Rodríguez-Solla 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(57):13104-13108
In this work, the first example of a radical stereodivergent reaction directed towards the stereoselective synthesis of both (R*,R*)- and (R*,S*)-2,2′-biflavanones promoted by samarium diiodide is reported. Control experiments showed that the selectivity of this reaction was exclusively controlled by the temperature. It was possible to generate a variety of 2,2′-biflavanones bearing different substitution patterns at the aromatic ring in good-to-quantitative yields, being both stereoisomers of the desired compounds obtained with total or high control of selectivity. A mechanism that explains both the generation of the corresponding 2,2′-biflavanones and the selectivity is also discussed. The structure and stereochemistry determination of each isomer was unequivocally elucidated by single-crystal X-ray diffraction experiments. 相似文献
7.
Dr. Tatsuya Nishimaru Dr. Kohei Eto Dr. Keita Komine Prof. Dr. Jun Ishihara Prof. Dr. Susumi Hatakeyama 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(33):7927-7934
The first total synthesis of lajollamycin B, a structurally novel nitro-tetraene spiro-β-lactone/γ-lactone antibiotic, is described. The convergent synthesis involves the construction of the C8′–C11′ nitrodienylstannane and its coupling with the segment prepared from the C1′–C7′ ω-iodoheptadienoic acid and the right-hand heterocyclic fragment, which has been utilized for our previous syntheses of oxazolomycin A. The revision of the geometry of the terminal Δ10′, 11′-double bond from E to Z is also described for the structure of natural lajollamycin B. 相似文献
8.
Total Synthesis,Stereochemical Assignment,and Field‐Testing of the Sex Pheromone of the Strepsipteran Xenos peckii 下载免费PDF全文
Dr. Huimin Zhai Michael Hrabar Regine Gries Prof. Gerhard Gries Prof. Robert Britton 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(18):6190-6193
The sex pheromone of the endoparasitoid insect Xenos peckii (Strepsiptera: Xenidae) was recently identified as (7E,11E)‐3,5,9,11‐tetramethyl‐7,11‐tridecadienal. Herein we report the asymmetric synthesis of three candidate stereostructures for this pheromone using a synthetic strategy that relies on an sp3–sp2 Suzuki–Miyaura coupling to construct the correctly configured C7‐alkene function. Comparison of 1H NMR spectra derived from the candidate stereostructures to that of the natural sex pheromone indicated a relative configuration of (3R*,5S*,9R*). Chiral gas chromatographic (GC) analyses of these compounds supported an assignment of (3R,5S,9R) for the natural product. Furthermore, in a 16‐replicate field experiment, traps baited with the synthetic (3R,5S,9R)‐enantiomer alone or in combination with the (3S,5R,9S)‐enantiomer captured 23 and 18 X. peckii males, respectively (mean±SE: 1.4±0.33 and 1.1±0.39), whereas traps baited with the synthetic (3S,5R,9S)‐enantiomer or a solvent control yielded no captures of males. These strong field trapping data, in combination with spectroscopic and chiral GC data, unambiguously demonstrate that (3R,5S,9R,7E,11E)‐3,5,9,11‐tetramethyl‐7,11‐tridecadienal is the X. peckii sex pheromone. 相似文献
9.
Total Synthesis and Structure Elucidation of JBIR‐39: A Linear Hexapeptide Possessing Piperazic Acid and γ‐Hydroxypiperazic Acid Residues 下载免费PDF全文
Dr. Masahito Yoshida Naoki Sekioka Dr. Miho Izumikawa Dr. Ikuko Kozone Dr. Motoki Takagi Dr. Kazuo Shin‐ya Prof. Dr. Takayuki Doi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(7):3031-3041
The total synthesis and stereochemical structural elucidation of JBIR‐39, containing four nonproteinogenic piperazic acid (Piz) residues, is reported. The synthesis includes Sc(OTf)3‐catalyzed acylation of a Piz(γ‐OTBS) derivative with piperazic acid chloride, providing the desired Piz‐Piz(γ‐OTBS) dipeptide in high yield without epimerization. After assembling two additional Piz moieties and (S)‐isoleucic acid at the N‐terminus, amidation with the (R)‐α‐methylserine ester at the C‐terminus, and deprotection afforded the desired (2R,8S)‐hexapeptide, which is the assumed structure of JBIR‐39. Although the spectral data of the (2R,8S)‐hexapeptide was not identical to JBIR‐39, further synthesis of three stereoisomers confirmed the stereochemical structure of JBIR‐39 to be (2S,6S,8S,11R,16S,21R,26S,27S). 相似文献
10.
Engyodontochones,Antibiotic Polyketides from the Marine Fungus Engyodontium album Strain LF069 下载免费PDF全文
Prof. Dr. Bin Wu Dr. Jutta Wiese Arlette Wenzel‐Storjohann Susan Malien Dr. Rolf Schmaljohann Prof. Dr. Johannes F. Imhoff 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(22):7452-7462
Six new ( 2 , 4 – 8 ) and two known polyketides with a basic structure of an anthraquinone‐xanthone were isolated from mycelia and culture broth of the fungus Engyodontium album strain LF069. The structures and relative configurations of these compounds were established by spectroscopic means, and their absolute configurations were defined mainly by comparison of quantum chemical TDDFT calculated and experimental ECD spectra. Compounds 2 and 4 – 8 were given the trivial names engyodontochone A ( 2 ) and B–F ( 4 – 8 ). Compounds 5 – 8 represent the first example of a 23,28 seco‐beticolin carbon skeleton. The relative and absolute configurations of two known substances JBIR‐97/98 ( 1 ) and JBIR‐99 ( 3 ) were determined for the first time. All isolated compounds were subjected to bioactivity assays. Compounds 1 – 4 exhibited inhibitory activity against methicillin‐resistant Staphylococcus aureus (MRSA) that was 10‐fold stronger than chloramphenicol. 相似文献