全文获取类型
收费全文 | 197篇 |
免费 | 8篇 |
专业分类
化学 | 151篇 |
数学 | 31篇 |
物理学 | 23篇 |
出版年
2021年 | 4篇 |
2019年 | 3篇 |
2016年 | 5篇 |
2015年 | 2篇 |
2014年 | 7篇 |
2013年 | 7篇 |
2012年 | 9篇 |
2011年 | 15篇 |
2010年 | 3篇 |
2009年 | 10篇 |
2008年 | 12篇 |
2007年 | 7篇 |
2006年 | 8篇 |
2005年 | 7篇 |
2004年 | 7篇 |
2003年 | 5篇 |
2002年 | 4篇 |
2001年 | 2篇 |
2000年 | 4篇 |
1999年 | 2篇 |
1997年 | 3篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 8篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1970年 | 3篇 |
1969年 | 1篇 |
1968年 | 3篇 |
1932年 | 1篇 |
1931年 | 1篇 |
1930年 | 1篇 |
1919年 | 2篇 |
1902年 | 1篇 |
排序方式: 共有205条查询结果,搜索用时 15 毫秒
51.
Steinmetz H Irschik H Kunze B Reichenbach H Höfle G Jansen R 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(20):5822-5832
Two novel antibiotics, thuggacin A (1) and B (2), were isolated from the myxobacterium Sorangium cellulosum. 1 and 2 are unique thiazole-containing macrolides with side chains on both sides of the lactone group. Upon standing in solution, thuggacin A (1) rearranges by acyl migration of the lactone group to give a mixture with thuggacins B (2) and C (3). NOEs and vicinal coupling constants within the lactone ring provided distinct data for the generation of a structure model by PM3 calculations, which allowed an analysis of the conformation in solution and the relative configuration of six asymmetric centres. A minor sorangium metabolite was identified as 13-methyl-thuggacin A (4). Furthermore, two natural thuggacin variants, 5 and 6, were found in another myxobacterium, Chondromyces crocatus. In these variants, one side chain is replaced by a methyl group and a hydroxy group is repositioned to give a primary alcohol at the former methyl site, in an alpha position with respect to the thiazole ring. 1 proved to be active against clinical isolates and reference strains of Mycobacterium tuberculosis. Preliminary studies on the mechanism of action indicate inhibition of the cellular electron-transport chain. 相似文献
52.
53.
Ralf Goretzki Gerhard Fink Bernd Tesche Bernd Steinmetz Rainer Rieger Wolfram Uzick 《Journal of polymer science. Part A, Polymer chemistry》1999,37(5):677-682
With the development of methods to support metallocenes and methylaluminoxane cocatalysts on suitable carriers, it became possible to combine the specific advantages of homogeneous metallocene catalysis with those of heterogeneous Ziegler catalysts in olefin polymerization. By means of ethylene polymerization it could be shown that the method of supporting methylaluminoxane and metallocene on porous silica has a substantial influence on the progress of polymerization. In particular, fragmentation of catalyst particles during polymerization can be circumvented, maintaining the catalyst activity, if active catalyst sites are being generated on the particle surface only. A method of preparation for such newly designed supported metallocene catalysts is presented, where the active catalyst sites are located exclusively on the particle surface. Furthermore, the kinetics of ethylene polymerization and morphology properties prior to and after polymerization are discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 677–682, 1999 相似文献
54.
Steinmetz NF Lomonossoff GP Evans DJ 《Langmuir : the ACS journal of surfaces and colloids》2006,22(8):3488-3490
For the first time, decoration of surface-exposed carboxylate groups on Cowpea mosaic virus particles is reported, thus increasing the number and types of addressable surface groups on this nanoscaffold. First, the addressabilty of carboxylates was demonstrated using a carboxylate-selective fluorescent dye, N-cyclohexyl-N'-(4-(dimethylamino)naphthyl)carbodiimide. Second, it was shown that the virions can be decorated with approximately 180 redox active, methyl(aminopropyl)viologen moieties by coupling to the surface carboxylates. The display of multiple redox centers on the virus particle surface may lead to the development of novel electron-transfer mediators in redox catalysis, to biosensors, and to nanoelectronic devices such as molecular batteries. 相似文献
55.
Bringmann G Kajahn I Reichert M Pedersen SE Faber JH Gulder T Brun R Christensen SB Ponte-Sucre A Moll H Heubl G Mudogo V 《The Journal of organic chemistry》2006,71(25):9348-9356
The isolation and structural elucidation of three novel-type naphthylisoquinoline alkaloids, ancistrocladinium A and B (the latter along with its atropisomer), from a Congolese Ancistrocladus species collected in the habitat Yeteto is reported. Their structures, including all stereochemical features, were elucidated by spectroscopic, chemical, and chiroptical methods. Ancistrocladinium A and B are the first N,C-coupled naphthyldihydroisoquinoline alkaloids found in nature, i.e., with an iminium-aryl axis. Although ancistrocladinium A, which is N,8'-coupled, is configurationally stable at this axis, ancistrocladinum B and its rotational isomer are based on a hitherto unprecedented N,6'-coupling type, with a slow rotation about the hetero biaryl axis at room temperature; they thus occur as a 46:54 mixture of two configurationally semistable atropo-diastereomers. For the isomerization of (P)-ancistrocladinium B to its (M)-diastereomer and for the opposite direction, the Gibbs free energies of activation were determined to be DeltaG double dagger1 = 105.8 kJ mol-1 and DeltaG double dagger2 = 105.7 kJ mol-1, respectively. In addition, the compounds were shown to have promising antileishmanial activities. 相似文献
56.
Dr. Cédric Tresse Dr. Marc François-Heude Vincent Servajean Rubal Ravinder Clémence Lesieur Lucie Geiben Dr. Louis Jeanne-Julien Vincent Steinmetz Dr. Pascal Retailleau Dr. Emmanuel Roulland Prof. Dr. Jean-Marie Beau Dr. Stéphanie Norsikian 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(16):5230-5239
We give a full account of the total synthesis of tiacumicin B (Tcn-B), a natural glycosylated macrolide with remarkable antibiotic properties. Our strategy is based on our experience with the synthesis of the tiacumicin B aglycone and on unique 1,2-cis-glycosylation steps. We used sulfoxide anomeric leaving-groups in combination with a remote 3-O-picoloyl group on the donors that allowed highly β-selective rhamnosylation and noviosylation that rely on H-bond-mediated aglycone delivery. The rhamnosylated C1–C3 fragment was anchored to the C4–C19 aglycone fragment by a Suzuki–Miyaura cross-coupling. Ring-size-selective Shiina macrolactonization provided a semiglycosylated aglycone that was engaged directly in the noviolysation step with a virtually total β-selectivity. Finally, a novel deprotection method was devised for the removal of a 2-naphthylmethyl ether on a phenol, and efficient removal of all the protecting groups provided synthetic tiacumicin B. 相似文献
57.
E. Ziegler Heidrun Mittelbach W. Steiger 《Monatshefte für Chemie / Chemical Monthly》1970,101(4):1059-1067
Zusammenfassung Brombernsteinsäure reagiert mit Benzalanilin inDMF/POCl3 zum 2,2-Diphenyl-4,4-biisochinolin-3,3 (2H, 2H)-dion (1a). Brombernsteinsäuredichlorid gibt mit dem Anil vorerst ein Addukt, welches beim Erhitzen in Nitrobenzol unter Abgabe von H2O, HBr und H2 ebenfalls in1 a übergeht. Analog können die Derivate1b-1k hergestellt werden, die in Lösung intensiv fluoreszieren.
J. R. Geigy AG, Basel, zum Patent angemeldet, CH 3-3253 am 3. Februar 1970. 相似文献
Syntheses of heterocycles, CXLVIII: concerning the chemistry of bi-isoquinolones
Bromosuccinic acid reacts with benzylidene aniline inDMF/POCl3 yielding 2.2-diphenyl-4.4-biisoquinoline-3.3 (2H, 2H)-dione (1a). Bromosuccinyldichloride and benzylidene aniline give firstly an adduct, which loses H2O, HBr and H2 to yield1 a, if heated in nitrobenzene. The derivatives1b-1k are obtained in a similar way and show in solution an intensive fluorescence.
J. R. Geigy AG, Basel, zum Patent angemeldet, CH 3-3253 am 3. Februar 1970. 相似文献
58.
59.
THE ROLE OF SOLAR UV-B IN GROWTH REGULATION OF CRESS (Lepidium sativum L.) SEEDLINGS 总被引:2,自引:0,他引:2
Abstract— Action spectra for growth reduction within the 260 nm to 305 nm waveband were measured for hypocotyls and roots of young etiolated cress ( Lepidium sativum ) seedlings. The action spectra show increasing photon effectiveness with decreasing wavelength and resemble those due to DNA damage. Using short term irradiations, photoreceptors absorbing in the visible range were found to be without influence. As no photoreactivation could be found and the seedlings showed no outward signs of damage, this growth effect may be due to a UV photoreceptor. A modeling calculation was carried out to estimate the effectiveness of solar UV-B on this response both under present conditions and under reduced ozone levels. Even under present conditions, solar UV-B could be involved in regulating growth during the period immediately after germination. 相似文献
60.
The present work was aimed at isolating human serum amyloid A, (SAA), an acute-phase protein mainly complexed to high density lipoproteins, directly from human plasma without sequential ultracentrifugation of lipoproteins and subsequent delipidation of the apolipoprotein moiety. Hydrophobic-interaction fast-protein liquid chromatography on Octylsepharose, using stepwise gradient elution profiles under dissociating conditions, followed by fast-protein liquid-gel permeation chromatography on a Superdex TM75 column revealed a higher than 95% purity of isolated SAA. Further purification of SAA from coeluting apolipoproteins C and A-II was achieved by preparative isoelectric focusing between pH5-7 using a Rotofor apparatus. Separation of the main SAA isoforms, SAA1 (pI 6.5) and SAA1 des-Arg (pI 6.0, lacking the N-terminal arginine), was achieved by anion-exchange fast-protein liquid chromatography on a Fractogel EMD DEAE 650-S column. The purity of the SAA1 and SAA1 des-Arg isoforms, thus isolated, was checked by immunochemical techniques and amino acid analysis. With the described method various SAA isoforms can be isolated, purified and separated directly from human plasma/serum without prior ultracentrifugation. 相似文献