全文获取类型
收费全文 | 3568篇 |
免费 | 112篇 |
国内免费 | 11篇 |
专业分类
化学 | 2695篇 |
晶体学 | 46篇 |
力学 | 42篇 |
数学 | 181篇 |
物理学 | 727篇 |
出版年
2022年 | 18篇 |
2021年 | 20篇 |
2020年 | 42篇 |
2019年 | 50篇 |
2018年 | 49篇 |
2017年 | 24篇 |
2016年 | 65篇 |
2015年 | 60篇 |
2014年 | 77篇 |
2013年 | 180篇 |
2012年 | 163篇 |
2011年 | 199篇 |
2010年 | 121篇 |
2009年 | 101篇 |
2008年 | 200篇 |
2007年 | 176篇 |
2006年 | 175篇 |
2005年 | 171篇 |
2004年 | 144篇 |
2003年 | 152篇 |
2002年 | 141篇 |
2001年 | 91篇 |
2000年 | 88篇 |
1999年 | 55篇 |
1998年 | 38篇 |
1997年 | 39篇 |
1996年 | 46篇 |
1995年 | 32篇 |
1994年 | 35篇 |
1993年 | 47篇 |
1992年 | 44篇 |
1991年 | 48篇 |
1990年 | 52篇 |
1989年 | 34篇 |
1988年 | 48篇 |
1987年 | 45篇 |
1986年 | 42篇 |
1985年 | 67篇 |
1984年 | 51篇 |
1983年 | 28篇 |
1982年 | 44篇 |
1981年 | 47篇 |
1980年 | 34篇 |
1979年 | 41篇 |
1978年 | 42篇 |
1977年 | 52篇 |
1976年 | 37篇 |
1975年 | 28篇 |
1974年 | 22篇 |
1973年 | 23篇 |
排序方式: 共有3691条查询结果,搜索用时 15 毫秒
51.
A general synthetic scheme for tamoxifen-type tetrasubstituted olefins based on the novel Cu-catalyzed carbomagnesation across alkynyl(2-pyridyl)silane has been developed. A wide array of electronically and structurally diverse tetrasubstituted olefins can be prepared in a regiocontrolled, stereocontrolled, and diversity-oriented manner. Noteworthy features are that (i) the three aryl groups, which are believed to be important (essential) for anti-estrogenic activity, can be varied at will because they all stem from readily available aryl iodides, and (ii) any stereo- and regioisomers can, in principle, be prepared by simply changing the applying order of aryl iodides into the sequence. 相似文献
52.
Kei Yoshida Junko Kusaki Katsunobu Ehara Shiro Saka 《Applied biochemistry and biotechnology》2005,123(1-3):795-806
Japanese beech (Fagus crenata Blume), its cell wall components, and model compounds were treated by supercritical water (380°C, 100 MPa) for 5 s using a batch-type reactor to investigate the production behavior of low molecular weight organic acids. It was found that cellulose and hemicellulose were decomposed to formic acid, pyruvic acid, glycolic acid, acetic acid, and lactic acid, whereas lignin was barely decomposed to such organic acids under the given conditions. However, after prolonged treatment (380°C, 100 MPa, 4 min) of lignin, some organic acids were recovered owing perhaps to the decomposition of the propyl side chain of lignin. It was additionally revealed that the predominant organic acid recovered was acetic acid, which might be derived from the acetyl group of hemicellulose in Japanese beech. 相似文献
53.
Kunihisa Yoshida Yoshimasa Toyo-Oka Kazusada Takeda 《Journal of heterocyclic chemistry》1995,32(2):701-702
The electrooxidation of 1-methylpyrazole in methanol containing sodium cyanide produced 1-methyl-pyrazole-4-carbonitrile 2 and ?5-carbonitrile 3 in yields of 23 and 8%, respectively (2e-oxidation products), together with 4-methoxy-1-methylpyrazole-5-carbonitrile 4 (4e-oxidation product, 4%). 相似文献
54.
Motozaki T Sawamura K Suzuki A Yoshida K Ueki T Ohara A Munakata R Takao K Tadano K 《Organic letters》2005,7(11):2265-2267
[reaction: see text]. We have completed the total synthesis of natural (+)-tubelactomicin A (1), a 16-membered macrolide antibiotic. This Letter presents a highly efficient synthesis of the upper-half segment (C14-C24) and the completion of the total synthesis featuring a high-yielding Stille coupling for the connection of the upper-half and lower-half segments and Mukaiyama macrolactonization for the construction of the entire structure of 1. 相似文献
55.
Structure-activity relationships of a new antifungal imidazole, AFK-108, and related compounds 总被引:1,自引:0,他引:1
Hori K Sakaguchi A Kudoh M Ishida K Aoyama Y Yoshida Y 《Chemical & pharmaceutical bulletin》2000,48(1):60-64
Fungicidal activity of widely used imidazole antifungal drugs in topical applications is not so strong in spite of their fungistatic activities against dermatophytes and pathogenic yeasts. In order to improve fungicidal activity of imidazole antifungal agents, a series of novel imidazole derivatives having a hydrophobic substituent derived from isoprenoid were synthesized. The efficacy of these compounds was evaluated with respect to direct cell-membrane damaging activity, ergosterol biosynthesis inhibition, minimum growth-inhibitory concentration (MIC) and therapeutic effect for experimental dermatophytosis of guinea pigs. Among the newly synthesized compounds, the geranyl derivative named AFK-108 (2a) showed the highest in vivo fungicidal activity with both cell membrane damaging activity and ergosterol biosynthesis inhibition in vitro. 相似文献
56.
Itoh F Yoshioka Y Yukishige K Yoshida S Ootsu K Akimoto H 《Chemical & pharmaceutical bulletin》2000,48(9):1270-1280
The glutamic acid moiety of N-[4-[3-(2,4-diamino-7H-pyrrolo[2,3-d]pyrimidin-5-yl)propyl]benzoyl]-L-g lutamic acid (1b, TNP-351) and the related compound (1a), was replaced with various N(omega)-acyl-, sulfonyl-, carbamoyl- and aryl-2,omega-diaminoalkanoic acids, and the inhibitory effects of the resulting products (9, 11, 14, 18, 21, 23, 25, 30, 36) on dihydrofolate reductase (DHFR), the growth of murine fibrosarcoma Meth A cells, and methotrexate-resistant human CCRF-CEM cells, were examined. Compounds (9a-f) acylated with a hemiphthaloyl group were efficiently synthesized by coupling pyrrolo[2,3-d]pyrimidine carboxylic acids (7a,b) and N(omega)-phthaloyl 2,omega-diaminoalkanoic acid methyl esters (6a-c) and subsequent hydrolysis. The other N(omega)-acyl- and sulfonyl-ornithine analogs (21, 23, 25) were synthesized by acylation of free amino intermediates (19a,b) derived from tert-butoxycarbonyl-ornithine analogs (17a,b). A free ornithine analog (18) did not strongly inhibit Meth A cell growth, whereas all N(omega)-acyl-, sulfonyl-, carbamoyl- and aryl-ornithine analogs (9, 11, 21, 23, 25, 30, 36) exhibited much more potent inhibitory activities against both DHFR and Meth A cell growth. In particular, compounds 9c, 21k and 36a also showed remarkable growth-inhibitory activities against methotrexate-resistant CCRF-CEM cells. These results demonstrate that the potent inhibitory activities of N(omega)-masked ornithine analogs against the growth of Meth A cells and methotrexate-resistant CCRF-CEM cells, results from effective uptake via reduced folate carrier and their potent DHFR inhibition. 相似文献
57.
Chesney A Bryce MR Yoshida S Perepichka IF 《Chemistry (Weinheim an der Bergstrasse, Germany)》2000,6(7):1153-1159
Two conceptually different routes to transient 1,2-diselones are reported: 1) via ring fragmentation of the 1,4,2-diselenazine system 6, and 2) by the tributylphosphane-induced depolymerisation of the shelf-stable organoselenium polymer 15. Evidence for the intermediacy of 1,2-diselone species 7 and 16 is provided in both cases by in situ trapping with dimethyl acetylenedicarboxylate (DMAD) to yield 1,4-diselenin derivatives. The route via 15 is especially expedient and trapping of 16 is efficient. Subsequent reactions of adduct 17 afford [1,2-ethanediylbis(diphenylphosphane)] [5,6-bis(methoxycarbonyl)-1,4-diselenin-2,3-dithiolato]nickel(IV) (20). Theoretical calculations at Hartree-Fock (HF) and Moller-Plesset electron-correlated levels (MP2) suggest that the cyclic 1,2-diselenete structure 7c is significantly more stable than the acyclic 1,2-diselone structure 7a. For the bicyclic system 16, the difference in energy between the cyclic and acyclic structures is considerably reduced due to the conformational rigidity imposed by the fused 1,3-dithiole ring. In contrast, the acyclic structures of the 1,2-dithione analogues 13a and 25a are computed to be more stable than their corresponding cyclic 1,2-dithiete structures 13c and 25c. 相似文献
58.
Takashima H Mimura N Ohkubo T Yoshida T Tamaoki H Kobayashi Y 《Journal of the American Chemical Society》2004,126(14):4504-4505
Distributed computing has been implemented to the solution structure determination of endothelin-1 to evaluate efficiency of the method for NMR constraint-based structure calculations. A key target of the investigation was determination of the C-terminal folding of the peptide, which had been dispersed in previous studies of NMR, despite its pharmacological significances. With use of tens of thousands of random initial structures to explore the conformational space comprehensively, we determined high-resolution structures with good convergences of C-terminal as well as previously defined N-terminal structures. The previous studies had missed the C-terminal convergence because of initial structure dependencies trapped in localized folding of the N-terminal region, which are strongly constricted by two disulfide bonds. 相似文献
59.
A simple high-performance liquid chromatographic method was developed for the simultaneous determination of the therapeutic levels of acyclovir and ganciclovir in human plasma. After precipitation of plasma proteins with 6% perchloric acid, acyclovir and ganciclovir were simultaneously determined by reversed-phase chromatography with spectophotometric detection at 254 nm. The peak heights for acyclovir and ganciclovir were linearly related to their concentrations ranging from 0.063 to 2.080 micro g/mL. The recovery was 100.48-102.84% for acyclovir and 99.26-103.07% for ganciclovir. The intra- and inter-day relative standard deviation values were in the range 0.186-8.703% for acyclovir and 0.137-6.424% for ganciclovir. The detection limits for both compounds were 0.01 micro g/mL determined as the signal-to-noise ratio of 3. The present method is applicable to therapeutic monitoring during antiviral medication. 相似文献
60.
Yoshio Ueno Yutaka Takeuchi Jinya Koshitani Takatoshi Yoshida 《Journal of heterocyclic chemistry》1981,18(3):645-647
4-Acyl-1,2-dimethyl-3H-phenothiazin-3-ones were prepared by the photochemical reaction of 1,2-dimethyl-3H-phenothiazin-3-one with aldehydes. The structures of the newly prepared compounds were determined by elemental analysis, spectroscopic methods (ir, nmr and ms) and comparison with a sample prepared by an alternate route. 相似文献