全文获取类型
收费全文 | 4175篇 |
免费 | 252篇 |
国内免费 | 21篇 |
专业分类
化学 | 3201篇 |
晶体学 | 16篇 |
力学 | 80篇 |
数学 | 537篇 |
物理学 | 614篇 |
出版年
2023年 | 52篇 |
2022年 | 73篇 |
2021年 | 84篇 |
2020年 | 135篇 |
2019年 | 109篇 |
2018年 | 63篇 |
2017年 | 79篇 |
2016年 | 174篇 |
2015年 | 150篇 |
2014年 | 175篇 |
2013年 | 199篇 |
2012年 | 308篇 |
2011年 | 259篇 |
2010年 | 149篇 |
2009年 | 142篇 |
2008年 | 230篇 |
2007年 | 217篇 |
2006年 | 193篇 |
2005年 | 197篇 |
2004年 | 185篇 |
2003年 | 122篇 |
2002年 | 157篇 |
2001年 | 68篇 |
2000年 | 61篇 |
1999年 | 51篇 |
1998年 | 58篇 |
1997年 | 50篇 |
1996年 | 54篇 |
1995年 | 38篇 |
1994年 | 32篇 |
1993年 | 31篇 |
1992年 | 42篇 |
1991年 | 24篇 |
1990年 | 27篇 |
1989年 | 34篇 |
1988年 | 33篇 |
1987年 | 30篇 |
1986年 | 23篇 |
1985年 | 27篇 |
1984年 | 31篇 |
1983年 | 26篇 |
1982年 | 21篇 |
1981年 | 38篇 |
1980年 | 24篇 |
1979年 | 21篇 |
1978年 | 17篇 |
1977年 | 27篇 |
1976年 | 10篇 |
1975年 | 12篇 |
1970年 | 13篇 |
排序方式: 共有4448条查询结果,搜索用时 0 毫秒
61.
Helmut Bahrmann Klaus Bergrath Han -Jerg Kleiner Peter Lappe Christoph Naumann Dieter Peters Dieter Regnat 《Journal of organometallic chemistry》1996,520(1-2):97-100
BINAS is a new, very efficient ligand for propylene hydroformylation. BINAS is made by the sulfonation of NAPHOS. Different synthetic routes to NAPHOS are discussed. A new two step synthesis starting from 2,2′-bis(bromomethyl)-1,1′-binaphthyl is described. 相似文献
62.
Seger C Sturm S Mair ME Ellmerer EP Stuppner H 《Magnetic resonance in chemistry : MRC》2005,43(6):489-491
2D NMR-derived 1H and 13C NMR signal assignments of six structurally closely related cucurbitacin derivatives are presented. The investigated 2-O-beta-D-glucopyranosylcucurbitacins I, J, K, and L were obtained from Citrullus colocynthis (L.) Schrader whereas the aglyca cucurbitacin E and I were isolated from Ecballium elaterium L. 相似文献
63.
Sodium [2-13C,2-2H3]acetate was incorporated into cytochalasin B( 1 ) by Phoma exigua and into cytochalasin D ( 2 ) by Zygosporium masonii. The 13C-NMR. and 2H-NMR. of the metabolites showed that most of the deuterium was lost except at carbon atoms which are in polyketide chain-initiating units. 相似文献
64.
Optically Active 3-Amino-2H-azirines as Synthons for Enantiomerically Pure αα-Disubstiuted α-Amino Acids: Syntheses of Isovaline Synthons and a Segment of Trichotoxin A-50 The synthesis of a novel 3-amino-2-methyl-2-[2-(phenylsulfonyl)ethyl]-2H-azirine derivative 12 with a chiral substituent at the amino group is described. Chromatographic separation of the diastereoisomer mixture gave pure diastereoisomers which, after an electrochemical cleavage of the phenylsulfonyl group, yielded the (S)- and (R)-isovalin (Iva) synthons 13a and 13b , respectively. The absolute configuration of the precursor molecule 12b was established by X-ray crystallography. The Iva synthons were successfully used in the synthesis of the C-terminal pentapeptide Z-Leu-Aib-(R)-Iva-Gln-Valol of the peptaibole Trichotoxin A-50 and its epimer. 相似文献
65.
Hafnium tetrachloride reacts with the tris(trimethylsilyl)silyl potassium tmen adduct (1) to form a [tris(trimethylsilyl)silyl]trichlorohafnium tmen complex (2); reaction of 2 with 2,6-dimethylphenylisonitrile leads to insertion into the silicon hafnium bond (4). 相似文献
66.
Chiral [2H] -labelled methylene groups flanked by two double bonds within (poly)unsaturated fatty acids are readily available from trans-2,3-epoxy[2,3-2H2] alk-4-yn-l-ols, obtained in their turn by asymmetric epoxidation of the corresponding (E)-[2,3-2H2] alk-2-en-4-yn-l-ols (see Scheme 3). The procedure is exemplified for (8S,3Z,6Z,9Z)-[7,8-2H2] trideca-3,6,9-trienoic acid ((8S)- 11 ) and (8R)- 11 (Scheme 4) as well as for (5S,3Z,6Z)-[4,5?2H2]deca-3,6-dienoic acid ((5S)- 13 ) and (5R)- 13 (Scheme 5). 相似文献
67.
Gaul C Njardarson JT Shan D Dorn DC Wu KD Tong WP Huang XY Moore MA Danishefsky SJ 《Journal of the American Chemical Society》2004,126(36):11326-11337
The first asymmetric total synthesis of (+)-migrastatin (1), a macrolide natural product with anti-metastatic properties, has been accomplished. Our concise and flexible approach utilized a Lewis acid-catalyzed diene aldehyde condensation (LACDAC) to install the three contiguous stereocenters and the trisubstituted (Z)-alkene of migrastatin (2 + 3 --> 21). Construction of the two remaining stereocenters and incorporation of the glutarimide-containing side chain was achieved by an anti-selective aldol addition of propionyl oxazolidinone 28 to angelic aldehyde 27, followed by a Horner-Wadsworth-Emmons (HWE) coupling of 32 with glutarimide aldehyde 5. Finally, the assembly of the macrocycle was realized by a highly (E)-selective ring-closing metathesis (35 --> 37). Utilizing the power of diverted total synthesis (DTS), a series of otherwise inaccessible analogues was prepared and evaluated for their potential as tumor cell migration inhibitors in several in vitro assays. These studies revealed a dramatic increase in activity when the natural motif was considerably simplified, presenting macrolactones 45 and 48, as well as macrolactam 55, macroketone 60, and CF(3)-alcohol 71 as promising anti-metastatic agents. 相似文献
68.
Constrained peptides are promising next-generation therapeutics. Peptide stapling is a particularly attractive technique to generate constrained macrocycles with improved biological activity and metabolic stability. We introduce a biocompatible two-component stapling approach based on the reagent 2,6-dicyanopyridine and a pseudo-cysteine amino acid. Stapling can proceed either directly on-resin during solid-phase synthesis or following isolation of the linear peptide. The stapling reaction is orthogonal to natural amino acid side chains and completes in aqueous solution at physiological pH, enabling its direct use in biochemical assays. We performed a small screening campaign of short peptides targeting the Zika virus protease NS2B-NS3, allowing the direct comparison of linear with in situ stapled peptides. A stapled screening hit showed over 28-fold stronger inhibition than its linear analogue, demonstrating the successful identification of constrained peptide inhibitors.A synthetically straightforward and biocompatible peptide-stapling strategy that can be used directly in biochemical assays to identify constrained enzyme inhibitors. 相似文献
69.
1-(2′-Deoxy-5′-O-dimethoxytrityl-′-D -ribofuranosyl)-1 H-benzimidazole 3′-[(p-chlorophenyl)(2-cyanoethyl) phosphate] ( 6 ) has been synthesized from 1-(β-D -ribofuranosyl)-1H-benzimidazole ( 3b ) using regiospecific 2′-deoxygenation. The latter compound was obtained by glycosylation of benzimidazole with the D -ribose derivative 2 leading exclusively of the β-D -anomer. 相似文献
70.
Christoph Elschenbroich Fabian Gerson Hiroaki Ohya-Nishiguchi Christoph Wydler Axel Nissen 《Helvetica chimica acta》1977,60(8):2530-2538
Proton hyperfine data are reported for the radical anions of 1,8-di (propyn-1-yl)-naphthalene (I), 7,8,12,13-tetradehydro-10,11-dihydro-9H-cyclodeca[d,e]naphthalene (II) and 2,2′-di(propyn-1-yl)-biphenyl (III), as well as of 5,6,11,12-tetradehydro-7,8,9,10-tetrahydro-dibenzo[a,c]cyclodecene (IV) and its 8,8,9,9-tetradeuterio-derivative (IV-d4). The triple bonds in I and II can be regarded as roughly parallel, while those in IV (and IV-d4) may be considered as crossed. The π-spin distributions in I? to IV? are discussed in terms of simple MO models which suggest a weekly bonding interaction between the acetylenic fragments in IV?, in contrast to III? where such an effect appears to be negligible. The importance of an analogous interaction in I? and II? is difficult to deduce, since its inclusion into a MO model does not substantially affect the π-spin distribution in these radical anions. 相似文献