全文获取类型
收费全文 | 61148篇 |
免费 | 1856篇 |
国内免费 | 164篇 |
专业分类
化学 | 37796篇 |
晶体学 | 376篇 |
力学 | 995篇 |
综合类 | 5篇 |
数学 | 11220篇 |
物理学 | 12776篇 |
出版年
2023年 | 394篇 |
2022年 | 604篇 |
2021年 | 826篇 |
2020年 | 886篇 |
2019年 | 910篇 |
2018年 | 1138篇 |
2017年 | 1073篇 |
2016年 | 2044篇 |
2015年 | 1712篇 |
2014年 | 1740篇 |
2013年 | 3770篇 |
2012年 | 3659篇 |
2011年 | 3670篇 |
2010年 | 2452篇 |
2009年 | 2181篇 |
2008年 | 3232篇 |
2007年 | 3041篇 |
2006年 | 2739篇 |
2005年 | 2676篇 |
2004年 | 2258篇 |
2003年 | 1918篇 |
2002年 | 1655篇 |
2001年 | 1286篇 |
2000年 | 1228篇 |
1999年 | 888篇 |
1998年 | 696篇 |
1997年 | 642篇 |
1996年 | 806篇 |
1995年 | 578篇 |
1994年 | 657篇 |
1993年 | 610篇 |
1992年 | 619篇 |
1991年 | 526篇 |
1990年 | 565篇 |
1989年 | 461篇 |
1988年 | 455篇 |
1987年 | 438篇 |
1986年 | 413篇 |
1985年 | 546篇 |
1984年 | 525篇 |
1983年 | 404篇 |
1982年 | 407篇 |
1981年 | 415篇 |
1980年 | 364篇 |
1979年 | 335篇 |
1978年 | 314篇 |
1977年 | 314篇 |
1976年 | 325篇 |
1974年 | 284篇 |
1973年 | 305篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Isidoro Izquierdo María T. Plaza Rafael Robles Concepcin Rodríguez 《Tetrahedron: Asymmetry》1996,7(12)
Bishydroxylation of methyl
1 with osmium tetraoxide proceeded with extremed high diastereoselectivity to give only methyl
2. Configurations of the new stereogenic centers (C-2,3) in 2 were determined by degradation of the C-5,6,7,8 fragment to the well-known methyl
7. Transformation of 2 into the required
10, was achieved by a methodology that implied, protection to 8, reduction of the ester group in 8 to a hydroxymethyl group in 9, and finally deprotection to the free
10. On the other hand, epoxidation reaction on
11 afforded only the corresponding 2,3-anhydro derivative 12 with
configuration, as could be demonstrated by degradation to (S)-1,2,4-trimetoxybutane 16, which synthesis is reported herein. 相似文献
Full-size image
Full-size image
Full-size image
Full-size image
Full-size image
Full-size image
992.
Chiral 1,3-perhydrobenzoxazines 1, 2, and 9-14, prepared by condensation of 8-(benzylamino)menthol with different aldehydes, react with alkylmagnesium bromides and trimethylaluminum leading to the open amino alcohols 3a-d, 4a-d, and 15-20 in excellent chemical yields and good to excellent diastereomeric excess. The sequential elimination of the menthol appendage by heating with P(2)O(5) and the benzyl group by hydrogenolysis lead to primary amines 7a-d, 8a-d, and 27-30 in excellent chemical yields and ee. The addition of the alkyl group from the Grignard derivatives and the methyl group from the trimethylaluminum occurs from opposite sides of the heterocycle, yielding the final primary amines with the same stereochemistry. 相似文献
993.
Fustero S Navarro A Díaz D de La Torre MG Asensio A Sanz F González ML 《The Journal of organic chemistry》1996,61(25):8849-8859
Reaction of azaenolates of 2-alkyl-oxa(thia)zolines 6 with imidoyl chlorides 7 as electrophiles to furnish masked N-substituted beta-enamino acid derivatives 1-2 in 70-90% yield is described. Alternative routes are discussed. Compounds 1-2 generally appear in one tautomeric form, imino or enamino, depending on the nature of the imidoyl chloride. The configuration of the enamino moiety (Z) and the conformation (s-cis) of compounds 1-2 obtained were established by an NMR study and unequivocally set by nuclear Overhauser effect difference experiments. An X-ray structure of compound 1e is also reported, showing a strong intramolecular NH.N hydrogen bond. Ab initio calculations (HF/3-21G and HF/3-21+G) have been carried out on several representative examples (1e, 1p, and 1l) in an attempt to support and provide the correct geometry of these derivatives. Structural considerations among the possible isomers of compounds 1 are discussed. From these studies it was concluded that the theoretical calculations agree with the experimental results. In addition, a very simple one-pot procedure for the preparation of masked N-substituted alpha-alkylated beta-enamino acid derivatives 2 from 6, 7, and different alkyl halides (R(3)Y) is described. 相似文献
994.
To develop solid-phase synthesis of phosphinic peptides, different FmocXaaPsi{PO(OAd)CH(2)}XaaOH building blocks have been prepared, where Fmoc is (fluorenylmethoxy)carbonyl. In this respect, the protection of the hydroxyphosphinyl function in these phosphinic dipeptides by the adamantyl group turns out to be convenient. The phosphinic adamantyl esters are completely stable in basic conditions and can be removed under relatively mild acidic conditions. Using these building blocks, despite the bulkiness of the adamantyl group, no particular problem of coupling was observed during the solid-phase synthesis of phosphinic peptides by the Fmoc strategy. The developed methodology is of particular interest to facilitate the development of potent inhibitors of zinc-metalloproteases. 相似文献
995.
The conformational preferences of N-acetyl-N'-methyl-alpha-methyl-beta-L-aspartamide, which is the model compound for helical poly(beta-L-aspartate)s, have been determined by ab initio SCF-MO computations. Two driving patterns have been found for the existing 13 minimum energy conformations: (i) intramolecular hydrogen bonding interactions of both amide-amide and amide-ester type; and (ii) repulsive interactions between the four oxygen atoms contained in the molecule. Self-consistent reaction-field (SCRF) calculations based on the method proposed by Miertus, Scrocco, and Tomasi have been performed in order to evaluate the effect of the solvent on the conformational preferences of the compound subject of study. Water and carbon tetrachloride were the solvents chosen for this purpose, and results have been discussed and interpreted on the basis of their electronic structures. The conclusions drawn from this study are of assistance to understand some features of the conformational transitions experimentally found in poly(beta-L-aspartate)s. 相似文献
996.
The most obvious symmetry breaking in Nature is the left-right asymmetry of living beings: sugars and amino acids are almost exclusively represented by one of their stereoisomers (D-isomer for sugars and L-isomer for amino acids) at the expense of other possible isomers. In our experiment we studied the aqueous solutions of tartaric acid, alanine, and cysteine in the function of temperature by positron lifetime spectroscopy. The results were compared with those obtained in pure water under similar conditions. We always observed a sharp minimum of the positron lifetime at around 41 °C in water and in the solutions of the natural stereo-isomers. On the other hand, the same minimum occurred at considerably lower temperatures in the solutions of the unnatural isomers. 相似文献
997.
The flame expansion factor (ratio of volumes of flame gases before and after burning) has been determined by mercury vapour introduction. The method is based on the determination of atomic concentration ratio in the flame and the flameless gas jet using the same sample introduction rate. A good agreement has been found between the expansion factors determined by the proposed method and those calculated by the gas law. 相似文献
998.
Anhydrites(II)- CaSO4 were prepared from two waste gypsums (PRECHEZA Perov, FOSFA Potorná). The samples of anhydrites(II) with sulphate activators were tested on spillage, beginning and the end of setting time, bending and pressure strength. Both sodium and potassium sulphates accelerated setting of anhydrite, more impressively that prepared from Potorná gypsum. The addition of Na2SO4 influenced the strength of this anhydrite very favourably. On the contrary K2SO4 influenced favourably the strength of Perov anhydrite. Results confirm the necessity to evaluate properties of such materials with respect to their origin. 相似文献
999.
1000.