全文获取类型
收费全文 | 590篇 |
免费 | 5篇 |
专业分类
化学 | 446篇 |
晶体学 | 7篇 |
力学 | 6篇 |
数学 | 21篇 |
物理学 | 115篇 |
出版年
2021年 | 4篇 |
2019年 | 3篇 |
2016年 | 6篇 |
2015年 | 8篇 |
2014年 | 16篇 |
2013年 | 29篇 |
2012年 | 23篇 |
2011年 | 28篇 |
2010年 | 19篇 |
2009年 | 19篇 |
2008年 | 26篇 |
2007年 | 31篇 |
2006年 | 32篇 |
2005年 | 34篇 |
2004年 | 33篇 |
2003年 | 20篇 |
2002年 | 22篇 |
2001年 | 6篇 |
2000年 | 10篇 |
1999年 | 9篇 |
1998年 | 4篇 |
1997年 | 8篇 |
1995年 | 5篇 |
1994年 | 3篇 |
1993年 | 10篇 |
1992年 | 8篇 |
1991年 | 11篇 |
1990年 | 7篇 |
1989年 | 4篇 |
1988年 | 6篇 |
1987年 | 8篇 |
1986年 | 6篇 |
1985年 | 10篇 |
1984年 | 9篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1981年 | 13篇 |
1980年 | 10篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1974年 | 9篇 |
1973年 | 13篇 |
1972年 | 4篇 |
1968年 | 7篇 |
1967年 | 3篇 |
1961年 | 3篇 |
1960年 | 2篇 |
排序方式: 共有595条查询结果,搜索用时 546 毫秒
1.
2.
Kiyoshi Tanemura Tsuneo Suzuki Takaaki Horaguchi Mikio Sudo 《Journal of heterocyclic chemistry》1991,28(2):305-309
A new heterocycle, furo[4,3,2-de][1]benzopyran ( 2 ), was synthesized. A key step in the sequence was the allylic bromination of 3,4-dihydrofuro[4,3,2-de][1]benzopyran ( 8 ) to give 3-bromo-3,4-dihydrofuro[4,3,2-de][1]-benzopyran ( 10 ) using N-bromosuccinimide under irradiation and high dilution conditions. Bromide 10 was dealt with 1,8-diazabicyclo[5.4.0]undec-7-ene to afford compound 2 . Several reactions of 2 were examined. Protonation of 2 in trifluoroacetic acid occurred at the 2-position to form a pyrylium ion 12 . Catalytic hydrogenation of 2 with palladium on charcoal proceeded smoothly to give 8 . Reduction of 2 by sodium and ethanol afforded 3-ethyl-4-hydroxybenzofuran ( 14 ). Electrophilic substitutions of 2 such as formylation, acetylation, and bromination, occurred easily at the 2-position. The above results show that compound 2 has both properties of benzofuran and 4-methylenepyran. 相似文献
3.
Regioselective allylation reactions using crotyl Grignard reagent-CeCl3 systems are described. Regioselectivity depends on the lanthanide salts: α-product was predominantly produced with light rare earth elements such as La, Ce, Pr, Nd, whereas γ-product was formed when heavy rare earth elements were used. 相似文献
4.
Dihydroboronium derivatives of (S,S)-1,2-bis(t-butylmethylphosphino)ethane (t-Bu-BisP*) were prepared and used as chiral diphosphine ligand precursors in Rh-catalyzed asymmetric hydrogenation of methyl (Z)-acetamidocinnamate to afford the hydrogenation product in up to 94% enantioselectivity. 相似文献
5.
Bond IA Budding E Conway MJ Fenton KB Fujii H Fujii Z Fujimoto M Hasegawa H Hayashida N Honda M Hotta N Humble JE Kabe S Kasahara K Kifune T Masaike A Matsubara Y Mitsui K Miura Y Mori M Murakami K Muraki Y Nagano M Nakamura K Nakamura T Norris PM Ohashi Y Okada A Saito T Sakata M Sato H Shibata S Shima M Storey JR Tanimori T Teshima M Torii S Uchino K Watase Y Woodhams MD Yamamoto Y Yock PC Yuda T 《Physical review letters》1988,61(20):2292-2295
6.
Tsunehisa Hirashita Yousuke Hayashi Kazuma Mitsui Shuki Araki 《Tetrahedron letters》2004,45(16):3225-3228
Vinyl and methylindium ate complexes (indates) were prepared and both the tendency of immigration and regioselectivity toward cinnamyl bromide were investigated. The vinyl group was more preferably transferred than the Me group, giving a regioisomeric mixture of SN2 and SN2′ products. The ratio of SN2/SN2′ selectivity can be controlled by solvents; in the presence of polar solvents, such as N-butylpyrrolidone (NBP) and THF, the SN2′ product was mainly obtained, whereas the SN2 product was selectively prepared in solutions containing hexane. The vinylindium compound, generated by the reaction of allylic-type diindium reagents with imine, was also converted to the corresponding vinyl indate, which was allowed to react with allyl chloride to give a three-component coupling product. 相似文献
7.
Enhancement of electronic excitation energy transfer in the colloidal crystals of colloidal silica suspensions doped with fluorescent dyes 总被引:1,自引:0,他引:1
Kiyoshi Shibata Hiroshi Kimura Akira Tsuchida Tsuneo Okubo 《Colloid and polymer science》2006,285(2):127-133
The efficiency of electronic excitation energy transfer from photo-excited rhodamine 110 (Rh110, energy donor) to rhodamine B (RhB, energy acceptor) in an exhaustively deionized colloidal silica suspension has been studied. This colloidal suspension shows Bragg reflection due to the formation of colloidal crystals and the Bragg-peak wavelength is controllable by the volume fraction of the silica spheres. When the Bragg-peak wavelength matches with the fluorescence band of Rh110, a depletion was observed in the Rh110 fluorescence spectrum. This means the fluorescence of Rh110 is partially trapped due to the Bragg reflection inside the crystal lattice. In the coexistence of RhB, the enhancement of RhB fluorescence intensity was observed. These facts clearly indicate the trapped photon energy of Rh110 is efficiently transferred to RhB within the colloidal crystals. The quantitative measurements showed that the enhancement of the transfer efficiency is 20% (or slightly more) in the present experimental conditions. 相似文献
8.
Takaaki Horaguchi Hiroaki Yagoh Kiyoshi Tanemura Tsuneo Suzuki 《Journal of heterocyclic chemistry》1986,23(3):657-663
4,5-Dihydro-3H-naphtho[1,8-bc]furans 4 and 6 which have various substituents (R1 and R2) have been synthesized from 8-oxo-5,6,7,8-tetrahydro-1-naphthyloxyacetic acids 1 and 3 or their ethyl esters 2 . The reaction of acids 1 and 3 with sodium acetate in acetic anhydride gave a mixture of furans 4 and 6 and lactones 5 and 7 . The ratios of the products were varied according to the types of substituents (R1 and R2) in acids 1 and 3 . As the substituent R1 (R2 = hydrogen) in acids 1 was changed from hydrogen to a methyl, ethyl or isopropyl group, production of furans 4 became more difficult. However, when a phenyl group was used as the substituent, furan 4 was obtained in good yield. Similarly, as the substituent R2 (R1 = hydrogen) in acids 1 was changed from hydrogen to a methyl, ethyl or isopropyl group, furan formation was more difficult. In contrast, acids 3 which had electron-withdrawing substituents such as chlorine, bromine or a nitro group at the 4-position afforded furans 6 in good yield. 4,5-Dihydro-3H-naphtho[1,8-bc]furans 4 and 4,5-dihydro-3H-naphtho[1,8-bc]furan-2-carbocylic acids 8 were synthesized from the reaction of esters 2 and potassium hydroxide in dioxane. When the substituents R1 or R2 in esters 2 were varied from hydrogen to a methyl, ethyl or isopropyl group the total yields of furans 4 and furancarboxylic acids 8 were reduced. 相似文献
9.
(+)-Steganacin was synthesized in a new and highly specific asymmetric pathway based on the novel application of chiral γ-lactone as a chiral synthon. By this synthesis the absolute stereochemistry of natural (?)-steganacin could be determined in unequivocal way. 相似文献
10.
Substituted alkylcyclohexanones were hydrogenated over several transition metal catalysts. The hydrogenation with freshly prepared Raney Ni gave predominantly the axial alcohols but catalyst to which sodium hydroxide was added or aged catalyst increased the stereoselectivity. In contrast to the PtO2 hydrogenation in which the equatorial alcohols were favoured, more axial alcohols were obtained than the equatorial counterparts on Pt-black. On Pd catalyst the isomeric ratios of the cyclohexanols changed as the reaction progressed. This may be due to a decrease in the amount of hydrogen available caused by the strong adsorption of ketones on the catalyst. Rh catalyst gave more of the axial alcohols than any other catalyst used in the present study. 相似文献