全文获取类型
收费全文 | 2175篇 |
免费 | 163篇 |
国内免费 | 188篇 |
专业分类
化学 | 2372篇 |
晶体学 | 4篇 |
力学 | 4篇 |
综合类 | 4篇 |
数学 | 96篇 |
物理学 | 46篇 |
出版年
2024年 | 2篇 |
2023年 | 21篇 |
2022年 | 26篇 |
2021年 | 39篇 |
2020年 | 63篇 |
2019年 | 60篇 |
2018年 | 67篇 |
2017年 | 72篇 |
2016年 | 59篇 |
2015年 | 73篇 |
2014年 | 133篇 |
2013年 | 259篇 |
2012年 | 101篇 |
2011年 | 140篇 |
2010年 | 120篇 |
2009年 | 115篇 |
2008年 | 125篇 |
2007年 | 128篇 |
2006年 | 135篇 |
2005年 | 157篇 |
2004年 | 155篇 |
2003年 | 136篇 |
2002年 | 57篇 |
2001年 | 27篇 |
2000年 | 56篇 |
1999年 | 31篇 |
1998年 | 18篇 |
1997年 | 20篇 |
1996年 | 12篇 |
1995年 | 12篇 |
1994年 | 24篇 |
1993年 | 13篇 |
1992年 | 23篇 |
1991年 | 5篇 |
1990年 | 11篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1975年 | 1篇 |
1970年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有2526条查询结果,搜索用时 15 毫秒
31.
Roselis A. Landaeta Aponte Andreas Luxenburger Scott A. Cameron Alex Weymouth-Wilson Richard H. Furneaux Lawrence D. Harris Benjamin J. Compton 《Molecules (Basel, Switzerland)》2022,27(7)
Bile acid receptors have been identified as important targets for the development of new therapeutics to treat various metabolic and inflammatory diseases. The synthesis of new bile acid analogues can help elucidate structure–activity relationships and define compounds that activate these receptors selectively. Towards this, access to large quantities of a chenodeoxycholic acid derivative bearing a C-12 methyl and a C-13 to C-14 double bond provided an interesting scaffold to investigate the chemical manipulation of the C/D ring junction in bile acids. The reactivity of this alkene substrate with various zinc carbenoid species showed that those generated using the Furukawa methodology achieved selective α-cyclopropanation, whereas those generated using the Shi methodology reacted in an unexpected manner giving rise to a rearranged skeleton whereby the C ring has undergone contraction to form a novel spiro–furan ring system. Further derivatization of the cyclopropanated steroid included O-7 oxidation and epimerization to afford new bile acid derivatives for biological evaluation. 相似文献
32.
Crown ether-type macrocycles consisting of an enantiopure biarenol derivative and an oligoethylene glycol were synthesized by the Lewis acid-mediated tandem Claisen rearrangement. This is the first example of the successful application of the tandem Claisen rearrangement to the synthesis of enantiopure macrocyclic biarenol derivatives. The enantiopure macrocyclic biarenols were found to form 1:1 complexes with amino acid salts and to discriminate their chirality. 相似文献
33.
《Arabian Journal of Chemistry》2020,13(1):1345-1354
We here report the synthesis of novel chalcone-sulfonamide compounds based on the hybridization at 2′ position and nitro substitution at the side chalcone phenyl ring followed by tandem cyclization into quinolinone derivatives and then a further aldol condensation only as a function of the reaction time. Therefore, for the first time, we have controlled the sequential preparation of chalcone-sulfonamide hybrids, quinolinones and then (E)-3-ene-2,3-dihydroquinolinones simply stopping reaction over increasing time periods. Furthermore, a new molecular scaffold based on a chalcone-(bis)sulfonamide hybrid has been gotten through changing the sequence of coupling reactions and catalyst. This study means practical and useful ways of constructing in high yields new biologically active compounds bearing diversified molecular scaffolds. 相似文献
34.
An efficient synthesis of various α‐halo,α‐allylic aldehydes from α,α‐dihalo ketones using both cyclic (3‐bromocyclohex‐1‐ene zinc bromide and (Z)‐3‐bromocyclobut‐1‐ene zinc bromide) and acyclic (allylzinc bromide and cinnamylzinc bromide) type of allylic organozinc bromide with DMF as base is described. A possible reaction mechanism is also proposed. 相似文献
35.
Achim Häußermann Dr. Frank Rominger Prof. Dr. Bernd F. Straub 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(44):14174-14185
A sterically shielded 3‐substituted zwitterionic N,N‐dimethylisotryptammonium carboxylate has been synthesized by consecutive chemoselective double alkylation of indole. The carboxylate undergoes a quantitative and unusually facile decarboxylation in dimethyl sulfoxide (DMSO) or dimethyl formamide (DMF) at room temperature. The breaking of a nearly equidistant hydrogen bond by solvent molecules initiates heterolytic C? C cleavage. The decarboxylation rate decreases with increasing CO2 partial pressure, proving the competitiveness of protonation and re‐carboxylation of the carbanionic intermediate. Corresponding spiro compounds containing silylene and stannylene moieties show high thermal stability. Addition of an excess of methyllithium to the sodium salt triggers a reaction sequence comprising a deprotonation, carboxylate transfer, and nucleophilic trapping of the rearranged carboxylate by another equivalent of methyllithium. Hydrolytic work‐up of the geminal diolate leads to an acetyl product. The role of the sodium counterion and the mechanism of the rearrangement have been unraveled by deuteration experiments. 相似文献
36.
A new approach to caerulomycins A and E via a 6-methyl-2-(2-pyridyl)-4H-pyran-4-one is described. The pyranone precursor is prepared by Claisen condensation of acetylacetone enol ether with ethyl picolinate. 相似文献
37.
The PESs of systems including deactivated silylenes (SiHHal, SiHal2, Hal = F, Cl, and 2-silaimidazol-2-ylidene, SiN2H2C2H2) and buta-1,3-diene have been studied using G3(MP2)//B3LYP method. Two major reaction channels, (2 + 1) and (4 + 1) cycloaddition reactions, leading to 2-vinylsiliranes and silacyclopent-3-enes, respectively, as well as [1,3]-sigmatropic rearrangements between 2-vinylsiliranes and the corresponding silacyclopent-3-enes, have been considered in detail. Reactivity of silylenes toward buta-1,3-diene decreases in the following series: SiHHal > SiHal2 > SiN2H2C2H2, which is reflected in increase of the reaction barriers for both cycloaddition reactions and in decrease of exothermicity of the formation of the corresponding products. The (4 + 1) cycloaddition is preferable for SiHal2 and SiN2H2C2H2 and can compete with (2 + 1) cycloaddition for SiHHal. [1,3]-Sigmatropic rearrangement is important for isomerization of 2-vinylsiliranes to the corresponding silacyclopent-3-enes for all systems studied, except the SiCl2 system. 相似文献
38.
A facile synthesis of tri and tetracyclic frameworks containing [1,8]naphthyridin-2-one skeleton from the Baylis-Hillman alcohols via the Johnson-Claisen rearrangement, followed by the treatment with Fe/AcOH in simple one-pot multi-step process is described. 相似文献
39.
Mathew W.C. Robinson 《Tetrahedron》2010,66(43):8377-8382
Epoxides undergo a highly efficient Meinwald rearrangement in the presence of catalytic quantities of copper(II) tetrafluoroborate to give carbonyl compounds in high yields and with excellent selectivity. The low toxicity and ease of handling of this reagent make it an attractive alternative to the more corrosive or costly Lewis acids frequently employed. 相似文献
40.