全文获取类型
收费全文 | 11733篇 |
免费 | 1735篇 |
国内免费 | 1303篇 |
专业分类
化学 | 6442篇 |
晶体学 | 98篇 |
力学 | 490篇 |
综合类 | 77篇 |
数学 | 1029篇 |
物理学 | 3143篇 |
无线电 | 3492篇 |
出版年
2024年 | 34篇 |
2023年 | 270篇 |
2022年 | 267篇 |
2021年 | 389篇 |
2020年 | 420篇 |
2019年 | 361篇 |
2018年 | 346篇 |
2017年 | 318篇 |
2016年 | 459篇 |
2015年 | 504篇 |
2014年 | 613篇 |
2013年 | 743篇 |
2012年 | 884篇 |
2011年 | 947篇 |
2010年 | 692篇 |
2009年 | 690篇 |
2008年 | 730篇 |
2007年 | 662篇 |
2006年 | 614篇 |
2005年 | 544篇 |
2004年 | 456篇 |
2003年 | 402篇 |
2002年 | 470篇 |
2001年 | 404篇 |
2000年 | 300篇 |
1999年 | 297篇 |
1998年 | 279篇 |
1997年 | 243篇 |
1996年 | 282篇 |
1995年 | 205篇 |
1994年 | 195篇 |
1993年 | 133篇 |
1992年 | 120篇 |
1991年 | 105篇 |
1990年 | 74篇 |
1989年 | 80篇 |
1988年 | 62篇 |
1987年 | 55篇 |
1986年 | 23篇 |
1985年 | 34篇 |
1984年 | 13篇 |
1983年 | 14篇 |
1982年 | 10篇 |
1981年 | 14篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1974年 | 1篇 |
1969年 | 1篇 |
1957年 | 3篇 |
1936年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 409 毫秒
981.
该文建立了Hersch-Pfluger偏差函数ψK(r)和第二类完全椭圆积分ε(r)之间的关系. 通过对完全椭圆积分及某些初等函数的组合的单调性和凹凸性的研究获得了完全椭圆积分的一些不等式, 并且藉此得到Hersch-Pfluger偏差函数ψK(r)的几个渐进精确的上界估计. 相似文献
982.
Synthesis and Characterization of Ti-ZSM-5 in a Non-alkaline Medium in the Presence of Fluoride Ions
Ti-ZSM-5 was synthesized by hydro thermal crystallization in the presence of fluoride via using a non-alkaline medium. pH values were 5~7. SEM showed perfect Ti-ZSM-5 crystals and a large single crystal growing from the favourable medium. Substitution of titanium for silicon in the ZSM-5 framework led to a decrease of crystal size and of the length/width ratio. Electron microprobe analysis indicated a homogeneous distribution of titanium in the ZSM-5 framework. The unit cell parameters of the Ti-ZSM-5 determined by XRD increased with an increase in titanium content in the framework. TiO4tetrahedron vibrations were found in the IR spectrum. Si(1Ti) peakwas seen in the 29Si MAS NMR spectrum at -1O1ppm(from TMS) and 13CMAS NMR analysis verified the effect of (C3H7)4N F- occluded in thechannels. XPS study on the precursors, calcined and H2O2 adsorbed Ti-ZSM-5 was performed and some interesting results were observed. 相似文献
983.
A facile procedure for the synthesis.of quinoxalines is being reported starting from benzil and 1,2-diaminobenzene. Thereactions were carried out catalyst-free, solvent-free and under microwave irradiation conditions in high yield (84-98%) with short time (3-6 rain) and environmental benign, as well as convenient operation. The structures of all the compounds have been confirmed on the basis of their IR, 1H NMR, and/or 13C NMR, mass spectral data. 相似文献
984.
985.
本体聚合反应过程中聚甲基丙烯酸甲酯增长自由基的ESR研究 总被引:3,自引:0,他引:3
采用TM110谐振腔和φ2mm样品管,在17℃室温条件下成功地记录了MMA本体聚合反应过程中增长自由基的ESR谱。当把DMA加入到MMA和BPO中后,立即抽取0.17ml混合液到φ2mm样品管并记谱。以后每隔2分钟记谱一次,波谱从13(5+8)条线逐渐变成9(5+4)条线。我们用阻碍振荡模型和构象重叠模型作了模拟。从全部谱图看,前者似更合理些。ESR实验表明:在聚合过程前期,自由基浓度基本保持不变,但从聚合中期的某一时刻开始,浓度剧增,它正好同步地与本体聚合反应的自加速效应相对应,而且其变化规律和单体转化率相平行。最后,我们用微波功率饱和方法观测到9线谱的协同自旋跳跃所产生的卫线,证明了主导的电子自旋晶格弛豫机理来自电子一核自旋间的偶极偶合角调制。 相似文献
986.
Bing-Hui Yang Jian-Hua Huang Feng-Yi Zhou Jian-Shao Bao Xia-Fei Lao Lian-Fen Wu Nan-Zhen Shen Shu-Ju Wang Zai-Wan Yang Ren-Rong Lu Chuan-Zhong Tu Yu Wang 《中国化学》1986,4(4):358-369
This article described the preparation and the protection of 3′-DMP and dihydrouridine (Dr) as well as the synthesis of four oligoribonucleotides composed of them. DMP and Dr were obtained by hydrogenation of 3′-UMP and Ur under acidic conditions in the presence of platinum dioxide. They were monomethoxytritylated and benzoylated to (MeOTr)-Dbzp and (MeOTr) Dbzs, respectively. The latter was converted to Dbzs, by demonomethoxytritylation. The oligoribonucleotides containing DMP or Dr—ApGpD, DpApG, ApGpDpC and ApGpDpCpGpG were synthesized via phosphodiester approach and DCC was used as condensing reagent. DpApG was also synthesized via phosphotriester approach and TPST, MSTe, MSNI and MSNT were used as condensing reagents for a preliminary comparison of the coupling yields. These synthetic oligoribonucleotides were checked for purity and nucleotide sequences as usual. ApGpDpCpGpG and DpApG had been used for enzymatic synthesis of ApGpDpCpGpGpDpApG, which had been in turn successfully used for the total syntheses of the 5′-half molecule and the whole molecule of yeast alanine t-RNA 相似文献
987.
988.
Chaoying Wan Feng Zhao Xujin Bao Bala Kandasubramanian Matt Duggan 《Journal of Polymer Science.Polymer Physics》2009,47(2):121-129
Polyamide 12/Trisilanolphenyl‐POSS (PA 12/POSS) composites were prepared via melt‐compounding. The effect of polyhedral oligomeric silsesquioxane (POSS) on crystalline structure and crystalline transition of PA 12 was investigated by wide‐angle X‐ray diffraction (WAXD) and real time fourier transform infrared spectroscopy (FTIR). WAXD results indicated that PA 12 crystallized into γ‐form as slowly cooling from melt and the presence of POSS did not influence the crystalline structure of PA 12. Both PA 12 and PA 12/POSS composites underwent Brill transitions when they were heated from room temperature to melt point. Real time FTIR patterns showed that an absorption band at 697 cm?1 ascribed to Amide V (α) mode was emerged along with the disappearance of Amide VI (γ) band at 628 cm?1 with the increase of the temperature for PA 12 and PA 12/POSS composites, which suggested that the γ‐form crystalline has transformed into α form. The Brill bands were identified and the transformed mechanism was discussed based on the real FTIR results. The addition of POSS enhanced the tensile strength and thermal stability of PA 12. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 121–129, 2009 相似文献
989.
Yue‐Qi Mo Xian‐Yu Deng Xi Jiang Qiu‐Hong Cui 《Journal of polymer science. Part A, Polymer chemistry》2009,47(13):3286-3295
Poly(3,6‐silafluorene) is a typical wide band‐gap conjugated polymer with ultraviolet light emission. The blue electroluminescence from the 3,6‐silafluorene‐based copolymers via intrachain energy transfer was reported in this study. The monomer containing vinylene, anthracene, and tri‐arylamine moieties incorporated into the poly(3,6‐silafluorene) backbone can form efficient deep‐blue emitting copolymers with EL efficiency of 1.1–1.9%. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3286–3295, 2009 相似文献
990.
The asymmetric total synthesis of (+)-xestoquinone and (+)-adociaquinones A and B was achieved in 6–7 steps using an easily accessible meso-cyclohexadienone derivative. The [6,6]-bicyclic decalin B–C ring and the all-carbon quaternary stereocenter at C-6 were prepared via a desymmetric intramolecular Michael reaction with up to 97% ee. The naphthalene diol D–E ring was constructed through a sequence of Ti(Oi-Pr)4-promoted photoenolization/Diels–Alder, dehydration, and aromatization reactions. This asymmetric strategy provides a scalable route to prepare target molecules and their derivatives for further biological studies.The asymmetric total synthesis of (+)-xestoquinone and (+)-adociaquinones A and B was achieved in 6–7 steps using an easily accessible meso-cyclohexadienone derivative.Various halenaquinone-type natural products with promising biological activity have been isolated from marine sponges of the genus Xestospongia1 from the Pacific Ocean. (+)-Halenaquinone (1),2,3 (+)-xestoquinone (2), and (+)-adociaquinones A (3) and B (4)4,5 bearing a naphtha[1,8-bc]furan core (Fig. 1) are the most typical representatives of this family. Naturally occurring (−)-xestosaprol N (5) and O (6)6,7 have the same structure as 3 and 4 except for a furan ring, while a naphtha[1,8-bc]furan core can also be found in fungus-isolated furanosteroids (−)-viridin (7) and (+)-nodulisporiviridin E (8)8,9 (Fig. 1). Halenaquinone (1) was first isolated from the tropical marine sponge Xestospongia exigua2 and it shows antibiotic activity against Staphylococcus aureus and Bacillus subtilis. Xestoquinone (2) and adociaquinones A (3) and B (4) were firstly isolated, respectively, from the Okinawan marine sponge Xestospongia sp.4a and the Truk Lagoon sponge Adocia sp.,4b and they show cardiotonic,4a,c cytotoxic,4b,i antifungal,4i antimalarial,4j and antitumor4l activities. These compounds inhibit the activity of pp60v-src protein tyrosine kinase,4d topoisomerases I4e and II,4f myosin Ca2+ ATPase,4c,g and phosphatases Cdc25B, MKP-1, and MKP-3.4h,kOpen in a separate windowFig. 1Structure of halenaquinone-type natural products and viridin-type furanosteroids.Owing to their diverse bioactivities, the synthesis of this family of natural compounds has been extensively studied, with published pathways making use of Diels–Alder,3a,d,e,5a–c,e,g furan ring transfer,5b Heck,3b,c,5f,7,9b,d palladium-catalyzed polyene cyclization,5d Pd-catalyzed oxidative cyclization,3f and hydrogen atom transfer (HAT) radical cyclization9c reactions. In this study, we report the asymmetric total synthesis of (+)-xestoquinone (2), (−)-xestoquinone (2′), and (+)-adociaquinones A (3) and B (4) (Fig. 1).The construction of the fused tetracyclic B–C–D–E skeleton and the all carbon quaternary stereocenter at C-6 is a major challenge towards the total synthesis of xestoquinone (2) and adociaquinones A (3) and B (4). Based on our retrosynthetic analysis (Scheme 1), the all-carbon quaternary carbon center at C-6 of cis-decalin 12 could first be prepared stereoselectively from the achiral aldehyde 13via an organocatalytic desymmetric intramolecular Michael reaction.10,11 The tetracyclic framework 10 could then be formed via a Ti(Oi-Pr)4-promoted photoenolization/Diels–Alder (PEDA) reaction12–16 of 11 and enone 12. Acid-mediated cyclization of 10 followed by oxidation state adjustment could be subsequently applied to form the furan ring A of xestoquinone (2). Finally, based on the biosynthetic pathway of (+)-xestoquinone (2)4b,5c and our previous studies,7 the heterocyclic ring F of adociaquinones A (3) and B (4) could be prepared from 2via a late-stage cyclization with hypotaurine (9).Open in a separate windowScheme 1Retrosynthetic analysis of (+)-xestoquinone and (+)-adociaquinones A and B.The catalytic enantioselective desymmetrization of meso compounds has been used as a powerful strategy to generate enantioenriched molecules bearing all-carbon quaternary stereocenters.10,11 For instance, two types of asymmetric intramolecular Michael reactions were developed using a cysteine-derived chiral amine as an organocatalyst by Hayashi and co-workers,11a,b while a desymmetrizing secondary amine-catalyzed asymmetric intramolecular Michael addition was later reported by Gaunt and co-workers to produce enantioenriched decalin structures.11c Prompted by these pioneering studies and following the suggested retrosynthetic pathway (Scheme 1), we first screened conditions for organocatalytic desymmetric intramolecular Michael addition of meso-cyclohexadienone 13 (Table 1) in order to form the desired quaternary stereocenter at C-6. Compound 13 was easily prepared on a gram scale via a four-step process (see details in the ESI†).Attempts of organocatalytic desymmetric intramolecular Michael additiona
Open in a separate windowaAll reactions were performed using 13 (5.8 mg, 0.03 mmol, 1.0 equiv., and 0.1 M) and a catalyst at room temperature in analytical-grade solvents, unless otherwise noted.bThe yields and diastereoisomeric ratios (d.r.) were determined from the crude 1H NMR spectrum of 14 using CH2Br2 as an internal standard, unless otherwise noted.cThe enantiomeric excess (e.e.) values were determined by chiral high-performance liquid chromatography (Chiralpak IG-H).dCompound 13: 9.6 mg, 0.05 mmol, and 0.1 M.eIsolated combined yield of 14a + 14b.fCompound 13: 192 mg, 1.0 mmol, and 0.1 M.gThe d.r. values decreased to 1 : 1 after purification by silica gel column chromatography.hCompound 13: 1.31 g, 6.82 mmol, and 0.1 M.iIsolated combined yield of 12a + 12b.jThe d.r. values were determined from the crude 1H NMR spectrum of 12 obtained from the one-pot process.We initially investigated the desymmetric intramolecular Michael addition of 13 using (S)-Hayashi–Jørgensen catalysts,17 and found that the absolute configuration of the obtained cis-decalin was opposite to the required stereochemistry of the natural products (see Table S1 in the ESI†). In order to achieve the desired absolute configuration of the angular methyl group at C-6, (R)-cat.I was used for further screening. In the presence of this catalyst, the intramolecular Michael addition afforded 14a (96% e.e.) and 14b (75% e.e.) in a ratio of 10.3 : 1 and 52% combined yield (entry 1, Table 1). We assumed that the enantioselectivity of the reaction was controlled by the more sterically hindered aromatic group of (R)-cat.I, which protected the upper enamine face and allowed an endo-like attack by the si-face of cyclohexadienone, as shown in the transition state TS-A (Table 1). In order to increase the yield of this reaction and improve the enantioselectivity of 14b, we further screened solvents and additives. Increasing the catalyst loading from 0.5 to 1.0 equivalents and screening various reaction solvents did not improve the enantiomeric excess of 14b (entries 2–7, Table 1). Therefore, based on previous studies,11d,e we added 5.0 equivalents of acetic acid (AcOH) to a solution of compound 13 and (R)-cat.I in toluene, which improved the enantiomeric excess of 14b to 95% with a 60% combined yield (entry 8, Table 1). And, the stability of (R)-cat.I has also been verified in the presence of AcOH (see Table S2 in the ESI†). Further adjustment of the (R)-cat.I and AcOH amount and ratio (entries 9–12, Table 1) indicated that 0.2 equivalents each of (R)-cat.I and AcOH were the best conditions to achieve high enantioselectivity for both 14a and 14b, and it also increased the reaction yield (entry 12, Table 1). The enantioselectivity was not affected when the optimized reaction was performed on a gram scale: 14a (97% e.e.) and 14b (91% e.e.) were obtained in 80% isolated yield (entry 12, Table 1). We also found that the gram-scale experiments needed a longer reaction time which led a slight decrease of the diastereoselectivity. The purification of the cyclized products by silica gel flash column chromatography indicated that the major product 14a was epimerized and slowly converted to the minor product 14b (entry 11, Table 1). Both 14a and 14b are useful in the syntheses because the stereogenic center at C-2 will be converted to sp2 hybridized carbon in the following transformations. Therefore, the aldehyde group of analogues 14a and 14b was directly protected with 1,3-propanediol to give the respective enones 12a and 12b for use in the subsequent PEDA reaction.Afterward, we selected the major cyclized cis-decalins 12a and 12a′ (obtained by using (S)-cat.I in desymmetric intramolecular Michael addition, see Table S1 in the ESI†) as the dienophiles to prepare the tetracyclic naphthalene framework 10 through a sequence of Ti(Oi-Pr)4-promoted PEDA, dehydration, and aromatization reactions (Scheme 2). When using 3,6-dimethoxy-2-methylbenzaldehyde (11) as the precursor of diene, no reaction occurred between 12a/12a′ and 11 under UV irradiation at 366 nm in the absence of Ti(Oi-Pr)4 (Scheme 2A). In contrast, the 1,2-dihydronaphthalene compounds 16a and 16a′ were successfully synthesized when 3.0 equivalents of Ti(Oi-Pr)4 were used. Based on our previous studies,13a,e the desired hydroanthracenol 15a was probably generated through the chelated intermediate TS-B and the cycloaddition occurred through an endo direction (Scheme 2B).18 The newly formed β-hydroxyl ketone groups in 15a and 15a′ could then be dehydrated with excess Ti(Oi-Pr)4 to form enones 16a and 16a′. These results confirmed the pivotal role of Ti(Oi-Pr)4 in this PEDA reaction: it stabilized the photoenolized hydroxy-o-quinodimethanes and controlled the diastereoselectivity of the reaction.Open in a separate windowScheme 2PEDA reaction of 11 and enone 12.Subsequent aromatization of compounds 16a and 16a′ with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) at 80 °C afforded compounds 10a and 10a′ bearing a fused tetracyclic B–C–D–E skeleton. The stereochemistry and absolute configuration of 10a were confirmed by X-ray diffraction analysis of single crystals (Scheme 3). The synthesis of (+)-xestoquinone (2) and (+)-adociaquinones A (3) and B (4) was completed by forming the furan A ring. Compound 10 was oxidized using bubbling oxygen gas in the presence of t-BuOK to give the unstable diosphenol 17a, which was used without purification in the next step. The subsequent acid-promoted deprotection of the acetal group led to the formation of an aldehyde group, which reacted in situ with enol to furnish the pentacyclic compound 18 bearing the furan A ring. The stereochemistry and absolute configuration of 18 were confirmed by X-ray diffraction analysis of single crystals (Scheme 3). Further oxidation of 18 with ceric ammonium nitrate afforded (+)-xestoquinone (2) in 82% yield. Following the same reaction process, (−)-xestoquinone (2′) was also synthesized from 10a′ in order to determine in the future whether xestoquinone enantiomers differ in biological activity. Further heating of a solution of (+)-xestoquinone (2) with hypotaurine (9) at 50 °C afforded a mixture of (+)-adociaquinones A (3) (21% yield) and B (4) (63% yield). We also tried to optimize the selectivity of this condensation by tuning the reaction temperature and pH of reaction mixtures (see Table S3 in the ESI†). The 1H and 13C NMR spectra, high-resolution mass spectrum, and optical rotation of synthetic (+)-xestoquinone (2), (+)-adociaquinones A (3) and B (4) were consistent with those data reported by Nakamura,4a,g Laurent,4j Schmitz,4b Harada5a,c and Keay.5dOpen in a separate windowScheme 3Total synthesis of (+)-xestoquinone and (+)-adociaquinones A and B. 相似文献
Entry | Cat. (equiv.) | Additive (equiv.) | Solvent | Time | Yield/d.r. at C2b | e.e.c |
---|---|---|---|---|---|---|
1 | (R)-cat.I (0.5) | — | Toluene | 10.0 h | 52%/10.3 : 1 | 14a: 96%; 14b: 75% |
2 | (R)-cat.I (1.0) | — | Toluene | 4.0 h | 60%/10.0 : 1 | 14a: 93%; 14b: 75% |
3 | (R)-cat.I (1.0) | — | MeOH | 4.0 h | 47%/5.5 : 1 | 14a: 86%; 14b: −3% |
4 | (R)-cat.I (1.0) | — | DCM | 10.0 h | 28%/24.0 : 1 | 14a: 91%; 14b: 7% |
5 | (R)-cat.I (1.0) | — | Et2O | 10.0 h | 22%/22.0 : 1 | 14a: 91%; 14b: 65% |
6 | (R)-cat.I (1.0) | — | MeCN | 10.0 h | 12%/2.6 : 1 | 14a: 90%; 14b: 62% |
7 | (R)-cat.I (1.0) | — | Toluene/MeOH (2 : 1) | 4.0 h | 47%/10.0 : 1 | 14a: 87%; 14b: −38% |
8d | (R)-cat.I (1.0) | AcOH (5.0) | Toluene | 4.0 h | 60%e/2.1 : 1 | 14a: 96%; 14b: 95% |
9d | (R)-cat.I (0.5) | AcOH (2.0) | Toluene | 6.0 h | 75%e/4.0 : 1 | 14a: 97%; 14b: 91% |
10d | (R)-cat.I (0.5) | AcOH (0.2) | Toluene | 6.0 h | 73%e/4.3 : 1 | 14a: 96%; 14b: 92% |
11f | (R)-cat.I (0.5) | AcOH (0.2) | Toluene | 6.0 h | 75%e/8.0 : 1g | 14a: 95%; 14b: 93% |
12h | (R)-cat.I (0.2) | AcOH (0.2) | Toluene | 9.0 h | 80%i/6.0 : 1j | 14a: 97%; 14b: 91% |