全文获取类型
收费全文 | 173篇 |
免费 | 1篇 |
专业分类
化学 | 147篇 |
力学 | 6篇 |
数学 | 14篇 |
物理学 | 7篇 |
出版年
2021年 | 2篇 |
2020年 | 1篇 |
2016年 | 1篇 |
2015年 | 2篇 |
2014年 | 6篇 |
2013年 | 3篇 |
2012年 | 2篇 |
2011年 | 2篇 |
2010年 | 3篇 |
2009年 | 2篇 |
2008年 | 1篇 |
2007年 | 3篇 |
2006年 | 1篇 |
2004年 | 5篇 |
2003年 | 5篇 |
2001年 | 1篇 |
2000年 | 7篇 |
1999年 | 6篇 |
1998年 | 1篇 |
1997年 | 2篇 |
1996年 | 7篇 |
1995年 | 6篇 |
1994年 | 5篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1991年 | 5篇 |
1990年 | 8篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 16篇 |
1986年 | 3篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1982年 | 8篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 1篇 |
1970年 | 2篇 |
1964年 | 1篇 |
排序方式: 共有174条查询结果,搜索用时 15 毫秒
81.
Summary A reversed-phase HPLC method has been developed suitable for a reliable quality control of pharmaceutical and dietary formulations containing the synthetic sweeteners aspartame and saccharin. The proposed method is able to separate acesulfame, aspartame and saccharin, and their impurities such as 5-benzyl-3,6-dioxo-2-piperazineacetic acid (the major degradation product of aspartame) and 4-sulphamoylbenzoic acid,o- andp-toluenesulphonamides (the synthesis impurities of saccharin). A convenient solid-phase extraction (SPE) procedure using C-18 sorbent, was also developed for the determination of potential saccharin impurities. 相似文献
82.
The title sponge is shown to contain eight new sesquiterpenoids for which a common, unusual biogenetic origin is postulated. The compounds are shown to be: (–)-(1R*,4R*)-3-(3′-furyl)methyl-2-p-menthen-7-yl acetate ((–)- 8b ); two diols separated as the monoacetates (–)-(1S*,4R*)-3-(3′-furyl)methyl-l-hydroxy-2-p-menthen-7-yl acetate ((–)- 13a ) and the (–)-(1R*,4R*)-epimer (–)- 13b , the two C(4)-epimeric 4-ethoxy-3-(1′(7′),2′-p-menthadien-3′-yl)methyl-2-buten-4-olides ((+)- 14a and (–)- 14b ), (–)-3-(3′-furyl)methyl-7-nor-2-p-menthen-l-one ((–)- 11 ), (–)-(3Z)-1-(3′-furyl)-4,8-dimethylnona-3, 7-dien-2-yl acetate ((–)- 17 ), and (+)-3-(5′,7′-seco-2′(10′)-pinen-7′-yl)methylfuran ((+)- 15 ). 相似文献
83.
The structure of rogioldiol A ((?)- 1 ), isolated from the red seaweed Laurencia microcladia, was determined. Employing the exciton-coupling technique for rogioldiol A p-bromobenzoate ( 2 ), the absolute configuration at C(9) of (?)- 1 was assigned, and, together with extensive NMR experiments, the absolute configuration at C(10) and preferred conformations of (?)- 1 were determined. The absolute configuration of the hetero-substituted cyclohexane ring was deduced in analogy from the X-ray structure of 4 , a derivative of the aldehyde 3 , which was isolated from the same seaweed and is believed to be a degradation product of (?)- 1 . 相似文献
84.
The Mediterranean stolonifer Sarcodictyon roseum (= Rolandia rosea) (Cnidaria, Anthozoa, Alcyonaria, Stolonifera, Clavulariidae) is shown to contain two novel diterpenoidic alcohols esterified by (E)-N(1)-methyl-urocanic acid (= E)-3-(l-methyl-lH-imidazol-4-yl)acrylic acid). They are sarcodictyin A ( = (?)-(4R,4a,R, 7R,10S,11S,12aR,lZ,5E,8Z)-7,10-epoxy-3,4,4a,7,10,11,12,12a-octahydro-7-hydroxy-6-(methoxycarbonyl)-1,10-dimethyl-4-(1-methylethyl)benzocyclodecen-11-yl (E)-3-(1-methyl-lH-imidazol-4-yl)acrylate; (?)- 1 ) and sarco-dictyin B (the 6-(ethoxycarbonyl analogue; (?)- 2 ). The assignment of the structures is mainly based on 1D- and 2D-NMR data, as well as on chemical transformations of (?)- 1 , such as transesterification with MeONa/MeOH giving methyl (E)-N(1)-methylurocanate ( 3 ) and the free alcohol (+)- 4 and reduction with LiAlH4 followed by benzoylation giving dibenzoate 7. Absolute configurations are based on Horeau's method of esterification of (+)- 4 . 相似文献
85.
It is shown here that the stoloniferan coral Sarcodictyon roseum of east Pyrenean waters contains four novel diterpenoids, sarcodictyin C ((?) -3 ), D ((?) -4 ), E ((+)- 5 ), F ((+)- 6 ), which are related to sarcodictyin A ( = (?)-(4R,4aR,7R,10S,11S,12aR,1Z,5E,8Z-7,10-epoxy-3,4,4a,7,10,11,12,12a-octahydro-7-hydroxy-6-(methyoxycarbonyl)-1,10-dimethyl-4-(1-methylethyl)-benzocyclodecen-11-yl (E)-N1-methylyrocanate; ((?)? 1 ), previously isolated from the same coral. Sarcodictyin C ((?) -3 ) and D ((?) -4 ) and the 3α-hydroxy and 3α-acetoxy derivatives of (?) -1 ), sarcodictyin E ((+) -5 ) is the (Z)-urocanate isomer of (?) -3 ), and sarcodictyin F ((+) -6 ) is the 1α-hydroxy-2-ene isomer of (?) -3 . In all cases, the nine-membered ring is locked, and the molecule stabilized, by the urocanic appendage; when this is removed in MeOH/KOH, the C(11)–O? function is free to attack at C(5), and retro-condensations then lead to the ring-contracted butenolides 11 (from (?) -3 ) or 10 (from(?) -1 ) with extrusion of the hydroxyfuran nucleus (Scheme 3). Under the same conditions, with (?) -3 , the C(3)-O? group competitively attacks at C(5), the hydroxyfuran nucleus is expelled, and aldehyde 14 is formed. Peculiarly, in the reaction of (?) -3 with MeOD/KOD, the ring-contracted butenolide 17 contains D at the 4′-ax position. The sarcodictyins are unique in these chemical properties, not shared by the cladiellanes which have the same C-skeleton. 相似文献
86.
Temperature-dependent NMR spectra indicate that the α-chamigren-3-ones (?) -11 , (+) -12 , (+) -14 (?) -15 , (+) -16, 18 , and 19 bearing equatorial halogen atoms at C(8) and C(9) undergo slow conformational flipping of the envelope-shaped enone ring, while the cyclohexane ring is maintained in the chair conformation. The α-chamigren-3-ols (+) -20 and (+) -21 , obtained by hydride reduction of (+) -12 , behave similarly, with slow half-chair inversion of the cyclohexenol ring. In each case, both conformers are about equally populated and detectable by NMR, except in the case of (+) -15 , where repulsive interactions between Br? C(2) and Heq?C(7) make the population of the conformer 15b with Me—C(5) faced to Hax?C(10) so low that it escapes direct 1H-NMR detection. The energy barriers to these conformational motions are viewed to arise mainly from repulsive interactions between Me—C(5) and the axial H-atoms at C(8) and C(10), while, contrary to previous beliefs, no twist-boat conformations of the cyclohexane ring intervene. Similar conclusions hold for the 4,5-epoxides of both (?) -6 and (+) -7 . Clean Jones oxidatio of (?) -2 to 17 , where the CH2?C(5) bond is maintained, and acid dehydration-isomerization of the α-chamigrene (+) -21 to the β-chamigrene (+) -24 , reflect the special stability of β-chamigrenes, providing a reason for their frequent occurrence in nature. 相似文献
87.
Antonio Guerriero Michele D'Ambrosio Vincenzo Cuomo Fortunato Vanzanella Francesco Pietra 《Helvetica chimica acta》1989,72(3):438-446
Further investigation of global extracts from cultures of the marine deuteromycete Dendryphiella salina leads to the isolation of three novel trinor-eremophilanes esterified by branched C9-carboxylic acids, dendryphiellin B (= (+)-(1R*,2S*,7R*,8aR*)-1,2,6,7,8,8a-hexahydro-7-hydroxy-1,8a-dimethyl-6-oxonaphthalen-2-yl (6R*, 2E, 4E)-6-hydroxy-6-methylocta-2,4-dienoate; (+)- 2 ), dendryphiellin C (=(+)-(1R*, 2S*, 7R*, 8aR*)-1,2,6,7,8,8a-hexa-hydro-7-hydroxy-1,8a-dimethyl-6-oxonaphthalen-2-yl (6S, 2E, 4E)-6-methylocta-2,4-dienoate; (+)- 3 )), and dendryphiellin D (=(+)-(1R*, 2S*, 7R*, 8aR*)-1,2,6,7(8,8a-hexahydro-7-hydroxy-1,8a-dimethyl-6-oxonaphthalen-2-yl (6R*,2E,4E)-6-(hydroxymethyl)octa-2,4-dienoate; (+)- 4 ). An intact eremophilane, dendryphiellin E ( 5 ), and its ethanolysis product dendryphiellin F whose absolute configuration is represented by structural formula (+)- 6 are also isolated from the above extracts. Dendryphiellin E exists as an open form 5a in equilibrium with a closed form 5b . A similar equilibrium exists between the open form 8a and the closed form 8b of a non-esterified eremophilane, dendryphiellin G ( 8 ), which is isolated too from the above extracts and proves structurally related to the cyclic portion of 5 . Finally, the free, branched C9-carboxylic acids dendryphiellic acid A ((+)- 9 ) and B ((+)- 10 ) which correspond to side chains of the above esterified terpenes are also isolated from the above extracts. 相似文献
88.
Michele D'Ambrosio Antonio Guerriero Francesco Pietra Marina Ripamonti Ccile Debitus Jean Waikedre 《Helvetica chimica acta》1996,79(3):727-735
Agelastatin A ( 1 ), an unusual alkaloid of the axinellid sponge Agelas dendromorpha from the Coral Sea, can be selectively acetylated (→ 7 ) or methylated at OH? C(8a) (→ 4 ), peracetylated (→ 8 ) or permethylated at OH? C(8a), NH(5), and NH(6) (→ 5 ), or, finally, subjected to C(9)? C(8a) (→ 14 ) or C(5b)? C(8a) β-elimination (→ 11–13 ), in a regiospecific manner or not, depending on the reaction conditions. Under acidic conditions, compound 12 adds H2O or MeOH, regioselectively though not endo/exo stereoselectively, giving transoid/cisoid mixtures 1/18 or 4/19 , respectively. Similarly 11 or 13 add MeOH to give mixtures (?)- 2/20 or 15/16 , respectively. Compound 13 also adds AcOH giving mixture 8/17 . The intermediate cisoid form obtained on treatment of 21 with H3O+ undergoes N(5)? N(6) bridging affording pentacyclic 22 which constitutes a proof for the cisoid configuration. From conformational studies, rules are devised that allow assigning the configuration of these compounds from NMR data. In vitro comparative cytotoxicity assays of these compounds show that for high cytotoxic activity, such as of 1 in vivo, unsubstituted OH? C(8a), H? N(5), H? N(6) moieties are needed in the natural B/D transoid configuration. 相似文献
89.
S. Della Pietra V. Della Pietra L. Alvarez-Gaumé 《Communications in Mathematical Physics》1987,109(4):691-700
For {
y
},y, a one parameter family of invertible Weyl operators of possibly non-zero index acting on spinors over an even dimensional compact manifoldX, we express the phase of the chiral determinant det
–
in terms of the invariant of a Dirac operator acting on spinors over ×X.Supported in part by NSF Grant No. PHY-82-15249Supported in part by NSF Grant PHY 8605978 and the Robert A. Welch Foundation 相似文献
90.
Pietra R. Sabbioni E. Springer A. Ubertalli L. 《Journal of Radioanalytical and Nuclear Chemistry》1982,69(1-2):365-379
Severe analytical problems are associated with the analysis of heavy metals at very low levels in biological samples. This
impose a high degree of sophistication of the NAA involving treatment of the sample before irradiation such as chemical separations,
physical treatments and biochemical fractionation, the experimental evaluation of interfering nuclear reaction as well as
the development of post-irradiation radiochemical separation. This paper treats of the problem related to the NAA of trace
elements in environmental biochemical toxicology focussing attention of the sources of the elements during the analysis. Typical
results and short discussion for every step of the analysis are reported. 相似文献