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981.
Five new substances, panicein-A, -B1, -B2, -B3 and C, containing an aromatic sesquiterpenoid moiety linked to a quinol or a quinone system, have been isolated from the sponge Halichondria panicea. The structures of these compounds were elucidated from their chemical and spectral properties. 相似文献
982.
983.
Andrea Goti Stefano Cicchi Alberto Brandi
K. Michal Pietrusiewicz
《Tetrahedron: Asymmetry》1991,2(12):1371-1378The cycloadditions of nitrones with 2,3-dihydro-1-phenyl-1H-phosphole 1-oxide give a single cycloadduct deriving from a highly diastereoselective approach of the nitrone anti to the phenyl ring of phospholene oxide. When the chiral gliceraldehyde derived nitrone is used, only two diastereoisomers are produced in 1.7:1 ratio. The structural assignment based on NMR data and X-ray analysis of the major isomer established a trans C3–C4 stereochemistry (derived from endo TS with respect to nitrone) and a C3–C4′ relative stereochemistry of threo type in the major isomer and erythro in the minor one. Therefore, each enantiomer of phospholene oxide 6 gives exclusively one cycloadduct with five contiguous stereogenic centres in an established and predictable absolute configuration. The difference of reactivity of the two enantiomers allowed a partial kinetic resolution of the racemic phospholene oxide, affording (+)-(S) enantiomer with 90% enantiomeric excess. 相似文献
984.
A novel pentacyclic saturated sesterpene condensed with hydroxyhydroquinone moiety, which we named disidein (1), has been isolated from the marine sponge Disidea pallescens as disulfate sodium calcium salt. A minor accompaining compound, which seems a likely precursor of disidein, has been identified as 2-pentaprenylbenzoquinone (7). 相似文献
985.
Sokrates T. Pantelides Bernhard Fischer Roger A. Pollak Thomas H. Di Stefano 《Solid State Communications》1977,21(11):1003-1006
Theoretical calculations are reported for SiO2, GeO2 and the intermediate composition Si.5Ge.5O2 which reproduce the main observed features and trends in experimental photoemission spectra. The agreement between the two establishes the importance of band theory in understanding the electronic structure of these materials, and demonstrates that detailed quantitative predictions are feasible for such complex materials in terms of the empirical tight-binding method. The calculations further establish that the structure in the valence bands is determined mainly by nearest- neighbor oxygen-oxygen interactions. 相似文献
986.
987.
Emanuela Licandro Stefano Maiorana Antonio Papagni Daniele Tarallo Alexandra M. Z. Slawin David J. Williams 《Journal of heterocyclic chemistry》1990,27(4):1103-1107
This paper reports the first example of Wittig reaction on arylazomethylenetriphenylphosphoranes, producing the new cinnolines 6 and 7 . A new unexpected rearrangement reaction producing quinoline 8 is also reported. The structure of compound 8 was determined by X-ray analysis. 相似文献
988.
Fabio Marchesoni Massimo Sparpaglione Stefano Ruffo Paolo Grigolini 《Physics letters. A》1982,88(3):113-116
New theoretical insights into the generalized Langevin approach to the onset of chaos are given. To make it possible to compare theory with “experiments” a fast computer algorithm is set up. 相似文献
989.
Alessandro De Robertis Concetta De Stefano Carmelo Rigano Silvio Sammartano 《Journal of solution chemistry》1990,19(6):569-587
The protonation constants of 21 carboxylic acids (formic, acetic, propionic, benzoic, phenoxyacetic, salicylic, oxalic, malonic, succinic, itaconic, malic, tartaric, oxydiacetic, thiodiacetic, thiodipropionic, phthalic, maleic, citric, 1,2,3-tricarboxylic, 1,2,4-tricarboxylic and 1,2,4,5-tetracarboxylic), have been determined potentiometrically, by pH-metric measurements, at several temperatures and ionic strengths, 5T55°C, 0<11 mol-dm–3, using tetraethylammonium iodide as background salt. General equations for the dependence on ionic strength of thermodynamic parameters have been found. The statistical significance of results and the possibility of using a simple model for the thermodynamics of carboxylic acids protonation, is discussed. 相似文献