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181.
A simple transformation of the eight-carbon side chain of a natural spirostan sapogenin into the cephalostatin north 1 spiroketal moiety is described. This methodology, based on an intramolecular hydrogen abstraction reaction promoted by alkoxy radicals, permits the synthesis of C-22 and C-25 stereoisomers of the dioxaspiro[4.4]nonane cephalostatin ring system. The acid-catalyzed isomerization of the spirocenter in the different isomers is studied. [reaction: see text] 相似文献
182.
Díaz-Requejo MM Caballero A Belderraín TR Nicasio MC Trofimenko S Pérez PJ 《Journal of the American Chemical Society》2002,124(6):978-983
In situ prepared copper catalysts Tp(X)Cu (Tp(X) = homoscorpionate) catalyze the olefin cyclopropanation reaction using ethyl diazoacetate as the carbene source. Very high values of both activity and diastereoselectivity toward the cis isomer have been obtained for styrene, alpha-methylstyrene, 1-hexene, 1-octene, vinyl acetate, n-butyl vinyl ether, 2,5-dimethyl-2,4-hexadiene, and 3,3-dimethyl-1-butene. The effect of the temperature in the diastereoselectivity was almost negligible within the range -10 to +30 degrees C. Kinetic studies have allowed us to propose that the homoscorpionate ligand might act in a dihapto form during the catalytic process. This transformation seems to operate under kinetic control, where the formation of the cis isomer would govern the reaction rate. 相似文献
183.
Díaz-Requejo MM Belderraín TR Nicasio MC Trofimenko S Pérez PJ 《Journal of the American Chemical Society》2002,124(6):896-897
A series of catalysts of general formula TpXCu (TpX = homoscorpionate ligand) promote the insertion of :CHCO2Et (ethyl diazoacetate as the carbene source) into the C-H bonds of cycloalkanes and cyclic ethers in moderate to high yield. A correlation between the steric hindrance of these catalysts and the yield of the transformation has been observed. 相似文献
184.
M. I. Sández A. Suárez M. A. Rios M. C. Balo F. Fernández Carmen López 《Photochemistry and photobiology》1996,64(3):486-491
Abstract— Various newly synthesized substances containing the same chromophore as PRODAN were subjected to a spectroscopic study. The new substances, ω-[6-( N -meth-yl- N -alkylamino)naphthoyl]alcanoic acids (MANA), possess polar groups and alkyl chains of variable length that make them potentially useful as fluorescent probes in membrane studies. This paper reports on their spectroscopic properties in various pure solvents. Their Stokes shifts are given and related to different physicochemical properties of the solvents. A theoretical study of the behavior of the new substances is also presented. 相似文献
185.
Benito Alcaide Carmen Lpez-Mardomingo Berta Lpez-Sez Rafael Prez-Ossorio Joaquín Plumet 《Journal of heterocyclic chemistry》1985,22(2):289-291
Chiral α-aminoalcohols obtained by stereoselective reduction of α-iminoketones have been condensed with oxalyl chloride to afford 4-substituted 5,6-diphenylmorpholine-2,3-diones having two or three chiral centers. 相似文献
186.
Alexander McKillop Alan Henderson Partha S. Ray Carmen Avendano Encarnacion G. Molinero 《Tetrahedron letters》1982,23(33):3357-3360
Ethyl carboethoxyformimidate is shown to be a versatile reagent for the synthesis of a variety of mono- and bicyclic heterocyclic systems. 相似文献
187.
Filip X Tripon C Filip C 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2005,176(2):239-243
A new heteronuclear decoupling mechanism under fast magic-angle spinning MAS is introduced. It is based on refocusing the coherences responsible for the dephase of low-gamma nuclei ((13)C, (15)N) transverse spin-polarization in the presence of strongly dipolar-coupled protons, and has the advantage that can be implemented by pulsed techniques, with all the benefits resulting from a reduced duty cycle compared with conventional decoupling by continuous rf irradiation. The decoupling efficiency of a simple rotor-synchronized Hahn-echo pulse train is analyzed both theoretically and experimentally. It was found that a substantial improvement in sensitivity and resolution can be achieved in compounds with small (1)H chemical shielding parameters even at moderate sample spinning, and some interesting applications are shown. It is also shown that much faster spinning frequencies, or alternative refocusing sequences, are needed for applications on rigid organic solids, i.e., in systems with larger (1)H chemical shifts. 相似文献
188.
This article reviews the most relevant chemical and structural aspects that influence the spin-crossover phenomenon (SCO). Special attention is focussed on the recent development of SCO coordination polymers. The different approaches currently being explored in order to achieve multifunctionality in SCO materials are discussed. 相似文献
189.
Canevali C Mari CM Mattoni M Morazzoni F Nodari L Ruffo R Russo U Scotti R 《The journal of physical chemistry. B》2005,109(15):7195-7202
The mechanism of NO interaction with nanosized Ru(Pd,Pt)-doped SnO(2) was studied by electron paramagnetic resonance, M?ssbauer, and electric resistance measurements. Three steps were proposed for the reaction between the semiconductor oxide and the gaseous component: (i) the formation of bielectronic oxygen vacancies (V(o)) in SnO(2); (ii) their single-ionization (V(o)(*)) with injection of electrons into the SnO(2) conduction band; (iii) the subsequent transfer of electrons from V(o)(*) to [Ru(Pd,Pt)](4+). The last process induces the formation of further oxygen vacancies which reduce the transition metal centers to lower oxidation states; the redox processes is enhanced and the electrical resistance in transition metal-doped SnO(2) is stronger modified with respect to the undoped material. 相似文献
190.
Liu JF Ye P Sprague K Sargent K Yohannes D Baldino CM Wilson CJ Ng SC 《Organic letters》2005,7(15):3363-3366
[reaction: see text]. Total syntheses of deoxyvasicinone (1), mackinazolinone (2), and 8-hydroxydeoxyvasicinone (3) via novel microwave-assisted domino reactions, as well as a novel three-component one-pot total synthesis of isaindigotone (5) promoted by microwave irradiation, are reported. The efficient reaction process enabled us to rapidly access related natural product derivatives and to identify a new class of cytotoxic agents. 相似文献