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The first diastereoselective synthesis of
xylo-(2R,3S,4S)-C18-phytosphingosine (1) has been achieved by double stereodifferentiation of enantiomerically enriched terminal olefin 14 using (DHQD)2–PHAL ligand in an asymmetric dihydroxylation with a diastereomeric ratio of 83:17. This phytosphingosine was fully characterized by the physical and spectral data of the corresponding tetraacetate 21. 相似文献
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Toshiyuki OhfusaAtsushi Nishida 《Tetrahedron》2011,67(10):1893-1906
Several bicyclic compounds were synthesized by the Diels-Alder reaction using aminodiene and a cyclic dienophile. The stereochemistries of the obtained adducts were determined by X-ray crystallography or NMR analysis. The stereoselectivity of this Diels-Alder reaction was based on the interaction of molecular orbitals between the diene and dienophile. The reactivities of these Diels-Alder reactions were estimated, and the generality of this reaction is discussed. 相似文献
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The aldol reaction between acetone and 4-nitrobenzaldehyde catalyzed by single l-prolinamide and its zinc complexes has been studied. An increase in the rate and the stereoselectivity of the reaction has been shown by using zinc derivatives. A mechanistic proposal, based on NMR and ESI studies, has been put forward to explain the experimental data: zinc-prolinamide complexes catalyze the reaction following the general mechanism of stereoselective enamine nucleophilic addition to the acceptor aldehyde. Zn2+ prevents the nonspecific base-catalyzed reaction by diminishing the basicity of the amine nitrogen of prolinamide. 相似文献
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The stereoselectivity of some Diels-Alder reactions was investigated by means of ABEEM-oTr model. Combined with local hard-soft and acid-base (HSAB) principle, we made reasonable explanation by calculating local softness of atom and bond regions for the stereoselectivity of four Diels-Alder reactions involving cyclopentadiene. 相似文献
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Dmitry Yu. Murzin Irina L. Simakova 《Journal of molecular catalysis. A, Chemical》2008,286(1-2):156-161
Kinetic peculiarities of stereoselectivity in hydrogenation of fatty acids are discussed. The reaction mechanism should be able to account for independence of the reaction rate on hydrogen pressure and conversion, stereoselectivity on conversion as well as an increase in cis/trans ratio with pressure increase. Various mechanisms advanced in the literature for formation of cis- and trans mono-unsaturated acids and their derivatives are considered, showing behavior contradicting with experiments. A reaction mechanism is proposed, which is consistent with experimental observations. 相似文献
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Catalytic properties of a series of iron(III)-salen (salen=N,N'-bis(salicylidene)ethylenediamine dianion) and related complexes in asymmetric sulfoxidation reactions, with iodosylarenes as terminal oxidants, have been explored. These catalysts have been found to efficiently catalyze oxidation of alkyl aryl sulfides to sulfoxides with high chemoselectivity (up to 100 %) and moderate-to-high enantioselectivity (up to 84 % with isopropylthiobenzene and iodosylmesitylene), the TON (TON=turnover number) approaching 500. The influence of the ligand (electronic and steric effects of the substituents), oxidant, and substrate structures on the oxidation stereoselectivity has been investigated systematically. The structure of the reactive intermediates (complexes of the type [Fe(III)(ArIO)(salen)] and the reaction mechanism have been revealed by both mechanistic studies with different iodosylarenes and direct in situ (1)H NMR observation of the formation of the reactive species and its reaction with the substrate. 相似文献
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Venkatachalam Tamilmani Durairajan Senthilnathan Ponnambalam Venuvanalingam 《Journal of Chemical Sciences》2008,120(2):225-236
Acridizinium cation (ACR) and 2,3-dimethylisoquinolinium ion (DMIQ) undergo [2 + 4+] addition with dienophiles methyl vinyl ether (MVE) and propylene (PY) and the addition takes place across the diene unit
containing the cationic centre and the dienophile acts as the electron donor. These reactions have two regiochemical paths
and in each of them two possible stereo adducts could be formed. DFT calculations performed at B3LYP/6-31G(d) level have shown
that the reactions pass through concerted mechanism and the TSs are highly asynchronous. Methoxy group in the dienophile can
take up cis or trans orientation with respect to the double bond and in that trans orientation of the methoxy group is preferred. Calculations further show that syn 2 adduct is kinetically and thermodynamically more favoured in both the reactions in excellent agreement with the experimental
observations. ACR is found to be more reactive than DMIQ as a diene and as a dienophile MVE is found to be more reactive than
PY. Computed bond orders establish that the syn 2 transition states are the most ‘reactant’ like. Though the reactions have both electrostatic control and frontier orbital
control the former dominates in the initial stages of the reaction.
Presented in the Theoretical Chemistry Symposium 2006 held during 11–13 December 2006 in Tiruchirappalli, India 相似文献
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Didier Bruyère 《Tetrahedron》2004,60(18):4007-4017
We have studied the remarkable dependence of the stereochemistry of the cyclization on the double bond geometry and of the effect of the bulkiness of the nucleophile on the regiochemistry of the palladium mediated cyclization of alkenes bearing aryl bromides and nucleophiles. In contrast, the cyclization of the acetylenic homologous substrates is not dependent on the nature of the nucleophile. 相似文献
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Tahar AyadYves Génisson Alexandre VerduMichel Baltas Liliane Gorrichon 《Tetrahedron letters》2003,44(3):579-582
The use of a trans α,β-epoxyaldehyde as a precursor of d-ribo-phytosphingosines was studied. The highly stereoselective alkylation of the aldehyde with a diorganozinc reagent was ensured through double asymmetric induction. The regioselective opening of the epoxide in turn gives access to an azide from which a C10 phytosphingosine can be easily prepared. 相似文献
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The substrate selectivity and stereoselectivity of a series of ketoreductases were evaluated toward the reduction of two sets of β-ketoesters. Both the structural variety at β-position and the substituent at α-position greatly affected the activity and stereoselectivity of these ketoreductases. For the first set of β-ketoesters, at least one ketoreductase was found that catalyzed the formation of either (d) or (l) enantiomer of β-hydroxyesters from each substrate with high optical purity, with the only exception of ethyl (d)-3-hydroxy-3-phenylpropionate. For the second set of β-ketoesters with α-substituents, the situation is more complex. More commonly, a ketoreductase was found that formed one of the four diastereomers in optically pure form, with only a few cases in which enzymes could be found that formed two or more of the diastereomers in high optical purity. The continued development of new, more diverse ketoreductases will create the capability to produce a wider range of single diastereomers of 2-substituted-3-hydroxy acids and their derivatives. 相似文献
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Hyunji Lee You-Kyoung Lee Dong-Guk Kim Mi-Sun Son Tae-gyu Nam Byeong-Seon Jeong 《Tetrahedron letters》2014
An asymmetric synthetic route for 1-iodofarnesane, a key intermediate for tocopherol side chain synthesis, starting from (+)-(R)-citronellal was developed. 1-Iodofarnesane was prepared through eight steps in about 50% overall yield, and asymmetric transfer hydrogenation of the enal with a chiral organocatalyst was conducted as a stereoinduction step. To measure the stereoinduction level and optical purity of the product, a convenient analytical method was developed in which a phenylcarbamate derivative of the C15 alcohol was found to be suitable to give proper polarity and UV-activity for chiral UV-HPLC analysis. 相似文献
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Jie Chen 《Tetrahedron》2008,64(37):8899-8906
An efficient electrophilic iodocyclization of alkylidenecyclopropyl ketones with N-iodosuccinimide (NIS) or I2 in aqueous CH3CN affording 3-oxabicyclo[3.1.0]hexan-2-ols is described. NIS is a better electrophilic iodocyclization reagent than I2. Four chiral centers were formed within one step. The stereochemistry was established by the X-ray diffraction studies of compounds 2e-2h, 2n, and 2c. It is quite interesting to observe that the substituent of the cyclopropane ring plays an important role in determining the relative stereochemistry at the 4-position: with R2 being an acyl or ester group a mixture of (1S∗,2R∗,4S∗,5R∗)-2 (major) and (1R∗,2R∗,4R∗,5R∗)-2 (minor) was formed with moderate selectivity while the reaction of the substrates with R2 being sulfonyl and p-methylphenylsulfonyl or R1 being phenyl afforded (1R∗,2R∗,4S∗,5S∗)-2 or (1S∗,2R∗,4S∗,5R∗)-2f as the only product. The reaction is general for a range of different substrates to afford the products in moderate to high yields. 相似文献
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由于其结构多样性,有机磷化合物作为配体、催化剂、辅助剂、添加剂、底物以及试剂等在不对称反应中均获得了成功的应用,从而使有机磷试剂在不对称反应研究领域占有举足轻重的地位.本文全面介绍了本课题组近几年来围绕有机磷试剂在不对称反应中的应用,在醛的不对称硅氰化反应、内消旋环氧烷不对称开环反应、潜手性酮的不对称硼烷还原、不对称Friedel-Crafts烷基化、对映选择Mitsunobu、不对称aza-Morita-Baylis-Hillman、不对称aza-Henry以及硝基烯的不对称Michael加成反应等方面所取得的一些研究结果. 相似文献