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1.
Michael W. Fennie 《Tetrahedron》2005,61(26):6249-6265
Metal complexes of C2-symmetric Lewis acid/Lewis base salen ligands provide bifunctional activation resulting in rapid rates in the enantioselective addition of diethylzinc to aldehydes (up to 92% ee). Further experiments probed the reactivity of the individual Lewis acid and Lewis base components of the catalyst and established that both moieties are essential for asymmetric catalysis. These catalysts are also effective in the asymmetric addition of diethylzinc to α-ketoesters. This finding is significant because α-ketoesters alone serve as their own ligands to accelerate racemic 1,2-carbonyl addition of Et2Zn and racemic carbonyl reduction. The latter proceeds via a metalloene pathway, and often accounts for the predominant product. Singular Lewis acid catalysts do not accelerate enantioselective 1,2-addition over these two competing paths. The bifunctional amino salen catalysts, however, rapidly provide enantioenriched 1,2-addition products in excellent yield, complete chemoselectivity, and good enantioselectivity (up to 88% ee). A library of the bifunctional amino salens was synthesized and evaluated in this reaction. The utility of the α-ketoester method has been demonstrated in the synthesis of an opiate antagonist. 相似文献
2.
An efficient strategy enabling the construction of polysubstituted 1,2-dihydroquinoline derivatives mediated by the commercially available, economical, and environmentally benign iodine (3 mol %) under mild conditions was developed. Various arylamines and α-ketoesters could participate in this cascade reaction, and the desired products were obtained in excellent yields up to 98%. 相似文献
3.
A simple and efficient transesterification process for the synthesis of β-ketoester derivatives has been achieved by the reaction of methyl β-ketoester with higher alcohols at 110 °C under solvent-free, catalyst-free conditions and its application in synthesis of 3,4-dihydropyrimidin-2(1H)-ones C-5 ester derivatives via Biginelli reaction has been described. 相似文献
4.
The (132R)-methoxycarbonyl group of methyl pheophorbide a, one of the chlorophyll-a derivatives, was converted to a methyl group through methylation at the C132 position followed by removal of the methoxycarbonyl group. The methylation of the C132 carboanion gave a 4:1 mixture of methyl 132-methyl-pheophorbide a and its 132-epimer. The successive pyrolysis of the major methylated product afforded methyl (132R)-methyl-pyropheophorbide a with a small amount of its (132S)-epimer. The substitution effects at the C132 position including stereochemistry were discussed on the basis of 1D/2D NMR, UV–vis absorption, and circular dichroism spectroscopic analyses as well as molecular modeling simulation. 相似文献
5.
Han-Feng Cui 《Tetrahedron letters》2010,51(17):2374-1548
A Lewis acid-catalyzed one-pot sequential transformation of β-keto esters, aromatic aldehydes, and NCS/NBS was reported. The reaction proceeds by way of Knoevenagel condensation/Nazarov cyclization/halogenation to give α-chloro- and α-bromo-β-keto esters in moderate yields with high diastereoselectivities. However, several aromatic aldehydes with electron-withdrawing substituents afforded unexpected α,β′-dichloro-β-keto esters in good to high yields. 相似文献
6.
Cristian L. HarrisonMariusz Krawiec Raymond E. ForslundWilliam A. Nugent 《Tetrahedron》2011,67(1):41-47
A simple route was developed for the synthesis of optically enriched β-amino-α-keto amides. The highly diastereoselective addition of alkyl dithiolanecarboxylates to optically enriched sulfinimines affords the corresponding β-amino-α-keto esters in which the ketone carbonyl group is protected as a dithiolane. Amidation of the ester functionality with a primary amine proceeds readily at room temperature. Treatment with HCl cleaves the N-S bond of the sulfinyl group to provide a free amine functionality, which can then be incorporated into a peptide. Removal of the dithiolane protecting group under oxidative conditions proceeds without epimerization as exemplified by a model dipeptide. 相似文献
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8.
Aline de Souza Ramos Joyce Benzaquem Ribeiro Leonardo Vazquez Selma Gomes Ferreira Leite Francisco Radler de Aquino Neto 《Tetrahedron letters》2009,50(52):7362-6129
The enantioselective reduction of ethyl benzoylacetate (EBA) into ethyl (S)-3-hydroxy-3-phenylpropanoate (S-HPPE) by nine yeast strains and three filamentous fungi strains is described. The conversion obtained was in the range 0-89% and the enantiomeric excess was 100% in many cases. Conversion levels were higher when the reduction was performed with microorganisms immobilized in calcium alginate and enantioselectivity remained excellent. Some reaction’s conditions of bioreduction by immobilized cells of Rhodotorula rubra were studied using a 25-2 fractional factorial design. 相似文献
9.
An efficient one-pot four-step domino reaction of substituted β-ketoesters has been optimized giving rise to a large panel of C2-substituted 3-methylcyclohex-2-enones, an important scaffold for the preparation of various initiators for cationic or radical cyclizations. The developed methodology is quite general and applicable to a wide range of β-ketoester substrates, allowing the introduction of various functionalities at the C2 position of the 3-methylcyclohex-2-enones, in good to excellent yields. 相似文献
10.
Copper(I) promoted Huigsen 1,3-dipolar cycloaddition of terminal alkyne with azide is utilized as an efficient protocol for the one-pot five-component synthesis of glycoside annulated dihydropyrimidinone derivatives with 1,2,3-triazol linkage accomplished by tert-butyl β-ketoester, arylaldehyde, urea, propargyl alcohol, and glycosyl azide through the combination of transesterification and Biginelli reaction in aqueous medium. This protocol provides an access to generate scaffolds with molecular diversity from readily available starting materials. 相似文献