Search results for "enantioselective synthesis"
showing 10 items of 418 documents
Dual stereocontrolled alkylation of aldehydes with polystyrene-supported nickel complexes derived from α-amino amides
2015
Nickel(II) complexes derived from α-amino amide ligands anchored to gel-type and monolithic polymers act as efficient catalysts for the enantioselective addition of dialkylzinc reagents to aldehydes. Similar to the analogous homogeneous systems, dual stereocontrol in addition products can be achieved by controlling the stoichiometry of the immobilized nickel complex. Aromatic and aliphatic aldehydes were alkylated in good yields with enantioselectivities comparable to those obtained with the homogeneous analogues. These polymer-supported catalysts offer significant advantages as no metal leaching is observed and they can be easily recovered from the reaction mixture by simple filtration and…
Supported Ionic Liquid Asymmetric Catalysis. A New Method for Chiral Catalysts Recycling. The Case of Proline-Catalyzed Aldol Reaction.
2004
A new method for chiral catalysts recycling, based on the supported ionic liquid asymmetric catalysis concept, has been developed. This concept involves the treatment of a monolayer of covalently attached ionic liquid on the surface of silica gel with additional ionic liquid. These layers serve as the reaction phase in which the homogeneous chiral catalyst is dissolved. As first application of this concept the L-proline-catalyzed aldol reaction has been carried out. Good yields and ee values, comparable with those obtained under homogeneous conditions have been obtained. Moreover, this material shows high regenerability.
Advances towards highly active and stereoselective simple and cheap proline-based organocatalysts
2010
Ten 4-acyloxy-L-prolines were screened as catalysts at loadings of 2–0.1 mol-% for the direct asymmetric aldol reaction in water by using variable amounts of water. Among them, a new catalyst, the L-proline carrying a trans-4-(2,2-diphenylacetoxy) group, and a catalyst previously synthesized by us, the L-proline carrying a trans-4-(4-phenylbutanoyloxy) group, were found to be excellent catalysts for the aldol reaction between cyclohexanone or cyclopentanone and substituted benzaldehydes when employed in only 1 and 0.5 mol-%,respectively, at room temperature without additives. For such catalysts, high turnover numbers were obtained, which are among the highest values obtained for enamine org…
Amino-Acid-Based Polymerizable Surfactants for the Synthesis of Chiral Nanoparticles
2016
Amino-acid-based chiral surfactants with polymerizable moieties are synthesized, and a versatile approach to prepare particles thereof with a chiral surface functionality is presented. As an example of an application, the synthesized particles are tested for their ability as nucleating agents in the enantioselective crystallization of amino acid conglomerate systems, taking rac-asparagine as a model system. Particles resulting from chiral surfactants with different tail groups are compared and the results demonstrate that only the chiral nanoparticles made of the polymerizable surfactant are able to act efficiently as nucleation agent in enantioselective crystallization.
Prolin-benzylester als chirale Hilfsgruppen in asymmetrischen 1,3-dipolaren Cycloadditionen mit Nitriloxiden
1990
Proline Benzyl Esters as Chiral Auxiliaries in Asymmetric 1,3-Dipolar Cycloadditions N-Acryloylproline benzyl ester (1b) reacts with nitrile oxides at temperatures from −78 to 0°C to give isoxazolines 4/5 with diastereomeric ratios of ca. 3:1 and with the (5S) isomer as the predominating. The corresponding N-methacryloylproline benzyl ester (1c) gives inferior results. The diastereoselectivity is not influenced by the temperature and the substituents of the nitrile oxides. To prove the absolute configuration of the predominating stereoisomer, a reaction sequence is developed which makes use of an enantioselective hydrolysis with pig liver esterase as the key step to generate an optically ac…
Enantioselective transport of amino acid through supported chiral liquid membranes
1993
Abstract Two chiral alcohols, nopol and (2S)-(−)-methyl-1-butanol, immobilized in the pores of a polyethylene film were used for the anantioselective transport of amino acid hydrochloride. The degree of stereoselectivity of the permeation process varied from 0.39 to 1.52 depending on both the type of the chiral membrane phase and the properties of the amino acid. The evaluation of the membrane ability to separate stereoisomers was performed by the step-by-step analysis by considering the effect of amino acids' hydrophobicity, polarity and degree of chirality. For the purpose of this paper, the chirality measure was introduced. It was demonstrated that the enantioselectivity was affected mai…
ChemInform Abstract: Base-Catalyzed Isomerization of 2-Isoxazolines Enables a Two-Step Enantioselective Synthesis of β-Hydroxynitriles from Enals.
2011
Treatment of chiral isoxazoles with catalytic amounts of DBU results in efficient formation of optically active β-hydroxynitriles.
Diastereomeric control of enantioselectivity: evidence for metal cluster catalysis
2014
Enantioselective hydrogenation of tiglic acid effected by diastereomers of the general formula [(μ-H)2Ru3(μ3-S)(CO)7(μ-P–P*)] (P–P* = chiral Walphos diphosphine ligand) strongly supports catalysis by intact Ru3 clusters. A catalytic mechanism involving Ru3 clusters has been established by DFT calculations. peerReviewed
From Overstoichiometric to Substoichiometric Enantioselective Protonation with 2-Sulfinyl Alcohols: A View in Perspective
2005
A general study of the enantioselective protonation of prochiral enolates with 2-sulfinyl alcohols is reported. The modification of reaction conditions to reduce drastically the amount of chiral proton source needed to obtain a good enantiomeric excess is reported. The effects of the different factors controlling the stereoselectivity are clearly established. Different protocols for enolate generation are compared.
Stereoselective Synthesis of the Antiprotozoal Lactone Passifloricin A and Seven Isomers Thereof
2004
The conjugated delta-lactone passifloricin A, a natural product with antiprotozoal activity, and seven isomers thereof have been synthesized in enantiopure form. It has been shown in this way that the proposed structure for the natural compound was erroneous. The correct structure is now evidenced. Key steps of the syntheses were asymmetric Brown-type aldehyde allylations and ring-closing metatheses.