Search results for "ASYM"
showing 10 items of 2151 documents
Frontispiece: Hierarchy of Asymmetry at Work: Chain-Dependent Helix-to-Helix Interactions in Supramolecular Polymers
2018
ChemInform Abstract: Carbohydrates as Chiral Templates: Diastereoselective Synthesis of N- Glycosyl-N-homoallylamines and β-Amino Acids from Imines.
2010
Complexing properties and chirality of carbohydrates were utilized in diastereoselective reactions of 0-pivaloylated N-galactosylimines with allylsilanes and -stannaries. With dyltrimethylsilane in the presence of SnCl4, imines 2 of aromatic and heteroaromatic aldehydes were converted to homdylamines 3, giving ratios of diastereomers higher than 7:1. No addition products derived from α-anomeric aromatic imines were formed. Aliphatic homdylamines 3 were synthesized by using allyltributylstannane in the presence of SnCl4. Both α- and β-anomeric aliphatic imines reacted with the allylstannane. They gave the same ratio of diastereomers and showed the same sense of asymmetric induction.
Base-Dependent Stereodivergent Intramolecular Aza-Michael Reaction: Asymmetric Synthesis of 1,3-Disubstituted Isoindolines
2013
The nucleophilic addition (A(N)) / intramolecular aza-Michael reaction (IMAMR) process on Ellman's tert-butylsulfinyl imines, bearing a Michael acceptor in the ortho position, is studied. This reaction affords 1,3-disubstituted isoindolines with a wide range of substituents in good yields and diastereoselectivities. Interestingly, careful choice of the base for the aza-Michael step allows either the cis or the trans diastereoisomers to be exclusively obtained. This stereodivergent cyclization has enabled the synthesis of C2-symmetric bisacetate-substituted isoindolines. In addition, bisacetate isoindolines bearing two well-differentiated ester moieties are also noteworthy because they may a…
Asymmetric hydrogenation of an α-unsaturated carboxylic acid catalyzed by intact chiral transition metal carbonyl clusters – diastereomeric control o…
2020
Twenty clusters of the general formula [(μ-H)2Ru3(μ3-S)(CO)7(μ-P–P*)] (P–P* = chiral diphosphine of the ferrocene-based Walphos or Josiphos families) have been synthesised and characterised. The clusters have been tested as catalysts for asymmetric hydrogenation of tiglic acid [trans-2-methyl-2-butenoic acid]. The observed enantioselectivities and conversion rates strongly support catalysis by intact Ru3 clusters. A catalytic mechanism involving an active Ru3 catalyst generated by CO loss from [(μ-H)2Ru3(μ3-S)(CO)7(μ-P–P*)] has been investigated by DFT calculations. peerReviewed
A Simple Organocatalytic Enantioselective Synthesis of Pregabalin
2009
This paper describes a new procedure for the enantioselective synthesis of the important anticonvulsant drug Pregabalin, which shows biological properties as the (S) enantiomer only. The key step of the synthetic sequence is the Michael addition reaction of Meldrum's acid to a nitroalkene mediated by a quinidine derived thiourea. A variety of novel catalysts bearing different groups at the thiourea moiety were synthesized and tested. The most successful catalyst that incorporates a trityl substituent provided up to 75 % ee of (S)-4. The conjugate addition reaction was carried out on a multigram scale with low loadings of catalyst (10 mol-%). Moreover, the catalyst can be recycled showing th…
“Nonsolvent” Applications of Ionic Liquids in Organocatalysis
2012
This chapter is organized in three sections (see Figure 10.1 ): 2. Immobilization of ionic liquids and organocatalysts 2.1 Covalently attached “ ionic liquid ” moieties as supports 2.2 Covalently attached “ ionic liquid ” moieties as linkers 2.3 Covalently attached “ ionic liquid ” moieties as organocatalysts
1,2-Asymmetric Induction in Diastereoselective Zwitterionic Aza-Claisen Rearrangements: Key Steps in Optically Active Alkaloid Synthesis
2013
The zwitterionic aza-Claisen rearrangement of optically active N-allylpyrrolidines and α-phenoxyacetyl fluorides proceeds with complete simple diastereoselectivity (internal asymmetric induction) and complete 1,2-asymmetric induction to generate a new C–C bond adjacent to a chiral C-N-Boc functionality. The resulting γ,δ-unsaturated amides were cyclised to give the corresponding pyrrolizidinones, which enabled the determination of the relative configuration of the stereotriads. Vinyl group degradation and a final lactam reduction gave an optically active analogue of (+)-petasinine (a pyrrolizidine alkaloid). Furthermore, the stereotriad-containing amides should be useful key intermediates f…
ChemInform Abstract: Water in Stereoselective Organocatalytic Reactions
2009
In this review, recent advances in asymmetric organocatalytic reactions carried out with variable amounts of water, from substoichiometric to a large excess (reaction medium), are discussed. We also summarize several proposed mechanisms for the different possibilities of the action of water both in the increased activity of the catalyst and in the asymmetric induction. Finally, the application of this catalytic methodology to the enantioselective synthesis of valuable compounds through enamine or iminium catalysis is presented.
ChemInform Abstract: Reversal of Asymmetric Induction in Stereoselective Strecker Synthesis on Galactosyl Amine as the Chiral Matrix.
1989
Abstract The reversal of the direction of asymmetric induction in Lewis acid catalyzed Strecker synthesis using the 2,3,4,6-tetra-O-pivaloyl-β-D-galactopyranosyl amine 1 is described. In isopropanol or tetrahydrofuran 1 had given ( R )-diastereomers of the corresponding α-amino nitriles preferably. However, in chloroform in the presence of heterogeneous zinc chloride the same auxiliary alternatively leads to an excess of the ( S )-diastereomers.