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New Stereocenters (new + stereocenter)
Selected AbstractsEnders' SAMP-Hydrazone as Traceless Auxiliary in the Asymmetric 1,4-Addition of Cuprates to EnonesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 13 2010Karsten Sammet Abstract Conjugate additions of Gilman cyanocuprates to (S)- N -amino-2-(methoxymethyl)pyrrolidine (SAMP)-hydrazones 4, 5 derived from cyclic and acyclic ,,,-unsaturated ketones were investigated. A protocol utilizing copper(II) sulfate/ammonium chloride was evolved, which allowed cleavage of SAMP (S)- 1 under the hydrolysis and work-up conditions, followed by recovery of the auxiliary with ethylenediaminetetraacetic acid (EDTA). The enantioselectivity of cuprate additions was dominated for cyclic SAMP-hydrazones 4 by the cuprate alkyl substituent and the ring size, in the case of acyclic arylidene SAMP-hydrazones 5, however, by the nature of the aryl substituent. Electron-donating substituents gave poor enantiomeric excesses, whereas electron-withdrawing groups provided excellent ee values of 98,99%. The configuration of the new stereocenter was determined to be (R). Moreover, a reaction sequence was developed which integrates a tandem 1,4-addition/methylation and traceless hydrolytic cleavage of the auxiliary (S)- 1 in a one-pot reaction, resulting in enantiomerically pure methyl ketones 11,13, each of them with>99% ee. [source] Indium-Promoted Acyloxyallylation Reaction of Azetidine-2,3-diones in Aqueous Media: A New Route to Densely Functionalized 3-Substituted 3-Hydroxy-,-lactams,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 26 2008Benito Alcaide Abstract Densely functionalized 3-substituted 3-hydroxy-,-lactams have been obtained by acyloxyallylation reaction of azetidine-2,3-diones with 3-bromopropenyl acetate or benzoate in aqueous media promoted by indium under Barbier conditions. Two new stereocenters were formed; the stereochemistry at the new C-3 quaternary center was fully controlled by placing a bulky chiral substituent at C-4. However, poor diastereoselectivities were observed in the new allylicstereocenter formed (up to 58,% de).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Asymmetric Counterion Pair Catalysis: An Enantioselective Brønsted Acid-Catalyzed ProtonationADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 7-8 2008Magnus Rueping Abstract A new asymmetric Brønsted acid-catalyzed cascade reaction involving a 1,4-addition, enantioselective protonation and 1,2-addition has been developed. This organocatalytic cascade not only provides for the first time 3- and 2,3-substituted tetrahydroquinolines and octahydroacridines in good yields with high dia- and enantioselectivities under mild reaction conditions but additionally represents the first example of a chiral Brønsted acid-catalyzed protonation reaction in an organocatalytic domino reaction. Furthermore, the new Brønsted acid-catalyzed hydride-proton-hydride transfer cascade can be applied to prepare new molecular scaffolds with up to three new stereocenters in an efficient one-pot reaction sequence. [source] General model of assignment of the relative stereochemistry in the Nicholas reaction products resulting from chiral dicobalthexacarbonyl-1-alkoxy-propargylium cationsMAGNETIC RESONANCE IN CHEMISTRY, Issue 4 2002Angel M. Montaña Abstract The Nicholas reaction of chiral 1-alkoxy-dicobalthexacarbonyl-propargylium cations with linear and/or cyclic silyl enol ethers resulted in the formation of two pairs of diastereoisomers (syn -1, syn -2 and anti -1, anti -2), depending on the relative stereochemistry of the two new stereocenters formed in the reaction products. Here, we present a model to establish the relative stereochemistry of those stereocenters on the basis of a correlation of their 1H and 13C NMR spectra, together with their conformational analysis and a study of the stereoelectronic interactions conditioning the chemical shifts and coupling constants. The importance of this method is based on its applicability to the assignment of the relative stereochemistry of non-separable components of a diastereomeric mixture or, when diastereomers are non-crystallizable oily products, not suitable for X-ray diffraction analysis. Copyright © 2002 John Wiley & Sons, Ltd. [source] One-Pot Catalytic Asymmetric Cascade Synthesis of Cycloheptane DerivativesCHEMISTRY - A EUROPEAN JOURNAL, Issue 9 2008Jan Vesely Dr. Abstract A regiospecific, highly chemo-, diastereo-, and enantioselective one-pot catalytic cascade synthesis of cycloheptane derivatives is presented. In this chiral-amine-catalyzed asymmetric process, six new bonds and five new stereocenters were formed with excellent stereocontrol (>25:1 d.r. and 98, >99,ee). [source] | ||||||||||