Search results for "stereo"
showing 10 items of 6147 documents
Installation of Amine Moieties into a Polycyclic Anodic Product Derived from 2,4-Dimethylphenol
2011
When 2,4-dimethylphenol is anodically treated, a dehydrotetramer with four contiguous stereocentres is readily obtained on a multi-gram scale. The substitution of a 2,4-dimethyl-phenoxy fragment by several amines was demonstrated, and the best results were obtained with primary amines. Optically pure α-chiral aliphatic amines yield diastereomeric mixtures that can be separated in most cases. The basic amine causes a partial hemiketal-opening of the bisbenzofuran moiety leading to an equilibrium within an α,β-unsaturated cyclohexenone. This dynamic behaviour occurs on the time scale of NMR spectroscopy and is also found by X-ray analysis providing a consistent picture.
Stereoselective Synthesis of Bromopiperidinones and their Conversion to Annulated Heterocycles
2009
N-Galactopyranosyl- and N-glucopyranosyl imines of aliphatic, aromatic and heteroaromatic aldehydes react with 1-methoxy-3-trimethylsilyloxy-1,3-butadiene in a domino Mannich-Michael reaction cascade to give 2-substituted 5,6-dehydro-piperidin-4-ones with high diastereoselectivity. Treatment of these cyclic enaminones with N-bromo-succinimide yields the corresponding 2-substituted 5-bromo-5,6-dehydropiperidinones which react with L-SelectrideⓇ or methylcuprate to afford the saturated bromo-piperidinones with high diastereoselectivity. Condensation reactions of these products with thioamides afford thiazolopiperidines
ChemInform Abstract: Enantioselective Mannich Reaction of β-Keto Esters with Aromatic and Aliphatic Imines Using a Cooperatively Assisted Bifunctiona…
2015
The title reaction tolerates aliphatic and aromatic substituents on both reaction partners, affording products (III) as a mixture of inseparable diastereomers with low or without any diastereoselectivity.
Organocatalytic asymmetric addition of naphthols and electron-rich phenols to isatin-derived ketimines: highly enantioselective construction of tetra…
2015
A quinine-derived thiourea organocatalyst promoted the highly enantioselective addition of naphthols and activated phenols to ketimines derived from isatins. The reaction afforded chiral 3-amino-2-oxindoles with a quaternary stereocenter in high yields (up to 99%) with excellent enantioselectivity (up to 99% ee). To the best of our knowledge, this transformation is the first highly enantioselective addition of naphthols to ketimines
Asymmetric Synthesis of Dihydropyranones with Three Contiguous Stereocenters by an NHC‐Catalyzed Kinetic Resolution
2021
An oxidative NHC-catalyzed kinetic resolution (KR) of racemic mixtures is presented. The developed reaction furnishes tricyclic dihydropyranones with three contiguous stereocenters in excellent dia- and enantioselectivity, with good-to-moderate yields. Mechanistic studies indicate that the rate-determining step of the reaction is the formation of the Breslow intermediate, while the selectivity determining step occurs later in the mechanism. The presented methodology enables rapid synthesis of complex structures in a single step.
Squarate and croconate in designing one- and two-dimensional oxamidato-bridged copper(II) complexes: synthesis, crystal structures and magnetic prope…
2001
Abstract The reaction of squarate (C4O42–, dianion of 3,4-dihydroxycyclobut-3-ene-1,2-dione) and croconate (C5O52–, dianion of 4,5-dihydroxycyclopent-4-ene-1,2,3-trione) with the dinuclear 〚Cu2(apox)〛2+ copper(II) complex 〚H2apox = N,N’-bis(3-aminopropyl)oxamide〛 in aqueous solution affords the compounds of formula 〚Cu2(apox)(C4O4)(H2O)2〛n·n H2O (1) and 〚Cu4(apox)2(C5O5)2〛·6 H2O (2). Crystals of 1 are monoclinic, space group C2/c, with a = 12.5527(9), b = 7.4161(6), c = 18.5198(12) A, β = 100.578(6)° and Z = 4. Crystals of 2 are triclinic, space group P 1 with a = 9.732(5), b = 9.795(2), c = 10.285(3) A, α = 84.95(2), β = 77.30(3), γ = 85.61(3)° and Z = 1. The structure of complex 1 consist…
Tantalocenehydridephosphorus chemistry.
2002
Abstract The aim of this paper is to look for a better knowledge of the behaviour of bent tantalocenes that bear hydrides, phosphorus PR2X (R=Me, Ph; X=H, lone pair) and cyclopentadienyl (Cp=C5H5, Cp′=C5H2tBu(Me)2, Cp*=C5Me5) ligands. An orbital control of regioselectivity of insertion of the PR2 phosphide fragment of chlorophosphines PR2Cl into the central TaH bond of trihydrides Cp2TaH3 leading to the formation of metallophosphonium cations is discussed. Neutralisation of cationic complexes with strong bases leads either to the Ta(V)–phosphide or to the Ta(III)–phosphine species depending on the nature of the cyclopentadienyl ligand; good electron donor Cp′ and Cp* rings favour the forma…
Enantioselective Synthesis of P-Chirogenic Phosphorus Compounds via the Ephedrine-Borane Complex Methodology
2008
The stereoselective synthesis of P-chirogenic organophosphorus compounds using the versatile ephedrine-methodology is reported. The principle of the method is based on the diastereoselective preparation of 1,3,2-oxazaphospholidine borane 3 from ephedrine 2 and regio- and stereoselective ring-opening of the heterocycle leading to an aminophosphine borane 4. Finally, these ring-opening products 4 are easily transformed into P-chirogenic organophosphorus compounds, by subsequent highly stereoselective reactions involving electrophilic or nucleophilic strategies.
Exploring the potential of some yeast strains in the stereoselective synthesis of aldol reaction products and its reduced 1,3-dialcohol derivatives
2013
Abstract The behavior of two yeast strains has been studied under different conditions. Both microorganims catalyzed the aldol reaction between activated aldehydes and acetone when a large amount of the latter was present in the reaction medium producing, with moderate stereoselectivity, the aldol product with the R configuration. No reduction of any of the products present in the medium was detected. On the other hand, the carbonyl group of the racemic aldol was reduced to produce chiral 1,3-dialcohol derivatives when water was employed as the only solvent. In this case, the resolution of the racemic starting material was also possible with one of the biocatalysts, and the aldol was recove…
A Novel Approach to Dihydropyrrolones from Iron-Substituted α,β-Unsaturated Imines and Organolithium and Grignard Reagents: Developments, Mechanisms …
2000
Reactions of N-sulfonylimines, derived from Cp(CO)2Fe-substituted (Z)-enals and benzenesulfonamide, with Grignard or organolithium reagents furnish N-sulfonyldihydropyrrolones. When N-sulfinylimines and organometallic reagents are used, unprotected α,β-unsaturated γ-lactams are formed exclusively. Recent mechanistic studies and diastereoselective variations for the synthesis of chiral 5-substituted dihydropyrrolones from iron-substituted azadienes are discussed here.