Search results for "Stereocenter"
showing 10 items of 123 documents
Chiral complexes of titanium containing a linked amido-cyclopentadienyl ligand: synthesis, structure, and asymmetric imine hydrogenation catalysis
2000
Abstract A series of mono- and disubstituted derivatives (−)-(S)-Ti(η5:η1-C5Me4SiMe2NCHMePh)(X)Cl (X=CH2SiMe3, BH4) and (−)-(S)-Ti(η5:η1-C5Me4SiMe2NCHMePh)X2 (X=OSO2CF3, OiPr, Me, CH2Ph) was prepared from (−)-(S)-Ti(η5:η1-C5Me4SiMe2NCHMePh)Cl2 without significant racemization at the stereogenic center. The monosubstituted complexes are formed as mixtures of diastereomers. One diastereomeric monoalkyl (STi, SC)-Ti(η5:η1-C5Me4SiMe2NCHMePh)(CH2SiMe3)Cl was characterized by X-ray single crystal structure analysis. When the (−)-(S)-NCHMePh group is attached to planar chiral ring moieties 3-tBuC5H3, C9H6, and C9H5(SiMe3)-3 and coordinated at the titanium center, diastereomeric mixtures are formed…
Asymmetric Synthesis of Amino-Bis-Pyrazolone Derivatives via an Organocatalytic Mannich Reaction.
2017
A new series of N-Boc ketimines derived from pyrazolin-5-ones have been used as electrophiles in asymmetric Mannich reactions with pyrazolones. The amino-bis-pyrazolone products are obtained in excellent yields and stereoselectivities by employing a very low loading of 1 mol % of a bifunctional squaramide organocatalyst. Depending on the substitution at position 4 of the pyrazolones, the new protocol allows for the generation of one or two tetrasubstituted stereocenters, including a one-pot version combing the Mannich reaction with a base-mediated halogenation. peerReviewed
A surprising switch in absolute configuration of anti-inflammatory macrolactones.
2016
Oxacyclododecindione-type macrolactones exhibit highly potent anti-inflammatory activities even at nanomolar concentration. After the determination of the relative configuration of the stereocenters at C14 and C15 by total synthesis of 4-dechloro-14-deoxyoxacyclododecindione and 14-deoxyoxacyclododecindione, the absolute configuration has now been assigned by X-ray crystallography. Surprisingly, the absolute configuration is (14S,15R) which differs for C15 from that of the well-known derivatives of (S)-curvularin. The biological activities of both enantiomers of 14-deoxyoxacyclododecindione, obtained by racemic synthesis and optical resolution, were investigated and the ring conformation of…
Understanding the Mechanism of the Intramolecular Stetter Reaction. A DFT Study
2012
The mechanism of the N-heterocyclic carbene (NHC)-catalyzed intramolecular Stetter reaction of salicylaldehyde 1 to yield chromanone 3 has been theoretically studied at the B3LYP/6-31G** level. This NHC-catalyzed reaction takes place through six elementary steps, which involve: (i) formation of the Breslow intermediate IN2; (ii) an intramolecular Michael-Type addition in IN2 to form the new C-C s bond; and (iii) extrusion of the NHC catalyst from the Michael adduct to yield chromanone 3. Analysis of the relative free energies in toluene indicates that while formation of Breslow intermediate IN2 involves the rate-determining step of the catalytic process, the intramolecular Michael-type addi…
Enantioselective zinc-mediated conjugate alkynylation of saccharin-derived 1-aza-butadienes
2020
The enantioselective 1,4-alkynylation of conjugated imines derived from saccharin with aryl- and alkyl-substituted terminal alkynes has been achieved. The reaction mediated by diethylzinc in the presence of a catalytic amount of a bis(hydroxy)malonamide chiral ligand provides the corresponding imines bearing a propargylic stereocenter with moderate yields and fair to excellent enantioselectivities.
Asymmetric Synthesis of Spirocyclic β-Lactams through Copper-Catalyzed Kinugasa/Michael Domino Reactions
2018
The first copper-catalyzed highly chemo-, regio-, diastereo-, and enantioselective Kinugasa/Michael domino reaction for the desymmetrization of prochiral cyclohexadienones is described. In the presence of a chiral copper catalyst, alkyne-tethered cyclohexadienones couple with nitrones to generate the chiral spirocyclic lactams with excellent stereoselectivity (up to 97 % ee, >20:1 dr). The new method provides direct access to versatile highly functionalized spirocyclic β-lactams possessing four contiguous stereocenters, including one quaternary and one tetra-substituted stereocenter.
Enantioselective Addition of Nitromethane to 2-Acylpyridine N-Oxides. Expanding the Generation of Quaternary Stereocenters with the Henry Reaction
2014
[EN] The direct asymmetric Henry reaction with prochiral ketones, leading to tertiary nitroaldols, is an elusive reaction so far limited to a reduced number of reactive substrates such as trifluoromethyl ketones or alpha-keto carbonyl compounds. Expanding the scope of this important reaction, the direct asymmetric addition of nitromethane to 2-acylpyridine N-oxides catalyzed by a BOX-Cu(II) complex to give the corresponding pyridine-derived tertiary nitroaldols having a quaternary stereogenic center with variable yields and good enantioselectivity, is described.
Catalytic Enantioselective Friedel–Crafts Reactions of Naphthols and Electron-Rich Phenols
2016
The enantioselective Friedel–Crafts reaction is a powerful tool for the construction of benzylic stereocenters in a stereodefined manner. Significant advances have already been achieved with heteroarenes such as indoles and pyrroles; however, the reaction with homoarenes is less developed. This short review covers the most relevant literature on enantioselective Friedel–Crafts reactions with naphthols and phenols. 1 Introduction 2 Friedel–Crafts Reactions Involving 1,2-Nucleophilic Addition to C=X Bonds 3 Friedel–Crafts Reactions Involving Conjugate Nucleophilic Addition to Electrophilic C=C Bonds 4 Friedel–Crafts Reactions Involving π-Allylic Complexes as Electrophiles 5 ipso-Friedel–Craft…
Synthesis of an enantiopure 2-arylcyclohexanols from prochiral enol acetates by an enantioselective protonation/diastereoselective reduction sequence
2003
Abstract The enantioselective protonation with 2-sulfinyl alcohols of lithium enolates of 2-arylcyclohexanones with different substituents on the phenyl group takes place with excellent enantioselectivities (89–99%). Chiral 2-phenylcyclohexanone and 2-arylcyclohexanones carrying electron donor substituents on the aromatic ring are converted into the corresponding trans -2-arylcyclohexanols by diastereoselective reduction with sodium naphthalenide in the presence of acetamide. The stereochemical integrity of the tertiary stereocenter is fully preserved using this reduction procedure. Interestingly, the chiral proton source is not consumed in the synthesis.
Organocatalytic Enantioselective Alkylation of Pyrazol-3-ones with Isatin-Derived Ketimines: Stereocontrolled Construction of Vicinal Tetrasubstitute…
2016
A quinine-derived thiourea catalysed the enantioselective addition of 4-substituted pyrazolones to isatin-derived ketimines, providing a variety of aminooxindole-pyrazolone adducts containing congested vicinal tetrasubstituted stereocentres with excellent outcomes (up to 98% yield, >20:1 dr and 98% ee).