Showing papers by "Xiaohua Liu published in 2010"

Asymmetric three-component inverse electron-demand aza-Diels-Alder reaction: efficient synthesis of ring-fused tetrahydroquinolines.

Abstract: The reaction of aromatic and aliphatic aldehydes, 2-aminophenols and cyclopentadiene (III) is catalyzed by Sc(OTf)3 and a chiral N,N′-dioxide catalysts to afford the target tricyclic products with high yields and selectivities.

Catalytic Asymmetric Bromoamination of Chalcones: Highly Efficient Synthesis of Chiral α-Bromo-β-Amino Ketone Derivatives

Abstract: Title reaction proceeds with excellent regio- and stereoselectivities in the presence of a chiral N,N′-dioxide—Sb(III) catalyst.

Chiral bisguanidine-catalyzed inverse-electron-demand hetero-Diels-Alder reaction of chalcones with azlactones.

Abstract: A new type of C2-symmetric chiral bisguanidine was designed as a highly efficient catalyst in the inverse-electron-demand hetero-Diels−Alder reaction of chalcones with azlactones for the first time. A wide variety of γ,δ-unsaturated δ-lactone derivatives with α-quaternary-β-tertiary stereocenters were obtained in high yields (up to 88%) with excellent enantioselectivities (up to 99% ee). Hydrogen bonds were considered to be crucial for the activation and stereoinduction of the reaction. In all cases, the major was HDA adducts, which were obtained as a single diastereomer along with little amount of Michael addition products.

Catalytic Asymmetric Roskamp Reaction of α-Alkyl-α-diazoesters with Aromatic Aldehydes: Highly Enantioselective Synthesis of α-Alkyl-β-keto Esters

Abstract: The first catalytic enantioselective Roskamp reaction of α-alkyl-α-diazoesters with aromatic aldehydes was realized using a simple chiral N,N′-dioxide−scandium(III) complex. Remarkably, with 0.05 mol % catalyst, the reaction was performed well over a series of substrates, giving the desired products chemoselectively in excellent yields (up to 99%) and enantioselectivities (up to 98% ee) under mild conditions. This protocol provides a promising method for the synthesis of chiral α-alkyl-β-keto esters and 1,3-diols.

Highly enantioselective synthesis of 1,3-bis(hydroxymethyl)-2-oxindoles from unprotected oxindoles and formalin using a chiral NdIII complex

Abstract: Highly enantioselective synthesis of 1,3-bis(hydroxymethyl)-2-oxindoles was established through chiral NdIII-N,N′-dioxide in up to 93% yield and >99% ee under mild conditions. The key features of this approach include using cheap and readily available formalin as a hydroxymethylation C1 unit and unprotected 3-substituted-2-oxindoles as nucleophiles directly. The initial C-aldol product from 3-substituted-2-oxindole was converted to the corresponding 1,3-bis(hydroxymethyl)-2-oxindole derivative immediately under the reaction conditions. Moreover, the catalyst was water-tolerant, and ligand and Nd(OTf)3 could be easily recovered and reused with no loss in catalytic activity and enantioselectivity. The synthetic utility of this methodology was accomplished with the synthesis of key intermediates of physostigmine and coerulescine.

Asymmetric Diels–Alder and Inverse‐Electron‐Demand Hetero‐Diels–Alder Reactions of β,γ‐Unsaturated α‐Ketoesters with Cyclopentadiene Catalyzed by N,N′‐Dioxide Copper(II) Complex

Abstract: Highly enantioselective Diels-Alder (DA) and inverse-electron-demand hetero-Diels-Alder (HDA) reactions of β,γ-unsaturated α-ketoesters with cyclopentadiene catalyzed by chiral N,N'-dioxide-Cu(OTf)(2) (Tf=triflate) complexes have been developed. Quantitative conversion of β,γ-unsaturated α-ketoesters and excellent diastereoselectivities (up to 99:1) and enantioselectivities (up to >99% ee) were observed for a broad range of substrates. Both aromatic and aliphatic β,γ-unsaturated α-ketoesters were found to be suitable substrates for the reactions. Moreover, the chemoselectivity of the DA and HDA adducts were improved by regulating the reaction temperature. Good to high chemoselectivity (up to 94%) of the DA adducts were obtained at room temperature, and moderate chemoselectivity (up to 65%) of the HDA adducts were achieved at low temperature. The reaction also featured mild reaction conditions, a simple procedure, and remarkably low catalyst loading (0.1-1.5 mol%). A strong positive nonlinear effect was observed.

Highly Enantioselective Zinc-Catalyzed Friedel–Crafts Alkylation of Indoles with Ethyl Trifluoropyruvate

Abstract: A highly enantioselective Friedel–Crafts alkylation of indoles with ethyl trifluoropyruvate has been developed using N,N′-dioxide-zinc(II) complexes. Both enantiomers of the desired adducts were obtained by the use of enantiomeric ligands in excellent results (up to 99% yield and 98% ee) within 0.5 h under mild conditions. On the basis of the experimental results, a proposed working model was proposed to explain the origin of the asymmetric induction.

Catalytic Asymmetric Bromoamination of Chalcones: Highly Efficient Synthesis of Chiral α-Bromo-β-amino Ketone Derivatives.

Abstract: Title reaction proceeds with excellent regio- and stereoselectivities in the presence of a chiral N,N′-dioxide—Sb(III) catalyst.

Application of l-thiazolidine-4-carboxylic acid monolayer in electrochemical determination of copper(II)

Abstract: L-Thiazolidine-4-carboxylic acid monolayer was prepared on gold electrodes through the self-assembly approach. Such novel thioether-based monolayer could efficiently preconcentrate Cu2+, which provided a simple, stable and reproducible method for the determination of Cu2+. The modified electrodes were stable enough to be continuously used for one week (more than 30 times regeneration) with lower than 10% decrease in the response. They retained their initial activity for more than one month if used once a day. The calibration curve was linear for Cu2+ from 0.6 to 158.8 μg L−1 with a detection limit of 0.38 μg L−1. The relative standard deviation was 3.2% for a series of six successive measurements. The proposed method was applied in the determination of Cu2+ in mineral water and human hair samples.

Catalytic Asymmetric Roskamp Reaction of α‐Alkyl‐α‐diazoesters with Aromatic Aldehydes: Highly Enantioselective Synthesis of α‐Alkyl‐β‐keto Esters.

Abstract: The first catalytic enantioselective Roskamp reaction of α-alkyl-α-diazoesters with aromatic aldehydes was realized using a simple chiral N,N′-dioxide−scandium(III) complex. Remarkably, with 0.05 mol % catalyst, the reaction was performed well over a series of substrates, giving the desired products chemoselectively in excellent yields (up to 99%) and enantioselectivities (up to 98% ee) under mild conditions. This protocol provides a promising method for the synthesis of chiral α-alkyl-β-keto esters and 1,3-diols.

Asymmetric three-component inverse electron-demand aza-Diels–Alder reaction: efficient synthesis of ring-fused tetrahydroquinolines

Abstract: The reaction of aromatic and aliphatic aldehydes, 2-aminophenols and cyclopentadiene (III) is catalyzed by Sc(OTf)3 and a chiral N,N′-dioxide catalysts to afford the target tricyclic products with high yields and selectivities.

Asymmetric Three-Component Inverse Electron-Demand Aza-Diels—Alder Reaction: Efficient Synthesis of Ring-Fused Tetrahydroquinolines.

Abstract: The reaction of aromatic and aliphatic aldehydes, 2-aminophenols and cyclopentadiene (III) is catalyzed by Sc(OTf)3 and a chiral N,N′-dioxide catalysts to afford the target tricyclic products with high yields and selectivities.

Amide-Based Bifunctional Organocatalysts in Asymmetric Reactions

Abstract: A series of amide-based bifunctional organocatalysts have been utilized in asymmetric reactions. Prolinamide analogues have been designed to catalyze asymmetric reactions via enamine intermediates with the assistance of tunable hydrogen bonding. In addition, the combination of various Lewis base functional groups, such as guanidine, N-oxide and phosphane, with an amide moiety have also constituted efficient bifunctional organocatalysts with unique reactivity and selectivity.

Highly Enantioselective Insertion of Carbenoids into N—H Bonds Catalyzed by Copper(I) Complexes of Binol Derivatives.

Abstract: A novel and readily available BINOL-derived Copper(I) catalyst is developed for the enantioselective synthesis of amines under mild reaction conditions.