Copper Catalyzed Enantioselective Alkylation of Pyrrole with β,γ-Unsaturated α-Ketoesters: Application to One-Pot Construction of the Seven-Membered Ring by Merging a Gold Catalysis

Synthesis of chiral systems featuring the pyrrole unit: a review

Synthetic strategies towards pyrroles within chiral frameworks are summarised, focussing on reports published 2010-2023. The synthesis of pyrroles featuring substituents bearing chiral centres are summarised, as are those whereby pyrroles are located within axially chiral systems courtesy of restricted bond rotation.

Construction of chiral N,O-hemiaminals via a copper-catalyzed enantioselective Michael/N-hemiacetalization cascade reaction

An efficient Michael/N-hemiacetalization cascade reaction of 5-aminoisoxazoles with β,γ-unsaturated α-ketoesters was developed under the catalysis of a chiral copper complex. A series of optically pure six-membered ring N,O-hemiaminals were obtained with excellent yields (up to 96% yield) and high enantioselectivities (up to 98% ee). The possible transition state was supported by DFT calculations and thereby the corresponding mechanism was proposed.

Enantioselective Friedel-Crafts Alkylation of Indoles with β,γ-Unsaturated α-Ketoesters Catalyzed by New Copper(I) Catalysts

New Cu(I) catalysts are effective in enantioselective Friedel-Crafts alkylation of a variety of indoles with different β,γ-unsaturated α-ketoesters. A control study shows that such a catalyst system is less sensitive to air, and the reactions can be carried out without special cares such as glovebox operation or moisture/oxygen-free conditions. Preliminary computation results suggest that there exists π-π stacking between the substrate and the catalyst, and such an interaction seems to play a role in stabilizing the reaction intermediate and enhancing the stereoselectivity of the reactions. The desired products can be obtained in up to 98% yield at 99% enantiomeric excess. The same high enantioselectivity can be observed when the reaction is carried in a gram scale, indicating a good scalability of the catalyst system in enantioselective Friedel-Crafts alkylation of different indoles with β,γ-unsaturated α-ketoesters.

Synthetic Access to α-Oxoketene Aminals by the Nucleophilic Addition of Enol Silane-Derived Palladium(II) Enolates to Carbodiimides

Synthetically important α-oxoketene aminal intermediates can now be accessed from readily available and inexpensive carbodiimides as starting materials via the nucleophilic addition of palladium enolates derived from enol silane precursors. This operationally simple method features mild reaction conditions, including open air atmosphere, ligand-free metal catalysis, broad substrate scope, and multi-gram scalability. Select synthetic applications that take advantage of the enamine character of α-oxoketene aminals and involve C-nucleophilic additions to electrophilic systems, including an α,β-unsaturated ester, an azo dicarboxylate, an aralkyl halide, and an aldehyde, are demonstrated.

Enantioselective Direct Synthesis of C3-Hydroxyalkylated Pyrrole via an Amine-Catalyzed Aldol/Paal-Knorr Reaction Sequence

Creating functionality with chirality at position C3 of pyrrole is challenging. An operationally simple organocatalytic method has been developed to generate functionality with a chiral tertiary/quaternary stereocenter at position C3 of pyrrole. The process proceeds through an amine-catalyzed direct aldol reaction of succinaldehyde with various acceptor carbonyls, followed by a Paal-Knorr reaction with a primary amine in the same pot. A series of chiral C3-hydroxyalkylated N-alkyl/Ar/H-pyrroles have been synthesized for the first time with good to high yields and excellent enantioselectivity.

Copper-Catalyzed Stereoselective [4 + 2] Cycloaddition of β,γ-Unsaturated α-Keto Esters and 2-Vinylpyrroles in Water

An asymmetric [4 + 2] cycloaddition of β,γ-unsaturated α-keto esters with 2-vinylpyrroles in water was developed under the catalysis of a kind of copper complex with a low loading. A series of optically pure 3,4-dihydro-2H-pyran derivatives could be obtained in excellent yields, with high diastereoselectivities and enantioselectivities. The corresponding mechanism was proposed, which was supported by DFT calculations.

Organocatalyzed Enantioselective Conjugate Addition of Boronic Acids to β,γ-Unsaturated α-Ketoesters

We report herein the (R)-3,3'-Br₂-BINOL-catalyzed enantioselective conjugate addition of organic boronic acids to β,γ-unsaturated α-ketoesters to generate the corresponding Michael addition products in moderate to high yields and with moderate to excellent enantioselectivities (up to 99% ee). This catalytic system features characteristics of mild reaction conditions, high efficiency, and tolerance to alkenylboronic acids and heteroarylboronic acids.

Gold/scandium bimetallic relay catalysis of formal [5+2]- and [4+2]-annulations: access to tetracyclic indole scaffolds

Regiodivergent formal [5+2]- and [4+2]-annulation reactions of indole derivatives with 2-(2-alkynyl)aryl cyclopropane-1,1-diesters (ACPs) have been developed. A series of tetracyclic indole derivatives were delivered in a 77% average yield with excellent regioselectivities enabled by Au(I)/Sc(III) bimetallic relay catalysis. A gram-scale reaction and further transformation of the resulting tetracyclic indoles demonstrated the practical utility of this protocol. Moreover, the photophysical properties of the obtained multicyclic compounds were also investigated.

Catalytic Gold Chemistry: From Simple Salts to Complexes for Regioselective C-H Bond Functionalization

Gold coordinated to neutral phosphines (R₃ P), N-heterocyclic carbenes (NHCs) or anionic ligands is catalytically active in functionalizing various C-H bonds with high selectivity. The sterics/electronic nature of the studied C-H bond, oxidation state of gold and stereoelectronic capacity of the coordinated auxiliary ligand are some of the associated selectivity factors in gold-catalyzed C-H bond functionalization reactions. Hence, in this review a comprehensive update about the action of different types of gold catalysts, from simple to sophisticated ones, on C-H bond reactions and their regiochemical outcome is disclosed. This review also highlights the catalytic applications of Au(I)- and Au(III)-species in creating new opportunities for the regio- and site-selective activation of challenging C-H bonds. Finally, it also intends to stress the potential applications in selective C-H bond activation associated with a variety of heterocycles recently described in the literature.

Stereoselective Copper-Catalyzed Direct Aldol Reaction of β, γ-Unsaturated α-Ketoesters with Coumaran-3-Ones

An efficient direct aldol reaction between coumaran-3-ones and β, γ-unsaturated α-ketoesters by virtue of a chiral copper complex is developed. A series of coumaran-3-one derivatives containing chiral tertiary alcohol structures are obtained in excellent yields and stereoselectivities.

Copper-catalyzed enantioselective Mukaiyama aldol reaction of silyl enol ethers with isatins

A highly enantioselective Mukaiyama aldol reaction of silyl enol ethers with isatins catalyzed by chiral copper complexes was developed. A series of chiral 3-substituted 3-hydroxy-2-oxindoles bearing a tetra-substituted center could be obtained exclusively with high yields (up to 95%) and excellent enantioselectivities (up to 99%). In particular, water was essential to improve the diastereoselectivity.

Hetero-bimetallic cooperative catalysis for the synthesis of heteroarenes

Review covering the synthesis of 5- and 6-membered as well as condensed heteroarenes, focussing on the combinations in cooperative catalytic systems in strategies used to achieve selectivity and also highlights the mode of action for the cooperative catalysis leading to the synthesis of commercially and biologically relevant heteroarenes.

Enantioselective Hetero-Diels-Alder Reaction and the Synthesis of Spiropyrrolidone Derivatives

An efficient enantioselective hetero-Diels-Alder reaction was developed under catalysis of a chiral copper complex. A variety of spiropyrrolidones, which bear a tetra-substituted carbon stereocenter, can be obtained in good yields with excellent enantioselectivities by virtue of this method. Furthermore, a substrate-dependent reaction pathway was proposed on the basis of the isolated intermediates.

Copper-Catalyzed Enantioselective Henry Reaction of β,γ-Unsaturated α-Ketoesters with Nitromethane in Water

A highly enantioselective Henry reaction of β,γ-unsaturated α-ketoesters with nitromethane in water by virtue of chiral copper complexes has been developed. A series of unsaturated β-nitro-α-hydroxy esters bearing tetrasubstituted carbon stereocenters were obtained exclusively with high yields and excellent enantioselectivities. This method could avoid tedious anaerobic anhydrous manipulation and reduce the environmental pollution caused by organic solvents.

Copper-Catalyzed Chemoselective and Enantioselective Friedel-Crafts 1,2-Addition of Pyrrole with β,γ-Unsaturated α-Ketoesters

A Friedel-Crafts alkylation of pyrrole was developed to afford the β,γ-unsaturated α-hydroxy esters bearing a quaternary stereogenic center with good enantioselectivities and yields. This protocol represents the first report of 1,2-addition of Friedel-Crafts alkylation of pyrrole to β,γ-unsaturated α-ketoesters.

Copper(II)-Catalyzed Benzylic C(sp3)-H Aerobic Oxidation of (Hetero)Aryl Acetimidates: Synthesis of Aryl-α-ketoesters

A straightforward method is developed in this paper for the synthesis of α-ketoesters through copper-catalyzed aerobic oxidation of (hetero)aryl acetimidates using molecular oxygen as a sustainable oxidant. The reaction represents the first example of the direct synthesis of aryl-α-ketoesters from arylacetimidates through the aerobic oxidation of a benzylic C(sp3)-H (C═O) bond in moderate to good yield. This transformation occurs under mild reaction conditions with a wide range of substrates and utilizes a readily available oxidant and catalyst. The synthetic utility of this transformation is demonstrated through scaled-up synthesis. A plausible reaction mechanism is also proposed.

Organocatalyzed Asymmetric Michael Addition of 1-Acetylindolin-3-ones to β,γ-Unsaturated α-Ketoesters: An Access to Chiral Indolin-3-ones with Two Adjacent Tertiary Stereogenic Centers

Asymmetric Michael addition of 1-acetylindolin-3-ones to β,γ-unsaturated α-ketoesters was investigated for the synthesis of chiral indolin-3-ones with two adjacent tertiary stereogenic centers. Under the catalysis of a chiral bifunctional squaramide derived from l-tert-leucine, a wide range of 1-acetylindolin-3-ones and β,γ-unsaturated α-ketoesters were well-tolerated in this transformation to provide the corresponding novel densely functionalized chiral indolin-3-one derivatives in high yield with excellent diastereo- and enantioselectivity under mild reaction conditions.