Enantioselective Synthesis of 2-Amino-1,1-diarylalkanes Bearing a Carbocyclic Ring Substituted Indole through Asymmetric Catalytic Reaction of Hydroxyindoles with Nitroalkenes

Recent Advances in the Catalytic Asymmetric Friedel-Crafts Reactions of Indoles

Functionalized chiral indole derivatives are privileged and versatile organic frameworks encountered in numerous pharmaceutically active agents and biologically active natural products. The catalytic asymmetric Friedel-Crafts reaction of indoles, catalyzed by chiral metal complexes or chiral organocatalysts, is one of the most powerful and atom-economical approaches to access optically active indole derivatives. Consequently, a wide range of electrophilic partners including α,β-unsaturated ketones, esters, amides, imines, β,γ-unsaturated α-keto- and α-ketiminoesters, ketimines, nitroalkenes, and many others have been successfully employed to achieve a plethora of functionalized chiral indole moieties. In particular, strategies for C-H functionalization in the phenyl of indoles require incorporation of a directing or blocking group in the phenyl or azole ring of indole. The discovery of chiral catalysts which can control enantiodiscrimination has gained a great deal of attention in recent years. This review will provide an updated account on the application of the asymmetric Friedel-Crafts reaction of indoles in the synthesis of diverse chiral indole derivatives, covering the timeframe from 2011 to today.

Brønsted acid catalyzed remote C6 functionalization of 2,3-disubstituted indoles with β,γ-unsaturated α-ketoester

A metal-free catalytic approach for the remote C6-functionalization of 2,3-disubstituted indoles has been developed. In the presence of catalytic amounts of Brønsted acid, the β,γ-unsaturated α-ketoesters react with 2,3-disubstituted indoles at the C6 position selectively. Under mild reaction conditions, a range of C6-functionalized indoles were prepared with good yields and excellent regioselectivity. This methodology provides a concise and efficient route for the synthesis of C6-functionalized indole derivatives.

Enantioselective Friedel-Crafts reaction of hydroxyarenes with nitroenynes to access chiral heterocycles via sequential catalysis

Naphthols, hydroxyindoles and an activated phenol are reacted with differently substituted (E)-nitrobut-1-en-3-ynes using the commercially available Rawal's chiral squaramide. The corresponding β-nitroalkynes were obtained with good yields and excellent enantioselectivities. Moreover, dihydronaphthofurans can be accessed via silver catalysed cyclization in a tandem one-pot procedure, with high preservation of the optical purity.

Organocatalytic dimensions to the C–H functionalization of the carbocyclic core in indoles: a review update

A review highlighting important research findings in remote C–H activation processes using effectual organocatalytic perspectives. The challenging indole carbocyclic ring positions were successfully accessed with proper regio- and stereocontrols.

Stereoselective Synthesis of Dihydrofuranoindoles via the Friedel-Crafts Alkylation/Annulation Cascade Process

A stereoselective annulation protocol was developed to construct dihydrofuranoindoles from readily available starting materials. In the presence of a bifunctional squaramide, the Friedel-Crafts alkylation/annulation cascade process occurred smoothly to provide dihydrofuranoindoles in 26-95% isolated yields exclusively as trans-diastereomers (38-99% ee). This catalytic protocol was compatible with a range of structurally distinct hydroxyindoles bearing the hydroxyl group at different positions, providing four kinds of dihydrofuranoindoles. Moreover, gram-scale synthesis and further synthetic manipulation of the product were also demonstrated.

Asymmetric synthesis of spirooxindole–pyranoindole products via Friedel–Crafts alkylation/cyclization of the indole carbocyclic ring

Enantioselective Friedel–Crafts alkylation/cyclization of the indole carbocyclic ring with isatylidene malononitriles was performed using a new bifunctional catalyst.

An Atropo-enantioselective Synthesis of Benzo-Linked Axially Chiral Indoles via Hydrogen-Bond Catalysis

A variety of axially chiral biaryldiols were synthesized in good yields with excellent atropo-enantioselectivities through construction of axially chiral indoles catalyzed by asymmetric hydrogen-bond donors. In addition, the new axially chiral compounds were proved to be efficient and practical catalysts for asymmetric catalysis. The strategy not only provides a novel method to synthesize axially chiral compounds but also extends the scope of chiral catalysts.

Organocatalytic enantioselective functionalization of indoles in the carbocyclic ring with cyclic imines

An organocatalytic enantioselective functionalization in the carbocyclic ring of indoles with benzoxathiazine 2,2-dioxides is described using a quinine-derived bifunctional organocatalyst.

Organocatalytic Aza-Friedel-Crafts/Lactonization Domino Reaction of Naphthols and Phenols with 2-Acetamidoacrylate to Naphtho- and Benzofuranones Bearing a Quaternary Center at the C3 Position

N-Acetyl ketimine generated from methyl 2-acetamidoacrylate was explored to develop an unprecedented domino aza-Friedel-Crafts/lactonization reaction with naphthols and phenols (including 5-hydroxyindoles). This novel method requires a catalyst loading of only 5 mol % of a phosphoric acid catalyst and provides a new series of 3-NHAc-naphtho- and benzofuranone derivatives bearing tetra-substituted stereogenic centers in moderate-to-good yields. The enantioselective variant using BINOL-derived phosphoric acids was also explored with 1-naphthol, providing the desired product with moderate enantioselectivities (up to 99:1 following recrystallization).