Solvent‐Mediated C3/C7 Regioselective Switch in Chiral Phosphoric Acid‐Catalyzed Enantioselective Friedel‐Crafts Alkylation of Indoles with α‐Ketiminoesters

Enantioselective Friedel-Crafts Reaction for the Synthesis of 4,7-Difunctionalized Indoles Featuring a Chiral Heteroatom-Substituted Quaternary Carbon at the C7 Position

Enantioselective functionalization of 4-aminoindoles at the site-specific C7 position via a chiral phosphoric acid-catalyzed Friedel-Crafts reaction with cyclic thioimidates was developed. This approach enables the formation of 4,7-difunctionalized indoles incorporating the chiral N,S-acetal motif with excellent yield and enantioselectivity (up to 99% yield and >99% ee). The protocol is also compatible with isatin-derived ketoimines for the highly enantioselective synthesis of 4,7-difunctionalized indole derivatives bearing aza-quaternary carbon at the C7 position. The synthetic potential was demonstrated by gram-scale experiments and versatile transformations of the products, with the method characterized by low catalyst loading, site specificity, excellent enantioselectivity, and a broad substrate scope.

Controlling Stereoselectivity with Noncovalent Interactions in Chiral Phosphoric Acid Organocatalysis

Chiral phosphoric acids (CPAs) have emerged as highly effective Brønsted acid catalysts in an expanding range of asymmetric transformations, often through novel multifunctional substrate activation modes. Versatile and broadly appealing, these catalysts benefit from modular and tunable structures, and compatibility with additives. Given the unique types of noncovalent interactions (NCIs) that can be established between CPAs and various reactants─such as hydrogen bonding, aromatic interactions, and van der Waals forces─it is unsurprising that these catalyst systems have become a promising approach for accessing diverse chiral product outcomes. This review aims to provide an in-depth exploration of the mechanisms by which CPAs impart stereoselectivity, positioning NCIs as the central feature that connects a broad spectrum of catalytic reactions. Spanning literature from 2004 to 2024, it covers nucleophilic additions, radical transformations, and atroposelective bond formations, highlighting the applicability of CPA organocatalysis. Special emphasis is placed on the structural and mechanistic features that govern CPA-substrate interactions, as well as the tools and techniques developed to enhance our understanding of their catalytic behavior. In addition to emphasizing mechanistic details and stereocontrolling elements in individual reactions, we have carefully structured this review to provide a natural progression from these specifics to a broader, class-level perspective. Overall, these findings underscore the critical role of NCIs in CPA catalysis and their significant contributions to advancing asymmetric synthesis.

Regio- and Enantioselective N-Heterocyclic Carbene-Catalyzed Annulation of Aminoindoles Initiated by Friedel-Crafts Alkylation

Chiral indoles annulated on the benzene ring are unique and significant in natural and medicinal compounds. However, accessing these enantioenriched molecules has often been overlooked. The present study introduces an organocatalytic protocol to access these compounds efficiently, demonstrated by substrate scope, functional group tolerance, and using only 1 mol % of a chiral conjugated acid catalyst. Additionally, the study explores regioselectivity, gram-scale reactions, and follow-up transformations, underscoring the method's potential.

N-Heterocyclic Carbene-Catalyzed Regio- and Enantioselective C7-Alkylation of 4-Aminoindoles with α-Bromoenals

The first carbene-catalyzed regio- and enantioselective indole C7-alkylation reaction between 4-aminoindoles and α-bromoenals is disclosed. The corresponding indole products could be obtained in moderate to good yields with good to excellent enantioselectivities. The evaluation of antibacterial activity against Psa revealed that nine of the C7-functionalized indoles exhibited superior inhibitory activities compared to the positive controls TC and BT. Our approach provides an efficient strategy to introduce a chiral chain into the C7 position of indole compounds, with potential applications evaluated in pesticide development.

Modular Synthesis of α-Aryl-α-Heteroaryl α-Amino Acid Derivatives via a Copper-Catalyzed Cross-Dehydrogenative-Coupling Reaction Using Air as the Sole Oxidant

A novel copper-catalyzed cross-dehydrogenative-coupling (CDC) process of arylglycine derivatives with N-heteroarenes for the straightforward synthesis of α-aryl-α-heteroaryl α-amino acid scaffolds has been successfully developed. This protocol exhibits a broad substrate scope with good functional group compatibility by utilizing air as the sole oxidant. The use of the reaction is also displayed through the late-stage functionalization of arylglycines bearing natural compounds or drug motifs.

B(C6F5)3-mediated direct intramolecular C7-alkenylation of N-propargylindoles

B(C6F5)3-mediated direct C7-alkenylation of N-propargylindoles without directing groups was developed. This reaction proceeds via the π-activation of the alkynyl group with B(C6F5)3/Friedel-Crafts alkenylation/proton transfer reaction sequence. Interestingly, C7-alkenylation products could further convert into the fused indoles by deprotonation and finally polyaromatic N-heterocycles by the hydride abstraction.

Synthesis of α,α-Diaryl-α-amino Acid Precursors by Reaction of Isocyanoacetate Esters with o-Quinone Diimides

A novel procedure for the synthesis of α,α-diaryl-α-amino acid derivatives has been developed. Silver oxide catalyzes the conjugate addition of α-aryl isocyanoacetates to o-quinone diimide, affording the corresponding α,α-diarylisocyano esters in excellent yields and regioselectivities in short reaction times. Acid hydrolysis of the isocyano group provides the corresponding amino acids bearing a diarylated tetrasubstituted carbon atom. The reaction is also amenable to the synthesis of α-alkyl-α-arylisocyano esters, while the reaction with 3-hydroxy o-quinone diimides provides 4H-benzo[e][1,3]oxazines via a conjugate addition/cyclization process.

Aromatic C-H bond functionalization through organocatalyzed asymmetric intermolecular aza-Friedel-Crafts reaction: a recent update

The aza-Friedel-Crafts reaction allows an efficient coupling of electron-rich aromatic systems with imines for the facile incorporation of aminoalkyl groups into the aromatic ring. This reaction has a great scope of forming aza-stereocenters which can be tuned by different asymmetric catalysts. This review assembles recent advances in asymmetric aza-Friedel-Crafts reactions mediated by organocatalysts. The mechanistic interpretation with the origin of stereoselectivity is also explained.

Chiral phosphoric acid-catalyzed enantioselective aza-Friedel-Crafts reaction of naphthols and electron-rich phenols with 2-aryl-3H-indol-3-ones

The enantioselective aza-Friedel-Crafts reaction is one of the most straightforward and efficient strategies for constructing a new carbon-carbon bond bearing quaternary stereocenter in organic synthesis, but the catalytic asymmetric aza-Friedel-Crafts reaction of naphthols/phenols with cyclic-ketimines attached to a neutral functional group remains still relatively unexplored. Herein, a highly enantioselective aza-Friedel-Crafts reaction of cyclic-ketimines and naphthols/phenols has been realized using a chiral phosphoric acid catalyst. A variety of chiral aminonaphthols (chiral indolin-3-ones) containing a quaternary stereocenter at the C2 position were obtained with excellent outcomes (up to 97% yield, 98% ee). Moreover, the synthetic utility of the enantiomerically enriched chiral aminonaphthols was demonstrated in some efficient transformations. According to the experimental results, a possible transition state model has been proposed to rationalize the origin of asymmetric induction.

Modular Synthesis of α,α-Diaryl α-Amino Esters via Bi(V)-Mediated Arylation/SN2-Displacement of Kukhtin–Ramirez Intermediates

We report a concise and modular approach to α,α-diaryl α-amino esters from readily available α-keto esters. This mild, one-pot protocol proceeds via ketone umpolung, with in situ formation of a Kukhtin–Ramirez intermediate preceding sequential electrophilic arylation by Bi(V) and S_N2 displacement by an amine. The methodology is compatible with a wide range of anilines and primary amines - including derivatives of drugs and proteinogenic amino acids - Bi(V) arylating agents, and α-keto ester substrates.

Enantioselective Friedel-Crafts Alkylation Reaction of Pyrroles with N-Unprotected Alkynyl Trifluoromethyl Ketimines

Developed herein is an enantioselective Friedel-Crafts alkylation reaction of N-unprotected alkynyl trifluoromethyl ketimines with pyrroles catalyzed by chiral phosphoric acid to furnish chiral primary α-trifluoromethyl-α-(2-pyrrolyl)propargylamines with high enantioselectivity. Transformation of the alkynyl group of the adducts afforded optically active α-trifluoromethylated amines bearing various substituents such as alkyl, alkenyl, enyne, and triazole without loss of optical purity.

Amide phosphonium salt catalyzed enantioselective Mannich addition of isoxazole-based nucleophiles to β,γ-alkynyl-α-ketimino esters

An enantioselective Mannich addition of 3,5-disubstituted 4-nitroisoxazoles to β,γ-alkynyl-α-ketimino esters promoted by an amide phosphonium salt-based catalyst has been developed. N-Cbz-protected ketimino esters with various aryl substituents attached to the alkyne unit were reacted with a series of isoxazoles with different substitution patterns. Chiral tertiary propargylic amine products were obtained with moderate to good yields and enantioselectivities. TIPS- and cyclopropyl-substituted alkynyl ketimines were also examined in the current system and the desired products were obtained with moderate yields and enantioselectivities. The potential scalability and utility of the current protocol were demonstrated by carrying out a relatively larger scale reaction followed by further transformations.

An overview of the applications of chiral phosphoric acid organocatalysts in enantioselective additions to CO and CN bonds

Since 2004, chiral phosphoric acids (CPAs) have emerged as highyl efficient organocatalysts, providing excellent results in a wide reaction scope. In this review, the applications of CPA for enantioselective additions to CO and CN bonds are covered.

Divergent synthesis of 4-amino indoles with free amine groups via tandem reaction of 2-alkynylanilines

A dearomatization method for divergent synthesis of 4-amino indoles having N1 or C4 free amine groups from 2-alkynylanilines was reported.

Organocatalytic asymmetric synthesis of quaternary α-isoxazole–α-alkynyl amino acid derivatives

Chiral phosphoric acid catalyzed enantioselective addition of 5-amino-isoxazoles with β,γ-alkynyl-α-ketimino esters provided good yields and excellent enantioselectivities.

Asymmetric C-H and N-H functionalization of Indoles involving Central Chirality via Chiral Phosphoric Acid Catalysis

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The C-H and N-H functionalization of indoles is an interesting area of research that has a useful impact on organic synthesis due to the availability of chiral indole scaffolds in the discovery of drugs, synthetic bioactive compounds, and natural products. The chiral phosphoric acid catalysts (CPAs) have proven to be a powerful and versatile class of enantioselective organocatalysts. Many asymmetric syntheses of organic compounds have been carried out with these catalysts in C–C and C-N bond formation reactions, and great progress has been reported. By 2011, several reviews were published covering some important topics and recent achievements in this field. Therefore, in this review, the most recent advances, research breakthroughs with key examples involving mechanisms of CPA-catalyzed C-H and N-H functionalization of indoles to form central chirality via Friedel Crafts, Michael type, and rearrangement reactions were reviewed and reported.

Synthesis of Proposed Structure of Aaptoline B via Transition Metal-Catalyzed Cycloisomerization and Evaluation of Its Neuroprotective Properties in C. Elegans

A concise synthesis of the proposed structure of aaptoline B, a pyrroloquinoline derived from a marine sponge, was accomplished. A key feature of this synthesis is the versatile transition metal-catalyzed cycloisomerization of N-propargylaniline to construct a quinoline skeleton. However, the spectral data of the synthesized aaptoline B did not agree with those of previous studies. The structure of the synthesized aaptoline B was confirmed using a combined 2D NMR analysis. Furthermore, we assessed the possible neuroprotective potential of aaptoline B using the C. elegans model system. In this study, aaptoline B significantly improved the viability and the morphology of dopaminergic neurons of nematodes under MPP+ exposure conditions. We also found that MPP+-induced motor deficits in nematodes were efficiently restored by aaptoline B treatment. Our findings demonstrate the neuroprotective effects of aaptoline B against MPP+-induced dopaminergic neuronal damage. Further studies are underway to explain its pharmacological mechanism.

Rate Dependence on Inductive and Resonance Effects for the Organocatalyzed Enantioselective Conjugate Addition of Alkenyl and Alkynyl Boronic Acids to β-Indolyl Enones and β-Pyrrolyl Enones

Two key factors bear on reaction rates for the conjugate addition of alkenyl boronic acids to heteroaryl-appended enones: the proximity of inductively electron-withdrawing heteroatoms to the site of bond formation and the resonance contribution of available heteroatom lone pairs to stabilize the developing positive charge at the enone β-position. For the former, the closer the heteroatom is to the enone β-carbon, the faster the reaction. For the latter, greater resonance stabilization of the benzylic cationic charge accelerates the reaction. Thus, reaction rates are increased by the closer proximity of inductive electron-withdrawing elements, but if resonance effects are involved, then increased rates are observed with electron-donating ability. Evidence for these trends in isomeric substrates is presented, and the application of these insights has allowed for reaction conditions that provide improved reactivity with previously problematic substrates.

Palladium-Catalyzed Direct and Specific C-7 Acylation of Indolines with 1,2-Diketones

The indole scaffold is a ubiquitous and useful substructure, and extensive investigations have been conducted to construct the indole framework and/or realize indole modification. Nevertheless, the direct selective functionalization on the benzenoid core must overcome the high activity of the C-3 position and still remains highly challenging. Herein, a palladium-catalyzed direct and specific C-7 acylation of indolines in the presence of an easily removed directing group was developed. This strategy usually is considered as a practical strategy for the preparation of acylated indoles because indoline can be easily converted to indole under oxidation conditions. In particular, our strategy greatly improved the alkacylation yield of indolines for which only an unsatisfactory yield could be achieved in the previous studies. Furthermore, the reaction can be scaled up to gram level in the standard reaction conditions with a much lower palladium loading (1 mol %).

Organocatalytic regio-, diastereo- and enantioselective γ-additions of isoxazol-5(4H)-ones to β,γ-alkynyl-α-imino esters for the synthesis of axially chiral tetrasubstituted α-amino allenoates

The chiral phosphoric acid catalyzed regio-, diastereo- and enantioselective reaction of isoxazol-5(4H)-ones with β,γ-alkynyl-α-imino esters has been developed.

An enantioselective aza-Friedel-Crafts reaction of 5-aminoisoxazoles with isatin-derived N-Boc ketimines

By employing a chiral phosphoric acid as a catalyst, an enantioselective aza-Friedel-Crafts reaction of 5-aminoisoxazoles with isatin-derived N-Boc ketimines was realized. The reaction provided a wide variety of novel 3-isoxazole 3-amino-oxindoles with good yields (up to 99%) and moderate to good enantioselectivities (up to 99%). The absolute configuration of one product was assigned by X-ray crystal structural analysis and a plausible reaction mechanism was proposed. In addition, a scale-up reaction was performed successfully. Finally, one product was subjected to Suzuki-Miyaura coupling with phenylboronic acid to afford the product in a moderate yield without erosion of the enantioselectivity.