Regio and Enantioselective Organocatalytic Friedel-Crafts Alkylation of 4-Aminoindoles at the C7-Position

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.

Enantioselective de novo construction of 3‑oxindoles via organocatalyzed formal [3 + 2] annulation from simple arylamines

The de novo construction of enantioenriched 3-hydroxyindolenines and 3-oxindoles from easily available starting materials has been highly desired. Herein, an enantioselectively intermolecular direct [3 + 2] annulation of aryl amine with 2,3-diketoesters to construct 3-hydroxyindolenines with a chiral tertiary alcohol has been disclosed. The results of control experiments and DFT calculation revealed that π - π interaction plays a pivotal role in the enantioselectivity-determining process of [3 + 2] annulation. The following unusual concerted [1,2]-ester migration provides a family of chiral 3-oxindoles in good to excellent yields with excellent enantioselectivity.

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.

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.

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.

Biocatalytic decarboxylative Michael addition for synthesis of 1,4-benzoxazinone derivatives

The Candida antarctica lipase B (Novozym 435) is found to catalyze a novel decarboxylative Michael addition in vinylogous carbamate systems for the synthesis of 1,4-benzoxazinone derivatives. The reaction goes through Michael addition, ester hydrolysis and decarboxylation. A possible mechanism is suggested, with simultaneous lipase-catalyzed Michael addition and ester hydrolysis. The present methodology offers formation of complex products through multi-step reactions in a one pot process under mild and facile reaction conditions with moderate to high yields (51–90%) and no side product formation. The reaction seems to be is a great example of enzymatic promiscuity.

Synthesis of C7-Functionalized Indoles through an Aromaticity Destruction-Reconstruction Process

A process for the synthesis of C7-functionalized indoles using para-substituted 2-alkynylanilines as starting materials was reported. The process involves a dearomatization, an 1,2-addition by organic lithium or Grignard reagents, an aromatization-driven allylic rearrangement, and a cyclization. A variety of groups including alkyl, aryl, alkenyl, or alkynyl groups were selectively installed at the C7 site of indoles leading to the formation of 2,5,7-trisubstituted indoles.

Catalytic Atroposelective C7 Functionalisation of Indolines and Indoles

Axially chiral atropisomeric compounds are widely applied in asymmetric catalysis and medicinal chemistry. In particular, axially chiral indole- and indoline-based frameworks have been recognised as important heterobiaryl classes because they are the core units of bioactive natural alkaloids, chiral ligands and bioactive compounds. Among them, the synthesis of C7-substituted indole biaryls and the analogous indoline derivatives is particularly challenging, and methods for their efficient synthesis are in high demand. Transition-metal catalysis is considered one of the most efficient methods to construct atropisomers. Here, we report the enantioselective synthesis of C7-indolino- and C7-indolo biaryl atropisomers by means of C-H functionalisation catalysed by chiral RhJasCp complexes.

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.

Iridium(iii)-catalyzed two-fold C–H alkylation of BINOLs with allyl alcohols

Ir(iii)-Catalyzed C–H alkylation of BINOL units has been well examined by using allyl alcohols as coupling partners.

Catalytic Asymmetric C-7 Friedel-Crafts Alkylation/N-Hemiacetalization of 4-Aminoindoles

A unique catalytic asymmetric C-7 Friedel-Crafts alkylation/N-hemiacetalization cascade reaction of 4-aminoindoles with β,γ-unsaturated α-keto esters has been described. Using a chiral magnesium H₈-BINOL-derived bis(phosphate) complex as catalyst, the resulting functionalized 1,7-annulated indole scaffolds are obtained in high yields (up to 98%) and with good to excellent enantioselectivities (up to 99%) and diastereoselectivities (up to >20:1) under mild reaction conditions.

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 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.

Rh(iii)-Catalyzed one-pot three-component cyclization reaction: rapid selective synthesis of monohydroxy polycyclic BINOL derivatives

A Rh(iii)-catalyzed three-component C–H bond functionalization protocol has been successfully applied to access complex polycyclic BINOL derivatives in which the formation of intermediate amides occurred in situ from aldehydes and amines.

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.

Rapid Synthesis of Alkenylated BINOL Derivatives via Rh(III)-Catalyzed C-H Bond Activation

Modification of BINOL units has been well examined via Rh-catalyzed C-H activation and functionalization reactions by using ester carbonyls as directing groups and alkenes as coupling partners. The one-pot strategy was an efficient protocol for the rapid synthesis of BINOL derivatives with retention of optical purity.

Hydride transfer enabled switchable dearomatization of indoles in the carbocyclic ring and the pyrrole ring

Hydride transfer enabled the first success of the regioselective dearomatization of indoles in the carbocyclic ring and the pyrrole ring, which was induced by ortho-quinone methides and vinylogous iminium intermediates, respectively.

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.

4-Aminoindoles as 1,4-bisnucleophiles for diversity-oriented synthesis of tricyclic indoles bearing 3,4-fused seven-membered rings

A straightforward access to tricyclic indoles bearing 3,4-fused seven-membered rings was established by using 4-aminoindoles as 1,4-bisnucleophiles in three-component reactions.

Brønsted Acid-Promoted Friedel-Crafts Alkylation/Cyclization of (7-Hydroxynaphthalenyl)pyrrole or (2-Hydroxyphenyl)pyrroles with Isatins for the Construction of Pyrrolospirooxindole Derivatives

An efficient trifluoroacetic acid-catalyzed cascade Friedel-Crafts alkylation/cyclization of 1-(7-hydroxynaphthalenyl)pyrrole or 1-(2-hydroxyphenyl)pyrroles with isatins has been developed, providing practical access to a variety of biologically important pyrrole-containing spirooxindoles.