Catalytic asymmetric C-7 Friedel-Crafts reaction of 4-aminoindoles: construction of N-substituted quaternary carbon atoms

A novel and enantioselective C-7 Friedel-Crafts alkylation reaction of 4-amino indoles with α-aryl enamides catalyzed by chiral H8-BINOL-derived phosphoric acid has been developed. The established method gives access to a wide variety of chiral amine derivatives bearing a quaternary carbon stereocenter (23 examples) in good-to-high yields and enantioselectivities (up to 96% yield, >99% ee) under mild reaction conditions.

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.

Asymmetric aza-Friedel-Crafts reaction of newly developed ketimines: access to chiral indeno[1,2-b]quinoxaline architectures

The functionalized 11-amino-indeno[1,2-b]quinoxaline scaffold is a pivotal structural motif in diverse bioactive compounds. In this study, we developed a chiral phosphoric acid catalysed enantioselective aza-Friedel-Crafts reaction between indeno[1,2-b]quinoxalin-11-imines and indoles. This methodology enables the synthesis of chiral hybrid architectures incorporating both 11-aminoindeno[1,2-b]quinoxaline and indole moieties, achieving exceptional synthetic efficiency (up to 99% yield) with outstanding stereocontrol (up to 99% ee).

Synthesis of 3-indolyl all-carbon quaternary centers via Rh/Brønsted acid co-catalyzed three-component reactions of azoalkenes with indoles and diazoacetates

Highly diastereoselective three-component reactions of azoalkenes with indoles and diazoacetates have been developed via a Rh/Brønsted acid co-catalyzed strategy. These transformations provide efficient construction of a range of 3-indolyl all-carbon quaternary centers in good yields and diastereoselectivities. This reaction is proposed to proceed through Michael-type trapping of zwitterionic intermediates generated from metal carbenes and indoles.

Noncovalent Interaction-Based Probe Design for PET-Facilitated Fluorescence Sensing of Synthetic Cannabinoids

Due to the structural diversity and rapid iteration of synthetic cannabinoids (SCs), their detection presents a challenging issue. Here, based on the structure and physicochemical property analysis of a typical SC, MDMB-CHMICA, four fluorescent probes were designed by introducing the recognition groups and fluorescence regulation groups on carbazole. It is found that the electron-withdrawing and conjugation-extending effect of the nitro group reduced the LUMO energy level and thereby narrowed the HOMO-LUMO energy gap, resulting in a red-shift of the fluorescence emission. As a result, the intramolecular charge transfer mechanism of the probe helps to lead to stronger fluorescence with a greater charge transfer distance. Two probes with stronger fluorescence show multiple noncovalent interactions with MDMB-CHMICA and efficient fluorescence quenching sensing through photoinduced electron transfer. This study is expected to shed light on the exploration of fluorescent probes from the analytes' physicochemical nature and would be helpful for new psychoactive substance detection.

Synthesis of 3-Substituted Indoles by Yonemitsu Three-Component Reactions Accelerated in Microdroplet/Thin Film

3-Substituted indoles are an important framework of many drugs, agricultural chemicals, functional materials, and bioactive compounds. Malononitrile-based three-component Yonemitsu reactions are attractive choices for the synthesis of 3-substituted indole derivatives but suffered from long reaction time and harsh conditions (e.g., elevated temperature and special catalysts/solvents) in conventional conversions. In this study, we developed a metal-free, efficient, mild, and wide microdroplet/thin-film method to construct 18 3-substituted indoles from various substituted aromatic aldehydes and indoles with good reaction yields (42-69%) and fantastic reaction acceleration (1.32 × 103 rate acceleration factor relative to the bulk reaction). By spraying 0.8 mol L-1 reactants at 300 μL min-1, the rate of the microdroplet/thin film product was scaled up to 1.72 g h-1. Overall, the microdroplet/thin-film method offered several advantages including high efficiency, mild conditions, wide scope, and gram-scale ability, making it attractive for synthesizing 3-substituted indoles under the requirements of sustainable chemistry.

Enantioselective Synthesis of Aminals Via Nickel-Catalyzed Hydroamination of 2-Azadienes with Indoles and N-Heterocycles

New methods for the enantioselective synthesis of N-alkylated indoles and their derivatives are of great interest because indoles are pivotal structural elements in biologically active molecules and natural products. They are also versatile intermediates in organic synthesis. Among well-established asymmetric hydroamination methods, the asymmetric hydroamination with indole-based substrates is a formidable challenge. This observation is likely due to the reduced nucleophilicity of the indole nitrogen. Herein, a unique nickel-catalyzed enantio- and branched-selective hydroamination of 2-azadienes with indoles and structurally related N-heterocycles is reported for the generation of enantioenriched N,N-aminals. Salient features of this reaction include good yields, mild reaction conditions, high enantioselectivities, and broad substrate scope (60 examples, up to 96% yield and 99% ee). The significance of this approach with indoles and other N-heterocycles is demonstrated through structural modification of natural products and drug molecules and the preparation of enantioenriched N-alkylated indole core structures. Mechanistic studies reveal that olefin insertion into a Ni-H bond in the hydroamination is the enantio-determining step and oxidative addition of the N-H bond may be the turnover-limiting step.

Radical-Initiated Dearomative Annulation of Tryptamine-Derived Isocyanides: Selective Synthesis of CF3-Substituted β-Aza-spiroindolines and β-Carbolines

A mild approach for synthesizing CF3-substituted β-aza-spiroindolines and β-carbolines from tryptamine-derived isocyanides via site-selective radical annulations has been reported. The crucial role of C2 substituents in the selective annulation process has been clarified. The approach shows good generality and practical applicability, highlighting its effectiveness and versatility.

Asymmetric electrophilic functionalization of amino-substituted heteroaromatic compounds: a convenient tool for the enantioselective synthesis of nitrogen heterocycles

The catalytic asymmetric electrophilic functionalization of the less reactive N-heteroaromatic compounds has been reported using the approach of the introduction of an exocyclic amino substituent. This strategy has allowed enantioselective Friedel-Crafts alkylation in pyrazoles, isoxazoles and isothiazoles, as well as in aminoindoles, aminobenzofurans and aminobenzothiophenes. Several stereoselective methods have been used for the 1,4-addition or 1,2-addition of these heteroaromatic compounds to different electrophiles employing organocatalysts or chiral metal complexes. The activating exocyclic amino substituent has also been used as a nucleophile in tandem reactions, including formal cycloadditions ([3+2] and [3+3]), for the synthesis of highly functionalized chiral nitrogen heterocycles.

Engineering the L-tryptophan metabolism for efficient de novo biosynthesis of tryptophol in Saccharomyces cerevisiae

Tryptophol (IET) is a metabolite derived from L-tryptophan that can be isolated from plants, bacteria, and fungi and has a wide range of biological activities in living systems. Despite the fact that IET biosynthesis pathways exist naturally in living organisms, industrial-scale production of IET and its derivatives is solely based on environmentally unfriendly chemical conversion. With diminishing petroleum reserves and a significant increase in global demand in all major commercial segments, it becomes essential to develop new technologies to produce chemicals from renewable resources and under mild conditions, such as microbial fermentation. Here we characterized and engineered the less-studied L-tryptophan pathway and IET biosynthesis in the baker's yeast Saccharomyces cerevisiae, with the goal of investigating microbial fermentation as an alternative/green strategy to produce IET. In detail, we divided the aromatic amino acids (AAAs) metabolism related to IET synthesis into the shikimate pathway, the L-tryptophan pathway, the competing L-tyrosine/L-phenylalanine pathways, and the Ehrlich pathway based on a modular engineering concept. Through stepwise engineering of these modules, we obtained a yeast mutant capable of producing IET up to 1.04 g/L through fed-batch fermentation, a ~ 650-fold improvement over the wild-type strain. Besides, our engineering process also revealed many insights about the regulation of AAAs metabolism in S. cerevisiae. Finally, during our engineering process, we also discovered yeast mutants that accumulate anthranilate and L-tryptophan, both of which are precursors of various valuable secondary metabolites from fungi and plants. These strains could be developed to the chassis for natural product biosynthesis upon introducing heterologous pathways.

Rh-Catalyzed C-H Alkynylation of Indole Derivatives with Silver(I)-Controlled Regiodivergence

We have disclosed silver(I)-induced switching of regioselectivity in rhodium-catalyzed C-H alkynylation of indole derivatives with the help of a pivaloyl directing group by tuning C-H metalation modes. The judicious choice of AgOAc, Ag2O, and Ag2CO3 affords an array of C2-alkynylated indoles, C4-alkynylated indoles, and C2,C4-dialkynylated indoles, respectively. The synthetic utility of the alkyne fragment is demonstrated by derivatization into valuable indole-based compounds.

Efficient assembly and anti-tumor evaluation of novel polycyclic [1,2-a]-fused indoles

Structurally diverse cyclopenta[4,5]pyrrolo[1,2-a]indoles heterocycles were smoothly constructed in good to excellent yields (up to 99 %) with excellent diastereoselectivities (>19:1 dr) through a novel and facile strategy based on BF3-catalyzed Friedel-Crafts alkylation/Aldol/Dehydrative cyclization cascade reaction. The anti-proliferative activity of these newly synthesized polycyclic indoles was screened, and all the functionalized reductive derivatives exhibited favorable anti-tumor activity. Notably, compound 4ae displayed the remarkable inhibitory activity against MCF-7 and HeLa cells with IC50 values of 4.62 μM and 7.71 μM, respectively. Mechanistically, the representative compound 4ae could effectively induce apoptosis of MCF-7 cells in crediting to up-regulate the relative expression of apoptotic protein BAX/Bcl-2, subsequently activate Pro-caspase 9 and cleave PARP, simultaneously block the cell cycle through down- and up-regulate the expression of cyclin B1 and p53, respectively. Moreover, compound 4ae also exhibited promising antineoplastic efficacy in subcutaneous MCF-7 xenograft mice which manifest significant shrunken tumors conspicuous nuclear apoptotic signal and minimal systemic toxicity. This strategy not only established a novel and efficient method for the assembly of structurally complex indole heterocycles, but also provided a series of compounds possessing attractive anti-cancer activity, which holds immense potential for future biomedical applications.

Diversity-oriented synthesis of indole-fused scaffolds and bis(indolyl)methane from tosyl-protected tryptamine

An efficient, diversity-oriented synthesis of indole-1,2-fused 1,4-benzodiazepines, tetrahydro-β-carbolines, and 2,2'-bis(indolyl)methanes was established starting from tosyl-protected tryptamine. These diverse privileged skeletons were controllably constructed by adjusting different hydride donors and Brønsted acids. A variety of indole-1,2-fused 1,4-benzodiazepines were facilely accessed using benzaldehydes bearing cyclic amines as hydride donors via a cascade N-alkylation/dehydration/[1,5]-hydride transfer/Friedel-Crafts alkylation sequence. The reaction site could be switched when benzaldehydes bearing an alkoxy moiety as hydride donors were used for the generation of tetrahydro-β-carbolines. On the other hand, the switchable synthesis of 2,2'-bis(indolyl)methanes could be achieved as well by applying p-TsOH·H2O as a catalyst. The reactions feature mild conditions, simple and practical operation, excellent efficiency and the use of EtOH as a green solvent. Using the concept of diversity-oriented, reagent-based synthesis, the inexpensive feedstock tryptamine was efficiently converted to three different types of privileged scaffolds, which facilitates rapid compound library synthesis for accelerating drug discovery.

Experimental and computational investigation of the α-amylase catalyzed Friedel-Crafts reaction of isatin to access symmetrical and unsymmetrical 3,3',3''-trisindoles

Trisindoles are of tremendous interest due to their wide range of biological activities. In this context, a number of methods have been reported in the past to synthesize 3,3',3''-trisindoles. However, most of the methods are only able to produce symmetrical 3,3',3''-trisindoles. Herein, we develop a sustainable and efficient approach to synthesize symmetrical as well as unsymmetrical 3,3',3''-trisindoles in a very selective manner using the α-amylase enzyme as a catalyst. Furthermore, various differently substituted isatin and indoles were used to prove the generality of the protocol and symmetrical or unsymmetrical 3,3',3''-trisindoles were obtained in 43-97% isolated yields. Next, a probable mechanism is proposed and investigated using molecular dynamics (MD) investigation to gain more insight into the role of residues available in the active site of the α-amylase enzyme. These studies revealed that Glu230, Lys209, and Asp206 in the active site of α-amylase play an important role in this catalysis. Moreover, the DFT studies suggested the formation of bisindole and alkylideneindolenine intermediates during the transformation. We synthesized four different biologically important 3,3',3''-trisindoles on a gram scale, which proved the robustness and scalability of this protocol.

Pd-Catalyzed Asymmetric Larock Indole Synthesis to Access Axially Chiral N-Arylindoles

Larock indole synthesis is one of the most straightforward and efficient methods for the synthesis of indoles; however, there has been no asymmetric version yet for the construction of indole-based axially chiral N-arylindoles since its initial report in 1991. Herein we report the first example of an asymmetric Larock indole synthesis by employing a chiral sulfinamide phosphine (SadPhos) ligand (Ming-Phos) with palladium. It allows rapid construction of a wide range of axially chiral N-arylindole compounds in good yields up to 98:2 er. The application of this unique chiral scaffold as an organocatalyst is promising. Furthermore, a kinetic study has revealed that the alkyne migratory insertion is the rate-determining step, which has been proven by the density functional theory (DFT) calculations. Additionally, DFT studies also suggest that the N-C dihedral difference caused by the steric hindrance of the ligand contributes to enantioselectivity control.

Regioselective C3Alkylation of Indoles for the Synthesis of Bis(indolyl)methanes and 3-Styryl Indoles

Herein, we describe an efficient transition-metal-free regioselective C3alkylation of indoles for the synthesis of bis(indolyl)methanes and 3-styryl indoles. Nitrobenzene is employed as the oxidant to oxidize the alcohols in the presence of a strong base and the reaction avoids the use of transition metals such as Ru and Mn. The protocol provides a favorable route to access biologically active compounds such as arundine, vibrindole A, and turbomycin B.

Controllable Synthesis of N-Heterocycles via Hydride Transfer Strategy-Enabled Formal [5 + 1] and [5 + 2] Cyclizations

The Brønsted acid-controlled switchable synthesis of indoline-fused tetrahydroquinolines and indole-fused benzazepines was developed through hydride transfer-enabled formal [5 + 1] and [5 + 2] cyclization reactions from indoles and N-alkyl o-aminobenzoketones. Indoline, furanone, and tetrahydroquinoline hybridized pentacyclic products were unprecedentedly accessed via a cascade condensation/hydride transfer/dearomatization-cyclization/deethylation/nucleophilic addition process. In addition, the undeveloped hydride transfer-involved [5 + 2] cyclizations were also realized for direct construction of indole-fused benzazepines.

Brønsted Acid-Catalyzed Tandem Double Friedel-Crafts Alkylation to Construct a Dihydrophenalene Skeleton Bearing an All-Carbon Quaternary Center

An efficient Brønsted acid-catalyzed tandem reaction has been developed for the construction of a dihydrophenalene skeleton bearing an all-carbon quaternary center. Starting with 2-naphthol-tethered ketones and indoles, the tandem reaction catalyzed by TsOH monohydrate proceeded smoothly with good to excellent efficiency through a double Friedel-Crafts alkylation process. Moreover, the synthetic utility of this method was demonstrated by easy gram-scale preparation and product transformations to fused hexacyclic compounds.

C3-Functionalization of indoles with α-heteroaryl-substituted methyl alcohols

The transition metal-free Cs2CO3/Oxone®-mediated C3-alkylation of indoles proceeds in moderate to high yields with a variety of C4-C7 functionalized indoles and is applicable to 2-, 3- and 4-hydroxymethyl pyridines and related electron-deficient heterocycles, permitting novel late-stage drug functionalizations. Preliminary mechanistic studies support a hydrogen autotransfer-type chain process starting with an initial oxidation of the alcohol to the corresponding aldehyde, followed by a subsequent condensation onto indole and reduction/hydride delivery from another equivalent of the primary alcohol.

An Axially Chiral Styrene-Phosphine Ligand for Pd-Catalyzed Asymmetric N-Alkylation of Indoles

An efficient palladium-catalyzed enantioselective direct N-alkylation of indoles using a novel type of axially chiral styrene-phosphine ligand SJTU-PHOS-1 was developed. This reaction demonstrated good functional group compatibility and a wide range scope of substrates in mild conditions. Moreover, the DFT calculations expounded the coordination mode of the metal catalyst and the axially chiral styrene-phosphine ligand in the enantioselectivity control.

Catalytic asymmetric indolization by a desymmetrizing [3 + 2] annulation strategy

A new catalytic asymmetric indolization reaction by a desymmetrizing [3 + 2] annulation strategy is developed. The reaction proceeds via a rhodium-catalyzed enantioposition-selective addition/5-exo-trig cyclization/dehydration cascade between ortho-amino arylboronic acids and 2,2-disubstituted cyclopentene-1,3-diones to produce N-unprotected cyclopenta[b]indoles bearing an all-carbon quaternary stereocenter in high yields with good enantioselectivities. A quantitative structure-selectivity relationship (QSSR) model was established to identify the optimal chiral ligand, which effectively controlled the formation of the stereocenter away from the reaction site. Density functional theory (DFT) calculations, non-covalent interaction analysis, and Eyring analysis were performed to understand the key reaction step and the function of the ligand.

3-Nitroindoles Serving as N-Centered Nucleophiles for Aza-1,6-Michael Addition to para-Quinone Methides

An unprecedented N-alkylation of 3-nitroindoles with para-quinone methides was developed for the first time. Using potassium carbonate as the base, a wide range of structurally diverse N-diarylmethylindole derivatives were obtained with moderated to good yields via the protection group migration/aza-1,6-Michael addition sequences. The reaction process was also demonstrated by control experiments. Different from the previous advances where 3-nitrodoles served as electrophiles trapping by various nucleophiles, the reaction herein is featured that 3-nitrodoles is defined with latent N-centered nucleophiles to react with ortho-hydrophenyl p-QMs for construction of various N-diarylmethylindoles.

Synthesis of azepane-fused pyrano[3,2-b]indoles by Lewis acid-catalysed oxa Diels-Alder reactions

The dihydroazepino[1,2-a]indole diones 3 are tricyclic oxindole-type enones which are readily accessible by catalytic photooxygenation of cyclohepta[b]indoles 1 followed by dehydration. Lewis acid-catalysed oxa Diels-Alder reactions of enones 3 with enol ethers 4 were developed that lead to novel tetracyclic azepane-fused pyrano[3,2-b]indoles 5, with high stereoselectivity and under mild reaction conditions.

Rhodium(I)-Catalyzed Direct Enantioselective C-H Functionalization of Indoles

A highly regiospecific vinylogous carbene insertion protocol for direct asymmetric C-H functionalization of indoles with arylvinyldiazoacetates has been developed. Under the catalysis of simple Rh(I)/chiral diene complexes, the reaction occurs solely at the vinylogous position of the vinylcarbenoid with exceptional E selectivity and enantiocontrol. It provides an efficient way to obtain an interesting class of chiral indole scaffolds bearing an α,β-unsaturated ester unit and a gem-diaryl carbon stereocenter in good yields (≤99%) with excellent enantioselectivities (≤96%) at room temperature.

Organocatalytic Regio- and Enantioselective Allylic Alkylation of Indolin-2-imines with MBH Carbonates toward 3-Allylindoles

An organocatalytic asymmetric C3-allylic alkylation of indolone-2-imines with MBH carbonates has been developed for the first time. As opposed to previous reports, an "interrupted" annulation was achieved, affording 3-allylindoles in generally high yields with excellent stereoselectivities. The representative scale-up reaction and transformation of 3-allylindoles were examined. A possible mechanism was also proposed.

Annulation of Indole-2-Carboxamides with Bicycloalkenes Catalyzed by Ru(II) at Room Temperature: An Easy Access to β-Carboline-1-one Derivatives under Mild Conditions

Herein, we report the annulation of indole-2-carboxamides with bicycloalkenes, to synthesize β-carboline-1-one derivatives under mild conditions. The commercially available ruthenium catalyst was used for the reaction. This reaction tolerates a wide range of functional groups and affords a good yield of β-carboline-1-one derivatives. A reversible cyclometalation pathway was found to be operative in the mechanistic study.

Asymmetric dearomative cyclopropanation of naphthalenes to construct polycyclic compounds

Catalytic asymmetric dearomatization (CADA) reactions is an important synthetic method for constructing enantioenriched complex cyclic systems from simple aromatic feedstocks. However, the CADA reactions of nonactivated arenes, such as naphthalenes and benzenes, have been far less explored than those of electronically activated arenes, such as phenols, naphthols and indoles. Herein, we disclose an asymmetric dearomative cyclopropanation of naphthalenes for the rapid construction of polycyclic compounds. With chiral dirhodium carboxylate as a catalyst, the dearomative cyclopropanation proceeded smoothly under mild conditions and afforded benzonorcaradiene-containing tetracycles in good yield and high enantioselectivity (up to 99% ee). Three stereogenic centers, including two all-carbon quaternary centers, were created in the dearomatization reaction. Moreover, a variety of functional groups are well-tolerated in the reaction. The products could be readily converted into other complex polycycles while maintaining the high ee value.

Enantioselective synthesis of N-alkylindoles enabled by nickel-catalyzed C-C coupling

Enantioenriched N-alkylindole compounds, in which nitrogen is bound to a stereogenic sp³ carbon, are an important entity of target molecules in the fields of biological, medicinal, and organic chemistry. Despite considerable efforts aimed at inventing methods for stereoselective indole functionalization, straightforward access to a diverse range of chiral N-alkylindoles in an intermolecular catalytic fashion from readily available indole substrates remains an ongoing challenge. In sharp contrast to existing C-N bond-forming strategies, here, we describe a modular nickel-catalyzed C-C coupling protocol that couples a broad array of N-indolyl-substituted alkenes with aryl/alkenyl/alkynyl bromides to produce chiral N-alkylindole adducts in single regioisomeric form, in up to 91% yield and 97% ee. The process is amenable to proceed under mild conditions and exhibit broad scope and high functional group compatibility. Utility is highlighted through late-stage functionalization of natural products and drug molecules, preparation of chiral building blocks.

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.

Switchable Reductive N-Trifluoroethylation and N-Trifluoroacetylation of Indoles with Trifluoroacetic Acid and Trimethylamine Borane

The metal-free reductive N-trifluoroethylation and N-trifluoroacetylation of indoles have been developed. Bench stable and inexpensive trimethylamine borane and trifluoroacetic acid (TFA) were utilized as the reductive and fluorinating reagents, respectively. These transformations were switchable on the basis of altering the loading of trimethylamine borane and TFA. Preliminary experiments indicated indoline was the common intermediate in these two transformations.

Weak Chelation-Assisted C4-Selective Alkylation of Indoles with Cyclopropanols via Sequential C-H/C-C Bond Activation

A Rh-catalyzed weak chelation-guided C4-alkylation of indoles has been accomplished using cyclopropanols as an alkylating agent via the cascade C-H and C-C bond activation. The substrate scope, functional group tolerance, and late-stage mutation of drug molecules are the important practical features.

Catalytic Atroposelective Electrophilic Amination of Indoles

Reported here is the first catalytic atroposelective electrophilic amination of indoles, which delivers functionalized atropochiral N-sulfonyl-3-arylaminoindoles with excellent optical purity. This reaction was furnished by 1,6-nucleophilic addition to p-quinone diimines. Control experiments suggest an ionic mechanism that differs from the radical addition pathway commonly proposed for 1,6-addition to quinones. The origin of 1,6-addition selectivity was investigated through computational studies. Preliminary studies show that the obtained 3-aminoindoles atropisomers exhibit anticancer activities. This method is valuable with respect to enlarging the toolbox for atropochiral amine derivatives.

Organocatalytic formal [3 + 3] cyclization of α-(6-indolyl) propargylic alcohols

With the aid of acetic acid, a 1,10-conjugate addition-mediated formal [3 + 3] cyclization of alkynyl indole imine methides formed in situ from α-(6-indolyl) propargylic alcohols with 1,3-dicarbonyl compounds such as 4-hydroxycoumarins and cyclohexane-1,3-diones was developed, which provided robust access to a wide range of pyranocoumarin and pyran derivatives containing an indole skeleton with high efficiency under mild conditions.

3,4-Annulated Indoles via Tandem Cyclopropane Ring-Opening/Conia-ene and Michael Addition/Conia-ene Reactions

4-Alkynyl indoles, when treated with a benzylidene malonate or a donor-acceptor cyclopropane in the presence of a zinc halide, furnished 3,4-hexannylated or 3,4-heptannulated products, respectively, in fair to excellent yields. The reaction proceeds via a tandem addition/Conia-ene cyclization.

Oxidative Addition to the N-C Bond Vs Formation of the Zwitterionic Intermediate in Platinum(II)-Catalyzed Intramolecular Annulation of Alkynes to Form Indoles: Mechanistic Studies and Reaction Scope

In this study, Pt(II)-catalyzed intramolecular translocation annulation of ortho-alkynylamides to the formation of indoles is presented, where a proposed intermediacy of zwitterionic intermediate has been substantiated over the oxidative addition. We focused our attention on Pt(II)-catalyzed aminoacylation of alkynes both theoretically and experimentally using low boiling solvent where the formation of deacylation product was suppressed simultaneously. One-step intramolecular [1,3]-acyl migration from the zwitterionic intermediate is highly unlikely, which imparts a high energy barrier of +99.0 kcal mol^-1. Another possible approach involving oxidative addition to the N-C bond, migratory insertion to alkyne, and subsequent reductive elimination is also explored through DFT studies to justify the reaction consequence. However, based on the computational studies, it is suggested that initial zwitterion formation is highly favored over oxidative addition. We suggest the formation of an acylium intermediate, which can further react with indol-3-ylplatinum species in an intramolecular manner, albeit within the same solvent cage to form 3-acyl indoles.

Oxidative N-Heterocyclic Carbene-Catalyzed Intramolecular Friedel-Crafts Alkylation of Indoles for the Synthesis of Spirocyclic Indolenines

The stereoselective intramolecular dearomatizing spirocyclization of indoles via oxidative N-heterocyclic carbene (NHC) catalysis to afford indolenines bearing an all-carbon quaternary center at the 3-position is reported. The reaction proceeds via the intramolecular nucleophilic addition of the indole to an in situ generated α,β-unsaturated acyl azolium. The cyclized indolenine bearing an acyl azolium functionality is trapped by a suitable external nucleophile that does not efficiently react with the α,β-unsaturated acyl azolium via direct acylation.

Dearomative Iodocyclization of N-(o-Alkynyl)aryl Isoindole

We present a dearomative iodocyclization of N-(o-alkynyl)aryl isoindole here, which affords various biologically active benzoindoleazine skeletons containing alkenyl iodine. The products can further undergo cycloaddition or coupling reactions to afford a series of highly functionalized N-fused polycyclic scaffolds.

Synthesis of functionalized indoles via cascade benzannulation strategies: a decade's overview

Indoles are one of the most prominent aromatic heterocycles in the organic chemistry field. Due to their widespread presence in various natural products, alkaloids, drugs, approved medicines, etc. the synthesis and functionalization of indoles are of great interest. This review emphasizes recent developments and techniques in the domino cascade cyclization process in the last decade starting from the various building blocks. In particular, this review depicts several intriguing benzannulation methods of creating a benzene ring on a pre-existing pyrrole nucleus in an inter/intramolecular fashion under metal-catalyzed/metal-free approaches. Different subsections focus on gradual timely developments in this complementary area and a detailed analysis of the mechanisms and reactivity patterns. As a complementary method, this review gives a significant incentive to various annulation strategies and also gives an overview of the remaining challenges and upcoming possibilities.

Phosphine-Mediated Sequential [2+4]/[2+3] Annulation to Construct Pyrroloquinolines

A domino [2+4]/[2+3] sequential annulation reaction of MBH carbonates with N-unprotected indoles has been developed to provide various pyrroloquinoline derivatives in ≤94% yield and 20:1 dr. The reaction could be either mediated by stoichiometric PCy₃ or catalyzed by R₃PO via P^III/P^V═O redox cycling in the presence of phenylsilane. This method assembles polycyclic 1,7-fused indoles in one step diastereoselectively.

One-Pot Three-Component Coupling Reaction of α-Amino Aryl Ketones, Indoles, and Perbromomethane Under Mild Conditions

A simple and efficient one-pot three-component cascade reaction of α-amino aryl ketones, indoles, and CBr₄ in moderate to good yields has been developed. This new strategy exhibits excellent mild reaction conditions and step-economy, easily accessible reactants, and simultaneous construction of three different new bonds (C=N, C–C, and N-Br) in a single step. It is worth noting that the protocol developed provides a simple and practical tool for the construction of diverse indole-containing heterocyclic frameworks, indicating its potential applications in medicinal and material chemistry.

Aymmetric Aza-Friedel-Crafts Reaction of Isatin-Derived Ketimines with Indoles Catalyzed by a Chiral Phase-Transfer Catalyst

A highly enantioselective aza-Friedel-Crafts reaction of 1H-indoles with isatin-derived N-Cbz-ketimines catalyzed by quinine-derived phase-transfer catalysts was developed. A series of chiral 3-aminobisindole compounds containing a tetrasubstituted stereocenter were constructed by this protocol in high yields (82-91%) and moderate to excellent enantioselectivities (46-94% ee).