Synthesis of Dihydropyridinone-Fused Indoles and α-Carbolines via N-Heterocyclic Carbene-Catalyzed [3 + 3] Annulation of Indolin-2-imines and Bromoenals

Novel access to α-carbolines with biological applications

In recent years, the 9H-pyrido[2,3-b]indole nuclei, also named α-carboline which is found in many organic compounds such as natural products, pharmaceuticals, and materials, have intensively stimulated the research of new synthetic pathways. After a brief report published in 2015 describing novel accesses and biological applications of α-carbolines, this update reports between 2015 and 2023 on the emergence of original syntheses to this heterocyclic nucleus. Examples representing these processes are described and the biological activities of α-carbolines are mentioned when they have been prepared for therapeutic purposes.

Mechanisms and Origins of Regio- and Stereoselectivities in NHC-Catalyzed [3 + 3] Annulation of α-Bromoenals and 5-Aminoisoxazoles: A DFT Study

Density functional theory (DFT) calculations were conducted to explore the mechanisms and origins of regio- and stereoselectivities underlying the [3 + 3] annulation reaction between α-bromoenals and 5-aminoisoxazoles with N-heterocyclic carbene (NHC) as the catalyst. The reaction occurs in nine steps: (1) nucleophilic addition of NHC to α-bromoenal, (2) Breslow intermediate formation through 1,2-proton transfer, (3) debromination, (4) α,β-unsaturated acyl azolium intermediate formation via 1,3-proton transfer, (5) addition of α,β-unsaturated acyl azolium intermediate to 5-aminoisoxazole, (6) deprotonation, (7) protonation, (8) ring closure, and (9) elimination of NHC. For the fifth step, 1,2-addition suggested in the experiment was not supported by our results. Instead, we found that Michael addition is energetically the most feasible pathway and the stereo-controlling step that preferentially provides the S-configuration product. DFT-computed results and experimental findings agree well. Analysis of distortion/interaction reveals that lower distortion energy leads to stability of the transition state corresponding to the S-configuration product. Global reactivity index analysis indicates that the behavior of the NHC catalyst differs significantly before and after the Breslow intermediate debromination. Before debromination, the nucleophilicity of α-bromoenal is enhanced by addition to NHC. However, after debromination, the α,β-unsaturated acyl azole generates and acts as an electrophilic reagent.

A solvent controlled regioselective synthesis of 2- and 4-substituted α-carbolines under palladium catalysis

A facile method for the chemodivergent synthesis of α-carbolines 1via palladium catalyzed [3+3] annulations of tosyliminoindolines 6 with α, β-unsaturated aldehydes 7 is described. Mechanistically, this cascade reaction proceeds through either a carba-Michael (in DMF) or aza-Michael (in NMA) pathway followed by intramolecular cyclization of the intermediate. A preliminary photo-physical study on selected products is also reported.

NHC-Catalyzed Enantioselective Transformations Involving α-Bromoenals

α-Haloenals, especially, α-bromoenals considered as one of the important building blocks in organic synthesis. They can participate in various (3+2)-, (3+3)-, (3+4)-, and (2+4)-annulation reactions with other organic molecules in the presence of an NHC catalyst to produce enantioenriched carbo-, and heterocyclic compounds. Herein, we have described NHC-catalyzed enantioselective transformations of α-bromoenals in the synthesis of various heterocycles, and carbocycles, as well as acyclic organic compounds.

Asymmetric synthesis of chiral pyrazolo[3,4-b]pyridin-6-ones under carbene catalysis

A structurally diverse set of chiral pyrazolo[3,4-b]pyridin-6-ones was efficiently prepared in excellent yields with excellent enantioselectivities via N-heterocyclic carbene-catalyzed oxidative [3 + 3] annulation of enals with pyrazol-5-amines. The reaction features mild reaction conditions, a broad substrate scope, and easy scale-up.

N-Heterocyclic Carbene-Catalyzed Atroposelective Synthesis of Axially Chiral α-Carbolinones via Heterocycle Formation

An NHC-catalyzed atroposelective synthesis of axially chiral α-carbolinones from α,β-unsaturated iminoindole derivatives and α-chloroaldehydes was developed. The reaction proceeds through a cascade process including [4 + 2] annulation and then oxidative dehydrogenation with concomitant central-to-axial chirality conversion under mild conditions. The developed method opens a new avenue to efficiently access axially chiral α-carbolinones in moderate to good enantioselectivities.

NHC-catalyzed [3 + 3] cycloaddition of α-bromoenals with nitroketene aminals or nitroketene N,S-acetals: synthesis of nitro-containing dihydropyridin-2-ones

An N-heterocyclic carbene (NHC)-catalyzed [3 + 3] cycloaddition of α-bromoenals with nitroketene aminals or nitroketene N,S-acetals has been developed. This methodology provides an efficient strategy for the construction of valuable nitro-containing heterocyclic compounds. This protocol features mild reaction conditions, easily available starting materials, broad substrate scope and easy scalability.

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.

Enantioselective Synthesis of Pyrazolo[3,4-b]pyridone Derivatives with Antifungal Activities against Phytophthora capsici and Colletotrichum fructicola

A chiral NHC-catalyzed [3 + 3] cycloaddition reaction is developed for the efficient synthesis of pyrazolo[3,4-b]pyridones in generally excellent yields and optical purities. The R, S, and racemic forms of these molecules are systematically studied via in vitro tests that detect antifungal activity against Phytophthora capsici and Colletotrichum fructicola. Chiral compounds (R)-3i, (R)-3j, and (R)-3p are identified to have excellent inhibitory effects against P. capsici and C. fructicola.

Kinetic Resolution Approach to the Synthesis of C-N Axially Chiral N-Aryl Aminomaleimides via NHC-Catalyzed [3 + 3] Annulation

Chiral NHC-catalyzed kinetic resolution of N-aryl aminomaleimides allowing the synthesis of C-N axially chiral N-aryl aminomaleimides via remote chirality control is presented. The catalytically generated α,β-unsaturated acylazoliums from 2-bromoenals underwent selective [3 + 3] annulation with one of the enantiomers of maleimide to furnish fused-dihydropyridinone (bearing axial/central chirality, up to 6:1 dr, >99:1 er) leaving the enantioenriched opposite enantiomer (up to >99:1 er). Studies on C-N bond rotation barrier and dependence on temperature are also provided.

Enantioselective Synthesis of δ-Carbolinones via N-Heterocyclic Carbene Catalysis

An enantioselective synthesis of δ-carbolinones was accomplished through N-heterocyclic carbene-catalyzed formal [3+3] annulation of 3-aminoindoles with 2-bromoenals. This protocol offers a rapid and efficient synthetic approach for accessing a wide range of enantioenriched structurally interesting δ-carbolinones under mild reaction conditions.

Design, synthesis, and biological evaluation of tetrahydro-αcarbolines as Akt1 inhibitors that inhibit colorectal cancer cells proliferation

A series of densely functionalized THαCs were designed and synthesized as Akt1 inhibitors. Organocatalytic [3+3] annulation between indolin-2-imines 1 and nitroallylic acetates 2 provided rapid access to this pharmacologically interesting framework. In vitro kinase inhibitory abilities and cytotoxicity assays revealed that compound 3af was the most potent Akt1 inhibitor, and mechanistic study indicated that compound 3af suppressed the proliferation of colorectal cancer cells via inducing apoptosis and autophagy. Molecular docking suggested that the indole fragment of 3af was inserted into the hydrophobic pocket of Akt1 protein, and the H-bond between 3af and residue Lys179 also contributed to the stable binding. This article provides an efficient strategy to design and synthesize biologically important compounds as novel Akt1 inhibitors.

Organocatalytic asymmetric synthesis of multifunctionalized α-carboline-spirooxindole hybrids that suppressed proliferation in colorectal cancer cells

An asymmetric organocatalytic cascade reaction has been reported for the rapid assembly of multifunctionalized α-carboline-spirooxindole hybrids, which suppressed proliferation in colorectal cancer cells.

Recent advances of N-heterocyclic carbenes in the applications of constructing carbo- and heterocyclic frameworks with potential biological activity

In recent years, N-heterocyclic carbenes (NHCs) have established themselves as a masterful and promising type of organocatalyst for the speedy construction of medicinally and biologically significant molecules from common and accessible small molecules. In particular, various cyclic scaffolds, including carbocycles and heterocycles, have been synthesized using NHCs via cycloaddition reaction. An exhaustive review focused on the chemistry of NHCs in these cyclic molecules has yet to be reported. In this contribution, a general picture of the utilization of NHCs in constructing twelve kinds of bioactive cyclic skeletons is firstly presented. We provide a systematic and comprehensive overview from the perspective of cycloaddition reactions; moreover, the limitations, challenges, and future prospects were discussed.

Divergent Reactivity of δ- and β'-Acetoxy Allenoates with 2-Sulfonamidoindoles via Phosphine Catalysis: Entry to Dihydro-α-carboline, α-Carboline, and Spiro-cyclopentene Motifs

The reactivity of 2-sulfonamidoindoles with acetoxy allenoates under phosphine catalysis depends on the disposition of the acetoxy (OAc) group on the allenoate. In the temperature-controlled [3 + 3] annulations, δ-acetoxy allenoates afforded dihydrocarboline and carboline scaffolds with carbon-nitrogen nucleophilic 2-sulfonamidoindoles, in which allenoate serves as a β-, γ-, and δ-carbon donor. At room temperature (25 °C), dihydro-α-carboline motifs were obtained exclusively through Michael addition, 1,4-proton shift, isomerization, 1,2-proton transfer, phosphine elimination, and aza-Michael addition. The higher temperature (80 °C) cascade protocol using Ph₃P-Cs₂CO₃ combination involves addition-elimination, aza-Claisen rearrangement, tosyl migration, and aromatization as key steps to give α-carbolines containing tosyl functionality at the γ-carbon. In contrast, with β'-acetoxy allenoate, 2-sulfonamidoindole acts only as a carbo-nucleophile in (p-tolyl)₃P-directed [4 + 1] spiro-annulation, leading to five-membered spiro-carbocyclic motifs essentially as single diastereomers (dr >20:1) via chemoselective carbo-annulation.

Metal-Free and Regioselective Synthesis of Functionalized α-Carbolines via [3 + 3] Annulation of Morita-Baylis-Hillman Acetates of Nitroalkenes with Iminoindolines

A facile, metal-free method for the synthesis of substituted α-carbolines from secondary Morita-Baylis-Hillman (MBH) acetates of nitroalkenes is presented. The cascade reaction of MBH acetates with tosyliminoindolines occurs regioselectively to form various α-carbolines with a wide substrate scope. The reaction involves mild conditions, and the products are formed in high yields within a short reaction time. The amenability of the reaction to scale up and synthetic applications of the products have been demonstrated.

N-Heterocyclic carbene-catalyzed [3 + 3] annulation of bromoenals with 2-aminochromones to access chromeno[2,3-b]pyridinones

N-Heterocyclic carbene-catalyzed [3 + 3] annulation of bromoenals with 2-aminochromones has been successfully developed. A structurally diverse set of chromeno[2,3-b]pyridinones was efficiently constructed in acceptable to excellent yields. The reaction features mild reaction conditions, a broad substrate scope, and easy scale-up.

An Environmentally Benign Cascade Reaction of 1,1-Enediamines (EDAMs) for Site-Selective Synthesis of Highly Functionalized 2,10-Dihydro-1H-imidazo[1',2':1,6]pyrido[2,3-b]indoles and Pyrroles

A novel protocol for the synthesis of pyrido[2,3-b]indoles (α-carbolines, 3) from (E)-3-(2-oxo-2-phenylethylidene) indolin-2-one derivatives 1 and 1,1-enediamine (EDAM) 2a via an unexpected cascade reaction in ethanol was developed. Pyrido[2,3-b]indole derivatives 4 were obtained by the same reaction, albeit by stirring the mixture for a longer period of time (about 48 h). As a result, two kinds of functionalized α-carbolines 3 and 4 were synthesized by the facile reaction of the (E)-3-(2-oxo-2-phenylethylidene)indolin-2-one derivatives and 2-(nitromethylene)imidazolidine under basic conditions (Cs₂CO₃) in ethanol. In addition, a diverse array of EDAM substrates (2b-2k) were tested in this reaction to afford the expected target compounds 5. This protocol is suitable for the combinatorial and parallel syntheses of natural-like products, including highly functionalized α-carbolines and pyrroles, especially 2-oxoindolin-3-yl pyrroles. This approach features several advantages, such as being a simple and practical operation (requiring only filtration and washing without column chromatography), furnishing excellent yields (72-98%), and producing diverse libraries of target compounds with potential biological activities.

Cascade C–N bond cleavage of amides/intramolecular amination reactions: an atom economical way to α-cabolin-4-ones

A novel one-pot procedure for the synthesis of 2,3-disubstituted α-carbolin-4-ones through successive C–N bond cleavage and intramolecular amination reactions involving a 1,2-acyl migration has been developed.

Benzannulation strategies for the synthesis of carbazoles, indolocarbazoles, benzocarbazoles, and carbolines

This review presents a critical and authoritative analysis of several exciting benzannulation approaches developed in the past decade for the construction of carbazoles, indolocarbazoles, benzocarbazoles, and carbolines.

Design and Efficient Synthesis of RalA Inhibitors Containing the Dihydro-α-carboline Scaffold

Ras-related protein RalA is a member of the Ras small GTPases superfamily. Its activation plays an important role in regulating tumor initiation, invasion, migration, and metastasis. In this study, we designed a new type of RalA inhibitor containing a dihydro-α-carboline scaffold. The structurally new dihydro-α-carboline derivatives could be efficiently synthesized in good yields through a newly developed three-component [3+2+1] cyclization reaction. Evaluation of the biological activity showed that some of the dihydro-α-carboline derivatives can inhibit RalA/B and proliferative activities of NSCLC cell lines. The 4-(pyridin-3-yl)-dihydro-α-carboline compound (3 o) was found to be the most potent derivative, with IC₅₀ values of 0.43±0.03, 0.64±0.07, 0.93±0.10, and 1.54±0.15 μM against A549, H1299, H460, and H1975 cells, respectively. Mechanism investigation suggested that 3 o inhibits the RalA/B activation of A549, down-regulates Bcl-2, stimulates cytochrome c and PARP cleavage, and induces cell apoptosis. A molecular docking study revealed that 3 o can form stable hydrogen bonds with residues of RalA. Moreover, amide-π and alkyl-π interactions also contributed to the affinity between 3 o and RalA.

Bifunctional Brønsted Base Catalyzed [3 + 3] Annulations of Indolin-2-imines and α,β-Unsaturated Imides: An Enantioselective Approach to α-Carbolinones

Asymmetric construction of α-carbolinones with easily available starting materials has recently attracted considerable attention from the synthesis community, and the development of effective catalysis for this target is in great demand. Here, a bifunctional Brønsted base catalyzed asymmetric [3 + 3] cyclization of indolin-2-imines and α,β-unsaturated N-acylated succinimides was developed by using the strategy of noncovalent bonding catalysis. With this organocatalytic protocol, a variety of tetrahydro-α-carbolinones bearing different substituents were synthesized with up to 99% yield and up to 96:4 er.

Construction of Dihydropyrido[2,3-d]pyrimidine Scaffolds via Aza-Claisen Rearrangement Catalyzed by N-Heterocyclic Carbenes

N-Heterocyclic carbenes (NHCs) catalyzing aza-Claisen rearrangement of α,β-unsaturated enals with cyclic vinylogous amides under oxidative conditions generating potentially biologically active dihydropyridinone-fused uracils have been developed. This strategy represents a unique NHC-activation-based path with the use of 6-aminouracils as stable α,β-diEWG cyclic vinylogous amides for the efficient synthesis of bicyclic N-unprotected lactams similar to those in many useful drugs.

Synthesis of Trifluoromethyl- and Ester Group-Substituted α-Carbolines via Iron-Catalyzed Tandem Cyclization Reaction

An efficient approach to prepare trifluoromethyl-α-carbolines and ester group-substituted α-carbolines via the tandem cyclization reaction of 2-(2-aminophenyl)acetonitriles and trifluoromethyl 1,3-diones or β,γ-unsaturated α-ketoesters was reported. The transformation proceeded smoothly in the presence of catalytic environmental-benign iron salts, which are used to prepare the desired products in moderate to good yields.

Enantioselective NHC-Catalyzed [3+3] Annulation of α-Bromoenals with 2-Aminobenzimidazoles

A chiral carbene-catalyzed [3+3] annulation of α-bromoenals with 2-aminobenzimidazoles providing pyrimido[1,2-a]benzimidazoles has been described. This protocol features a broad scope and good functional group tolerance. Biological studies indicated that the formed pyrimido[1,2-a]benzimidazole exhibited moderate cytotoxic activity against tumor cells.

Controllable synthesis of pyrido[2,3-b]indol-4-ones or indolo[3,2-b]quinolines via formal intramolecular C(sp2)-H functionalization

A novel Fe-catalyzed protocol for the controllable synthesis of pyrido[2,3-b]indol-4-ones or indolo[3,2-b]quinolines has been developed by using indole-2-carboxylic derivatives as starting materials. Indole-2-carboxenamines were transformed into pyrido[2,3-b]indol-4-ones through intramolecular N-H/C-H coupling, in which a carbonyl 1,2-migration was involved. Whereas, when indole-2-carboxarylamines were employed, indolo[3,2-b]quinolones were produced through direct N-H/C-H coupling. The desired products were obtained under mild reaction conditions in moderate to good yields with wide substrate scope. The natural product quindolinone was conveniently prepared by this reaction.

Highly Enantioselective Synthesis of Functionalized Glutarimide Using Oxidative N-Heterocyclic Carbene Catalysis: A Formal Synthesis of (-)-Paroxetine

A simple yet highly effective approach toward enantioselective synthesis of trans-3,4-disubstituted glutarimides from readily available starting materials is developed using oxidative N-heterocyclic carbene catalysis. The catalytic reaction involves a formal [3 + 3] annulation between enals and substituted malonamides enabling the production of glutarimide derivatives in a single chemical operation via concomitant formation of C-C and C-N bonds. The reaction offers easy access to a broad range of functionalized glutarimides with excellent enantioselectivity and good yield. Synthetic application of the method is demonstrated via formal synthesis of (-)-paroxetine and other bioactive molecules.

Facile Synthesis of 2,2-Diacyl Spirocyclohexanones via an N-Heterocyclic Carbene-Catalyzed Formal [3C + 3C] Annulation

A novel strategy for the construction of 2,2-diacyl spirocyclohexanones 3 has been demonstrated on the basis of an NHC-catalyzed [3C + 3C] annulation of potassium 2-oxo-3-enoates with 2-ethylidene 1,3-indandiones. Furthermore, enantioenriched 3 was obtained in good to excellent yields with good enantioselectivities when chiral N-heterocyclic carbene (NHC) was employed. Notably, ring opening of the resulting 2,2-diacyl spirocyclohexanones 3 with hydrazine led to the formation of phthalazinones in good to excellent yields.

(Dynamic) Kinetic Resolution of Enamines/Imines: Enantioselective N-Heterocyclic Carbene Catalyzed [3+3] Annulation of Bromoenals and Enamines/Imines

The enantioselective N-heterocyclic carbene catalyzed [3+3] annulation of α-bromoenals by dynamic kinetic resolution (DKR) of enamines and normal resolution of α,α-disubstituted imines were developed. The corresponding substituted dihydropyridones were isolated in good yields with excellent diastereo- and enantioselectivities, and a high selective factor (up to 83) was realized for the resolution of α,α-disubstituted imines.

Direct and Enantioselective Synthesis of N-H-Free 1,5-Benzodiazepin-2-ones by an N-Heterocyclic Carbene Catalyzed [3+4] Annulation Reaction

An NHC-catalyzed formal [3+4] annulation of α,β-unsaturated acylazoliums with protecting-group-free aryl 1,2-diamines was developed for a direct and highly enantioselective synthesis of 4-aryl N-H-free 1,5-benzodiazepin-2-ones. This methodology offers an efficient and rapid access to a wide range of enantioenriched target compounds from easily accessible starting materials. The protocol is also scalable and the desired products can easily undergo subsequent N-functionalization to afford diverse N-substituted derivatives. Additionally, a mechanism was proposed to explain the high enantioselectivity in this process.

Chiral NHC-catalyzed 1,3-dipolar [3 + 2] cycloaddition of azomethine imines with α-chloroaldehydes for the synthesis of bicyclic pyrazolidinones

An NHC-catalyzed [3 + 2] cycloaddition reaction of azomethine imines and α-chloroaldehydes was developed for the synthesis of chiral bicyclic pyrazolidinone derivatives.