Recent advances in the dearomative functionalisation of heteroarenes

Reductive Transamination of Pyridinium Salts to N-Aryl Piperidines

Saturated N-heterocycles are found in numerous bioactive natural products and are prevalent in pharmaceuticals and agrochemicals. While there are many methods for their synthesis, each has its limitations, such as scope and functional group tolerance. Herein, we describe a rhodium-catalyzed transfer hydrogenation of pyridinium salts to access N-(hetero)aryl piperidines. The reaction proceeds via a reductive transamination process, involving the initial formation of a dihydropyridine intermediate via reduction of the pyridinium ion with HCOOH, which is intercepted by water and then hydrolyzed. Subsequent reductive amination with an exogenous (hetero)aryl amine affords an N-(hetero)aryl piperidine. This reductive transamination method thus allows for access of N-(hetero)aryl piperidines from readily available pyridine derivatives, expanding the toolbox of dearomatization and skeletal editing.

Photochemical dearomative skeletal modifications of heteroaromatics

Dearomatization has emerged as a powerful tool for rapid construction of 3D molecular architectures from simple, abundant, and planar (hetero)arenes. The field has evolved beyond simple dearomatization driven by new synthetic technology development. With the renaissance of photocatalysis and expansion of the activation mode, the last few years have witnessed impressive developments in innovative photochemical dearomatization methodologies, enabling skeletal modifications of dearomatized structures. They offer truly efficient and useful tools for facile construction of highly complex structures, which are viable for natural product synthesis and drug discovery. In this review, we aim to provide a mechanistically insightful overview on these innovations based on the degree of skeletal alteration, categorized into dearomative functionalization and skeletal editing, and to highlight their synthetic utilities.

Dearomative Construction of 2D/3D Frameworks from Quinolines via Nucleophilic Addition/Borate-Mediated Photocycloaddition

Dearomative construction of multiply-fused 2D/3D frameworks, composed of aromatic two-dimensional (2D) rings and saturated three-dimensional (3D) rings, from readily available quinolines has greatly contributed to drug discovery. However, dearomative cycloadditions of quinolines in the presence of photocatalysts usually afford 5,6,7,8-tetrahydroquinoline (THQ)-based polycycles, and dearomative access to 1,2,3,4-THQ-based structures remains limited. Herein, we present a chemo-, regio-, diastereo-, and enantioselective dearomative transformation of quinolines into 1,2,3,4-THQ-based 6-6-4-membered rings without any catalyst, through a combination of nucleophilic addition and borate-mediated [2+2] photocycloaddition. Detailed mechanistic studies revealed that the photoexcited borate complex, generated from quinoline, organolithium, and HB(pin), accelerates the cycloaddition and suppresses the rearomatization that usually occurs in conventional photocycloaddition. Based on our mechanistic analysis, we also developed further photoinduced cycloadditions affording other types of 2D/3D frameworks from isoquinoline and phenanthrene.

Chemoselective Diazine Dearomatization: The Catalytic Enantioselective Dearomatization of Pyrazine

Despite much progress in the area of dearomatization, the enantioselective dearomatization of heterocycles is limited to those with a single heteroatom. Here we report a highly enantioselective copper-catalyzed dearomatization of pyrazine, a diazine, leading to chiral C-substituted piperazines. When exposed to a chloroformate and an alkyne in the presence of a catalyst derived from a copper salt and the chiral ligand StackPhos, pyrazine is readily dearomatized to provide a 2,3-disubstituted dihydropyrazine as single diastereomer in high enantiomeric excess. Mechanistic studies support a noninnocent involvement of chloride ion preventing a second iminium alkynylation, thus enabling subsequent functionalization at the second reactive site. The synthetically useful dihydropyrazine products, obtained in up to 95% yield and 99% ee, can be further manipulated to form optically active C-substituted piperazines and C1-symmetric 1,2-diamines.

Regiodivergent Arylation of Pyridines via Zincke Intermediates

An arylation protocol for pyridines is described, via the ring-opened Zincke intermediate. Treatment of pyridines with triflic anhydride and a secondary amine produces an azahexatriene species, which undergoes regioselective Pd-catalyzed arylation at the putative C4 position. Recyclization then provides the pyridine products. Alternatively, metal-free arylation with a diaryliodonium salt is selective for the pyridine meta-position, affording a regiodivergent approach to pyridine biaryls from a common intermediate.

Oxidative Dearomatization of Pyridines

Dearomatization of pyridines is a well-established synthetic approach to access piperidines. Although remarkably powerful, existing dearomatization processes have been limited to the hydrogenation or addition of carbon-based nucleophiles to activated pyridiniums. Here, we show that arenophile-mediated dearomatizations can be applied to pyridines to directly introduce heteroatom functionalities without prior substrate activation. The arenophile platform in combination with olefin oxidation chemistry provides access to dihydropyridine cis-diols and epoxides. These previously elusive compounds are now readily accessible and can be used for the downstream preparation of diversely functionalized piperidines.

Solvent-controlled halohydroxylation or C3-C2 coupling of pyridinium salts through an interrupted dearomative reduction

Herein, we report an efficient and easily operable method to halohydroxylate pyridiniums through an interrupted dearomative reduction strategy. In this process, we make the most of the halide anion from the pyridinium salts by performing the reaction in DMSO without the need of external HX added. Notably, by changing the solvents from DMSO into Et2O, the bimolecular C3-C2 coupling occurs successfully.

Photochemical Reduction of Quinolines with γ-Terpinene

The saturation of aromatic scaffolds is valuable for the synthesis of complex rings. Herein, we demonstrate a process for photochemical dearomative reduction of quinolines. The process involves capture of a quinoline excited state with γ-terpinene. Importantly, the reaction is chemoselective as other easily reduced functionalities such as halogens or alkenes do not undergo reduction. The mechanism of the reaction has also been investigated. Finally, the generality of the approach towards other substrates is demonstrated.

Copper(I)-Catalyzed Dearomatization of Benzofurans with 2-(Chloromethyl)anilines through Radical Addition and Cyclization Cascade

Herein, we described a copper(I)-catalyzed dearomatization of benzofurans with 2-(chloromethyl)anilines to prepare various tetrahydrobenzofuro[3,2-b]quinolines and 2-(quinolin-2-yl)phenols in good to excellent yields through radical addition and an intramolecular cyclization process. Mechanistic studies revealed that 2-(chloromethyl)anilines served as radical precursors. The present method features broad substrate scope, good functional group tolerance, quinoline scaffold diversity, and radical addition dearomatization of benzofurans.

Construction of Bridged Benzazepines via Photo-Induced Dearomatization

Bridged benzazepine scaffolds, possessing unique structural and physicochemical activities, are widespread in various natural products and drugs. The construction of these skeletons often requires elaborate synthetic effort with low efficiency. Herein, we develop a simple and divergent approach for constructing various bridged benzazepines by a photocatalytic intermolecular dearomatization of naphthalene derivatives with readily available α-amino acids. The bridged motif is created via a cascade sequence involving photocatalytic 1,4-hydroaminoalkylation, alkene isomerization and cyclization. Interestingly, the diastereoselectivity can be regulated through different reaction modes in the cyclization step. Moreover, aminohydroxylation and its further bromination have also been demonstrated to access highly functionalized bridged benzazepines. Preliminary mechanistic studies have been performed to get insights into the mechanism. This method provides a divergent synthetic approach for construction of highly functionalized bridged benzazepines, which have been otherwise difficult to access.

Dearomatization of Pyridines: Stereoselective Synthesis of Functionalized [1,2,4]Triazolo[4,3-a]pyridines

The stereoselective preparation of functionalized [1,2,4]triazolo[4,3-a]pyridines from N-tosylhydrazones and pyridines was developed through the dearomatization of pyridines. The current transformation features good step- and atom-economy, high diastereoselectivity, and the efficient formation of four new carbon-heteroatom bonds in the corresponding product tetrahydro pyridines.

Bifunctional Squaramide-Catalyzed Asymmetric Cascade Reaction of Benzothiazoles with 2-Nitroallylic Acetates or Nitroenynes

We describe here an organocatalytic asymmetric cascade formal [3 + 3] cycloaddition of benzothiazoles with 2-nitroallylic acetates and nitroenynes. This dearomative methodology provided a facile and efficient strategy for the construction of a broad range of valuable benzothiazolopyridines bearing two adjacent stereogenic centers in moderate to good yields with good to excellent stereocontrol.

Rhodium-Catalyzed Asymmetric Functionalization of Quinoxalinium Salts

We report a rhodium-catalyzed asymmetric addition of aryl and alkenyl boronic acids to quinoxalinium salts that generates dihydroquinoxalines with high enantioselectivity. Functionalization of the reaction products, dihydroquinoxaline, allows the preparation of tetrahydroquinoxalines with various substitution patterns.

Synthesis of Pyrroloindolines via N-Heterocyclic Carbene Catalyzed Dearomative Amidoacylation of Indole Derivatives

Pyrroloindoline is a privileged heterocyclic motif that is widely present in many natural products and pharmaceutical compounds. Herein, we report an amidyl radical-mediated dearomatization for synthesizing a series of pyrroloindolines via N-heterocyclic carbene catalysis. In this organocatalytic process, the Breslow enolate served as both a single electron donor and an acyl radical equivalent to assemble C3a-acyl pyrroloindolines with a broad substrate scope. Sequential reduction of the indole derivatives provided the analogues of (±)-desoxyeseroline, which exhibited potential anticancer activity.

Diastereoselective construction of carbo-bridged polyheterocycles by a three-component tandem annulation reaction

By a hydroamination-induced tandem annulation process, we herein report a new three-component reaction for room temperature construction of carbo-bridged polyheterocycles with exclusive diastereoselectivity, which features readily available feedstocks, catalyst-free conditions, good substrate and functionality compatibility, no need for transition metal catalysts, and high step and atom efficiency. The products are formed via initial formation of 1,2-dihydro-3H-pyrazol-3-one nucleophiles from but-2-ynedioates and hydrazine followed by 2,4-difunctionalization of N-heteroarenium salts. Given that the obtained products possess structurally important tetrahydroquinoline and pyranopyrazole motifs, the developed chemistry is anticipated to be further applied to the discovery of functional molecules including biomedical ones.

Catalyst-Controlled Enantioselective and Regiodivergent Addition of Aryl Boron Nucleophiles to N-Alkyl Nicotinate Salts

Dihydropyridines are versatile building blocks for the synthesis of pyridines, tetrahydropyridines, and piperidines. Addition of nucleophiles to activated pyridinium salts allows synthesis of 1,2-, 1,4-, or 1,6-dihydropyridines; however, this process often leads to a mixture of constitutional isomers. Catalyst-controlled regioselective addition of nucleophiles to pyridiniums has the potential to solve this problem. Herein, we report that the regioselective addition of boron-based nucleophiles to pyridinium salts can be accomplished by the choice of a Rh catalyst.

One-pot synthesis of 4-(imidazol-1-yl)indole derivatives through a sequential dearomatization and Ag-catalyzed cyclization/Cs2CO3-mediated addition/aromatization reaction

A convenient one-pot assembly of 4-(imidazol-1-yl)indole derivatives from easily accessible o-alkynylanilines and imidazoles has been developed. The sequential dearomatization and Ag(I)-catalyzed cyclization/Cs₂CO₃-mediated conjugate addition/aromatization cascade reactions exhibit high efficiency and excellent selectivity. The combined use of a silver(I) salt and cesium carbonate is significant for facilitating this domino transformation. The 4-(imidazol-1-yl)indole products could be easily converted to the corresponding derivatives and might be valuable in biological chemistry and medicinal science.

Dearomative intermolecular [2 + 2] photocycloaddition for construction of C(sp3)-rich heterospirocycles on-DNA

DNA-encoded library (DEL) screens have significantly impacted new lead compound identification efforts within drug discovery. An advantage of DELs compared to traditional screening methods is that an exponentially broader chemical space can be effectively screened using only nmol quantities of billions of DNA-tagged, drug-like molecules. The synthesis of DELs containing diverse, sp³-rich spirocycles, an important class of molecules in drug discovery, has not been previously reported. Herein, we demonstrate the synthesis of complex and novel spirocyclic cores via an on-DNA, visible light-mediated intermolecular [2 + 2] cycloaddition of olefins with heterocycles, including indoles, azaindoles, benzofurans, and coumarins. The DNA-tagged exo-methylenecyclobutane substrates were prepared from easily accessible alkyl iodides and styrene derivatives. Broad reactivity with many other DNA-conjugated alkene substrates was observed, including unactivated and activated alkenes, and the process is tolerant of various heterocycles. The cycloaddition was successfully scaled from 10 to 100 nmol without diminished yield, indicative of this reaction's suitability for DNA-encoded library production. Evaluation of DNA compatibility with the developed reaction in a mock-library format showed that the DNA barcode was maintained with high fidelity, with <1% mutated sequences and >99% amplifiable DNA from quantitative polymerase chain reaction (PCR) and next generation sequencing (NGS).

Photoredox-Enabled Dearomatization of Protected Anilines: Access to Cyclohexadienone Imines with Contiguous Quaternary Centers

A photoredox-enabled alkylative dearomatization of protected anilines is reported. Under Ir catalysis and light irradiation, an N-carbamoyl-protected aniline and an α-bromocarbonyl compound could be simultaneously activated, and the two resulting radical species then recombine with each other to afford a dearomatized cyclohexadienone imine as the major product. A series of such imines with contiguous quaternary carbon centers were prepared, which can be further converted into cyclohexadienones, cyclohexadienols, and cyclohexyl amines.

Synthesis of Dihydropyridine Spirocycles by Semi-Pinacol-Driven Dearomatization of Pyridines

The identification of the beneficial pharmacokinetic properties of aza-spirocycles has led to the routine incorporation of these highly rigid and three-dimensional structures in pharmaceuticals. Herein, we report an operationally simple synthesis of spirocyclic dihydropyridines via an electrophile-induced dearomative semi-pinacol rearrangement of 4-(1'-hydroxycyclobutyl)pyridines. The various points for diversification of the spirocyclization precursors, as well as the synthetic utility of the amine and ketone functionalities in the products, provide the potential to rapidly assemble medicinally relevant spirocycles.