Diastereoselective trifunctionalization of pyridinium salts to access structurally crowded azaheteropolycycles

Reaction strategies for the meta-selective functionalization of pyridine through dearomatization

The pyridine moiety is a crucial structural component in various pharmaceuticals. While the direct ortho- and para-functionalization of pyridines is relatively straightforward, the meta-selective C-H functionalization remains a significant challenge. This review highlights dearomatization strategies as a key area of interest in expanding the application of meta-C-H functionalization of pyridines. Dearomatization enables the meta-functionalization through various catalytic methods that directly generate dearomatization products, and some products can be rearomatized back to pyridine derivatives. Furthermore, this article also covers the dearomatization of multiple positions of pyridine in the synthesis of polycyclic compounds. It offers a comprehensive overview of the latest advancements in dearomatization at different positions of pyridine, aiming to provide a valuable resource for researchers in this field. It also highlights the advantages and limitations of existing technologies, aiming to inform a broader audience about this important field and foster its future development.

Recent Strategies in the Nucleophilic Dearomatization of Pyridines, Quinolines, and Isoquinolines

Dearomatization reactions have become fundamental chemical transformations in organic synthesis since they allow for the generation of three-dimensional complexity from two-dimensional precursors, bridging arene feedstocks with alicyclic structures. When those processes are applied to pyridines, quinolines, and isoquinolines, partially or fully saturated nitrogen heterocycles are formed, which are among the most significant structural components of pharmaceuticals and natural products. The inherent challenge of those transformations lies in the low reactivity of heteroaromatic substrates, which makes the dearomatization process thermodynamically unfavorable. Usually, connecting the dearomatization event to the irreversible formation of a strong C-C, C-H, or C-heteroatom bond compensates the energy required to disrupt the aromaticity. This aromaticity breakup normally results in a 1,2- or 1,4-functionalization of the heterocycle. Moreover, the combination of these dearomatization processes with subsequent transformations in tandem or stepwise protocols allows for multiple heterocycle functionalizations, giving access to complex molecular skeletons. The aim of this review, which covers the period from 2016 to 2022, is to update the state of the art of nucleophilic dearomatizations of pyridines, quinolines, and isoquinolines, showing the extraordinary ability of the dearomative methodology in organic synthesis and indicating their limitations and future trends.

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.

Phosphine Catalyzed Michael-Type Additions: The Synthesis of Glutamic Acid Derivatives from Arylidene-α-amino Esters

The reaction of arylidene-α-amino esters with electrophilic alkenes to yield Michael-type addition compounds is optimized using several phosphines as organocatalysts. The transformation is very complicated due to the generation of several final compounds, including those derived from the 1,3-dipolar cycloadditions. For this reason, the selection of the reaction conditions is a very complex task and the slow addition of the acrylic system is very important to complete the process. The study of the variation in the structural components of the starting imino ester is performed as well as the expansion of other electron-poor alkenes. The crude products have a purity higher than 90% in most cases without any purification. A plausible mechanism is detailed based on the bibliography and the experimental results. The synthesis of pyroglutamate entities, after the reduction of the imino group and cyclization, is performed in high yields. In addition, the hydrolysis of the imino group, under acidic media, represents a direct access to glutamate surrogates.

Diverse Synthesis of Fused Polyheterocyclic Compounds via [3 + 2] Cycloaddition of In Situ-Generated Heteroaromatic N-Ylides and Electron-Deficient Olefins

[3 + 2] Cycloaddition reactions of heteroaromatic N-ylides with electron-deficient olefins have been developed. The heteroaromatic N-ylides, in situ generated from N-phenacylbenzothiazolium bromides, can smoothly react with maleimides under very mild conditions, affording fused polycyclic octahydropyrrolo[3,4-c]pyrroles in good-to-excellent isolated yields. This reaction concept could also be extended to 3-trifluoroethylidene oxindoles and benzylidenemalononitriles as electron-deficient olefins for accessing highly functionalized polyheterocyclic compounds. A gram-scale experiment was also carried out to verify the practicability of the methodology.

Tunable vicinal, geminal diphosphorylation and C-N bond phosphorylation of enaminones toward divergent phosphorylated ketone derivatives

This paper reports the trifunctionalization reactions of tertiary enaminones in the fashion of selective gem- and vicinal diphosphorylation, leading to the tunable synthesis of α,α- and α,β-diphosphoryl ketones. In addition, the C-N bond phosphorylation with improved substrate tolerance has been achieved.

Dearomative cyclization of pyridines/isoquinolines with cyclopropenones: access to indolizinones and benzo-fused indolizinones

Dearomatization reactions provide a rapid approach to construct complicated molecules that are difficult to synthesize by traditional methods from simple aromatic compounds. Herein, we report an efficient dearomative [3+2] cycloaddition reaction of 2-alkynyl pyridines with diarylcyclopropenones, leading to the synthesis of densely functionalized indolizinones in moderate to good yields under metal-free conditions. In addition, this strategy can also be employed in dearomative cyclization of isoquinolines to access a variety of benzo-fused indolizinones. Density functional theory (DFT) calculations revealed that an appropriate substituent at the 2-position of pyridine is crucial to the dearomatization process.

Organophosphine as an Alkyl Transfer Shuttle for the Direct β-Alkylation of Chalcones Using Alkyl Halides

We report an efficient alkyl transfer strategy for the direct β-alkylation of chalcones using commercially available alkyl bromides as alkyl reagents. In this transformation, the ortho-phosphanyl substituent in the chalcones is crucial for controlling their reactivity and selectivity. It also serves as a reliable alkyl transfer shuttle to transform electrophilic alkyl bromides into nucleophilic alkyl species in the form of quaternary phosphonium salts and transfer the alkyl group effectively to the β-position of the chalcones. This alkyl transfer strategy can be further extended to the alkenylation of ortho-phosphanyl benzaldehydes to assemble functionalized polyenes.

Recent advances in the dearomative functionalisation of heteroarenes

Dearomatisation reactions of (hetero)arenes have been widely employed as efficient methods to obtain highly substituted saturated cyclic compounds for over a century. In recent years, research in this area has shifted towards effecting additional C-C bond formation during the overall dearomative process. Moving away from classical hydrogenation-based strategies a wide range of reagents were found to be capable of initiating dearomatisation through nucleophilic addition (typically a reduction) or photochemically induced radical addition. The dearomatisation process gives rise to reactive intermediates which can be intercepted in an intra- or intermolecular fashion to deliver products with significantly increased molecular complexity when compared to simple dearomatisation. In this Perspective recent examples and strategies for the dearomative functionalisation of heteroaromatic systems will be discussed.

Substrate-directed divergent synthesis of fused indole polycycles through Rh(II)-catalyzed cascade reactions of bis(diazo)indolin-2-ones

Herein, we report a substrate-directed diverse synthetic strategy toward two kinds of structurally intriguing fused indole polycycles through Rh(II)-catalyzed cascade reactions of bis(diazo)indolin-2-ones with enaminones. The subtle structural changes in enaminones lead to different reactivities. Cyclic enaminones produce indolo[2,3-b]indoles, whereas acyclic ones afford oxazolo[3,2-a]indoles.

Diastereoselective construction of bridged piperidines through an interrupted dearomative reduction

An interrupted dearomative reduction strategy was developed to transform planar chalcone-based pyridinium salts into structurally intriguing bridged piperidines in a completely regio- and diastereoselective manner. This reaction proceeded successfully by using cheap and easily accessible NaBH₄ as the reductant under mild conditions without exclusion of oxygen or use of special equipment.

One-pot catalyst-free synthesis of benzopyrroxazine derivatives by a mutually activated sequential annulation process

A one-pot catalyst-free reaction of o-hydroxyaryl azomethine ylides, vinyl pyridines and paraformaldehyde for the synthesis of benzopyrroxazines is reported, which offers a straightforward and atom-economical procedure for the preparation of benzopyrroxazine derivatives in moderate to excellent yields under mild conditions. A self-catalyzed [3 + 2] annulation through a mutual activation method and a sequential non-catalyzed [5 + 1] annulation process contribute to this strategy. The corresponding control experiments have been conducted to reveal the mechanism of this reaction.

A combination of polarity reversal, Diels-Alder cycloaddition and skeletal remodeling to access pyridine-fused nitrones

An unprecedented cascade strategy consisting of polarity reversal, normal electron-demand Diels-Alder cycloaddition and skeletal remodeling was developed to construct novel pyridine-fused nitrones in up to 82% yield. The key to the success was the umpolung process, which transformed the electron-deficient 3-nitropyridinium ring into a reactive, π-extended cyclic nitroalkene, serving as a rarely reported hetero-diene to participate in normal Diels-Alder cycloadditions.

Dearomative Periphery Modification of Quinolinium Salts to Assemble Ring-Encumbered Pyrrolidine-Tetrahydroquinoline Polycycles

We report an unexpected dearomative periphery modification strategy for transforming quinolinium salts into structurally crowded pyrrolidine-tetrahydroquinoline polycyclic systems with complete regio- and diastereoselectivity. Importantly, the reaction pathway was regulated by simply tuning the substituents, achieving substituent-directed divergent synthesis. The notable features of this transformation include readily available starting materials, green conditions, a simple workup procedure, high bond- and ring-forming efficiency, and substituent-directed diverse synthesis.

Regio- and stereoselective synthesis and evaluation of densely functionalized bispiro[oxindole-isoxazole-indandione] hybrids as anticancer agents

This is the combination of three key pharmacophores through bispiro quaternary carbon atoms and evaluation of their anticancer activity.