Recent advances in difunctionalization of alkenes using pyridinium salts as radical precursors

Strategies for Using Quaternary Ammonium Salts as Alternative Reagents in Alkylations

Alkylation reactions are pivotal in organic chemistry, with wide-ranging utilization across various fields of applied synthetic chemistry. However, conventional reagents employed in alkylations often pose substantial health and exposure risks. Quaternary ammonium salts (QAS) present a promising alternative for these transformations offering significantly reduced hazards as they are non-cancerogenic, non-mutagenic, non-flammable, and non-corrosive. Despite their potential, their use in direct organic transformations remains relatively unexplored. This review outlines strategies for utilizing QAS as alternative reagents in alkylation reactions, providing researchers with safer approaches to chemical synthesis.

Visible-Light-Induced Acylative Pyridylation of Styrenes

A visible-light-induced photocatalyst-free acylative pyridylation of styrenes with 4-acyl-1,4-dihydropyridines (DHPs) and 4-cyanopyridines has been described, featuring mild reaction conditions, a broad substrate scope, and good functional group tolerance. The reaction could also be performed under sunlight irradiation albeit with a slightly lower conversion. 4-Acyl-1,4-DHPs serve a dual role, acting as both a photoreductant to reduce the cyanopyridine to its radical anion intermediate and a radical precursor to produce the acyl radical. The mechanism was especially elucidated through the Hammett analysis, with the quadratic linear regression analysis by using radical dual parameters, σmb and σjj·. The findings from Hammett analysis further demonstrate that the rate-limiting step of the process is the single electron transfer between 4-acyl-1,4-DHPs and 4-cyanopyridines.

Construction of β-Oximino Phosphorodithioates via (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl-Promoted Difunctionalization of Alkenes with tert-Butyl Nitrite, P4S10, and Alcohols

A (2,2,6,6-tetramethylpiperidin-1-yl)oxyl-mediated difunctionalization of alkenes with tert-butyl nitrite, P4S10, and alcohols has been developed for the synthesis of β-oximino phosphorodithioates. The reaction goes through a radical pathway with the successive installation of phosphorodithioate and an oxime group. This four-component protocol offers a practical approach to constructing a variety of β-oximino phosphorodithioates in moderate to good yields with favorable functional group tolerance.

Confinement Effect in Metal-Organic Framework Cu3()2 for Enhancing Shape Selectivity of Radical Difunctionalization of Alkenes

The radical difunctionalization of alkenes plays a vital role in pharmacy, but the conventional homogeneous catalytic systems are challenging in selectivity and sustainability to afford the target molecules. Herein, the famous readily available metal-organic framework (MOF), Cu3(BTC)2, has been applied to cyano-trifluoromethylation of alkenes as a high-performance and recyclable heterogeneous catalyst, which possesses copper(II) active sites residing in funnel-like cavities. Under mild conditions, styrene derivatives and various unactivated olefins could be smoothly transformed into the corresponding cyano-trifluoromethylation products. Moreover, the transformation brought about by the active copper center in confined environments achieved regio- and shape selectivity. To understand the enhanced selectivity, the activation manner of the MOF catalyst was studied with control catalytic experiments such as FT-IR and UV-vis absorption spectroscopy of substrate-incorporated Cu3(BTC)2, which elucidated that the catalyst underwent a radical transformation with the intermediates confined in the MOF cavity, and the confinement effect endowed the method with pronounced selectivities.

Divergent Synthesis of F- and CF3-Containing N-Fused Heterocycles Enabled by Fragmentation Cycloaddition of β-CF3-1,3-Enynes with N-Aminopyridiniums Ylides

The two novel cyclization modes of β-CF3-1,3-enynes are presented herein for the divergent construction of F- and CF3-containing N-fused heterocycles. Fluorinated pyrazolo[1,5-a]pyridines were afforded from β-CF3-1,3-enynes with N-aminopyridiniums ylides via detrifluoromethylative [2 + 3] cyclizations, followed by fluorine transfer from a CF3 unit. Whereas reaction with N-aminoisoquinoliniums ylides gave CF3-substituted pyrrolo[2,1-a]isoquinoline by unprecedented fragmentation [3 + 2]-cycloadditions. Additionally, gram-scale experiments and synthetic utility are demonstrated by further derivatization of fluorinated heterocycles.

Iodonium based regioselective double nucleophilic alkene functionalization of a hydrofluoroolefin scaffold

Herein, we report a modular regioselective alkene difunctionalization strategy based on the use of hydrofluoroolefin (HFO) gas as fluorous feedstock material. The transformation of the HFO gas to iodonium salt creates vicinal electrophilic sites readily available for a broad range of nucleophiles.

Photocatalytic Unsymmetrical Diamination of Styrenes, Indoles, and Benzofurans Facilitated by Benzotriazolyl and Iminyl Radicals

Utilizing energy transfer catalysis, this research employed the bifunctional reagents benzotriazole carboxylic acid oxime esters to simultaneously generate benzotriazole and imine radicals. The synthesis of two distinct C-N bonds in a single conversion is showcased through radical addition and radical-radical cross-coupling processes between benzotriazole carboxylic acid oxime ester and olefins. This process facilitates the intermolecular two-component unsymmetrical diamination reaction of olefins. Using this approach, more than 40 benzotriazole-containing molecules were successfully synthesized using styrene, indole, and benzofuran as acceptors, with yields ranging from moderate to excellent.

Visible-Light-Mediated Divergent and Regioselective Vicinal Difunctionalization of Styrenes with Arylazo Sulfones

Activated by visible light, arylazo sulfones can serve as multifaceted reactants and are employed in diazenylation, sulfonylation, and arylation reactions under (photo)catalyst-free conditions. Such versatile reactivity enabled us to develop an operationally simple, regioselective, and tunable difunctionalization of styrenes with arylazo sulfones to produce α-sulfonyl arylhydrazones and 1,2-alkoxyarylated products in moderate to excellent yields. Furthermore, such difunctionalized products have been exploited as key building blocks for the synthesis of various heterocycles.

Chemo- and Regioselective Alkylation of Pyridine N-Oxides with Titanacyclopropanes

While titanacyclopropanes are used to react mainly with ester, amide, and cyano to undergo cyclopropanation, herein they react preferentially with pyridine N-oxide to accomplish C2-H alkylation beyond these functionalities with double regioselectivity. After being pyridylated at the less hindered C-Ti bond, the remaining C-Ti bond of titanacyclopropanes can be further functionalized by various electrophiles, allowing facile introduction of complex alkyls onto the C2 of pyridines. Its synthetic potential has been demonstrated by late-stage diversification of drugs.

Three-Component Photochemical 1,2,5-Trifunctionalizations of Alkenes toward Densely Functionalized Lynchpins

Radical remote 1,n-difunctionalization reactions (n > 2) of alkenes are powerful tools to efficiently introduce functional groups with selected distances into target molecules. Among these reactions, 1,5-difunctionalizations are an important subclass, leading to sought-after scaffolds, but typically suffer from tailored starting materials and strict limitations for the formed functional group in 2-position. Seeking to address these issues and to make radical 1,5-difunctionalizations of alkenes more applicable, we report a novel three-component 1,2,5-trifunctionalization reaction between imine-based bifunctional reagents and two distinct alkenes, driven by visible light energy transfer-catalysis. Key to achieving this selective one-step installation of three different functional groups via the choreographed formation of four bonds was the utilization of a 1,2-boron shift and the rigorous capitalization of radical polarities and stabilities. Thorough mechanistic studies were carried out, and the synthetic utility of the obtained products was demonstrated by various downstream modifications. Notably, in addition to the functionalization of individual functional groups, their interplay gave rise to a unique array of cyclic products.

Photochemical Oximesulfonylation of Alkenes Using Sulfonyl-Oxime-Ethers as Bifunctional Reagents

Utilization of oxime ethers as bifunctional reagents remains unknown. Herein, we present a mechanistically distinct strategy that enables oximesulfonylation of olefins using sulfonyl-oxime-ethers as bifunctional reagents under metal-free photochemical conditions. Via concomitant C-S and C-C bond formation, the process permits incorporation of oxime and sulfonyl groups into olefins in a complete atom-economic fashion, providing rapid access to multi-functionalized β-sulfonyl oxime ethers with good yields and stereoselectivity. The method is amenable to functionalization of complex bioactive molecules and is shown to be scalable. A radical chain mechanism initiated via photochemical Hydrogen Atom Transfer (HAT) mediated N-O bond cleavage is suggested for the process, based on our results on mechanistic investigations.

Photoredox Catalyzed [3 + 2]-Annulation Reaction of Pyridinium 1,4-Zwitterionic Thiolates with Alkenes: Synthesis of Dihydrothiophenes

Pyridinium 1,4-zwitterionic thiolates are usually used to develop ionic annulation reactions. However, radical reactions were rare. We developed a photoredox catalyzed [3 + 2]-annulation reaction of pyridinium 1,4-zwitterionic thiolates with alkenes, disclosed the new reactivity of pyridinium 1,4-zwitterionic thiolate, and provided a new synthetic method for dihydrothiophene.

Synthesis of 2,3-Fused Quinazolinones via the Radical Cascade Pathway and Reaction Mechanistic Studies by DFT Calculations

An efficient radical cascade cyclization of unactivated alkenes toward the synthesis of a series of ring-fused quinazolinones has been developed in moderate to excellent yields using commercially available ethers, alkanes, and alcohols, respectively, under a base-free condition in a short time without a transition metal as catalyst. Notably, the transformations can be carried out with the advantages of a broad substrate scope and high atomic economy. Density functional theory calculations and wavefunction analyses were performed to elucidate the radical reaction mechanism.

Metal-Free Amino(hetero)arylation and Aminosulfonylation of Alkenes Enabled by Photoinduced Energy Transfer

β-(Hetero)arylethylamines are privileged structural motifs found in many high-value organic molecules, including pharmaceuticals and natural products. To construct these important molecular skeletons, previous methods are mainly achieved by amino(hetero)arylation reaction with the aid of transition metals and preactivated substrates. Herein, we report a metal-free and photoinduced intermolecular amino(hetero)arylation reaction for the single-step installation of both (hetero)aryl and iminyl groups across alkenes in an efficient and regioselective manner. This method shows broad scope (up to 124 examples) and excellent tolerance of various olefins─from the simplest ethylene to complex multisubstituted alkenes can all participate in the reaction. Furthermore, aminosulfonylation of alkenes can be also conducted in the presence of sodium bisulfite as the SO2 source.

Lewis Acid Promoted Vicinal Oxytrifluoromethylselenolation of Alkenes

Herein, we have developed a metal-free, Lewis acid promoted vicinal oxytrifluoromethylselenolation of alkenes using trifluoromethyl selenoxides as electrophilic trifluoromethylselenolation reagents and alcohols as nucleophiles. With less steric and good nucleophilic solvents (such as ethanol and methol), Tf₂O-catalyzed oxytrifluoromethylselenolation could be realized, while stoichiometric Tf₂O was required to promote full transformation with less nucleophilic and steric solvents (such as isopropanol and tert-butanol). The reaction featured good substrate scope, functional group compatibility, and diastereoselectivity. This method could be further applied to oxytrifluoromethylselenolation, aminotrifluoromethylselenolation with stoichiometric nucleophiles under modified conditions. A mechanism involving a seleniranium ion was proposed based on the preliminary results.

Dual Photoredox/Copper Catalyzed Fluoroalkylative Alkene Difunctionalization

A method for performing radical difunctionalization of alkenes using fluorinated halides and a nucleophilic component (thiolate and iodide anions) is described. These nucleophilic fragments serve as redox active groups for performing subsequent transformations. The difunctionalization reaction involves photoredox generation of fluorinated radicals, their addition to the double bond followed by copper-promoted C-S or C-I bond formation. The primary products can be further coupled with silyl enol ethers, α-(trifluoromethyl)styrenes, and trimethylsilyl cyanide affording a variety of fluorinated compounds.

Remote Radical 1,3-, 1,4-, 1,5-, 1,6- and 1,7-Difunctionalization Reactions

Radical transformations are powerful in organic synthesis for the construction of molecular scaffolds and introduction of functional groups. In radical difunctionalization reactions, the radicals in the first functionalized intermediates can be relocated through resonance, hydrogen atom or group transfer, and ring opening. The resulting radical intermediates can undertake the following paths for the second functionalization: (1) couple with other radical groups, (2) oxidize to cations and then react with nucleophiles, (3) reduce to anions and then react with electrophiles, (4) couple with metal-complexes. The rearrangements of radicals provide the opportunity for the synthesis of 1,3-, 1,4-, 1,5-, 1,6-, and 1,7-difunctionalization products. Multiple ways to initiate the radical reaction coupling with intermediate radical rearrangements make the radical reactions good for difunctionalization at the remote positions. These reactions offer the advantages of synthetic efficiency, operation simplicity, and product diversity.

Photochemical and Atom-Economical Sulfonylimination of Alkenes with Bifunctional N-Sulfonyl Ketimine

An organo-photocatalytic sulfonylimination of alkenes was developed by employing readily available N-sulfonyl ketimines as bifunctional reagents. This transformation, featuring prominent functional group tolerance, provides a direct and atom-economic approach for the synthesis of valuable β-amino sulfone derivatives as a single regioisomer. In addition to terminal alkenes, internal alkenes participate in this reaction with high diastereoselectivity. N-Sulfonyl ketimines with aryl or alkyl substituents were found to be compatible with this reaction condition. This method could be applied in the late-stage modifications of drugs. Additionally, a formal insertion of alkene into cyclic sulfonyl imine was observed, affording a ring expansion product.

Visible-Light-Induced Monofluoroalkenylation and gem-Difluoroallylation of Inactivated C(sp3)-H Bonds via 1,5-Hydrogen Atom Transfer (HAT)

The direct monofluoroalkenylation of C(sp³)-H bonds is of great importance and quite challenging. Current methods have been restricted to the monofluoroalkenylation of activated C(sp³)-H bonds. Here, we reported the photocatalyzed C(sp³)-H monofluoroalkenylation of inactivated C(sp³)-H bonds with gem-difluoroalkenes via 1,5-hydrogen atom transfer. This process shows good functional group tolerance, such as halides (F, Cl), nitrile, sulfone, ester, and pyridine, and good γ-selectivity. Moreover, this method succeeds in the photocatalyzed gem-difluoroallylation of inactivated C(sp³)-H with α-trifluoromethyl alkenes.

Photocatalytic 1,2-Iminosulfonylation and Remote 1,6-Iminosulfonylation of Olefins

A new class of iminosulfonylation reagents were developed and extensively used in the 1,2-iminosulfonylation of various olefins. Olefins containing bioactive molecules, such as indomethacin, gemfibrozil, clofibrate, and fenbufen, afforded the desired iminosulfonylation products in synthetically useful yields. Furthermore, the first remote 1,6-iminosulfonylation of alkenes was realized by using oxime ester bifunctionalization reagents. Overall, more than 40 structurally diverse β-imine sulfones were obtained in moderate to excellent yields.

Photoinduced Remote C(sp3)-H Imination Enabled by Vinyl Radical-Mediated 1,5-Hydrogen Atom Transfer

A vinyl radical-mediated 1,5-hydrogen atom transfer strategy for remote C(sp³)-H imination under visible-light-induced photochemical metal-free conditions afforded diverse γ-imino alkenes with excellent stereoselectivity. Oxime ester-based bifunctional reagents provided not only nucleophilic alkyl radicals for radical addition reactions with electron-deficient alkynes but also long-lived steady-state imine radicals for trapping alkyl radicals following the intramolecular 1,5-hydrogen migration of unstable olefin radicals.

Recent advances in the application of Langlois' reagent in olefin difunctionalization

In this review, we summarise the recent applications of Langlois' reagent in the radical-mediated difunctionalization of alkenes. Among the various trifluoromethylation reagents, Langlois' reagent is an exceptional compound, and many important organic transformations have been realized by employing such reagents. Various organic transformations of Langlois' reagent, especially in radical chemistry, have been developed in recent years. This review describes several key activation methods for Langlois' reagent in the difunctionalization of alkenes by showcasing selected cornerstone research areas and related mechanisms to stimulate the interest of readers in promoting the wider development and application of Langlois' reagent.

Visible-light-induced reactions of methylenecyclopropanes (MCPs)

Diverse, visible-light-induced transformations of methylenecyclopropanes (MCPs) have been reported in recent years, attracting significant attention from synthetic chemists. As readily accessible strained molecules, MCPs have sufficient reactivity to selectively generate different target products, through reactions with various radical species upon visible-light irradiation under regulated reaction conditions. These transformations can be classified into three subcategories of reaction pathway, forming ring-opened products, cyclopropane derivatives, and alkynes. These products include pharmaceutical intermediates and polycyclic/heterocyclic compounds that are challenging to obtain using traditional methods. This review summarizes the recent advancements in this field.

Synthesis, antitubercular evaluation, and molecular docking studies of hybrid pyridinium salts derived from isoniazid

In the quest to develop potent inhibitors for Mycobacterium tuberculosis, novel isoniazid-based pyridinium salts were designed, synthesized, and tested for their antimycobacterial activities against the H₃₇ Rv strain of Mycobacterium tuberculosis using rifampicin as a standard. The pyridinium salts 4k, 4l, and 7d showed exceptional antimycobacterial activities with MIC₉₀ at 1 µg/mL. The in vitro cytotoxicity and pharmacokinetics profiles of these compounds were established for the identification of a lead molecule using in vivo efficacy proof-of-concept studies and found that the lead compound 4k possesses LC₅₀ value at 25 µg/mL. The in vitro antimycobacterial activity results were further supported by in silico studies with good binding affinities ranging from -9.8 to -11.6 kcal/mol for 4k, 4l, and 7d with the target oxidoreductase DprE1 enzyme. These results demonstrate that pyridinium salts derived from isoniazid can be a potentially promising pharmacophore for the development of novel antitubercular candidates.

Substituent-Controlled Regioselective Photoinduced Cyclization of N-Allylbenzamides with N-Sulfonylaminopyridinium Salts

The annulation reactions of N-allylbenzamides with N-sulfonylaminopyridinium salts were developed under metal-free photoinduced mild conditions. Substituent-controlled sulfonaminoarylation and sulfonaminooxylation of benzamides were realized: N-allylbenzamides lead to benzosultams, while N-(2-phenylallyl)benzamides give sulfonamidylated oxazoline derivatives. Control experiments indicated that those reactions undergo a radical pathway with arylsulfonamidyl radicals as the intermediates. The aryl C-H bond functionalization in arylsulfonamidyl was involved for the first time to give benzosultams.

N-Functionalized Pyridinium Salts: A New Chapter for Site-Selective Pyridine C-H Functionalization via Radical-Based Processes under Visible Light Irradiation

The radical-mediated C-H functionalization of pyridines has attracted considerable attention as a powerful tool in synthetic chemistry for the direct functionalization of the C-H bonds of the pyridine scaffold. Classically, the synthetic methods for functionalized pyridines often involve radical-mediated Minisci-type reactions under strongly acidic conditions. However, the site-selective functionalization of pyridines in unbiased systems has been a long-standing challenge because the pyridine scaffold contains multiple competing reaction sites (C2 vs C4) to intercept free radicals. Therefore, prefunctionalization of the pyridine is required to avoid issues observed with the formation of a mixture of regioisomers and overalkylated side products.Recently, N-functionalized pyridinium salts have been attracting considerable attention in organic chemistry as promising radical precursors and pyridine surrogates. The notable advantage of N-functionalized pyridinium salts lies in their ability to enhance the reactivity and selectivity for synthetically useful reactions under acid-free conditions. This approach enables exquisite regiocontrol for nonclassical Minisci-type reactions at the C2 and C4 positions under mild reaction conditions, which are suitable for the late-stage functionalization of bioactive molecules with greater complexity and diversity. Over the past five years, a variety of fascinating synthetic applications have been developed using various types of pyridinium salts under visible light conditions. In addition, a new platform for alkene difunctionalization using appropriately designed N-substituted pyridinium salts as bifunctional reagents has been reported, offering an innovative assembly process for complex organic architectures. Intriguingly, strategies involving light-absorbing electron donor-acceptor (EDA) complexes between pyridinium salts and suitable electron-rich donors further open up new reactivity under photocatalyst-free conditions. Furthermore, we developed enantioselective reactions using pyridinium salts to afford enantioenriched molecules bearing pyridines through single-electron N-heterocyclic carbene (NHC) catalysis.Herein, we provide a broad overview of our recent contributions to the development of N-functionalized pyridinium salts and summarize the cornerstones of organic reactions that successfully employ these pyridinium salts under visible light conditions. The major advances in the field are systematically categorized on the basis of the pyridines' N-substituent, N-X (X = O, N, C, and SO₂CF₃), and its reactivity patterns. Furthermore, the identification of new activation modes and their mechanistic aspects are discussed by providing representative contributions to each paradigm. We hope that this Account will inspire broad interest in the continued innovation of N-functionalized pyridinium salts in the exploration of new transformations.

Selective Three-Component 1,2-Aminoalkoxylation of 1-Aryl-1,3-dienes by Dual Photoredox and Copper Catalysis

A three-component 1,2-aminooxygenation reaction of 1,3-dienes by dual photoredox and copper catalysis is described. This protocol uses N-aminopyridinium salts as N-centered radical precursors and nucleophilic alcohols as oxygen sources, providing modular and practical access to 1,2-aminoalkoxylation products with good yields and regioselectivity. Preliminary mechanistic studies support the radical property of the reaction and the involvement of N-centered radical intermediates.

Intermolecular C-H Aminocyanation of Indoles via Copper-iodine Cocatalyzed Tandem C-N/C-C Bond Formation

An efficient copper-iodine cocatalyzed intermolecular C-H aminocyanation of indoles with a broad substrate scope has been developed for the first time. This method enables highly step-economic access to 2-amino-3-cyanoindoles in moderate to good yields and provides a complementary strategy for the regioselective difunctionalization of carbon═carbon double bonds of interest in organic synthesis and related areas. Mechanistic studies suggest that these transformations are initiated by iodine-mediated C2-H amination with azoles, followed by copper-catalyzed C3-H cyanation with ethyl cyanoformate.

Visible-Light-Driven [3 + 2]/[4 + 2] Annulation Reactions of Alkenes with N-Aminopyridinium Salts

The annulation reactions of benzoamidyl radicals with alkenes were realized under visible light irradiation with fac-Ir(ppy)₃ as catalyst and N-aminopyridinium salts as benzoamidyl radical precursors. The reaction can deliver two distinct types of products: in the case of vinyl arenes, [3 + 2] annulation product dihydrooxazoles were yielded exclusively; when alkyl-substituted alkenes were used, on the other hand, it afforded [4 + 2] annulation product dihydroisoquinolinones. Factors determining the reaction consequence were elucidated by DFT calculations.

Photocatalytic difluoromethylarylation of unactivated alkenes via a (hetero)aryl neophyl-like radical migration

Photoredox-catalyzed addition of the difluoromethylradical to unactivated alkenes has been found to trigger neophyl-like aryl and heteroaryl migrations which allowed the construction of a diverse series of difluoromethyl ketones. The reaction featured mild reaction conditions and broad substrate scope.

A Convenient Synthesis towards 2-Bromo-2-(methoxy(phenyl)methyl)malononitrile and 2-Iodo-2-(methoxy(phenyl)methyl)malononitrile

This short note elaborates a concise protocol for the synthesis of two novel vicinal haloethers bearing a malonontrile group, 2-bromo-2-(methoxy(phenyl)methyl)malononitrile (1) and 2-iodo-2-(methoxy(phenyl)methyl)malononitrile (2). The structures of the synthesized compounds were confirmed by 1H, 13C-NMR spectroscopy. The final products indicate that methanol not only acts as solvent but also participates in and dominates the reaction result.

Oxalates as Activating Groups for Tertiary Alcohols in Photoredox-Catalyzed gem-Difluoroallylation To Construct All-Carbon Quaternary Centers

Construction of challenging and important all-carbon quaternary centers has received growing attention. Herein, with oxalates as activating groups for tertiary alcohols, we report photoredox-catalyzed gem-difluoroallylation to construct all-carbon quaternary centers enabled by efficient tertiary radical addition to α-trifluoromethyl alkenes. This transformation shows good functional group tolerance for both α-trifluoromethyl alkenes and oxalates. Moreover, this strategy is also successfully applied to the synthesis of monofluoralkenes from the corresponding electron-rich gem-difluoroalkenes and cesium tertiary alkyl oxalates under modified conditions.

Recent Advances on the Synthesis of 2,3-Fused Quinazolinones

As one of the most important structural units in pharmaceuticals and medicinal chemistry, quinazolinone and its derivatives exhibit a wide range of biological and pharmacological activities, including anti-inflammatory, antitubercular, antiviral,...

Self-[3+2] annulation reaction of pyridinium salts: synthesis of N-indolizine-substituted pyridine-2(1H)-ones

A self-[3+2] annulation reaction of pyridinium salts has been developed for the synthesis of N-indolizine-substituted pyridine-2(1H)-ones.