New applications of pyridinium ylides toward heterocyclic synthesis

Nucleophile-Controlled Regiodivergent Domino Reactions of Enetriones with γ-Bromocrotonates: Access to 1,3-Dienic Esters and Tetrasubstituted Pyrans

Herein, we developed an efficient nucleophile-controlled regiodivergent domino reaction between enetriones and γ-bromocrotonates. This method allowed for the rapid synthesis of a range of 1,3-dienic esters and tetrasubstituted pyrans under metal-free conditions. In the presence of pyridine, a SN2 substitution/Michael addition/elimination sequence formed 1,3-dienic esters in satisfactory yields with high E-stereoselectivities. Alternatively, a SN2 substitution/Michael addition/cyclization/cyclopropanation/cyclopropane ring-opening process forged tetrasubstituted pyrans in good yields with the help of Et3N. It is interesting to note that the site-selective reactions of γ-bromocrotonates at the α- or γ-position were readily realized by modulating pyridine and Et3N. Furthermore, the simple pyridine and Et3N act as both nucleophiles in SN2 substitution reactions and Lewis bases in deprotonation processes.

Synthesis of Functionalized Indolizines through 1,3-Dipolar Cycloaddition of Zwitterionic Ketenimines and Pyridinium Salts

A straightforward and efficient strategy for the synthesis of fully functionalized indolizines has been developed through a transition metal- and oxidant-free [3 + 2] cycloaddition reaction of zwitterionic ketenimines and pyridinium salts. This versatile method proceeds under mild conditions, affording functionalized indolizines in moderate to good yields. This efficient approach involves an intermolecular [3 + 2] cycloaddition, followed by enamine/imine tautomerization and aromatization. Notably, this method demonstrates broad functional group compatibility and allows for facile scalability, making it a valuable tool for the synthesis of indolizine-based frameworks in organic and medicinal chemistry.

Visible-light-induced difunctionalization of β-CF2H-1,3-enynes to access CF2H-containing all-carbon quaternary centers

An efficient and regioselective method for the introduction of a highly functionalized CF2H-containing quaternary carbon center at the C-2 position of pyridines has been described. This method proceeds via [3 + 2] cycloaddition of β-CF2H-1,3-enynes with pyridinium ylides, followed by a light-induced aza-Norrish II rearrangement. The salient features of this present protocol include mild reaction conditions, operational simplicity, and excellent C2 site selectivity. Furthermore, the synthetic utility of this method is demonstrated by the downstream functionalization of the resulting products.

DNA-compatible one-pot synthesis of multi-substituted dihydrofuran via pyridinium ylide-mediated cyclization

Synthesis of chemically diverse heterocyclic scaffolds in DNA-encoded libraries is highly demanded. We herein reported a convenient one-pot multi-component on-DNA synthetic strategy to afford multi-substituted 2,3-dihydrofuran scaffolds via pyridinium ylide-mediated cyclization. This reaction exhibited modest to excellent conversions for a broad range of DNA-conjugated aldehydes, β-ketonitriles and pyridinium salts under mild reaction conditions. Furthermore, the compatibility of this strategy with DEL construction was verified by enzymatic DNA ligation, PCR amplification and mock library synthesis.

Highly Diastereoselective Synthesis of 2,3-Dihydropyrroles via Formal [4 + 1] Annulation Reaction of α,β-Unsaturated Imines and in Situ Generated Pyridinium Ylide

We report a cascade approach for the synthesis of 2,3-dihydropyrroles derivatives via a formal [4 + 1] annulation reaction of α,β-unsaturated imines with in situ generated pyridinium ylides. Importantly, this protocol is compatible with diverse substituted imines as well as pyridinium ylides, constructing 2,3-dihydropyrroles with excellent yield and selectivity. Thereafter, the Merrifield resin-supported pyridinium ylide as a potential C1 synthon was also employed in our strategy and reused several times, resulting in products with excellent yield and diastereoselectivity.

Oxy-pyridinium Ylides Mediated 1,4-Pyridyl/Aryl Translocation

Molecular rearrangement via carbene transfer is a powerful tool to access molecular diversity. Herein, we describe an efficient approach to selective pyridyl/aryl relocation via a rhodium-catalyzed aminoarylation of diazo compounds, providing a promising strategy to access ortho-pyridyl N-alkylated pyridone scaffolds in a single operation. This reaction features the novel reactivity of oxy-pyridinium ylide, rhodium-associated five-membered transition state, and 1,4-pyridyl/aryl relocation. A computational study discloses the initial oxy-pyridinium ylide formation, keto-enol tautomerization, and 1,4-pyridyl migration to complete the whole rearrangement.

Electron donor-acceptor complex enabled photocascade strategy for the synthesis of trans-dihydrofuro[3,2-c]chromen-4-one scaffolds via radical conjugate addition of pyridinium ylide

A visible-light-induced photocascade strategy is disclosed for the synthesis of trans-dihydrofuro[3,2-c]chromen-4-one scaffolds. The photocascade consists of electron donor-acceptor (EDA) complex enabled formation of arylidene coumarinone, followed by 1,4-radical conjugate addition (1,4-RCA) of an in situ generated pyridinium ylide radical (PyYR) towards diastereoselective formation of the trans-dihydrofuro[3,2-c]chromen-4-one scaffold in good to excellent yield. Thorough mechanistic investigations comprising photophysical, spectroscopic, electrochemical and DFT studies provide further insights into the reaction mechanism.

Base-Mediated Chemodivergent [4 + 1] and [2 + 1] Cycloadditions of N-Alkylpyridiniums and Enones

Divergent synthesis of structurally different products from the same kinds of starting materials is highly synthetically useful but very challenging. Herein, we reported a base-mediated chemodivergent [4 + 1] and [2 + 1] cycloaddition of N-alkylpyridinium and enone under mild conditions, leading to furan-fused bicycles with high diastereoselectivity and spirobicycles, respectively, from moderate to high yields. N-Alkylpyridinium salts were modular nucleophilic transfer reagents and C1 synthons, which underwent tandem Michael addition to the α,β-unsaturated ketones and cyclization under the base conditions. Late-stage derivatization of 4-propyldicyclohexylanone from an important industrial raw of liquid crystal display (LCD) screens was realized. In vitro, compound 3f exhibited good activities against human colon cancer cells (HCT116) with IC50 values in 9.82 ± 0.27 μM. Further biological evaluations investigated the mechanism of the effective inhibition of cell growth, including apoptosis ratio detection, cell cycle analysis, and migration capacity of HCT116 cells. In apoptosis effect studies, complex 3f increased the percentage of apoptotic HCT116 cells to 26.8% (15 μM).

The protocatechuic acid-based deep eutectic solvent-mediated green synthesis of 1,2,4,5-tetrasubstituted imidazoles

A novel protocatechuic acid-based deep eutectic solvent (ETPPBr/PCA-DES) was prepared by mixing ethyltriphenylphosphonium bromide (ETPPBr) and protocatechuic acid (PCA = 3,4-dihydroxybenzoic acid), and its structure was fully investigated by using the FT-IR, TGA/DTA, densitometer, eutectic point and 1H NMR techniques. Different molar ratios of ETPPBr to PCA were examined and the eutectic point phase diagram showed that the best ratio for the synthesis of the new DES is the one-to-one ratio of the two starting materials (ETPPBr and PCA). Then, the novel DES was used as a new and capable catalyst for the green synthesis of diverse 1,2,4,5-tetrasubstituted imidazoles a1-a12 from the four-component condensation reaction of phenanthrene-9,10-dione, aromatic amine, aromatic aldehyde, and ammonium acetate with high yields and very short reaction times. High yields and very short reaction times are two advantages of our proposed method compared with the previous reported methods.

Pyridinium Ylide-Mediated Diastereoselective Synthesis of Spirocyclopropanyl-pyrazolones via Cascade Michael/Substitution Reaction

We have devised a highly diastereoselective formal [2 + 1] annulation reaction of arylidene/alkylidine-pyrazolones with in situ-generated supported as well as standard pyridinium ylides to construct spirocyclopropanyl-pyrazolones. The cascade approach exhibits a wide range of functional group tolerance, gram-scale capability, and substrate versatility. A diverse range of spirocyclic cyclopropanes was synthesized extensively with both mediators, and the supported pyridine was reused in subsequent cycles. Density functional theory calculations confirmed the formation of spirocyclopropane as the lower energy pathway.

Organocatalyzed Enantio- and Diastereoselective Formal Domino 1,3-Dipolar Cycloaddition/Rearrangement: Synthesis of Chiral Pyrrolo-thiazine-2-carbaldehydes

An efficient approach for the synthesis of chiral pyrrolo[1,2-d][1,4]thiazine-2-carbaldehydes is achieved via formal 1,3-dipolar cycloaddition/rearrangement reactions of benzothiazolium salt and α,β-unsaturated aldehydes, utilizing an asymmetric organocatalyst. This process results in the formation of fluorescent, highly enantioenriched chiral molecules with three contiguous stereogenic centers, one of which is a chiral quaternary center, with excellent yields and enantio- and diastereoselectivity. A computational study demonstrated the understanding of the reaction mechanism. The synthetic utility of this protocol was successfully employed for gram scale synthesis. Fluorescent and in silico studies showed the application of the present methodology.

One-pot C-N/C-C bond formation and oxidation of donor-acceptor cyclopropanes with tetrahydroisoquinolines: access to benzo-fused indolizines

One-pot C-N/C-C bond formation of donor-acceptor cyclopropanes (DACs) with tetrahydroisoquinolines (THIQs) has been achieved to furnish benzo-fused indolizines. These reactions involve a MgI2-catalyzed ring opening of DACs and oxidative annulation using Mn(OAc)3·2H2O. The substrate scope and functional group diversity are the important practical features.

Tandem Reactions of Electrophilic Indoles toward Indolizines and Their Subsequent Transformations through Pd(II)-Mediated C-H Functionalization to Access Polyring-Fused N-Heterocycles

A simple and efficient synthetic approach for generating a library of structurally novel indolizines has been developed via sequential 1,3-dipolar cycloaddition-ring opening processes. Using this methodology, a series of indolizines bearing different substituents were made in moderate to good yields. The presence of two functionalizable C-H bonds in these indolizine motifs makes them attractive for accessing fused indolizine scaffolds. In this line, we have introduced palladium-mediated site-selective C-H functionalizations, where the N-center and the two C-H centers of the indolizine moiety can be readily functionalized to generate fused N-heterocycles. Utilizing a Pd-mediated dual C-H activation of 5-benzoyl-substituted indolizine afforded 6H-indeno-indolizine, and a tetracene, viz., indolizino[2,1-b]indoles, was produced in the same substrate by the Pd-catalyzed selective C-H amination in the presence of oxygen.

Green Light Promoted Iridium(III)/Copper(I)-Catalyzed Addition of Alkynes to Aziridinoquinoxalines Through the Intermediacy of Azomethine Ylides

This manuscript describes the development of alkyne addition to the aziridine moiety of aziridinoquinoxalines using dual Ir(III)/Cu(I) catalytic system under green light-emitting diode (LED) photolysis (λmax =525 nm). This mild method features high levels of chemo- and regioselectivity and was used to generate 30 highly functionalized substituted dihydroquinoxalines in 36-98 % yield. This transformation was also carried asymmetrically using phthalazinamine-based chiral ligand to provide 9 chiral addition products in 96 : 4 to 86 : 14 e.r. The experimental and quantum chemical explorations of this reaction suggest a mechanism that involves Ir(III)-catalyzed triplet energy transfer followed by a ring-opening reaction ultimately leading to the formation of azomethine ylide intermediates. These azomethine intermediates undergo sequential protonation/copper(I) acetylide addition to provide the products.

Pyridinium-Derived Mesoionic N-Heterocyclic Olefins (py-mNHOs)

Mesoionic polarization allows access to electron-rich olefins that have found application as organocatalysts, ligands, or nucleophiles. Herein, we report the synthesis and characterization of a series of 3-methylpyridinium-derived mesoionic olefins (py-mNHOs). We used a DFT-supported design concept, which showed that the introduction of aryl groups in the 1-, 2-, 4-, and 6-positions of the heterocyclic core allowed the kinetic stabilization of the novel mesoionic compounds. Tolman electronic parameters indicate that py-mNHOs are remarkably strong σ-donor ligands toward transition metals and main group Lewis acids. Additionally, they are among the strongest nucleophiles on the Mayr reactivity scale. In reactions of py-mNHOs with electron-poor π-systems, a gradual transition from the formation of zwitterionic adducts via stepwise to concerted 1,3-dipolar cycloadditions was observed experimentally and analyzed by quantum-chemical calculations.

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.

(3+2) Cycloaddition of Heteroaromatic N-Ylides with Sulfenes

A (3+2) cycloaddition of heteroaromatic N-ylides with sulfenes, which are generated in situ from sulfonyl chlorides, has been developed. A variety of ylides were transformed into the corresponding sulfone-embedded N-fused heterocycles in high yields. Hexafluoroisopropyl mesylate was demonstrated to be a suitable reactant for quinolinium ylides. Furthermore, this cycloaddition could be performed with an ylide prepared by a Cu-catalyzed ylide transfer reaction in a one-pot manner, extending the substrate scope to an unisolable ylide.

Sulfonium Salt Reagents for the Introduction of Deuterated Alkyl Groups in Drug Discovery

The pharmacokinetics of pharmaceutical drugs can be improved by replacing C-H bonds with the more stable C-D bonds at the α-position to heteroatoms, which is a typical metabolic site for cytochrome P450 enzymes. However, the application of deuterated synthons is limited. Herein, we established a novel concept for preparing deuterated reagents for the successful synthesis of complex drug skeletons with deuterium atoms at the α-position to heteroatoms. (dn -Alkyl)diphenylsulfonium salts prepared from the corresponding nondeuterated forms using inexpensive and abundant D2 O as the deuterium source with a base, were used as electrophilic alkylating reagents. Additionally, these deuterated sulfonium salts were efficiently transformed into dn -alkyl halides and a dn -alkyl azide as coupling reagents and a dn -alkyl amine as a nucleophile. Furthermore, liver microsomal metabolism studies revealed deuterium kinetic isotope effects (KIE) in 7-(d2 -ethoxy)flavone. The present concept for the synthesis of deuterated reagents and the first demonstration of a KIE in a d2 -ethoxy group will contribute to drug discovery research based on deuterium chemistry.

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.

Recent Progress in Heterocycle Synthesis: Cyclization Reaction with Pyridinium and Quinolinium 1,4-Zwitterions

Heteroarene 1, n-zwitterions are powerful and versatile building blocks in the construction of heterocycles and have received increasing attention in recent years. In particular, pyridinium and quinolinium 1,4-zwitterions have been widely studied and used in a variety of cyclization reactions due to their air stability, ease of use, and high efficiency. Sulfur- and nitrogen-based pyridinium and quinolinium 1,4-zwitterions, types of emerging heteroatom-containing synthons, have attracted much attention from chemists. These 1,4-zwitterions, which contain multiple reaction sites, have been successfully used in the synthesis of three- to eight-membered cyclic compounds over the last decade. In this review, we present the exciting progress made in the field of cyclization reactions of sulfur- and nitrogen-based pyridinium and quinolinium 1,4-zwitterions. Moreover, the mechanistic insights, the transition states, some synthetic applications, and the challenges and opportunities are also discussed. We hope to provide an overview for synthetic chemists who are interested in the heterocycle synthesis from cyclization reaction with pyridinium and quinolinium 1,4-zwitterions pyridinium and quinolinium 1,4-zwitterions.

Chemo- and Regioselective Multiple C(sp2 )-H Insertions of Malonate Metal Carbenes for Late-Stage Functionalizations of Azahelicenes

Chiral quinacridines react up to four times, step-by-step, with α-diazomalonates under RuII and RhII catalysis. By selecting the catalyst, [CpRu(CH3 CN)3 ][PF6 ] (Cp=cyclopentadienyl) or Rh2 (oct)4 , chemo and regioselective insertions of derived metal carbenes are achieved in favor of mono- or bis-functionalized malonate derivatives, respectively, (r.r.>49 : 1, up to 77 % yield, 12 examples). This multi-introduction of malonate groups is particularly useful to tune optical and chemical properties such as absorption, emission or Brønsted acidity but also cellular bioimaging. Density-functional theory further elucidates the origin of the carbene insertion selectivity and also showcases the importance of conformations in the optical response.

Synthesis and Chemistry of Ammonioethenyl and Phosphonioethenyl Ligands in Zwitterionic Dirhenium Carbonyl Complexes

New zwitterionic dirhenium carbonyl complexes containing ammonioethenyl and phosphonioethenyl ligands have been synthesized and studied. The reaction of Re₂(CO)₁₀ with C₂H₂ and Me₃NO yielded the dirhenium complex Re₂(CO)₉(NMe₃) (6) and the new zwitterionic complex Re₂(CO)₉[η¹-E-2-CH═CH(NMe₃)] (7). Compound 6 was characterized structurally and was found to have a NMe₃ ligand in an equatorial coordination site cis to a long Re-Re single bond, Re-Re = 3.0938(2) Å. Compound 7 can be obtained from the reaction of 6 with ethyne (C₂H₂) formally by the insertion of ethyne into the Re-N bond to the NMe₃ ligand. Compound 7 contains a 2-trimethylammonioethenyl ligand, ^-CH═CH(⁺NMe₃), that is formally a zwitterion having a positive charge on the nitrogen atom and a negative charge on the terminal carbon atom. When coordinated to rhenium by the terminal ethenyl carbon atom, the negative charge on the ^-CH═CH(⁺NMe₃) carbon atom is formally transferred to the rhenium atom. The reaction of Re₂(CO)₁₀ with C₂H₂ and NEt₃ in the presence of Me₃NO yielded the new dirhenium complex Re₂(CO)₉[η¹-E-2-CH═CH(NEt₃)] (8) together with some 6 and 7. Compound 8 is structurally similar to 7, but it contains a NEt₃ group in the ammonioethenyl ligand in the place of the NMe₃ group in 7. Reactions of 7 with PMePh₂ and PPh₃ yielded the zwitterionic 2-arylphosphonioethenyl-coordinated dirhenium carbonyl complexes, Re₂(CO)₉[η¹-E-2-CH═CH(PPh₂Me)] (9a) and Re₂(CO)₉[η¹-E-2-CH═CH(PPh₃)] (9b), and the zwitterionic 1-phosphonioethenyl ligand in the dirhenium carbonyl complexes, Re₂(CO)₉[η¹-1-C(PPh₂Me)(═CH₂)] (10a), Re₂(CO)₈[μ-η²-1-C(PPh₂Me)(═CH₂)] (11a), and Re₂(CO)₈[[μ-η²-1-C(PPh₃)(CH₂)] (11b). Compound 10a was converted to 11a and the new compound Re₂(CO)₇(μ-H)[μ-η²-1-(CH₂C)P(Ph)(Me)(o-C₆H₄)], (12) by decarbonylation using Me₃NO. Compound 12 contains an ortho-metalated phenyl ring. The new products 6,7, 8, 9b, 10a, 11a, 11b and 12 were characterized structurally by single-crystal X-ray diffraction analyses.

Tandem Michael addition/elimination – novel reactivity of pyridinium ylides in reaction with electron-deficient alkenes

Novel reactivity of pyridinium ylides in reaction with 4-oxothiazolidin-5-ylidenes as electron-deficient alkenes involving a tandem Michael addition/elimination process was developed to give double bond functionalization products in mild conditions.

Oxidative [3+2]-annulation of nitroalkenes and azolium ylides in the presence of Cu(II): efficient synthesis of [5,5]-annulated N-fused heterocycles

A facile synthesis of [5,5]-annulated N-fused heterocycles – pyrrolo[2,1-b]thiazoles and pyrrolo[1,2-b]indazoles via oxidative [3+2]-annulation between nitroalkenes and azolium ylides in the presence of copper (II) was developed. The reaction proceeds...

Formal (3 + 1 + 1) Carboannulation of Morita-Baylis-Hillman Carbonates with Pyridinium Ylides: Access to Spiro-Cyclopentadiene Oxindoles

An efficient formal (3 + 1 + 1) carboannulation strategy of Morita-Baylis-Hillman (MBH) carbonates with pyridinium ylides was developed for constructing diversely functionalized spiro-cyclopentadiene oxindoles. The reaction initiates with an S_N2' olefination of MBH carbonates with pyridinium ylides. The in situ generated dienes then engage in a challenging (4 + 1) ylide carboannulation, which has been rarely reported before. The reaction features broad substrate scope as well as high chemo- and regioselectivity. (3 + 1 + 1) carboannulation products could be easily transformed into interesting spiro-cyclopenta[c]furan oxindoles.

Detection of Ylide Formation between an Alkylidenecarbene and Acetonitrile by Femtosecond Transient Absorption Spectroscopy

Femtosecond laser flash photolysis of 3-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-ylidene)tetrahydrofuran produces singlet 3-oxacyclopentylidenecarbene which reacts with acetonitrile solvent to form an ylide. This is the first direct detection of ylide formation by an alkylidenecarbene. This new type of ylide was observed to have a broad absorption band in the visible region with λ_max ∼450 nm and a lifetime of ∼13.5 ps. As with other "conventional" carbenes (the divalent carbon atom is separately bound to two substituents), this ylide formation method could be also useful for detecting alkylidenecarbenes, especially those that do not absorb at wavelengths suitable for direct observation. Furthermore, the mechanisms by which 3-oxacyclopentylidenecarbene forms the ylide and the overall favorability of ylide formation, vis-à-vis ring expansion of the carbene to strained 3-oxacyclohexyne, were supported by results from density functional theory calculations.

Three-Component Condensation of Pyridinium Ylides, β-Ketonitriles, and Aldehydes with Divergent Regioselectivity: Synthesis of 4,5-Dihydrofuran-3- and 2H-Pyran-5-carbonitriles

A library of trans-4,5-dihydrofuran-3-carbonitriles was synthesized in a diastereoselective manner in good yields by the three-component reaction of β-ketonitriles, carbonyl- and semistabilized pyridinium ylide precursors, and aldehydes in the presence of piperidine. This one-pot transformation generates two C-C and one C-O bond and proceeds through a cascade Knoevenagel condensation, a Michael addition, and intramolecular S_N2 cyclization. Formation of cyclopropanecarbonitrile derivatives, which in some cases were obtained as major products, was found to be a competing reaction. The use of arylglyoxals changes regioselectivity and leads to 2-hydroxy-2H-pyran-5-carbonitriles.

20 Years of Forging N-Heterocycles from Acrylamides through Domino/Cascade Reactions

Acrylamides are versatile building blocks that are easily obtained from readily available starting materials. During the last 20 years, these valuable substrates bearing a nucleophilic nitrogen atom and an electrophilic double bond have proven to be efficient domino partners, leading to a wide variety of complex aza-heterocycles of synthetic relevance. In this non-exhaustive review, metal-free and metal-triggered reactions followed by an annulation will be presented; these two approaches allow good modulation of the reactivity of the polyvalent acrylamides.

1 Introduction

2 Metal-Free Annulations

2.1 Domino Reactions Triggered by a Michael Addition

2.2 Domino Reactions Triggered by an Aza-Michael Addition

2.3 Domino Processes Triggered by an Acylation Reaction

2.4 Domino Reactions Triggered by a Baylis–Hillman Reaction

2.5 Cycloadditions and Domino Reactions

2.6 Miscellaneous Domino Reactions

3 Metal-Triggered/Mediated Annulations

3.1 Zinc-Promoted Transformations

3.2 Rhodium-Catalyzed Functionalization/Annulation Cascades

3.3 Cobalt-Catalyzed Functionalization/Annulation Cascades

3.4 Ruthenium-Catalyzed Functionalization/Annulation Cascades

3.5 Iron-Catalyzed Functionalization/Annulation Cascades

3.6 Palladium-Catalyzed Functionalization/Annulation Cascades

3.7 Copper-Catalyzed Transformations

3.8 Transition Metals Acting in Tandem in Domino Processes

4 Radical Cascade Reactions

5 Conclusion

[3 + 2]-Annulation of gem-Difluoroalkenes and Pyridinium Ylides: Access to Functionalized 2-Fluoroindolizines

A [3 + 2]-annulation of gem-difluoroalkenes and pyridinium ylides was developed employing ambient air as the sole oxidant in an open-vessel manner, affording a series of multifunctionalized 2-fluoroindolizines in moderate to good yields. In this reaction, gem-difluoroalkene acts as a C2 synthon and entirely avoids the competitive addition-elimination process, which provides facile access to 2-fluoroindolizines.

Synthesis of novel pyridinium 1,5-zwitterions and their reactivity with isatin-based α-(trifluoromethyl)imines: a sulfur-controlled domino reaction

A new stable pyridinium 1,5-zwitterion, generated from azadiene and 4-dimethylaminopyridine, was isolated. The reaction of pyridinium 1,5-zwitterion and isatin-derived α-(trifluoromethyl)imine was regulated by temperature.

Efficient synthesis of tetra- and penta-substituted benzenes via a domino annulation reaction of a pyridinium ylide and chalcone o-enolate

A very simple and highly efficient protocol for synthesizing tetra- and penta-substituted benzene derivatives has been developed.

Highly stereoselective dearomative [3 + 2] cycloadditon of cyclic pyridinium ylides to access spiro-indolizidine scaffolds

A novel type of pyridinium salt bearing a EWG on the pyridine was developed as an efficient pyridinium ylide precursor in the [3 + 2] cycloaddition with nitroolefins to construct various spiro-indolizidine scaffolds via a dearomative pathway.