In Situ Generation of Quinolinium Ylides from Diazo Compounds: Copper-Catalyzed Synthesis of Indolizine

Rhodium(III)-Catalyzed Intramolecular Annulation and Aromatization for the Synthesis of Pyrrolo[1,2-a]quinolines

A rhodium(III)-catalyzed protocol for the synthesis of pyrrolo[1,2-a]quinolines through intramolecular annulation of o-alkynyl amino aromatic ketones and subsequent aromatization is reported. This transformation builds the pyrrole and quinoline moieties of the pyrrolo[1,2-a]quinoline in one pot and achieves a flexible introduction of different substituent groups at 4- and 5-positions on products that were difficult to prepare by other means. The reaction proceeds smoothly on a gram scale, and the products are amenable to downstream synthetic manipulations.

Transition metal-free annulative vinylene transfer via the 1,3-dipolar reaction of N-ylides: access to benzo-fused indolizines

An efficient metal-free annulative vinylene transfer protocol for the synthesis of benzo-fused indolizines via 1,3-dipolar cycloadditions of N-ylides with vinylene carbonate has been developed. Vinylene carbonate serves as an acetylene surrogate without any external oxidant involved. This transformation leads to the direct construction of versatile benzo-fused indolizine derivatives in moderate to good yields.

Synthesis of Pyrrolo[2,1-a]isoquinolines through Cu-Catalyzed Condensation/Addition/Oxidation/Cyclization Cascade

We have developed a copper-catalyzed synthesis of pyrrolo[2,1-a]isoquinolines with terminal alkynes, aldehydes, and tetrahydroisoquinolines. A variety of pyrrolo[2,1-a]isoquinolines have been prepared in 17-69% yield via a condensation/Mannich-type addition/oxidation/cyclization cascade sequence. Modifications through simple chemical transformations provided highly functionalized molecules containing a privileged framework.

TBAI-Catalyzed S-H and N-H Insertion Reactions of α-Diazoesters with Thiophenols and Amines under Metal-Free Conditions

Mild, convenient, and effective TBAI-catalyzed S-H and N-H insertion reactions of α-diazoesters with thiophenols and aromatic amines under metal-free conditions have been described, furnishing a straightforward and general platform for the synthesis of various thioethers and 2-amino-2-oxoacetates in moderate to excellent yields. Moreover, this strategy features simple operation, mild conditions, broad substrate scope, and easy scale-up.

Recent applications of quinolinium salts in the synthesis of annulated heterocycles

Quinoline derivatives are frequently found in natural products and biologically active compounds, however, construction of quinoline fused polyheterocycles is the challenging goal in synthetic organic chemistry. In this regard, quinolinium salts meet the demand to a great level, as they can be synthesized readily and employed effectively for the rapid construction of condensed heterocyclic core. The present review focuses on recent (2015-2021) applications of different quinolinium salts that react with suitable partners to access diverse annulated products. Most of the reactions discussed here involve easily available starting materials, operationally simple, high atom efficiency and environmentally benign. Mechanistic aspects of representative transformations have also been highlighted for better understanding of reaction pathway.

Recent progress in the synthesis of pyrrolo[2,1-a]isoquinolines

Pyrrolo[2,1-a]isoquinolines occur frequently in a large number of bioactive natural products and pharmaceutically important molecules. The synthesis of pyrrolo[2,1-a]isoquinoline derivatives is an easy and useful way to produce artificial molecules with potential applications. A huge number of excellent methodologies for constructing pyrrolo[2,1-a]isoquinolines have been reported in the last decade. This review summarizes the recent advances in this research field covering from 2011 to 2021.

Rh(ii)-catalyzed synthesis of 5H-isochromeno[3,4-b]indolizines from 4-diazoisochroman-3-imines and pyridines

A Rh(ii)-catalyzed (3 + 2) annulation of pyridines with 4-diazoisochroman-3-imines furnished 5H-isochromeno[3,4-b]indolizines with moderate to good yields and complete regioselectivity.

Iodine-Mediated Domino Cyclization for One-Pot Synthesis of Indolizine-Fused Chromones via Metal-Free sp3 C-H Functionalization

An efficient method for the synthesis of new indolizine-fused chromones has been accomplished from ethyl (E)-3-(2-acetylphenoxy)acrylates and pyridines in a "one-pot" manner. Facile operation in open-air, metal-free, and mild conditions renders this protocol particularly practical and attractive. Moreover, this method can simultaneously construct two molecular fragments of chromone and indolizine. Scale-up experiment and the construction of natural products further prove the practicability of this strategy.

One-Pot Synthesis of Chromone-Fused Pyrrolo[2,1-a]isoquinolines and Indolizino[8,7-b]indoles: Iodine-Promoted Oxidative [2 + 2 + 1] Annulation of O-Acetylphenoxyacrylates with Tetrahydroisoquinolines and Noreleagnines

An iodine-promoted one-pot cascade oxidative annulation reaction has been developed for the synthesis of chromone-fused-pyrrolo[2,1-a]isoquinolines and indolizino[8,7-b]indoles from o-acetylphenoxyacrylates, tetrahydroisoquinolines, and noreleagnines. This process underwent a logical approach to both chromone-fused-pyrrolo[2,1-a]isoquinolines and chromone-fused-indolizino[8,7-b]indoles isolamellarin derivatives. Manipulations of l-menthol and dl-α-tocopherol demonstrate the applications of this strategy.

Multicomponent Reactions Involving Diazo Reagents: A 5-Year Update

This review summarizes recent developments in multicomponent reactions of diazo compounds. The role of diazo reagent and the type of interaction between components was analyzed to structure the discussion. In contrast to previous reviews on related topics mostly focused on metal catalyzed transformations, a substantial amount of organocatalytic or catalyst-free methodologies is covered in this work.

Blue Light-Emitting Diode-Mediated In Situ Generation of Pyridinium and Isoquinolinium Ylides from Aryl Diazoesters: Their Application in the Synthesis of Diverse Dihydroindolizine

Blue light-emitting diode-mediated environmentally sustainable three component reactions among pyridine/isoquinoline 1/2, aryl diazoesters 3, and acrylic ester/3-alkenyl oxindoles 5/6 provide various dihydroindolizines 7 to 9 in excellent yield. The principle of the strategy is photolytic generation of nitrogen ylides from N-heteroarenes and aryl diazoesters and their subsequent [3 + 2] cycloaddition reaction with dipolarophiles. Detailed mechanistic analysis of the transformation through control experiments establishes this strategy as the foundation for the photolytic multicomponent reaction.

Assembly of functionalized π-extended indolizine polycycles through dearomative [3+2] cycloaddition/oxidative decarbonylation

Reported herein is an unexpected construction of functionalized π-extended indolizine polycycles through a one-pot two-step cascade process comprising the base-promoted dearomative [3+2] cycloaddition of quinilinium salts and 3-alkenyl oxindoles, followed by a DDQ-mediated oxidative decarbonylation. Moreover, we could achieve the substrate-controlled diverse synthesis of structurally strained cyclopropane spirooxindole by using pyridinium salts as starting materials.

A novel three-component reaction for constructing indolizine-containing aliphatic sulfonyl fluorides

A copper-catalyzed three-component reaction for transforming quinolines, isoquinolines and pyridines to a class of indolizine-containing alkyl sulfonyl fluorides was developed.

CuBr/NHPI co-catalyzed aerobic oxidative [3 + 2] cycloaddition-aromatization to access 5,6-dihydro-pyrrolo[2,1-a]isoquinolines

An efficient and enviromentally friendly CuBr/NHPI co-catalyzed aerobic oxidative [3 + 2] cycloaddition-aromatization cascade was realized with N-substituted tetrahydroisoquinolines and electron-deficient olefins. Under the mild conditions, the reaction proceeded smoothly and displayed excellent functional group tolerance, affording 5,6-dihydro-pyrrolo[2,1-a]isoquinolines in good to high yields. This protocol exhibits a broad substrate scope to both N-alkyl tetrahydroisoquinolines and dipolarophile substrates.

Aziridinium Ylides: Underutilized Intermediates for Complex Amine Synthesis

Harnessing the chemistry of onium ylide intermediates generated from transition metal catalysis is a powerful strategy to convert simple precursors into complex scaffolds. While the chemistry of onium ylides has been studied for over three decades, transformations of aziridinium ylides have just recently emerged as a versatile way to exploit the strain of these reactive intermediates to furnish densely functionalized N-heterocycles in a highly stereocontrolled manner. Herein, we provide a short overview of the key concepts and recent developments in this area, with a focus on how mechanistic studies to delineate the factors controlling the reactivity of aziridinium ylides can stimulate fruitful future investigations.

Iron-catalyzed [3 + 2]-cycloaddition of in situ generated N-ylides with alkynes or olefins: access to multi-substituted/polycyclic pyrrole derivatives

An iron-catalyzed one-pot three-component reaction of 1-substituted benzimidazoles with diazoacetates and electron-deficient alkynes or alkenes has been reported. Mechanistically, the reaction goes through a 1,3-dipolar cycloaddition of catalytically generated benzimidazolium N-ylides with various activated alkynes or alkenes, leading to multi-substituted and polycyclic fused pyrrole derivatives, respectively.

Copper-catalyzed formal [1 + 2 + 2]-annulation of alkyne-tethered diazoacetates and pyridines: access to polycyclic indolizines

A copper-catalyzed formal [1 + 2 + 2]-annulation of alkyne-tethered diazo compounds with pyridines, which affords polycyclic fused indolizine derivatives with broad substrate generality, has been reported.

Copper-Catalyzed Cascade Cyclization Reactions of Diazo Compounds with tert-Butyl Nitrite and Alkynes: One-Pot Synthesis of Isoxazoles

A novel copper-catalyzed [3 + 2] cycloaddition reaction of alkynes with nitrile oxides generated in situ from the coupling reaction of copper carbene and nitroso radical has been developed. The three-component reaction provides a simple and efficient method for the construction of isoxazoles in a highly regioselective manner in a single step. On the basis of the experimental results and density functional theory calculations, a catalytic cycle (Cu^I-Cu^II-Cu⁰-Cu^I) for this cascade cyclization reaction is proposed.

Metal-Free Aminothiation of Alkynes: Three-Component Tandem Annulation toward Indolizine Thiones from 2-Alkylpyridines, Ynals, and Elemental Sulfur

A metal-free three-component annulation reaction for the synthesis of indolizine thiones via tandem C-C/C-N/C-S bond formation was developed. Various 2-alkylpyridines with aromatic ynals and elemental sulfur proceeded smoothly under catalyst-free conditions, and the desired products were obtained in moderate to excellent yields.

Synthesis of pyrrolo[1,2-a]quinolines by formal 1,3-dipolar cycloaddition reactions of quinolinium salts

Quinolinium salts, Q+-CH2-CO2Me Br- and Q+-CH2-CONMe2 Br- (where Q = quinoline), were prepared from quinolines. Deprotonation of these salts with triethylamine promoted the reaction of the resulting quinolinium ylides (formally azomethine ylides) with electron-poor alkenes by conjugate addition followed by cyclization or by [3 + 2] dipolar cycloaddition. The pyrroloquinoline products were formed as single regio- and stereoisomers. These could be converted to other derivatives by Suzuki-Miyaura coupling, reduction or oxidation reactions.

Transition-metal-free catalyzed [3+2] cycloadditions/oxidative aromatization reactions for the synthesis of annulated indolizines

A transition-metal-free catalyzed [3+2] cycloadditions/oxidative aromatization three-component reactions for direct construction of annulated indolizines was reported.