Palladium-Catalyzed Difunctionalization of Alkenes by Relay Coupling with Propargylic Pyridines: Synthesis of Indolizine and Indolizinone-Containing Bisheterocycles

Metal-Free Synthesis of Functionalized Indolizines via a Cascade Michael/SN2/Aromatization Reaction of 2-Alkylazaarene Derivatives with Bromonitroolefins

A transition metal-free domino Michael/SN2/aromatization annulation of 2-pyridylacetates with bromonitroolefins has been developed. A wide range of substrates containing various substituted groups was compatible with the present methodology and afforded functionalized indolizines with moderate to excellent yield (up to 99% yield). In addition, the potential practicality of the method stood out through scale-up reactions and further transformations to other valuable compounds. In our view, this study is an essential complement for the rapid construction of indolizine derivatives through a metal-free strategy.

Domino Approach to Heterocycles-Based Unsymmetrical Triarylmethanes through Heteroannulation of 2-(2-Enynyl)-pyridines with Enaminones

Herein, we report a copper-catalyzed protocol to access unsymmetrical triarylmethanes containing both indolizine and the chromone scaffolds in the same molecule via a 5-endo-dig cyclization of 2-(2-enynyl)-pyridines followed by reaction with 2-hydroxyaryl enaminones. A variety of 2-hydroxyaryl enaminones and 2-(2-enynyl)-pyridines were subjected to reaction under the optimal reaction conditions, and the respective triarylmethanes were obtained in good to excellent yields.

Dual Nickel/Photoredox-Catalyzed Arylsulfonylation of Allenes

The nickel/photoredox dual catalysis system is an efficient conversion platform for the difunctionalization of unsaturated hydrocarbons. Herein, we disclose the first dual nickel/photoredox-catalyzed intramolecular 1,2-arylsulfonylation of allenes, which can accurately construct a C(sp2)-C(sp2) bond and a C(sp3)-S bond. The reaction exhibits excellent chemoselectivity and regioselectivity, allowing modular conformations of a diverse series of 3-sulfonylmethylbenzofuran derivatives. Control experiments showed that the bipyridine ligand is crucial for the formation of a stable σ-alkyl nickel intermediate, providing the possibility for sulfonyl radical insertion. Meanwhile, the electrophilic sulfonyl radical facilitates further oxidative addition of the σ-alkyl nickel intermediate and inhibits addition with allenes. In addition, control experiments, cyclic voltammetry tests, Stern-Volmer experiments, and density functional theory calculations afford evidence for the Ni(0)/Ni(I)/Ni(II)/Ni(III) pathway in this 1,2-arylsulfonylation.

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.

Stereoselective Synthesis of Indolyl-C-glycosides Enabled by Sequential Aminopalladation and Heck Glycosylation of 2-Alkynylanilines with Glycals

An efficient and general approach for the synthesis of indolyl-C-glycosides via aminopalladation and subsequent Heck-type glycosylation of easily available 2-alkynylanilines and glycals has been developed. This protocol features excellent stereoselectivity, a broad substrate scope, and mild reaction conditions. In addition, 2,3-pseudoglycals also successfully participated in this cascade reaction, affording C2/C3-branched indolyl glycosides with high regio-/stereoselectivity. The utility of this protocol was also demonstrated by a large-scale reaction and diversified synthetic transformations of the desired products.

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.

Palladium-catalyzed cyclizative cross coupling of ynone oximes with 2-haloaryl N-acrylamides for isoxazolyl indoline bis-heterocycles

Metal-catalyzed cyclizative cross-coupling reactions have attracted enormous attention due to their unique cascade nature. We demonstrated, herein, a dual-cyclizative coupling of ynone oxime ethers with acrylamides for the synthesis of methylene-linked isoxazolyl 2-oxindoles. The cascade was triggered by a palladium(II)-catalyzed ynone oxime ether cyclization, which underwent a Heck-type coupling intercepted by an aryl iodide insertion. Control experiments were carried out to understand the mechanism.