Organocatalytic Regiodivergent C−C Bond Cleavage of Cyclopropenones: A Highly Efficient Cascade Approach to Enantiopure Heterocyclic Frameworks

Asymmetric Organocatalysis in the Remote (3 + 2)-Cycloaddition to 4-(Alk-1-en-1-yl)-3-cyanocoumarins

The application of organocatalytic bifunctional activation in the remote (3 + 2)-cycloaddition between 4-(alk-1-en-1-yl)-3-cyanocoumarins and imines derived from salicylaldehyde is demonstrated. Products, bearing two biologically relevant units, have been obtained with good chemical and stereochemical efficiency. The stereochemical outcome of the process results from the application of a quinine-derived catalyst. Selected transformations of the cycloadducts leading to further chemical diversity have been demonstrated.

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.

Catalyst-Controlled Chemodivergent [3+3] and [3+2] Formal Cycloadditions of Azomethine Ylides with Diphenylcyclopropenone

Chemodivergent cycloadditions of azomethine ylides with diphenylcyclopropenone involving either Cu-catalyzed [3+3] or Ag-catalyzed [3+2] processes have been developed. These transformations provide a highly efficient method for the preparation of a variety of aromatic substituted dihydropyridinones and dihydropyrrolones with excellent regio and diasteroselectivities.

Rh(III)-Catalyzed spiroannulation of ketimines with cyclopropenones via sequential C-H/C-C bond activation

An unprecedented Rh(III)-catalyzed [3+3]-spiroannulation of ketimines with cyclopropenones to access spiro[4,5]dienones has been developed. Sequential C-H/C-C bond activation and subsequent nucleophilic addition are disclosed in this process. This procedure represents the first example of the construction of spirolactams utilising cyclopropenones as 3C synthons. The remarkable advantages of this protocol are excellent chemo- and regio-selectivity, wide functional group tolerance, high reaction yields, and tolerance towards H₂O.

Phosphine-Catalyzed Dearomative [3 + 2] Cycloaddition of Benzoxazoles with a Cyclopropenone

The triphenylphosphine-catalyzed dearomative [3 + 2] cycloaddition of benzoxazoles with 1,2-diphenylcyclopropenone is herein described. The reaction scope, mechanism, and possible future applications of this rare organocatalyzed cycloaddition are herein discussed.

Heterocycles from cyclopropenones

Great attention has been paid to cyclopropenones as they are present in many natural sources.

Perfluoroalkyl-Promoted Synthesis of Perfluoroalkylated Pyrrolidine-Fused Coumarins with Methyl β-Perfluoroalkylpropionates

Employing the methyl β-perfluoroalkylpropionate as the Michael acceptor, an efficient approach for the synthesis of perfluoroalkylated pyrrolidine-fused coumarins has been achieved. A tandem reaction involving [3 + 2] cycloaddition and intramolecular transesterification was proposed for the mechanism. The enhanced electrophilicity resulting from the strong electron-withdrawing ability of the perfluoroalkyl group was crucial for this tandem reaction.

Enantioselective palladium-catalyzed C(sp2)-C(sp2) σ bond activation of cyclopropenones by merging desymmetrization and (3 + 2) spiroannulation with cyclic 1,3-diketones

Catalytic asymmetric variants for functional group transformations based on carbon–carbon bond activation still remain elusive. Herein we present an unprecedented palladium-catalyzed (3 + 2) spiro-annulation merging C(sp²)–C(sp²) σ bond activation and click desymmetrization to form synthetically versatile and value-added oxaspiro products. The operationally straightforward and enantioselective palladium-catalyzed atom-economic annulation process exploits a TADDOL-derived bulky P-ligand bearing a large cavity to control enantioselective spiro-annulation that converts cyclopropenones and cyclic 1,3-diketones into chiral oxaspiro cyclopentenone–lactone scaffolds with good diastereo- and enantio-selectivity. The click-like reaction is a successful methodology with a facile construction of two vicinal carbon quaternary stereocenters and can be used to deliver additional stereocenters during late-state functionalization for the synthesis of highly functionalized or more complex molecules.

An unprecedented palladium-catalyzed (3 + 2) spiro-annulation merging C–C bond activation and desymmetrization was developed for the enantioselective construction of synthetically versatile and value-added oxaspiro products with up to 95% ee.

Phosphine-catalyzed sequential (2+3)/(2+4) annulation of γ-vinyl allenoates: access to the synthesis of chromeno[4,3-b]pyrroles

A phosphine-catalyzed cascade (2+3)/(2+4) cyclization reaction of γ-vinyl allenoates with aldimine esters has been developed to provide a series of chromeno[4,3-b]pyrrole derivatives that contain three contiguous stereogenic centers. The method gives a good yield, excellent chemoselectivity and diastereoselectivity under mild conditions.

Diastereoselective trifunctionalization of pyridinium salts to access structurally crowded azaheteropolycycles

A highly diastereoselective dearomative trifunctionalization of pyridinium salts with multifunctional o-hydroxyl aromatic azomethine ylides has been established, which not only provided a convenient and rapid method to assemble challenging and architecturally crowded chroman-pyrrolidine-hydrogenated pyridine fused pentacycles, but also represented a rare successful example of the higher-order multifunctionalization of pyridiniums.

Regiodivergent Organocatalytic Reactions

Organocatalysts are abundantly used for various transformations, particularly to obtain highly enantio- and diastereomeric pure products by controlling the stereochemistry. These applications of organocatalysts have been the topic of several reviews. Organocatalysts have emerged as one of the very essential areas of research due to their mild reaction conditions, cost-effective nature, non-toxicity, and environmentally benign approach that obviates the need for transition metal catalysts and other toxic reagents. Various types of organocatalysts including amine catalysts, Brønsted acids, and Lewis bases such as N-heterocyclic carbene (NHC) catalysts, cinchona alkaloids, 4-dimethylaminopyridine (DMAP), and hydrogen bond-donating catalysts, have gained renewed interest because of their regioselectivity. In this review, we present recent advances in regiodivergent reactions that are governed by organocatalysts. Additionally, we briefly discuss the reaction pathways of achieving regiodivergent products by changes in conditions such as solvents, additives, or the temperature.

Hydroxyl-group-activated azomethine ylides in organocatalytic H-bond-assisted 1,3-dipolar cycloadditions and beyond

1,3-Dipolar cycloaddition constitutes a powerful means for the synthesis of five-membered heterocycles. Recently, the potential of this field of chemistry has been expanded by the employment of organocatalytic activation strategies. One group of substrates, namely imines derived from salicylaldehydes, is particularly useful. Additional activation via intramolecular H-bonding interactions offered by the presence of an ortho-hydroxyl phenolic group in their structure results in increased reactivity of these reactants. Furthermore, it can be utilized in subsequent reactions creating chemical and stereochemical diversity. This minireview provides a summary of the recent progress in this field of organocatalysis and indicates other important applications of hydroxyl-group-activated azomethine ylides in asymmetric organocatalysis.

Palladium/Copper Cocatalyzed C-H Activation and C-C Bond Regioselective Cleavage Reaction for the Synthesis of Fused Chromeno Quinolines

A method for the synthesis of fused chromeno quinolines by the palladium/copper cocatalyzed C-H bond activation and C-C bond cleavage reaction has been developed. A variety of fused chromeno quinoline derivatives could be synthesized by coumarin derivatives and anilines through the C-C bond cleavage/C-H functionalization/C-C bond formation process. The MTT assay was used to evaluate the chromeno quinolines against the human cervix tumor cells (HeLa), 3c and 9c without coordination metals showed good inhibition effect.

Synergistic promotion by intramolecular hydrogen bonding: a bi-functionally catalyzed cascade reaction for the synthesis of enantiopure chromenopyrrolidines

An effective organocatalytic asymmetric cascade reaction of o-hydroxy aromatic aldimines and β,γ-unsaturated-α-ketoesters was developed.