Three-Component Fusion to Pyrazolo[5,1-a]isoquinolines via Rh-Catalyzed Multiple Order Transformation of Enaminones

A facile and practical method for the synthesis of fused tricyclic pyrazolo[5,1-a]isoquinolines has been realized via the reactions of enaminones, hydrazine hydrochloride, and internal alkynes. By means of Rh catalysis, the extraordinary high-order bond functionalization, including the transformation of aryl C-H, ketone C═O, and alkenyl C-N bonds in the enaminones, marks the major feature of the cascade reactions. The results disclose the individual advantage of enaminones in the design of novel and efficient synthetic methods.

Ultrasound-Promoted Synthesis of α-Thiocyanoketones via Enaminone C═C Bond Cleavage and Tunable One-Pot Access to 4-Aryl-2-aminothiazoles

Ultrasound has been successfully employed to promote the thiocyanation of the C═C bond in enaminones for the synthesis of α-thiocyanoketones and 2-aminothiazoles. The reactions of enaminones with ammonium thiocyanate provide α-thiocyanoketones with ultrasound irradiation at room temperature. More interestingly, simply further heating the vessel after ultrasonic irradiation leads to the selective synthesis of 2-aminothiazoles with an unconventional 4-aryl substructure.

Base-Promoted Annulative Difluoromethylenation of Enaminones with BrCF2CO2Et toward 2,2-Difluorinated 2,3-Dihydrofurans

A practical method for the synthesis of 2,2-difluorinated 2,3-dihydrofurans has been established via the [4 + 1] annulation of enaminones and BrCF₂CO₂Et with Na₂CO₃ promotion. This new protocol does not employ any transition metal reagent and enables the annulative difluoromethylation by the partial cleavage of the C═C double bond. In addition, the further treatment with hydrochloric acid in one pot leads to β-keto enoic acids (4-oxo-2-butenoic acids) via a formal enaminone C-N carboxylation.

Copper-catalyzed C-H/N-H annulation of enaminones and alkynyl esters for densely substituted pyrrole synthesis

Herein, the copper-catalyzed annulation of enaminones with alkynyl esters for the facile synthesis of different pyrroles with a 2,3,4,5-tetrasubstituted structure has been developed. With Cu(OAc)₂ as the only catalyst, the tunable synthesis of 2-vinyl and 2,3-dicarboxyl-functionalized pyrroles has been achieved by using terminal and internal alkynyl esters, respectively. The synthesis of 2-vinyl pyrroles represents the first example accessing 2-vinyl substituted pyrroles via direct cascade reactions involving vinylation and pyrrole ring formation.

Recent developments in the synthesis of nitrogen-containing heterocycles from β-aminovinyl esters/ketones as CC-N donors

Nitrogen-containing heterocycles are ubiquitous fragments of numerous natural products, pharmaceuticals, designed bioactive drug candidates and agrochemicals. During the past few decades, these compounds have received considerable attention from the synthetic chemistry community, and great efforts have been focused on the development of concise and efficient methods for the synthesis of these heterocyclic skeletons. In this review, we summarize a diverse range of synthetic methods employing β-aminovinyl esters(ketones) as key CC-N-synthons to furnish useful bioactive heterocyclic frameworks, such as quinolines, pyridines, pyrazines, pyrroles, indoles, oxazoles, imidazoles, thiazoles, isothiazoles, pyrazoles, triazoles, and azepines, thus offering new opportunities and expanding the toolbox of synthetic chemistry reactions.