Harnessing Cyclopropenium Salts in Phase-Transfer Catalysis: A Cascade Reaction of Enaminones and Coumarins for the Construction of Functionalized Quinoline-2,5-Diones

Cyclopropenium salts, being the smallest class of aromatic compounds, have garnered extensive scientific interests across several realms of study. Herein, we report a phase-transfer catalyzed cascade reaction of cyclohexane-1,3-dione-derived enaminones and 3-aromacyl coumarins utilizing triaminocyclopropenium salts. This reaction efficiently synthesized a diverse set of quinoline-2,5-diones in good yields (78%-92%) and notably exhibited excellent diastereoselectivities, often furnishing the products as a single diastereoisomer. The successful gram-scale reaction and structural modifications of the product fully demonstrated the application potential of this protocol. Mechanistic studies implied that the triaminocyclopropenium ion transported the carbonate ion from the solid phase to the liquid phase to abstract protons, thus dramatically accelerating the reaction process.