N-Heterocyclic Carbene (NHC) Catalyzed Annulations of δ-Acetoxy Allenoates and Enaminones: A Time-Controlled Regiodivergent Synthesis of Hexahydroquinolines

An NHC (N-heterocyclic carbene) catalyzed [3 + 3] annulation of δ-acetoxy allenoates and enaminones has been developed. This strategy provides a facile and convenient approach for the synthesis of 1,4-dihydropyridine and 3,4-dihydropyridine-based hexahydroquinoline derivatives (51 examples, up to 77% yield). This protocol features a broad substrate scope, good functional group tolerance, and mild reaction conditions. Interestingly, the regioselectivity of products is time-controlled. At the beginning of the reaction, 1,4-dihydropyridines are rapidly formed, and 3,4-dihydropyridines are obtained with the extension of the reaction time.