Ring-Strain-Driven Annulation of Cyclopropenones with Hydrazones via Nucleophilic Phosphine Catalysis.

Herein, we demonstrate a ring-strain-driven annulation of cyclopropenones with hydrazones via simple phosphine catalysis, providing an efficient route to structurally diverse 1,3,4-oxadiazolines and spiro-1,3,4-oxadiazolines under mild conditions. The reaction proceeds through the formation of a reactive α-ketenyl phosphorus ylide intermediate, generated by the interaction of cyclopropenone with a nucleophilic phosphine catalyst. Notably, an intriguing acyl group migration-mediated [3+2] annulation is observed with hydrazones derived from linear ketones, while a ring-opening addition/cyclization cascade takes place with cyclic hydrazones.