Synthesis of Spiropentadiene Pyrazolones by Rh(III)-Catalyzed Formal sp C-H Activation/Annulation.

A Rh-catalyzed enol-directed formal sp C-H activation/annulation of α-arylidene pyrazolones with alkynes has been developed. This reaction provides a convenient route to synthesize spiropentadiene pyrazolones in good to excellent yields at room temperature, exhibiting good functional group tolerance, gram scalability, and high regioselectivity. Of note, the α-arylidene pyrazolone was introduced as a novel C3 synthon in C-H activation/annulation.