Metal-free cyclization of unsaturated hydrazones for the divergent assembly of pyrazolones and pyrazolines.

An oxidative cyclization of β,γ-unsaturated hydrazones for the divergent assembly of pyrazolones and pyrazolines under metal-free conditions is presented. This divergent synthetic strategy was achieved by controlling the reaction atmosphere. A number of pyrazolines were obtained via 5-exo-trig cyclization when the reaction was performed under N2. Whereas when the reaction was conducted under O2, diverse functionalized pyrazolones were achieved via oxidative cleavage of the C[double bond, length as m-dash]C double bond.