Synthesis of spiro‐isoxazoline‐pyrazolones via formal [4 + 1] cycloaddition between α‐bromooximes and 4‐bromo‐pyrazolones

An efficient [4 + 1] cycloaddition between nitrosoalkenes generated from α‐bromooximes and 4‐bromo‐pyrazolone has been developed, and a serious of spiro‐isoxazoline‐pyrazolones were obtained in good to excellent yields with a wide substrate scope under mild conditions. This reaction led to the assembly of pharmaceuticaly relevant pyrazolone and isoxazolin nuclei into a structurally novel spirocyclic skeleton.