Highly Regioselective [3 + 2] Annulation of Azomethine Imines with 1-Alkynyl Fischer Carbene Complexes to Functionalized <i>N</i>,<i>N</i>-Bicyclic Pyrazolidin-3-ones

The highly regioselective [3 + 2] cycloaddition of azomethine imines to 1-alkynyl Fischer carbene complexes has been successfully realized under mild conditions. Oxidative demetalation of the newly formed pyrazolo-pyrazolone carbene complexes with pyridine-N-oxide or ceric ammonium nitrate efficiently afforded pyrazolo-pyrazolone derivatives as well as cycloprop-2-enone and trisubstituted 1H-pyrazoles in some cases, providing a novel route to versatile functionalized N,N-bicyclic pyrazolidin-3-ones.