Facile Synthesis of Polysubstituted Spiro‐Pyrazolone‐Pyrrolines via (1 + 4) Annulations of 4‐Halo Pyrazolones With 1‐Heterodienes

ABSTRACT A facile and mild synthetic route to polysubstituted spiro‐pyrazolone–pyrrolidinones has been developed via a formal (1 + 4) cycloaddition between 4‐halopyrazolones and acyclic conjugated imines. The reactions proceed in high yields (up to 95%) with excellent regioselectivity, delivering the target spirocyclic compounds that incorporate both pyrazolone and dihydropyrrole moieties. Moreover, the novel triazaspiro[4.4]nonene framework, which features two fused five‐membered azaheterocycles, shows promising potential as an enzyme inhibitor, as supported by preliminary molecular docking studies. The structure of the representative product was unambiguously confirmed by single‐crystal X‐ray diffraction analysis.