P(NMe ₂ ) ₃ -Mediated Reductive (4 + 1) Annulation Reaction of Isatins with Oxindole-Derived α,β-Unsaturated Imines and Diastereoselective Synthesis of 2,3-Di(spirooxindolyl)pyrrolines

Herein, we report a P(NMe₂)₃-mediated (4 + 1) annulation reaction between substituted isatins and oxindole-derived α,β-unsaturated imines. This method facilitates the diastereoselective synthesis of a variety of structurally diverse 2,3-di(spirooxindolyl)pyrrolines featuring two vicinal tetrasubstituted carbon-stereocenters in moderate to high yields with exclusive diastereoselectivity. Mechanistically, the reaction is proposed to proceed via a Kukhtin-Ramirez adduct-initiated reductively deoxygenative annulation pathway, comprising an intermolecular nucleophilic 1,4-addition and subsequently followed by an intramolecular stereoselective SN2-type substitution. This strategy provides a practical and complementary synthetic approach to forge biologically relevant 2,3-di(spirooxindolyl)pyrrolidine scaffolds from readily accessible starting materials.