Phosphine-Catalyzed (3 + 2) Annulation of δ-Acetoxy Allenoates with 2-Sulfonamidomalonate: Synthesis of Highly Substituted 3-Pyrrolines and Mechanistic Insight.

A mild and efficient synthetic protocol for 3-pyrrolines via the phosphine-catalyzed (3 + 2) annulation of δ-acetoxy allenoates with 2-sulfonamidomalonate is reported. The asymmetric version (up to 83% ee) is also achieved by using phosphine ( R)-SITCP as the catalyst. Mechanistic experiments disclose that the involved deprotonation of amide N-H and aza-addition to vinyl phosphonium might proceed in a concerted manner.