Phosphine‐Catalyzed [3+2] and [4+3] Annulation Reactions of C,N‐Cyclic Azomethine Imines with Allenoates

Phosphine‐catalyzed [3+2] and [4+3] annulation reactions of C,N‐cyclic azomethine imines with allenoates have been developed to give a variety of pharmaceutically attractive tetrahydroisoquinoline derivatives in moderate to excellent yields. The two distinct reaction pathways, [3+2] and [4+3] cyclization, depend on the nature of the nucleophilic phosphine and the allenoate. Generally, for α‐alkylallenoates, the reactions always proceed with [3+2] cyclization as the major pathway no matter what phosphine was used; for α‐ArCH 2 ‐substituted allenoates, the reaction pathway was controlled by the phosphine catalyst used.