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 a-alkylallenoates, the reactions always proceed with [3+2]?cyclization as the major pathway no matter what phosphine was used; for a-ArCH2-substituted allenoates, the reaction pathway was controlled by the phosphine catalyst used.