Asymmetric cyclization reactions of allenoates with imines or α,β-unsaturated ketones catalyzed by organocatalysts derived from cinchona alkaloids

Lewis base-catalyzed cyclization reactions of allenoates with electron-deficient olefins and imines have been demonstrated by the preparation of biologically active natural products and pharmaceutically interesting substances and have emerged as powerful synthetic tools in the rapid construction of cyclic molecular complexity. In contrast to phosphine-containing Lewis bases, nitrogen-containing Lewis base amines display markedly different reaction profiles; however, this area is not well-developed. Herein we summarize the recent progress in this emerging field and outline the challenges ahead.

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-ArCH₂-substituted allenoates, the reaction pathway was controlled by the phosphine catalyst used.