Discussion Addendum For: Phosphine‐Catalyzed [4 + 2] Annulation: Synthesis of Ethyl 6‐Phenyl‐1‐Tosyl‐1,2,5,6‐Tetrahydropyridine‐3‐Carboxylate

Abstract

Abstract There has been remarkable progress in organophosphine‐catalyzed reactions, especially the processes involving annulations, because of their potential application in building 5–8‐membered cyclic products. The phosphine‐catalyzed [4 + 2] annulation between 2‐alkyl‐but‐2,3‐dienoates and aldimines has become a powerful tool in the construction of substituted tetrahydropyridine derivatives. This discussion addendum focuses on new developments of the [4 + 2] annulation–including its asymmetric versions and synthetic utility–since 2003. The potential of the allene‐imine [4 + 2] annulation reaction in the total synthesis of natural product was illustrated in the formal synthesis of alstonerine. The discussion addendum presents the application of [4 + 2] annulation reaction. The [4 + 2] annulations between allene and imine will continue to find use in valuable product syntheses, especially those for which mild conditions, robust efficiency, and high selectivity are paramount.