Synthesis of 1,1-Spirocyclic Tetrahydroisoquinoline Systems

Significance

A new strategy for the construction of 1,1-spirocyclic tetrahydroisoquinolines, representing the framework of Erythrina alkaloids, is reported. The route involves the interception of the cationic species resulting from AgBF₄-mediated electrocyclic ring expansion of a gem-dihalocyclopropane, by an N-anion derived by LiHMDS deprotonation. Although the fundamental reaction is known, its use for the construction of this type of heterocyclic system has not been reported previously. The subsequent 5-exo-trig radical cyclization and Cl-radical elimination process is based on the work of Knapp et al. (Tetrahedron Lett. 1990, 31, 5397-5400).