Zinc(II)-Catalyzed [2+2+1] Annulation of Internal Alkenes, Diazooxindoles, and Isocyanates to Access Spirooxindoles.

Zinc(II)-catalyzed [2+2+1] annulation of internal alkenes, diazooxindoles, and isocyanates was successfully developed for the construction of multisubstituted spirooxindoles. This multicomponent transformation involves generation of a sulfur-containing spirocyclic intermediate from the [4+1] annulation of diazooxindole to sulfonyl isocyanate, which subsequently reacts as a 1,3-dipole with the internal alkene, that is, α-oxo ketene dithioacetal, to furnish a formal [2+2+1] annulation in a one-pot manner. This synthetic protocol features a low-toxicity main group metal catalyst, readily available reagents, and ≤96% yields, offering an efficient route to multisubstituted spirooxindole derivatives.