Diastereodivergent Synthesis of Pentacyclic Spiroindolines via a Magnesium(II)-Catalyzed Cascade Reaction of N,N '-Cyclic Azomethine Imines with Indolyl-Substituted Isocyanides.

Magnesium(II)-catalyzed cascade reactions of N,N '-cyclic azomethine imines with indolyl-substituted isocyanides are reported herein. The method exhibited a high functional group tolerance and broad substrate scope. A series of anti -pentacyclic spiroindolines containing N,N '-fused heterocycle skeletons were obtained in up to 82% yield with 8.5:1 dr under mild reaction conditions. Intriguingly, a sequential HOAc-mediated protonation results in a diastereoenriched epimerization, which gives rise to the syn -pentacyclic spiroindolines as the sole isomers.