Brucine Diol–Copper-Catalyzed Asymmetric Synthesis of endo-Pyrrolidines: The Mechanistic Dichotomy of Imino Esters

Enantio- and diastereodivergent approaches to pyrrolidines are described by using catalyst- and substrate-controlled reaction pathways. A concerted endo-selective [3 + 2]-cycloaddition pathway is developed for the reaction of methyl imino ester, whereas endo-pyrrolidines with an opposite absolute stereochemical outcome are prepared by using the stepwise reaction pathway of tert-butyl imino ester. The development of catalyst- and substrate-controlled stereodivergent approaches highlights the inherent substrate–catalyst interactions in the [3 + 2]-cycloaddition reactions of metalated azomethine ylides.