Organocatalytic Enantioselective Formal [4 + 2] Cycloaddition of Enones with Cyclic <i>N</i>‑Sulfonylimines and Methylene Chromene for Chiral Spirocyclic Compounds

Abstract

A highly enantioselective synthesis of spirocycles and bridged rings has been developed through a formal [4 + 2] cycloaddition reaction between enones and <i>N</i>-sulfonylimines. The unprecedented strategy has been realized utilizing <i>N</i>-sulfonylimine as a novel dienophile through enamine–iminium tautomerism of <i>N</i>-sulfonylimine. In addition, a γ,ε-regioselective cycloaddition reaction proceeded by employing methylene chromene species as dienophiles