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