Unexpected exo selectivity for an intramolecular Diels-Alder reaction involving a doubly-activated δ-pentenolide dienophile

The intramolecular Diels-Alder cyclization of a homochiral doubly-activated δ-pentenolide system is described, proceeding predominantly via an exo transition state to provide an advanced precursor toward unique stereochemical analogs of (+)-symbioimine.