2,3-Bis(phenylsulfonyl)-1,3-butadiene as a Reagent for the Synthesis of the Azatricyclic Core of (±)-Halichlorine

An efficient stereocontrolled route to the azatricyclic core of an advanced halichlorine intermediate is described. Reaction of the oxime derived from 2-(oxo-cyclopentyl)acetic acid ethyl ester with 2,3-bis(phenylsulfonyl)-1,3-butadiene gives rise to a 7-oxa-1-azanorbornane cycloadduct in high yield. The formation of the bicyclic isoxazolidine arises from conjugate addition of the oxime onto the diene to afford a transient nitrone that then undergoes an intramolecular dipolar cycloaddition. Treatment of the cycloadduct with 5% Na/Hg results in reductive nitrogen-oxygen bond cleavage to furnish a spirocyclic piperidinone, which Was further elaborated to an advanced intermediate employed in an earlier synthesis of halichlorine.