Redox-Neutral Dearomative Spirocyclization of Isoxazoline N-Oxides: A Route to Structural Analogs of Tyrosine-Derived Sponge Metabolites

A novel synthetic route to spirocyclic isoxazolines based on a redox-neutral dearomative cyclization of 3-benzyl-substituted isoxazoline N-oxides has been developed. The reaction likely proceeds through isoxazoline ring opening with generation of an unstable N-oxoiminium cation. The latter acts as an O-electrophile in the cyclization involving the ipso-position of the aryl ring. The reaction exhibits a broad substrate scope and affords spirocyclic isoxazolines featuring novel substitution patterns compared with those accessible by synthetic methods or found in natural sources.