Studies toward the total synthesis of marine natural products having an azaspiro ring system

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Toward the total synthesis of spirolides, marine natural products with an azaspirocyclic system, the construction of E and BCD ring systems of these molecules have been explored. Diels-Alder reaction of the diene, prepared from L-glutamic acid, with a 7-membered α-methylene lactone exhibited complete regioselectivity and modest exo and diastereoface selectivity to provide the desired exo cycloadduct as the major product. Since all attempts to obtain the desired dispiroketal isomer from an acyclic precursor by tandem double hemiketal formation/hetero-Michael addition sequence met with failure, we turned our attention to the transannular reaction, and designed and synthesized a macrocyclic substrate.