Synthesis of the zoanthamine ABC ring system: Some surprises from intramolecular Diels-Alder reactions

In connection with the total synthesis of the marine alkaloids zoanthamine and norzoanthamine, an elaborate model study was conducted starting from (-)-carvone. In nine steps alkenyl iodides corresponding to the C-11-C-24 fragment of zoanthamine were obtained. The alkenyl iodides were coupled to various stannanes (C-6-C-10 fragment) via the Corey modification of the Stille reaction, affording a variety of Diels-Alder precursors. An interesting and highly unexpected cascade reaction sequence was observed during the screening of an intramolecular Diels-Alder reaction. generating a novel tetracyclic framework. A slight, modification in the Diels-Alder precursor allowed the desired cycloaddition to take place, giving the cycloadduct in 87% yield.