Biomimetic Synthetic Studies on the Bruceol Family of Meroterpenoid Natural Products

A biomimetic approach to total synthesis can offer several benefits, including the development of cascade reactions for the rapid generation of molecular complexity, and guidance in the structure revision of old natural products and the anticipation of new ones. Herein, we describe how a biomimetic synthesis of bruceol, a pentacyclic meroterpenoid, led to the anticipation, isolation, and synthesis of isobruceol. The key step in the synthesis of both bruceol and isobruceol was an intramolecular hetero-Diels-Alder reaction of an o-quinone methide that was formed by dearomatization of an electron-rich chromene. The synthesis of an elusive biosynthetic intermediate also allowed a concise synthesis of eriobrucinol via a photochemical [2 + 2] cycloaddition. Furthermore, some speculation on the biosynthesis of prenylated bruceol derivatives inspired the development of a Claisen/Cope/Diels-Alder cascade reaction. We also report the generation of halogenated bruceol derivatives and the synthesis of several protobruceol natural products using singlet oxygen ene reactions.