Efficient Synthesis of Octahydrophenanthrene Derivatives with Mild Cascade Reactions of Isochromenylium Tetrafluoroborates and Bifunctional Styrenes

A highly
efficient catalyst-free cascade reaction of air-stable
isochromenylium tetrafluoroborates and bifunctional styrenes containing
a 1,3-diketone moiety has been developed, affording the corresponding
single diastereomeric ocatahydrophenanthrene derivatives (21 examples,
up to 86% yield). A cascade process of [4 + 2]-cyclization and subsequent
intramolecular nucleophilic addition is proposed to generate the three
new C–C bonds diastereoselectively in the reactions.