Studies towards the synthesis of FCRR toxin: an expeditious entry into 7–5–6 ring systems via [5+2] oxidopyrylium-alkene cycloaddition

Synthetic studies towards the diterpene natural product FCRR toxin have been undertaken. An intermolecular [5+2] oxidopyrylium-alkene cycloaddition reaction was employed to construct the 7–5–6 tricyclic framework. The reaction proceeded with very high regio- and stereoselectivity and the bridging ether was reductively cleaved to unmask the carbocycle.