Oxidative Polycyclization versus the “Polyepoxide Cascade”: New Pathways in Polyether (Bio)synthesis?

By which route is monensin A biosynthesized? Is an (E.E.E.) triene first epoxidized and the polyepoxide then transformed into the natural product in a cascade of ring openings and ring closures? Or does a (Z.Z.Z.) triene undergo successive oxidative cyclizations to give monensin A directly? New results from McDonald and Towne, who carried out oxidations of (Z) dienes with pyridinium chlorochromate, support the second route.