Manganese(III)‐Promoted Tandem Oxidation and Cyclization of β‐Keto Ester Derivatives of Terpenoids

A new type of terpenoid cyclization directed by a β‐keto ester moiety has been developed, which proceeded by manganese(III)‐initiated oxidation of the β‐keto ester, followed by an intramolecular hetero Diels–Alder reaction with the terpenoid chain. This reaction produces polycyclic dihydropyrans in high yields and stereoselectivities under mild conditions.