A Diastereoselective Formal Synthesis of Berkelic Acid

A formal synthesis of berkelic acid is reported. The strategy employs the combination of a chiral exocyclic enol ether and an achiral isochromanone to afford the chroman spiroketal core via a base-triggered generation and cycloaddition of an o-quinone methide intermediate. Other key steps include equilibration of the spiroketal, intramolecular benzylic oxidation, and lactone addition/hemiketal reduction; all occur with good diastereoselectivity.