Synthesis of the spiroketal fragment of bistramide A via an exocyclic enol ether

An efficient synthesis of the spirocyclic fragment 1 of bistramides is reported. An olefination reaction of lactone 4 with sulfone 5 gave the enol ether 3, which upon cyclization in acidic media provided the spiroketal ring system. This compound was then converted into the C19–C36 fragment of the bistramides via successive Julia–Kocienski and Horner–Emmons olefinations.