Versatile Construction of 6‑Substituted cis-2,8-Dioxabicyclo[3.3.0]octan-3-ones: Short Enantioselective Total Syntheses of Cheloviolenes A and B and Dendrillolide C

A short enantioselective synthesis of 6-substituted cis-2,8-dioxabicyclo­[3.3.0]­octan-3-ones is described. The pivotal step is coupling of a tertiary radical generated directly from a tertiary alcohol with a 3-chloro-5-alkoxybutenolide. This strategy is applied toward scalable 14–15 step syntheses of three rearranged spongian diterpenoids: cheloviolenes A and B and dendrillolide C.