Short Enantioselective Total Syntheses of Cheloviolenes A and B and Dendrillolide C via Convergent Fragment Coupling Using a Tertiary Carbon Radical

The development of
a convergent fragment coupling strategy for
the enantioselective total syntheses of a group of rearranged spongian
diterpenoids that harbor the cis-2,8-dioxabicyclo­[3.3.0]­octan-3-one
unit is described. The key bond disconnection relies on a late-stage
fragment coupling between a tertiary carbon radical and an electron-deficient
alkene to unite two ring systems and form two new stereocenters, one
of which is quaternary, in a stereoselective and efficient manner.
This strategy is applied toward scalable 14–15 step syntheses
of three rearranged spongian diterpenoids, cheloviolenes A and B,
and dendrillolide C.