An enantioselective total synthesis of the marine tricyclic polyketide ascospiroketal B, previously isolated from the marine-derived fungus Ascochyta salicorniae, was accomplished in 21 steps by using an improved route. The intriguing 5,5-spiroketal-cis-fused-γ-lactone core was constructed through rearrangement of an epoxide, in conjunction with an acid-mediated spiroketalization.

Ascospiroketal B was isolated from a marine-derived fungus as a structurally unique polyketide possessing a rare tricyclic core including 5,5-spiroketal-γ-lactone. An asymmetric total synthesis of ent-ascospiroketal B was achieved using an original synthetic route. The synthesis included the stereoselective construction of 5,5-spiroketal for ascospiroketal B and stereocontrolled construction of a quaternary asymmetric carbon by rearrangement of a trisubstituted epoxide.

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