Synthesis of the Fully Functionalized Core Structure of the Antibiotic Abyssomicin C

[reaction: see text] The fully functionalized core structure 23 of abyssomicin C (6) containing an oxabicyclooctane ring and a tetronate was prepared via a Diels-Alder approach. After hydroxylation of lactone 10 to the alpha-hydroxylactone 12, lactone opening led to the hydroxy ester 16. A directed epoxidation furnished the desired syn-epoxide 20. Acetylation of the tertiary hydroxyl group, followed by intramolecular Claisen condensation, gave directly the core structure 23.