Total Synthesis of Marine Sesquiterpene Quinones (+)‐Cyclospongiaquinone‐1 and (–)‐Dehydrocyclospongiaquinone‐1 with a Tetracyclic Benzo[ a ]xanthene Skeleton

Two marine sesquiterpene quinones, (+)‐cyclospongiaquinone‐1 and (–)‐dehydrocyclospongiaquinone‐1, were efficiently synthesized in 10 and 12 steps and overall yields of 33 and 24 %, respectively, from commercially available (+)‐sclareolide. The synthetic strategy involves the following crucial steps: (i) assembly of the carbon skeleton by coupling the decalin segment to the aromatic moiety; (ii) one‐step construction of the dihydropyran ring by acid‐induced acetonide deprotection/ether cyclization of an olefinic decalin intermediate that contains the aromatic moiety; (iii) formation of the p ‐benzoquinone moiety by oxidation of a p ‐methoxyphenolic derivative to produce (+)‐cyclospongiaquinone‐1; and (iv) oxidative dehydrogenation at the C‐12–C‐12a bond of a tetracyclic intermediate to obtain (–)‐dehydrocyclospongiaquinone‐1.