Total Syntheses of Marine Furanosesquiterpenoids, Tubipofurans

The first total syntheses of marine furanosesquiterpenoids, tubipofuran (1) and 15-acetoxytubipofuran (2), have been achieved via the common intermediate 3. The syntheses were begun by our newly improved fused 3-methylfuran construction method by the reaction of allenic sulfonium salt with the enolate anion of a cyclic 1,3-diketone. Using this fused furan synthesis, multigram quantities of bicyclic 3-methylfurans were readily obtained in only one step. The cis-fused furanodecalin system was constructed by the regioselective Diels–Alder reaction of benzofuranquinone 5 and Danishefsky diene 4. The Diels–Alder adduct 9 was transformed into the intermediate 3 by sequential radical deoxygenations of the two hydroxy groups derived from benzofuranquinone 5.