Synthetic Photochemistry. LVII. Facile Photochemical Construction of Hexahydro-as-indacene Skeleton, a Carbon Framework of Ikarugamycin, from High-Pressure Diels–Alder Adducts of Tropones–Cyclopentenone

The high-pressure Diels–Alder reaction of tropones with 2–cyclopenten-1-one afforded isomeric mixtures of adducts. Upon UV irradiation, these adducts isomerized to hexahydro-as-indacene, via [1,3] acyl migration, and tetracyclo[7.3.0.03,7.06,8]undecene derivatives through a ketene intermediate via a di-π-methane rearrangement. The former tricyclic system corresponds to the carbon skeleton of ikarugamycin. An “exo [6+4] adduct” of tropone and 1,4-cyclopentadien-1-ol, an enol form of 2-cyclopentenone, was obtained as a by-product.