Photochemical High‐yield Preparation of Tricyclo [3.3.0.0 2,8 ]octan‐3‐ones. Potential Synthons for Polycyclopentanoid Terpenes and Prostacyclin Analogs. Preliminary Communication

Three‐step syntheses and the resolution into the enantiomers are reported for the tricyclo[3.3.0.0 2,8 ]octan‐3‐ones 7–9 , which are destined to serve as synthons for polycyclopentanoid terpenes and prostacyclin analogs. Routine overall yields of ca. 75% for 7 , 40% for 8 , and 46% for 9 are obtained, with 2‐chloroacrylonitrile and 1,3‐cyclohexadiene (for 7) and 1‐methyl‐1,4‐cyclohexadiene (for 8 and 9 ) as the starting materials. The key step is the triplet‐sensitized oxadi‐π‐methane photorearrangement of the β,γ‐unsaturated ketones 1–3 which can be achieved in 80–90% yields of isolated product and quantum yields of 0.5–1.0. The racemates of both ketone 1 and its photoisomer 7 have been resolved via chromatographic separation of suitable diastereoisomeric acetal mixtures. On the other hand, sensitization of 1 with an optically active donor, (‐)‐ 14 , gave only an impractical maximum enantiomeric excess of 10% (‐)‐ 7 .