Base-Promoted Rearrangement of 1,5-Dibromopentacyclo[5.3.0.02,5.03,9.04,8]decane-6,10-dione: Easy Entry of a Novel Cage System, 10-Oxa-9-oxopentacyclo[5.3.0.02,4.03,6.05,8]decane

Abstract The Diels-Alder dimerization of 2-bromo-2,4-cyclopentadienone ethylene acetal (2) gave endo-2,7-dibromodicyclopentadiene-1,8-dione 1,8-bis (ethylene acetal) (3) as a minor product in a 4.5% yield. Ultraviolet irradiation of 5 led to dimerization product 7. Deacetalization was accomplished by treating with concentrated sulfuric acid to give 1,5-dibromopentacyclo[5.3.0.02,5.03,9.04,8]decane-6,10-dione (9). Compound 9 was converted into a new cage system, 10-oxa-9-oxopentacyclo[5.3.0.02,4.03,6.05,8]decane, in a high yield by treating with 5% aqueous potassium hydroxide at 80 °C.