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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.

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The conversion of 1,5-dibromopentacyclo[5.3.0.02,5.03,9.04,8]decane-6,10-dione (1) to 10-oxa-9-oxopentacyclo [5.3.0.02,4.03,6.05,8]decane-3-carboxylic acid (6) on treatment with 5% aqueous potassium hydroxide was shown to proceed through 1, exo-7-dibromo-9-oxotetracyclo[4.3.0.02,5.03,8]nonane-endo-4-carboxylic acid (2). Treatment of 1 with the base in heavy water gave rise to 2-d-6 deuterated at the 2-position, but 2 gave the protium analog 6 after the same treatment. These results are discussed in relation to reactions of similar compounds which have been previously described in the literature.