4.2.0]octane-3,5-dione, 90296-03-0; (la,6a,78)-6-fluoro-7,8,8trimethyl-2,4-diazabicyclo[4.2.0]octane-3,5-dione, 90365-49-4; (la,6a,8a)-6-fluoro-7,7,8-trimethyl-2,4-diazabicyclo[4.2.0]octane-3,5-dione, 90296-04-1; (la,6a,88)-6-fluoro-7,7,8-trimethyl-2,4-diazabicyclo[4.2.0]octane-3,5-dione, 90365-50-7; isopropenyl J. Org. Chem.The photocycloaddition products of &fluorouracil and enol acetates undergo fragmentation reactions to afford 5-substituted uracils in good yields. Unexpectedly, certain 5-fluorouracil-olefin photocycloaddition products react with 3 equiv of potassium tert-butoxide in tert-butyl alcohol to give high yields of 5-substituted uracils. Kinetic, labeling, and product studies of this novel reaction support a reaction mechanism in which the dianion of the cycloadduct rearranges to a spirocyclopropane intermediate which then affords the &substituted uracil.