An Unusual Substituent Effect on Elimination vs Fragmentation Reactions of the Dianions of 5-Fluorouracil-Alkene Photoadducts. Preparation of Cyclobutane-Annelated Uracils

Article abstract of DOI:10.1021/jo00242a036

Acetone-sensitized photocycloadditions of 5-fluorouracil to methyl vinyl ether, 2-methoxypropene, and ketene dimethyl acetal give good yields of the respective 8,8-disubstituted-6-fluoro-2,4-diazabicyclo<4.2.0>octane-3,5-diones.Whereas the dianions of cycloadducts of 5-fluorouracil and alkyl-substituted alkenes afford high yields of 5-substituted uracils, the products from 2-methoxypropene and ketene dimethyl acetal give predominantly cyclobutane-annelated uracils derived from syn elimination of hydrogen fluoride.Deuterium labeling studies support the proposed mechanism of elimination.Heating 8,8-dimethoxy-2,4-diazabicyclo<4.2.0>oct-1(6)-ene-3,5-dione, obtained from hydrogen fluoride elimination of the 5-fluorouracil-ketene dimethyl acetal photoadduct, in the presence of N-phenylmaleimide or dimethyl acetylenedicarboxylate, gives Diels-Alder adducts derived from trapping the thermally generated diene.