Product Studies of the Photocycloaddition Reactions of 5-Chloro- and 5-Fluorouracil Derivatives and Olefins. An Interesting and Useful Effect of Fluorine an Regioselectivity

Article abstract of DOI:10.1021/jo00189a017

The acetone-sensitized photochemical cycloadditions of 1,3-dimethyl-5-chlorouracil (1a), 5-chlorouracil (1b), 1,3-dimethyl-5-fluorouracil (4), and 5-fluorouracil (7) have been studied.The 5-chlorouracil systems 1a and 1b gave complex, acidic product mixtures on attemped photocycloaddition to enol acetates.Only tetramethylethylene underwent clean photocycloaddition with 1a and 1b, affording a mixture of cycloadduct and 1,3-dimethyl-5-(1,1,2-trimethyl-2-propenyl)-2,4(1H,3H)-pyrimidinedione (3).By contrast, 5-fluorouracil derivatives 4 and 7 underwent clean photochemical cycloaddition to isopropenyl acetate, cyclopentenyl acetate, and cyclohexenyl acetate to form endo, exo mixtures of the head-to-tail photoadducts.In its photocycloaddition to simple olefins, 5-fluorouracil showed much higher preference for formation of the head-to-tail photoadduct than do other simple uracil derivatives or cyclohexenone.The reaction of 5-fluorouracil with isobutylene, methylencyclopentane, methylenecyclohexane, and methylenecycloheptane gave nearly exclusive the head-to-tail regioisomer.The ratio of head-to-tail to head-to-head isomers for the reaction of 5-fluorouracil with 1-methylcyclopentene, 2-methyl-2-butene, and propene was 89:11, 85:15, and 76:24, respectively.Extensive 19F NMR data are reported for the photocycloadducts.