Abstract
Photolyses of 5-substituted 1, 3-dimethyluracils (1a-e : a, R=F; b, R=Cl; c, R=H; d, R=CH3; e, R=p-xylyl) in p-xylene in the presence of trifluoroacetic acid (TFA) afforded 5, 6-dihydro-1, 3-dimethyl-6-p-methylbenzyluracils (3a-d), 5, 6-dihydro-1, 3-dimethyl-5-p-methylbenzyluracils (4a-e), and 5, 6-dihydro-1, 3-dimethyluracils (5a-e) in varying ratios. It is suggested that the 6-isomers (3) are derived from the O(4)-protonated electron adducts of 1, while 4 and 5 are the products from the C(6)-protonated electron adducts. The ratio of (4+5) vs. 3 depends on the ionization potentials of the O(4)-protonated intermediates. The formation of 4+5 increases with increasing concentration of TFA.
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