PROTONATION OF THE ELECTRON ADDUCTS OF PYRIMIDINE-DERIVATIVES

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),Is. 3 depends on the ionization potentials of the O(4)-protonated intermediates. The formation of 4+5 increases with increasing concentration of TFA.