Dimethyluracil, photolysis

The rate of dimethyluracil photolysis in water is independent of pH,64 and neutral salt concentration HCN does not have any effect upon the photolysis rate. These elements of photochemical behavior are all strikingly different from the behavior of uracil. 

The hydrate formed by photolysis of this substance is one of the few such products (the others are uracil hydrate, 5-fluorouracil hydrate, and uridine hydrate) that have actually been isolated and compared with authentic material of known structure.7 It is nearly the only product formed in the photolysis, is definitely stable at room temperature and neutral pH, and the thermal reversal to dimethyluracil is nearly quantitative. These properties, as Moore observed, make the reaction ideal for mechanistic investigation. Burr and Park have investigated the reaction mechanism by measuring the photolysis rate of dimethyluracil in mixtures of water with several nonaqueous, nonreactive solvents as a function of water concentration.64 The photolysis rate for 10" iM DMU was found to be the same in water-saturated cyclohexane (about 5 x 10-3M in water) as in dry cyclohexane. The photolysis rate in dry, highly purified dioxane was quite insensitive to water, and it was observed that hydrate formation (measured by thin-layer chromatography and by thermal absorbance reversal) became appreciable only at water concentrations above 40%. 

It was found that the rate of hydrate formation jn the photolysis of dimethyluracil in acetonitrile was zero, and the rate in acetonitrile-water mixtures was a monotonic function of water concentration... 

Fig. 6. The percentage of dimethyluracil hydrate in the products of photolysis of dimethyluracil in acetonitrile-water mixtures. The percentage hydrate is normalized to 100% in pure water (Burr and Park64).

Another method for determining the presence of triplet intermediates in a photochemical reaction is direct spectroscopic evidence using flash photolytic methods. Little of this has yet been tried. Flash photolysis of 101,3-dimethyluracil solutions in water or in dry cyclohexane did not give any new absorption bands in the visible or near UV in the time interval 2-600 (xsec after the flash.108 This preliminary observation seems to weaken the case for a triplet intermediate and also seems to exclude any radical intermediates in the reaction of dimethyluracil with water. 

Photolysis of 6-chloro-l,3-dimethyluracil (150) with the 1,3,4-oxadiazole 151 afforded the pyrazolo[3,4-d]pyrimidine 152, with photoelimination of benzoyl chloride (80CB2566). 

Materials. Uracil (U), 1-ethyluracil (EU), 1-cyclohexyluracil (CU), and 1,3-dimethyluracil (DMU) were obtained from Cyclo Chemical Corporation uridine was from Calbiochem. They were used as obtained in working with them, no impurities were apparent, spectroscopically or chromatographically. Solutions for photolysis at a particular pH were prepared by diluting a 10"3M triple-distilled water stock solution of the uracil derivative tenfold with the appropriate phosphate buffer. Concentrations were determined by absorbance measurement, using a Perkin-Elmer Model 202 Spectrophotometer. The values of Amax, in n.m., and e measured for the four substances were uracil 259, 8.6 X 103 1-ethyl-uracil 267, 9 X 103 1-cyclohexyluracil 270, 9 X 103 uridine 265, 10 X 103. 

TABLE 105.1 Photolysis of 6-Chloro-l,3-dimethyluracil in Benzene in the Presence of Various Acids... 

Seki, K., Bando, Y., and Ohkura, K., Photolysis of 5-bromo-l,3-dimethyluracil in substituted benzenes, Chem. Lett., 195,1986. 

---