72 



D. SHUGAR 



dimethyluracil ; one of these is also found following prolonged irradiation 

 of uracil and Up. 142 



Wang has shown 147 that irradiation of the reversible photoproduct of 

 dimethyluracil, 4-hydroxy-l ,3-dimethylhydrouracil (VIII), leads to the 

 formation of A^iV'-dimethylmalonamide (XI) via dimethylbarbituric acid 

 (X), so that the reaction sequence starting from dimethyluracil (IX) would 

 be as follows: 



o 



o 



B 



CH 3 N 



/\ N /\ 

 CH 3 H 

 (IX) 

 Dimethyl- 

 uracil 



(+H2O) 



A 



CH 3 N 



o 



~H 



-OH 



~H 



[O] 



CH 3 



(VIII) 



4-Hydroxy-l,3- 



dimethvlhvdrouracil 











CH 3 N 



(-CO) 



CH 3 N 

 H 



/\ N /\ 



CH 3 ° 



(X) 



Dimethylbarbituric 



acid 



HN 



/\ 



CH 

 (XI) 

 A 7 , iV '-Dimethyl 

 malonamide 







At starting concentrations of (VIII) higher than 10~ 4 M, products other 

 than (XI) were also formed, a not unexpected result (see below). 



The demonstration of (X) as the photoproduct of (VIII) enables us to 

 formulate the reaction sequence leading to the formation of parabanic acid 

 (VI) from uracil (XII), mentioned above, 125 and which may be as follows: 



HN 



B 







+H2O 



/\ N /\ 



H H 



(XII) 



Uracil 







HN 

 A N , 



-H 

 H 

 -OH 



' ^H 



H 



(XIII) 



4-Hydroxyhydro- 



uracil 



[0] 



o 



A 



HN 



A N /\ 



H ° 



(XIV) 

 Barbituric 

 acid 



HN 



H ° 



(VI) 

 Parabanic 

 acid 



The nature of the final reaction step requires clarification. In addition 

 oxamide (V) is probably formed from (VI) by decarbonylation as in the 

 formation of (XI) from (X) above. 



147 S. Y. Wang, J. Am. Chem. Soc. 80, 6199 (1958). 



