Photooxygenation of 5‐aryl‐2,4‐diaminopyrimidines leading to 4‐amino‐1,3,5‐triazin‐2‐yl ketones and, in the presence of sodium borohydride, to 5,6‐dihyro‐4(3 H )‐pyrimidinones

The photosensitized oxygenation of 5‐aryl‐2,4‐diaminopyrimidines 1 in protic solvent led to the formation of the new 4‐amino‐1,3‐5‐triazin‐2‐yl ketones 2 in high yields. The structures of 2 were elucidated by spectroscopical means, especially by 13 C‐NMR and UV data. Photooxygenation of 2,4‐diamino‐5‐( p ‐chlorophenyl)‐6‐ethylpyrimidine 1a under reductive conditions, e.g. In the presence of excess NaBH 4 , gave 2‐amino‐5‐( p ‐chlorophenyl)‐ t ‐6‐ethyl‐5,6‐dihydro‐ r ‐5‐hydroxy‐4( 3H )‐pyrimidinone ( 4a ), the structure of which was determined by X‐ray analysis. In the proposed mechanisms for both types of reactions, the dipolar ion 5 is assumed to be a common intermediate. For the new efficient synthesis of 1,3,5‐triazines from 2,4‐diaminopyrimidines, a 5‐aryl substituent seems to be essential.