A facile one‐pot regioselective synthesis of [1,2,4]triazolo[4,3‐a]5(1H)‐pyrimidinones via tandem Japp–Klingemann, Smiles rearrangement, and cyclization reactions

Coupling of active [(4‐oxo‐6‐phenyl‐3H‐pyrimidin‐2‐yl)thio]methine compounds (3) with diazotized anilines in the presence of base gave [1,2,4]triazolo[4,3‐a]pyrimidines (7). The latter products were also obtained by reactions of hydrazonoyl chlorides (10) with either 6‐phenyl‐2‐thiouracil (1) or the 2‐methylthio derivative 9. The mechanisms and the regiochemistry of the reactions studied are discussed.