Synthesis of 1,2,4‐Triazolo[4,3‐ a ]pyrimidine Derivatives by Cyclocondensation of a 2‐Thioxopyrimidin‐4(3 H )‐one with Hydrazonoyl Halides

The reaction of 1,2‐dihydro‐6‐(phenylamino)‐2‐thioxopyrimidin‐4(3 H )one ( 16 ) with N ‐arylhydrazonoyl halides 15 in CHCl 3 in the presence of Et 3 N under reflux afforded the corresponding 1,2,4‐triazolo[4,3‐ a ]pyrimidin‐5‐ones of type 20 in good yields ( Scheme 3 ). The structure of one of the derivatives, 20d , has been established by X‐ray crystallography. Conceivable reaction mechanisms are discussed in Schemes 3 and 4. The products of type 20 easily undergo reactions with electrophiles such as benzenediazonium chloride, chloroacetyl chloride, and NaNO 2 in AcOH to give 6‐phenylazo, 6‐chloroacetyl, and 6‐nitroso derivatives 21, 23 , and 25 , respectively ( Scheme 5 ).