Synthesis of 3‐(2‐chloro‐5‐nitroanilino and 4‐nitroanilino)‐1‐(2,4,6‐trichlorophenyl)‐2‐pyrazolin‐5‐ones

A new synthesis of 3‐anilino‐1‐aryl‐2‐pyrazolin‐5‐ones in which the pyrazolinone ring is built via NN bond formation is described. 2‐Cyano‐2′,4′,6′‐trichloroacetanilide 1 was converted to imino ether hydrochloride 2 which was reacted with anilines in methanol to produce N ‐arylimino ether 3a,b. Reaction of these N ‐arylimino ethers with hydroxylamine gave N ‐arylamidoximes 4a,b . An 1,2,4‐oxadiazol‐5‐one 6a was prepared from the N ‐arylamidoxime 4a and subjected to base‐induced rearrangement. The desired 3‐anilino‐pyrazolinone 7a was obtained only in a very low yield. However, O ‐acetylation of the N ‐arylamidoximes 4a,b followed by acid‐catalyzed ring closure and rearrangement in the presence of excess acetic anhydride gave a mixture of N ‐acetylanilinopyrazolinones ( e.g. 10 ) and 4‐acetyloxy‐3‐ N ‐acetylanilinopyrazoles ( e.g. 12 ) which upon acid hydrolysis afforded the 3‐anilinopyrazolinones 7a,b in better yield.