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

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AbstractA new synthesis of 3‐anilino‐1‐aryl‐2‐pyrazolin‐5‐ones in which the pyrazolinone ring is built via NN 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.