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 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.