Studies in the indazole series. Ring opening of some 1-Arylindazole-3-carboxylic acids during decarboxylation

Abstract

Indazole reacted readily with 4-fluoronitrobenzene and with 2-fluoro-5- nitro-benzonitrile to give the corresponding isomeric 1- and 2-nitroarylindazoles. The isomeric 2-(N-indazolyl)-5-nitrobenzonitriles behaved differently towards acid; the 1-derivative gave the corresponding 2-carboxylic acid whereas the 2-derivative gave a product which contains the hitherto unknown indolo[1,2-b]indazole ring system.     The decarboxylation of several 1-arylindazole-3-carboxylic acids in boiling quinoline gives not only the expected 1-arylindazole but also the product of heterocyclic ring fission, namely the isomeric N- arylanthranilonitrile. The proportion of this latter compound is particularly marked when the 1-aryl substituent carries an electron- withdrawing (nitro or chloro) group. If the 1-aryl substituent carries a nitro group, ring fission also occurs when the solid acid is heated above its melting point, but to a lesser extent than in refluxing quinoline.     The preparation of several of the 1-arylindazole-3-carboxylic acids was achieved by a route which had as its starting point the reaction of lead tetraacetate with an ethyl or methyl α-(arylhydrazono)phenylacet- ate to form the corresponding azoacetate. The failure of this reaction to occur when the arylhydrazone possessed an ortho-nitro group is discussed.