Fischer indole synthesis. The reaction of N′-alkyl-2,6-dialkylphenylhydrazines with cyclohexanone

Condensation of N′-methyl-2,6-dimethylphenylhydrazine with cyclohexanone in refluxing benzene gives 8,9-dimethyl-1,2,3,4-tetrahydrocarbazole along with water and ammonia. Yields of the 8,9-dimethyl-1,2,3,4-tetrahydrocarbazole are improved and general decomposition in the reaction mixture minimized if the hydrazine hydrochloride is used. In this case, however, the ammonia is obtained as ammonium chloride contaminated with ~6% of methylammonium chloride. N′-Ethyl-2,6-dimethylphenylhydrazine hydrochloride and N′-methyl-2,6-diethylphenylhydrazine hydrochloride condense with cyclohexanone to give 9-ethyl-8-methyl-1,2,3,4-tetrahydrocarbazole and 8-ethyl-9-methyl-1,2,3,4-tetrahydrocarbazole respectively along with ammonium chloride contaminated with a very small amount of ethylammonium chloride and methylammonium chloride respectively.The results are interpreted in terms of the formation first of the intermediate enamine then the dienone–imine.