Reaction of propionaldehyde with N′-methyl-2,6-dimethylphenylhydrazine hydrochloride. The dienone–imine intermediate in the Fischer indole synthesis and its conversion to the indole structure

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When N′-methyl-2,6-dimethylphenylhydrazine hydrochloride was condensed with propionaldehyde by azeotropic removal of water in refluxing benzene for 10–12 min, a solid mixture was obtained which contained no aromatic ring system but instead possessed features indicative of a dienone–imine structure. When this mixture, or the original combination of N′-methyl-2,6-dimethylphenylhydrazine hydrochloride and propionaldehyde, was heated in refluxing benzene for 20 h there was obtained 1,3,7-trimethylindole in 36 and 31% yield respectively. Evidence indicated that a substantial amount of 3,7-dimethylindole was also produced.The results support the view that an intermediate dienone–imine is formed as one step in the Robinson mechanism of the Fischer indole synthesis.