Further Evidence for the Dienone-Imine Intermediate in the Fischer Indole Synthesis: An Uncatalyzed Fischer Reaction Under Mild Conditions

Evidence has been presented for the transient formation of a dienone-imine intermediate in the Fischer indole synthesis analogous to the dienone intermediate found in the Claisen rearrangement. The isolation of methylamine hydrochloride, 8-chloro-1,2,3,4-tetrahydrocarbazole, and 5-amino-6-chloro-9-methyl-1,2,3,4-tetrahydrocarbazole from the room temperature, uncatalyzed condensation between cyclohexanone and N′-methyl-2,6-dichlorophenylhydrazine under enamine synthesis conditions can be rationalized only on the assumption of the occurrence of a dienone-imine intermediate. An example of a facile, uncatalyzed Fischer cyclization at room temperature has been described.