A Convenient Modification of the Fischer Indole Synthesis with a Solid Acid

A new one-pot version of the titled reaction involves heating a mixture of a carbonyl compound, a phenylhydrazine, and the cation exchange resin Amberlite IR 120 in refluxing ethanol. A variety of enolizable aldehydes, and ketones and several substituted phenylhydrazines could thus be converted to the corresponding indoles in excellent yields (70–88%). Reaction times were typically 6–10 h, with the resin being then filtered off and the product isolated after minimal workup.