Ring-Opening of Indoles: An Unconventional Route for the Transformation of Indoles to 1H‑Pyrazoles Using Lewis Acid

An unusual transformation of indoles to pyrazoles via an aromatic ring-opening strategy has been developed. The salient feature of this strategy involves the C2–N1 bond opening and concomitant cyclization reaction of the C2C3 bond of the indole moiety with the tosylhydrazone, which proceeds under transition-metal and ligand free conditions. This ring-opening functionalization of indoles provides a wide scope of differently substituted pyrazoles.