The Benzyne Fischer Indole Reaction

Significance

Reported is a new approach to the Fischer indole synthesis from the addition of N-tosylhydrazones 2 to arynes generated via fluoride activation of 2-(trimethylsilyl)phenyl triflates 1. The first step is the addition of N-tosylhydrazones into benzyne followed by Fischer cyclization, which has been performed by the addition of Lewis acid BF₃˙OEt₂ in the same reaction pot. A range of alkyl hydrazones (cyclic, acyclic, and benzylic) and substituted arynes may be used for this transformation. For the naphthyne intermediate, both the initial arylation and the subsequent cyclization are highly selective; however, for the 4-methylbenzyne, a mixture of three different isomeric indole products was obtained.