Sequential Hydroamination-Fischer Cyclization Route to Indoles

Significance

The synthesis of indole-2,3-di­carboxylates and 2-arylindole-3-carboxylates by a hydroamination-Fischer indole cyclization procedure from N-alkyl-N-aryl hydrazines and acetylene carboxylates is reported. The scope was limited to ready availability of the two substrates and the yields range from mediocre to good. Solvent, temperature, and stoichiometry of starting material factors were studied. Interestingly, slightly higher yields were observed in the overall one-pot process OR in the hydroamination step in the absence of titanium catalysis than in its presence. This is in accordance with previous observations (R. M. Acheson, J. M. Vernon J. Chem. Soc. 1962, 1148). The intermediate hydrazone could be isolated in the absence of ZnCl₂.