A reinvestigation of 4-hydroxyindole-6-carboxylate synthesis from pyrrole-2-carboxaldehyde: a facile synthesis of indoles and indolizines.

Cyclization of the Stobbe product 3 under the literature conditions (acetic anhydride/sodium acetate) affords both the indole 6 and the indolizine 8. The presence of base promotes the formation of indolizine products, and using acetic anhydride/triethylamine leads to indolizine products in good yield. Improved conditions for conversion to indoles 2, 5, and 6 are reported, and inconsistencies in some of the literature structure assignments and characterization data are noted. Copyright 2004 American Chemical Society