Ethyl 4,6-Dichloro-3-formyl-2-indolecarboxylate has been used as a reactant for the preparation of 3-[2-(3-aminophenyl)-2-carboxyvinyl]-4,6-dichloro-1H-indole-2-carboxylic acid to be used as a selective glycine-site N-methyl-D-aspartate (NMDA) receptor antagonist.