In an attempt to prepare indole-3-carbonyl chloride from indole-3-carboxylic acid by heating with thionyl chloride in chlorobenzene, we serendipitously discovered that the expected indole-3-carbonyl chloride is not obtained, but rather has trimerized to form 6,13,20-trioxo-5:21,7:12,14:19-trimethenotribenzo[b,f,k][1,6,11]triazacyclopentadecine, a hitherto unknown macrocyclic ring system. The reaction conditions and structural elucidation is discussed.