On the Baeyer-Emmerling Synthesis of Indigo

The first indigo synthesis is of interest since at that time the structure of indigo was unknown. So, there was not target molecule but a blurred target: synthesize a blue dye that by oxidation gives isatin. Thus, this synthesis stands between chemical insight and good luck. An unexpected fact was the formation of an unwanted coloured compound (indigo red) besides of indigo blue. However, the undesired compound resulted later to be beneficial according to Chinese medicine and now is used in Cancer Treatment. We update this synthesis providing formulas, systematic nomenclature and reaction mechanisms which are absent in the original communications. Critical comments are also given.