Synthesis of hipposudoric and norhipposudoric acids: the pigments responsible for the color reaction of the red sweat of Hippopotamus amphibius

Highly unstable pigments, hipposudoric acid and norhipposudoric acid, isolated from the red sweat of hippopotamus were synthesized using the Pschorr reaction for the construction of the fluorene nucleus as the key step and the careful oxidation in the last step.