Preparation of 5- and 6-Carboxyfluorescein

Condensation of resorcinol with 4-carboxyphthalic anhydride in methanesulfonic acid gave a mixture of 5- and 6-carboxyfluorescein stereoisomers. These were separated by recrystallization from methanol- or ethanol-hexane to give 5- and 6-carboxyfluorescein, each in over 98% purity.