Preparation of succinimidyl and pentafluorophenyl active esters of 5- and 6-carboxyfluorescein.

A mixture of 5- and 6-carboxyfluorescein was activated with 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride in the presence of either N-hydroxysuccinimide or pentafluorophenol to give the corresponding succinimidyl and pentafluorophenyl esters. The regioisomeric mixtures were separated to give the 5- and 6-succinimidyl and pentafluorophenyl active esters in > 98% purity.