N-9-fluorenylmethoxycarbonylpyroglutamate. Preparation of the acid, chloride and succinimidyl ester.

Fmoc-glutamic acid is converted by thionyl chloride into the dichloride, which spontaneously cyclizes to Fmoc-pyroglutamyl chloride. The latter is stable to water. Pure Fmoc-pyroglutamyl chloride is obtained by washing the reaction mixture with water, which destroys uncyclized dichloride by converting it into the 2-alkoxy-5(4H)-oxazolone that is readily hydrolyzed. Fmoc-pyroglutamic acid and succinimidyl ester are obtained from the chloride by acid hydrolysis and reaction with N-hydroxysuccinimide, respectively.