[PROBLEMS] To provide a process for producing a labeled precursor which is useful for the production of an amino acid compound labeled with a radioactive fluorine. [MEANS FOR SOLVING PROBLEMS] In the reaction process for introducing a leaving group to a mixture of a syn-OH form and an anti-OH form of FACBC, a base is allowed to present in the reaction system to produce a syn-leaving group adduct which is unreactive with the base and is highly stable and an anti-leaving group adduct which can be react with the base to form a water-soluble compound. By employing a purification method utilizing the difference in solubility between these adducts, the syn-leaving group adduct can be separated selectively. The base may be a linear or branched primary to tertiary alkylamine having 1 to 10 carbon atoms, a nitrogenated heterocyclic compound having 2 to 20 carbon atoms or a nitrogenated hetero-aromatic cyclic compound having 2 to 20 carbon atoms.
