A process for producing a labeled precursor which isuseful for production of a radioactive fluorine- labeled aminoacid compound is provided. In the reaction step forintroducing a leaving group to a mixture of syn-form andanti-form of FACBC, a base is allowed to present in thereaction system to produce a syn-leaving group adduct whichis unreactive with the base and is highly stable and ananti-leaving group adduct which can react with the base toform a water-soluble compound. By employing a purificationmethod utilizing such a difference in solubility, thesyn-leaving group adduct can be separated selectively. Thebase may be a linear-chain or branched-chain primary totertiary alkylamine having 1 to 10 carbon atoms, anitrogen-containing heterocyclic compound with 2 to 20 carbonatoms, and a nitrogen-containing hetero aromatic compoundwith 2 to 20 carbon atoms.
