A method of forming a thiohydantoin from an N-protected amino acid. The method employs a uronium or phosphonium compound to activate the terminal carboxyl group of the amino acid and a thiocyanate reagent to cyclize the activated amino acid to the thio-hydantoin. The thiohydantoin can be cleaved from its N-protecting group, for use in C-terminal peptide sequencing. Particularly preferred uronium compounds include salts of 2-chlorouronium. Preferred thiocyanate reagents include trimethylsilyl isothiocyanate and crown ether adducts of metallothiocyanates, such as the 18-crown-6 adduct of KSCN.