The invention relates to a process for preparation of radiopharmaceutical precursors, and in particular protected amino acid derivatives which are used as precursors for production of radiolabeled amino acids for use in in vivo imaging procedures such as positron emission tomography (PET). Particularly, the invention relates to a process for preparation of a precursor of the [18F]-1-amino-3-fluorocyclobutanecarboxylic acid ([18F] FACBC) PET agent and particularly to the work-up process of this precursor removing generated salts from the intermediate composition.