The present invention refers to reagents and methods for preparing a peptide sequence having a [18F]fluoro-aromatic amino acid side which may be further substituted, in particular a 4-[18F]fluoro-phenylalanine side chain in peptide sequences, by chemoselective radio-deoxyfluorination of an aromatic amino acid residue, in particular a tyrosine residue using a traceless-activating group and the reagents used in said process.