N,N,N-Trimethyl-5-[(2,3,5,6-tetrafluorophenoxy)-carbonyl]pyridin-2-aminium trifluoromethane-sulfonate a precursor for the synthesis of 2,3,5,6-tetrafluorophenyl 6-[18F]-fluoronicotinate

The synthesis, recrystallization, and X-ray deterimination of N,N,N-trimethyl-5-[(2,3,5,6-tetrafluorophenoxy)carbonyl]pyridin-2-aminium trifluoromethanesulfonate (PyTFP-precursor), C15H13F4N2O2+center dot CF3SO3-, is described. This triflate salt precursor is required for the synthesis of 2,3,5,6-tetrafluorophenyl 6-[F-18]-fluoronicotinate ([F-18]FPyTFP), a prosthetic group used to radiolabel peptides for positron emission tomography (PET), as peptides are increasingly being used as PET-imaging probes in nuclear medicine. Radiolabeling of peptides is typically done using a 'prosthetic group', a small synthon to which the radioisotope is attached in the first step, followed by attachment to the peptide in the second step. During the synthesis of the PyTFP-precursor, displacement of a Cl atom with trimethylamine gas and anion replacement with a triflate counter-ion is critical, as incomplete replacement would hinder radioisotopic incorporation of nucleophilic fluorine-18 and result in diminished radiochemical yields. The structural determination of the PyTFP-precursor by X-ray crystal-lography helped confirm the anion exchange of chloride with triflate.