Under N2, (S)-OH- [9]CPP (20 mg, 24 μmol, 1.0 equiv) was dissolved in anhydrous pyridine (2 mL). POCl3 (18.7 mg, 12 μL, 0.12 mmol, 5.0 equiv) was added to the solution and the mixture was heated at 60 °C during 24 h. After that time, H2O (2 mL) was added and heated at 60 °C for another 24 h. Then, the reaction was allowed to cool down to r.t. and the solvent was evaporated under reduced pressure. The crude was extracted three times with DCM using HCl (3 M) as aqueous phase and the combined organic layers were concentrated under reduced pressure without drying the organic phase. The resulting yellow solid was repeatedly washed with toluene in order to remove any traces of HCl and evaporated under reduced pressure to co-evaporate with the remaining traces of water. Final compound (S)-PO4H- [9]CPP was recovered as a light yellow solid (20 mg, 23 μmol, 98%).