cooled down to -78 °C. The freshly prepared solution of sodium naphthalenide (3.0 mL, 0.5 M, 1.50 mmol, 12 equiv) was added dropwise and the solution stirred at -78 °C for further 2 h. The reaction was quenched by adding a solution of iodine in THF. An aqueous solution of Na2S2O3 was added and the reaction was extracted with DCM and the combined organic layers were dried over MgSO4 and concentrated under reduced pressure. The crude product was then dissolved in anhydrous THF (20 mL) and methanol (10 mL) and stirred for 1 d at 70 °C together with Amberlyst ® 15 (0.20 g). After cooling down to room temperature, Amberlyst ® 15 was filtered of, the solution was concentrated under reduced pressure and purified by column chromatography (petroleum ether / DCM 3:2 → 1:1) to yield (S)-OH- [9]CPP as yellow solid (54 mg, 66 μmol, 52%).