A solution of Fmoc-Val-Pro-OH and doxorubicin. HCl (50 mg, 0.08 mmol) in DMSO (4 mL), was successively treated at room temperature with N-[(dimethylamino)1H-1,2,3-triazolo[4,5-b]pyridino-1-ylmethylene]-N-methylmethanaminium hexafluorophosphate N-oxide (HATU) (36.0 mg, 0.09 mmol) and diisopropylethylamine (DIEA) (29.5 μL, 0.17 mmol). The reaction mixture was stirred at room temperature overnight. Then, the solvent was lyophilized and the residue was dissolved in ethyl acetate and washed with citric acid (10%), NaHCO3 (10%) and brine. The organic layer was dried (Na2SO4) and evaporated to dryness to give F (45% yield).
