TBTU (88 mg; 0.274 mmol) was added to a solution of 1-[4-(aminomethyl)phenyl]-2,3-di-tert-butyloxycarbonyl-guanidine hydrochloride (8) (100 mg; 0.249 mmol), monobenzyl 2-phenylmalonate (67.3 mg; 0.249 mmol), HOBt (37 mg; 0.274 mmol) and triethylamine (104 μl; 0.747 mmol) in DMF (5 ml), and the solution was stirred at room temperature overnight. After stripping off the solvent under reduced pressure, the residue was taken up in ethyl acetate (20 ml) and washed 3× with 5% NaHCO3 solution, 3× with 0.5 M HCl and 1× with saturated NaCl solution. The organic phase was dried (Na2SO4) and the solvent was stripped off under reduced pressure. The product was recrystallized from ethanol/water.
