Dipeptide Boc-Arg(Tos)-Tyr(Bzl)-OCH3 : A solution of Boc-Arg(Tos)-OH (2.14 g, 5 mmol) and DPPA (1.37 g, 5 mmol) in DMF (100 mL) was cooled to -5° C. DIEA (2.6 mL, 15 mmol) and HCl.H-Tyr(Bzl)-OCH3 (1.44 g, 4.5 mmol) were added, each in one portion, to the cooled solution. The solution was stirred at -10° C. for 2h, allowed to stand at 4° C. for 18h, and then evaporated to dryness. The residue was dissolved in EtOAc (75 mL). The solution was washed sequentially with H2O, 1N HCl, 5% aqueous NaHCO3 and H2O, dried, and evaporated to dryness. The residue was dissolved in EtOAc. Addition of Et2O caused an oil to separate from the solution. The mixture was cooled (0° C.) and triturated. The solvent phase of the mixture was removed by decantation. Et2O was added to the residue. After warming the mixture, the preceding trituration and decantation treatment was repeated. Finally, the product was dried under reduced pressure to yield Boc-Arg(Tos)-Tyr(Bzl)-OCH3 (5.3 g).
