Boc-Arg(Ts)-Pro-NEtBn was prepared by a similar procedure to Example 1 from Boc-Pro-NEtBn and Boc-Arg(Ts). To the solution composed of the compound (1.00 g), chlorobenzene (5.8 ml) and DBF (1.5 ml), methanesulfonic acid (1.41 g, 9.8 equivalents) was added at room temperature, and the resulting mixture was stirred at room temperature for 24 hours, thereby the Boc group was removed (reaction conversion ratio: 100.0%). After the reaction, 10% aqueous sodium carbonate solution (12 ml) was added thereto, and the resulting mixture was stirred for 15 minutes for neutralization. Then, the aqueous layer was separated off, thereby a solution containing Arg(Ts)-Pro-NEtBn (8.58 g) was obtained.
