To a solution of Boc-DTrp-OBzl(1 g) in dichloromethane(8 ml) was added Bu4NHSO4 (8.62 mg) and pulverized NaOH(253 mg) under ice cooling. To the resulting suspension was added dropwise a solution of methyl chloroformate(599 mg) in dichloromethane(6 ml) under ice cooling over a period of 30 min. The reaction mixture was stirred under ice cooling for 30 min and at room temperature for 12 h, diluted with dichloromethane, washed successively with water, 5% KHSO., saturated NaHCO3 and saturated NaCl, dried over MgSO4, and filtered. The filtrate was concentrated under reduced pressure to give Boc-DTrp(COOCH3)-OBzl(1.42 g) as a colorless solid. The solid was hydrogenolized over a catalytic amount of 10% Pd/C in methanol under an atmosphere of hydrogen(under atmospheric pressure) to give Boc-DTrp(COOCH3).
