To a solution of Boc-D-Trp-OBzl (3.0 g) in methylene chloride (60 ml) were added powdered sodium hydroxide (1.52 g), ethyltrimethylammonium chloride (150 mg) and ethyl bromoacetate (2.54 g). The mixture was stirred overnight at room temperature, then powdered sodium hydroxide (0.61 g) and ethyl bromoacetate (0.63 g) were added. The mixture was stirred further for four and half an hour at room temperature and for two hours under reflux. After cooling, 1N-hydrochloric acid (53 ml) was added to the mixture, and the organic layer was washed with sodium chloride solution and dried with magnesium sulfate. After evaporation, the residue (4.87 g) was chromatographed on a silica gel column (60 g) eluting successively with chloroform and chloroform-ethyl acetate (4:1) to give a purified Boc-D-Trp(CH2CO2Et)-OBzl (4.14 g).
