Finely divided t-butoxycarbonyl-D-tryptophan (3 g, 10 mmol) was suspended in methylene chloride and benzyl alcohol (1.08 mL, 10 mmol) and 4-dimethylaminopyridine (0.12 g, 1 mmol) were added and stirred at room temperature. Solid 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (1.92 g, 10 mmol) was then added in three roughly equal portions over 5 minutes. The reaction mixture was stirred for 3 hours at room temperature during which time the reaction mixture became a homogeneous solution. The reaction mixture was poured into water (100 mL) and extracted with methylene chloride (2×50 mL). The combined methylene chloride layers were washed with 5% aqueous citric acid solution (100 mL) and 5% aqueous sodium bicarbonate solution (100 mL). The resulting methylene chloride layer was dried over magnesium sulfate, filtered and evaporated under vaccum to give an off-white solid. This solid material was chromatographed on silica gel using ethyl acetate/hexanes (2:3 v/v) as eluant. This afforded 3.56 g (91%) of the desired benzyl ester as a white amorphous powder.
