To a solution of N-(t-butoxycarbonyl)tryptophan (21.28 g, 70 mmol) in 140 ml of acetone was added triethylamine (9.76 ml, 7.08 g, 70 mmol) in a portionwise manner. The resulting mixture was stirred for about thirty-five minutes at room temperature and then stirred at 45° C. for 30 minutes under nitrogen. To the resulting mixture ethyl chloroformate (7.35 ml, 8.36 g, 77 mmol) was added with continued cooling and the reaction mixture was stirred for about one hour. Next, 2-chlorobenzylamine (9.30 ml, 10.9 g, 77 mmol) was added and the reaction mixture was then stirred until a white precipitate formed. The ice bath was removed and additional acetone was added. The reaction mixture was then stirred overnight.
