Boc-Ser-OtBu, 100 mg (0.38 mmol), 86 mg (0.38 mmol) of 5-amino-3-chloro-2-methylbenzenesulfonic acid and 37 mg (0.0127 mmol) of triphosgene were suspended in 1 ml of methylene chloride, and 66 μl (0.76 mmol) of DIEA was added to the suspension. After stirring at room temperature overnight, the solvent was removed by distillation. The mixture was purified according to the purification step A to give the protected title compound. The protected product obtained was dissolved in 2 ml of trifluoroacetic acid and stirred for 2 hours. The solvent was then removed by distillation and the mixture was purified according to the purification step A to give the title compound.
