47.0 g of H-Thr-OBzl. HCl in 300 ml of methylene chloride are added to a solution of 73.8 g of BOC-Ser-OH. Dicyclohexylammonium salt in 500 ml of methylene chloride and the mixture stirred for 10 minutes at room temperature and then cooled to -5°C. A solution of 40.1 g of dicyclohexylcarbodiimide in 90 ml of methylene chloride is added dropwise at this temperature. The mixture is stirred for 3 hours at -5°C and overnight at room temperature. After filtering off the dicyclohexylurea and dicyclohexylamine hydrochloride the solution is extracted by shaking three times with 0.1 N hydrochloric acid, twice with 20 % strength sodium chloride solution, once with 10 % strength sodium bicarbonate solution and twice with 20 % strength sodium chloride solution and is dried over sodium sulphate. The solution is concentrated to about 600 ml, cooled to 5°C and further dicyclohexylurea is filtered off. After evaporation to dryness, the residue is crystallised from ethyl acetate-hexane in order to purify it.
