5.1 g of (S)-2-(ethylsulfonylamino)-3-methyl-butyric acid and 5.7 ml of N-methylmorpholine in 200 ml of tetrahydrofuran are cooled, with stirring, to −20° C. 3.15 ml of isobutyl chloroformate are added dropwise thereto. When the addition is complete, the reaction mixture is stirred at −10° C. for 40 minutes. The mixture is again cooled to −20° C. 5.0 g of solid 2-(4-hydroxy-3-methoxy-phenyl)-ethylamine hydrochloride are introduced into the reaction mixture. The reaction mixture is stirred, without cooling, for a further 24 hours, the internal temperature gradually rising to room temperature. The reaction mixture is introduced into 300 ml of 2N hydrochloric acid. Extraction is carried out twice using 500 ml of ethyl acetate each time. The organic phases are washed twice using 100 ml of saturated sodium chloride solution each time, dried over magnesium sulfate and concentrated. The residue is purified by flash chromatography on silica gel with a mixture of ethyl acetate/n-hexane 3: 1, yielding (S)-2-(ethylsulfonylamino)-3methyl-butyric acid N-[2-(4-hydroxy-3-methoxyphenyl)-ethyl]-amide in the form of a colourless oil.
