Hydrochloric Acid → (2RS,3S)-2-hydroxy-3-[N-(2-methyl-2-propyloxy-carbonyl)amino]heptanecarboxylic acid

Subsequently, to 260 ml of dioxane solution of 38.7 g (160 mmol) of the (2RS,3S)-2-hydroxy-3-[N-(2-methyl-2-propyloxycarbonyl)amino]heptanenitrile, 133 ml of concentrated hydrochloric acid was added thereto and was stirred under the reflux condition. After 3 hours, the reaction solution was concentrated under reduced pressure. 100 ml of distilled water and 100 ml of dioxane were added to the residue. 100 ml of dioxane of 70 g (319 mmol) of di-tert-butyl carboxylate was added dropwise to the above-prepared reaction mixture under 0° C. After completion of dropping, the reaction solution was warmed to the room temperature and then stirred overnight. The reaction solution was concentrated under reduced pressure. The residue thus obtained was washed with ether in addition to distilled water. The organic layer was extracted with aqueous solution of 1N sodium hydroxide and this water layer thus obtained was mixed with the water layer previously obtained. The mixed water layers was adjusted to be acid (pH 2 degree) with the addition of potassium hydrogensulfate and was extracted with ethyl acetate. The organic layer was washed with 50% brine and dried over anhydrous magnesium sulfate and concentrated under reduced pressure, whereby 36.2 g of (2RS,3S)-2-hydroxy-3-[N-(2-methyl-2-propyloxy-carbonyl)amino]heptanecarboxylic acid was obtained.