The general procedure for the synthesis of (2S,3R)-2-(((benzyloxy)carbonyl)amino)-3-(tert-butoxy)butanoic acid from linaclotide impurity 23 was as follows: z-Thr(tBu)-OMe (169 g) was added to a 2 L beaker with 300 mL of acetone and 600 mL of 1,2-dichloroethane, and the pH was adjusted to 10-11 with 1 M sodium hydroxide solution. After completion of the reaction, the reaction was washed once with 500 mL of ether, 800 mL of ethyl acetate was added, and the pH was adjusted to 2-3 with aqueous citric acid solution.Subsequently, the reaction was washed with 300 mL of water, and then with 300 mL of saturated brine. The organic phase was dried over 100 g of anhydrous sodium sulfate, filtered and concentrated under reduced pressure. To the residue was added 500 mL of ether and dried to obtain Z-Thr(tBu)-OH (130 g) in granular form in 80.3% yield.
