(2R,3S)-2-(3-chloro-4-cyano-2-methylphenylamino)-3-hydroxybutanoic acid (5.0 g, 18.6 mmol), 4-iodobenzhydrazide (CAS 3915-95-2, 4.88 g, 18.6 mmol) and 1-hydroxybenzotriazole hydrate (2.85 g, 18.6 mmol) were placed in a 250 mL round bottomed flask. Anhydrous THF (120 mL) was added and the mixture was cooled to −20° C. N-(3-Dimethylaminopropyl)-N′-ethylcarbodimide HCl (5.35 g, 27.9 mmol) was added, followed by Et3N (3.9 mL, 28.0 mmol). The reaction mixture was stirred at −20° C. for h and room temperature for overnight. The reaction was quenched by adding water and extracted with EtOAc, which was washed with 5% aqueous NaOH, water, brine and dried over Na2SO4. The extracts were filtered and the filtrate was concentrated to provide a brown solid, which was purified by flash chromatography (80% EtOAc in hexanes) to afford the title compound (7.3 g, 77%), which was used directly in the next step.
