Hydrochloric Acid → (3R)-3-(cyclopentylmethyl)-4-oxo-4-{(5S)-5-[(2-pyridinylamino)carbonyl]-4,5-dihydro-1H-pyrazol-1-yl}butanoic acid, hydrochloride

To a 500 ml round bottom flask was added 1,1-dimethylethyl (3R)-3-(cyclopentylmethyl)-4-oxo-4-{(5S)-5-[(2-pyridinylamino)carbonyl]-4,5-dihydro-1H-pyrazol-1-yl}butanoate (29.1 g, 58.4 mmol) and 4 M HCl in 1,4-dioxane (240 mL, 960 mmol). The mixture was stirred at room temperature and monitored by LCMS. After 90 min, no starting material remained. The mixture was concentrated to dryness and co-evaporated with dioxane (2×15 mL) to afford the crude (3R)-3-(cyclopentylmethyl)-4-oxo-4-{(5S)-5-[(2-pyridinylamino)carbonyl]-4,5-dihydro-1H-pyrazol-1-yl}butanoic acid, hydrochloride (34.0 g, 58.5 mmol, 100% yield) as a light brown foamy solid that was used directly for the next step. LCMS: [M+H]+: 373.2.