Hydrochloric Acid → (2S,3S)-2-Amino-3-methyl-1-(3,3,4,4-tetrafluoro-pyrrolidin-1-yl)-pentan-1-one Hydrochloride

[(1S,2S)-2-Methyl-1-(3,3,4,4-tetrafluoro-pyrrolidine-1-carbonyl)-butyl]-carbamic acid tert-butyl ester (200 mg, 0.56 mmol) was dissolved in ethyl acetate (4 mL), cooled to about 0° C. and treated with gaseous hydrochloric acid for about 1 minute. After about 15 minutes at about 0° C. and about 30 minutes at room temperature, the mixture was concentrated to dryness and the solid was triturated with hexane, collected and dried under vacuum overnight (124 mg, 76%, melting point: greater than 250° C.).