Trifluoroacetic acid → (3S)-3-amino-4-cyclobutyl-N-cyclopropyl-2-hydroxy-butanamide trifluoroacetic acid salt

Tert-butyl-(2S)-1-cyclobutyl-4-(cyclopropylamino)-3-hydroxy-4-oxobutan-2-ylcarbamate (51 mg, 0.165 mmol) was dissolved in dichloromethane (3.0 mL) and trifluoroacetic acid (3.0 mL) was added. After stirring for 1 hour at room temperature, the reaction mixture was evaporated to dryness to give (3S)-3-amino-4-cyclobutyl-N-cyclopropyl-2-hydroxy-butanamide trifluoroacetic acid salt as a white solid. A solution of (2S,4R)-1-((S)-2-(3-tert-butylureido)-3,3-dimethylbutanoyl)-4-(7-methoxy-2-(1H-pyrazol-1-yl)quinolin-4-yloxy)pyrrolidine-2-carboxylic acid in dichloromethane/N,N-dimethylformamide (1:1, 6.0 mL) was added to the (3S)-3-amino-4-cyclobutyl-N-cyclopropyl-2-hydroxy-butanamide trifluoroacetic acid salt followed by O (7 azabenzotriazol 1 yl) 1,1,3,3 tetramethyl-uronium hexafluorophosphate (HATU) (75 mg, 0.198 mmol) and diisopropylethylamine (0.1 mL, 0.7 mmol). After 24 hours at room temperature, the reaction mixture was diluted with ethyl acetate and washed with 1N HCl, saturated sodium bicarbonate, and brine. The ethyl acetate layer was dried (magnesium sulfate), filtered and evaporated to dryness under reduced pressure. The crude product was then dissolved in dry dichloromethane (10.0 mL) and 1,1,1-triacetoxy-1,1-dihydro-1,2-benziodoxol-3(1H)-one (Dess-Martin periodinane reagent, 112 mg, 0.264 mmol) added. After stirring at room temperature for 2 hours, the reaction mixture was quenched with 0.26M sodium thiosulfate in saturated sodium bicarbonate and extracted with ethyl acetate. The combined ethyl acetate layers were then washed with saturated sodium bicarbonate and brine. Purification by preparative HPLC gave (2S,4R)-1-((S)-(3-tert-butylureido)-3,3-dimethylbutanoyl]-N-((S)-1-cyclobutyl-4-(cyclopropylamino)-3,4-dioxobutan-2-yl)-4-(7-methoxy-2-(1H-pyrazol-1-yl)-quinolin-4-yloxy)pyrrolidine-2-carboxamide in >99% purity by HPLC.