Sodium 2-aminooxazole-4-carboxylate (75 mg, 0.500 mmol), HOBt (67.5 mg, 0.500 mmol) and EDCI (96 mg, 0.500 mmol) were dissolved in DMF (0.5 mL). The mixture was stirred at rt for 0.5 h, then was added (R)-2-amino-1-((S)-4-(4-chlorophenyl)-4-hydroxy-3,3-dimethylpiperidin-1-yl)-3-methylbutan-1-one hydrochloride (188 mg, 0.500 mmol) and further stirred for 2 h. The reaction was quenched with water and stirred at rt for 2 h. The solids were filtered and rinsed with water to give 2-amino-N—((R)-1-((S)-4-(4-chlorophenyl)-4-hydroxy-3,3-dimethylpiperidin-1-yl)-3-methyl-1-oxobutan-2-yl)oxazole-4-carboxamide (169 mg, 0.376 mmol, 75% yield).
