To a solution of crude tert-butyl[(3S,5S,6R)-2-hydrazinylidene-6-methyl-5-phenylpiperidin-3-yl]carbamate (99 mg, 0.31 mmol) in dichloromethane (3.1 mL) was added 1-(trifluoromethyl)cyclopropanecarboxylic acid (58 mg, 0.37 mmol), 1-hydroxy-7-azabenzotriazole (4.3 mg, 0.031 mmol), N-methylmorpholine (82 μL, 0.75 mmol) and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (72 mg, 0.37 mmol). The mixture was stirred 1 h. Saturated aqueous bicarbonate was added, and the resulting mixture was extracted with ethyl acetate (3×). The combined organic extracts were washed with brine, dried over magnesium sulfate, filtered and concentrated. Acetonitrile (12.5 mL) and glacial acetic acid (18 μL, 0.31 mmol) were added and the resulting mixture stirred at 70° C. for 90 min. Saturated aqueous bicarbonate was added, and the resulting mixture was extracted with ethyl acetate (3×). The combined organic extracts were washed with brine, dried over magnesium sulfate, filtered and concentrated giving the crude title compound. MS 437.2 (M+1),
