To a solution of tert-butyl [(3S,5S)-5-(2,3-difluorophenyl)-2-oxopiperidin-3-yl]carbamate (300 mg, 0.92 mmol) in toluene (9.2 mL) was added Lawesson's Reagent (190 mg, 0.47 mmol). The mixture was stirred at 40° C. After 16 h, the mixture was cooled to ambient temperature and saturated aqueous sodium bicarbonate was added. Water was added, and the mixture was extracted with ethyl acetate (3×). The combined organic extracts were washed with brine, dried over magnesium sulfate, filtered and concentrated. Purification by reverse phase chromatography (C-18, 90% water/acetonitrile→5% water/acetonitrile with 0.1% trifluoroacetic acid) gave the title compound (226 mg). MS 365.3 (M+Na).
