Triethylamine → tert-butyl N-((2R)-1-oxobutan-2-yl)carbamate

A solution of (R)-tert-butyl 1-hydroxybutan-2-ylcarbamate (5.0 g, 26.4 mmol) in DMSO (50 mL) was chilled to 15° C., followed by the addition of triethylamine (10.99 mL, 79 mmol) and a solution of sulfur trioxide-pyridine complex (12.62 g, 79 mmol) in DMSO (50 mL) in succession. The mixture was stirred at 15° C. for 30 minutes, then allowed to warm to ambient temperature overnight. The reaction was quenched with 200 mL 1.0 M citric acid, and extracted twice with 250 mL MTBE. The organic phase was washed with 200 mL water and 200 mL brine, separated, dried over Na2SO4, filtered, and concentrated. The residue was chromatographed on a Grace Reveleris 80 g column, eluted with 0-35% EtOAc in Hexanes (40 mL/min) to provide the title compound (4.561 g, 24.36 mmol, 92.2% yield) as a light yellow solid. 1H NMR (300 MHz, DMSO-d6) δ 9.42 (d, J=0.7, 1H), 7.27 (d, J=6.8, 1H), 3.73 (dd, J=13.3, 7.8, 1H), 1.78-1.58 (m, 1H), 1.54-1.28 (m, 10H), 0.87 (t, J=7.4, 3H).