To a round bottom flask was added (R)-benzyl 1-(4,4-diethoxybutylamino)-3-mercapto-3-methyl-1-oxobutan-2-ylcarbamate (6.90 g, 16.2 mmol), DCM (50 mL) and TFA (1.87 mL, 24.3 mmol). The reaction was stirred at rt for 5 hr. The reaction was diluted with DCM (200 mL) and the organics was washed with saturated aqueous NaHCO3 (200 mL), water (200 mL) and saturated aqueous NaCl (200 mL). The organic layer was separated, dried over MgSO4, filtered and concentrated. The residue was purified using silica gel chromatography (ISCO system) eluting with a gradient of 0-100% EtOAc/Hex to give benzyl (3R,8aS)-2,2-dimethyl-4-oxohexahydro-2H-pyrrolo[2,1-b][1,3]thiazin-3-ylcarbamate (1.89 g, 5.65 mmol, 34.9% yield) as a clear oil. Anal. Calcd. for C12H22N2O3S m/z 334.3. found: 335.2 (M+H)+; 1H NMR (500 MHz, CDCl3) δ ppm 7.29-7.44 (m, 5H), 5.92 (d, J=7.15 Hz, 1H), 5.14 (s, 3H), 4.57 (d, J=7.70 Hz, 1H), 3.89-3.68 (m, 1H), 3.60-3.40 (m, 1H), 2.55-2.34 (m, 1H), 2.18-2.00 (m, 1H), 1.99-1.83 (m, 2H), 1.51 (s, 3H), 1.13 (s, 3H); 13C NMR (126 MHz, CDCl3) δ ppm 167.31, 157.00, 136.19, 128.54, 128.20, 128.08, 67.23, 60.46, 55.12, 48.93, 46.26, 33.10, 29.56, 27.70, 23.39.
