Trifluoroacetic acid (4 mL, 23.5 mmol) was added to a stirred solution of (R)-4-(tert-butoxy carbonyl)thiomorpholine-3-carboxylic acid (1.0 g, 4.05 mmol) in CH2Cl2 (20 mL), and the reaction mixture was stirred at room temperature for 2 h. Volatile byproducts were removed at reduced pressure and the residue was concentrated from toluene (2×20 mL) and dried under high vacuum for 30 min. The crude residue was dissolved in 1,4-dioxane (10 mL) and water (10 mL) was added followed by NaHCO3 (1.02 g, 12.15 mmol) and a solution of FMOC-Cl (1.04 g, 4.05 mmol) in 1,4-dioxane (10 mL). The reaction mixture was stirred at room temperature for 16 h. The mixture was concentrated under reduced pressure to remove volatile organic solvent and the residue was washed with MTBE (2×20 mL). The aqueous layer was acidified with 1N aqueous HCl to pH 2 and extracted with EtOAc (3×50 mL). The combined organic extracts were washed with brine solution (50 mL), dried over anhydrous Na2SO4, filtered and concentrated. The residue was purified by reverse phase column chromatography (C18; eluent: 10-100% acetonitrile/water) to afford the title compound (1.00 g) as a white semi-solid.
