To a solution of (D)-valine, tert-butyl ester (0.13 g, 0.74 mmol) and triethylamine (0.075 g, 0.74 mmol) in tetrahydrofuran/water (5 mL, 1:1) was added in one portion at room temperature 6,7,8,9-tetrahydro-dibenzofuran-3-sulfonyl chloride (0.20 g, 0.74 mmol). The reaction mixture was stirred at room temperature for 14 hours, followed by the addition of aqueous HCl (1 M, 5 mL) and ethyl acetate (10 mL). The organic phase was separated and washed with brine, dried (MgSO4), and concentrated to dryness to give the title compound as a white solid (0.24 g, 80%).
