To a solution of 1D (10.03 g, 40.7 mmol) in DMF (100 mL) was added (S)-(+)-mandelic acid (7.45 g, 48.9 mmol), followed by EDAC (9.37 g, 48.9 mmol), HOBt (7.49 g, 48.9 mmol) and NMM (16.1 mL, 146.1 mmol). The reaction mixture was stirred at RT overnight, then diluted with EtOAc (200 mL), washed with water (50 mL×3), 1N aqueous HCl (50 mL), water (50 mL) and brine, dried (Na2SO4) and concentrated. The residue was chromatographed (silica gel) eluting with EtOAc/hexane (2:1), and the purified compound crystallized from acetone (3×) to give the title compound as the (S, S) diastereoisomer (2.54 g, 20% yield).
