1-hydroxybenzotriazole (225 mg, 1.65 mmol) and EDC (315 mg, 1.65 mmol) were added sequentially to compound 50.3 (750 mg, 1.5 mmol) in dichloromethane (25 mL). The resulting mixture was stirred for 1 h before ammonium hydroxide (25% in water, 630 mg, 4.5 mmol) was added dropwise. The reaction was quenched after 30 min. with water (10 mL). The reaction mixture was extracted with dichloromethane (10 mL×3). The organic extracts were combined, dried over MgSO4 and concentrated under reduced pleasure. The product was purified via flash chromatography (50% ethyl acetate in hexanes) to yield 51.1 (720 mg) as a white solid. MS ESI (pos.) m/e: 500 (M+H). 1H NMR (400 MHz) (CDCl3) δ 7.72 (4H, s); 7.68 (1H, s); 7.57-7.48 (3H, m); 7.29-7.25 (2H, m); 7.0-6.97 (2H, d, J=6,8 Hz); 5.14 (2H, s); 3.91-3.88 (1H, m); 3.19-3.13 (1H, m); 2.60-2.55 (1H, m); 1.4 (9H, s).
