To a stirred solution of compound 3 (0.062 g, 0.21 mmol, 1.0 equiv) in methanol (0.560 mL) was added zinc powder (0.068 g, 1.04 mmol, 5.00 equiv) followed by ammonium chloride solution (55.3 mg, 1.04 mmol, 5.00 equiv) drop wise. After being stirred for 4 h at RT, DIPEA (48.0 μL, 0.260 mmol, 1.25 equiv) and ethyl chloroformate (20.0 μL, 0.210 mmol, 1.00 equiv) were added to reaction mixture at 10° C. and the stirring was continued for another 2 h at RT. After consumption of the starting material (by TLC), the reaction mixture was diluted with water (25 mL) and extracted with EtOAc (2×75 mL). The separated organic layer was dried over anhydrous Na2SO4, filtered and concentrated under reduced pressure to give crude; which was purified by neutral alumina column chromatography (EtOAc/Hexane 18:82) to afford to compound 4A (0.017 g, 21%) as an off-white solid. TLC: 50% EtOAc/Hexane (Rf: 0.50); 1H NMR (500 MHz, CDCl3): δ 7.30-7.27 (m, 2H), 7.00 (t, J=8.5 Hz, 2H), 6.88 (d, J=11.5 Hz, 1H), 6.26 (br s, 1H, Exc), 4.40 (d, J=6.0 Hz, 2H), 4.21 (q, J=7.0 Hz, 2H), 3.75 (br s, 1H, Exc), 3.50 (s, 2H, Exc), 1.30 (t, J=7.0 Hz, 3H); LC-MS: m/z=340 (M++1) at RT 3.46 (94.3% purity); UPLC purity: 95.2%
