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Acetic anhydride (1.42 mL, 15 mmol) and pyridine (1.7 mL, 21 mmol) were added sequentially to a solution of compound A1 (2.2 g, 10.5 mmol) in 20 mL of CH2Cl2. The reaction mixture was stirred at room temperature for 12 h and then evaporated to give a yellowish oil residue. The residue was purified by flash chromatography (eluting with 7→9% EtOAc in hexanes) to give compound A2 as a colorless oil (2.26 g; 9.0 mmol; 85.6% yield); 1H NMR (400 MHz, chloroform-D) δ ppm 1.88 (d, J=6.82 Hz, 3 H) 2.31 (s, 3H) 6.62 (q, J=6.82 Hz, 1H) 7.25 (t, J=8.46 Hz, 1H) 7.49 (dd, J=8.84, 5.05 Hz, 1H).

1. customevaporated
2. customto give a yellowish oil residue
3. customThe residue was purified by flash chromatography (eluting with 7→9% EtOAc in hexanes)