Powdered 4 Å molecular sieves (583 mg) and pyridinium dichromate (2.17 g, 5.77 mmol) were successively added to a stirred solution of intermediate 241 (1.46 g, 3.84 mmol) in of anhydrous DCM (38.5 mL) at 0° C. The mixture was stirred at rt for 15 h. More PDC (290 mg, 0.770 mmol) was added and the mixture was stirred for another 4 h, The diluted with ether and filtered through a celite pad. The filtrate was concentrated and the brown solid was purified by flash chromatography using EtOAc/DCM (7:93) affording the title compound 242 (551 mg, 41%) as a yellow solid. 1H NMR (CDCl3) δ (ppm): 7.72 (s, 1H), 6.99 (s, 2H), 6.86 (s, 1H), 4.06 (s, 3H), 4.00 (s, 3H), 3.93 (s, 3H), 3.84 (s, 6H). m/z: 378.4 (MH+).
