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To a stirred solution of (48) (7.56 g, 21.88 mmol) in anhydrous DMF (100 ml) in a water bath under an atmosphere of nitrogen, was portionwise added 60% sodium hydride (1.05 g, 26 mmol). After 30 min, oxygen gas was bubbled into the reaction mixture for 4 hr. The mixture was neutralized with aqueous saturated ammonium chloride solution. The crude product was extracted with ether and purified by silica gel column chromatography (eluent, ether:hexane (1:4)) to afford 5.87 g (80%) of a white solid. m.p. 119-120° C.; 1H NMR (200 MHz, CDCl3) δ: 1.19 (6H, d, J=7.1 Hz), 2.38 (3H, s), 2.80 (1H, m), 3.93 (3H, s), 4.06 (3H, s), 7.40-7.61 (3H, m).

1. customoxygen gas was bubbled into the reaction mixture for 4 hr
2. extractionThe crude product was extracted with ether
3. custompurified by silica gel column chromatography (eluent, ether:hexane (1:4))