In an alternative synthesis of (10), N-hydroxysuccinimide (558 mg, 4.8 mmol) was added to a solution of compound (6) (500 mg, 2.4 mmol) in CH2Cl2 (10 mL) and the reaction mixture cooled to 0° C. m-Chloroperbenzoic acid (1.62 g, 7.2 mmol, commercial grade: 77% in water) was added over a period of 10 min and the mixture allowed to stir at room temperature for 16 h. The reaction mixture was diluted with ether (50 mL) and washed with water (2×10 mL), saturated sodium bicarbonate solution (10 mL) and brine (10 mL). The organic layer was dried over anhydrous sodium sulfate, filtered and concentrated in vacuo to give the title compound (10) together with m-chlorobenzoic acid. The crude product mixture was purified by column chromatography on silica gel, eluting with 4:6 EtOAc:hexane. Residual m-chlorobenzoic acid was removed by repeated crystallizations from a mixture of tert-butyl methyl ether and hexane, resulting in analytically pure product (66 mg, 10%).
