The compound (4) (51.26 g, 155 mmol) prepared in above (3) was dissolved in acetonitrile (1.54L), and TEMPO (2, 2, 6, 6-tetramethylpiperidine-1-oxyl, 250 mg, 0.01 eq.) was added thereto. To the mixture was added dropwise 0.81 N aqueous sodium hypochlorite, which had been prepared by diluting 1.63 N aqueous sodium hypochlorite (150 ml) with water (75 ml), adjusting pH 8.6 with 1 N sulfuric acid, and adjusting the total volume to 300 ml, over 15 minutes, while maintaining the inner temperature between −1° C. and 8° C. After stirring for 25 minutes at this temperature, 1 N aqueous sodium sulfite (32 ml) was added. Subsequently, 79% sodium chlorite (27.48 g, 240 mmol) and 31% aqueous hydrogen peroxide (23.26 g, 212 mmol) were added under ice-cooling. The cooling bath was removed and the mixture was stirred for 2 hours. The reaction was diluted with water (1.5L), adjusted to pH 3 with 1 N hydrochloric acid and the deposited crystals were filtered, and washed twice with water (200 ml), acetonitrile (50 ml) to provide 32.4 g of crude crystals. The crude crystals (32.4 g) were suspended in acetonitrile (224 ml), refluxed for 15 minutes and cooled on ice. The crystals were filtered and washed with acetonitrile (65 ml) to provide 26.79 g of the title compound (6). Yield: 51.7%, mp 202-203° C.
