at u2212 19 9 to u2212 14 3 u00b0 c the suspension was stirred at u2212 20 2 to u2212 16 2 u00b0 c for 10 minutes and the precipitated crystals were filtered using a 6 5 cm u03c6 glass filter the precipitated crystals were washed with a mixed solvent of 31 5 ml of methanol and 3 5 ml of purified water the washed crystals were deliquored for 1 hour to obtain 6 81 g of wet crystals the wet crystals were dried under reduced pressure at room temperature for 3 hours and 20 minutes to obtain 6 81 g of crystals of the compound 1 as a white powder recovery 97 2 a 200 ml three necked flask was charged with the crystals of the compound 1 7 00 g obtained in example 1 28 0 ml of hexane and 28 0 ml of diisopropyl ether and the mixture was stirred at 30 0 to 35 0 u00b0 c for 36 hours 9 hours day stirred for 4 days the solution was left to stand at room temperature at night during this process part of the crystals were sampled from the suspension 1 to 3 days after the start of stirring in order to check the degree of suspension purification after completion of the suspension purification the suspension was stirred at 20 2 to 20 4 u00b0 c for 30 minutes and the precipitated crystals were filtered using a glass filter 17g3.