3 ml of formic acid and 3 ml of acetic anhydride were mixed and stirred at a room temperature, and to the mixture was added 1.41 g of the product of Example 172, and the mixture was stirred for one hour. The reaction mixture was added to 50 ml of ethyl acetate and 30 ml of saturated sodium carbonate aqueous solution with ice, and the mixture was stirred, and after foaming was terminated, the ethyl acetate layer was sequentially washed twice with water and once with a saturated sodium chloride aqueous solution, and dried over magnesium sulfate, filtered and evaporated under a reduced pressure. The resulting residue was purified on a silica gel column (silica gel 60 g, eluant: 2% methanol in chloroform), to obtain 1.49 g of the title compound as a mixture of two isomers (3:2) in a colorless amorphous form.
