To a solution of 791 mg of the compound obtained in Example 4 in 5 ml of anhydrous N,N-dimethylformamide (hereinafter abbreviated as DMF), 207 mg of potassium carbonate and a catalytic amount of 18-crown-6-ether were added, and the mixture was heated at 80° C. for 8.5 hours. To the mixture were added ethyl acetate and water. The mixture was shaken, and the organic layer was separated. The organic layer was washed with water and dried over anhydrous magnesium sulfate. The solvent was removed under reduced pressure. The residue was purified through silica gel column chromatography to yield the titled compound as crystals in quantitative yield. The physical data of the product were identical to those of the authentic specimen as described in Reference Example B-1.
