To a 5 ml acetonitrile solution of 190 mg of the compound (8) synthesized in Reference Example 8 were added 184 mg of the compound (4) synthesized in Reference Example 4 and 0.14 ml of triethylamine. This was heated at reflux for 3 hours. To the reaction was added 10 ml of ice water, then extraction was performed with ethyl acetate. The extract was dried, filtered, then concentrated under reduced pressure to obtain a residue which was then purified by silica gel column chromatography (methylene chloride:methanol=25:1) to obtain the above-identified compound (28) in an amount of 300 mg (yield of 98%).
