Into dimethylacetoamide (30 ml) was dispersed sodium hydroxide (content: 60%, 0.88 g, 22 mmol). To this were added IM-3 obtained in Example 1-2 (1.77 g, 10 mmol)/dimethylacetoamide (5 ml). This mixture was stirred for 10 minutes, then, to this was added IM-5 (4.5 g, 11 mmol) obtained as described above, and this mixture was stirred for three hours. To this was added water (10 ml), and this mixture was acidified with 1N water hydrochloride, and the reaction solution was concentrated. The concentrated mixture was refined with silica gel column chromatography to obtain 2.8 g of IM-6.
