Compound 145 (2.70 g, 7.02 mmol) obtained in Reference Example 88 was dissolved in 50 ml of tetrahydrofuran. To the solution was added 17.6 ml of 2N hydrochloric acid, and the mixture was heated under reflux for 2.5 hours. The reaction solution was neutralized with a 2N aqueous solution of sodium hydroxide under ice cooling, water was added thereto, and the deposited crystals were collected by filtration. The obtained crude crystals were recrystallized from 2-propanol to give 2.33 g (yield 98%) of Compound 175 as yellow grains.
