19 mmol of the compound from Example 15, 66.7 mmol of KOH, 100 ml of ethanol and 50 ml of water are introduced into a 500 ml single-necked flask fitted with reflux condenser and magnetic stirrer, and the mixture is refluxed for 6 hours. After cooling, the reaction mixture is carefully acidified in concentrated HCl solution and subsequently washed with water until neutral. The solvent is stripped off, and the residue is recrystallized a number of times from glacial acetic acid. The residue is then boiled with methanol and subsequently with ether and filtered off with suction. Yield: 90.5%.
