To 80 ml of a methanol suspension containing 4.31 of N-(2'-methoxycarbonylphenyl)-5,6,7,8tetrahydro-9-oxo-xanthene-2-carboxylic acid amide obtained in Example 43 was added 10 ml of an aqueous solution containing 0.48 g of sodium hydroxide. The mixture was heated for 3 hours under refluxing and stirring conditions. The solvent was then removed by distillation to give a residue which was dissolved in hot water and filtered. The filtrate was acidified with concentrated hydrochloric acid to separate a yellowish white deposit. The deposit was collected by filtration, washed with water and then air-dried. Finally, the deposit was recrystallized from acetic acid to obtain 2.96 g (yield: 71.8%) of N-(2'-carboxyphenyl)-5,6,7,8-tetrahydro-9-oxo-xanthene-2-carboxylic acid amide.
