Ethyl 7-(4-propoxyethoxyphenyl)-1,1-dioxo-2,3-dihydro-1-benzothiepine-4-carboxlate (11 g, 24.7447 mmol) was dissolved in 110 ml of tetrahydrofuran and 55 ml of methanol, and then 93.5 ml of water was added. After adding potassium carbonate (6.84 g, 49.49 mmol), the mixture was heated under reflux for 4.5 hours. Under reflux, 39.6 ml of a 3N hydrochloric acid and 40 ml of water were dropped. After cooling, the mixture was stirred for 1 hour while cooling in ice. After filtration, the residue was washed with 88 ml of water. After vacuum drying (40° C.), 9.4 g (Yield 91.2%) of 7-(4-propoxyethoxyphenyl)-1,1-dioxo-2,3-dihydro-1-benzothiepine-4-carboxylic acid was obtained as pale yellowish white crystals.
