To methyl 7-[4-(2-propoxyethoxy)phenyl]-1,1-dioxo-2,3-dihydro-1-benzothiepine-4-carboxylate (6.5 g) were added 1,2-dimethoxyethane (130 ml) and 6N hydrochloric acid (65 ml), and the mixture was refluxed at 100° C. for 18 hours, cooled to room temperature, extracted with ethyl acetate, washed with saturated brine and dried with magnesium sulfate. Under reduced pressure, the solvent was evaporated to give 7-[4-(2-propoxyethoxy)phenyl]-1,1-dioxo-2,3-dihydro-1-benzothiepine-4-carboxylic acid (5.0 g).
