To a suspension of 2-Acetylamino-4,5,6,7-tetrahydro-benzo[b]thiophene-3-carboxylic acid ethyl ester (140 g) in acetic acid (1.5 L) and water (1.5 L) was added gradually cerium (IV) sulfate 4 hydrate (900 g) with vigorously stirring. After the solution was stirred overnight, the solid was removed by filtration and the filtrate was concentrated. The solution was then neutralized with aqueous ammonia and extracted 3 times with ethyl acetate-tetrahydrofuran (=4:1). The organic layers were combined and evaporated in vacuo. The resulting solid was washed with tetrahydrofuran to provide the title compound (44 g) as a pale yellow solid.
