To a solution of 480 mg of (13E)-(11α,15S)-6,9-dioxo-11,15-bis(tetrahydropyran-2-yloxy)prost-13-enoic acid methyl ester (prepared as described in Example 1) in 1 ml of tetrahydrofuran was added 3 ml of 65% (v/v) aqueous acetic acid and the mixture was stirred at 45° C. for 3 hours. The reaction mixture was diluted with 16 ml of water and extracted with a mixture of ethyl acetate and n-hexane (1:1). The extract was washed with water, an aqueous solution of sodium bicarbonate and an aqueous solution of sodium chloride, dried over magnesium sulphate and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel using a mixture of ethyl acetate and n-hexane (1:1) as eluent to give 177 mg of the title compound having the following physical characteristics:
