Esterification of 16-phenoxy-17,18,19-20-tetranor-PGF2alpha-11,15-bistetrahydropyranyl ether (I) with diazomethane followed by fluorination with (diethylamino)sulfur trifluoride (DAST) in the presence of pyridine affords the 9beta-fluoro intermediate (II). Deprotection with acetic acid yields the 11,15-diol (III), which is finally converted to flunoprost by treatment with potassium hydroxide in ethanol - water.