position promote bicyclic ring formation the tautomerism between the hydroxy at the c 11 position and the keto moiety at the c 15 position shown below is especially significant in the case of compounds having a 13 14 single bond and two fluorine atoms the c 16 position accordingly in the specification and claims a compound of formula i represented as the bicyclic form also covers its mono cyclic tautomer for example bicyclic and monocyclic forms of a compound having a keto group at the c 15 position and halogen atoms at the c 16 position may be as follows further while the compounds used in the invention may be represented by a name based on the mono cyclic tautomer regardless of the presence or absence of the isomers it is to be noted that such structure or name does not intend to exclude the bicyclic type compound a preferred compound according to the invention in its monocyclic form can be named as 13 14 dihydro 15 keto 16 16 difluoro pge 1 or 13 14 dihydro 15 keto 16 16 difluoro 18 s methyl pge 1 according to conventional prostaglandin nomenclature the compound used in the present invention may be prepared by the method disclosed in u s pat no 5 284 858 and u s pat.