Prostanoic acid derivatives of the formula, ##STR1## wherein R.sub.1 is hydrogen or C.sub.1-4 alkyl, R.sub.2 is C.sub.4-8 alkyl and >C.dbd.Z is >C.dbd.0 or ##STR2## are prepared by reacting a cyclopentane of the formula, ##STR3## wherein R.sub.2 is defined above and R is hydrogen or tetrahydropyranyl, with Wittig reagent of the formula,(R.sub.3).sub.3 P.dbd.CH(CH.sub.2).sub.3 COOMwherein R.sub.3 is aryl and M is alkali metal and, if necessary, hydrolyzing, or by oxidizing a compound of the formula, ##STR4## wherein R.sub.1 and R.sub.2 are as defined above and R.sub.9 is tetrahydropyranyl group, with an oxidizing agent. The compounds (I) are useful as anti-ulcer agent.