Organothiocyclopentanes of the formula ##STR1## wherein E represents >C.dbd.O or ##STR2## in which Z' represents a hydrogen atom, a hydroxyl group or a protected hydroxyl group; X represents --S--, --CH.sub.2 --, --CH.dbd.; represents a single or double bond; R' represents a monovalent or divalent organic group containing 1 to 25 carbon atoms; R represents a monovalent organic group containing 1 to 25 carbon atoms; Z represents a hydrogen atom, a hydroxyl group or a protected hydroxyl group; and when both E and Z represent a hydroxyl group or a protected hydroxyl group, X represents --S--. Also, 2-organothio-2-cyclopentenones of the formula ##STR3## wherein R and Z are as defined above. The compounds of formula (I) are prepared by reacting 2,3-epoxy-cyclopentanone or its derivatives with mercaptans in the presence of basic compounds. The compounds of formula (II) are prepared by reacting the compounds of formula (I) with mercaptans or organocopper-lithium compounds, and optionally reducing the resulting products, and optionally protecting the hydroxyl group at their cyclopentane ring. The compounds of formula (I) have an action of inhibiting ulcer formation, and are useful intermediates for the compounds of formula (II). The compounds of formula (II) are monothia- or dithia-prostanoic acid derivatives, and exhibit various superior pharmacological actions.