Cyclopentanoids (I) of the formula: ##STR1## including stereoisomers are described along with a process for the preparation of I. In particular the preparation of prostanoids of the formula: ##STR2## wherein R.sub.1 is a alkyl group containing 1 to 8 carbon atoms and R.sub.2 CO.sub.2 R.sub.3 is an alkenyl ester group, R.sub.2 contains 2 to 6 carbon atoms and R.sub.3 is a lower alkyl group containing 1 to 6 carbon atoms is described. A particular prostaglandin prepared by the process is PGE.sub.2. The prostanoids have been demonstrated to have pharmacological activity in animals and humans. Novel intermediates of (I) are also described.