A method for preparing cyclopentanones or lactones is provided to be useful for preparation of prostagladins, to remove problems such as the long synthesis route and the removal of undesired isomers, and thus achieves a simple and economical synthesis route. The method for preparing a compound of the following formula II having an enantiomer purity higher than 95% e.e.(enantiomeric excess) comprises the steps of: reacting a compound of the following formula IV having an optical purity higher than 90% e.e. with cuprate； and optionally performing a de-protection reaction of the obtained compound to convert P1, P2, or both P1 and P2 into H. In the formulae, Z does not take part in coupling and de-protection reactions and is an optional group acting as a leaving group in reduction/lactonization reactions, R2 is a single bond, C1-4 alkylene or -CH2O- group, R3 is a C1-7-alkyl or aryl or aralkyl unsubstituted or substituted by C1-4 alkyl, halogen or trihalomethyl, X1 and X2 are independently hydrogen or P1 and P2, respectively, wherein P1 and P2 are protective groups for hydroxyl groups which are identical to or different from each other.
