The present invention relates to a novel process for preparing cis-4-O-protected-substituted-2-cyclopentenol derivatives comprising, a) dissolving a 4-O-protected-2-cyclopentenone in a suitable organic solvent; and b) treating the solution with a suitable Lewis acid and a suitable reducing agent at a temperature of from about -100.degree. C. to about 20.degree. C. The cis-4-O-protected-substituted-2-cyclopentenol derivatives are useful intermediates in the preparation of various cyclopentanyl and cyclopentenyl purine analogs which are useful as immunosuppressants and in the preparation of various prostaglandins.