Both inter and intramolecular Horner-Emmons reactions were examined as methods for the preparation of cyclopentenones and cyclopentenals. These enones and enals are useful as precursors to a number of natural products (and analogs) which include the jasmones, pyrethrins, and the prostaglandins. It was determined that the addition of cerium trichloride during the intermolecular Horner-Emmons reaction of aldehydes with 2-oxocyclopentyl diethyl-phosphonate significantly increased the yield of the expected cyclopentenones. In addition, a variety of new 5-substituted cyclopenten-1-als were selectively prepared by a simple two step process. The first step involved the Diels-Alder reaction of a number of electron-poor olefins with dimethyl 1-(1,3-butadienyl)phosphonate. The final step included ozonolysis of the Diels-Alder adducts, reduction of the ozonide, and intramolecular Horner-Emmons reaction to form six new 5-substituted cyclopenten-1-als
