Diethy]-Ş,3-dimethylglutarate condenses with furan-2-aldehyde and thiophene-2-aI- dehyde in presence of sodium hydride to give predominantly the (Z)-half ester (la,b) together with small amounts of the (E)-half eseter (2a,b). The structure of the (E)-half esters was based on thier cyclization to cyclopenteno-furan and cyclopenteno-thiophene derivatives (5a,b) res- pectively, as well as, spectroscopic evidence.