The replacement of the furanose ring by a cyclopentane in nucleosides generates a series of analogues known as carbocyclic nucleosides,which show increased chemical and enzymatic stability because of the lack of a glycosidic bond.1 However,the loss of such glycosidic bond leads to a significant change in conformation due to the absence of anomeric effect and gauche effect which help maintain the ribose in either 3-endo(north) or 2-endo (south) conformation in conventional nucleosides.2 Fusing a ring to the cyclopentane will be able to lock the embedded cyclopentane ring into a conformation similar to the conventional nucleosides.3 Herein,a synthetic study of conformation constrained carbocyclic nucleosides,a novel bicycle[5.4.0]nonane system,will be reported(Scheme).The carbanucleoside(1) could be derived from ketone 2 which would be formed by enone 3usingan intramolecular Diels Alder reaction as the key step.Enone 3would be derived from D-ribose(5).