Enantioselective synthesis and conformational study of cyclohexene carbocyclic nucleosides

Enantioselective synthesis of a new family of unsaturated six-membered carbocyclic nucleosides using (R)-(-)-can one as starting material is described. Introduction of the base moiety via Mitsunobu reaction proceeded regio- and stereoselectively and with good chemical yield, while the Pd-coupling approach failed. H-1 NMR study and molecular modeling show the adenine compound exists in an equilibrium of H-3(2) and H-2(3) conformers (ratio 7:3) in favor of the 3'-endo half-chair conformation, with the base oriented in a pseudoaxial position. This conformational preference can be explained by the pi --> sigma*(C1')-(N1) interaction involving the antibonding orbital of the C1'-N bond.