Semiempirical (AM1) and ab initio calculations of 2-X-adenine (X = H, F, Cl) and β-D-1-amino-2,3-didehydro-1,2,3-trideoxyribofuranose

We performed semiempirical (AM1) and ab initio (3-21G and 4-31G^**) calculations, with complete optimization of geometry, for 2-X-adenine (X = H, F, Cl) and selected conformers of β-D-1-amino-2,3-didehydro-1,2,3-trideoxyribo-furanose. Together (as 2-X-2',3'-didehydro-2',3'-dideoxyadenosine), or in separate conjunction with similar fragments, these structures belong to a group of 2',3'-dideoxynucleosides with potential anti-HIV activity.

The 2-X-adenine molecules are basically flat. For the halogenated species, especially the chlorine derivative, the results of the semiempirical method differ from the ab initio findings more widely than for adenine.

The results for the ribose derivative show the existence of conformers differing very little in energy. Ring puckering does not appear to depend on exocyclic torsion angles, structures with different conformations around the exocyclic C-C bond having similar ring conformations in all the cases analysed. The AMI method yielded geometries similar to those obtained ab initio with 3-21G.