Ab initio study of endo/exo and diastereofacial selectivities in Diels-Alder reactions between chiral butenolides and cyclopentadiene

Diels-Alder reactions of cyclopentadiene with crotonolactone and β-angelica lactone have been studied using ab initio methods at the MP3/6-31G*//RHF/3-21G and MP2/6-31G*//RHF/6-31G* levels. The transition states corresponding to the formation of different stereoisomers and diastereoisomers have been located. The results obtained show that the correct endo/exo selectivity is only obtained when polarization functions are included in the basis set. However, syn/anti selectivity is correctly described at all levels of calculation.