ORIGIN OF DIASTEREOSELECTIVITY IN THE NITRILE OXIDE CYCLOADDITIONS WITH OPPOLZERS CHIRAL SULTAMS - COULOMBIC INTERACTION AS THE KEY ROLE IN DIASTEREOFACIAL DIFFERENTIATION

Diastereofacial selectivity in the nitrile oxide cycloadditions with Oppolzer's chiral sultams cannot be explained with the conventional face shielding by sterically bulky groups. To find the origin of the diastereoselectivity, the transition states for the cycloadditions have been studied with semiempirical quantum mechanical calculations using the PM3 method. The calculated product ratios are in good agreement with experiment. We find that the activation barriers for the favored and disfavored transition states are strongly correlated with the Coulombic repulsions between the dipolar oxygen and the sultam oxygens. Such correlation is also found from ab initio calculations using the PM3 optimized geometries. In addition, when two oxygen atoms of the sulfone group in the sultam systems are removed, these isothiazolidine systems have very small energy differences between the top-facial and bottom-facial transition states, showing no diastereoselectivity. Therefore, we suggest that the electrostatic effect is a main factor governing the diastereofacial selectivity for the sultam systems. These systems can be considered as one of the most typical case of Hehre's claim that the electrostatic effect is a very general determinant of pi-facial selectivity.