Origin of diastereoselectivity in the thermal[4+2] cycloadditions of dienophiles derived from Oppolzer's sultams: Steric vs stereoelectronic influences

Comparative semi-empirical PM3 and nb initio STO 3-21G calculations on bornanesultam-derived dienophiles containing the structural moiety SO2-N-C(O)-X(alpha)=Y(beta) suggest that, among the conformers of low energy, the thermodynamically less stable SO2/C(O)-syn,C(O)/X=Y-s-cis conformation is also reactive in terms of LUMO level and atomic coefficients. Furthermore, the X(alpha), Y(beta) LUMO atomic coefficients are nonequivalent with respect to both X(alpha)-re and X(alpha)-si faces, and thus have, depending on the conformation, a matching or mismatching stereoelectronic influence with the co-operative steric effect. This dissymmetry is believed to result from the generalized anomeric effect of the N lone pair, itself anomerically stabilized and directed, in the absence of crucial steric interactions, by the pseudo-axial anti-periplanar S=O bond. Five N-acyl-substituted bornanesultams are discussed ((-)-1a: N-acryloyl, X=CH, Y=CH2: (-)-1b: N-crotonoyl, X=CH, Y=CHMe; (-)-1c: N,N'-fumaroyl, X=CH, Y=CH(C(O)-bornanesultam); 2n: N-glyoxyloyl, X=CH, Y=O; 2b: N-acylnitroso, X=N, Y=O). In this context, differences with toluenesultams 3 are pointed out. A previous report on N-(acylnitroso)-bornanesultam 2b is revisited, and the diastereoselectivity observed is shown to result from thermodynamic control.