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 ab initio STO 3‐21G calculations on bornanesultam‐derived dienophiles containing the structural moiety SO 2 NC(O)X(α) = Y(β) suggest that, among the conformers of low energy, the thermodynamically less stable SO 2 /C(O)‐ syn ,C(O)/X=Y‐s‐ cis conformation is also reactive in terms of LUMO level and atomic coefficients. Furthermore, the X(α), Y(β) LUMO atomic coefficients are nonequivalent with respect to both X(α)‐re and X(α)‐ 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 anemone effect of the N lone pair, itself anomerically stabilized and directed, in the absence of crucial steric interactions, by the pseudo‐axial anti ‐periplanar SO bond. Five N ‐acyl‐substituted bornanesultams arc discussed ((–)‐ 1a : N ‐acryloyl, XCH, YCH 2 ; (–)‐ 1b ; N ‐crotonoyl, XCH, YCHMe; (–)‐ 1c : N‐N ′‐fumaroyl, XCH, YCH(C(O)‐bornanesultam); 2a : N ‐glyoxyloyl, XCH, YO; 2b : N ‐acylnitroso, XN, YO). 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.