Loss of allyl cation configuration in cycloadditions to electron-rich conjugated dienes

The cycloaddition of W-configurated 1,3-dimethyl-2-oxyallyl cations to electron-rich cyclic conjugated dienes is accompanied by configurational losses and leads to cycloadducts with a,ß-configurated (equatorial, axial) methyl groups. For aromatic "dienes" such as benzene and N-alkylpyrroles, clectrophilic substitution competes strongly with [4 + 3 ? 7] cycloaddition which, nonetheless, can be accomplished by choice of an oxyallyl moiety with the proper blend of electrophilicity and nucleophilicity. It is suggested that formation of the seven-membered ring proceeds in two distinct stages, i.e., the stepwise pull-push mechanism involving formation of the first s bond in an electrophilic step and the second s bond in a nucleophilic step. © 1979 American Chemical Society.