ALLYL CATION ROUTE TO 7-MEMBERED RINGS .19. 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 α,β-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 σ bond in an electrophilic step and the second σ bond in a nucleophilic step. © 1979, American Chemical Society. All rights reserved.