Abstract
2,6-Dimethyl-5-heptenal (3) and 5-octenal (24) undergo concerted ene reactions with 1 equiv of Me2AlCl and cation-olefin cyclizations with 2 equiv of Me2AlCl, MeAlCl2, or EtAlCl2 to give a zwitterion which reacts to give several products including an ene adduct. 3,7-Dimethyl-6-octenal (12) and (Z)- and (E)-6-nonenal (18 and 19) undergo only ene reactions with all catalysts. The enones 31 and 32 formed by reaction of 3 and 12 with acetonylidenetriphenylphosphorane react analogously to 3 and 12 in the presence of Lewis acid. The effect of ring size on the nature of these cyclizations is explained on the basis of thermodynamic and kinetic data.