4 + 2] cycloadditions of N-alkenyl iminium ions: structurally complex heterocycles from a three-component Diels-Alder reaction sequence

N-Alkenyl iminium ions serve as conduits to three-component 4 + 2] cycloaddition reactions accessing structurally and stereochemically diverse piperidine derivatives. These cationic 2-azadienes participate in endo- or exo-selective 4 + 2] cycloadditions with electron-rich and neutral alkene dienophiles to generate a tetrahydropyridinium ion as the initial cycloadduct. In situ nucleophilic addition to the cycloaddition-derived iminium ion completes the three-component coupling sequence and affords a versatile synthesis of structurally complex piperidines.