[2,3]-Sigmatropic rearrangements of didehydropiperidinium ylids

The [2,3]-sigmatropic rearrangements of ammonium ylids derived from 1,2,5,6-tetrahydropyridine have been studied: both rearrangement and elimination processes are observed, with rearrangement favoured when aprotic solvents are used in the reaction. The presence of anion-stabilizing substituents on the nucleophilic carbon atom of the intermediate ylid species involved in the tranformation also engenders rearrangement; when certain aryl substituents or two anion-stabilizing groups are present, elimination is not observed, and electron-donating ylid substituents retard rearrangement whilst enhancing elimination.