A Low-Temperature Ammonium Ylid Rearrangement: Enhanced Reactivity Engendered by Rigidity

Sigmatropic rearrangement of tetrahydropyridine-­derived ammonium is a valuable method for the preparation of substituted prolines. These reaction normally require elevated temperatures to proceed, but bicyclic tetrahydropyridine-like ylid 1 undergoes rearrangement at -15 ˚C; the extra rigidity of the azabicyclo[3.3.0]octene system preorganizes the transition state and lowers the activation energy for rearrangement.