Chirality Transfer from Carbon to Nitrogen to Carbon via Cyclic Ammonium Ylides

Cyclic ammonium salts 2 were prepared by diastereoselective quaternization of nitrogen in the corresponding proline or threonine derivatives. Upon treatment with base, salts 2 underwent [1,2]- or [2,3]-shift to 3 with moderate to complete stereospecificity. The overall process entails chirality transfer from the original α carbon to the neighboring nitrogen and then back to the carbon.