Asymmetric [2,3]-Sigmatropic Rearrangement of Allylic Ammonium Ylides

摘要

The [2,3]-sigmatropic rearrangement of allylic ammonium ylides has become a powerful strategy for the synthesis of nitrogen heterocycles.1'2 The rearrangement is proposed to proceed via an envelope-like five-membered transition state,creating a C-C bond and at least one new stereogenic center (Scheme I).The relative stereochemical outcome is determined by steric and electronic interactions between the allyl moiety and the anion stabilizing group (G) in the endo and exo transition states.Asymmetric variants of this rearrangement have relied on intramolecular chirality transfer,an elegant example being Sweeney's report of a highly diastereo-selective rearrangement of various glycine-derived allylic ammonium salts using Oppolzer's chiral sultam as auxiliary.