Lewis Acid Mediated [2,3]-Sigmatropic Rearrangement of Allylic α-Amino Amides

Article abstract of DOI:10.1021/jo035618g

Boron trifluoride and BBr₃ mediated [2,3]-sigmatropic rearrangements of allylic α-amino amides have been developed affording secondary amines in good yields. (E)-Crotyl and (E)-cinnamyl α-amino amides 2b and 2c exhibit excellent syn-diastereoselectivity upon rearrangement with either Lewis acid. The allylic amine 2a forms upon treatment with BF ₃ or BBr₃ a five-membered heterocylic complex in which a single halide anion has been displaced by the carbonyl oxygen atom. The structures of the Lewis acid-amine complexes were elucidated using NMR spectroscopy. A plausible reaction mechanism, based on DFT calculations, is presented. Thus, BF₃- or BBr₃-complexed allylic amines 2 are shown to preferentially proceed, after deprotonation, via an endo transition state.