Oxidation of β-Anilinoacrylate Alkaloids Vincadifformine and Tabersonine by Fremy's Salt. A Mechanistic Insight into the Rearrangement of Aspidosperma to Hunteria Alkaloids

Article abstract of DOI:10.1021/jo00240a023

β-Anilinoacrylate Aspidosperma alkaloids vincadifformine (2a) and tabersonine (2b) react with Fremy's salt in aqueous acidic conditions via radical coupling at C-16.The resulting zwitterionic compounds 7a and 10 rearange to isoxazolidines 8 and then ultimately to azepino<2,3-b>indoles 9.The mechanism of these reactions is discussed, and the structures of 7a, 8b, and 9a were established by single-crystal X-ray analysis.Diazotization of amine 20b (X = NH2) affords fragmentation-cyclization products corresponding to eburnanes 18 and 21.This reaction mimics the skeletal rearrangement of Aspidosperma -> Hunteria alkaloids, and these findings support Wenkert's biogenetic proposal.