OXIDATION OF BETA-ANILINOACRYLATE ALKALOIDS VINCADIFFORMINE AND TABERSONINE BY FREMY SALT - A MECHANISTIC INSIGHT INTO THE REARRANGEMENT OF ASPIDOSPERMA TO HUNTERIA ALKALOIDS

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 rearrange 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.