Novel rearrangement of a 2-aryl-3-alkyl-3H-indol-3-ol to a 1,4,5,6-tetrahydro-2,6-methano-1-benzazocin-3(2H)-one with implications for the biosynthesis of aspernomine

reaction: see text] Nominine (1) and aspernomine (2) are two biologically important indole diterpenoids that arise from a common digeranylindole precursor. The skeletal relationship of these two natural products was not heretofore understood. We have observed a novel rearrangement of 2-(2-bromophenyl)-3-(3-butenyl)-3H-indol-3-ol (5) to 7, which contains the uncommon 1,4,5,6-tetrahydro-2,6-methano-1-benzazocin-3(2H)-one ring system, under acidic conditions. This rearrangement suggests that aspernomine (2) may arise biosynthetically from nominine (1).