Tricyclic Imidazolidin-4-ones by Witkop Oxidation of Tetrahydro-β-carbolines

1-Substituted
and 1,1-disubstituted tetrahydro-β-carbolines
undergo sodium periodate oxidative ring expansion in the presence
of formaldehyde and other aldehydes to form 5,6-dihydro-7H-1,4-methano­benzo­[e]­[1,4]­diazonine-2,7­(3H)-diones in 30–81% yield. In most cases, the reaction
to form this new 6/8/5-tricyclic ring system proceeds with high diastereoselectivity.
These benzannulated medium-ring keto imidazolidin-4-ones expand the
menu of tetrahydro-β-carboline oxidation products.