Transition-metal-catalyzed ring expansion of diazocarbonylated cyclic N-hydroxylamines: a new approach to cyclic ketonitrones.

Novel C-ethoxycarbonyl cyclic ketonitrones are synthesized from the Ag- or Cu-catalyzed ring expansion of ß-diazo cyclic hydroxylamines. The latter are themselves easily obtained by the addition of lithiated ethyl diazoacetate onto cyclic nitrones. The regioselective metal-catalyzed rearrangement of ß-diazo cyclic hydroxylamines proved highly efficient and resulted in a synthetically useful ring expansion to produce 6- or 7-membered ring functionalized nitrones. The outcome of the two steps, i.e. nucleophilic addition of α-diazoesters to nitrones and ring expansion, is a formal nitrone homologation.