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Ceric ammonium nitrate mediates the oxidative 5-endo radical−polar crossover reactions of β-enamide esters to give 5,5-C,O-disubstituted-γ-lactams. Trapping of the intermediate cations leads to 5-hydroxy- or 5-alkoxy-γ-lactams depending upon the reaction conditions. The methodology was used to synthesize the basic heterocyclic ring fragments of the natural products L-755,807, Quinolacticin C, and PI-091.