Oxidative 5-endo cyclization of enamides mediated by ceric ammonium nitrate.

[reaction: see text] Ceric ammonium nitrate mediates the oxidative 5-endo radical-polar crossover reactions of beta-enamide esters to give 5,5-C,O-disubstituted-gamma-lactams. Trapping of the intermediate cations leads to 5-hydroxy- or 5-alkoxy-gamma-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.