Unexpected N‐Demethylation of Oxymorphone and Oxycodone N‐Oxides Mediated by the Burgess Reagent: Direct Synthesis of Naltrexone, Naloxone, and Other Antagonists from Oxymorphone

N‐Oxides derived from oxycodone and O‐acyloxymorphone were treated with the Burgess reagent to provide the corresponding oxazolidines in excellent yields. Oxazolidines derived from O‐acyloxymorphone were further hydrolyzed to noroxymorphone, whose alkylation furnished naltrexone, naloxone, and nalbuphone, which can be converted to nalbuphine, the mixed agonist‐antagonist analgesic. The entire sequence from oxymorphone to the various antagonists was reduced to three one‐pot operations, proceeding in excellent overall yields. In addition, quaternary salts of the oxazolidines with allyl or cyclopropylmethyl groups in fixed equatorial configurations were synthesized. Complete spectral and experimental data are provided for all compounds.