Treatment of naltrexone, dihydromorphinone, or related opiates that contain a 6-keto group with acetic anhydride or tert-butyldimethylsilyl chloride under mild conditions afforded enol derivatives having a Δ6bond. Naltrexone also was found to undergo the Robinson annelation reaction with methyl vinyl ketone with facility and in high yield to give the β-hydroxy ketone 3, whose stereochemistry was determined by means of 2D NMR spectroscopy. Upon acid treatment, 3 was dehydrated to the α,β-unsaturated ketone 4. Acid-catalyzed equilibration of 4 afforded the unconjugated olefin 5. These studies suggest that the Δ6bond is more stable than one that is exocyclic to ring C of the opiate because it permits ring flattening which may partially relieve torsional ring strain. The Δ6bond also may relieve an eclipsing interaction between a C-6 exocyclic substituent and the vicinal furan oxygen.