A novel molecular oxygen-mediated Baeyer-Villiger-type oxidation specific to the morphinan skeleton has been discovered. The reaction of a ketone bearing a bicyclo[2.2.2]octenone skeleton with KOH (pellet) in refluxing tBuOH under an oxygen atmosphere (balloon) afforded unexpected cyclopropanecarboxylic acid and ketolactone derivatives. The reactive sites (carboxylic acid and ester) in those products are oriented over the C-ring in the morphinan skeleton, and thus such morphinan compounds would be useful intermediates to design ideal kappa opioid receptor agonists. (C) 2020 Elsevier Ltd. All rights reserved.