Acid-catalyzed reactions of sarcophytoxide, a marine cembranoid: An apparently enantio-directive reaction, unusual products and stereochemical reconsideration of epoxide-ketone rearrangement

Perchloric acid treatment of sarcophytoxide, a marine cembranoid possessing an epoxide, brought about epoxide-ketone rearrangement affording ketones. When the reaction time was long (22 h), a minor ketone that was antipodal to the ketone obtained in a short-time (10 min) reaction was formed. These puzzling findings, considering that the starting epoxide had three asymmetric carbons, were interpreted by surveying the structures of other ketonic products. The stereochemistry of a major ketone, which had been wrongly assigned, was corrected by extensive analyses of NMR spectra. The correct stereochemistry indicated that the epoxide-ketone rearrangement took a course via a cationic intermediate.