Dehydropethidine, prepared by the oxidation of pethidine with mercury(II) acetate, has been reduced with lithium aluminium hydride to 2-methyl-5-phenyl-2-aza-7-oxabicyclo[3,2,1]octane (7). The latter reacts with methyl vinyl ketone to give the corresponding 8-(2-acetylethyl)derivative (9), which is in equilibrium with its enamine form (10).