Ethyl 6,14-endo-ethenotetrahydrothebaine-7α-carboxylate is demethylated by hydrogen bromide in acetic acid to ethyl 6,14-endo-etheno-7,8-dihydromorphine-7α-carboxylate via the corresponding codeine derivative. 6,14-endo-Etheno-7,8-dihydro-7α(1-hydroxy-1-methylethyl)codeine is rearranged by base as well as by acid to a derivative of thebainone.