Stereochemistry and transformation of dihydrothebain in the bain synthesis from labelled thebaines

The complete stereochemistry of the Δ⁶-dihydrothebaine → 7-bromodihydrocodeinone dimethyl ketal → codeinone dimethyl ketal → thebaine sequence of transformations has been elucidated by NMR studies of the isotopically labeled compounds prepared using diimide-d₂, methyl hypobromite-[¹⁴C], and methyl hypobromite-d₃. Final elimination of methanol from codeinone dimethyl ketal proceeds in a stereospecific manner: cis under acid catalysis with POC1₃, and trans under alkaline catalysis with EtO^?. As a result of these reactions, variously isotopically labeled thebaines can be prepared.