Isothiocyanate and alpha-methylene-gamma-lactone analogues of 6,14-endo-ethenotetrahydrothebaine and -oripavine were prepared with the electrophilic groups being located at C-19 in the C-7 alpha-side chain. Isothiocyanates were prepared in the N-Me and N-CPM (N-cyclopropylmethyl) series, both as the phenols and 3-O-methyl ethers from the diastereomeric amines formed from reductive amination of thevinone (2) and N-(cyclopropylmethyl)northevinone (13). Although addition of the organozinc reagent from methyl alpha-bromomethacrylate to 25 failed, addition to 3-O-protected aldehydes 27 and 35 produced, after subsequent deprotection, alpha-methylene-gamma-lactones 29 and 37, respectively. In the opioid receptor displacement assays against [3H]bremazocine as the radiolabeled ligand, the phenolic compounds were most potent with N-CPM isothiocyanates 20 and 21 showing IC50s of 0.32 and 0.76 nM, respectively, and N-CPM alpha-methylene-gamma-lactone 37 having an IC50 = 1.0 nM. Compound 37 showed irreversible effects in the binding assay which were mu-selective, as demonstrated by analogous experiments using [3H]DAGO, and naloxone was found to protect against the irreversible effects. This observation suggests that a receptor-bound nucleophile is located at a position where it can readily reach the alpha-methylene group of lactone 37.