AbstractThe Diels‐Alder reaction of 7‐chloro‐6‐demethoxythebaine (8) and 7‐chloro‐5β‐methyl‐ ‐6‐demethoxythebaine (14) each with ethyl acrylate yielded four (14:6:4:1) and three adducts (15:4:1), respectively. The main adduct from diene 8 was the ethyl 6α,14α‐ethenoisomorphinan‐7α‐ ‐carboxylate 15, while diene 14 afforded the ethyl 6β,14β‐ethenomorphinan‐8β‐carboxylate congener 17 in which the dienophile has reacted from the opposite face. Cycloaddition of 8 and 14 with maleic anhydride, followed by esterification, yielded the 6α,14α‐ethenoisomorphinans 23 and 24, respectively, as the main adducts in both cases. In the case of diene 14 a considerable amount (20%) of the 6β,14β‐ethenomorphinan 25 was also isolated.The adducts 15, 16, and 19 were 3‐O‐demethylated using boron tribromide to their 3‐hydroxy esters 26, 28, and 30. Conversion of these esters into the tertiary alcohols 27, 29, and 31 was performed using methylmagnesium bromide. Compared to morphine and etorphine, the compounds 26–29 exhibit high affinity for all the opiate receptor subtypes with a higher selectivity for the μ‐receptor. The compounds 30 and 31 were inactive.The outcome of the cycloadditions is discussed in terms of the electronic influence of the 7‐chloro substituent in the presence or absence of the 5β‐methyl group.