Syntheses and opioid receptor binding properties of carboxamido-substituted opioids

A series of 15 novel opioid derivatives were made where the prototypic phenolic-OH group of traditional opioids was replaced by a carboxamido (CONH₂) group. For 2,6-methano-3-benzazocines and morphinans similar or, in a few instances, enhanced affinity for μ, δ and κ opioid receptors was observed when the OH → CONH₂ switch was applied. For 4,5α-epoxymorphinans, binding affinities for the corresponding carboxamide derivatives were much lower than the OH partner consistent with our pharmacophore hypothesis concerning carboxamide bioactive conformation. The active metabolite of tramadol and its carboxamide counterpart had comparable affinities for the three receptors.