Synthesis of new opioid derivatives with a propellane skeleton and their pharmacology. Part 2: Propellane derivatives with an amide side chain

We designed and synthesized propellane derivatives with a 6- or 7-amide side chain on the basis of the active conformation of the κ selective agonist nalfurafine. The 6-amides showed high affinities for the κ receptor, and one of the 6β-amides showed higher κ selectivity than nalfurafine. On the other hand, although the affinities of the 7-amides decreased compared to the 6-amides, some 7α-amides showed the highest selectivities for the κ receptor among the tested compounds. The affinities of 7β-isomers were extremely low, which was postulated to result from the shielding effect of the 7β-amide side chain against the lone electron pair on the 17-nitrogen. This is the first conformational information about the 7-amide side chain in propellane derivatives.