New analogues of epiboxidine incorporating the 4,5-dihydroisoxazole nucleus: Synthesis, binding affinity at neuronal nicotinic acetylcholine receptors, and molecular modeling investigations

A group of novel 4,5-dihydro-3-methylisoxitzolyl derivatives, structurally related to epiboxidine (= (1R,4S,6S)-6-(3-methylisoxazol-5-yl)-7-azzibicyclo[2.2.1]heptane), was prepared via 1,3-dipolar cyclo-addition of acetonitrile oxide to different olefins. Target compounds 1a and 1b, 2a and 2b, 3, 4, and 5 were tested for affinity at neuronal nicotinic heteromeric (alpha 4 beta 2) and homomeric (alpha 7) acetylcholine receptors. Notably, diastereoisomers 1a and 1b were characterized by a massive drop of the affinity at the alpha 4 beta 2 subtypes (K(i) values spanning the range 4.3-126 mu M), when compared with that of epiboxidine (Ki = 0.6 nM). Therefore, the replacement of the 3-methylisoxazole ring of epiboxidine with the 4.5-dihydro-3-methylisoxazole nucleus is detrimental for the affinity at alpha 4 beta 2 receptors. A comparable lack of affinity/selectivity for the two nACh R subtypes under study was evidenced for the remaining epiboxidine-related dihydroisoxazole derivatives 2a and 2b. and 3-5. Diastereoisomers 1a and 1b, and spirocyclic derivative 3 were docked into molecular models of the receptor subtypes under study, and their binding mode was compared with that of reference ligands endowed with high binding affinity.