Novel epibatidine-related analogues and epiboxidine enantiomers: synthesis and binding affinity at neuronal nicotinic acetylcholine receptor subtypes

Neuronal nicotinic acetylcholine receptors (nAChRs) belong to a heterogeneous family of ligand-operated ion channels, differently expressed in the central and peripheral nervous systems, which are assembled in a pentameric architecture to form a pore permeable to cations such as Na+, K+, or Ca++. Of the seventeen different nAChR subunits which have been cloned, twelve (a2 to a10 and b2 to b4) were found in the neuronal receptors of vertebrates. Different subunit combinations characterize the nAChRs expressed within the diverse areas of the central nervous system (CNS), and affect their biophysical and functional properties, such as ion selectivity, conductance, mean open-channel time, rate of desensitization as well as the sensitivity to neurotoxins. Ongoing investigations on the pharmacology and neurobiology of CNS nAChRs allowed a better understanding of their involvement in various neuropsychiatric pathologies, such as Alzheimer’s and Parkinson’s diseases, mild cognitive impairment (MCI), Tourette’s syndrome, schizophrenia, depression, anxiety, attention-deficit hyperactivity disorder (ADHD), and nicotine addiction. More than 90% of the heteromeric channels localized in the CNS contain the a4 and b2 subunits, whereas the most abundant homomeric channel is the a7 pentamer. Thus, research efforts have mainly focused on the discovery of selective a4b2 or a7 ligands to ascertain the physiological and pathophysiological relevance of these receptor subtypes. A significant impulse to the development of selective a4b2 nAChR ligands was given by the discovery of Epibatidine (−)-1, a natural toxin possessing an analgesic potency roughly one hundred times higher than that of morphine. Among the numerous Epibatidine-related analogues investigated so far, (±)-Epiboxidine 2, characterized by a 3-methylisoxazole ring, emerged as a potent a4b2 nicotinic receptor agonist, 10-fold less potent than Epibatidine as antinociceptive agent but 20-fold less toxic. Since we aimed at deepening the pharmacological profile of Epiboxidine, we developed a novel synthetic sequence for the racemate and extended this approach to the preparation of the enantiomers (1R,2S,4S)-2 and (1S,2R,4R)-2 in enantiomeric excess higher than 99%. The same approach allowed to prepare a group of Epibatidine-related unsaturated analogues represented by the general formula A (R=H, Me). The synthesis of target derivatives took advantage of a palladium-catalyzed Stille reaction involving racemic or enantiopure enoltriflates and the suitable tributylisoxazolylstannane. The synthetic details, the affinity profile of (+)-2 and (−)-2 and that of related analogues at a4b2 and a7 nAChRs will be outlined.