The enantiomers of epiboxidine: 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 (alpha2 to alpha10 and beta2 to beta4) 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 alpha4 and beta2 subunits, whereas the most abundant homomeric channel is the alpha7 pentamer. Thus, research efforts have mainly focused on the discovery of selective alpha4beta2 or alpha7 ligands to ascertain the physiological and pathophysiological relevance of these receptor subtypes. A significant impulse to the development of selective alpha4beta2 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 the presence of a 3-methylisoxazole ring, emerged as a potent alpha4beta2 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 synthesis of target derivatives took advantage of a palladium-catalyzed Stille reaction involving two enantiomeric enoltriflates and the suitable tributylisoxazolylstannane. The synthetic details, the affinity profile of (+)2 and (-)2 and that of related analogues at alpha4beta2 and alpha7 nAChRs will be presented and discussed.