Original article Synthesis of novel chiral D 2 -isoxazoline derivatives related to ABT-418 and estimation of their affinity at neuronal nicotinic acetylcholine receptor subtypes Clelia Dallanoce a, * , Pietro Magrone a , Carlo Matera a , Leonardo Lo Presti b , Marco De Amici a , Loredana Riganti c , Francesco Clementi c , Cecilia Gotti c , Carlo De Micheli a a Dipartimento di Scienze Farmaceutiche “Pietro Pratesi”, Università degli Studi di Milano, Via Mangiagalli 25, 20133 Milano, Italy b Dipartimento di Chimica Fisica ed Elettrochimica, Università degli Studi di Milano, Via Golgi 19, 20133 Milano, Italy c CNR, Istituto di Neuroscienze, Farmacologia Cellulare e Molecolare e Dipartimento di Farmacologia, Chemioterapia e Tossicologia Medica, Università degli Studi di Milano, Via Vanvitelli 32, 20129 Milano, Italy article info Article history: Received 22 June 2010 Received in revised form 1 September 2010 Accepted 2 September 2010 Available online 17 September 2010 Keywords: Neuronal nicotinic acetylcholine receptors ABT-418 1,3-Dipolar cycloaddition Nicotinic ligands Binding affinity abstract The enantiopure diastereomeric D 2 -isoxazoline derivatives (2S,5 0 R)-5ae10a and (2S,5 0 S)-5b, (2S,5 0 S)-9b, (2S,5 0 S)-11b, which are structural analogues of both ABT-418 2 and oxyimino ethers (S)-3 and (Z)-(S)-4, were synthesized through cycloaddition reactions involving nitrile oxides as 1,3-dipoles and (S)-N-Boc-2- vinylpyrrolidine-13 as the dipolarophile. The absolute configuration was unequivocally assigned to target compounds by means of an X-ray analysis. The derivatives under study were assayed at neuronal acetylcholine nicotinic receptors (nAChRs), where they showed a meaningful reduction in affinity at the heteromeric a4b2 subtype when compared to the reference molecules. Conversely, anti (2S,5 0 S)-5b and syn (2S,5 0 R)-10a isomers showed an affinity for the a7 nAChRs comparable to that observed for the model compound ABT-418. Ó 2010 Elsevier Masson SAS. All rights reserved. 1. Introduction Neuronal nicotinic acetylcholine receptors (nAChRs), which belong to the family of Cys-loop ligand-gated ion channels, are responsible for rapid excitatory cholinergic transmission. The various physiological functions regulated by nAChRs are due either to their synaptic activation or to their involvement in the modu- lation of neurotransmitter systems other than the cholinergic one [1,2]. Accordingly, nAChRs have been identified as major thera- peutic targets mainly for pathological conditions of the central nervous system, such as Alzheimer’s and Parkinson’s diseases, Tourette’s syndrome, schizophrenia, attention deficit/hyperactivity disorder (ADHD), pain, substance abuse, anxiety and depression [3e6]. In addition, some subtype selective radioligands with improved brain penetration are under development for mapping and recording the expression of central nAChRs [7,8]. Worth mentioning, an alternative approach to the design and synthesis of compounds selective for a specific nAChR subtype is to target its allosteric binding sites, which are usually less conserved than the acetylcholine recognition site (orthosteric site) [9,10]. The potential therapeutic impact of novel drug candidates tar- geting nAChRs is strictly reliant on their selectivity for a given subtype. Among the 12 different neuronal nAChRs characterized so far in the vertebrate brain [11,12], the most expressed are the het- eromeric a4b2 channels and the homomeric a7 channels, which represent preferred biological targets for the design of new selec- tive nicotinic agonists/partial agonists [1,5,11]. Among the various approaches involving the chemical manipulation of naturally occurring nicotinic ligands [1,11,13], structure activity studies on (S)-nicotine 1 , the active ingredient of tobacco, afforded ABT-418 2, a potent and rather selective a4b2 nAChR agonist, in which a 3-methylisoxazol-5-yl moiety bioisosterically replaced the pyri- dine ring of the parent compound (Fig. 1) [14]. Notably, ABT-418 has demonstrated signals of efficacy in adults with ADHD [15]. More recently, a group of ABT-418 analogues, in which an oxyimino ether moiety replaced the 3-methyl-5-isoxazolyl residue, was synthe- sized and tested [16]. In particular, the two-enantiopure oxime ethers (S)-3 and (Z)-(S)-4 (Fig. 1) displayed a moderate * Corresponding author. Tel.: þ39 02 50319327; fax: þ39 02 50319326. E-mail address: clelia.dallanoce@unimi.it (C. Dallanoce). Contents lists available at ScienceDirect European Journal of Medicinal Chemistry journal homepage: http://www.elsevier.com/locate/ejmech 0223-5234/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ejmech.2010.09.009 European Journal of Medicinal Chemistry 45 (2010) 5594e5601