1-Phenyl-3-azabicyclo[3.1.0]hexane derivatives as new ligands for sigma receptors

A series of 1-phenyl-3-azabicyclo[3.1.0]hexanes were synthesized as more conformationally restricted prototypical σ ligands 3-phenylpiperidines with the aim to developing new σ ligands. Compared with 3-phenylpiperidines reported by Largent et al ., binding data showed that conformational restriction was not detrimental for σ receptor affinity. Specifically, except for secondary amine 4 , all racemic 1-phenyl-3-azabicyclo[3.1.0]hexane derivatives ( 12 - 19 ) showed moderate to high affinity for both σ 1 and σ 2 receptors. Dextrorotatory isomers with the same configuration of 3-phenylpiperidines to C-1 carbon linked to the phenyl ring showed a better affinity and selectivity for σ 1 receptors compared to the respective levorotatory isomers. Compounds (+)- 14 and (+)- 15 displayed very high affinity for σ 1 (Ki = 0.9 and 2.3 nM respectively) but low selectivity for receptor subtypes. Compound (+)- 18 with N -phenethyl substituent embodies the highest selectivity for σ 1 receptors.

Several pharmacological studies showed that σ 2 receptors are expressed in high concentration in tumor cell lines and that they are involved in proliferation and cell viability. 9hus, selective σ 1 and σ 2 ligands with agonist or antagonist properties might be potential drugs for clinical treatment of memory and learning disorders, psychoses, cocaine abuse, dyskinesia induced by classical antipsychotic therapy and cancer.
] Taking into account these data and the synthetic opportunity to extend conformational restriction to prototypical σ ligands 3-phenylpiperidines 12 (Figure 1-2) we synthesized a series of 1-phenyl-3-azabicyclo[3.1.0]hexanesas possible new ligands for σ 1 and σ 2 receptors.Using a different synthetic approach, 1-phenyl-3-azabicyclo[3.1.0]hexanes4][15] Although some of these effects might be related to a possible interaction with σ receptors, to date no affinity binding data have been reported about these compounds.

Results and Discussion
Chemistry Racemic methyl 2-(bromomethyl)-1-phenylcyclopropanecarboxylate (±)-1 and its enantiomers (+)-and (-)-1 were synthesized according to a previously reported procedure. 11reatment of 1 with NaN 3 in DMF gave azido intermediate 2 (Scheme 1).Subsequently, reduction of 2 with sodium hydrogentelluride (NaTeH) 16 in ethanol and internal cyclization provided a good yield of lactam 3. The NaTeH was readily prepared from tellurium and NaBH 4 as reported by Barton and McCombie. 17 1.These data show that secondary amine 4 did not interact significantly with σ   and σ 2 receptors (Ki > 10,000).However, nitrogen substitution with propyl or 2-methyl-2-butene led to compounds (13 and 12 respectively) able to interact with σ receptors.Specifically, compound 12 showed a good affinity with a slight preference for σ 1 receptor subtypes.Ncyclohexyl substitution gave compound 14 with the highest affinity in the racemic series.These results were similar to the N-substitution on 3-phenylpiperidine, octahydrobenzo[f]quinoline and cis-benzomorphan derivatives 12,18 which revealed an increase in binding affinity when nitrogen of the secondary amines was substituted with bulk alkyl substituents.
Compared to 14, introduction of a methylene spacer between the nitrogen and cyclohexyl residue (15) decreased by six time the binding affinity for σ 1 receptors and to a greater extent for σ 2 .The substitution of cyclohexane of 15 with more bulk adamantane (16) induced a strong reduction of σ 1 and σ 2 receptor affinity.
The benzyl derivative 17 revealed a lower σ binding affinity compared with the respective cycloalkyl 15.Moreover, an increase of the methylenic chain in phenylethyl and phenylpropyl derivatives (18 and 19 respectively) induced different effects on affinity and selectivity.In particular, 18 showed an improved affinity and selectivity for σ 1 subtypes, whereas compound 19 displayed a slight preference for σ 2 receptors.
In conclusion, in these paper we present novel 1-phenyl-3-azabicyclo[3.1.0]hexanederivatives capable of interacting with moderate to high affinity with sigma receptors.Conformational restriction of phenyl-3-azabicyclo[3.1.0]hexanederivatives compared with 3phenylpiperidine did not produce compounds with high selectivity for σ receptor subtypes in this series but like (+)-and (-)-3-PPP (3-phenylpropylpiperidine) only a slight preference for σ 1 receptors.However, the very high affinity of compounds (+)-(1R,5S)-14 and (+)-(1S,5R)-15 gave a good starting point to design new potential σ 1 and σ 2 selective ligands.Radiology and binding assays σ 1 -Site binding assays were carried out on guinea pig brain membranes as previously reported. 19iefly, each tube containing 500 µg of membrane protein was incubated for 150 min at 37 °C with [ 3 H]-(+)-pentazocine (3 nM) in 50 mM Tris-HCl pH 7.4.Non-specific binding was determined using 10 µM haloperidol.The final volume of the assay samples was 1.0 mL.After incubation the samples were filter through a Schleicher & Schnell GF 6 glass fiber filter which had been pre-soaked for 1 h in a 0.5% poly(ethylenimine) solution.Filters were washed twice with 4 ml of ice-cold buffer before transfer to scintillation vials.
σ 2 -Site binding assays were carried out on guinea pig brain membranes, prepared as previously described by Mach et al. 20 .The membranes were incubated with [ 3 H]DTG [1,3-di-(2-tolyl)guanidine] (3 nM) in the presence of 400 nM (+)-SKF10,047 (to block binding to σ 1 sites).The final volume of the assay sample was 0.5 mL.Incubations proceeded for 2 h at room temperature in 50 mM Tris-HCl, pH 8.0.Non-specific binding was evaluated in the presence of 5 µM DTG.Each assay was terminated by the addition of ice-cold 10 mM Tris-HCl pH 8.0, followed by filtration through a poly(ethylenimine) (0.5% w/v) treated GF 6 glass fiber filter which were washed twice with 4 ml of ice-cold buffer before transfer to scintillation vials.The Ki values were calculated using the EBDA/LIGAND program 21 purchased from Elsevier/Biosoft.

Issue in Honor of Prof. Vincenzo Tortorella ARKIVOC 2004 (v) 156-169 ISSN 1424-6376 Page 162
Thin-layer chromatography (TLC) was performed on precoated silica gel 60 F 264 aluminum sheets (Merck); with visualization under UV light and in a iodine chamber.Melting points were determined in open capillary tubes on a Büchi melting point apparatus and are uncorrected.Infrared spectra (IR) were obtained using a 1600 FT-IR Perkin-Elmer spectrophotometer.All optical rotations were determined in CH 3 OH solution (C = 1) employing an Optical Activity Ltd. automatic polarimeter type AA-10.Elemental analyses (C, H, N) were done on a Carlo Erba Model 1106 elemental analyzer.°C was added (57.9 mg, 0.89 mmol) and the whole was stirred for 4 h.Water was added and the resulting mixture was evaporated under reduced pressure.The residue was partitioned between brine and AcOEt and the organic phase was dried (Na 2 SO 4 ) and evaporated to give, as an oil, 171 mg (theoretical yield) of azide 2.
© ARKAT USA, Inc an internal standard.

The handling of NaN 3 and derivatives might be dangerous. For this reason, the reaction was repeated several time on a reduced scale in order to reduce the risk of explosion.
and the mixture was basified with NaOH 2 M. The latter was extracted three times with a total of 100 ml of CHCl 3 .The organic extract was dried with anhydrous Na 2 SO 4 and evaporated under reduced pressure to give the crude base 4 which were purified on silica gel by flash chromatography using CHCl 3 /cyclohexane/EtOH (50:48:2) as eluent.The purified compound 4 (1.4 g, yield 90%) was dissolved in ether and treated with an ether solution of oxalic acid to give the oxalate salts as a white solid.The analytically pure samples were obtained by recrystallization (EtOH/ether).