Methylated analogues of methyl (R)-4-(3,4-dichlorophenylacetyl)- 3-(pyrrolidin-1-ylmethyl)piperazine-1-carboxylate (GR-89,696) as highly potent kappa-receptor agonists: stereoselective synthesis, opioid-receptor affinity, receptor selectivity, and functional studies

Synthesis of 8-aminomorphans with high KOR affinity

2-Azabicyclo[3.3.1]nonanes (morphans) with a (3,4-dichlorophenyl)acetyl group at 2-position and a pyrrolidino moiety at 8-position were designed as conformationally restricted analogs of piperidine-based KOR agonists. The synthesis started with 4-oxopiperidine-2-carboxylic acid comprising 13 reaction steps. At first the ketone 10 was transformed into diester 7 bearing a propionate side chain. Dieckmann condensation of diester 7 to afford bicyclic enolester 14 and subsequent Krapcho deethoxycarbonylation represent the key steps of the synthesis. The enantiomeric pyrrolidines (1S,5R,8R)-5a and (1R,5S,8S)-5a were separated by chiral HPLC. The eutomer (1S,5R,8R)-5a showed high KOR affinity (K_i = 18 nM) and selectivity over MOR, DOR and σ₂ receptors. It was concluded that the dihedral angle of the KOR pharmacophore N(pyrrolididine)-C-C-N(acyl) of (1S,5R,8R)-5a (68°) is close to the bioactive conformation of the flexible KOR agonist 3.

Synthesis and pharmacological evaluation of enantiomerically pure endo-configured KOR agonists with 2-azabicyclo[3.2.1]octane scaffold

Conformationally restricted bicyclic KOR agonists 10 with an endo-configured amino moiety were synthesized to analyze the bioactive conformation of conformationally flexible KOR agonists such as 2-5. A seven-step synthesis starting with (S)-configured 4-oxopiperidine-2-carboxylate 13 was developed. cis- and trans-configured diesters 12 were obtained in a 3 : 1 ratio via hydrogenation of the α,β-unsaturated ester 14. After establishment of the bicyclic scaffold, a diastereoselective reductive amination of ketone 11 provided exclusively the endo-configured bicyclic amines 10a,b. The 3 : 1 mixtures of enantiomers were separated by chiral HPLC, respectively, leading to enantiomerically pure KOR agonists (1S,5S,7R)-10a,b and (1R,5R,7S)-10a,b (ent-10a,b). The KOR affinity was determined in receptor binding studies with the radioligand [³H]U-69 593. The high KOR affinity of endo-configured amines 10a (K_i = 7 nM) and 10b (K_i = 13 nM) indicates that the dihedral angle of the KOR pharmacophoric element N(pyrrolidine)-C-C-N(phenylacetyl) of 42° is close to the bioactive conformation of more flexible KOR agonists. It should be noted that changing the configuration of potent and selective KOR agonists 10a and 10b led to potent and selective σ₁ ligands (e.g. ent-10aK_i(σ₁) = 10 nM).

Diastereoselective synthesis of conformationally restricted KOR agonists

In order to analyze the bioactive conformation of flexible KOR agonists the ethylenediamine KOR pharmacophore was conformationally constrained by incorporation into a bicyclic system. For this purpose, 2-azabicyclo[3.2.1.]octan-7-amines were designed, synthesized and pharmacologically evaluated. The primary amine 14 as first key intermediate was prepared in a six-step synthesis starting with methyl cyclopent-3-enecarboxylate 9. Whereas phenylacetamides failed to provide bicyclic compounds, the intramolecular nucleophilic substitution of the sulfonamide 25 was initiated by deprotonation with NaH affording the bicyclic compound 26 in 72% yield. The three-step introduction of the pharmacophoric pyrrolidine ring started with nucleophilic substitution of exo-configured tosylate 26 with NaN3, which unexpectedly occurred under retention of configuration leading to exo-configured azide 31. The final KOR agonists 35 and 36 with exo-configured amino moieties were obtained by removal of the N-tosyl moiety of 33 and introduction of the second pharmacophoric element by acylation with dihalophenylacetyl chlorides. The KOR affinity of the pyrrolidine 35a is in the high nanomolar range (Ki = 862 nM). The low KOR affinity is explained by a non-appropriate dihedral angle of 137°/141° of the N(pyrrolidine)-C-C-N(acyl) system. As observed for stereoisomers of potent KOR agonists, phenylacetamide 35a and more importantly sulfonamides 33a and 33b show moderate affinity at σ1 receptors (Ki = 109-208 nM).

Stereoselective synthesis of conformationally restricted KOR agonists based on the 2,5-diazabicyclo[2.2.2]octane scaffold

It has been postulated that the KOR affinity depends on the dihedral angle of the ethylenediamine pharmacophore. Herein, 2,5-diazabicyclooctanes bearing a pyrrolidino moiety in the 7-position were envisaged to study KOR agonists with a conformationally rigid ethylenediamine pharmacophore and thus a defined N(pyrrolidine)-C7-C1-N2 dihedral angle. The first approach with an intramolecular addition at the chiral sulfinylimines 9 failed to give bicyclic products. The key step in the second approach was a Dieckmann analogous cyclization providing mixed methyl silyl ketals 11a-e as key intermediates. The highest KOR affinity was found for the 2,5-dibenzyl substituted derivatives (S,R,S)-16a (K_i = 31 nM) and (R,S,R)-16a (K_i = 74 nM) with the pyrrolidine ring oriented towards N-5. The high KOR affinity of (S,R,S)-16a is unexpected, since the KOR pharmacophoric ethylenediamine system adopts a dihedral angle of about 160°, which is quite different from the angle of the energetically most favored conformer of the flexible and potent KOR agonist 2. (S,R,S)-16a represents a KOR agonist with moderate selectivity over MOR (8-fold) and DOR (5-fold), but high selectivity over both σ receptor subtypes. In the [³⁵S]GTPγS assay (S,R,S)-16a reacted as a full KOR agonist with an EC₅₀ value of 240 nM.

Design, synthesis and biological evaluation of novel tetrahydroisoquinoline quaternary derivatives as peripheral κ-opioid receptor agonists

A novel series of tetrahydroisoquinoline quaternary derivatives 4 were synthesized as peripheral κ-opioid receptor agonists. All the target compounds were evaluated in κ-opioid receptor binding assays, and compounds 4l, 4m, and 4n exhibited high affinity for κ-opioid receptor. Furthermore, compound 4l (κKi=0.94nM) produced potent antinociceptive activity in the mouse acetic acid-induced writhing assay, with lower sedative side effects than the parent compound MB-1c.

Synthesis and pharmacological evaluation of conformationally restricted κ-opioid receptor agonists

Additional N- and O-atoms in the bicyclic scaffold increase polarity and allow fine tuning of pharmacodynamic and pharmacokinetic properties of novel κ agonists.

Effects of polar κ receptor agonists designed for the periphery on ATP-induced Ca2+ release from keratinocytes

The very polar pyridylmethyl derivative 5a (log D_7.4 = 1.1) represents a potent and selective full κ-opioid receptor agonist (K_i = 0.13 nM, EC₅₀ = 33 nM), which reduced the release of Ca²⁺-ions into the cytoplasm in human keratinocytes.

Conformationally restricted κ-opioid receptor agonists: Synthesis and pharmacological evaluation of diastereoisomeric and enantiomeric decahydroquinoxalines

All diastereoisomeric decahydroquinoxalines representing conformationally restricted analogs of κ agonists U-50,488 and GR-89,696 have been prepared. Cis/trans configured compound 7 is by far the highest binding diastereoisomer with a Ki of 0.35 nM. Racemates 4, 6, and 7 were separated into enantiomers. (+)-(4aR,5S,8aS)-Configured enantiomer 7b was identified as a high affinity (Ki=0.25 nM) κ ligand with high selectivity over μ and δ receptors. It acts as full agonist with an EC50 value of 2.0 nM in the [(35)S]GTPγS assay, while enantiomer 7a showed an EC50 value of 1000 nM.

Synthesis and pharmacological evaluation of 5-pyrrolidinylquinoxalines as a novel class of peripherally restricted κ-opioid receptor agonists

5-Pyrrolidinyl substituted perhydroquinoxalines were designed as conformationally restricted κ-opioid receptor agonists restricted to the periphery. The additional N atom of the quinoxaline system located outside the ethylenediamine κ pharmacophore allows the fine-tuning of the pharmacodynamic and pharmacokinetic properties. The perhydroquinoxalines were synthesized stereoselectively using the concept of late stage diversification of the central building blocks 14. In addition to high κ-opioid receptor affinity they demonstrate high selectivity over μ, δ, σ1, σ2, and NMDA receptors. In the [35S]GTPγS assay full agonism was observed. Because of their high polarity, the secondary amines 14a (log D7.4=0.26) and 14b (log D7.4=0.21) did not penetrate an artificial blood-brain barrier. 14b was able to inhibit the spontaneous pain reaction after rectal mustard oil application to mice (ED50=2.35 mg/kg). This analgesic effect is attributed to activation of peripherally located κ receptors, since 14b did not affect centrally mediated referred allodynia and hyperalgesia.

Combination of cyclohexane and piperazine based κ-opioid receptor agonists: Synthesis and pharmacological evaluation of trans,trans-configured perhydroquinoxalines

Desymmetrization of the pseudochiral (2r)-configured cyclohexane-1,2,3-triamines 8 with dimethyl oxalate led to racemic aminoquinoxaline-2,3-diones 9. Selective introduction of the κ pharmacophoric structural elements pyrrolidine and 3,4-dichlorophenylacetamide with a two-carbon distance afforded conformationally restricted κ agonists 13-15 based on the quinoxaline ring system. In competitive radioligand receptor binding studies the benzylamine 13b, the secondary amine 14b, and the carbamate 15 displayed high κ receptor affinity. The Ki value of the lead compound derived methoxycarbonyl derivative 15 is 9.7nM. However, the κ affinity of 15 is exceeded by 13b and 14b with a basic functional group instead of the methoxycarbonyl group in 1-position of the quinoxaline system. The chlorine atoms of the dichlorophenylacetyl residue are essential, since the corresponding phenylacetyl analogs show considerably reduced κ affinity. The potent κ ligands 13b, 14b and 15 are selective over the related μ- and δ-opioid receptors, σ1, σ2 and NMDA receptors. In the [(35)S]GTPγS-binding assay 13b behaved as partial agonist with lower activity than U-69,593.

Bicyclic sigma receptor ligands by stereoselective Dieckmann analogous cyclization of piperazinebutyrate

Starting from racemic 2-aminoadipic acid (6) the piperazinedione 10 with a butyrate side chain was synthesized in four reaction steps. The four-carbon bridge was established upon deprotonation of 10 with LHMDS and subsequent trapping of the lithium alcoholate with Me(3)SiCl to give diastereoselectively the mixed methyl silyl ketal 12 in 94% yield. The relative configuration of the new center of chirality was determined by X-ray crystal structure analysis of 12. The high diastereoselectivity during the conversion of the butyrate 10 into the mixed methyl silyl ketal 12 supports the formation of the lithium alcoholate 11 as central intermediate. Stereoselective reduction of the ketone 13 with LiBH(4) led to the alcohol 14, which was benzylated and reduced to provide the final bicyclic products 16 and 17. Whereas the alcohol 16 shows only moderate affinity to both sigma receptor subtypes, the benzyl ether 17 represents a potent and selective sigma(1) receptor ligand (K(i) = 47 nM). Comparison of the sigma receptor affinities of 16 and 17 with the smaller homologues 5a and 5c clearly indicates that the size of the bridge and the O-substituent determines subtype selectivity.

Stereoselective synthesis and structure-affinity relationships of bicyclic kappa receptor agonists

Reductive amination of the bicyclic ketone 4 led diastereoselectively to endo-configured amines, which were transformed into the amides 7-10. The synthesis of the diastereomers 25 with an exo-configured amino moiety at position 6 was only successful after deactivation of both N-atoms of the 1,4-diazabicyclo[3.3.1]nonane system. The N-1-oxide 19 with an N-4-tosyl moiety was the crucial intermediate, which allows SN2 substitution with NaN3 under inversion of the configuration at position 6. Whereas the endo-configured pyrrolidine 7a (WMS-1302) revealed a kappa receptor affinity of 73 nM, the exo-configured diastereomer 25a was almost inactive at the kappa receptor (Ki > 1 microM). Replacement of the 3,4-dichlorophenylacetyl residue by other acyl and sulfonyl residues showed that it is essential for high kappa affinity. The kappa receptor affinities of the conformationally constrained pyrrolidines 7a and 25a were correlated with the dihedral angle N(pyrrolidine)-C-C-N(acetamide). A systematic conformational analysis of the potent but flexible kappa agonist 2 showed that a dihedral angle of 168 degrees (as in 25a) is energetically more disfavored than a dihedral angle of 58 degrees (7a). However, even the conformation with a dihedral angle of 58 degrees does not represent an energy minimum, which might explain the reduced kappa affinity of 7a.

Chemical function-based pharmacophore generation of selective kappa-opioid receptor agonists by catalyst and phase

Two chemical function-based pharmacophore models of selective kappa-opioid receptor agonists were generated by using two different programs: Catalyst/HypoGen and Phase. The best output hypothesis (Hypo1) of HypoGen consisted of five features: one hydrogen-bond acceptor (HA), three hydrophobic points (HY), and one positive ionizable function (PI). The highest scoring model (Hypo2) produced by Phase comprised four features: one acceptor (A), one positive ionizable function (P), and two aromatic ring features (R). These two models (Hypo1 and Hypo2) were then validated by test set prediction and enrichment factors. They were shown to be able to identify highly potent kappa-agonists within a certain range, and satisfactory enrichments were achieved. The features of these two pharmacophore models were similar and consistent with experiment data. The models produced here were also generally in accord with other reported models. Therefore, our pharmacophore models were considered as valuable tools for 3D virtual screening, and could be useful for designing novel kappa-agonists.

Synthesis of 1,4-Diazabicyclo[3.3.1]nonan-6-ones

1,4-Diazabicyclo[3.3.1]nonanes (aza-morphans) represent conformationally constrained piperazine derivatives. Herein, we report a six-step synthesis of the benzyl and allyl substituted bicyclic ketones 3a and 3b, which represent interesting building blocks for the synthesis of conformationally restricted receptor ligands. The key steps of the synthesis are the regioselective addition of ethyl acrylate to the piperazine 8, the sodium hexamethyldisilazide-induced Dieckmann cyclization of the diesters 10, and the decarboxylation of the enol esters 11 with dilute HCl. The complete sequence is only successful when a benzyl (10a) or allyl moiety (10b) is attached to N-1, since the tosyl derivative 10f failed to give a Dieckmann cyclization product, and the decarboxylation failed with the acyl derivatives 11c and 11d.

Chemical function-based pharmacophore development for novel, selective kappa opioid receptor agonists

In an effort to reduce or eliminate the centrally associated side effects produced by opioid analgesics there has been an interest in the preparation of peripherally acting opioid receptor agonists. These compounds would have very limited or no access to the central nervous system. As a first step towards developing peripheral kappa opioid receptor (KOP) agonists, we have developed a quantitatively predictive chemical function-based pharmacophore model of selective kappa opioid receptor agonists by using the HypoGen algorithm implemented in the Catalyst software. The input for HypoGen was a training set of 26 KOP agonists exhibiting K(i) values ranging between 0.015nM and 2300nM. The best output hypothesis consists of four features: one hydrophobic (HYD), one ring aromatic (RA), one hydrogen bond acceptor (HBA), and one positive ionizable (PI) function. The predictive power of the model could be demonstrated by internal and external validation of the generated hypothesis. The resulting Catalyst pharmacophore can be used concurrently for rapid virtual screening of chemical databases to identify novel, selective KOP agonists that may be easily restricted to target tissues by synthetic modification. It is anticipated that such an approach will lead to the generation of novel selective KOP agonists that are clinically useful for the treatment of pain through peripheral mechanisms.

Structure−Activity Relationships of αS1-Casomorphin Using AM1 Calculations and Molecular Dynamics Simulations

This paper investigates the structure-activity relationships of alphaS1-casomorphin (alphaS1-CM) using AM1 calculations and molecular dynamics (MD) simulations. Previous studies have shown that this peptide has remarkable opioid actions, and not only has a high affinity toward all three subtypes (kappa1-kappa3) of the kappa-opioid sites, but also inhibits the proliferation of the T47D human breast cancer cell line. The systematic conformer search performed by the AM1 calculations is based on the torsional angles of the Val2-Pro3 (omega2) and Phe4-Pro5 (omega4) amide bonds. The AM1 results reveal that the alphaS1-CM conformers strongly favor the cis/cis pair of the omega2/omega4 amide bonds in the minimized energy state. Furthermore, the picture of these stable conformers is found to be a strong interaction of the coulomb's force between two terminuses. MD simulations are performed to investigate the features of both the structural stability and pharmacological activity of alphaS1-CM in aqueous solution. The simulation results reveal that the omega2/omega4 amide bonds favor the cis/cis status in the stable state. Furthermore, the pharmacophoric distance between two aromatic rings is found to be 5.0 approximately 5.4A. The chi1 rotamers of the Tyr and Phe residues show a preference for gauche (-) and trans, respectively. The side chain rotamers of alphaS1-CM are competed to those of other opioid ligands with a known potency and selectivity for delta- and mu-opioid receptors. Finally, we address a likely kappa pharmacophore model compared to the delta pharmacophore model.

Novel malonamide derivatives as potent κ opioid receptor agonists

A novel series of malonamide derivatives was synthesized. These amides were shown to be potent and selective kappa opioid receptor agonists.

Synthesis of bridged piperazines with sigma receptor affinity

Bridged piperazines 4 were designed as conformationally restricted piperazine sigma receptor ligands. The chiral pool synthesis started from (S)-glutamate, which was transformed in five reaction steps into the piperazinediones 5 bearing a propionic acid ester side chain. A two-step Dieckmann analogous cyclization provided the bicyclic ketones 7 as key intermediates. The alcohols 8 were prepared by LiAlH4 reduction of the ketones 7. NaBH4 reduction, Williamson ether synthesis and LiAlH4 reduction led to the methyl and benzyl ethers 12 and 13. High sigma1 affinity is attained when one large substituent is introduced either at N-8 or O-2. The most potent sigma1 ligand in this series of compounds is the methyl ether 12b with the N-butyl substituent (K(i)=13.2 nM, selectivity sigma2:sigma1 = 16). Moreover, the N-methyl derivatives 13a (sigma2: K(i)=30.4 nM) and 12a (sigma2 preference) represent promising starting points for the development of potent and selective sigma2 ligands.

Synthesis of novel σ-receptor ligands from methyl α-d-mannopyranoside

For the first time a monosaccharide (methyl alpha-D-mannopyranoside) has been used as starting material for the synthesis of novel sigma-receptor ligands. The hept-3-ulopyranoside dimethyl ketals 14 and 15 were obtained from the nitrile 7 via two synthetic routes. After selective hydrolysis of the ketone dimethyl acetal, various amino substituents were introduced into position 3. High sigma(1)-receptor affinity and selectivity was attained with equatorially arranged amino substituents in position 3 and a dichlorophenylacetamide moiety in position 7. The anomeric mixture of dimethylamines 26alpha/beta displayed the highest sigma(1)-receptor affinity (K(i)=21nM) within this small series of test compounds.

Synthesis and receptor binding studies of 3-substituted piperazine derivatives

In order to find novel receptor ligands various substituents were introduced into the side chain in position 3 of the piperazine 5. During nucleophilic substitution of the hydroxy group of 5 aziridinium ions were formed, resulting in rearranged 1,4-diazepanes and piperazines as side products. 1,2-anellated piperazines 15, 18 and 19 were prepared by hydrogenation of the alpha,beta-unsaturated ester 13 and by condensation of the primary amine 16b with formaldehyde, respectively. Receptor binding studies with radioligands revealed that the phenylacetamide 17b interacts with moderate affinity (K(i) = 181 nM) and considerable selectivity with sigma(1) receptors.

Novel phenylamino acetamide derivatives as potent and selective kappa opioid receptor agonists

A novel series of phenylamino acetamide derivatives was synthesized. These amides were shown to be potent and selective kappa opioid receptor agonists.

Amino acid conjugates as κ opioid receptor agonists

A novel series of kappa (kappa) opioid receptor agonists were synthesized by incorporating the key structural features of known kappa opioid agonists while replacing the aryl acetamide portion with substituted amino acid conjugates. Compounds 3j (Ki = 6.7 nM), 3k (Ki = 3.6 nM), 3l (Ki = 4.6 nM), 3m (Ki = 0.83 nM) and 3o (Ki = 2 nM) possessed potent affinities for the kappa opioid receptor in vitro with reasonable selectivity over other opioid receptors.

Chiral, nonracemic (piperazin-2-yl)methanol derivatives with sigma-receptor affinity

Starting with the proteinogenic amino acid (S)-serine a series of chiral nonracemic (piperazin-2-yl)methanols 3 with various N-4 substituents is described. The key step in the synthesis of 3 is the reaction of the chloroacetamide 5 with various primary amines to yield the diastereomeric bicyclic piperazinediones cis-6 and trans-6. The scope and limitation of this transformation is thoroughly investigated. The alpha1- and sigma2-receptor affinities of the piperazines 3 are determined in receptor binding studies with guinea pig brain and rat liver membrane preparations using [3H]-labeled (+)-pentazocine and ditolylguanidine, respectively. It was found, that an additional phenyl residue in the N-4 substituent is favorable to high sigma1-receptor affinity. In this series the p-methoxybenzyl substituted piperazine 3d reveals the highest sigma1-receptor affinity (Ki=12.4 nM) with selectivity toward sigma2-, NMDA-, kappa-opioid, and mu-opioid receptors.

Recent advances in selective opioid receptor agonists and antagonists

Opioid analgesics provide outstanding benefits for relief of severe pain. The mechanisms of the analgesia accompanied with some side effects have been investigated by many scientists to shed light on the complex biological processes at the molecular level. New opioid drugs and therapies with more desirable properties can be developed on the bases of accurate insight of the opioid ligand-receptor interaction and clear knowledge of the pharmacological behavior of opioid receptors and the associated proteins. Toward this goal, recent advances in selective opioid receptor agonists and antagonists including opioid ligand-receptor interactions are summarized in this review article.

Conformationally constrained ethylenediamines: synthesis and receptor binding of 6,8-diazabicyclo[3.2.2]nonanes

The synthesis and receptor affinity of 6,8-diazabicyclo[3.2.2]nonanes representing conformationally constrained ethylenediamines are described. The Dieckmann analogous cyclization of the (piperazin-2-yl)propionate 9 provided the bicyclononane 10 only, when the first cyclization product was trapped with chlorotrimethylsilane. 10 was stereoselectively transformed into the bicyclic amines 19a,b and amides 22a,b, which were investigated in competition experiments with radioligands for their sigma(1)-, sigma(2)-, kappa-, and mu-receptor affinities. The (2R)-configured dimethylamine 19a showed promising sigma(1)-receptor affinity (K(i)=23.8 nM) and selectivity, whereas the (2S)-configured (dichlorophenyl)acetamide 22b displayed a sigma-receptor binding profile (sigma(1): K(i)=184 nM; sigma(2): K(i)=263 nM) very similar to the binding profile of the atypical antipsychotic BMY-14802 (26).

Novel spiropiperidines as highly potent and subtype selective sigma-receptor ligands. Part 1

A series of spiro[[2]benzopyran-1,4'-piperidines] and spiro[[2]benzofuran-1,4'-piperidines] of general structure 10 is prepared, and the affinity for sigma(1)- and sigma(2)-receptors is investigated by means of radioligand binding assays. The synthesis of the spiropiperidines 14a and 23 proceeds from bromine/lithium exchange of the bromoacetals 11 and 21, addition to piperidin-4-one 12a, and subsequent cyclization. Systematic variations of the substituent R at the nitrogen atom, the group X in position 3, and the ring size of the oxygen heterocycle are performed. The sigma(1)- and sigma(2)-receptor affinities are determined with guinea pig brain and rat liver membrane preparations using [(3)H]-labeled (+)-pentazocine and ditolylguanidine, respectively. Test results show that a benzyl residue at the piperidine nitrogen atom and a methoxy group in position 3 are advantageous for high sigma(1)-receptor affinity. In this series the 1'-benzyl-3-methoxy-3,4-dihydrospiro[[2]benzopyran-1,4'-piperidine] (14a) and the 1'-benzyl-3-methoxy-3H-spiro[[2]benzofuran-1,4'-piperidine] (23) are among the most potent sigma(1)-ligands interacting in the low nanomolar range with sigma(1)-receptors (14a, K(i) = 1.29 nM; 23, K(i) = 1.14 nM). Variation of the nitrogen substituent R from benzyl to H, alkyl, phenyl, or omega-phenylalkyl and the group X from methoxy to hydroxy, carbonyl, or alkyloxy led to reduced sigma(1)-receptor affinity. In addition to their high sigma(1)-receptor affinity, the spiropiperidines 14a and 23 display excellent selectivity toward sigma(2)-receptors (sigma(1)/sigma(2) = 2708 and 1130) and several other receptor and reuptake systems. Introduction of a polar hydroxy group in position 3 and elongation of the distance between the piperidine nitrogen atom and the phenyl moiety result in ligands with considerable sigma(2)-receptor affinity and therefore diminished sigma(1)/sigma(2)-receptor selectivity. The hemiacetalic 1'-(3-phenylpropyl)-3,4-dihydrospiro[[2]benzopyran-1,4'-piperidin]-3-ol (15e) represents the most active sigma(2)-receptor ligand in this series with a K(i) value of 83.1 nM.