Skip to main content
NCBI home page
RSC Medicinal Chemistry logoLink to RSC Medicinal Chemistry
. 2020 Dec 9;12(2):237–244. doi: 10.1039/d0md00307g

Synthesis and σ receptor affinity of spiro[[2]benzopyran-1,1′-cyclohexanes] with an exocyclic amino moiety in the 3′-position†

Elisabeth Kronenberg 1, Frauke Weber 1, Dirk Schepmann 1, Bernhard Wünsch 1,✉
PMCID: PMC8127328  PMID: 34046612

Abstract

The main functions of σ1 receptors include the modulation of release and reuptake of neurotransmitters, the regulation of ion channels and the influence on intracellular signaling through modulation of calcium levels. Due to these properties, σ1 receptors are interesting drug targets for the treatment of various neurological disorders, pain and cancer. In order to modify the distance between the pharmacophoric elements (the benzene ring of 2-benzopyran and an amino moiety), a set of spiro[[2]benzopyran-1,1′-cyclohexan]-3′-amines was synthesized. The key step of the synthesis was a Parham cyclization of 1-bromo-2-(2-bromoethyl)benzene (6) with the mono ketal 7 of cyclohexane-1,3-dione, which led in a one-pot reaction to the spirocyclic framework 8. Reductive amination of ketone 9 stereoselectively provided secondary amines cis-4, which were methylated to afford tertiary amines cis-5. Whereas spirocyclic compounds cis-4a and cis-5a bearing a benzyl moiety at the exocyclic amino moiety showed rather low σ1 affinity, the corresponding cyclohexylmethyl derivatives cis-4b and cis-5b exhibited low nanomolar σ1 affinity. The secondary amine cis-4b displayed the highest σ1 receptor affinity (Ki = 5.4 nM) in this series. Methylation of the secondary amine cis-4b led to a slightly decreased σ1 receptor affinity of cis-5b (Ki = 15 nM).

Novel spirocyclic σ1 ligands with defined distances between the basic amino moiety and the phenyl rings were designed, synthesized and pharmacologically evaluated, in order to analyze the structural features crucial for high σ1 affinity.graphic file with name d0md00307g-ga.jpg

1. Introduction

First described in 1976 by Martin et al., σ receptors were initially accounted to the opioid receptor family.1 Further investigation revealed a pharmacological profile deviating from the profile of opioid receptors.2,3 Thus, σ receptors became their own receptor class consisting of two subtypes termed σ1 and σ2 receptors.4 Both subtypes differ in their molecular weight, their distribution in different tissues and their ligand binding profile.

σ 1 receptors are localized in the central nervous system and in peripheral tissues including liver, kidney and heart.5,6 On the cellular level, σ1 receptors are found in the membrane of the endoplasmic reticulum and at mitochondria-associated membranes.7 In 2016, Kruse et al. published the X-ray crystal structure of the σ1 receptor revealing the receptor as a trimer in the solid state with one transmembrane domain in each protein. The carboxy and amino termini are located on opposite sides of the membrane.8,9σ1 receptors play an important role in the modulation of ion channels, neurotransmitter systems and intracellular calcium homeostasis.10,11

Due to their modulating properties and the high expression in the central nervous system, σ1 receptors are associated with the pathophysiology of several neurological disorders like Parkinson's and Alzheimer's disease, psychosis and depression.12–16σ1 receptor antagonists can be used for the treatment of neuropathic pain.17,18 Moreover, various human tumor cell lines show an increased σ1 receptor expression level compared to non-tumor cells and it is reported that σ1 receptors are involved in programmed cell death. Therefore, σ1 receptors are promising targets for the development of innovative cancer treatment and for tumor diagnosis.19–24

In contrast, much less is known about the σ2 receptor, which has a molecular weight of 21.5 kDa. Very recently, it could be isolated from calf liver preparations and was identified as endoplasmic reticulum resident transmembrane protein 97 (TMEM97).25σ2 receptors are found in the central nervous system26,27 and in several peripheral tissues, including liver and the gastrointestinal tract. In particular, high expression levels were detected in fast proliferating tumor cells. Thus, ligands addressing the σ2 receptor subtype are useful as antitumor agents and as tumor markers allowing tumor cells with a high σ2 receptor density to be visualized.28–30

Several σ1 receptor ligands based on various scaffolds have been reported in the literature.31 We are particularly interested in spirocyclic systems provided with a basic amino moiety. Due to the spirocyclic structure, such types of ligands are conformationally restricted allowing the decoration of the system with various substituents in a predefined three-dimensional orientation to each other. During the last few years, we have reported several spirocyclic piperidines with a very high σ1 receptor affinity and selectivity over the σ2 subtype.32–37 Pharmacophore models recommend a defined distance between the hydrophobic residues and the basic amino moiety.38,39 High σ1 affinity was achieved for spirocyclic piperidines 1 (Ki = 0.69 nM) and 2 (Ki = 1.3 nM) with an endocyclic amino moiety (piperidine ring, Fig. 1).32 For these types of spirocyclic piperidines, distances of 5.6–5.7 Å and 5.1–5.2 Å between the benzene ring of the 2-benzopyran system and the basic amino moiety (piperidine N-atom) were determined for conformations with equatorially and axially oriented phenyl moieties at the piperidine ring (spiro center), respectively.39,40

Fig. 1. Spirocyclic cyclohexanamines 4 and 5 with an exocyclic amino moiety in the 3′-position were derived from spirocyclic piperidines 1 and 2 with an endocyclic amino moiety and spirocyclic cyclohexanamine 3 with an exocyclic amino moiety in the 4′-position. The stereodescriptors cis and trans refer to the relative orientation of the O- and N-substituents at the central cyclohexane ring.

Fig. 1

Open in a new tab

According to the pharmacophore models, the distance between the benzene ring and the basic amino group should be longer, i.e. in the range of 6–10 Å. Shifting the basic functional group from inside the piperidine ring (1 and 2) at the 4′-position of a cyclohexane ring leads to increased distances between the benzene ring of the benzopyran system and the basic amino group. Depending on the equatorial or axial orientation of the phenyl moiety at the cyclohexane ring, the corresponding distances for trans-3 are 6.0–6.3 Å and 6.5–6.6 Å, respectively, and those for cis-3 are 7.2 Å and 6.3 Å, respectively. Although these distances fit exactly into the pharmacophore models, the increased distances led to a reduced σ1 affinity: the benzyl derivatives trans-3a and cis-3a exhibit Ki-values of 256 nM and 97 nM, respectively. However, replacement of the benzyl moiety at the N-atom of 3a with a cyclohexylmethyl moiety resulted in trans-3b and cis-3b displaying 10-fold and 5.5-fold increased σ1 affinity, respectively39,40 (Fig. 1 and Table 1).

σ 1 and σ2 receptor affinity of the spirocyclic cyclohexanes with an amino moiety in the 3′-position and reference compounds.

graphic file with name d0md00307g-u1.jpg
Compd. NR2 Distancea (Å) K i ± SEM [nM]
σ 1 σ 2
1 (ref. 32) — 5.6 - 5.7 Å

5.1–5.2 Å

 0.69 ± 0.17 99.7 ± 19.8
trans-3a (ref. 40) NHCH2C6H5 6.0–6.3 Å

6.5–6.6 Å,

 256 ± 72 13%c
cis-3a (ref. 40) NHCH2C6H5 7.2 Å

6.3 Å

 97 ± 4.8 503b
trans-3b (ref. 40) NHCH2C6H11 6.0–6.3 Å

6.5–6.6

 15 ± 2.6 99 ± 17
cis-3b (ref. 40) NHCH2C6H11 7.2 Å

6.3 Å

 18 ± 3.8 84 ± 19
cis-4a NHCH2C6H5 5.9–6.1 Å 95 ± 35 340b
cis-4b NHCH2C6H11 5.9–6.1 Å 5.4 ± 2.8 60 ± 40
cis-5a N(CH3)CH2C6H5 5.5–5,7 Å 278 ± 2 24%c
cis-5b N(CH3)CH2C6H11 5.5–5.7 Å 15 ± 4.6 34%c
(+)-pentazocine 5.4 ± 0.5 —
di-o-Tolylguanidine 71 ± 8 54 ± 8
Open in a new tab
a

Distance refers to the distance between the center of the benzene ring of the 2-benzopyran and the N-atom; the value in the first line defines the distance, when the benzene ring of the 2-benzopyran adopts the equatorial orientation at the cyclohexane ring and the second value describes the distance for the axial orientation of the benzene ring.

b

Result from one measurement.

c

Inhibition of radioligand binding (%) at a test compound concentration of 1 μM.

In this study, spirocyclic cyclohexanes of type 4 and 5 bearing an exocyclic amino moiety in the 3′-position were investigated. Attachment of the amino moiety at the 3′-position instead of the 4′-position leads to a reduced distance between the benzene moiety of the benzopyran ring and the basic amino moiety. Thus, for trans-4 and 5 and cis-4 and 5, distances of 6.2–6.3 Å (trans) and 5.9–6.1 Å (cis, axial amine)/5.5–5.7 Å (cis, equatorial amine), respectively, were calculated, which are very similar to the distances in the spirocyclic piperidines 1 and 2. Whereas only one substituent at the piperidine ring of spirocyclic piperidines 1 and 2 can be modified, a broader diversification of the exocyclic amino moiety by the introduction of one (secondary amines 4) or two substituents (tertiary amines 5) is possible.

2. Synthesis

1-Bromo-2-(2-bromoethyl)benzene (6) served as a starting material for the synthesis of 3′-substituted spirocyclic 2-benzopyrans cis-4a and b and cis-5a and b. The Parham cyclization41 was performed by treatment of aryl bromide 6 with n-BuLi and subsequently with monoethylene ketal-protected cyclohexane-1,3-dione 7.42,43 Due to the high CH-acidity of cyclohexane-1,3-dione, one carbonyl moiety had to be protected before the reaction with aryllithium. In the Parham cyclization, an alcoholate was formed by the reaction of the aryllithium intermediate with ketone 7. A subsequent intramolecular SN2 reaction of the alcoholate with the bromoethyl moiety led to the spirocyclic compound 8 in 28% yield. Ketone 9 was obtained in 85% yield by hydrolysis of ethylene ketal 8 with 2 M HCl in diethyl ether (Scheme 1).

Scheme 1. Synthesis of spirocyclic cyclohexanes cis-4a and b and cis-5a and b with an exocyclic amino function in the 3′-position. Reagents and reaction conditions: (a) THF, n-BuLi, cyclohexane-1,3-dione monoethylene ketal (7), 5 min, −88 °C, 1 h, rt, 28%. (b) Et2O, 2 M HCl, 2 d, reflux, 85%. (c) RNH2, CH3CO2H, NaBH(OAc)3, THF, 24 h, rt, 90% (cis-4a), 80% (cis-4b). (d) Formalin 37%, NaBH(OAc)3, CH2Cl2, 3 h, rt, 92% (cis-5a), 88% (cis-5b). Only one enantiomer of the racemic mixtures is shown. The stereodescriptor cis refers to the relative orientation of the O- and N-substituents at the central cyclohexane ring.

Scheme 1

Open in a new tab

Reductive amination of ketone 9 with benzylamine and cyclohexylmethylamine in the presence of reducing agent NaBH(OAc)3 44 and one equivalent of HOAc provided the secondary amines cis-4a (90%) and cis-4b (80%). This reaction proceeded with high diastereoselectivity, since only the cis-configured secondary amines cis-4a and cis-4b were formed. The high diastereoselectivity is explained by an H-bond between the intermediate iminium ion and the axially oriented O-atom of the benzopyran ring. This H-bond stabilizes the transition state leading to an axially oriented amino moiety in the 3′-position by equatorial transfer of hydride from NaBH(OAc)3. The ddd at 2.32 ppm for the equatorially oriented 2′-Heq, the dd at 1.79 ppm for the axially oriented 2′-Hax and the rather narrow signal for 3′-Heq at 2.98–3.04 ppm (width of 20 Hz) exclude an axial orientation of 3′-H and thus proves unequivocally the axial position of the benzylamino moiety of cis-4a.

The secondary amines cis-4a and cis-4b were finally methylated with formalin and NaBH(OAc)3 44 to afford the tertiary amines cis-5a (92%) and cis-5b (88%), respectively. Unexpectedly, an inversion of the chair conformation of the cyclohexane ring was observed for the tertiary amines cis-5. The tt for 3′-H at 3.01 ppm (J = 9.9/4.4 Hz) with two large coupling constants of 9.9 Hz towards the axially oriented neighbor protons in the 2′- and 4′-positions confirms the axial orientation of 3′-Hax. Thus, the NR2-moiety has to adopt the equatorial position. The conformational change occurring during methylation is explained by the stabilizing H-bond between the NH moiety and the axially oriented O-atom of the benzopyran ring of secondary amine 4. After methylation, this H-bond is no longer possible leading to a conformational change with two substituents NR2 and OR in the energetically favored equatorial position.

3. σ 1 and σ2 receptor affinity

The σ1 and σ2 receptor affinity of the synthesized spirocyclic 2-benzopyrans was determined in competition experiments with radioligands.45–47 Briefly, homogenates of guinea pig brain and rat liver served as sources of σ1 and σ2 receptors, respectively. [3H]-(+)-pentazocine was used as a radioligand in the σ1 assay. The nonspecific binding was determined with an excess of nonlabeled (+)-pentazocine. Due to the missing availability of a selective σ2 receptor radioligand, [3H]-di-o-tolylguanidine (DTG) was used as an unselective radioligand in the presence of an excess of (+)-pentazocine to mask the σ1 receptors. Compounds with high affinity were tested three times (n = 3).45–47 For compounds with low σ affinity, only the inhibition of the radioligand binding at a concentration of 1.0 μM is given.

The spirocyclic 2-benzopyrans cis-4a and cis-5a bearing a benzylamino moiety in the 3′-position show a similar low σ1 affinity as the 2-benzopyrans trans-3a and cis-3a with the benzylamino moiety located in the 4′-position. (Table 1) The crucial distance between the center of the benzene ring and the N-atom of cis-4a (5.9–6.1 Å) and cis-5a (5.5–5.7 Å) is shorter than the distance in trans-3a and cis-3a (6.0–7.2 Å), but close to the distance in the high-affinity spirocyclic piperidine derivative 1 (5.1 Å/5.6 Å). It is assumed that the low affinity of cis-4a and cis-5a compared to the high affinity of 1 is due to the different orientation of the pharmacophoric structural elements, in particular, the benzylamino moiety relative to the benzopyran system.

As observed for the spirocyclic cyclohexanes trans-3b and cis-3b, replacement of the benzyl moiety with a cyclohexylmethyl moiety at the amino group led to considerably increased σ1 affinity of cis-4b and cis-5b. At 5.4 nM and 15 nM, their Ki values are in the low nanomolar range and comparable with the Ki values of the cyclohexylmethylamines trans-3b and cis-3b. However, both compounds do not reach the subnanomolar affinity of the spirocyclic piperidine 1 (Ki = 0.69 nM).

In contrast to the result of recent studies,40 transformation of the secondary amines cis-4a and cis-4b into methylated tertiary amines cis-5a and cis-5b led to 3-fold decreased σ1 receptor affinity.

Spirocyclic cyclohexanes 4 and 5 with an amino moiety in the 3′-position display a very low σ2 affinity indicating the high selectivity for σ1 over σ2 receptors. N-Methylation of secondary amines cis-4 leading to tertiary amines cis-5 resulted in a particularly low σ2 affinity and thus high σ1:σ2 selectivity.

Since the σ receptor was originally classified as an opioid receptor, the affinity of the cyclohexylmethylamines cis-4b and cis-5b towards the opioid receptors MOR, KOR and DOR was determined in receptor binding studies. Up to a concentration of 1 μM, the tertiary methylamine cis-5b did not interact with all three opioid receptors. However, the most potent σ1 ligand cis-4b showed a moderate MOR (Ki = 696 ± 180 nM), DOR (Ki = 150 ± 108 nM), and KOR affinity (Ki = 407 ± 165 nM). The selectivity for the σ1 receptor over the three opioid receptors is at least 25-fold (over DOR). Also, the low-σ1-affinity benzylamines cis-4a and cis-5a did not display MOR affinity up to a concentration of 1 μM.

4. Conclusions

A small series of spirocyclic cyclohexanes with an amino moiety in the 3′-position was synthesized and pharmacologically evaluated. Unexpectedly, reductive amination of the spirocyclic cyclohexanone 9 stereoselectively provided only cis-configured amines cis-4 and cis-5. Therefore, only cis-configured amines were investigated.

Whereas the σ1 affinity of the benzylamines cis-4a and cis-5a is rather low, the σ1 affinity of the corresponding cyclohexylmethylamines cis-4b and cis-5b is in the low nanomolar range. The secondary amine cis-4b represents the most active σ1 ligand (Ki = 5.4 nM) of this series of ligands. However, the σ1 affinity of cis-4a does not reach the σ1 affinity of the spirocyclic piperidines 1 (Ki = 0.69 nM) and 2 (Ki = 1.3 nM). Since the distance of the benzene moiety of the 2-benzopyran and the N-atom is very similar for the spirocyclic piperidines 1 and 2 (5.1 Å/5.6 Å) and the cyclohexanes cis-4 (5.9–6.1 Å) and cis-5 (5.5–5.7 Å), it was concluded that in addition to the correct distance, the orientation of the pharmacophoric elements is crucial for high σ1 affinity. In particular, the H-bond donor or basic functional group of the spirocyclic cyclohexanes cis-4 and cis-5 adopts an orientation different from the spirocyclic piperidines 1 and 2. Although an equatorial orientation of the tertiary amines cis-5a and cis-5b was shown by 1H NMR spectroscopy (CDCl3), in a buffer system at pH 7.4, the amino moiety will be protonated and able to form the same H-bond as the secondary amines cis-4a and cis-4b. Altogether, further pharmacological properties of these types of spirocyclic σ1 ligands were not further investigated.

5. Experimental part

5.1. Chemistry and general methods

Oxygen and moisture sensitive reactions were carried out under nitrogen, dried with silica gel with a moisture indicator (orange gel, VWR, Darmstadt, Germany) and in dry glassware (Schlenk flask or Schlenk tube). Temperature was controlled with dry ice/acetone (−78 °C), ice/water (0 °C), a cryostat (Julabo TC100E-F, Seelbach, Germany), a magnetic stirrer MR 3001 K (Heidolph, Schwalbach, Germany) or an RCT CL (IKA, Staufen, Germany), together with a temperature controller EKT HeiCon (Heidolph) or a VT-5 (VWR) and PEG or silicone bath. All solvents were of analytical or technical grade quality. Demineralized water was used. CH2Cl2 was distilled from CaH2; THF was distilled from sodium/benzophenone; MeOH was distilled from magnesium methanolate. Thin layer chromatography (TLC): TLC silica gel 60 F254 on aluminum sheets (VWR). Flash chromatography (FC): silica gel 60, 40–63 μm (VWR); parentheses include: the diameter of the column (ϕ), the length of the stationary phase (l), and the eluent and fraction size (v). Melting point: melting point system MP50 (Mettler Toledo, Gießen, Germany), open capillary, uncorrected. MS: MAT GCQ (Thermo-Finnigan); EI = electron impact; Thermo Finnigan LCQ® ion trap mass spectrometer with an ESI = electrospray ionization interface. IR: IR spectrophotometer 480Plus FT-ATR-IR (Jasco). 1H NMR (400 MHz), 13C NMR (100 MHz): Mercury-400BB spectrometer (Varian); δ in ppm related to tetramethylsilane; coupling constants are given with 0.5 Hz resolution.

5.2. HPLC method for the determination of the purity

Equipment 1: pump: L-7100, degasser: L-7614, autosampler: L-7200, UV detector: L-7400, interface: D-7000, data transfer: D-line, data acquisition: HSM-software (all from Merck Hitachi, Darmstadt, Germany); equipment 2: pump: LPG-3400SD, degasser: DG-1210, autosampler: ACC-3000T, UV-detector: VWD-3400RS, interface: DIONEX UltiMate 3000, data acquisition: Chromeleon 7 (equipment and software from Thermo Fisher Scientific, Lauenstadt, Germany); column: LiChrospher® 60 RP-select B (5 μm), LiChroCART® 250–4 mm cartridge; flow rate: 1.0 mL min−1; injection volume: 5.0 μL; detection at λ = 210 nm; solvents: A: demineralized water with 0.05% (V/V) trifluoroacetic acid, B: CH3CN with 0.05% (V/V) trifluoroacetic acid; gradient elution (% A): 0–4 min: 90%; 4–29 min: gradient from 90% to 0%; 29–31 min: 0%; 31–31.5 min: gradient from 0% to 90%; 31.5–40 min: 90%. Unless otherwise mentioned, the purity of all the test compounds is greater than 95%.

5.3. Synthetic procedures

The synthesis of cyclohexane-1,3-dione monoethylene ketal (7)42,43 is described in the ESI.†

3,4-Dihydrospiro[[2]benzopyran-1,1′-cyclohexan]-3′-one ethylene ketal (8)

1-Bromo-2-(2-bromoethyl)benzene (6, 118 mg, 0.45 mmol) was dissolved in THF (5 mL) under a N2 atmosphere and the solution was cooled to −88 °C. Subsequently, n-BuLi (1.6 M in n-hexane. 0.33 mL, 0.93 mmol) was added dropwise. After 5 min at −88 °C, a solution of cyclohexane-1,3-dione monoethylene ketal (7, 77 mg, 0.49 mmol) in THF (2 mL)) was added rapidly and the mixture was stirred at −88 °C for 5 min and 2 h at rt. H2O (10 mL) was added and the mixture was extracted with Et2O (3 × 30 mL). The combined organic layers were dried (K2CO3), concentrated in vacuo and the residue was purified by FC (ϕ 3 cm, 20 cm, cyclohexane : ethyl acetate 4 : 1, 20 mL). Rf (cyclohexane : ethyl acetate 4 : 1 = 0.14). Pale yellow oil, yield 33 mg (28%). C16H20O3 (260.4). MS (EI): m/z (%) = 260 [M, 12], 232 [M–CH2 Created by potrace 1.16, written by Peter Selinger 2001-2019 CH2, 34], 217 [M–CH3CH2CH2*, 100], 159 [M–CH3CketalCH2*, 60], 146 [M–CH2Cketal, –CH2 Created by potrace 1.16, written by Peter Selinger 2001-2019 CH2, 61], 86 [C*H2C( Created by potrace 1.16, written by Peter Selinger 2001-2019 O + CH2CH2O), 31]. FT-IR: ṽ (cm−1) = 2945, 2921, 2862 (s, v, C–H, alkyl), 1492 (w, v, C Created by potrace 1.16, written by Peter Selinger 2001-2019 C, arom), 1087 [s, v, C–O), 765 (s, δ, C–H, o-disubst. arom). 1H NMR (CDCl3): δ (ppm) = 1.54–1.69 (m, 3H, CH2CH2CH2C Created by potrace 1.16, written by Peter Selinger 2001-2019 O), 1.89–2.07 (m, 4H, CH2CH2CH2C Created by potrace 1.16, written by Peter Selinger 2001-2019 O (3H), CCH2C(OCH2CH2O) (1H)), 2.19 (dt, J = 14.7/2.5 Hz, 1H, CCH2C(OCH2CH2O), 2.77 (“dt”, J = 16.2/5.1 Hz, 1H, ArCH2CH2O), 2.89 (ddd, J = 16.2/6.9/4.9 Hz, 1H, ArCH2CH2O), 3.81–4.15 (m, 6H, OCH2CH2O (4H), ArCH2CH2O (2H)), 7.07–7.20 (m, 4H, Ar–H). Purity (HPLC): 96.2%, tR = 17.3 min.

3,4-Dihydrospiro[[2]benzopyran-1,1′-cyclohexan]-3′-one (9)

Ketal 8 (85 mg, 0.33 mmol) was dissolved in Et2O (10 mL) and HCl (2 M, 10 ml) was added. The mixture was heated to reflux for 48 h. The mixture was extracted with Et2O (3 × 40 mL) and the combined organic layers were dried (K2CO3), concentrated in vacuo and the residue was purified by FC (ϕ 2 cm, 20 cm, cyclohexane : ethyl acetate 9 : 1, 10 mL). Rf (cyclohexane : ethyl acetate 9 : 1 = 0.12, cyclohexane : ethyl acetate 4 : 1 = 0.20). Colorless solid, mp 91 °C, yield 60 mg (85%). C14H16O2 (216.3). MS (EI): m/z (%) = 216 [M, 25], 188 [M–CH2 Created by potrace 1.16, written by Peter Selinger 2001-2019 CH2, 11], 173 [M–CH3CH2CH2*, 47], 159 [M–CH3C Created by potrace 1.16, written by Peter Selinger 2001-2019 OCH2*, 100]. FT-IR: ṽ (cm−1) = 3062, 3025 (w, v, C–H, arom), 2939, 2869 (s, v, C–H, Alkyl), 1712 (s, v, C Created by potrace 1.16, written by Peter Selinger 2001-2019 O), 1491 (w, v, C Created by potrace 1.16, written by Peter Selinger 2001-2019 C, arom), 1085 (s, v, C–O), 754 (s, δ, C-H, o-disubst. arom). 1H NMR (CDCl3): δ (ppm) = 1.93–2.01 (m, 1H, CH2CH2CH2C Created by potrace 1.16, written by Peter Selinger 2001-2019 O), 2.03–2.22 (m, 3H, CH2CH2CH2C Created by potrace 1.16, written by Peter Selinger 2001-2019 O), 2.34–2.42 (m, 1H, CH2CH2CH2C Created by potrace 1.16, written by Peter Selinger 2001-2019 O), 2.44–2.52 (m, 1H, CH2CH2CH2C Created by potrace 1.16, written by Peter Selinger 2001-2019 O), 2.72 (s, 2H, CH2C Created by potrace 1.16, written by Peter Selinger 2001-2019 O), 2.79 (“dt”, J = 16.2/5.5 Hz, 1H, ArCH2CH2O), 2.85 (“dt”, J = 16.3/5.5 Hz, ArCH2CH2O), 3.86 (ddd, J = 11.5/6.1/5.1 Hz, 1H ArCH2CH2O), 3.9 (ddd, J = 11.5/6.3/5.0 Hz, 1H, ArCH2CH2O), 7.11–7.25 (m, 4H, Ar–H). Purity (HPLC): 97.4%, tR = 15.49 min.

cis-N-Benzyl-3,4-dihydrospiro[[2]benzopyran-1,1′-cyclohexan]-3′-amine (cis-4a)

Ketone 9 (100 mg, 0.46 mmol) was dissolved in THF (5 mL). Benzylamine (79 μL, 0.70 mmol), acetic acid (27 μL, 0.46 mmol) and NaBH(OAc)3 (95%, 186 mg, 0.83 mmol) were added and the mixture was stirred for 24 h under a N2 atmosphere at rt. NaOH (1 M, 10 mL) was added and the mixture was extracted with CH2Cl2 (3 × 20 mL) and with Et2O (3 × 20 mL). The combined organic layers were dried (K2CO3), concentrated in vacuo and the residue was purified by FC (ϕ 2 cm, 20 cm, cyclohexane + 2% N,N-dimethylethanamine, 10 mL). Rf (cyclohexane + 2% N,N-dimethylethanamine = 0.17). Colorless oil, yield 128 mg (90%). C21H25NO (307.5). MS (ESI): m/z (%) = 308 [MH,100]. FT-IR: ṽ (cm−1) = 3341 (w, v, N–H), 3060, 3025 (w, C–H, arom), 2929, 2865 (s, v, C–H, alkyl), 1604, 1490 (w, C Created by potrace 1.16, written by Peter Selinger 2001-2019 C, arom), 1451 (m, δ, C–H, alkyl), 1089 (s, v, C–O), 751 (s, δ, C–H, o-disubst. arom), 729, 696 (s, δ, C–H, mono-subst. arom). 1H NMR (CDCl3): δ (ppm) = 1.45–1.54 (m, 2H, 4′-H, 5′-H or 6′-H), 1.70 (“td”, J = 13.2/3.6 Hz, 1H, 6′-H or 5′-H), 1.79 (dd, J = 14.6/4.6 Hz, 1H, 2′-H), 1.85–1.94 (m, 1H, 5′-H or 6′-H), 1.97 – 2.03 (m, 1H, 6′-H or 5′-H), 2.06 – 2.13 (m, 1H, 4′-H), 2.32 (ddd, J = 14.6/4.4/2.2 Hz, 1H, 2′-H), 2.76 – 2.88 (m, 2H, ArCH2CH2O), 2.98–3.04 (m, 1H, 3′-H), 3.79 (d, J = 13.2 Hz, 1H, ArCH2NH), 3.83 (d, J = 13.2 Hz, 1H, ArCH2NH), 3.88 (ddd, J = 11.2/5.6/5.1 Hz, 1H, ArCH2CH2O), 3.97 (ddd, J = 11.5/6.2/5.1 Hz, 1H, ArCH2CH2O), 7.06–7.12 (m, 2H, Ar–H), 7.13–7.18 (m, 2H, Ar–H), 7.20–7.25 (m, 1H, Ar–H), 7.28–7.34 (m, 2H, Ar–H), 7.35–7.39 (m, 2H, Ar–H). A signal for the NH-proton is not seen in the spectrum. 13C NMR (CDCl3): δ (ppm) = 16.6 (1C, C-5′), 29.3 (1C, 4′-C), 29.9 (1C, ArCH2CH2O), 37.9 (1C, 6′-C), 38.8 (1C, 2′-C), 51.3 (1C, NHCH2Ar), 52.0 (1C, 3′-C), 59.3 (1C, ArCH2CH2O), 77.1 (1C, 1′-C), 125.8 (1C, arom), 126.3 (1C, arom), 126.4 (1C, arom), 126.9 (1C, arom), 128.4 (2C, arom), 128.6 (2C, arom), 129.2 (1C, arom), 133.9 (1C, arom), 141.5 (1C, arom), 143.1 (1C, arom). Purity (HPLC): 99.1%, tR = 15.9 min.

cis-N-(Cyclohexylmethyl)-3,4-dihydrospiro[[2]benzopyran-1,1′-cyclohexan]-3′-amine (cis-4b)

Ketone 9 (37 mg, 0.17 mmol) was dissolved in THF (5 mL). Cyclohexylmethylamine (98%, 30 mg, 0.26 mmol in THF (2 mL)), acetic acid (10 μL, 0.17 mmol), and NaBH(OAc)3 (95%, 69 mg, 0.31 mmol) were added and the mixture was stirred for 24 h under a N2 atmosphere at rt. Subsequently, NaOH (1 M, 10 mL) was added and the mixture was extracted with Et2O (3 × 20 mL) and once with CH2Cl2 (3 × 20 mL). The combined organic layers were dried (K2CO3), concentrated in vacuo and the residue was purified by FC (ϕ 2 cm, 0 cm, cyclohexane: ethyl acetate + 1% N,N-dimethylethanamine, 10 mL). Rf (cyclohexane : ethyl acetate 9 : 1 + 1% N,N-dimethylethanamine = 0.11). Colorless oil, yield 43 mg (80%). C21H31NO (313.5). MS (ESI): m/z (%) = 314 [MH, 100]. FT-IR: ṽ (cm−1) = 3357 (w, v, N–H), 3059, 3019 (w, C–H, arom), 2919, 2848 (s, v, C–H, alkyl), 1489 (w, C Created by potrace 1.16, written by Peter Selinger 2001-2019 C, arom), 1448 (m, δ, C–H, alkyl), 1090 (s, v, C–O), 751 (s, δ, C–H, o-disubst. arom), 731, 668 (m, δ, monosubst. arom). 1H NMR (CDCl3): δ (ppm) = 0.86–0.97 (m, 2H, NCH2(cyclohexyl-H)), 1.10–1.32 (m, 3H, NCH2(cyclohexyl-H)), 1.42–1.52 (m, 3H, NCH2(cyclohexyl-H) (1H), 4′-H, 5′-H or 6′-H (2H)), 1.62 – 1.90 (m, 8H, NCH2(cyclohexyl-H) (5H), 2′-H, 5′-H, 6′-H, (3H)), 1.92–1.99 (m, 1H, 6′-H or 5′-H), 1.99–2.08 (m, 1H, 4′-H), 2.26 (ddd, J = 14.8/4.4/2.1 Hz, 1H, 2′-H), 2.36 (dd, J = 11.1/6.5 Hz, 1H, NCH2(cyclohexyl-H), 2.40 (dd, J = 11.1/7.0 Hz, 1H, NCH2(cyclohexyl-H), 2.75–2.88 (m, 2H, ArCH2CH2O), 2.92–2.97 (m, 1H, 3′-H), 3.88 (“dt”, J = 11.6/5.2 Hz, 1H, ArCH2CH2O), 3.94 (ddd, J = 11.5/7.0/4.7 Hz, 1H, ArCH2CH2O), 7.06–7.19 (m, 4H, Ar–H). A signal for the NH-proton is not seen in the spectrum. Purity (HPLC): 99.5%, tR = 17.4 min.

cis-N-Benzyl-N-methyl-3,4-dihydrospiro[[2]benzopyran-1,1′-cyclohexan]-3′-amine (cis-5a)

Benzylamine cis-4a (56.6 mg, 0.18 mmol) was dissolved in CH2Cl2 (3.5 mL) and formalin (37%, stab. with 10–15% MeOH, 274 μL, 3.68 mmol) and NaBH(OAc)3 (95%, 66 mg, 0.29 mmol) were added. The reaction mixture was stirred under a N2 atmosphere at rt for 3 h. Subsequently, H2O (10 mL) was added and the mixture was extracted with CH2Cl2 (4 × 20 mL) and once with Et2O (20 mL). The combined organic layers were dried (K2CO3), concentrated in vacuo and the residue was purified by FC (ϕ 2 cm, 30 cm, cyclohexane + 2% N,N-dimethylethanamine, 10 mL). Rf (cyclohexane + 2% N,N-dimethylethanamine = 0.31, cyclohexane + 1% N,N-dimethylethanamine = 0.13). Colorless oil, yield 55 mg (92%). C22H27NO (321.5). MS (ESI): m/z (%) = 322 [MH, 100]. FT-IR: ṽ (cm−1) = 3060, 3023 (w, v, C–H, arom), 2932, 2858 (s, v, C–H, alkyl), 2782 (m, v, N–CH3), 1603, 1490 (w, v, C Created by potrace 1.16, written by Peter Selinger 2001-2019 C, arom), 1450 (m, δ, C–H, alkyl), 1092 (s, v, C–O), 754 (s, δ, C–H, o-disubst. arom), 733, 698 (s, δ, C–H, mono-subst. arom). 1H NMR (CDCl3): δ (ppm) = 1.61–1.81 (m, 3H, 4′-H, 5′-H or 6′-H), 1.83–2.02 (m, 4H, 2′-H (1H), 4′-H, 5′-H or 6′-H (3H)), 2.17–2.21 (m, 1H, 2′-H), 2.21 (s, 3H, NCH3), 2.86 (t, J = 5.7 Hz, 2H, ArCH2CH2O), 3.01 (“tt”, J = 9.9/4.4 Hz, 1H, 3′-H), 3.54 (d, J = 13.5 Hz, 1H, NCH2Ar), 3.65 (d, J = 13.5 Hz, 1H, NCH2Ar), 3.94–4.03 (m, 2H, ArCH2CH2O), 7.08–7.18 (m, 3H, Ar–H), 7.20–7.33 (m, 6H, Ar–H). Purity (HPLC): 98.5%, tR = 15.9 min.

cis-N-(Cyclohexylmethyl)-N-methyl-3,4-dihydrospiro[[2]benzopyran-1,1′-cyclohexan]-3′-amine (cis-5b)

Cyclohexylmethylamine cis-4b (19 mg, 0.06 mmol) was dissolved in CH2Cl2 (3.5 mL) and formalin (37%, stab. with 10–15% MeOH, 90 μL, 1.20 mmol) and NaBH(OAc)3 (95%, 22 mg, 0.10 mmol) were added. The reaction mixture was stirred for 3 h under a N2 atmosphere at rt. H2O (10 mL) was added and the mixture was extracted with CH2Cl2 (4 × 20 mL). The combined organic layers were dried (K2CO3), concentrated in vacuo and the residue was purified by FC (ϕ 1.5 cm, cyclohexane + 1% N,N-dimethylethanamine, 20 cm, 10 mL). Rf (cyclohexane + 2% N,N-dimethylethanamine = 0.31, (cyclohexane + 1% N,N-dimethylethanamine = 0.15). Colorless oil, yield 18 mg (88%). C22H33NO (327.6). MS (ESI): m/z (%) = 328 [MH, 100]. IR: ṽ (cm−1) = 3067, 3021 (w, C–H, arom), 2920, 2848 (s, v, C–H, alkyl), 2786 (m, v, N–CH3), 1488 (w, C Created by potrace 1.16, written by Peter Selinger 2001-2019 C, arom), 1448 (m, δ, C–H, alkyl), 755 (s, δ, C–H, o-disubst. arom), 733 (m, δ, C–H, mono-subst. arom). 1H NMR (CDCl3): δ (ppm) = 0.75–0.89 (m, 2H, NCH2(cyclohexyl-H)), 1.08–1.28 (m, 3H, NCH2(cyclohexyl-H)), 1.30–1.40 (m, 1H, NCH2(cyclohexyl-H)), 1.46–1.56 (m, 1H, 4′-H), 1.60–1.76 (m, 8H,(cyclohexyl-H) (4H), 2′-H, 4′-H, 5′-H, 6′-H (4H)), 1.80–1.92 (m, 2H, (cyclohexyl-H) (1H), 5′-H (1H)), 1.96–2.05 (m, 1H, 6′-H), 2.12–2.22 (m, 3H, 2′-H (1H), NCH2cyclohexyl (2H)) 2.24 (s, 3H, NCH3), 2.86 (t, J = 5.9 Hz, 2H, ArCH2CH2O), 2.95 (tt, J = 11.4/3.9 Hz, 1H, 3′-H), 3.93–4.03 (m, 2H, ArCH2CH2O), 7.08–7.19 (m, 3H, Ar–H), 7.33–7.39 (m, 1H, Ar–H). 13C NMR (CDCl3): δ (ppm) = 21.1 (1C, 5′-C), 26.6 (2C, cyclohexyl-C), 26.7 (1C, 4′-C), 27.2 (1C, cyclohexyl-C), 29.7 (1C, ArCH2CH2O), 32.3 (2C, cyclohexyl-C), 36.7 (1C, cyclohexyl-C), 37.2 (1C, 6′-C), 39.3 (1C, NCH3), 39.7 (1C, 2′-C), 59.3 (1C, ArCH2CH2O), 59.4 (1C, 3′-C), 60.6 (1C, NHCH2Ar), 76.5 (1C, 1′-C), 125.6 (1C, arom), 126.0 (1C, arom), 126.4 (1C, arom), 129.5 (1C, arom), 133.9 (1C, arom), 144.0 (1C, arom). Purity (HPLC): 99.0%, tR = 16.9 min.

5.4. Receptor binding studies

The σ1 and σ2 affinities were recorded as described in references.45–47 Details of the assays are given in the ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Supplementary Material

MD-012-D0MD00307G-s001

Acknowledgments

This work was supported by the Deutsche Forschungsgemeinschaft, which is gratefully acknowledged.

†

Electronic supplementary information (ESI) available: Contains the procedure for the synthesis of mono ketal 7, details on the receptor binding studies, 1H and 13C NMR spectra of the synthesized compounds and HPLC chromatograms of all the test compounds confirming their purity. See DOI: 10.1039/d0md00307g

References

  1. Martin W. R. Eades C. G. Thomson J. A. Huppler R. E. Gilbert P. E. The effects of morphine- and nalorphine- like drugs in the nondependent and morphine-dependent chronic spinal dog. J. Pharmacol. Exp. Ther. 1976;197:517–532. [PubMed] [Google Scholar]
  2. Tam S. W. Naloxone-inaccessible in rat central nervous system. Proc. Natl. Acad. Sci. U. S. A. 1983;80:6703–6707. doi: 10.1073/pnas.80.21.6703. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Vaupel D. B. Naltrexone fails to antagonize the σ effects of PCP and SKF 10,047 in the dog. Eur. J. Pharmacol. 1983;92:269–274. doi: 10.1016/0014-2999(83)90297-2. [DOI] [PubMed] [Google Scholar]
  4. Bowen W. D. Hellewell S. B. McGarry K. A. Evidence for a multi-site model of the rat brain sigma receptor. Eur. J. Pharmacol. 1989;163:309–318. doi: 10.1016/0014-2999(89)90200-8. [DOI] [PubMed] [Google Scholar]
  5. Hellewell S. B. Bruce A. Feinstein G. Orringer J. Williams W. Bowen W. D. Rat liver and kidney contain high densities of sigma 1 and sigma 2 receptors: characterization by ligand binding and photoaffinity labelling. Eur. J. Pharmacol. 1994;268:9–18. doi: 10.1016/0922-4106(94)90115-5. [DOI] [PubMed] [Google Scholar]
  6. Ela C. Barg J. Vogel Z. Hasin Y. Eilam Y. Sigma receptor ligands modulate contractility, Ca++ influx and beating rate in cultured cardiac myocytes. J. Pharmacol. Exp. Ther. 1994;269:1300–1309. [PubMed] [Google Scholar]
  7. Hayashi T. Su T.-P. Sigma-1 receptor chaperones at the ER-mitochondrion interface regulate Ca(2+) signaling and cell survival. Cell. 2007;131:596–610. doi: 10.1016/j.cell.2007.08.036. [DOI] [PubMed] [Google Scholar]
  8. Schmidt H. R. Zheng S. Gurpinar E. Koehl A. Manglik A. Kruse A. C. Crystal structure of the human σ1 receptor. Nature. 2016;532:527–530. doi: 10.1038/nature17391. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Schmidt H. R. Betz R. M. Dror R. O. Kruse A. C. Structural basis for σ1 receptor ligand recognition. Nat. Struct. Mol. Biol. 2018;25:981–987. doi: 10.1038/s41594-018-0137-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Lupardus P. D. Wilke R. A. Aydar E. Palmer C. P. Chen Y. Ruoho A. E. Jackson M. B. Membrane-delimited coupling between sigma receptors and K+ channels in rat neurohypophysial terminals requires neither G-Protein nor ATP. J. Physiol. 2000;526:527–539. doi: 10.1111/j.1469-7793.2000.00527.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  11. Hong W. C. Yano H. Hiranita T. Chin F. T. McCurdy C. R. Su T. P. Amara S. G. Katz J. L. The sigma-1 receptor modulates dopamine transporter conformation and cocaine binding and may thereby potentiate cocaine self-administration in rats. J. Biol. Chem. 2017;292:11250–11261. doi: 10.1074/jbc.M116.774075. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Cobos E. J. Entrena J. M. Nieto F. R. Cendan C. M. Del Pozo E. Pharmacology and Therapeutic Potential of Sigma(1) Receptor Ligands. Curr. Neuropharmacol. 2008;6(4):344–366. doi: 10.2174/157015908787386113. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Hayashi T. Tsai S. Y. Mori T. Fujimoto M. Su T. P. Targeting ligand-operated chaperone sigma-1 receptors in the treatment of neuropsychiatric disorders. Expert Opin. Ther. Targets. 2011;15:557–577. doi: 10.1517/14728222.2011.560837. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Hayashi T. Su T. P. Sigma-1 receptor ligands – Potential in the treatment of neuropsychiatric disorders. CNS Drugs. 2004;18(5):269–284. doi: 10.2165/00023210-200418050-00001. [DOI] [PubMed] [Google Scholar]
  15. Maurice T. Su T. P. The pharmacology of sigma-1 receptors. Pharmacol. Ther. 2009;124(2):195–206. doi: 10.1016/j.pharmthera.2009.07.001. [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. Urani A. Roman F. J. Pham V. L. Su T. P. Maurice T. The Antidepressant-Like Effect Induced by σ1-Receptor Agonists and Neuroactive Steroids in Mice Submitted to the Forced Swimming Test. J. Pharmacol. Exp. Ther. 2001;298:1269–1279. [PubMed] [Google Scholar]
  17. Diaz J. L. Cuberes R. Berrocal J. Contijoch M. Christmann U. Fernandez A. Port A. Holenz J. Buschmann H. Laggner C. Serafini M. T. Burgueno J. Zamanillo D. Merlos M. Vela J. M. Almansa C. Synthesis and biological evaluation of the 1-arylpyrazole class of σ(1) receptor antagonists: identification of 4-{2-[5-methyl-1-(naphthalen-2-yl)-1H-pyrazol-3-yloxy]ethyl}morpholine (S1RA, E-52862) J. Med. Chem. 2012;55:8211–8224. doi: 10.1021/jm3007323. [DOI] [PubMed] [Google Scholar]
  18. Wünsch B. The σ1 receptor antagonist S1RA is a promising candidate for the treatment of neurogenic pain. J. Med. Chem. 2012;55:8209–8210. doi: 10.1021/jm3011993. [DOI] [PubMed] [Google Scholar]
  19. Aydar E. Palmer C. P. Djamgoz M. B. Sigma receptors and cancer: possible involvement of ion channels. Cancer Res. 2004;64:5029–5035. doi: 10.1158/0008-5472.CAN-03-2329. [DOI] [PubMed] [Google Scholar]
  20. van Waarde A. Rybczynska A. A. Ramakrishnan N. K. Ishiwata K. Elsinga P. H. Dierckx R. A. J. O. Potential applications for sigma receptor ligands in cancer diagnosis and therapy. Biochim. Biophys. Acta. 2015;1848(10):2703–2714. doi: 10.1016/j.bbamem.2014.08.022. [DOI] [PubMed] [Google Scholar]
  21. Kim F. J. Maher C. M. Sigma1 pharmacology in the context of cancer. Handb. Exp. Pharmacol. 2017;244:237–308. doi: 10.1007/164_2017_38. [DOI] [PubMed] [Google Scholar]
  22. Spruce B. A. Campbell L. A. McTavish N. Cooper M. A. Appleyard M. V. O'Neill M. Howie J. Samson J. Watt S. Murray K. McLean D. Leslie N. R. Safrany S. T. Ferguson M. J. Peters J. A. Prescott A. R. Box G. Hayes A. Nutley B. Raynaud F. Downes C. P. Lambert J. J. Thompson A. M. Eccles S. Small molecule antagonists of the sigma-1 receptor cause selective release of the death program in tumor and self-reliant cells and inhibit tumor growth in vitro and in vivo. Cancer Res. 2004;64:4875–4886. doi: 10.1158/0008-5472.CAN-03-3180. [DOI] [PubMed] [Google Scholar]
  23. Vilner B. J. John C. S. Bowen W. D. Sigma-1 and Sigma-2 Receptors Are Expressed in a Wide Variety of Human and Rodent Tumor Cell Lines. Cancer Res. 1995;55:408–413. [PubMed] [Google Scholar]
  24. Hashimoto K. Ishiwata K. Sigma receptor ligands: possible application as therapeutic drugs and radiopharmaceuticals. Curr. Pharm. Des. 2006;12:3857–3876. doi: 10.2174/138161206778559614. [DOI] [PubMed] [Google Scholar]
  25. Alon A. Schmidt H. R. Wood M. D. Sahn J. J. Martin S. F. Kruse A. C. Identification of the gene that codes for the σ2 receptor. Proc. Natl. Acad. Sci. U. S. A. 2017;114:7160–7165. doi: 10.1073/pnas.1705154114. [DOI] [PMC free article] [PubMed] [Google Scholar]
  26. Terada K. Migita K. Matsushima Y. Kamei C. Sigma-2 receptor as a potential therapeutic target for treating central nervous system disorders. Neural Regener. Res. 2019;14:1893–1894. doi: 10.4103/1673-5374.259609. [DOI] [PMC free article] [PubMed] [Google Scholar]
  27. Katnik C. Garcia A. Behensky A. A. Yasny I. E. Shuster A. M. Seredenin S. B. Petrov A. V. Cuevas J. Activation of σ1 and σ2 receptors by afobazole increases glial cell survival and prevents glial cell activation and nitrosative stress after ischemic stroke. J. Neurochem. 2016;139:497–509. doi: 10.1111/jnc.13756. [DOI] [PubMed] [Google Scholar]
  28. Megalizzi V. Le Mercier M. Decaestecker C. Sigma receptors and their ligands in cancer biology: overview and new perspectives for cancer therapy. Med. Res. Rev. 2012;32:410–427. doi: 10.1002/med.20218. [DOI] [PubMed] [Google Scholar]
  29. Mach R. H. Zeng C. Hawkins W. G. The σ2 receptor: a novel protein for the imaging and treatment of cancer. J. Med. Chem. 2013;56:7137–7160. doi: 10.1021/jm301545c. [DOI] [PMC free article] [PubMed] [Google Scholar]
  30. Zeng C. Rothfuss J. Zhang J. Chu W. Vangveravong S. Tu Z. Pan F. Chang K. C. Hotchkiss R. Mach R. H. Sigma-2 ligands induce tumor cell death by multiple signaling pathways. Br. J. Cancer. 2012;106:693–701. doi: 10.1038/bjc.2011.602. [DOI] [PMC free article] [PubMed] [Google Scholar]
  31. Collina S. Gaggeri R. Marra A. Bassi A. Negrinotti S. Negri F. Rossi D. Sigma receptor modulators: A patent review. Expert Opin. Ther. Pat. 2013;23:597–613. doi: 10.1517/13543776.2013.769522. [DOI] [PubMed] [Google Scholar]
  32. Maier C. A. Wünsch B. Novel Spiropiperidines as Highly Potent and Subtype Selective σ-Receptor Ligands. Part 1. J. Med. Chem. 2002;45:438–448. doi: 10.1021/jm010992z. [DOI] [PubMed] [Google Scholar]
  33. Maier C. A. Wünsch B. Novel σ Receptor Ligands. Part 2. SAR of Spiro[[2]benzopyran-1,4`-piperidines] and Spiro[[2]-benzofuran-1,4`-piperidines] with Carbon Substituents in Position 3. J. Med. Chem. 2002;45:4923–4930. doi: 10.1021/jm020889p. [DOI] [PubMed] [Google Scholar]
  34. Schläger T. Schepmann D. Lehmkuhl K. Holenz J. Vela J. M. Buschmann H. Wünsch B. Combination of two pharmacophoric systems: synthesis and pharmacological evaluation of spirocyclic pyranopyrazoles with high σ1 receptor affinity. J. Med. Chem. 2011;54:6704–6713. doi: 10.1021/jm200585k. [DOI] [PubMed] [Google Scholar]
  35. Oberdorf C. Schepmann D. Vela J. M. Buschmann H. Holenz J. Wünsch B. Thiophene bioisosteres of spirocyclic σ receptor ligands: Relationships between substitution pattern and σ receptor affinity. J. Med. Chem. 2012;55:5350–5360. doi: 10.1021/jm300302p. [DOI] [PubMed] [Google Scholar]
  36. Meyer C. Schepmann D. Yanagisawa S. Yamaguchi J. Dal Col V. Laurini E. Itami K. Pricl S. Wünsch B. Thiopehene Pd-catalyzed direct C-H bond functionalization of spirocyclic σ1 ligands: Generation of a pharmacophore model and analysis of the reverse binding mode by docking into a 3D homology model of the σ1 receptor. J. Med. Chem. 2012;55:8047–8065. doi: 10.1021/jm300894h. [DOI] [PubMed] [Google Scholar]
  37. Miyata K. Möller G. Schepmann D. Wünsch B. Pyridine analogues of spirocyclic σ1 receptor ligands. Bioorg. Med. Chem. 2014;22:4277–4284. doi: 10.1016/j.bmc.2014.05.033. [DOI] [PubMed] [Google Scholar]
  38. Ablordeppey S. Y. Fischer J. B. Law H. Glennon R. A. Probing the proposed phenyl-A region of the sigma-1 receptor. Bioorg. Med. Chem. 2002;10:2759–2765. doi: 10.1016/S0968-0896(02)00096-2. [DOI] [PubMed] [Google Scholar]
  39. Rack E. Fröhlich R. Schepmann D. Wünsch B. Design, synthesis and pharmacological evaluation of spirocyclic σ1 receptor ligands with exocyclic amino moiety (increased distance 1) Bioorg. Med. Chem. 2011;19:3141–3151. doi: 10.1016/j.bmc.2011.04.002. [DOI] [PubMed] [Google Scholar]
  40. Kronenberg E. Weber F. Brune S. Schepmann D. Almansa C. Friedland K. Laurini E. Pricl S. Wünsch B. Synthesis and structure-affinity relationships of spirocyclic benzopyrans with exocyclic amino moiety. J. Med. Chem. 2019;62:4204–4217. doi: 10.1021/acs.jmedchem.9b00449. [DOI] [PubMed] [Google Scholar]
  41. Parham W. E. Jones L. D. Sayed Y. A. Selective halogen-lithium exchange in bromophenylalkyl halides. J. Org. Chem. 1976;41:1184–1186. doi: 10.1021/jo00869a022. [DOI] [Google Scholar]
  42. Hayashi T. Takagi H. Masuda H. Ogoshi H. Synthesis and structure of new cis-1,3-dihydroxycyclohexane derivative having four convergent hydroxy groups. J. Chem. Soc., Chem. Commun. 1993:346–365. [Google Scholar]
  43. Takagi H. Hayashi T. Mizutani H. Ogoshi H. Synthesis and structure of tetraols with convergent and divergent arrays of hydroxy groups. J. Chem. Soc., Perkin Trans. 1. 1999:1885–1892. doi: 10.1039/A809834D. [DOI] [Google Scholar]
  44. Abdel-Magid A. F. Mehrman S. J. A Review on the Use of Sodium Triacetoxyborohydride in the Reductive Amination of Ketones and Aldehydes. Org. Process Res. Dev. 2006;10:971–1031. doi: 10.1021/op0601013. [DOI] [Google Scholar]
  45. Hasebein P. Frehland B. Lehmkuhl K. Fröhlich R. Schepmann D. Wünsch B. Synthesis and pharmacological evaluation of like- and unlike-configured tetrahydro-2-benzazepines with the α-substituted benzyl moiety in the 5-position. Org. Biomol. Chem. 2014;12:5407–5426. doi: 10.1039/C4OB00510D. [DOI] [PubMed] [Google Scholar]
  46. Meyer C. Neue B. Schepmann D. Yanagisawa S. Yamaguchi J. Würthwein E.-U. Itami K. Wünsch B. Improvement of σ1 receptor affinity by late-stage C-H-bond arylation of spirocyclic lactones. Bioorg. Med. Chem. 2013;21:1844–1856. doi: 10.1016/j.bmc.2013.01.038. [DOI] [PubMed] [Google Scholar]
  47. Miyata K. Schepmann D. Wünsch B. Synthesis and σ receptor affinity of regioisomeric spirocyclic furopyridines. Eur. J. Med. Chem. 2014;83:709–716. doi: 10.1016/j.ejmech.2014.06.073. [DOI] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

MD-012-D0MD00307G-s001
MD-012-D0MD00307G-s001.pdf (1.2MB, pdf)

Articles from RSC Medicinal Chemistry are provided here courtesy of Royal Society of Chemistry

RESOURCES