Design and Synthesis of a Piperidinone Scaffold as an Analgesic through Kappa-Opioid Receptor: Structure–Activity Relationship Study of Matrine Alkaloids

Abstract

The matrine-type alkaloid 4-dimethylamino-1-pentanoylpiperidine (3a) has an antinociceptive effect through its impact on the κ-opioid receptor (KOR). Derivatives of 3a were synthesized by altering its amide and tertiary amine groups, and were evaluated for their antinociceptive effects. The results indicated that the distance between these groups on 3a was optimal for the antinociceptive effect. The effects obtained with compounds 8 and 9 indicated that the relative configuration of the 3- and 4-substituents influenced the effect mediated through the KOR.

Narcotic analgesics such as morphine are administered for pain relief to cancer patients. Most narcotic analgesics are µ-opioid receptor (MOR) agonists and have adverse effects such as addiction,¹⁾ respiratory depression,²⁾ and constipation.^3,4) Although stimulation of the κ-opioid receptor (KOR) results in significant analgesia, KOR agonists do not suffer from the same adverse effects as MOR agonists. Many KOR agonists, including ethylketocyclazocine, U-50,488H, and nalfurafine (TRK-820), have been developed and investigated for their analgesic, anti-inflammatory and antipruritic activity^5,6) (Fig. 1). However, these agonists suffer from dose-limiting dysphoria, sedation, and psychotomimetic effects.^7,8) Consequently, the development of a KOR agonist that does not cause adverse effects is important.

Fig. 1. Structure of Conventional κ-Opioid Receptor Agonists and (+)-Matrine Derivatives

We previously reported that (+)-matrine (1) and (+)-allomatrine (2), typical matrine-type lupine alkaloids produced by Sophora Leguminosae, have antinociceptive properties identical to those of pentazocine.⁹⁾ The effects of 1 were mediated mainly through activation of the KOR and partially through the MOR, and those of 2 were mediated only through the KOR.¹⁰⁾ Furthermore, we found that neither 1 nor 2 provided the activation in the guanosine-5′-O-(3-[³⁵S]thio)trisphosphate ([³⁵S]GTPγS) binding assay with the membranes of spinal cord, indicating that the supraspinal antinociceptive actions induced by 1 and 2 were not caused by direct stimulation of the KOR.¹¹⁾ Although intracerebroventricular pretreatment with an antiserum against dynorphin A (1–17) did not affect the antinociceptive effect induced by subcutaneous (s.c.) treatment of 1 and 2, the antinociceptive effect was greatly attenuated by intrathecal (i.t.) pretreatment with an antiserum against dynorphin A (1–17) in mice. This suggested that the antinociceptive effect induced by s.c. treatment of 1 and 2 occurred without binding to the KOR in the ventricles of the brain, in where might stimulate the descending dynorphinergic neuron and production of dynorphin in the spinal cord.

Because the pharmacological mechanism of action and chemical structures of 1 and 2 differ from conventional KOR agonists, we performed structure–activity relationship (SAR) studies for 1 and 2 and their antinociceptive activity using modifications of the A–D ring systems. The SAR studies showed that the amide group, tertiary amine group, and C-ring of 1 and 2 were important in determining their activity. 4-Dimethylamino-1-pentanoylpiperidine (3) was identified as a lead compound through the antinociceptive effect of 1.^12–14) We hypothesized that the level of the antinociceptive effect would be affected by the distance between the amide and amino groups, which are important structural components of compound 3. To investigate this, model compounds (4–6) were designed to study the effect of the spatial relationship between the amide and amine groups on the agonistic activity (Fig. 2). We also identified that compound 7, 3-Bn analogue of compound 3 with a trans-configuration, has antinociceptive effects through KOR which exhibited higher antinociceptive activity than compound 3.¹⁵⁾ Because compound 7 exists as a rotamer by amide group, and we anticipated that the antinociceptive effects could be different from the rotamers. To investigate this, we designed and synthesized compounds 8 and 9 with the carbonyl group of the amide incorporated into piperidine. A SAR study was carried out to clarify the effect of the positional relationships between the benzyl and carbonyl group for the rotamers. The antinociceptive effects of the analogues of compound 3 (4–6, 8, 9) were evaluated using acetic acid-induced abdominal contraction tests in mice.

Fig. 2. Structure of Lead Compound 3 Derivatives Converted Amine and Amide Position

Synthesis

The synthetic routes for compounds 4a and b–6a and b are shown in Chart 1. Compounds 4a and b were prepared with pentanoyl chloride or benzoyl chloride from 1-methylpiperidine (10). The syntheses of 5a and b began with N-benzylation of commercially available 4-carboxamidopiperidine (11). Amide 12 was converted to the amine by reduction with LiAlH₄,¹⁶⁾ and treated with HCHO and HCO₂H to yield 13. After debenzylation using Pd(OH)₂, acylation of the secondary amine gave the target compounds 5a and b. Compounds 6a and b were synthesized from 1-benzyl-4-piperidone (14) using the Horner–Wadsworth–Emmons reaction to give olefin 15. Reduction of 15 using NaBH₄,¹⁷⁾ followed by reduction of the nitrile and N,N-dimethylation, afforded tertiary amine 18. After debenzylation, acylation of the secondary amine gave the target compounds 6a and b.

Chart 1

Reagents and reaction conditions: (a) RCOCl, DMAP, Et₃N, CH₂Cl₂, 0°C, 94–97%; (b) BnBr, NaHCO₃, toluene, reflux, 57%; (c) LiAlH₄, Et₂O, reflux; (d) HCHO, HCO₂H, 100°C, two steps, 83%; (e) Pd(OH)₂, H₂, EtOH, rt; (f) RCOCl, Et₃N, CH₂Cl₂, rt, two steps, 72–91%; (g) (EtO)₂POCH₂CN, K₂CO₃, THF, reflux, 94%; (h) NaBH₄, MeOH, Pyridine, reflux, 79%; (i) LiAlH₄, THF, 0°C; (j) HCHO, HCO₂H, 100°C, two steps, 61%.

The synthetic routes for cis- and trans-8a and b are shown in Chart 2. The β-keto lactam 19 was prepared from β-alanine according to an established procedure.^18,19) Condensation of 19 with benzaldehyde, followed by hydrogenation with Pd–C and treatment under acidic conditions gave the 3-benzyl derivative (20).²⁰⁾ Reductive amination of 20 using dimethylamine and NaCNBH₃ afforded cis-8a as a single diastereomer. Treatment of cis-8a with potassium tert-butoxide produced the diastereomers cis-8b and trans-8b, which could be separated. The relative configurations of these diastereomers were assigned using coupling constants from ¹H-NMR data. The results suggested that cis-8a could be epimerized to trans-8a under basic conditions. Therefore, epimerization of cis-8a to trans-8a was attempted using potassium tert-butoxide.

Chart 2

Reagents and reaction conditions: (a) PhCHO, piperidine, benzene, reflux; (b) Pd–C, H₂, MeOH, rt; (c) HCl aq, EtOAc, rt, three steps, 99%; (d) Me₂NH, Ti(Oi-Pr)₄, THF, reflux, then NaCNBH₄, AcOH, rt, 72%; (e) 1-iodopentane, t-BuOK, THF, rt, 74% (dr=2 : 1); (f) t-BuOK, THF, 0°C, 42%.

Chart 3 shows the synthetic routes used to prepare cis- and trans-9a and b. Ketone 21 was prepared from 19 according an established procedure.¹⁹⁾ To prepare cis-9a, enaminone 22 was obtained from ketone 21 and then reduced with PtO₂. The relative configuration of cis-9a was confirmed by X-ray crystallography (Fig. 3). To prepare trans-9a and b, ketone 21 was stereoselectively reduced with L-selectride to yield alcohol 23,²¹⁾ which was submitted to a Mitsunobu reaction using diphenylphosphoryl azide (DPPA), diisopropyl azodicarboxylate (DIAD), and triphenylphosphine (TPP).²²⁾ The prepared cis-9a and trans-9a were converted to cis-9b and trans-9b, respectively, by N-pentylation.

Chart 3

Reagents and reaction conditions: (a) Me₂NH, benzene, reflux, 92%; (b) PtO₂, H₂, MeOH, rt, 94%; (c) 1-bromopentane, KOH, DMSO, rt, 66%; (d) L-selectride, toluene, −78°C, 99%; (e) DIAD, DPPA, Ph₃P, THF, 0°C to rt; (f) Pd–C, H₂, HCHO, MeOH, two steps, 69%; (g) 1-iodopentane, t-BuOK, THF, rt, 59%.

Fig. 3. ORTEP Drawing of cis-9a

In order to investigate the antinociception between racemic and chiral compounds, (−)-trans-8a and b were synthesized by alternative plan as shown in Chart 4. Diastereoselective benzylation of 24,^23,24) which was derived from L-aspartic acid, provided 25 with a 3,4-trans configuration. Deprotection of the N-Boc group under acidic conditions, and hydrogenation by Pd(OH)₂ in the presence of HCHO, gave (−)-trans-8a. Compound (−)-trans-8b was obtained by N-pentylation²⁵⁾ before debenzylation.

Chart 4

Reagents and reaction conditions: (a) BnBr, LDA, THF, −78°C to rt, 52%; (b) HCl aq, EtOAc, rt, 78%; (c) Pd(OH)₂, H₂, HCHO, MeOH, rt, 58–71%; (d) 1-bromopentane, KOH, DMSO, rt, 66%.

Biological Assay

The antinociceptive effects of compounds 4a, b–6a and b, 8a and b, and 9a and b were evaluated in acetic acid-induced abdominal contraction assays (writhing tests). The results for 4a and b–6a and b are shown in Fig. 4A. Compared with 3a and b, the effects of 5a and b and 6a and b were greatly attenuated, while the effects of 4a and b were similar to those of 3a and b. To investigate if the effects of 3a and b and 4a and b were mediated through the KOR, we attempted to antagonize these effects by pretreatment with the KOR antagonist norbinaltorphimine (norBNI) (Fig. 4B). Pretreatment with norBNI resulted in a large dampening of the effect of 3a, but the effect of 4a and b was not dampened. These results show that the distance between the amide and amine groups is important for antinociceptive effects through the KOR, which is similar effects to what was found for 1 and 2.¹⁰⁾

Fig. 4. The Antinociceptive Effects of s.c. Administration of Compounds 3a and b–6a and b (30 mg/kg, s.c.) in the Writhing Test in Mice

Each mouse was injected i.p. with 0.7% acetic acid in a volume of 10 mL/kg 30 min after administration of the test drug dissolved in saline. After 10 min mice were observed for an additional 10 min during which abdominal contractions were counted. The % antinociception was calculated from the mean number of contractions in each test group and control group. Each column represents the mean±S.E.M. of 10 mice in each group. (A) ** p<0.01 versus the compound 3a. ^## p<0.01, ^### p<0.001 versus the compound 3b. (B) The blockage of antinociceptive effects of 3a and b and 4a and b (100 mg/kg, s.c.) by pretreatment of KOR antagonist nor-BNI (3.2 mg/kg, s.c.) at 4 h before administration test drug. ** p<0.01, *** p<0.001 versus the compounds 3a and b alone.

The results for cis- and trans-8a and b, 9a and b are shown in Fig. 5A. Compounds cis-8a, trans-8a and trans-9a and b showed high antinociceptive effects compared with all the other compounds tested. Since trans-9a showed the highest activity, it was selected to investigate the selectivity of opioid receptor by a writhing test. Its effect was evidently dampened with norBNI pretreatment (Fig. 5B). Comparison of the effects of these compounds indicated that the relative configuration of the dimethylamino and benzyl groups was more important than the positional relationship between the carbonyl group on the amide and the benzyl group in determining the antinociceptive effect. Therefore, the structural differences by the rotamers might not be as large as we expected.

Fig. 5. (A) The Antinociceptive Effects of s.c. Administration of Compounds 8 and 9 (30 mg/kg) in the Writhing Test in Mice; (B) The Blockage of Antinociceptive Effects of trans-9a (30 mg/kg, s.c.) by Pretreatment of KOR Antagonist nor-BNI (3.2 mg/kg, s.c.) at 4 h before Administration Test Drug; (C) The Antinociceptive Effects of Racemic and Optical Active trans-8a and b (30 mg/kg, s.c.)

(A) Each column represents the mean±S.E.M. of 10 mice in each group. (B) *** p<0.001 versus the compound trans-9a alone.

Evaluation of the antinociceptive effect of (−)-trans-8a and b showed it had a similar or inferior activity compared with the racemic mixture of (±)-trans-8a and b (Fig. 5C). Therefore, the (+)-enantiomer would probably have a similar antinociceptive effect to (−)-trans-8a and b.

In summary, the antinociceptive properties through the KOR depended on the distance between the amide and amine group. The relative configurations of the dimethylamino and benzyl groups in the (+)-matrine derivatives was important for high antinociceptive activity. These results will be useful for development of analgesics through KOR.

Experimental

Chemistry

¹H- (400 MHz) and ¹³C-NMR (100 MHz) spectra were obtained on a Bruker AVIII-400 instrument, and chemical shifts are reported in ppm on the δ-scale from internal tetramethylsilane. MS spectra were measured with a JEOL JMS D-600 and JMS T-100LP spectrometer by using the chemical ionization (CI) with isobutene, the electron impact (EI) methods, and electrospray ionization (ESI) methods. All melting points were measured with a Yanagimoto Micro melting point apparatus without collection. IR spectra were recorded on Perkin-Elmer Spectrum Two. Optical rotation were taken with a JASCO-DIP-370 polarimeter at room temperature (rt). Column chromatography was performed on Silica gel 60 (100–210 µm, Kanto Chemical Co., Inc.). The X-ray diffraction analysis were carried out on Rigaku RAXIS RAPID.

4-N-Methyl-1-N-pentanoylpiperazine (4a)

Pentanoyl chloride (3.61 g, 30.0 mmol), Et₃N (6.06 g, 59.9 mmol) and N,N-dimethyl-4-aminopyridine (DMAP) (0.24 g, 1.99 mmol) were added to a solution of 10 (2.00 g, 20.0 mmol) in CH₂Cl₂ (19 mL) at 0°C under the nitrogen atmosphere. The reaction mixture was stirred for 2 h, poured on 10% aqueous NaOH and extracted with CH₂Cl₂. The combined organic layer was dried over anhydrous Na₂SO₄. After filtration, the solvent was evaporated, and the residue was purified by chromatography on SiO₂ (CH₂Cl₂–MeOH=12 : 1) to give 4a (3.47 g, 18.8 mmol, 94%) as a light yellow oil. ¹H-NMR (CDCl₃) δ: 0.93 (3H, t, J=7.3 Hz), 1.37 (2H, sext, J=7.3 Hz), 1.55–1.67 (2H, m), 2.29–2.40 (6H, m), 2.30 (3H, s), 3.48 (2H, t, J=5.0 Hz), 3.63 (2H, t, J=5.0 Hz). ¹³C-NMR (CDCl₃) δ: 12.9, 21.5, 26.4, 31.9, 40.4, 44.5, 45.0, 53.8, 54.3, 170.3. IR (film) cm⁻¹: 1640. High resolution (HR)-MS (EI) m/z: Found 184.1598 (Calcd for C₁₀H₂₀N₂O (M⁺) 184.1575).

1-N-Benzoyl-4-N-methylpiperazine (4b)

Prepared according to procedure for the preparation of 4a, 97% yield; ¹H-NMR (CDCl₃) δ: 2.33 (3H, s), 2.32–2.46 (4H, br), 3.43 (2H, br), 3.79 (2H, br), 7.40 (5H, br). ¹³C-NMR (CDCl₃) δ: 41.3, 45.3, 46.9, 54.2, 126.6, 128.0, 128.9, 135.1, 169.4. IR (film) cm⁻¹: 1620. HR-MS (EI) m/z: Found 204.1253 (Calcd for C₁₂H₁₆N₂O (M⁺) 204.1262).

1-N-Benzyl-4-piperidinecarboxamide (12)

Benzyl bromide (20.4 mL, 23.5 g, 172 mmol) was added to a suspension of 11 (20.0 g, 156 mmol) and NaHCO₃ (23.5 g, 280 mmol) in toluene (320 mL). After being stirred at reflux for 5 h, the reaction mixture was filtrated. The solid was recrystallized with n-hexane–EtOAc to yield 12 (19.3 g, 88.4 mmol, 57%) as a white solid. ¹H-NMR (CD₃OD) δ: 1.62–1.80 (4H, m), 2.03 (2H, dt, J=12.0, 4.0 Hz), 2.20 (1H, m), 2.93 (2H, br), 3.51 (2H, s), 7.23–7.33 (5H, m). ¹³C-NMR (CD₃OD) δ: 29.5, 43.4, 53.9, 64.1, 128.4, 129.3, 130.7, 138.3, 180.8. IR (KBr) cm⁻¹: 1590, 1640. HR-MS (EI) m/z: Found 218.1401 (Calcd for C₁₃H₁₈N₂O (M⁺) 218.1419).

1-N-Benzyl-4-(N,N-dimethylaminomethyl)piperidine (13)

To a stirred suspension of LiAlH₄ (630 mg, 16.5 mmol) in Et₂O (25 mL) was added dropwise 12 (2.40 g, 11.0 mmol) in Et₂O (10 mL) at rt under nitrogen atmosphere. The reaction mixture was stirred at reflux for 8 h, and 10% aqueous NaOH was added. After filtration through Celite pad, the aqueous layer was extracted with Et₂O, and the combined organic layer was dried over anhydrous Na₂SO₄ and the solvent was evaporated. The resulting residue was used in the subsequent reaction without further purification. Ninety percent formic acid (5.06 g, 110 mmol) and 36% formaldehyde (4.59 mL, 1.65 g, 55.0 mmol) were added to the residue. After being stirred at 100°C for 14 h, the reaction mixture was evaporated to remove formic acid and formaldehyde, alkalized with 10% aqueous NaOH and extracted with CH₂Cl₂. The combined organic layer was dried over anhydrous Na₂SO₄ and evaporated. The residue was purified by chromatography on SiO₂ (CH₂Cl₂–MeOH–28% NH₄OH=60 : 9 : 1) to yield 13 (2.26 g, 9.17 mmol, 83%) as a light yellow oil. ¹H-NMR (CDCl₃) δ: 1.22 (2H, ddt, J=12.7, 11.5, 3.6 Hz), 1.38–1.52 (1H, m), 1.70 (2H, dm, J=12.7 Hz), 1.94 (2H, dt, J=11.5, 2.3 Hz), 2.10 (2H, d, J=7.1 Hz), 2.18 (6H, s), 2.88 (2H, br d, J=11.5 Hz), 3.49 (2H, s), 7.20–7.32 (5H, m). ¹³C-NMR (CDCl₃) δ: 30.3, 33.4, 45.4, 53.1, 62.9, 65.7, 126.2, 127.5, 128.4, 138.0. IR (film) cm⁻¹: 1630. HR-MS (EI) m/z: Found 232.1934 (Calcd for C₁₅H₂₄N₂ (M⁺) 232.1939).

4-(N,N-Dimethylaminomethyl)-1-N-pentanoylpiperidine (5a)

Pd(OH)₂ (80 mg, 20% w/w) was added to a solution of 13 (400 mg, 1.62 mmol) in EtOH (6 mL). The reaction mixture was stirred at rt under hydrogen atmosphere (1 atm) for 24 h. The suspension was filtered off, and the solvent was evaporated. The resulting residue was used in the subsequent reaction without further purification. Et₃N (0.67 mL, 4.83 mmol) and pentanoyl chloride (0.29 mL, 2.43 mmol) were added to a solution of the residue in CH₂Cl₂ (6 mL) at 0°C. After being stirred at rt for 10 h, the reaction mixture was alkalized with 10% aqueous NaOH, and extracted with CH₂Cl₂. The organic layer was dried over anhydrous Na₂SO₄ and evaporated. The result residue was purified by chromatography on SiO₂ (CH₂Cl₂–MeOH–28% NH₄OH=80 : 9 : 1) to yield 5a (317 mg, 1.40 mmol, 86%) as a light yellow oil. ¹H-NMR (CDCl₃) δ: 0.93 (3H, t, J=7.3 Hz), 0.98–1.18 (2H, m), 1.29–1.43 (2H, m), 1.54–1.96 (5H, m), 2.10 (2H, d, J=6.6 Hz), 2.20 (6H, s), 2.32 (2H, t, J=7.6 Hz), 2.54 (1H, dt, J=13.2, 2.9 Hz), 2.99 (1H, dt, J=13.2, 2.9 Hz), 3.85 (1H, br d, J=13.2 Hz), 4.62 (1H, br d, J=13.2 Hz). ¹³C-NMR (CDCl₃) δ: 12.9, 21.5, 26.5, 29.4, 30.3, 32.0, 33.3, 40.6, 44.7, 44.9, 64.7, 170.1. IR (film) cm⁻¹: 1630. HR-MS (EI) m/z: Found 226.2027 (Calcd for C₁₃H₂₆N₂O (M⁺) 226.2045).

1-N-Benzoyl-4-(N,N-dimethylaminomethyl)piperidine (5b)

Prepared according to procedure for the preparation of 5a, 91% yield; ¹H-NMR (C₆D₆, 80°C) δ: 0.97 (2H, dq, J=13.0, 4.3 Hz), 1.31–1.45 (1H, m), 1.51 (2H, br d, J=13.0 Hz), 1.89 (2H, d, J=7.1 Hz), 2.04 (6H, s), 2.61 (2H, dt, J=12.6, 2.8 Hz), 4.16 (2H, br d, J=12.6 Hz), 7.04–7.42 (5H, m). ¹³C-NMR (C₆D_6, 80°C) δ: 31.1, 34.9, 45.1, 45.9, 65.9, 127.4, 128.4, 129.3, 137.7, 169.7. IR (film) cm⁻¹: 1600, 1625. HR-MS (EI) m/z: Found 246.1702 (Calcd for C₁₅H₂₂N₂O (M⁺) 246.1732).

(1-N-Benzylpiperidin-4-ylidene)acetonitrile (15)

Potassium carbonate (3.65 g, 26.4 mmol) was added to a solution of diethyl cyanomethanephosphonate (4.97 mL, 31.7 mmol) in tetrahydrofuran (THF) (6 mL) under nitrogen atmosphere. The reaction mixture was stirred at rt for 15 min, warmed to reflux for 20 min. After being cooled to rt, 14 (5.00 g, 26.4 mmol) was added and refluxed for 12 h. The reaction mixture was alkalized with 10% aqueous potassium carbonate and extracted with ethyl acetate. The combined organic layer was dried over anhydrous Na₂SO₄ and evaporated. The resulting residue was purified by chromatography on SiO₂ (n-hexane–EtOAc=1 : 1) to yield 15 (5.18 g, 24.4 mmol, 94%) as a white solid. mp 85°C. ¹H-NMR (CDCl₃) δ: 2.37–2.41 (2H, m), 2.51–2.66 (6H, m), 3.56 (2H, s), 5.11 (1H, s), 7.26–7.38 (5H, m). ¹³C-NMR (CDCl₃) δ: 32.5, 34.9, 53.5, 62.1, 92.9, 116.5, 127.1, 128.2, 137.9, 164.9. IR (KBr) cm⁻¹: 1600, 1630, 2220. HR-MS (EI) m/z: Found 212.1343 (Calcd for C₁₄H₁₆N₂ (M⁺) 212.1313).

(1-N-Benzylpiperidin-4-yl)acetonitrile (16)

Sodium borohydride (236 mg, 6.25 mmol) was added to a solution of 15 (1.06 g, 5.00 mmol) in pyridine–MeOH (10 mL; 3 : 1) and heated at 120°C for 2 h. After being cooled to rt, H₂O was added and the solvent was removed, then the residue was filtered with Et₂O. After being alkalized with 10% aqueous NaOH, the reaction mixture was filtrated and extracted with Et₂O. The combined organic layer was dried over anhydrous Na₂SO₄, and evaporated. The resulting residue was purified by chromatography on SiO₂ (n-hexane–EtOAc=1 : 3) to yield 16 (0.85 g, 3.95 mmol, 79%) as a light yellow oil. ¹H-NMR (CDCl₃) δ: 1.43 (2H, dt, J=11.7, 3.4 Hz), 1.57–1.78 (3H, m), 1.99 (2H, dt, J=11.7, 2.3 Hz), 2.28 (2H, d, J=6.4 Hz), 2.91 (2H, br d, J=11.7 Hz), 3.50 (2H, s), 7.24–7.35 (5H, m). ¹³C-NMR (CDCl₃) δ: 23.5, 31.2, 32.7, 52.6, 62.7, 118.2, 126.6, 127.8, 128.6, 138.0. IR (film) cm⁻¹: 1630, 2250. HR-MS (EI) m/z: Found 214.1492 (Calcd for C₁₄H₁₈N₂ (M⁺) 214.1470).

1-N-Benzyl-4-(2-N,N-dimethylaminoethyl)piperidine (18)

To a stirred suspension of LiAlH₄ (2.66 g, 70.1 mmol) in THF (40 mL) was added dropwise 16 (10.0 g, 46.7 mmol) at 0°C under nitrogen atmosphere and stirred at 0°C for 2 h. After being quenched with 10% aqueous NaOH, the reaction mixture was filtrated with EtOAc. The combined organic layer was dried over anhydrous Na₂SO₄ and evaporated to give crude product. The resulting residue was used in the subsequent reaction without further purification. 90% Formic acid (21.5 g, 467 mmol) and 36% formaldehyde (19.5 mL, 233 mmol) was added to the crude, and the reaction mixture was stirred at 100°C for 14 h. After being evaporated, the reaction mixture was alkalized with 10% NaOH and extracted with CH₂Cl₂. The combined organic layer was dried over anhydrous Na₂SO₄ and evaporated. The resulting residue was purified by chromatography on SiO₂ (CHCl₃–MeOH–28% NH₄OH=60 : 9 : 1) to yield 18 (6.95 g, 28.2 mmol, 61%) as a light yellow oil. ¹H-NMR (CDCl₃) δ: 1.22–1.43 (5H, m), 1.62–1.66 (2H, dm, J=9.2 Hz), 1.89–1.98 (2H, m), 2.20 (6H, s), 2.23 (2H, t, J=7.6 Hz), 2.85 (2H, br d, J=11.5 Hz), 3.47 (2H, s), 7.19–7.31 (5H, m). ¹³C-NMR (CDCl₃) δ: 32.0, 33.4, 34.1, 45.1, 53.3, 56.8, 63.0, 126.3, 127.5, 128.5, 138.2. IR (film) cm⁻¹: 1630. HR-MS (EI) m/z: Found 246.2081 (Calcd for C₁₆H₂₆N₂ (M⁺) 246.2096).

4-(2-N,N-Dimethylaminoethyl)-1-N-pentanoylpiperidine (6a)

Prepared according to procedure for the preparation of 5a, 72% yield; ¹H-NMR (C₆D₆, 80°C) δ: 0.93 (3H, t, J=7.3 Hz), 1.01–1.19 (2H, m), 1.25–1.45 (4H, m), 1.48–1.77 (5H, m), 2.21 (6H, s), 2.25–2.34 (4H, m), 2.53 (1H, dt, J=13.2, 2.9 Hz), 2.99 (1H, dt, J=13.2, 2.9 Hz), 3.83 (1H, br d, J=13.2 Hz), 4.60 (1H, br d, J=13.2 Hz). ¹³C-NMR (C₆D₆, 80°C) δ: 12.7, 21.3, 26.2, 30.8, 31.6, 31.7, 32.8, 33.0, 40.5, 44.2, 44.6, 55.6, 169.6. IR (film) cm⁻¹: 1620. HR-MS (EI) m/z: Found 240.2188 (Calcd for C₁₄H₂₈N₂O (M⁺) 240.2201).

1-N-Benzoyl-4-(2-N,N-dimethylaminoethyl)piperidine (6b)

Prepared according to procedure for the preparation of 5a, 76% yield; ¹H-NMR (C₆D₆, 80°C) δ: 0.90–1.03 (2H, m), 1.18–1.38 (5H, m), 2.09 (2H, t, J=8.0 Hz), 2.08 (6H, s), 2.56 (2H, dt, J=13.2, 2.9 Hz), 4.14 (2H, br), 7.08–7.16 (5H, m). ¹³C-NMR (C₆D₆, 80°C) δ: 32.8, 34.5, 34.6, 45.3, 45.5, 57.3, 127.5, 128.3, 129.2, 137.8, 169.6. IR (film) cm⁻¹: 1580, 1625. HR-MS (EI) m/z: Found 260.1904 (Calcd for C₁₆H₂₄N₂O (M⁺) 260.1889).

3-Benzylpiperidine-2,4-dione (21)

Benzaldehyde (2.00 mL, 19.6 mmol) and piperidine (2.00 mL, 20.2 mmol) were added to a solution of 20 (4.00 g, 18.8 mmol) in benzene (50 mL). After being stirred at reflux for 4 h, the reaction mixture was cooled until rt and evaporated. The residue was dissolved in MeOH (30 mL) and Pd–C (1.0 g, 10% (w/w)) was added. After being stirred under hydrogen atmosphere (4 atm) at rt for 20 h, the reaction mixture was filtrated and concentrated to give brown oil. Six molar HCl (50 mL) was added to a solution of the residual oil in EtOAc (50 mL). After being stirred at rt for 3 h, the reaction mixture was evaporated, alkalized with NaHCO₃ aq, and extracted with CHCl₃. The combined organic layer was dried over anhydrous Na₂SO₄ and evaporated. The resulting residue was purified by chromatography on SiO₂ (CHCl₃–MeOH=20 : 1) to yield 21 (3.80 g, 18.7 mmol, 99%) as a white solid. mp 158–160°C. ¹H-NMR (CDCl₃) δ: 2.33 (1H, ddd, J=17.3, 9.3, 5.8 Hz), 2.50 (1H, dt, J=17.3, 5.3 Hz), 3.03 (1H, ddd, J=13.3, 9.3, 5.3 Hz), 3.27–3.40 (3H, m), 3.42 (1H, t, J=5.3 Hz), 7.16–7.24 (5H, m). ¹³C-NMR (CDCl₃) δ: 31.4, 36.2, 38.5, 59.1, 126.8, 128.5, 128.6, 129.6, 139.0, 171.1, 204.9. IR (KBr) cm⁻¹: 1480, 1590, 1660, 3370. MS (ESI+) m/z: 204 (M+H⁺, base peak). HR-MS (ESI+) m/z: Found 204.0982 (Calcd for C₁₂H₁₄NO₂ (M+H⁺) 204.1025).

cis-3-Benzyl-4-N,N-dimethylaminopiperidin-2-one (cis-8a)

Dimethylamine (4.50 mL, 9.00 mmol, 2 M in MeOH) and titanium tetraisopropoxide (0.68 mL, 2.30 mmol) were added to a solution of 21 (203 mg, 1.00 mmol) in THF (2 mL). After being stirred at reflux for 18 h, acetic acid (0.90 mL, 15.8 mmol) and sodium cyanoborohydride (189 mg, 3.00 mmol) were added at rt. After being stirred for 1 h, the reaction mixture was alkalized with 1 M aqueous NaOH and filtered. The filtrate was extracted with CHCl₃ and evaporated. The resulting residue was purified by chromatography on SiO₂ (CHCl₃–MeOH=20 : 1) to yield cis-8a (166 mg, 0.72 mmol, 72%) as a light yellow solid. mp 103–104°C. ¹H-NMR (CDCl₃) δ: 1.74–1.89 (2H, m), 2.26 (6H, s), 2.61 (1H, dt, J=8.9, 6.2 Hz), 2.73 (1H, q, J=6.2 Hz), 2.98 (1H, dd, J=13.7, 6.2 Hz), 3.12 (1H, dd, J=13.7, 6.2 Hz), 3.23 (1H, dddd, J=12.3, 10.5, 5.8, 1.7 Hz), 3.29–3.35 (1H, m), 5.88 (1H, br), 7.16–7.20 (1H, m), 7.25–7.32 (4H, m). ¹³C-NMR (CDCl₃) δ: 21.6, 32.8, 40.2, 42.9, 47.2, 61.1, 126.6, 128.8, 130.3, 141.8, 176.2. IR (KBr) cm⁻¹: 1460, 1630, 1670, 3290. MS (ESI+) m/z: 233 (M+H⁺, base peak). HR-MS (ESI+) m/z: Found 233.1611 (Calcd for C₁₄H₂₁N₂O (M+H⁺) 233.1654).

cis- and trans-3-Benzyl-4-N,N-dimethylamino-1-N-pentylpiperidin-2-one (cis-8b and trans-8b)

1-Iodopentane (171 mg, 0.86 mmol) and t-BuOK (0.72 mL, 0.72 mmol, 1 M in THF) were added to a solution of cis-8a (166 mg, 0.72 mmol) in THF (2 mL). After being stirred at rt for 30 min, the reaction mixture was quenched with brine and extracted with CHCl₃. The combined organic layer was dried over anhydrous Na₂SO₄ and evaporated. The resulting residue was purified by chromatography on SiO₂ (CHCl₃–MeOH=30 : 1) to yield cis-8b (93 mg, 0.32 mmol, 45%) and trans-8b (61 mg, 0.21 mmol, 29%) perspective as a light yellow oil. cis-8b: ¹H-NMR (CDCl₃) δ: 0.90 (3H, t, J=7.3 Hz), 1.24–1.37 (4H, m), 1.52 (2H, quin, J=7.3 Hz), 1.72–1.89 (2H, m), 2.22 (6H, s), 2.58 (1H, dt, J=9.1, 6.2 Hz), 2.74 (1H, q, J=6.2 Hz), 2.96 (1H, dd, J=13.7, 6.2 Hz), 3.10 (1H, dd, J=13.7, 6.2 Hz), 3.17–3.26 (3H, m), 3.45 (1H, dt, J=13.4, 7.6 Hz), 7.15–7.19 (1H, m), 7.23–7.27 (2H, m), 7.30–7.32 (2H, m). ¹³C-NMR (CDCl₃) δ: 13.9, 19.8, 22.3, 26.6, 28.9, 34.1, 40.4, 45.0, 46.6, 47.3, 59.3, 125.8, 127.8, 130.0, 139.6, 170.8. IR (film) cm⁻¹: 1450, 1640. MS (ESI+) m/z: 303 (M+H⁺, base peak). HR-MS (ESI+) m/z: Found 303.2460 (Calcd for C₁₉H₃₁N₂O (M+H⁺) 303.2436). trans-8b: ¹H-NMR (CDCl₃) δ: 0.88 (3H, t, J=7.2 Hz), 1.17–1.23 (2H, m), 1.27–1.34 (2H, m), 1.42–1.51 (2H, m), 1.59 (1H, ddt, J=13.1, 10.4, 4.9 Hz), 1.84–1.90 (1H, m), 2.22 (6H, s), 2.36–2.42 (1H, m), 2.69 (1H, dt, J=9.1, 5.0 Hz), 2.90–2.97 (1H, m), 3.05–3.19 (3H, m), 3.32 (1H, dd, J=13.2, 5.0 Hz), 3.54 (1H, ddd, J=13.2, 8.5, 6.7 Hz), 7.15–7.19 (1H, m), 7.21–7.27 (4H, m). ¹³C-NMR (CDCl₃) δ: 14.0, 20.0, 22.5, 26.8, 29.0, 34.3, 40.6, 45.2, 46.7, 47.4, 59.5, 125.9, 127.9, 130.1, 139.8, 171.0. IR (film) cm⁻¹: 1600, 1640. MS (EI) m/z: 302 (M⁺, base peak). HR-MS (EI) m/z: Found 302.2364 (Calcd for C₁₉H₃₀N₂O (M⁺) 302.2358).

trans-3-Benzyl-4-N,N-dimethylaminopiperidin-2-one (trans-8a)

t-BuOK (3.18 mL, 3.18 mmol, 1 M in THF) was added dropwise to a solution of cis-8a (368 mg, 1.59 mmol) in THF (20 mL) at rt under nitrogen atmosphere. After being stirred for 21 h, the reaction mixture was quenched with H₂O and extracted with CHCl₃. The combined organic layer was dried over anhydrous Na₂SO₄ and evaporated. The residue was purified by chromatography on SiO₂ (CHCl₃–MeOH=10 : 1) to yield trans-8a (154 mg, 0.66 mmol, 42%) as a yellow oil. ¹H-NMR (CDCl₃) δ: 1.63 (1H, ddt, J=13.2, 10.1, 4.9 Hz), 1.86–1.92 (1H, m), 2.23 (6H, s), 2.39–2.45 (1H, m), 2.72 (1H, dt, J=9.4, 5.1 Hz), 2.92–2.99 (1H, m), 3.12 (1H, dd, J=13.3, 5.1 Hz), 3.24–3.30 (2H, m), 5.89 (1H, br), 7.17–7.29 (5H, m). ¹³C-NMR (CDCl₃) δ: 19.5, 33.7, 39.3, 40.7, 46.4, 59.2, 126.0, 128.1, 130.0, 139.5, 174.0. IR (film) cm⁻¹: 1600, 1660. MS (EI) m/z: 232 (M⁺), 84 (base peak). HR-MS (EI) m/z: Found 232.1557 (Calcd for C₁₄H₂₀N₂O (M⁺) 232.1575).

5-Benzyl-4-N,N-dimethylamino-5,6-dihydro-1H-pyridin-2-one (23)

Dimethylamine (4.5 mL, 9.0 mmol, 2 M in MeOH) was added to a solution of 22 (305 mg, 1.50 mmol) in benzene (10 mL). After being stirred at reflux for 18 h, the reaction mixture was evaporated. The resulting residue was purified by chromatography on SiO₂ (CHCl₃–MeOH=10 : 1) to yield 23 (298 mg, 1.29 mmol, 86%) as a light yellow solid. mp 184–187°C. ¹H-NMR (CDCl₃) δ: 2.67–2.70 (1H, m), 2.81 (1H, dd, J=13.9, 4.1 Hz), 2.93 (1H, dd, J=13.9, 10.8 Hz), 2.96 (6H, s), 3.08 (1H, ddd, J=12.3, 4.7, 1.7 Hz), 3.35 (1H, dd, J=12.3, 3.8 Hz), 4.69 (1H, s), 4.98 (1H, br), 7.15–7.38 (5H, m). ¹³C-NMR (CDCl₃) δ: 35.3, 36.8, 38.9, 41.2, 87.7, 126.3, 128.4, 128.8, 138.7, 161.7, 170.3. IR (film) cm⁻¹: 1580, 1600, 1640. MS (EI) m/z: 230 (M⁺, base peak). HR-MS (EI) m/z: Found 230.1410 (Calcd for C₁₄H₁₈N₂O (M⁺) 230.1419).

cis-5-Benzyl-4-N,N-dimethylaminopiperidin-2-one (cis-9a)

PtO₂ (50 mg) was added to a solution of 23 (184 mg, 0.80 mmol) in MeOH (20 mL). After being stirred at rt for 12 h under hydrogen atmosphere (6.8 atm), the reaction mixture was filtrated through Celite pad and evaporated. The resulting residue was purified by chromatography on SiO₂ (CHCl₃–MeOH–28% NH₄OH=90 : 9 : 1) to yield cis-9a (174 mg, 0.75 mmol, 94%) as a yellow solid. mp 150–152°C. ¹H-NMR (C₆D₆) δ: 1.76–1.84 (2H, m), 1.92 (6H, s), 2.29 (1H, dd, J=17.3, 10.9 Hz), 2.36 (1H, dd, J=13.8, 11.4 Hz), 2.54 (1H, dd, J=17.3, 6.4 Hz), 2.59 (1H, dd, J=12.7, 3.4 Hz), 2.84 (1H, ddd, J=12.7, 3.9, 2.6 Hz), 2.91 (1H, dd, J=13.8, 2.4 Hz), 6.99–7.15 (5H, m), 7.87 (1H, br). ¹³C-NMR (C₆D₆) δ: 29.7, 34.4, 37.2, 42.1, 43.0, 62.4, 126.2, 128.7, 129.4, 141.3, 171.6. IR (KBr) cm⁻¹: 1660. MS (EI) m/z: 232 (M⁺, base peak). HR-MS (EI) m/z: Found 232.1568 (Calcd for C₁₄H₂₀N₂O (M⁺) 232.1575).

cis-5-Benzyl-4-N,N-dimethylamino-1-N-pentylpiperidin-2-one (cis-9b)

The solution of cis-9a (122 mg, 0.52 mmol) in dimethyl sulfoxide (DMSO) (8 mL) was added to a suspension of powder potassium hydroxide (56 mg, 1.00 mmol) in DMSO (3 mL) under nitrogen atmosphere. After being stirred at rt for 5 min, 1-bromopentane (0.10 mL, 0.78 mmol) was added. After being stirred for 1 h, the reaction mixture was quenched with H₂O and extracted with CHCl₃. The combined organic layer was dried over anhydrous Na₂SO₄ and evaporated. The resulting residue was purified by chromatography on SiO₂ (CHCl₃–MeOH–28% NH₄OH=200 : 9 : 1) to yield cis-9b (114 mg, 0.38 mmol, 73%) as a light yellow oil. ¹H-NMR (C₆D₆) δ: 0.87 (3H, t, J=7.2 Hz), 1.11–1.47 (6H, m), 1.83–1.88 (1H, m), 1.93–1.96 (1H, m), 1.96 (6H, s), 2.27 (1H, dd, J=13.8, 11.8 Hz), 2.33 (1H, dd, J=17.3, 11.3 Hz), 2.57–2.65 (2H, m), 2.68 (1H, dd, J=12.5, 2.6 Hz), 2.99 (1H, dd, J=13.8, 2.4 Hz), 3.23–3.37 (2H, m), 6.99–7.17 (5H, m). ¹³C-NMR (C₆D₆) δ: 14.2, 22.8, 27.3, 29.4, 30.0, 34.9, 37.9, 43.0, 47.8, 47.8, 62.7, 126.3, 128.7, 129.4, 141.4, 167.1. IR (film) cm⁻¹: 1600, 1650. MS (EI) m/z: 302 (M⁺), 98 (base peak). HR-MS (EI) m/z: Found 302.2352 (Calcd for C₁₉H₃₀N₂O (M⁺) 302.2358).

cis-5-Benzyl-4-hydroxypiperidin-2-one (24)

L-Selectride (3.92 mL, 3.92 mmol, 1 M in THF) was added dropwise to a solution of 22 (200 mg, 0.98 mmol) in toluene (10 mL) at –78°C under nitrogen atmosphere. After being stirred for 18 h, the reaction mixture was quenched with saturated aqueous NH₄Cl and extracted with EtOAc. The combined organic layer was dried over anhydrous Na₂SO₄ and evaporated. The resulting residue was purified by chromatography on SiO₂ (CHCl₃–MeOH–28% NH₄OH=100 : 9 : 1) to yield 24 (198 mg, 0.96 mmol, 99%) as a white solid. mp 183–185°C. ¹H-NMR (CDCl₃) δ: 1.93 (1H, d, J=3.6 Hz), 2.10–2.18 (1H, m), 2.48 (1H, dd, J=18.0, 3.1 Hz), 2.55 (1H, dd, J=18.0, 3.8 Hz), 2.67 (1H, dd, J=13.6, 7.6 Hz), 2.82 (1H, dd, J=13.6, 7.7 Hz), 3.14 (1H, ddd, J=11.5, 5.3, 3.6 Hz), 3.40 (1H, t, J=11.5 Hz), 4.07 (1H, br), 5.76 (1H, br), 7.19–7.26 (3H, m), 7.30–7.34 (2H, m). ¹³C-NMR (CDCl₃) δ: 35.5, 39.6, 40.3, 65.7, 126.7, 128.8, 129.1, 139.2, 170.9. IR (KBr) cm⁻¹: 1430, 1660, 3200, 3300. MS (ESI+) m/z: 206 (M+H⁺), 180 (base peak). HR-MS (ESI+) m/z: Found 206.1168 (Calcd for C₁₂H₁₆NO₂ (M+H⁺) 206.1181).

trans-5-Benzyl-4-N,N-dimethylaminopiperidin-2-one (trans-9a)

Diisopropyl carboxylate (0.58 mL, 2.92 mmol) was added to a solution of triphenylphosphine (1.10 g, 4.16 mmol) in THF (30 mL) at 0°C under nitrogen atmosphere. After being stirred for 20 min, the solution of 24 (600 mg, 2.92 mmol) in THF (10 mL) and diphenylphosphoryl azide (1.34 mL, 6.21 mmol) were added dropwise. After being stirred at 0°C for 3 h, the reaction mixture was warmed to rt for 30 min, quenched with H₂O, and extracted with EtOAc. The combined organic layer was dried over anhydrous Na₂SO₄ and evaporated. Pd–C (430 mg, 10% (w/w)) was added to a solution of the resulting residue in MeOH (30 mL). After being stirred under hydrogen atmosphere (1 atm) for 4 h, 36% formaldehyde (11.3 mL, 132 mmol) was added to the reaction mixture and it was stirred further for 18 h. Palladium catalyst was removed by filtration and the filtrate was evaporated. The resulting residue was purified by chromatography on SiO₂ (CHCl₃–MeOH–28% NH₄OH=180 : 9 : 1) to yield trans-9a (470 mg, 2.02 mmol, 69%) as a white solid. mp 146–147°C. ¹H-NMR (CDCl₃) δ: 2.04–2.16 (1H, m), 2.32 (6H, s), 2.33–2.42 (2H, m), 2.49 (1H, dd, J=17.2, 5.5 Hz), 2.73 (1H, dt, J=9.8, 5.5 Hz), 2.89–2.94 (1H, m), 3.14 (1H, ddd, J=12.0, 4.8, 4.0 Hz), 3.20 (1H, dd, J=13.9, 3.7 Hz), 5.74 (1H, br), 7.15–7.17 (2H, m), 7.20–7.23 (1H, m), 7.28–7.31 (2H, m). ¹³C-NMR (CDCl₃) δ: 28.4, 29.1, 29.8, 36.4, 38.4, 40.1, 44.8, 61.4, 126.5, 128.7, 129.1, 139.6, 172.8. IR (KBr) cm⁻¹: 1040, 1500, 1670, 3190. MS (ESI+) m/z: 233 (M+H⁺), 162 (base peak). HR-MS (ESI+) m/z: Found 233.1621 (Calcd for C₁₄H₂₁N₂O (M+H⁺) 233.1654).

trans-5-Benzyl-4-N,N-dimethylamino-1-N-pentylpiperidin-2-one (trans-9b)

Prepared according to procedure for the preparation of cis-8b, 59% yield; ¹H-NMR (CDCl₃) δ: 0.85 (3H, t, J=7.2 Hz), 1.15–1.33 (4H, m), 1.38–1.46 (2H, m), 2.06–2.16 (1H, m), 2.30 (6H, s), 2.34–2.42 (2H, m), 2.50 (1H, dd, J=17.0, 5.5 Hz), 2.67 (1H, dt, J=9.8, 5.5 Hz), 2.92 (1H, dd, J=12.3, 9.6 Hz), 3.07 (1H, dd, J=12.3, 5.0 Hz), 3.14–3.21 (1H, m), 3.29 (1H, dt, J=13.4, 7.6 Hz), 7.17–7.24 (3H, m), 7.29–7.33 (2H, m). ¹³C-NMR (CDCl₃) δ: 14.1, 22.5, 26.9, 29.1, 36.5, 39.0, 40.1, 47.0, 50.5, 61.5, 126.4, 128.6, 129.1, 139.7, 169.6. IR (film) cm⁻¹: 1040, 1180, 1450, 1650. MS (ESI+) m/z: 303 (M+H⁺, base peak). HR-MS (ESI+) m/z: Found 303.2452 (Calcd for C₁₉H₃₁N₂O (M+H⁺) 303.2436).

(3R,4S)-3-Benzyl-4-N,N-dibenzylaminopiperidin-2-one (27)

Six molar HCl (5 mL) was added to a solution of 26 (245 mg, 0.51 mmol) in EtOAc (5 mL). After being stirred at rt for 3 h, the reaction mixture was diluted with H₂O and extracted with CHCl₃. The combined organic layer was dried over anhydrous Na₂SO₄ and evaporated. The resulting residue was purified by chromatography on SiO₂ (CHCl₃–MeOH–28% NH₄OH=600 : 9 : 1) to yield 27 (152 mg, 0.40 mmol, 78%) as a light yellow solid. mp 45–48°C. [α]_D²⁵ +36.8 (c=0.92, CHCl₃). ¹H-NMR (CDCl₃) δ: 1.64 (1H, dq, J=11.8, 4.6 Hz), 2.03–2.07 (1H, m), 2.65 (1H, dt, J=11.9, 3.0 Hz), 2.78–2.83 (1H, m), 2.91–2.98 (2H, m), 3.12–3.20 (1H, m), 3.25 (1H, dd, J=13.4, 4.6 Hz), 3.42 (2H, d, J=13.6 Hz), 3.82 (2H, d, J=13.6 Hz), 6.17 (1H, br), 6.64–6.66 (2H, m), 7.01–7.10 (3H, m), 7.20–7.36 (10H, m). ¹³C-NMR (CDCl₃) δ: 21.0, 34.1, 39.0, 46.6, 53.9, 54.8, 125.7, 127.0, 127.7, 128.2, 129.0, 129.7, 138.8, 139.5, 175.1. IR (film) cm⁻¹: 1600, 1660. MS (EI) m/z: 384 (M⁺), 236 (base peak). HR-MS (EI) m/z: Found 384.2191 (Calcd for C₂₆H₂₈N₂O (M⁺) 384.2202).

(3R,4S)-3-Benzyl-4-N,N-dimethylaminopiperidin-2-one ((−)-trans-8a)

Pd(OH)₂ (300 mg, 20% (w/w)) and 36% formaldehyde (1.00 mL, 11.7 mmol) were added to a solution of 27 (137 mg, 0.36 mmol) in MeOH (10 mL). After being stirred under hydrogen atmosphere (4 atm) at rt for 15 h, palladium catalyst was removed by filtration and the filtrate was evaporated. The resulting residue was purified by chromatography on SiO₂ (CHCl₃–MeOH–28% NH₄OH=80 : 9 : 1) to yield (−)-trans-8a (58 mg, 0.25 mmol, 71%) as a yellow oil. [α]_D²⁵ −50.0 (c=0.46, CHCl₃).

(3R,4S)-3-Benzyl-4-N,N-dibenzylamino-1-N-pentylpiperidin-2-one (28)

The solution of (−)-trans-8a (137 mg, 0.35 mmol) in DMSO (3 mL) was added to a suspension of powder potassium hydroxide (31 mg, 0.55 mmol) in DMSO (2 mL) under nitrogen atmosphere. After being stirred at rt for 15 min, to the suspension added 1-bromopentane (44 µL, 0.35 mmol) and it was stirred further for 3 h. The reaction mixture was diluted with H₂O and extracted with CHCl₃. The combined organic layer was dried over anhydrous Na₂SO₄ and evaporated. The resulting residue was purified by chromatography on SiO₂ (n-hexane–EtOAc=9 : 2) to yield 28 (106 mg, 0.23 mmol, 66%) as a light yellow oil. [α]_D²⁸ +14.1 (c=0.50, CHCl₃). ¹H-NMR (CDCl₃) δ: 0.85 (3H, t, J=7.3 Hz), 1.08–1.15 (2H, m), 1.22–1.40 (4H, m), 1.61 (1H, dq, J=11.8, 4.4 Hz), 2.01–2.06 (1H, m), 2.65 (1H, dt, J=12.0, 3.0 Hz), 2.81–2.94 (3H, m), 3.01–3.08 (2H, m), 3.28 (1H, dd, J=12.9, 4.5 Hz), 3.37–3.44 (1H, m), 3.40 (2H, d, J=13.6 Hz), 3.82 (2H, d, J=13.6 Hz), 6.56 (2H, dd, J=8.1, 1.3 Hz), 6.98–7.07 (3H, m), 7.26–7.36 (10H, m). ¹³C-NMR (CDCl₃) δ: 13.9, 21.5, 22.4, 26.8, 29.0, 34.8, 45.3, 47.1, 47.5, 54.0, 55.2, 125.7, 127.1, 127.7, 128.4, 129.1, 130.0, 139.0, 139.7, 171.3. IR (film) cm⁻¹: 1600, 1640. MS (EI) m/z: 454 (M⁺), 363 (base peak). HR-MS (EI) m/z: Found 454.2970 (Calcd for C₃₁H₃₈N₂O (M⁺) 454.2984).

(3R,4S)-3-Benzyl-4-N,N-dimethylamino-1-N-pentylpiperidin-2-one ((−)-trans-8b)

Prepared according to procedure for the preparation of cis-9b, 58% yield; [α]_D²⁵ −67.5 (c=0.38, CHCl₃).

Acetic Acid-Induced Abdominal Contraction Assay (Writhing Test)

Evaluation for antinociceptive effects were carried out by acetic acid writhing test using male ICR mice (Tokyo Laboratory Animals Science, Tokyo, Japan) weighing approximately 25–35 g (5–6 weeks old). Mice had free access to food and water in an animal room that was maintained at 24±1°C with a 12 h light–dark cycle (lights on 8:00 a.m.). Each mice were injected i.p. with 0.7% acetic acid in a volume of 10 mL/kg 30 min after administration of the test drug dissolved in saline. After 10 min mice were observed for an additional 10 min during which abdominal contractions were counted. The % antinociception was calculated from the mean number of contractions in each test group and control group (% antinociception=[(mean control responses−test responses)/(mean control responses)]×100). The statistical significance of differences between groups was assessed with analysis of variance followed by the Bonferroni–Dunn test or Student t-test.

Acknowledgments

This work was MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2014–2018 (Grant No. S1411019).

Conflict of Interest

The authors declare no conflict of interest.

Supplementary Materials

The online version of this article contains supplementary materials.

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