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Abstract

Hydroxy-α-sanshool was synthesized in a 13% overall yield through eight steps, which included two Wittig reactions that were used to form the carbon skeleton with ethyl 2-oxoacetate and 2E,4E-hexadienal being reacted with the appropriate ylides. Impurities in the processes could easily be separated. Ethyl 6-hydroxy-2Z-hexenoate was converted to its E-isomer with catalysis by I₂ and 2E,6Z,8E,10E-dodecatetraenoic acid was crystallized from a solution in 1% ethyl acetate in n-hexane.

Keywords

Hydroxy-α-sanshool Wittig reaction synthesis

Introduction

The sanshools are representative unsaturated fatty acid amides found in Zanthoxylum (Zanthoxylum bungeanum Maxim.) and include α-sanshool, β-sanshool, γ-sanshool, δ-sanshool, and homologues containing one hydroxy group in the amino fragment (Figure 1).^1–3 Hydroxy-α-sanshool is the main numb flavoring substance in Zanthoxylum, and its content directly determines the degree of numbness of Zanthoxylum, so establishing the quality of the Zanthoxylum.^4,5

Figure 1.

The sanshools.

Hydroxy-α-sanshool has been found to have several bioactivities. It acts as an agonist of transient receptor potential vanilloid type-1 (TRPV-1) and transient receptor potential ankyrin-1 (TRPA-1)⁶ and is a selective blocker of some two-pore domain potassium channels (KCNK): TASK-1 (KCNK3), TASK-3 (KCNK9), and TRESK (KCNK18).^7,8 The process of separating hydroxy-α-sanshool from Zanthoxylum is tedious and gives low yields.⁹

Igarashi et al.¹⁰ synthesized hydroxy-α-sanshool using a Suzuki–Miyaura coupling (SMC) (see Scheme 1). This SMC-based route involved the synthesis of complex building blocks and involved non-readily available starting materials. Sonnet et al.¹¹ synthesized α-sanshool using ethyl 4,6,8-decatrienoate as an intermediate although the process needs harsh conditions and involved the reduction of an acid to an aldehyde. Wu et al.¹² synthesized hydroxy-α-sanshool using 4-bromobutyraldehyde as a key intermediate. However, the starting material 4-bromo-1-butanol is unstable during its preparation, storage, and oxidation; therefore, 4-bromobutyraldehyde is difficult to prepare on a large scale.

Scheme 1.

Reported processes in the literatures

The synthesis of Wu et al. was the easiest to carry out although the preparation of ethyl 6-bromo-2E-hexenoate is not straightforward. Although 4-bromobutyraldehyde can be prepared by reducing ethyl 4-bromobutyrate, the reaction must be performed in the absence of water and at a low temperature.^13–15 An alternative method is the bromination of ethyl 6-hydroxy-2E-hexenoate.^16–19 6-Hydroxy-2E-hexenoate can be prepared from tetrahydrofuran-2-ol which is obtained by reduction of γ-butyrolactone using diisobutylaluminium hydride (DIBALH) albeit at an extremely low temperature or from 4-hydroxy-1-butyraldehyde. This can be obtained by oxidation of 1,4-butanediol with pyridinium chlorochromate (PCC) or MnO₂,^20–22 but the yield is low and the purification difficult.

Considering the difficult preparation of 4-bromobutyraldehyde or 4-hydoxybutyraldehyde, a new route starting from glyoxylic acid was designed (Scheme 2). Ethyl (E)-6-bromohex-2-enoate 4 can be prepared from the hydroxyester 3 by bromination. In turn, ester 3 can be prepared by a Wittig reaction involving the phosphonium salt 1 and ethyl 2-oxoacetate, followed by cleavage of the acetate and isomerization of the alkene. Phosphonium salt 1 can be prepared by PPh₃ reacted with 4-bromobutyl acetate which was synthesized from tetrahydrofuran and acetyl bromide.²³ The isomerization of the Z-isomer of 3 into its E-isomer was catalyzed by iodine. This novel process avoids the use of 4-bromobutyraldehyde. The reaction work-ups are simple, the yields are high, and the starting materials are easily available.

Scheme 2.

Designed synthetic route in our work

Results and discussion

Chemistry

According to the literature,²³ the impurities in 4-bromobutyl acetate are 1,4-dibromobutane and 1,4-diacetoxybutane. 1,4-Diacetoxybutane can be easily removed following the preparation of the quaternary phosphonium salt 1. The 1,4-dibromobutane was transferred into a mixture of a single quaternary phosphonium salt and a double quaternary phosphonium salt. The mono quaternary phosphonium salt leads to the target intermediate 4 after isomerization. The double quaternary phosphonium salt may participate in a single Wittig reaction to give a product that can be removed from the reaction mixture because of its insolubility in n-hexane and in a double Wittig reaction whose product can be separated in the subsequent preparation of phosphonium salt 5. Other impurities can also be easily removed during the preparation of the quaternary phosphonium salt 5.

The phosphonium salt 1 was obtained in a 93% yield from 4-bromobutyl acetate and PPh₃.²⁴ Hex-2-enoate 2 was obtained through the Wittig reaction of phosphonium salt 1 and ethyl 2-oxoacetate in a 76% yield (2E:2Z = 32:68). The hex-2-enoate 2 was ethanolyzed and the product isomerized to ethyl (E)-6-hydroxyhex-2-enoate 3 in a 78% yield (2E:2Z = 86:14). The hydroxyester 3 was reacted with CBr₄ and PPh₃ to give the corresponding bromoester 4 in a 81% yield.²⁴ This bromoester 4 was reacted with PPh₃ in acetonitrile to give the phosphonium salt 5 in a 92% yield and the long-chain ester 6 was prepared from phosphonium salt 5 and 2E,4E-hexa-2,4-dienal in the presence of Cs₂CO₃ in a 79% yield (2E,6Z: 2E,6E = 56:44). The ester 6 was hydrolyzed and the product crystallized to afford 2E,6Z-acid 7 in a 45% yield and a 2E,6Z-purity of 99%. Finally, hydroxy-α-sanshool 8 was obtained in a 82% yield by the coupling reaction of the acid 7 with 1-amino-2-methyl-2-propanol catalyzed by 2-1H-Benzotriazole-1-yl)-1,1,3,3-tetramethyluronium (HBTU) and Et₃N.

We tried to record the nuclear magnetic resonance (NMR) spectrum of our synthetic hydroxy-α-sanshool 8 in CDCl₃ to compare it with the data reported in the literature. However, in our hands, hydroxy-α-sanshool 8 is unstable in this solvent. However, the high-performance liquid chromatography (HPLC) of our synthetic sanshool 8 was consistent with that of the natural product and the NMR spectrum of our sample in dimethyl sulfoxide (DMSO)-d₆ was supportive of the assigned structure.

Conclusion

In summary, hydroxy-α-sanshool was synthesized in eight steps using two Wittig reactions as the key assembly steps. The cis-isomer of ethyl 6-hydroxyhex-2-enoate can be isomerized to the desired trans-isomer using I₂ as the catalyst. 2E,6Z,8E,10E-Dodecatetraenoic acid was crystallized from a solution in 1% ethyl acetate in n-hexane. The work-ups were simple and the overall yield of the product was high. All the intermediate and target compounds were characterized by ¹H NMR, ¹³C NMR, and MS spectra.

Experiment

All the reagents were purchased from commercial suppliers without further purification unless otherwise specified. ¹H NMR and ¹³C NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer. NMR spectra were recorded in DMSO-d₆ solutions at room temperature (20 °C ± 2 °C). ¹H and ¹³C chemical shifts are quoted in parts per million downfield from TMS. Supplemental MS spectra were recorded on a Bruker Esquire 3000 instrument. High-resolution mass spectra (HRMS) were obtained on a MicrOTOF-Q II mass spectrometer with a supplemental source (Waters, Manchester). As for known compounds, only ¹H NMR and ¹³C NMR spectra were confirmed with previously reported literature, and the main intermediates were characterized by ¹H NMR, ¹³C NMR spectra, and mass spectra.

4-Bromobutyl acetate

It was prepared according to the reported procedure.²³ light yellow liquid 27.6 g, yield 85%. ¹H NMR (400 MHz, DMSO-d₆, ppm) δ 4.03 (t, J = 6.8 Hz, 2H), 3.42 (t, J = 6.4 Hz, 2H), 2.01 (s, 3H), 1.81–1.88 (m, 2H), 1.50–1.57 (m, 2H). ¹³C NMR (101 MHz, DMSO-d₆, ppm) δ 171.2, 67.7, 36.0, 31.6, 29.9, 25.8.

[4-(Acetyloxy)butyl]triphenylphosphonium bromide (1)

4-Bromobutyl acetate (23.5 g, 0.12 mol) and PPh3 (30 g, 0.12 mol) were added to 250 mL acetonitrile and refluxed overnight. Acetonitrile was distilled off under reduced pressure to obtain a pale yellow viscous oil. After 150 mL ether was added, the mixture was frozen for 1 h, to give a white solid, the solid was then filtered and washed with ethyl acetate and ether, respectively, to afford 1 as a white powder, 51.3 g, 93%.^{24
¹}H NMR (600 MHz, DMSO-d₆, ppm): δ 7.85–7.73 (m, 15H), 4.13–4.06 (t, J = 7.1 Hz, 2H), 2.05–1.71 (s, 3H), 1.71–1.53 (m, 2H), 1.19 (m, 4H). ¹³C NMR (151 MHz, DMSO-d₆, ppm) δ 165.8, 147.6, 135.2, 134.0, 133.9, 130.6, 122.4, 119.0, 118.4, 60.1, 32.3, 20.8, 20.2, 14.5.

(Z/E)Ethyl 6-acetoxy-2-hexenoate (2)

To a solution of 1 (50 g, 0.11 mol) and ethyl 2-oxoacetate (11.2 g, 0.11 mol) in CH₂Cl₂ (250 mL) was added Cs₂CO₃ (71.2 g, 0.22 mol), and the mixture was refluxed for 24 h under nitrogen atmosphere. CH₂Cl₂ was distilled and 250 mL of n-hexane was added to the residue, the mixture was then stirred for 30 min, and the cake was filtered and washed twice with 200 mL of n-hexane. n-Hexane was distilled to obtain a yellow oil. The oil was distilled under vacuum, and the 120 °C–125 °C fraction (15 torr) was collected to afford 2 as a colorless oil, 16.6 g, 76% (2E:2Z = 32:68).^{16
¹}H NMR (400 MHz, DMSO-d₆, ppm): δ (E)-isomer 6.89 (dt, J = 15.7 Hz, 6.9 Hz, 1H), 5.88 (d, J = 15.7 Hz, 1H), 4.15–4.09 (q, 2H), 3.99 (t, J = 6.5 Hz, 2H), 2.29–2.21 (m, 2H), 2.00 (s, 3H), 1.73 (p, J = 6.9 Hz, 2H), 1.21 (t, J = 7.1 Hz, 3H). ¹³C NMR (100 MHz, DMSO-d₆, ppm): δ 170.8, 166.0, 148.9, 121.8, 63.5, 60.2, 28.5, 26.9, 21.1, 14.5. HRMS (Supplemental) m/z: Anal. calcd for C₁₀H₁₆O₄ [M + Na]⁺ 223.0941; found 223.0943.

Ethyl 6-hydroxy-2E-hex-2-enoate (3)

The strong acidic styrene type cation exchange resin 732 (15 g) was added into a solution of compound 2 (14.5 g, 71.92 mmol) dissolved in 80 mL ethanol and refluxed overnight. The reaction was cooled to room temperature, and then the resin was filtered and washed twice with 30 mL ethanol. The ratio of E- and Z-isomers was 67:33. 10 Drops of I₂ in ethanol solution (0.1 g/mL) were added to the reaction mixture that was refluxed for 3 h. Ethanol was removed to afford a brown oil. 100 mL CH₂Cl₂ was added to the oil, which was washed with water (3 × 20 mL), and the organic phase was dried over anhydrous sodium sulfate. The CH₂Cl₂ was distilled to obtain 3 as a light yellow oil (E:Z = 86:14), which could be used directly in the next reaction, 8.9 g, 78% (2E:2Z=86:14).^{15
¹}H NMR (400 MHz, CDCl₃, ppm): δ 6.93 (dt, J = 15.7, 6.9 Hz, 1H), 5.80 (d, J = 15.6 Hz, 1H), 4.19–4.11 (q, J = 7.1 Hz, 2H), 3.61 (t, J = 5.4 Hz, 2H), 2.26 (m, 2H), 1.73–1.64 (m, 2H), 1.27 (t, J = 7.1 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃, ppm): δ 166.6, 148.5, 121.4, 61.5, 60.1, 30.7, 28.3, 14.0. HRMS (Supplemental) m/z: Anal. calcd for C₈H₁₄O₃ [M + H]⁺ 159.0140; found 159.0141.

Ethyl 6-bromo-2E-hexenoate (4)

CBr₄ (4.4 g, 12.64 mmol) was added to a solution of PPh₃ (13.2 g, 50.56 mmol) in 50 mL CH₂Cl₂ cooled in ice bath, and the reaction was stirred for 1 h. To the reaction was added a solution of compound 3 (8.0 g, 50.56 mmol) in 10 mL CH₂Cl₂, the reaction was stirred for 2 h in ice salt bath and then another 2 h at room temperature. The solvent was distilled off to obtain a viscous yellow solid, to the viscous solid was added 50 mL n-hexane and stirred vigorously until the mixture was dispersed into a granular solid, the solid was filtered and washed with n-hexane (2 × 20 mL), and n-hexane was evaporated under vacuum to give 4 as a light yellow oil, which was used directly in the next reaction, 9.1 g, 81%.^{12
¹}H NMR (400 MHz, CDCl₃, ppm): δ 6.89 (dt, J = 15.7, 7.0 Hz, 1H), 5.86 (d, J = 15.6 Hz, 1H), 4.17 (q, J = 7.1 Hz, 2H), 3.40 (t, J = 6.5 Hz, 2H), 2.40–2.32 (m, 2H), 2.03–1.97 (m, 2H), 1.27 (t, J = 7.2 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃, ppm): δ 166.2, 146.6, 122.4, 60.1, 32.4, 30.6, 30.2, 14.1. HRMS (Supplemental) m/z: Anal. calcd for C₈H₁₃BrO₂ [M + H]⁺ 221.0172; found 221.0174.

(6-Ethoxyl-6-oxo-4E-hexenyl)triphenylphosphonium bromide (5)

Compound 4 (8.8 g, 39.98 mmol) and PPh₃ (10.5 g, 39.98 mmol) were added to 50 mL acetonitrile, and the reaction mixture was refluxed overnight. Acetonitrile was distilled off under reduced pressure to obtain a pale yellow viscous oil. 100 mL ether was added to the oil and frozen for 1 h in the refrigerator. The solid was filtered and washed twice with ethyl acetate and ether, respectively, to afford 5 as a white powder, 17.9 g, 92%.¹² m.p. 142–144 °C. ¹H NMR (400 MHz, DMSO-d₆, ppm): δ 7.86–7.72 (m, 15H), 6.86 (dt, J = 15.7, 6.7 Hz, 1H), 5.93–5.82 (d, J = 15.7 Hz, 1H), 4.09 (q, J = 7.1 Hz, 2H), 3.67–3.59 (m, 2H), 2.40 (m, 2H), 1.76–1.67 (m, 2H), 1.19 (t, J = 7.1 Hz, 3H). ¹³C NMR (100 MHz, DMSO-d₆, ppm): δ 165.6, 147.4, 135.1, 135.0, 133.8, 133.7, 130.4, 130.3, 122.2, 119.0, 118.1, 59.9, 32.2, 32.0, 28.8, 28.6, 20.8, 14.3. HRMS (Supplemental) m/z: Anal. calcd for C₂₆H₂₈O₂P⁺ 403.1821; found 403.1819.

Ethyl 2E,6Z,8E,10E-dodecatetraenoate (6)

To a solution of 5 (14.9 g, 30.76 mmol) and 2E,4E-hexadienal (3.0 g, 30.76 mmol) in CH₂Cl₂ (100 mL) was added Cs₂CO₃ (20.1 g, 61.52 mmol) and stirred for 24 h at 45 °C under nitrogen atmosphere. After cooled to room temperature, celite (10.0 g) was added to the reaction mixture, which was stirred for 1 h and filtered, the cake was washed with CH₂Cl₂ (3 × 50 mL), and the combined filtrate was concentrated in vacuum. n-Hexane (100 mL) was added to the mixture, the mixture was filtered, the cake was washed with n-hexane (3 × 20 mL), and the solvent was evaporated in vacuum to afford 6 as a colorless oil, which was used directly in the next reaction, 5.4 g, 79% (the percentage of 2E,6Z isomer in the mixture was 56%).^{12
¹}H NMR (400 MHz, DMSO-d₆, ppm): δ 6.92–6.82 (dt, J = 15.5, 6.5 Hz, 1H), 6.27–5.95 (m, 4H), 5.87 (d, J = 15.7 Hz, 1H), 5.78–5.65 (m, 1H), 5.37 (m, 1H), 4.09 (q, J = 7.1 Hz, 2H), 2.29 (m, 4H), 1.73 (d, J = 6.4 Hz, 3H), 1.19 (t, J = 7.1 Hz, 3H). ¹³C NMR (100 MHz, DMSO-d₆, ppm): δ 166.0, 149.1, 133.7, 132.3, 130.2, 130.1, 129.9, 126.0, 121.8, 60.1, 31.8, 26.2, 18.5, 14.5. HRMS (Supplemental) m/z: Anal. calcd for C₁₄H₂₀O₂ [M + Na]⁺ 243.1356; found 243.1359.

2E,6Z,8E,10E-dodecatetraenoic acid (7)

Compound 6 (5.0 g, 22.70 mmol) and NaOH (1.8 g, 45.39 mmol) were added to a mixture of 120 mL water and 60 mL methanol and refluxed for 2 h. After cooled to room temperature, methanol was removed, and the reaction mixture was extracted with diethyl ether(3 × 50 mL). The aqueous phase was acidified with 1 mol/L HCl to pH = 1, which was then extracted with CH₂Cl₂ (3 × 50 mL). The organic layer was washed with brine, and dried over Na₂SO₄. CH₂Cl₂ was concentrated in vacuum to afford a yellowish viscous solid. The crude product was recrystallized twice from 1% ethyl acetate in hexane to obtain 7 as a white solid, 2.0 g with 45% yield (the purity of 2E,6Z-isomer was 99%).¹² m.p. 93.5–95 °C (lit.11 93.5–95 °C); ¹H NMR (400 MHz, DMSO-d₆, ppm): δ 12.11 (s, 1H), 6.81 (dt, J = 15.6, 6.5 Hz, 1H), 6.46–6.35 (m, 1H), 6.23–6.11 (m, 2H), 6.02 (t, J = 11.0 Hz, 1H), 5.81–5.69 (m, 2H), 5.36 (dt, J = 7.7, 6.2 Hz, 1H), 2.34–2.23 (m, 4H), 1.76–1.72 (d, J = 7.1 Hz, 3H). ¹³C NMR (100 MHz, DMSO-d₆, ppm): δ 167.1, 148.0, 133.3, 131.9, 129.9, 129.7, 129.5, 125.6, 122.3, 31.4, 25.9, 18.1. HRMS (Supplemental) m/z: Anal. calcd for C₁₂H₁₆O₂ [M + Na]⁺ 215.1048; found 215.1051.

Hydroxy-α-sanshool (8)

To a mixture of 7 (1.7 g, 8.84 mmol), 1-amino-2-methyl-2-propanol (1.2 g, 13.46 mmol), and triethylamine (3.5 g, 34.59 mmol) in CH₂Cl₂ (20 mL) was added HBTU (4.9 g, 12.92 mmol) and stirred for 1 h at room temperature. 50 mL water was added to the reaction mixture, which was then extracted with CH₂Cl₂ (3 × 30 mL), the organic phase was washed with 1 mol/L HCl, 5% NaHCO₃, and brine, respectively, and dried with Na₂SO₄, CH₂Cl₂ was evaporated to afford a colorless oil. Petroleum ether (20 ml) was added to the oil, the mixture was placed for 2 h in the refrigerator, which was then violently stirred for 10 min, the formed solid was filtered and dried in vacuum to obtain 8 as a white solid, 1.9 g, 82%.¹² The solid turned dark brown during heating and dissolves at 84–90 °C. ¹H NMR (400 MHz, DMSO-d₆, ppm): δ 7.76–7.71 (m, 1H), 6.65–6.57 (m, 1H), 6.45–6.34 (m, 1H), 6.29–5.94 (m, 4H), 5.73 (dq, J = 13.8, 6.8 Hz, 1H), 5.43–5.33 (m, 1H), 4.44 (s, 1H), 3.08 (d, J = 6.0 Hz, 2H), 2.25 (dq, J = 13.8, 7.5 Hz, 4H), 1.74 (d, J = 6.8 Hz, 3H), 1.04 (s, 6H). ¹³C NMR (101 MHz, DMSO-d₆, ppm): δ 165.3, 141.7, 133.3, 132.0, 130.2, 129.7, 129.4, 125.7, 125.1, 69.6, 49.8, 31.5, 27.4, 27.4, 26.4, 18.2. HRMS (Supplemental) m/z: Anal. calcd for C₁₆H₂₅NO₂ [M + Na]⁺ 286.1778; found 286.1781.

Supplemental Material

sj-pdf-1-chl-10.1177_1747519820974323 – Supplemental material for Synthesis of hydroxy-α-sanshool

Supplemental material, sj-pdf-1-chl-10.1177_1747519820974323 for Synthesis of hydroxy-α-sanshool by Jianjun Zhou, Yan Xiao, Taiping Chen, Jiyu Gao, Wencai Huang and Zicheng Li in Journal of Chemical Research

Footnotes

Acknowledgements

The authors thank State Key Laboratory of Biotherapy, West China Hospital, Sichuan University, for ¹H NMR, ¹³C NMR, and high-resolution mass spectra (HRMS) determination.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was financially supported by Sichuan Provincial Administration of Traditional Chinese Medicine 2018HJZX04.

ORCID iD

Zicheng Li

Supplemental material

Supplemental material for this article is available online.

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