Synthesis of Benzothiophene-Fused Pyran Derivatives via Piperidine Promoted Domino Reaction

Li, Shihang; Yu, Aimin; Li, Jianfa; Meng, Xiangtai

doi:https://doi.org/10.1155/2019/4361410

Heteroatom Chemistry

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Abstract Introduction Materials and Methods Results and Discussion Conclusions Experimental Data Availability Conflicts of Interest Acknowledgments Supplementary Materials References Copyright Related Articles

Research Article | Open Access

Volume 2019 | Article ID 4361410 | https://doi.org/10.1155/2019/4361410

Synthesis of Benzothiophene-Fused Pyran Derivatives via Piperidine Promoted Domino Reaction

Shihang Li,¹Aimin Yu,¹Jianfa Li,¹and Xiangtai Meng¹

Academic Editor: David Barker

Received01 Dec 2018

Accepted20 Mar 2019

Published02 May 2019

Abstract

A new domino reaction between thioaurones and malononitrile has been reported. This reaction allows efficient access to benzothiophene-fused pyran derivatives in good yields under mild reaction conditions. The substrate scope is broad; a series of benzothiophene-fused pyran derivatives have been synthesized.

1. Introduction

Fused pyran ring existing in both numerous natural products and synthetic compounds is an important heteroatom framework [1–5], which demonstrate great function on pharmacological activities, antibacterial, antiviral, anticoagulant, antianaphylactic, anticancer, diuretic activities, neurodegenerative disorders, and so on [6–10]. Recently, 2-aminochromenes are found to be employed as pigments, cosmetics, and agrochemicals [11–13]. Furthermore, the therapeutically effect on immune system diseases and diabetic complications entitled by substituted 2-amino-benzochromenes have been proved [14]. To date, there have been only limited methods to construct of a 2-amino-3-cyan-pyranskeleton. Klimochkin’s group developed a convenient one-step synthesis of 4-unsubstituted 2-amino-4H-chromene-2-carbonitriles from quaternary ammonium salts (Scheme 1(a)) [15].

Subsequently, Takaki’s research group was given an efficient synthetic strategy for 2-amino-4H-chromenes from photochemical generated o-quinone methides and malononitrile (Scheme 1(b)) [16]. After that, Rao’s group also designed and synthesized a series of pyran derivatives in good yields by utilizing Baylis–Hillman chemistry (Scheme 1(c)) [17]. For the past decades, there was a rapid development on the organic small molecule catalyzed domino reaction. During our ongoing investigation of domino reactions, our research group has developed many domino reactions on thioaurone. Many benzothiophene-fused heterocycles were synthesized (Scheme 2(a)) [18–22]. Herein, we will report another new domino reaction between thioaurone and malononitrile. To our surprise, a series of benzothiophene-fused pyran derivatives were obtained (Scheme 2(b)).

2. Materials and Methods

Material 1 was synthesized and reported in our previous work [18, 19] and 2 was purchased from commercial access.

General synthetic procedure for 3 was as follows: under Ar atmosphere, to a solution of 1 (0.2 mmol) in dichloromethane (DCM) (2.0 mL) 2 (0.4 mmol) and piperidine (10 mol%) were added and the mixture was stirred at room temperature for 2 h. After extraction with DCM, the organic layer was washed with saturated aqueous NaCl and dried over MgSO₄ and then concentrated under reduced pressure. The residue was purified through flash column chromatography on silica gel (petroleum ether/ethyl acetate = 1:1 to 5:1) to afford the desired product 3.

3. Results and Discussion

The reaction between thioaurone 1a and malononitrile 2 in dichloromethane as the solvent under reflux was first performed. Unfortunately, no product was detected by TLC (Table 1, entry 1). Then piperidine was added as a catalyst to promote the reaction. To our surprise, the reaction could give a quickly and cleanly conversion and the product was obtained in a 70% yield (Table 1, entry 2). The structure of the product 3a was established by X-ray crystallography (Figure 1) [23]. Encouraged by this result, the solvent effect was examined to optimize the reaction condition. There was only a feebly variation of the yield given by the different solvent such as chloroform, acetonitrile, tetrahydrofuran, and ethyl alcohol; the reaction afforded the yields of 68%-73% after stirring at the corresponding reflux temperature (Table 1, entries 3-6). When selecting toluene as the solvent, there was a negative effect on the conversion; the yield dropped to 57% (Table 1, entry 7). In the screening process, the additive effect of acetic acid was also screened. Insignificantly, there was no visible fluctuation on the yield (Table 1, entry 8). As the reflux temperature provided a moderate yield, the reaction was performed at room temperature (Table 1, entries 9-11). After attempting the above studies, the best reaction condition is at room temperature using piperidine as catalyst, and the yield up to 83% (entry 10).

With the best reaction conditions in hand, the substrate scope was examined with a series of thioaurone 1. Firstly, the ethyl ester on the R² functional group switched to a benzyl ester, leading to the desired product in yield of 52% (Table 2, entry 2). Subsequently, thioaurone 1 with aromatic groups on the 1 was also examined, for example, o- and m-chloro substituted 1c and 1d, p-methyl substituted 1e. And as a consequence, the para methyl-substituted substrate was not given an optimistic effect, but the other two were tolerated well and excellent; the yield was reached to 83% and 99%, respectively (Table 2, entries 3-5). Furthermore, the effect of R¹ was also studied. When using halogen atom to replace the methyl on the C5 position, the fluoro and bromo substituted substrates were given the corresponding products in 60% and 59% yields, respectively (Table 2, entries 6-7). Substrate 1h, bearing a 6-MeO group (R¹), also worked well and furnished the desired product in 71% yield. In addition, 7-Cl-substituted substrate 1i was also screened in this domino reaction. The corresponding product 3i was obtained in yield of 80% (Table 2, entry 9).

In order to explore the domino reaction scope, ethyl 2-cyanoacetate (2b) was used in this domino reaction (Scheme 3). To our surprise, the corresponding product 4 was obtained in yield of 63%. The structure of 4 was confirmed by X-ray crystal structure analysis (Figure 2) [23].

4. Conclusions

In conclusion, a novel piperidine-catalyzed + domino reaction between thioaurone and malononitrile was developed. A number of benzothiophene ring fused 2-amino-3-cyano-pyran derivatives were obtained in good yields. The product structure was identified by NMR, HRMS, and X-ray crystal structure.

5. Experimental

The ¹H- and ¹³C-NMR spectrum were recorded at ambient temperature on Bruker 400 instruments. All spectra were referenced to CDCl₃ (¹H δ 7.26 ppm and ¹³C NMR δ 77.00 ppm) and DMSO-d₆ (¹H δ 2.50 ppm and ¹³C NMR δ 39.52 ppm). HRMS were obtained on Waters Xevo Q-TOF MS with ESI resource. Melting points were measured on a RY-I apparatus and are reported to be uncorrected.

Ethyl 2-amino-3-cyano-8-methyl-4H-benzothieno[3,2-b]pyran-4-carboxylate (3a). Yellow solid, m.p. 182-184°C; IR (KBr): 3411, 3332, 2362, 2336, 2192, 1719, 1653, 669 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ = 7.60 (d, J = 8.3 Hz, 1H, Ar-H), 7.45 (s, 1H, Ar-H), 7.20 (d, J = 8.3 Hz, 1H, Ar-H), 4.91 (s, 2H, N), 4.62 (s, 1H, CH), 4.33 – 4.22 (m, 2H, OCH₂CH₃), 2.46 (s, 3H, Ar-C), 1.34 (t, J = 7.1 Hz, 3H, OCH₂C) ppm; ¹³C NMR (100 MHz, CDCl₃) δ = 169.8 (COOEt), 160.8, 139.4, 134.8, 133.8, 129.0, 127.6, 122.4, 119.7, 119.1, 110.9, 62.3 (OCH₂CH₃), 55.0 (CCN), 40.3 (CH), 21.4 (Ar-CH₃), 14.2 (OCH₂CH₃) ppm; ESI-HRMS [M+H] calcd. for C₁₆H₁₅N₂O₃S 315.0798, found 315.0801.

Benzyl2-amino-3-cyano-8-methyl-4H-benzothieno[3,2-b]pyran-4-carboxylate (3b). White solid, m.p. 177-179°C; IR (KBr): 3378, 3325, 3211, 2360, 2342, 2205, 1739, 1587, 1540, 734, 799 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ = 7.60 (d, J = 8.3 Hz, 1H, Ar-H), 7.46 (s, 1H, Ar-H), 7.37 (m, 5H, Ar-H), 7.20 (d, J = 8.3 Hz, 1H, Ar-H), 5.23 (s, 2H, PhC), 4.90 (s, 2H, N), 4.69 (s, 1H, CH), 2.45 (s, 3H, Ar-C) ppm; ¹³C NMR (100 MHz, CDCl₃) δ = 169.7 (COOBn), 160.8, 139.4, 135.0, 134.8, 133.82, 129.0, 128.6, 128.5, 128.4, 127.7, 122.4, 119.7, 110.7, 68.0 (OCH₂Bn), 54.9 (CCN), 40.3 (CH), 21.4 (Ar-CH₃) ppm; ESI-HRMS [M+H] calcd. for C₂₁H₁₇N₂O₃S 377.0954, found 377.0957.

2-amino-4-(2-chlorophenyl)-8-methyl-4H-benzothieno[3,2-b]pyran-3-carbonitrile (3c). Red solid, m.p. 236-238°C; IR (KBr): 3482, 3321, 3284, 2360, 2200, 1650, 1581, 863, 800, 763, 745 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ = 7.58 – 7.50 (m, 2H, Ar-H), 7.40 (d, J = 8.3 Hz, 1H, Ar-H), 7.33 – 7.29 (m, 1H, Ar-H), 7.25 – 7.16 (m, 3H, Ar-H), 5.58 (s, 1H, CH), 4.79 (s, 2H, N), 2.47 (s, 3H, Ar-C) ppm; ¹³C NMR (100 MHz, DMSO) δ = 161.5, 140.9, 138.7, 135.0, 133.3, 132.4, 130.7, 130.3, 129.8, 129.2, 128.5, 127.8, 123.5, 120.4, 119.6, 117.0, 54.8 (CCN), 37.4 (CH), 21.5 (Ar-CH₃) ppm; ESI-HRMS [M+H] calcd. for C₁₉H₁₄N₂OSCl 353.0510, found 353.0515.

2-amino-4-(3-chlorophenyl)-8-methyl-4H-benzothieno[3,2-b]pyran-3-carbonitrile (3d). White solid, m.p. 204-205°C; IR (KBr): 3470, 3322, 2360, 2342, 2199, 1661, 1581, 807, 799 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ = 7.61 – 7.47 (m, 2H, Ar-H), 7.28 (d, J = 7.8 Hz, 2H, Ar-H), 7.25 (s, 1H, Ar-H), 7.19 (d, J = 7.3 Hz, 2H, Ar-H), 4.93 (s, 1H, CH), 4.80 (s, 2H, N), 2.48 (s, 3H, Ar-C) ppm; ¹³C NMR (100 MHz, CDCl₃) δ = 159.8, 144.8, 138.4, 134.9, 133.8, 130.2, 129.2, 128.2, 127.8, 127.4, 125.9, 122.6, 119.8, 119.3, 117.3, 60.2 (CCN), 39.9 (CH), 21.5 (Ar-CH₃) ppm; ESI-HRMS [M+H] calcd. for C₁₉H₁₄N₂OSCl 353.0510, found 353.0513.

2-amino-8-methyl-4-(p-tolyl)-4H-benzothieno[3,2-b]pyran-3-carbonitrile (3e). White solid, m.p. 249-251°C; IR (KBr): 3466, 3314, 2360, 2199, 1660, 1584, 1400, 872, 804 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ = 7.57 – 7.48 (m, 2H, Ar-H), 7.21 – 7.10 (m, 5H, Ar-H), 4.92 (s, 1H, CH), 4.71 (s, 2H, N), 2.48 (s, 3H, Ar-C), 2.32 (s, 3H, Ar-C) ppm; ¹³C NMR (100 MHz, CDCl₃) δ = 139.9, 138.1, 137.6, 134.7, 133.8, 129.6, 129.3, 127.4, 127.1, 122.56 119.7, 119.5, 118.5, 61.2 (CCN), 39.7 (CH), 21.5 (Ar-CH₃), 21.1 (Ar-CH₃) ppm; ESI-HRMS [M+H] calcd. for C₂₀H₁₇N₂OS 333.1056, found 333.1058.

Ethyl 2-amino-3-cyano-8-fluoro-4H-benzothieno[3,2-b]pyran-4-carboxylate (3f). Gray solid, m.p. 165-167°C; IR (KBr): 3424, 3372, 3327, 2198, 1739,1720, 1659, 1586, 854 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ = 7.66 (dd, J = 8.8, 4.5 Hz, 1H, Ar-H), 7.31 (dd, J = 8.7, 2.2 Hz, 1H, Ar-H), 7.13 (td, J = 8.8, 2.4 Hz, 1H, Ar-H), 4.97 (s, 2H, N), 4.64 (s, 1H, CH), 4.36 – 4.21 (m, 2H, OCH₂CH₃), 1.35 (t, J = 7.1 Hz, 3H, OCH₂C) ppm; ¹³C NMR (100 MHz, CDCl₃) δ =169.6 (COOEt), 160.8 (d, J = 242.3 Hz), 160.6, 139.3 (d, J = 4.3 Hz), 131.8 (d, J = 1.7 Hz), 129.7 (d, J = 9.8 Hz), 124.1 (d, J = 9.2 Hz), 118.9, 114.7 (d, J = 25.2 Hz), 113.3, 105.7 (d, J = 24.5 Hz), 62.5 (OCH₂CH₃), 54.7 (CCN), 40.3 (CH), 14.2 (OCH₂CH₃) ppm; ESI-HRMS [M+H] calcd. for C₁₅H₁₂N₂O₃SF 319.0547, found 319.0551.

Ethyl 2-amino-8-bromo-3-cyano-4H-benzothieno[3,2-b]pyran-4-carboxylate (3g). Yellow solid, m.p. 195-197°C; IR (KBr): 3460, 3366, 3314, 2194, 1740, 1720, 1652, 1583, 859, 874 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ = 7.75 (s, 1H, Ar-H), 7.58 (d, J = 8.6 Hz, 1H, Ar-H), 7.45 (d, J = 8.6 Hz, 1H, Ar-H), 4.99 (s, 2H, N), 4.64 (s, 1H, CH), 4.37 – 4.23 (m, 2H, OCH₂CH₃), 1.36 (t, J = 7.1 Hz, 3H, OCH₂C) ppm; ¹³C NMR (100 MHz, CDCl₃) δ = 169.5 (COOEt), 160.5, 138.8, 135.2, 130.2, 129.0, 124.1, 122.6, 118.9, 112.8, 62.5 (OCH₂CH₃), 54.9 (CCN), 40.2 (CH), 14.2 (OCH₂CH₃) ppm; ESI-HRMS [M+H] calcd. for C₁₅H₁₂N₂O₃SBr 378.9747, found 378.9751.

Methyl2-amino-3-cyano-7-methoxy-4H-benzothieno[3,2-b]pyran-4-carboxylate (3h). Red solid, m.p. 187-189°C; IR (KBr): 3447, 3384, 3350, 2196, 1743, 1651, 1584, 848, 831 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ = 7.54 (d, J = 8.8 Hz, 1H, Ar-H), 7.19 (d, J = 2.1 Hz, 1H, Ar-H), 7.00 (dd, J = 8.8, 2.1 Hz, 1H, Ar-H), 4.90 (s, 2H, N), 4.61 (s, 1H, CH), 3.86 (s, 3H, Ar-OC), 3.82 (s, 3H, COOC) ppm; ¹³C NMR (100 MHz, CDCl₃) δ = 170.4 (COOCH₃), 160.8, 158.6, 139.4, 138.3, 122.7, 120.5, 119.1, 114.9, 107.7, 105.4, 55.7 (COOCH₃), 55.0 (CCN), 53.0 (Ar-OCH₃), 40.1 (CH) ppm; ESI-HRMS [M+H] calcd. for C₁₅H₁₃N₂O₄S 317.0591, found 317.0598.

2-amino-6-chloro-4-phenyl-4H-benzothieno[3,2-b]pyran-3-carbonitrile (3i). Yellow solid, m.p. 231-233°C; IR (KBr): 3345, 3314, 3282, 2205, 1647, 1584, 1172, 817, 784 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ = 7.64 (dd, J = 6.6, 2.2 Hz, 1H, Ar-H), 7.37 (m, 4H, Ar-H), 7.30 (m, 3H, Ar-H), 4.99 (s, 1H, CH), 4.78 (s, 2H, N) ppm; ¹³C NMR (100 MHz, CDCl₃) δ = 159.5, 142.3, 138.6, 135.7, 130.6, 129.0, 128.4, 128.1, 127.6, 126.1, 125.2, 119.7, 119.1, 118.3, 60.9 (CCN), 40.1 (CH) ppm; ESI-HRMS [M+H] calcd. for C₁₈H₁₂N₂OSCl 339.0353, found 339.0354.

Diethyl 2-amino-8-methyl-4H-benzothieno[3,2-b]pyran-3,4-dicarboxylate (4). White solid, m.p. 139-141°C, IR (KBr): 3367, 3271, 2979, 2913, 1724, 1685, 1631, 804, 874 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ = 7.55 (d, J = 8.4 Hz, 1H, Ar-H), 7.38 (s, 1H, Ar-H), 7.12 (dd, J = 8.4, 2.0 Hz, 1H, Ar-H), 6.64 (br, 2H, N), 4.78 (s, 1H, CH), 4.10-4.29 (m, 4H, 2 × OCH₂CH₃), 2.41 (s, 3H, Ar-C), 1.31 (t, J = 7.2 Hz, 3H, OCH₂C), 1.26 (t, J = 7.2 Hz, 3H, OCH₂C) ppm; ¹³C NMR (100 MHz, CDCl₃) δ = 172.2 (COOEt), 169.0 (COOEt), 160.6, 139.1, 134.3, 133.5, 129.4, 127.0, 122.2, 119.5, 112.8, 72.8 (CCOOEt), 61.4 (OCH₂CH₃), 59.7 (OCH₂CH₃), 40.4 (CH), 21.3 (Ar-CH₃), 14.3 (OCH₂CH₃), 14.3 (OCH₂CH₃) ppm; ESI-HRMS [M+H] calcd. for C₁₈H₂₀NO₅S 362.1057, found 362.1068.

Data Availability

The copies of NMR spectra data used to support the findings of this study are included within the supplementary information file(s) (available here).

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Grant no. 21403154), the Natural Science Foundation of Tianjin (Grant no. 13JCYBJC38700), and the Tianjin Municipal Education Commission (Grants nos. 20120502, 20180KJ137). Xiangtai Meng is grateful for the support from the 131 Talents Program of Tianjin and Training Project of Innovation Team of Colleges and Universities in Tianjin (TD13-5020). Shihang Li is grateful for the support from the National Innovation Training Program of Undergraduates (no. 201710060149).

Supplementary Materials

NMR spectra of all new compound (PDF) and crystallographic data for compound 3a and 4 (CIF). (Supplementary Materials)

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