Synthesis of Novel Thiazoles Based on (+)-Usnic Acid

Abstract

A series of usnic acid derivatives containing a thiazole ring with an amide substituent were synthesized. The convenient method for synthesis of these compounds is a reaction of 14-bromousnic acid with N-acylthioureas. Acylation of aminothiazole does not lead to the targeted compound.

1. Introduction

(+)-Usnic acid 1 (Figure 1) is an accessible metabolite of lichens. Its biological activity is very diverse and is of interest for pharmaceuticals. Its wide distribution in various types of lichens, the simplicity of the isolation procedure from plant materials, and the high optical purity of the extracted compound draw the attention of researchers as a basis for the development of new pharmacological agents [1]. Usnic acid derivatives containing thiazole fragments in ring A exhibit a wide range of biological activities; in particular, those containing an aminothiazole substituent (compound 2) exhibit antimycobacterial properties [2], and hydrazonothiazoles (compounds 3 and 4) exhibit antiviral activity [3] and enhance the cytostatic efficiency in anticancer therapy [4] (Figure 1).

However, the hydrazone structural fragment of compounds 3 and 4 is an undesirable structural element, since it belongs to common pan-assay interference compounds (PAINS) [5]. PAINS are chemical compounds that often give false positive results in high-throughput screens [6]. PAINS tend to react nonspecifically with numerous biological targets rather than specifically affecting one desired target.

The goal of this work was the synthesis of an analog of compounds 3 and 4 containing thiazole and aryl(hetaryl) substituents connected not by a hydrazone but by an amide linker. Such a functional group is poorly reactive and resistant to hydrolysis. In addition, the presence of an electronegative nitrogen atom, as well as an oxygen atom, capable of forming hydrogen bonds, determines the use of the amide structural motif for the creation of protein-targeted drugs. Amides commonly occur in drugs, and have both basic and acidic possibilities. However, their basic pK_a values are very low, and their acidic pK_a values are very high, and, unless affected by substitution, they are always unionized at physiological pH [7]. Amides are commonly used in biologically active compounds, with a broad variety of biotechnological, agricultural, and medical applications. Amides and their derivatives are associated with a broad range of biological activities, including anticonvulsant, antitubercular, antimicrobial, analgesic and anti-inflammatory, insecticidal, antitumor, fibrinolytic, and antiplatelet aggregator activities [8,9,10,11,12,13,14]. Amide bonds are well known in biological and natural pathways. Amides are one of the most essential functionalities of anticonvulsant drugs in chemical building blocks [15].

2. Results and Discussion

The usnic acid derivative with an aminothiazole fragment 6 (Scheme 1), which was obtained according to the method described in work [2], is a convenient starting material for the synthesis of usnic acid derivatives, with a thiazole ring with amide linker 5. However, the acylation reaction of usnic acid derivative 6 with benzoyl chloride in the presence of triethylamine did not lead to the production of the target derivative 5. As a result of this reaction, the mixture of initial reagents was isolated (Scheme 1). This may be due to the fact that the amino group of derivative 6 was deactivated. The electron pair of the amino group is conjugated to the 4-phenylthizole π-system.

Since this approach did not lead to the desired result, a series of compounds 5a–e with a variation in the type of substituent at the carbonyl atom of the amide fragment were prepared, starting from 14-bromousnic acid 7 and the corresponding acylthiourea. For this purpose, at the first step, a series of monoacylated thioureas 8a–e was synthesized (Scheme 2). The synthesis was carried out starting from freshly prepared acyl chlorides, which were introduced into the reaction with sodium thiocyanate, followed by formation of acylisothiocyanate. This fact is consistent with the hard and soft Lewis acids and bases theory: the carbon atom of the acyl group is a hard acid and the nitrogen atom of the thiocyanate anion is a hard base. The resulting intermediate is then reacted with aqueous ammonia. The last step involves the attack of the ammonia nitrogen atom on the carbon atom of the isothiocyanate group. As a result of this reaction, compounds 8a–e were isolated with 28–40% yields. Bromousnic acid 7 was synthesized by the reaction of usnic acid with two equivalents of bromine in dioxane [3,4]. The resulting acylthioureas 8a–e were reacted with the usnic acid derivative 7 in methanol, as a result of which the target compounds 5a–e were isolated with a yield of 80–92% (Scheme 2). Structure of compounds 5a-e had been confirmed using ¹H and ¹³C NMR and mass spectroscopy (see Supplementary Materials).

3. Materials and Methods

The analytical and spectral studies were conducted at the Chemical Service Center for the collective use of the Siberian Branch of the Russian Academy of Science.

The ¹H and ¹³C-NMR spectra for solutions of the compound in CDCl₃ or DMSO-d₆ were recorded on a Bruker AV-400 spectrometer (Bruker Corporation, Karlsruhe, Germany; operating frequencies 400.13 MHz for ¹H and 100.61, for ¹³C). The residual signals of the solvent were used as references (δ_H 2.50, δ_C 39.52 for DMSO-d₆, δ_H 7.24, δ_C 76.90 for CDCl₃). The mass spectra (ionizing-electron energy 70 eV) were recorded on a DFS Thermo Scientific high-resolution mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). Electron impact ionization was used in the measurement of mass spectra. Thin-layer chromatography was performed on TLC Silica gel 60F₂₅₄ (Merck KGaA, Darmstadt, Germany). Synthetic starting materials and reagents were acquired from Reachem (Moscow, Russia). (+)-Usnic acid was obtained from Zhejiang Yixin Pharmaceutical Co., Ltd., (Lanxi, China). All chemicals were used as described unless otherwise noted.

The atom numbers in the compound provided for the assignment of signals in the NMR spectra correspond to the traditional numeration for usnic acid.

The synthesis of the bromo-substituted derivatives for 7 was performed by reaction of usnic acid with bromine in dioxane [3].

The Synthesis of Compounds 5a–e

(A)

A total of 1 mmol of carboxylic acid was placed in a flask with a solution of oxalyl chloride in methylene chloride (2 mmol in 2 mL). A few drops of DMF were added to the mixture and left to be stirred at room temperature for one hour. Afterwards, the solvent was removed using a rotary evaporator. The resulting mixture was dissolved in 10 mL of acetone, and the solution was heated to 50 °C. Sodium thiocyanate (1 mmol) was added to the solution. After 15 min, the solution was cooled, and the precipitate that formed (sodium chloride) was filtered off. The resulting solution was evaporated on a rotary evaporator. An aqueous solution of ammonia (120 µL, 30% in water) was added to the resulting mixture and left to be stirred for 30 min. The precipitate that formed was filtered off, washed with water, and dried in air.

(B)

A total of 1 mmol of bromousnic acid 7 was dissolved in 35 mL of methanol. The corresponding acylthiourea 8a–e (1 mmol) was added to the solution. The solution was stirred for 1 h. Then, the solution was diluted with water, and the precipitate that formed was filtered, washed with water and dried in air.

(R)-N-(4-(8-acetyl-1,3,7-trihydroxy-2,9a-dimethyl-9-oxo-9,9a-dihydrodibenzo[b,d] furan-4-yl)thiazol-2-yl)benzamide (5a).

Yellow amorphous powder. Yield: 95%. ¹H NMR (CDCl₃, δ): 1.76 (3H, s, H-15), 2.10 (3H, s, H-10), 2.67 (3H, s, H-12), 5.96 (1H, s, H-4), 7.48 (3H, m, H-20 and H-14), 7.60 (1H, m, H-21), 7.35 (1H, d, J = 7.35, H-14), 9.92 (1H, s, NH), 10.30 (1H, s, OH-9), 12.27 (1H, bs, OH-7), 18.82 (1H, s, OH-3). ¹³C NMR (CDCl₃, δ): 8.18 (C-10), 27.74 (C-15), 32.03 (C-12), 59.29 (C-9b), 96.85 (C-9a), 97.41 (C-4), 103.51 (C-6), 105.11 (C-2), 108.91 (C-8), 109.16 (C-14), 127.25 (C-19), 128.83 (C-20), 130.96 (C-18), 133.02 (C-21), 142.47 (C-13), 151.42 and 151.52 (C-7 and C-9), 156.1 and 156.29 (C-5a and C-16), 164.14 (C-17), 180.39 (C-4a), 191.54 (C-3), 197.94 (C-11), 201.33 (C-11). IR (cm⁻¹): 3500, 3140, 1681, 1627, 1544, 1456, 1301, 1180. HRMS: m/z 504.0991 [M]+ (calcd for (C₂₆H₂₀O₇N₂³²S)⁺: 504.0986).

(R)-N-(4-(8-acetyl-1,3,7-trihydroxy-2,9a-dimethyl-9-oxo-9,9a-dihydrodibenzo[b,d] furan-4-yl)thiazol-2-yl)-4-bromobenzamide (5b).

Yellow amorphous powder. Yield: 92%. ¹H NMR (CDCl₃, δ): 1.75 (3H, s, H-15), 1.96 (3H, s, H-10), 2.64 (3H, s, H-12), 5.95 (1H, s, H-4), 7.43 (1H, s, H-14), 7.50 (2H, AB-syst, 12.43 (1H, bs, OH-7), 18.81 (1H, s, OH-3). ¹³C NMR (CDCl₃, δ): 8.25 (C-10), 27.80 (C-15), 32.09 (C-12), 59.27 (C-9b), 96.88 (C-9a), 97.58 (C-4), 103.86 (C-6), 105.18 (C-2), 108.81 (C-8), 109.41 (C-14), 128.04 (C-21), 128.89 (C-19), 129.74 (C-18), 132.01 (C-20), 142.33 (C-13), 151.49 and 151.61 (C-7 and C-9), 155.82 and 156.30 (C-5a and C-16), 163.35 (C-17), 180.32 (C-4a), 191.59 (C-3), 197.92 (C-11), 201.42 (C-11). IR (cm⁻¹): 3594, 3147, 1681, 1627, 1556, 1456, 1303, 1180. HRMS: m/z 582.0092 [M]+ (calcd for (C₂₆H₁₉O₇N₂⁷⁹Br³²S)⁺: 582.0091).

(R)-N-(4-(8-acetyl-1,3,7-trihydroxy-2,9a-dimethyl-9-oxo-9,9a-dihydrodibenzo[b,d] furan-4-yl)thiazol-2-yl)-5-methylfuran-2-carboxamide (5c).

Yellow amorphous powder. Yield: 89%. ¹H NMR (CDCl₃, δ): 1.54–2.10 (9H, bs, H-10 and H-15 and H-22), 2.65 (3H, s, H-12), 6.00 (1H, s, H-4), 6.07 (1H, bs, H-20), 7.09 (1H, bs, H-19), 7.35 (1H, s, H-14), 10.17 (1H, s, OH-9), 13.13 (1H, bs, OH-7), 18.83 (1H, s, OH-3). ¹³C NMR (CDCl₃, δ): 7.69 (C-10), 13.19 (C 22), 27.80(C-15), 32.09 (C-12), 59.43 (C-9b), 97.17 (C-9a), 97.46 (C-4), 103.99 (C-6), 105.28 (C-2), 109.07 (C-8 and C-20), 109.47 (C-14), 118.73 (C-19), 141.92 (C-18), 143.73 (C-13), 151.30 and 151.61 (C-7 and C-9), 154.6 (C-21), 155.85 (C-5a), 156.06 and 156.29 (C-16 and C-17), 180.79 (C-4a), 191.70 (C-3), 198.24 (C-11), 201.30 (C-11). IR (cm⁻¹): 3500, 3139, 1680, 1623, 1542, 1456, 1303, 1176. HRMS: m/z 508.0931 [M]+ (calcd for (C₂₅H₂₀O₈N₂³²S)⁺: 508.0935).

(R)-N-(4-(8-acetyl-1,3,7-trihydroxy-2,9a-dimethyl-9-oxo-9,9a-dihydrodibenzo[b,d] furan-4-yl)thiazol-2-yl)-5-bromofuran-2-carboxamide (5d).

Yellow amorphous powder. Yield: 82%. ¹H NMR (DMSO-d₆, δ): 1.70 (3H, s, H-15), 2.06 (3H, s, H-10), 2.59 (3H, s, H-12), 6.17 (1H, s, H-4), 6.92 (1H, AB-syst, J = 3.58, H-20), 7.58 (2H, m, H-14 and H-19), 10.30 (1H, s, OH-9), 12.54 (1H, s, NH), 12.86 (1H, s, OH-7), 18.81 (1H, bs, OH-3). ¹³C NMR (DMSO-d₆, δ): 8.51 (C-10), 27.69 (C-15), 31.79 (C-12), 59.18 (C-9b), 96.16 (C-9a), 97.46 (C-4), 103.61 (C-6), 105.19 (C-2), 109.07 (C-8), 109.20 (C-20), 114.97 (C-14), 119.53 (C-19), 127.53 (C-21), 141.78 (C-13), 147.23 (C-18), 150.76 and 151.43 (C-7 and C-9), 154.46 (C-16), 156.21 and 156.68 (C-5a and C-16), 180.46 (C-4a), 191.44 (C-3), 198.15 (C-11), 201.14 (C-11). IR (cm⁻¹): 3521, 3137, 1679, 1621, 1554, 1469, 1305, 1182. HRMS: m/z 571.9877 [M]+ (calcd for (C₂₄H₁₇O₈N₂⁷⁹Br³²S)⁺: 571.9877).

(R)-N-(4-(8-acetyl-1,3,7-trihydroxy-2,9a-dimethyl-9-oxo-9,9a-dihydrodibenzo[b,d] furan-4-yl)thiazol-2-yl)-5-bromothiophen-2-carboxamide (5e).

Yellow amorphous powder. Yield: 90%. ¹H NMR (CDCl₃, δ): 1.74 (3H, s, H-15), 1.94 (3H, s, H-10), 2.66 (3H, s, H-12), 5.98 (1H, s, H-4), 6.96 (1H, bs, H-20), 7.33 (1H, bs, H-19), 7.43 (1H, s, H-14), 10.15 (1H, s, NH), 10.29 (1H, s, OH-9), 12.47 (1H, bs, OH-7), 18.83 (1H, s, OH-3). ¹³C NMR (CDCl₃, δ): 8.27 (C-10), 27.78 (C-15), 32.04 (C-12), 59.32 (C-9b), 96.92 (C-9a), 97.61 (C-4), 104.05 (C-6), 105.23 (C-2), 108.85 (C-8), 109.61 (C-14), 121.45 (C-21), 129.76 (C-20), 131.18 (C-19), 137.18 (C-18), 142.41 (C-13), 151.64 and 151.71 (C-7 and C-9), 155.86 (C-17), 155.88 (C-5a), 157.57 (C-16), 180.42 (C-4a), 191.61 (C-3), 197.95 (C-11), 201.42 (C-11). IR (cm⁻¹): 3500, 3139, 1682, 1624, 1548, 1454, 1301, 1176. HRMS: m/z 587.9654 [M]+ (calcd for (C₂₄H₁₇O₇N₂⁷⁹Br³²S₂)⁺: 587.9655).