(3a,8b)-5-Acetyl-3a-fluoro-6,8-dihydroxy-7,8b-dimethyl-3,3a-dihydrofuro[3,2-b]benzofuran-2(8bH)-one

Abstract

Usnetic acid is a dibenzofuran-2-ylacetic acid that can be obtained by alkaline degradation of a secondary lichen metabolite—usnic acid. In the present paper, the product of the reaction of usnetic acid with a mild fluorinating agent, Selectfluor^®, was obtained. The structure of the product was proved by a set of physical methods, including ¹H, ¹³C, ¹⁹F NMR, HMBC, HSQC, HRMS and IR spectroscopy.

1. Introduction

Usnic acid 1 is a major secondary metabolite of lichens of the genera Usnea, Cladonia, Evernia and others. It has a wide range of biological activities, including antibacterial, antiviral, anticancer, anti-inflammatory and analgesic [1,2]. It is known that the synthesis of derivatives of usnic acid leads to compounds with more pronounced activity against serious diseases such as tuberculosis, influenza and malaria [2].

Usnetic acid 2 is a derivative of usnic acid 1 (Scheme 1). The structure of usnetic acid includes a dibenzofuran-2-ylacetic acid backbone, which contains a phloroglucinol moiety. In general, the methods of usnetic acid 2 synthesis consist of the sequential degradation of the usnic acid backbone, the key step of which is the treatment of usnic acid 1 or its degradation products with concentrated solutions of potassium hydroxide (20–75%) [3,4,5,6,7].

Despite the variety of functional groups, the chemistry of usnetic acid 2 is poorly explored and is limited to the works of the early 20th century, in which the reactions of decarboxylation, alkaline degradation, methylation and acylation were studied [3,5]. At the same time, these transformations were used exclusively to prove and confirm the structure of usnic acid 1 and its degradation products, including usnetic acid 2.

Selectfluor^® is a fluorinating reagent (Scheme 2). Fluorination with its use can take place via both radical and electrophilic mechanisms. Selectfluor^® is known to react with compounds containing double bonds, including benzofurans [8,9]. In this case, the reaction proceeds with the participation of the double bond of the furan cycle. As a result, a fluorine “cation” and a nucleophile in the reaction mixture, such as methanol, are added to this bond.

In the present work, a new usnetic acid derivative has been obtained by the reaction of usnetic acid 2 with Selectfluor^®.

2. Results and Discussion

Usnetic acid 2 was obtained as described in [6] (Scheme 1). Briefly, (+)-usnic acid 1 was dissolved in an aqueous solution of 75% potassium hydroxide, and the mixture was then stirred at 90 °C for one hour. The compound obtained was purified by column chromatography. Usnetic acid 2 was then reacted with two equivalents of Selectfluor^® in an acetonitrile–water mixture (6:1) (Scheme 3). The reaction was carried out for 24 h at room temperature. Product 3 was purified by column chromatography.

The structure of compound 3 was proposed on the basis of NMR (¹H, ¹³C and ¹⁹F) spectroscopy and HRMS data (Figure 1 and sup. mat.). A single set of signals was observed in the ¹H NMR spectrum, indicating the presence of a single compound. Change in the signal of the methylene group was observed: it appeared as an AB spin system in which one of the hydrogen atoms interacted with the fluorine atom J_H-F = 11.5 Hz. This may be due to conformational difficulties that arose because of the inclusion of a methylene group in the new lactone cycle structure. A change in the chemical shifts of the methyl groups from 2.04, 2.24 and 2.65 ppm to 1.98, 1.89 and 2.58 ppm, respectively, is also observed. One of these, at 1.89 ppm, appears to be a doublet with J_H-F = 3.2 Hz, which corresponds to the methyl group in the 3-position of benzofuran. The ¹⁹F NMR spectrum shows one signal at −117.2 ppm as a doublet of doublets (J_F-H = 3.2 and 11.5 Hz). In ¹³C NMR, the absence of signals of furan carbon atoms (112 and 145 ppm) is observed, which shift to 91 and 123 ppm, respectively. The methylene carbon atom shifted from 32 ppm to 37 ppm. The methyl group of the furan cycle moved from 9 md to 18 md, with an 8 Hz doublet. The carbon atom of the methyl substituent of the furan cycle moved to 18 ppm from 9 ppm.

In the IR spectrum, the absence of signal 3600 compound 3, which refers to the O-H bond of the carboxyl group, is observed. A shift in the valence vibrations of the C=O carboxyl signal from 1700 to 1770 is also observed, which is characteristic of ester formation. The signal of the C-F bond cannot be determined due to the complexity of the spectrum.

We suppose that the fluorine atom and the methyl group are on the same side of the furanone cycle of derivative 3. The first stage of the transformation is probably the addition of fluorine to the double bond of the furanone cycle, which leads to the formation of the benzyl cation (Scheme 4). In this case, the attack of the oxygen atom of the carboxyl group can be realized only from the same side of the cycle on which it is located. Thus, the compound 3 obtained by us is a mixture of enantiomers 3a and 3b.

A similar situation was observed in [10]. During the palladium-catalyzed reaction of benzofuran-2-ylacetic acid or homolog compounds with phenylboronic acid, lactonization was observed (Scheme 5). The authors of this work also proposed a mechanism with the formation of a similar carbocation, which is then attacked by the oxygen atom of the carboxyl group to form a racemic mixture. The furan cycle substituent (hydrogen atom) and the new aryl substituent are on the same side of the furan cycle. In some examples, the structures of the obtained compounds were confirmed by X-ray structural analysis. Crystallography is a future perspective to confirm our proposed enantiomer structure of compound 3.

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, ¹³C and ¹⁹F-NMR spectra for solutions of the compound in DMSO-d₆ were recorded on a Bruker AV-400 spectrometer (Bruker Corporation, Karlsruhe, Germany; operating frequencies 400.13 MHz for ¹H, 100.61 MHz for ¹³C, 282.40 MHz for ¹⁹F). Hexafluorobenzene C₆F₆ (δ_F −162.9 ppm) and residual DMSO-d₅ (δ_H 2.50, δ_C 39.52) served as internal standards. The mass spectra (ionizing electron energy 70 eV) were recorded on a Thermo Electron Corporation DFS (Thermo Electron, Karlsruhe, Germany) instrument. The IR spectra were measured on a Bruker Vector 22 IR (Bruker Corporation, Billerica, MA, USA) spectrophotometer. Melting points were determined on an Electrothermal IA 9100 (Electrothermal, Chelmsford, UK) apparatus (1 °C/min). Thin-layer chromatography was performed on TLC silica gel 60F₂₅₄ (Merck KGaA, Darmstadt, Germany). (+)-Usnic acid was obtained from Zhejiang Yixin Pharmaceutical Co., Ltd., (Lanxi, China). All chemicals were used as described unless otherwise noted.

3.1. Usnetic Acid Preparation

The synthesis of usnetic acid 2 was performed by reaction of usnic acid 1 with aqueous solution of potassium hydroxide 75%, as described in [4].

2-(7-acetyl-4,6-dihydroxy-3,5-dimethylbenzofuran-2-yl)acetic acid (1).

Light-brown powder. Yield 36%. ¹H NMR (DMSO-d₆, δ): 2.04 (3H, s, CH₃-5), 2.24 (3H, s, CH₃-3), 2.65 (3H, s, CH₃-acetyl), 3.73 (2H, s, CH₂), 10.03 (1H, bs, OH-4), 13.77 (1H, s, OH-6). ¹³C NMR (DMSO-d₆, δ): 8.07 (CH₃-5), 9.53 (CH₃-3), 30.47 (CH₃-acetyl), 31.88 (CH₂), 100.76 (C-7), 104.94 (C-5), 111.38 (C-3a), 112.14 (C-3), 144.48 (C-2), 152.67 (C-7a), 156.96 (C-4), 161.65 (C-6), 170.60 (COOH), 200.06 (C=O-acetyl). HRMS: m/z 278.0784 [M]⁺ (calcd. for (C₁₄H₁₄O₆)⁺: 278.0785). IR (cm⁻¹): 3558, 3367, 3232, 2960, 2931, 2726, 2595, 1706, 1619, 1494, 1400, 1375, 1344, 278, 1218, 1143, 1122, 1097, 1076, 1012, 887, 815, 752, 599, 484.

3.2. Usnetic Acid Fluorination

Usnetic acid 1 (200 mg, 0.72 mmol) was dissolved in 14 mL CH₃CN:H₂O mixture (6:10). To the resulting solution, Selectfluor^® (520 mg, 1.55 mmol) was added. The mixture was stirred at room temperature for 24 h. After that, 100 mL of distilled water was added, and the resulting mixture was extracted with EtOAc (15 mL, 3 times), dried with sodium sulfate, and then evaporated. The solid was chromatographed over silica gel (60–200 mesh) using CHCl₃ eluent.

(3aR,8bS)-5-acetyl-3a-fluoro-6,8-dihydroxy-7,8b-dimethyl-3,3a-dihydrofuro[3,2-b]benzofuran-2(8bH)-one and (3aS,8bR)-5-acetyl-3a-fluoro-6,8-dihydroxy-7,8b-dimethyl-3,3a-dihydrofuro[3,2-b]benzofuran-2(8bH)-one (3)

Light gray powder. Yield 80%. M.p. 239 °C with decomposition. ¹H NMR (DMSO-d₆, δ): 1.90 (3H, d, J_H-F = 3.2 Hz, CH₃-8b), 1.98 (3H, s, CH₃-7), 2.58 (3H, s, CH₃-acetyl), 3.55 (1H, d, J_H-H = 18.5 Hz, CH₂), 3.88 (1H, dd, J_H-H = 18.5 Hz, J_H-F = 11.5 Hz, CH₂), 10.59 (1H, bs, OH-8), 13.68 (1H, s, OH-6). ¹³C NMR (DMSO-d₆, δ): 7.86 (CH₃-7), 17.55 (d, J_C-F = 7.6 Hz, CH₃-8b), 31.05 (CH₃-acetyl), 36.59 (d, J_C-F = 29.8 Hz, CH₂), 90.71 (d, J_C-F = 7.6 Hz, C-8b), 100.44 (C-5), 103.75 (C-8a), 105.91 (C-7), 123.00 (d, J_C-F = 246.7 Hz, C-3a), 158.93 (d, J_C-F = 1.7 Hz, C-4), 159.70 (C-8), 164.26 (C-6), 169.91 (d, J_C-F = 14.5 Hz, -COOR), 201.31 (C=O-acetyl). HRMS: m/z 296.0689 [M]⁺ (calcd. for (C₁₄H₁₃O₆F₁)⁺: 296.0691). IR (cm⁻¹): 3600-3000, 3006, 2948, 1783, 1764, 1625, 1498, 1429, 1400, 1371, 1344, 1299, 1263, 1249, 1197, 1160, 1147, 1126, 1106, 1043, 79, 958, 943, 914, 889, 873, 856, 829, 782, 732, 711, 688, 634, 615, 584, 489.

4. Conclusions

In this work, it was shown that the reaction of usnetic acid with Selectfluor^®, besides the addition of a fluorine “cation”, leads to the formation of a new tricycle compound containing a lactonic cycle. This approach may allow us to carry out reactions with other electrophiles to obtain similar lactones. The obtained derivative 3, due to its similarity to usnic acid, can be considered as a substance with potentially high bioactivity, which is the subject of further study.