New Heterocyclic Derivatives of Usnic Acid †

: New heterocyclic derivatives of usnic acid have been obtained by the reaction of bromous-nic acid with CS 2 -based nucleophiles. A series of compounds with dithiolane, 1,3,4-thiadiazine, and thiophene fragments was synthesized.


Introduction
Usnic acid 1 ( Figure 1) is a well-known secondary lichen metabolite, the derivatives of which, like itself, show a wide spectrum of biological activity: antibacterial, analgesic, immunomodulatory, antitumor, etc. [1][2][3]. Studies carried out over the past decades indicate that derivatives of usnic acid containing a heterocyclic substituent in the A ring are promising pharmacological agents for the treatment of a number of serious diseases such as cancer and tuberculosis [4,5]. One of the most popular platforms for heterocycle substituent formation is a bromine derivative of usnic acid 2 ( Figure 1). Previously, it was shown that the direction of the substitution reaction of the bromine atom varies with type of nucleophile [6]. The reaction with N-and O-nucleophiles leads to an intramolecular nucleophilic substitution reaction with the formation of furanone 3 ( Figure 1) [6], whereas reaction with 1,3-S,N-binucleophiles (thioureas, thioamides, thiosemicarbazones) leads to a desired substitution bromine atom with S, followed by the reaction of the N-center with the carbonyl group with the formation of substituted thiazoles 4 ( Figure 1) [4,5].
In this work, we developed a technique for the synthesis of other than thiazole types of heterocyclic derivatives of usnic acid by the reaction compound 1 with CS2-based nucleophiles. A series of compounds with dithiolane 5 and thiophene 6 and 7 fragments was synthesized.

Results and Discussion
Dithiolane 6 was prepared by the reaction of bromoderivative 2 with 1,3-S,S-binuclephile sodium N,N-dimethyldithiocarbamate (Scheme 1). The reaction was carried out in DMF, the substitution product for bromine atom 10 was precipitated with water. The reaction of the second nucleophilic center with the carbonyl group proceeds in the presence of concentrated sulfuric acid and leads to the formation of the dithiolan ring. The yield of compound 6 in two stages was 67%. In the absence of a reactive second nucleophilic center, the reaction of bromusnic acid 2 with the S-nucleophile proceeds without the involvement of the carbonyl group C13=O; the thiophene ring is formed on the adjacent methylene carbon atom. Thus, we obtained heterocyclic derivatives of usnic acid 7 and 8 containing a thiophene substituent.
Alkylated (by methyl iodide or benzyl bromide action) dicyanodithioacetates 10a,b and a dithioacetic acid derivative 11 were synthesized in situ by the reaction of carbon disulfide with malonodinitrile or diethyl malonate, respectively, in the presence of alkylating agents and potassium hydroxide Scheme 2. To the resulting mixture was added the bromoderivative of usnic acid 2 and left to stir for 3 h at room temperature (for 7a,b) and with heating (for 8) of the reaction products, the 7a,b and 8 compounds precipitated upon the addition of water and acidification with aqueous hydrochloric acid (Scheme 2). Compounds 7a,b and 8 were isolated in 90%, 50%, and 90% yields, respectively.

Methods
Reagent-grade solvents were redistilled prior to use. Synthetic starting materials, reagents, and solvents were purchased from Sigma-Aldrich, Acros Organics, and AlfaAesar. The usnic acid bromine derivative was obtained by the bromination of usnic acid as described in the paper [4].
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 1 H and 13 C-NMR spectra for solutions of the compounds in CDCl3 were recorded on a Bruker AV-400 spectrometer (400.13 and 100.61 MHz, respectively). The residual signals of the solvent were used as references (δH 2.48, δC 39.52 for DMSO-d6 and δH 7.27, δC 77.1 for CDCl3). The mass spectra (70 eV) were recorded on a DFS Thermo Scientific highresolution mass spectrometer. The melting points were measured using a Kofler heating stage. Merck silica gel (63-200 μ) was used for the column chromatography. Thin-layer chromatography was performed on TLC Silica gel 60F254 (Merck KGaA, Darmstadt, Germany). (6) (a) The usnic acid bromine derivative (2) (100 mg, 0.24 mmol) and sodium N,N-dimethyldithiocarbamate (38 mg, 0.24 mmol) were placed in a flask with 12 mL DMF. The reaction mixture was stirred for one hour at room temperature. Then, the solution was diluted with water and acidified (1 M HCI) until precipitation. The obtained precipitate was filtered, washed with water, and dried in the air.

Synthesis of Usnic Acid Dithiolanic Derivative
( (b) Compound 5 was placed in 5 mL of sulfuric acid. The obtained solution was stirred for 15 min without heating (TLC control). After that, the reaction mixture was poured into a glass with 30 mL of diethyl ether and the precipitate was filtered. The product was purified by column chromatography. Carbon sulfide (2.2 mmol) of sulfur carbon and malononitrile or diethylmalonate (1 mmol) was added to DMF (2 mL). A prepared solution of sodium hydroxide 1.1 mmol in 1 mL of water was added to the obtained solution. The mixture was left to stir in an ice bath for an hour. After that, methyliodide or benzylbromide (1 mmol) was added to the mixture. The resulting mixture was stirred for 30 min at room temperature. Bromine derivative 2 was added to the reaction mixture. The resulting mixture was stirred at room temperature for an hour. After that, the reaction mixture was diluted with water and acidified (1 M HCI). The obtained precipitate was filtered, washed with water, and dried.

Conclusions
In this paper, we developed a technique for the synthesis of new heterocyclic derivatives of usnic acid. The novel compounds can be considered as promising biologically active agents.