4-Amino-3-(1-{[amino(3-methyl-2-oxido-1,2,5-oxadiazol-4-yl)methylene]hydrazinylidene}ethyl)-1,2,5-oxadiazole 2-Oxide

Abstract

Functionally substituted 1,2,5-oxadiazole 2-oxides (furoxans) are important pharmaceutical scaffolds used for the preparation of various pharmacologically active substances. Furoxans bearing hydrazone functionality are considered as promising drug candidates for the treatment of neglected diseases. However, pharmacologically oriented hydrazones derived from (furoxanyl)amidrazones and acetylfuroxans have remained unknown so far. In this communication, a simple method for the synthesis of 4-amino-3-(1-{[amino(3-methyl-2-oxido-1,2,5-oxadiazol-4-yl)methylene]hydrazinylidene}ethyl)-1,2,5-oxadiazole 2-oxide is described. The structure of the synthesized compound was established by elemental analysis, high-resolution mass spectrometry, ¹H, ¹³C NMR and IR spectroscopy.

1. Introduction

Hydrazones are versatile bioactive molecules displaying a wide range of pharmacological activities, including antibacterial, antitubercular, antifungal, anticancer, etc. [1,2]. Such compounds usually have acceptable hydrolytic stability, low toxicity and may serve various biomedical applications [3,4]. In addition, hydrazones are useful synthons for the formation of carbon–nitrogen and nitrogen–heteroatom bonds, which is highly relevant for the creation of new methods of constructing functionally substituted nitrogen heterocycles [5].

In a series of nitrogen heterocycles, 1,2,5-oxadiazoles N-oxides (furoxans) became emergent scaffolds with strong application potential in various areas. Furoxans correspond to an important subclass of heterocyclic pharmaceutical ingredients capable of exogenous release of nitric oxide (NO) [6,7]. NO is a crucial gaseous signaling molecule that mediates various physiological processes and may be useful for cancer treatment [8,9,10,11]. Due to described NO-releasing properties, furoxans have a wide range of pharmacological activities including antiparasitic [12,13,14,15], antiaggregant [16,17,18] and anticancer [19,20] types of activity. Recently, a potential application of arylazofuroxans as organic photoswitchable NO-donors was proposed [21]. In addition, furoxan-based hydrazones are considered as promising drug candidates for the treatment of various neglected diseases including tuberculosis, leishmaniosis, schistosomiasis and Chagas’ disease [12,13,14,15,22]. Herein, we report the synthesis of 4-amino-3-(1-{[amino(3-methyl-2-oxido-1,2,5-oxadiazol-4-yl)methylene]hydrazinylidene}ethyl)-1,2,5-oxadiazole 2-oxide 1, which can be considered as a promising pharmacologically active compound for the treatment of neglected diseases.

2. Results and Discussion

Hydrazones are usually prepared via condensation of substituted hydrazines with carbonyl compounds. However, furoxanylhydrazines are unstable compounds that cannot be isolated, only generated in situ under rather harsh conditions [23]. Therefore, we decided to investigate the reactivity of more convenient and bench-stable (furoxanyl)amidrazones for the preparation of the corresponding hydrazones. (3-Methylfuroxan-4-yl)amidrazone 2 [24] and 3-acetyl-4-aminofuroxan 3 [25] were chosen as readily available compounds in the series of functionally substituted furoxan derivatives, which can be synthesized on a large scale. The condensation of substrates 2 and 3 was performed under mild reaction conditions in MeCN at 25 °C and resulted in the formation of target product 1 in a good yield (Scheme 1).

Scheme 1. Synthesis of hydrazone 1.

The structure of target product 1 was fully confirmed by elemental analysis, high-resolution mass spectrometry and ¹H, ¹³C NMR and IR spectroscopy (see Supplementary Materials). HRMS and elemental analysis confirm the chemical formula of compound 1. The ¹H NMR spectrum of 1 showed signals of both methyl groups at 2.36 ppm (methyl group at the furoxan ring) and 2.41 ppm (methyl group of the hydrazone functionality) and of both amino groups at 6.55 ppm (amino group at the furoxan ring) and 7.52 ppm (amino group of the amidrazone functionality). Characteristic signals of C-3 furoxan carbon atoms were observed in the ¹³C NMR spectrum at 109.8 and 113.3 ppm. The IR spectrum demonstrated characteristic bands of amino groups (3501 and 3386 cm⁻¹), C=N double bonds (1633 cm⁻¹) and furoxan rings (1598 and 1566 cm⁻¹).

In conclusion, we synthesized a new furoxan-based hydrazone representative, namely 4-amino-3-(1-{[amino(3-methyl-2-oxido-1,2,5-oxadiazol-4-yl)methylene]hydrazinylidene}ethyl)-1,2,5-oxadiazole 2-oxide 1, which is a promising pharmacologically oriented substance and a useful precursor of various polynitrogen heterocyclic systems.

3. Materials and Methods

General. All reactions were carried out in well-cleaned, oven-dried glassware with magnetic stirring. ¹H and ¹³C NMR spectra were recorded on a Bruker AM-300 (300.13 and 75.47 MHz, respectively) spectrometer (Billerica, MA, USA) and referenced to residual solvent peaks. The chemical shifts are reported in ppm (δ); coupling constants, J, are reported in Hertz. The IR spectra were recorded on a Bruker “Alpha” spectrometer (Münich, Germany) in the range 400–4000 cm⁻¹ (resolution 2 cm⁻¹). Elemental analyses were performed by the CHN Analyzer Perkin-Elmer 2400 (Waltham, MA, USA). High-resolution mass spectra were recorded on a Bruker microTOF spectrometer (Münich, Germany) with electrospray ionization (ESI). All measurements were performed in a positive (+MS) ion mode (interface capillary voltage: 4500 V) with scan range m/z: 50–3000. External calibration of the mass spectrometer was performed with Electrospray Calibrant Solution (Fluka). A direct syringe injection was used for all analyzed solutions in MeCN (flow rate: 3 μL min⁻¹). Nitrogen was used as a nebulizer gas (0.4 bar) and dry gas (4.0 L min⁻¹); the interface temperature was set at 180 °C. All spectra were processed using the Bruker DataAnalysis 4.0 software package (Billerica, MA, USA). Analytical and preparative thin-layer chromatography (TLC) was carried out on Merck 25 TLC silica gel 60 F₂₅₄ aluminum sheets. The visualization of the TLC plates was accomplished with a UV light. All solvents were purified and dried using standard methods prior to use. All standard reagents were purchased from Aldrich or Acros Organics and used without further purification. Amidrazone 2 [24] and 3-acetyl-4-aminofuroxan 3 [25] were prepared according to published procedures.

Synthesis of 4-amino-3-(1-{[amino(3-methyl-2-oxido-1,2,5-oxadiazol-4-yl)methylene]hydrazinylidene}ethyl)-1,2,5-oxadiazole 2-oxide 1. 3-Acetyl-4-aminofuroxan 3 (286 mg, 2 mmol) was added to a magnetically stirred solution of amidrazone 2 (314 mg, 2 mmol) in MeCN (5 mL) at 25 °C. The reaction mixture was stirred at 25 °C for 10 h until full consumption of the starting material according to TLC data (eluent CHCl₃-EtOAc, 75:25). Then, the solvent was evaporated, and the crude product was purified by preparative TLC (eluent CHCl₃-EtOAc, 80:20). Yield 412 mg (73%), yellow needles, mp. 229–230 °C (recrystallized from MeCN). R_f 0.12 (CH₂Cl₂-EtOAc, 90:10), 0.30 (CHCl₃-EtOAc, 80:20), 0.55 (CHCl₃-EtOAc, 50:50). IR spectrum (KBr), ν, cm⁻¹: 3501 (NH), 3386 (NH), 2958 (CH), 2931 (CH), 1633 (C=N), 1598 (furoxan), 1566 (furoxan), 1502, 1457, 1349, 1108, 1040, 858. ¹H NMR (DMSO-d₆, ppm): 2.36 (s, 3H), 2.41 (s, 3H), 6.55 (s, 2H), 7.52 (s, 2H). ¹³C NMR (DMSO-d₆, ppm): 10.4, 14.4, 109.8, 113.3, 148.8, 153.0, 153.1, 156.9. HRMS (ESI) Calcd. for C₈H₁₀N₈NaO₄ [M + Na]⁺: 305.0717, found: 305.0721. Anal. calcd. for C₈H₁₀N₈O₄ (%): C, 34.05; H, 3.57; N, 39.71. Found (%): C, 33.92; H, 3.65; N, 39.53.