Synthesis of New 1,2,4-Triazoles Containing Oxadiazole Moiety: Potential Intermediates for Preparation of Ribavirin Analogues

In this paper, we report synthesis of the new biheterocycles containing 1,2,4-triazole and 1,2,4oxadiazole or 1,3,4-oxadiazole moiety. Oxadiazole ring is bioisosteric replacement of carboxamide group. In future, these compounds will be used for enzymatic synthesis of ribavirin analogues. Structures of the final biheterocycles were confirmed by spectral and elemental analysis.


INTRODUCTION
Antiviral properties of ribavirin ( Fig. 1) were known since 1972 year [1]. Initially, it demonstrated activity against broad-spectrum of DNA and RNA viruses [1], that`s why it has been widely used in medicine. Currently, it used for treatment of influenza, severe respiratory syncytial virus (RSV) infection, Lassa fever virus infection, SARS coronavirus infections [2,3]. In

Short Research Article
combination with interferon-α, it used for therapy of hepatitis C virus (HCV) infection -major cause of cirrhosis and hepatocellular carcinoma [4]. However, side effects, including hemolytic anemia, and resistance of some viruses significantly limited its application [5,6].
Synthesis of new ribavirin structural analogues is most common strategy to overcome described limits. There are two routes of modification: carboxamide group of heterocyclic moiety or glycoside moiety. Phosphorylation of primary hydroxyl group is the main glycoside moiety modification [7]. The hydrazones, amidines, ketones and α-imino esters were obtained as a result of carboxamide group transformation [8,9].

General
The starting reagents and solvents were obtained from Sigma-Aldrich Company and used without further purification. The reactions were monitored by high-performance liquid chromatography (HPLC). Analysis was performed on Shimadzu LC 20 Prominence apparatus and Waters Xbridge C18, 50 mm × 4.6 mm column with water-acetonitrile eluent. The purity of compounds was also determined by HPLC. 1 H NMR spectra were recorded on a Bruker DRX500 (500 MHz) and were determined in DMSO-d 6 . 13 C NMR spectra were recorded at 125 MHz. Chemical shifts are reported in parts per million relative to the peak of tetramethylsilane (TMS) (0.00 ppm). Melting points were measured on Mel-Temp 3.0.

General Procedure for the Synthesis of 3-alkyl-5-(1H-1,2,4-triazole-3-yl)-1,2,4-oxadiazoles (8a-c)
To 0.011 mol of acid 2 in 20 mL of 1.4-dioxane was added 0.015 mol of EDC and resulted mixture was stirred for 30 min at RT. Further, 0.01 mol of amidoxime was added. Reaction mixture was stirred for 3 h at RT and for 6 h at 100 °С. Solvent was evaporated at reduced pressure, and then 50 mL of dichloromethane was added to the residue. Organic layer was separated, washed with 10% solution of sodium bicarbonate (1 x 25 mL) and water (2 x 25 mL), dried with sodium sulfate and evaporated at reduced pressure.

General Procedure for the Synthesis of 2-alkyl-5-(1H-1,2,4-triazole-3-yl)-1,3,4-oxadiazoles (9a-c)
a) 0.03 Mol of sodium bicarbonate and 0.022 mol of acyl chloride were added to solution of 0.02 mol of hydrazide 5 in 20 mL of acetone at 0°C. Reaction mixture was stirred for 4 h at RT and 100 mL of cold water was added. The obtained precipitate was filtered and used in the next step without additional purification. b) 0.01 Mol of diacylhydrazide from previous step was added to 0.02 mol of phosphorus oxychloride at 0°С. Reaction mixture was heated for 8 h at 100°С. After cooling to RT 50 g of the ice and 50 mL of dichloromethane was added. Organic layer was separated, washed with 10% solution of sodium bicarbonate (2 x 25 mL) and water (2 x 25 mL), dried with sodium sulfate and evaporated at reduced pressure.

RESULTS AND DISCUSSION
As part of our idea to replace the carboxamide group by 1,2,4-or 1,3,4-oxadiazole moiety, we synthesized 9 biheterocyclic compounds (8a-c, 9a-c, 10a-c). Route of synthesis is shown in Scheme 1 above. At first, we converted starting 1,2,4-triazolecarboxylic acid 2 into ester 3 and amide 4 by well-known methods. Further, hydrazide 5 and nitrile 6 were obtained from compounds 3 and 4 respectively. Treatment of the nitrile 6 with hydroxylamine hydrochloride alcohol solution and sodium bicarbonate was led to amidoxime 7.
1,3,4-Oxadiazoles 9a-c were prepared by twostep method. In the first step, hydrazide 5 was acylated with previously described acyl chlorides in the presence of sodium bicarbonate in acetone at room temperature. In the second step, obtained dihydrazides were heated with two equivalents of phosphorus oxychloride.
Structure of all synthesized compounds was confirmed by 1 H NMR data, and they purity's was determined by HPLC. Moreover, final biheterocycles were characterized by 13 C NMR spectroscopy and elemental analysis.
In future, all synthesized biheterocycles will be used for enzymatic synthesis of ribavirin analogues.

CONCLUSION
In this work, we report the synthesis of series of 9 new biheterocyclic compounds (8a-c, 9a-c, 10a-c), in which oxadiazole moiety is bioisosteric replacement of carboxamide group. In future, these compounds will be used for preparation of ribavirin analogues.