Heteronucleophiles as well as carbanionic reagents can be used to react with α-amido sulfones, thus giving the opportunity to prepare a large array of amino derivatives. Since, novel 1,3,4-oxadiazole-2-thiol derivatives can serve as potent nucleophiles, we employed 5-subsititued phenyl-1,3,4-oxadiazole-2-thiols as the nucleophilic source of nitrogen in the reaction with α-amido sulfones.
A series of N-substituted benzamides bearing 1,3,4-oxadiazol unit were prepared for the first time by the reaction of in situ generated protected imine from α-amido sulfones with 5-subsititued phenyl-1,3,4-oxadiazole-2-thiols as the source of nitrogen nucleophile. Some of the synthesized products displayed favourable antiviral activity against cucumber mosaic virus (CMV) in preliminary antiviral activity tests. The title compounds 5c, 5o and 5r revealed curative activity of 42.2%, 48.7% and 40.5%, respectively against CMV (inhibitory rate) compared to the commercial standard Ningnanmycin (53.4%) at 500 μg/mL.
A practical synthetic route to N-benzoyl-α-amido sulfones by the reaction of 5-subsititued phenyl-1,3,4-oxadiazole-2-thiols as the source of nitrogen nucleophiles with in situ generated protected imine from N-benzoyl-α-amido sulfones is presented. The reaction catalyzed by an inorganic base has considerable significance to exploit the potential of α-amido sulfones in organic synthesis.
α-Amido sulfones are known for their wide range of application in asymmetric synthesis . Different carbanionic nucleophiles can be reacted with α-amido sulfones, affording a wide variety of amino derivatives [2–7]. In addition, considerable research has been conducted on the reaction of nitrogen nucleophiles and α,β-unsaturated carbonyls [8–11]. Heteronucleophiles as well as carbanionic reagents can react with α-amido sulfones, thus giving the opportunity to prepare a large array of amino derivatives. Furthermore, a series of amino derivatives containing 1, 3, 4-oxadiazole ring were synthesized by the reaction of 1, 3, 4-oxadiazole-2-thiol with suitably substituted amines and formaldehyde in ethanol . Based on these reports, we executed an inorganic base-mediated reaction of 5-subsititued phenyl-1, 3, 4-oxadiazole-2-thiols as the source of nitrogen nucleophiles with N-benzoyl-α-amido sulfones to produce N, N-aminals. Interestingly, the resulting compounds bearing a 1, 3, 4-oxadiazole ring are often associated with significant fungicidal and insecticidal activities [13–15]. Nevertheless, as of today, there has been no report on antiviral activities of N-substituted benzamides bearing 1,3,4-oxadiazol unit. We report synthetic and antiviral studies of the title compounds in the ensuing sections.
Results and Discussion
The synthetic route to the title N-substituted benzamides 5 is shown in Scheme 1. In order to prepare the key electrophilic component N-benzoyl-α-amido sulfones 4 for the final reaction, a three components reaction involving aromatic aldehyde, benzamide and sodium sulfonate was used by modifying the procedure reported by Chemla . The reaction was fast, free of any significant side products formation and the pure product was isolated in moderate yield through recrystallization. The sulfones were characterized by 1H NMR, 13C NMR and IR spectral data. Finally, reaction of nitrogen nucleophiles 3 with N-benzoyl-α-amido sulfones in the presence of basic catalyst KOH in CH2Cl2 at room temperature afforded the desired N-substituted benzamides 5 bearing 1,3,4-oxadiazol group [Additional file 1]. The reaction conditions were optimized by taking compound 5a as the model. The effect of different solvents and bases was studied at room temperature with a fixed reaction time of 24 h (Table 1). Under these conditions, dichloromethane (CH2Cl2) provided the product in higher yield compared to toluene, tetrahydrofuran (THF) or acetonitrile. Amongst the various bases screened for the experiment, KOH gave the best result and higher conversion was achieved when 1.2 equiv of KOH was used instead of 1.0 equiv. Under optimized conditions, the isolated yield of the N-substituted benzamides 5a reached as high as 70% when the reaction mixture was stirred for 24 h in CH2Cl2 using 1.2 equiv of KOH.
Effect of different solvents and bases for the synthesis of 5aa
Yield (%) c
a Unless otherwise noted, reactions were carried out with 0.5 mmol (1.0 equiv) of 4a, 0.6 mmol (1.2 equiv) of 3a in 5.0 mL solvent and 1 mL H2O using 0.6 mmol (1.2 equiv) of basic catalyst at room temperature for 24 h. b Reaction was carried out with 0.5 mmol (1.0 equiv) of KOH at room temperature for 24 h. c Isolated yield after chromatographic purification.
Antiviral activity and structure-activity relationship
The results of in vivo antiviral activity studies of the N-substituted benzamides 5a-5u against CMV are given in Table 2. Ningnanmycin was used as the reference antiviral agent. Most of the compounds showed promising results in terms of curative bioactivities at 500 μg/mL. The comparison of the antiviral activity of the products with commercial reference leads to the following conclusions: (a) The antiviral activity is affected by the type of the substituents present in the compound. The compounds containing 2-fluorophenyl or 4-fluorophenyl group showed better anti-CMV activity compared with those derived from other groups. In particular, N-substituted benzamides 5o (R1 = o-F, R2 = p-Cl) and 5c (R1 = p-F, R2 = m-Cl) displayed moderate curative rates (48.7% and 42.2%, respectively) against CMV at the concentration of 500 μg/mL. These values were comparable to the curative rate (53.4%) shown by the commercial reference Ningnanmycin, and superior to other compounds bearing different substituents. (b) Substituents have a certain influence on the activity. Compared with the compounds 5o and 5c bearing suitably substituted aryl substituents, the compounds bearing an unsubstituted phenyl ring, such as 5a and 5n, showed lower inhibitory activities. (c) The structural modification caused by changing the substituents (R1 and R2) in the phenyl ring have a wide impact on anti-viral activity of the prepared compounds. Bioactivity of various compounds having the same substituents at different positions of the phenyl ring is various. Thus, amongst the compounds 5b, 5c, 5o and 5p containing the same substituents at different positions of the phenyl ring, the compound 5o carrying 4-chloro and 2-fluoro groups in their respective phenyl rings exhibited better bioactivity than others. Although these compounds, in general, exhibited slightly lower activity in comparison with the commercial reference Ningnanmycin at the concentration of 500 μg/mL, some suitably substituted N-substituted benzamides bearing 1,3,4-oxadiazol moiety showed favourable antiviral activity in the preliminary studies. Subtle structural variation might lead to enhancement of activity and should be the direction of future research.
Curative effect of the title compounds 5 against CMV in vivoa
Curative effect (%)
a All antiviral tests were carried out at the concentration of 500 μg/mL.
The melting points of the products were determined on an XT-4 binocular microscope (Beijing Tech Instrument Co., China) and were not corrected. The IR spectra were recorded on a Bruker VECTOR 22 spectrometer in a KBr disk. 1H NMR (500 MHz), 13C NMR (125 MHz) and 19F NMR (470 MHz) spectral analyses were performed on a JEOL-ECX 500 NMR spectrometer at room temperature using TMS as an internal standard and CDCl3 as the solvent. Elemental analysis was performed on an Elementar Vario-III CHN analyzer. Analytical TLC was performed on silica gel GF254. Column chromatographic purification was carried out using silica gel. All the solvents and materials were of analytical-grade. Intermediate 1, intermediate 2 and 5-subsititued phenyl-1,3,4-oxadiazole-2-thiol 3 were prepared according to the reported methods [17, 18] and used without further purifications [Additional file 2].
Antiviral biological assay
The leaves of Nicotiana tabacum L. inoculated with CMV were selected and grinded in phosphate buffer and filtered through double-layer pledget. The filtrate was centrifuged at 8000 g and the supernatant liquid was the crude extract of virus. The whole experiment was carried out at 4°C. Absorbance values were estimated at 260 nm using an ultraviolet spectro-photometer.
Preparation of medicaments
Tested compounds and 2% Ningnanmycin aqua used as a reference antiviral agent were first dissolved in minimum volume of N, N-dimethylformamide (DMF) and then diluted with distilled water containing 1% Tween 20 at 500 μg/mL concentration.
Curative effect of compounds against CMV in vivo
Growing leaves of Chenopodium amaranticolor of the same age were selected. Crude extracts of CMV were dipped and inoculated with a brush on the whole leaves, which were previously scattered with silicon carbide. The leaves were washed by water after inoculation for 0.5 h and then dried. The compound solution was smeared on the left side of leaves, and the solvent was smeared on the right side for control. All plants were cultivated at 28±1°C with an illumination of 10000 Lux. The local lesion numbers appearing 6-7 d after inoculation were counted. Three repetitions were conducted for each compounds. The inhibition rate of the compound was calculated according to the following formula (av denotes average):
We have demonstrated a general and practical route for the synthesis of the N-substituted benzamides bearing 1,3,4-oxadiazol moiety in the presence of an inorganic base as the catalyst. The reaction of 5-subsititued phenyl-1,3,4-oxadiazole-2-thiol which serves as the source of N-nucleophile with in situ generated protected imine from N-benzoyl-α-amido sulfones provides a ready access to a series of structurally diverse N, N-aminals. The antiviral tests indicated that some of the synthesized compounds possessed of moderately high curative activity against CMV. The structure of the target products needs to be optimized to enhance their antiviral activity. Further studies on mechanistic aspects, enantioselectivities and asymmetric variants of catalysts for this reaction are currently being investigated in our group.
We gratefully acknowledge the generous financial support received from the National Key Project for Basic Research (2010CB126105, 2010CB134504) and the National Natural Science Foundation of China (20872021).
State Key Laboratory Breeding Base of Green Pesticide and Agricultural Bioengineering, Key Laboratory of Green Pesticide and Agricultural Bioengineering, Ministry of Education, Research and Development Center for Fine Chemicals, Guizhou University
Petrini M: α-Amido sulfones as stable precursors of reactiveN-acylimino derivatives.Chem Rev 2005, 105:3949–3977.View Article
Murry JA, Frantz DE, Soheili A, Tillyer R, Grabowski EJJ, Reider PJ: Synthesis of α-amido ketones via organic catalysis: thiazolium-catalyzed cross-coupling of aldehydes with acylimines.J Am Chem Soc 2001, 123:9696–9697.View Article
Palomo C, Oiarbide M, Laso A, López R: Catalytic enantioselective aza-Henry reaction with broad substrate scope.J Am Chem Soc 2005, 127:17622–17623.View Article
Song J, Shih HW, Deng L: Asymmetric Mannich reactions within situgeneration of carbamate-protected imines by an organic catalyst.Org Lett 2007, 9:603–606.View Article
Gomez-Bengoa E, Linden A, Lόpez R, Múgica-Mendiola I, Oiarbide M, Palomo C: Asymmetric aza-Henry reaction under phase transfer catalysis: an experimental and theoretical study.J Am Chem Soc 2008, 130:7955–7966.View Article
Zhang H, Syed S, Barbas CF: Highly enantio- and diastereoselective Mannich reactions of glycine Schiff bases within situgeneratedN-Boc-imines catalyzed by a cinchona alkaloid thiourea.Org Lett 2010, 12:708–711.View Article
Mazzotta S, Gramigna L, Bernardi L, Ricci A: One-pot synthesis of optically active β-amino-α-methylene carbonyl derivatives from α-amidosulfones using quinine-based phase-transfer catalysts.Org Process Res Dev 2010, 14:687–691.View Article
Kobayashi S, Kakumoto K, Sugiura M: Transition metal salts-catalyzed aza-Michael reactions of enones with carbamates.Org Lett 2002, 4:1319–1322.View Article
Wabnitz TC, Spencer JB: A general, Brønsted acid-catalyzed hetero-Michael addition of nitrogen, oxygen, and sulfur nucleophiles.Org Lett 2003, 5:2141–2144.View Article
Palomo C, Oiarbide M, Halder R, Kelso M, Gómez-Bengoa E, García JM: Catalytic enantioselective conjugate addition of carbamates.J Am Chem Soc 2004, 126:9188–9189.View Article
Rowland GB, Zhan H, Rowland EB, Chennamadhavuni S, Wang Y, Antilla JC: Brønsted acid-catalyzed imine amidation.J Am Chem Soc 2005, 127:15696–15697.View Article
Koparır M, Çetin A, Cansız A: 5-Furan-2yl134oxadiazole-2-thiol, 5-furan-2yl-4H124 triazole-3-thiol and their thiol-thione tautomerism.Molecules 2005, 10:475–480.View Article
Wu J, Song BA, Chen HJ, Bhadury PS, Hu DY: Synthesis and antifungal activity of 5-chloro-6-phenylpyridazin-3(2H)-one derivatives.Molecules 2009, 14:3676–3687.View Article
Zheng XM, Li Z, Wang YL, Chen WD, Huang QH, Liu CX, Song GH: Syntheses and insecticidal activities of novel 2,5-disubstituted 1,3,4-oxadiazoles.J Fluorine Chem 2003, 123:163–169.View Article
Ram VJ, Vlietinck AJ: Chemotherapeutical agents. VII. synthesis and pesticidal activities of sulphides and sulphones derived from bis[4-aryl-1,2,4-triazoline-5-thione-3-yl]alkane and 5-phenyl-1,3,4-oxadiazole-2-thione.J Heterocycl Chem 1988, 25:253–256.View Article
Chemla F, Hebbe V, Normant JF: An easy synthesis of aliphatic and aromaticN-sulfonyl aldimines.Synthesis 2000, 32:75–77.View Article
Chen Z, Xu WM, Liu KM, Yang S, Fan HT, Bhadury PS, Hu DY, Zhang YP: Synthesis and antiviral activity of 5-(4-chlorophenyl)-1,3,4-thiadiazole sulfonamides.Molecules 2010, 15:9046–9056.View Article
Liu F, Luo XQ, Song BA, Bhadury PS, Yang S, Jin LH, Xue W, Hu DY: Synthesis and antifungal activity of novel sulfoxide derivatives containing trimethoxyphenyl substituted 1,3,4-thiadiazole and 1,3,4-oxadiazole moiety.Bioorg Med Chem 2008, 16:3632–3640.View Article
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.