Application of the Negishi Reaction in the Synthesis of Thiophene-Based Lignans Analogues with Leishmanicidal Effects

As lignanas tetraidrofurânicas são metabólitos secundários com reconhecida atividade antiprotozoária. Na literatura, há vários relatos sobre os efeitos anti-parasitários de análogos sintéticos de lignanas contendo pontes de enxofre. Neste trabalho, foi realizada a síntese de análogos tiofênicos de lignanas com o uso de uma estratégia sintética seletiva e de alto desempenho baseada na reação de acoplamento cruzado de Negishi. Os derivados sintéticos foram obtidos de maneira rápida e apresentaram alto grau de pureza e baixa toxicidade para uma linhagem celular de mamífero e atividade leishmanicida com diferentes potências.


Introduction
Leishmaniasis is a serious disease caused by different protozoan parasites of Leishmania genus and this disease can be represented by a large spectrum of symptoms such as cutaneous lesion and fatal visceral infection. 1 Data from World Health Organization (WHO) estimated that leishmaniasis threatens 350 millions of people and it is responsible for about 2 million clinical cases each year in 88 countries. 2 The countries with high prevalence of leishmaniasis are subtropical and development countries, for example, India, Sudan, Bangadesh, Nepal and Brazil. 2 The treatment of leishmaniasis is based on pentavalent antimony compounds, which may be represented by sodium stibogluconate (Pentostam ® ) or meglumine antimoniate (Glucantime ® ). These antimony compounds are widely prescribed despite their severe side effects in the heart, kidney, pancreas and liver, high cost, difficult administration and increase of parasite resistance. [3][4][5] Other drugs such as amphotericin B, pentamidine and metilfosine are used in leishmaniasis treatment. However, the clinical use of these drugs are limited because of their toxicity, adverse side effects and high cost. 4,6 Currently, there is an important demand for the development of new treatments against leishmaniasis, which have primarily features as inexpensive, potent, safe and easy administration. The drugs of choice for the treatment of leishmaniasis, such as pentavalent antimonials (sodium stibogluconate and meglumine antimonates), miltefosine, and pentamidine are toxic to the host. Some forms of administration (injections) or different formulations developed to reduce toxicity, increase the expense of the treatment and makes the use of the drug less practical in developing nations, where leishmaniasis are endemic and cost is a major factor. 7,8 Therefore, in order to solve these problems, studies aiming the discovery of new drugs against leishmaniasis are necessary and may provide treatments that could be more efficient for population and reduce the economic impact of this disease in endemic countries. Lignans and neolignanas are included in the potential natural products for the treatment of neglected diseases. 9,10 From the Brazilian biodiversity, there are some papers that report the anti-protozoal activity of lignans from Virola pavonis and Virola surinamensis. 11,12 For example, Barata et al. showed the highest selective activity for lignans analogues with sulphur bridges than those structures isolated from V. pavonis. 11 Recently, some new compounds containing the thiophene ring in their structure have demonstrated high leishmanicidal activity. For instance, Takahashi et al. have isolated thiophene derivatives with leishmanicidal activity from Porophyllum ruderale (Jacq.) Cass. These molecules have exhibited strong activity against promastigote and axenic amatigote forms of L. amazonensis. 13 15 Due to the growing interest on thiophene-based compounds presenting biological activity, the preparation of functionalized derivatives is of great importance. In this context, the construction of carbon-carbon bonds using selective strategies has been a major target on synthetic organic chemistry. Palladium catalysed coupling reactions are among the most important reactions for the functionalization of aromatics and heterocyclic substrates. 16 The Nobel Prize in Chemistry 2010 was awarded jointly to Richard F. Heck, Ei-ichi Negishi and Akira Suzuki for their contributions on the development of efficient and versatile palladium-catalyzed coupling reactions. 17 Among the strategies commonly used for doing new C-C bonds, the Negishi cross-coupling reaction has been highlighted, mainly in the preparation of biaryl compounds. 18 In addition, this strategy has already been successfully used for the arylation of tiophenes, and different methodologies have been applied to improve this reaction. 19 For example, Genov et al. have reported a greener protocol by using a microwave assisted Negishi cross-coupling reaction that led to the rapid enantioselective synthesis of binaphthalene derivatives in reasonable to excellent yields in few minutes. 20 In this work, we have applied the Negishi reaction in the synthesis of thiophene-based analogues of lignans 2-(4-chlorophenyl)-5-phenylthiophene (5a) and 2,5 bis(2-methoxyphenyl)thiophene (5b) and evaluated their leishmanicidal effect against Leishmania amazonensis promastigotes. In addition, we have also evaluated the impact of these compounds on mammalian cell viability.

Experimental
General procedure for the preparation of (3a-3b) To a 30 mL sealed flask equipped with a magnetic stirring bar and under a nitrogen atmosphere was added a solution of thiophene (5 mmol) in THF (10 mL). This solution was cooled to −78 °C and n-BuLi (5.5 mmol, 1.1 equiv; C = 2.47 mol L −1 ) was added dropwise. After the addition was complete, the reaction mixture was warmed to 0 °C and stirred at this temperature for 1 h. Then this solution was cooled to −40 °C and a solution of ZnCl 2 in THF was added (1 mol L −1 , 5.5 mmol, 5.5 mL). After 15 min, a solution of Pd 2 (dba) 3 (2 mol%) e P(ofuril) 3 (4 mol%) in THF (2.5 mL) and a solution of corresponding aryl halide (7.5 mmol, 1.5 equiv.) were added and irradiated at 100 °C, in a microwave Antoon Paar Monowave 3000. After 45 min, the reaction mixture was quenched with saturated aqueous NH 4 Cl (40 mL) and extracted with AcOEt (3 × 50 mL). The solvent was evaporated under vacuum and the crude product was purified by flash chromatography (Hex.). General procedure for the preparation of (5a-5b) To a 30 mL sealed flask equipped with a magnetic stirring bar and under a nitrogen atmosphere was added a solution of 2-aryl-thiophene (3a or 3b) (2.5 mmol) in THF (5 mL). This solution was cooled to −78 °C and n-BuLi (2.75 mmol, 1.1 equiv; C = 2.47 mol L −1 ) was added dropwise. After the addition was complete, the reaction mixture was warmed to 0 °C and stirred at this temperature for 1 h. Then this solution was cooled to −40 °C and a solution of ZnCl 2 in THF was added (1 mol L −1 , 2.75 mmol, 2.75 mL). After 15 min, a solution of Pd 2 (dba) 3 (2 mol%) e P(ofuril) 3 (4 mol%) in THF (2.5 mL) and a solution of corresponding aryl halide (3.75 mmol, 1.5 equiv.) were added and irradiated at 100 °C. After 45 min, the reaction mixture was quenched with saturated aqueous NH 4 Cl (30 mL) and extracted with AcOEt (3 × 30 mL). The solvent was evaporated under vacuum and the crude product was purified by flash chromatography (Hex/AcOEt).

Leishmanicidal assay
Promastigote forms of Leishmania amazonensis were cultured in Scheneider's medium (Sigma-Aldrich, USA) supplemented with 20% fetal bovine serum, 1% L-glutamine, 10 UI penicillin and 10 µg mL −1 streptomicin at 24 °C. The leishmanicidal activity of compounds was evaluated by colorimetric MTT assay (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide) was used as previously described by Mossman. 25 Briefly, log phase promastigotes of Leishmania amazonensis were seeded in 96-well tissue culture plates (1.0 × 10 6 well −1 ). Then, the parasites were treated with 37, 111, 222, 369 µmol L −1 of compound 5a or 34, 101, 202, 337 µmol L −1 of compound 5b previously diluted in Schneider's medium with dimethyl-sulfoxide (DMSO) for 48 h. Each concentration was performed in triplicate and two independent experiments were executed. The used concentration of DMSO was not higher than 0.1%. DMSO solution and amphotericin B (Amp, 2.2 µmol L −1 ) were used as negative and positive controls of cell death, respectively. After all treatments, cells were incubated with MTT solution (5 mg mL −1 ; 10 µL well −1 ) by 4 h at 24 °C. The plate was centrifuged at 400 × g for 10 min and the supernatants were then discarded. The formazan crystals produced by viable cells were solubilized with 200 µL of DMSO. The absorbance readout of each well was determined using a spectrophotometer (Spectramax Plus, Molecular Devices) at 570 nm. The leishmanicidal effects of compounds (5a and 5b) was determined by calculation of the 50% inhibitory (IC 50 ) using a logarithm regression analysis.

Mammalian cell viability assay
In order to evaluate the in vitro toxic effect of compounds 5a and 5b on mammalian cells, a cell lineage of mouse macrophage (J744A.1 cells) were used. J744A.1 cells (10 5 cells well −1 ) were incubated in 96-well plates at 37 °C and 5% CO 2 in complete RPMI 1640 media containing 10% of fetal bovine serum, 1% L-glutamine, 10 UI penicillin, and 10 µg mL −1 streptomycin. After 24 h, the cells were washed with PBS and treated with 37, 111, 222, 369 µmol L −1 of compound 5a or 34, 101, 202, 337 µmol L −1 of compound 5b previously diluted in RPMI-1640 medium with DMSO. The 96-well plates were incubated at CO 2 5%, and 37 °C, for 48 h. The used DMSO concentration was not higher than 0.1%. Macrophages cells viability was determined by the MTT assay, as described above.

Statistical analysis
Results are presented as mean ± S.D. of experiments. Differences between groups were evaluated by analyses of variance (one-way ANOVA) followed by Bonferroni's test. Statistical differences were considered to be significant at p < 0.05.

Results and Discussion
In our synthetic approach to thiophene-based analogues of lignans, we have planned to apply the Negishi crosscoupling reaction for the double arylation of the thiophene ring. Thus, we initiated our study by investigating the preparation of the organozinc reagent (2) and its palladium catalyzed cross-coupling reaction with aryl-iodines.
After the synthesis, we evaluated the leishmanicidal activity of two compounds (5a and 5b) on the promastigote forms of Leishmania amazonensis (Figure 1). The results showed that the parasites were distinct susceptible to treatment of both compounds. The treatment of promatigotes with compound 5a promoted a slight decrease in their cell viability compared to the positive control  (Amp). Moreover, all concentrations of compound 5a showed similar leishmanicidal effects and its IC 50 was 526 µM.
The efficiency of the leishmanicidal activity of compound 5b was greater than the activity of compound 5a, however its efficiency was moderate compared to Amp. The leishmanicidal effect of compound 5b on parasites was concentration-dependent, and the optimum effect was observed at ca. 202 µM, which showed similar activity to Amp (2.2 µM). Moreover, the IC 50 of compound 5b was around three times smaller (166 µM) than IC 50 of compound 5a (526 µM). Therefore, these results indicate that the compound 5b presents better leishmanicidal effects than compound 5a and this could be associated with the structural differences between these two compounds, since their substituent (methoxy) and/or the position of aromatic rings of these groups are different.
Several drugs used to treat leishmaniasis present significant toxic effects to the patients, such as amphotericin B. 19,20 Hence, the next step was to verify the impact of compounds 5a and 5b on the viability of J744A.1 macrophages. Interestingly, both compounds and negative control (non-treated macrophages) showed similar levels of cell death on macrophage (Figure 2). This set of results shows that the compounds 5a and 5b were less effective in causing death of leishmania compared to amphotericin B, but they were not toxic to J744A.1 cell line. The macrophage is a host cell target for leishmania and participates of immune defense against this parasite. 26 Thus, it is important for a good leishmanicidal activity to be selectively toxic against the parasite cell, causing no or little damage to host cells, as described for compound 5b (Figure 2). 27 In the literature, there are other studies showing that thiophene compounds present important activity against Leishmania spp. and other protozoans. For example, Gonzalez et al. have described the synthesis of bis-2,5-[4-guanidinophenyl]thiophenes, dicationic diguanidines related to the anti-protozoans furamidine and pentamidine, and their activity against some parasites. 28 In addition, a recent work from Bonano et al. 29 showed a screening of a compound library of estrogen receptor modulator analogs based on 2-arylbenzothiophene structure. It was described that optimal anti-leishmanial potency is dependent on the presence of two basic side chains in this skeleton, but the presence of phenols are not required for this activity. Takahashi et al. have demonstrated that two thiophene derivatives isolated from Porophyllum ruderale inhibited the growth of the promastigote and axenic amastigote forms of L. amazonensis. 11 Moreover, the same research group have also demonstrated the potential leishmanicidal effect of these compounds against intracellular amastigote forms. 30 However, in comparison to our data, the structures of these two thiophene, 5-methyl-2,2':5',2"-terthiophene and 5'-methyl-[5-4(4-acetoxy-1-butynyl)]-2,2' bi-thiophene, are very different from our compounds, mainly due to the presence of two or more rings in the thiophene structure. This fact can be related to the higher leishmanicidal effect from these two thiophenes when compared with our results.
Other study demonstrated the effect of many classes of diphenyl furans and thiophenes against Leishmania donovani in vitro. Among these synthetic analogues, the 2,5-diphenyl thiophenes with diamidines substituent were those that possess the most leishmanicidal activity from the compounds against L. donovani amastigote forms. 20 In these context, the results of our study may provide some insights as to structure activity relationship of 2,5-diphenyl thiophenes compounds. Our studies and those from Brendle et al. 20 suggest that the structure of a 2,5-diphenyl thiophene unit bearing aromatic rings appears to be essential for the leishmanicidal effect of these compounds. However, the presence of substituents on the phenyl rings interfere in the anti-parasite effects. The presence of methoxy group (5b) provide more leishmanicidal effects than the chloro substituent (5b). These data suggest that the presence of a methoxy group in the aromatic ring is important for the increase of the leishmanicidal activity. Also, these modifications did not Application of the Negishi Reaction in the Synthesis of Thiophene-Based Lignans Analogues J. Braz. Chem. Soc. 1912 induce cytotoxicity in the mammal cells. However, the presence of diamidines substituents in the thiophenes increased their leishmanicidal activity, but in some cases increased cytotoxicity in macrophage J774A.1 cell line were observed. 20 It is interesting to note that the thiophenes can be a source of compounds to the development of new drugs against leishmaniasis and other protozoan diseases. This fact has been reported in the patent WO 2009051796, 31 where diazaaryl thiophene and selenophene compounds showed significant leishmanicidal effect on amastigote forms of L. donovani and other protozoan such as Trypanossoma brucei and Plasmodium falciparum. Thus, the synthetic routes to obtain 2,5-diaryl thiophenes are important to help the development of new compounds to protozoan diseases. In addition, many authors showed the potential leishmanicidal or antiparasitic effects of several thiophene compounds isolated from plants, which confirm the importance of additional studies with this class of structures against protozoan agents. 11,20,28,30 Other important aspect of our study is that we demonstrated the leishmanicidal effect in Leishmania amazonensis. These parasites are responsible to induce localized cutaneous leishmaniasis (LCL) and diffuse cutaneous leishmaniasis (DCL), while other reported studies demonstrated leishmanicidal effect in Leishmania donovani, that is known to cause visceral leishmaniasis or mucosal leishmaniasis, mainly in immunocompromised patients (HIV-positive patients and other subjects immunosuppressive). 20,32 Biological differences between these parasites and the different mechanism of pathogenesis should be considered when comparing our studies and others in the literature.
Further studies are necessary to investigate the effect of 2,5-diphenyl thiophenes in leishmaniasis, mainly because it is a complex disease and involves modulation of the immune system. In addition, some aspects related to in vivo administration and efficacy of these compounds by different doses, routes and/or formulations should be evaluated, as absorption, leishmanicidal effect, half-life, excretion and possible toxicological effects. Therefore, the investigation of these 2,5-diphenyl thiophenes in an in vivo model of leishmaniasis and evaluation of impact of these drugs in a biological system, mainly in immune system is important to help in the development of new treatments to be used against leishmaniasis.

Conclusion
In summary, the application of the Negishi crosscoupling reaction in the microwave assisted synthesis of thiophene-based analogues of lignans has allowed the isolation of the desired 2,5-diarylthiophenes in good yields. In addition, the synthesized compounds appeared to induce selective leishmanicidal effect with low (compound 5a) or moderate (compound 5b) potencies compared to Amp. Therefore, this synthetic strategy will be helpful for the development of new drugs against cutaneous leishmaniasis. The scope of this methodology and its applicability toward the fast synthesis of other bioactive thiophenes with improved leishmanicidal activity are currently being investigated in our laboratories.