Synthesis and phytotoxic activity of 1,2,3-triazole derivatives

ARTICLE

Synthesis and phytotoxic activity of 1,2,3-triazole derivatives

Thiago F. Borgati^I,*; Rosemeire B. Alves^I,*; Róbson R. Teixeira^II; Rossimiriam P. de Freitas^II; Thays G. Perdigão^II; Silma F. da Silva^II; Aline Aparecida dos Santos^II; Alberto de Jesús O. Bastidas^III

^IDepartamento de Química, Universidade Federal de Minas Gerais, Av. Antônio Carlos, 6627, 31270-9001 Belo Horizonte-MG, Brazil

^IIDepartamento de Química, Universidade Federal de Viçosa, Av. P. H. Rolfs, s/n, 36570-000 Viçosa-MG, Brazil

^IIILaboratório de Química Ecológica, Departamento de Química, Facultad de Ciencias, Universidad de Los Andes, Núcleo Universitario Pedro Rincón Gutiérrez, La Hechicera, Mérida 5101-A, Venezuela

ABSTRACT

Thirteen triazole derivatives bearing halogenated benzyl substituents were synthesized using the Cu-catalyzed azide-alkyne cycloaddition (CuAAC), a leading example of the click chemistry approach, as the key step. The biological activity of the compounds was evaluated, and it was found that these compounds interfere with the germination and radicle growth (shoots and roots) of two dicotyledonous species, Lactuca sativa and Cucumis sativus, and one monocotyledonous species, Allium cepa. The compounds showed predominantly inhibitory activity related to the evaluated species mainly at the concentration of 10^-4 mol L^-1. Some of them presented inhibitory activity comparable to 2,4-D (2,4-dichlorophenoxyacetic acid), used as positive control.

Keywords: herbicides, 1,2,3-triazoles, click chemistry, phytotoxicity

RESUMO

Uma série de treze derivados triazólicos contendo grupos benzila-halogenados foi sintetizada utilizando-se como etapa chave a cicloadição azida-alcino catalisada por Cu(I) (CuAAC), transformação comumente descrita como reação click. A atividade biológica destes compostos foi avaliada, e verificou-se que estes compostos interferem na germinação e no crescimento radicular (brotos e raízes) das espécies Allium cepa (cebola), modelo de monocotiledônea, e Cucumis sativus (pepino) e Lactuca sativa (alface), modelos de dicotiledôneas. Os compostos apresentaram atividade predominantemente inibitória com relação às espécies avaliadas principalmente na concentração de 10^-4 mol L^-1, sendo que alguns deles foram tão ativos quanto o 2,4-D (ácido 2,4-diclorofenoxiacético), o controle positivo.

Introduction

Weeds can be defined in several ways, for example, as plants that grow where humans do not wish them to be. Vegetable species that grow in the wrong place, in the wrong quantity or at the wrong time can also be considered weeds. Another definition of a weed is a species whose utility has not been identified.^1-4 More often than not, weeds interfere with human activities such as agriculture. Weeds compete with crops for nutrients, water and physical space, and may harbor insect and disease pests. Thus, weeds are capable of greatly reducing both crop quality and yield, and therefore, weed control is highly desirable.^5,6

Although there are several ways to weed, the use of chemicals (known as herbicides or weed killers) is currently the most cost efficient and reliable weed control method utilized by farmers.^1-3 Currently, there are several active compounds available to control weeds, but it is still necessary to identify new herbicides to overcome weed resistance problems resulting from pressure of selection.^7-10 In addition, due to the public's concerns about the environment, modern herbicides should have a favorable combination of properties, such as a high level of herbicidal activity, a low application rate, crop tolerance and low toxicity to mammals.

In the search and development for new herbicides as well as other agrochemicals and pharmaceuticals, heterocyclic compounds play an important role. The heterocyclic core is frequently part of the pharmacophore responsible for the observed biological activity.^11,12 The heterocyclic portion of a compound can have beneficial effects in terms of its physicochemical properties, conferring lipophilicity and solubility values in the optimal range for uptake and bioavailability. Moreover, heterocycles are ideal bioisosteres of other homocyclic rings, heterocyclic rings and several different functional groups. In many cases, this bioisosterism can result in compounds with improved biological efficacy.^12-14 Nitrogen-containing heterocycles are representatives of this class of organic compounds that stand out due to their abundance in nature and great significance in biochemistry. These structural subunits exist in many natural compounds such as vitamins, hormones, antibiotics and alkaloids, in addition to being found in pharmaceuticals, herbicides, dyes and many other compounds.¹⁴ Triazoles are one of the most studied classes of nitrogen heterocycles. Triazole derivatives have a wide range of applications and are used as explosives, drugs and agrochemicals. The 1,2,4-triazole core has been found to be an integral part of therapeutically interesting compounds that display significant antibacterial, central nervous system (CNS) stimulative, sedative, antifungal and antitumor activities.¹⁴ It is also worth to mention that all triazole derivatives are synthetic.¹⁵

Another class of organic compounds widely employed as pesticides is the halogen-containing heterocycles. These compounds are generally more polar than their homocyclic analogs and possess lower n-octanol-water partition coefficients. Consequently, halogen-containing heterocycles are often more environmentally mobile. In addition to their use as pesticides, halogen-containing heterocycles have also been used as pharmaceuticals, dyes and explosives.

The past 30 years have witnessed a period of significant expansion in the research and development of halogenated compounds to be used as agrochemicals.^16-18 The primary advantages of using these compounds are their economic viability and high efficacy. This high efficacy makes these compounds environmentally safe and user friendly because they are used at very low concentrations. Interestingly, there has been an increase in the number of commercial products containing mixed-halogen compounds. The extrapolation of the current trend indicates that an increase in the number of fluorine-substituted agrochemicals throughout the twenty-first century is to be expected. QSAR (quantitative structure-activity relationship) studies have shown that fluorinated benzyl moieties with fragments such as CHF₂O- or CF₃O- are very active; therefore, the synthesis of compounds containing these fragments is a primary goal of modern agrochemistry.¹⁹

Because of the importance of heterocycles and halogens in the development of new agrochemicals and as well as our interest in the chemistry of triazoles^20-23 and in the preparation of bioactive compounds that can be used as new active ingredients to control weeds,^24-27 our group synthesized novel 1,2,3-triazoles bearing halogenated benzyl moieties, and then, evaluated their phytotoxic activities.

Experimental

Materials and methods

All of the solvents used were purified by distillation. Commercially available benzyl alcohol, 4-fluorobenzyl alcohol, 4-chlorobenzyl alcohol, 4-bromobenzyl alcohol, 4-iodobenzyl alcohol, 3,4-difluorobenzyl alcohol, 4-(trifluoromethyl)benzyl alcohol, 4-(trifluoromethoxy)benzyl alcohol, 5-bromo-2-chlorobenzyl alcohol, 2,4,6-trichlorobenzyl alcohol, pent-4-yn-1-ol, prop-2-yn-1-ol, triethylamine, methanesulfonyl chloride, sodium ascorbate, sodium azide and copper(II) sulfate were purchased from Aldrich (USA) and utilized without further purification. The ¹H and ¹³C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance DPX 200 spectrometer at 200 MHz using CDCl₃ as the solvent and TMS (tetramethylsilane) as the internal standard, unless otherwise stated. The NMR data are presented as follows: chemical shift, d in ppm, multiplicity, number of protons, proton assignments and J in Hz. Multiplicities are indicated by the following abbreviations: s (singlet), d (doublet), dd (double doublet), t (triplet), m (multiplet), qn (quintet) and brs (broad signal). Mass spectra were recorded on a Shimadzu GC MS-QP5050A instrument using direct insertion along with the electrospray ionization method and a quadrupole analyzer. Infrared spectra were recorded on a Spectra One Perkin-Elmer spectrophotometer, fitted with a Paragon ATR accessory. Melting points were determined using an MQAPF-301 melting point apparatus (Microquimica, Brazil) and are uncorrected. The progress of the reactions was monitored by thin layer chromatography (TLC). Column chromatography was performed over silica gel (60-230 mesh).

Synthesis

4-Fluorobenzyl methanesulfonate (2b)

To a 50 mL round bottom flask, 4-fluorobenzyl alcohol (126 mg, 1 mmol), dichloromethane (5 mL) and triethylamine (280 mL, 2.0 mmol) were added, and the mixture was cooled to -50 ºC. Subsequently, methanesulfonyl chloride (120 mL, 1.4 mmol) was added to the flask, and the mixture was stirred vigorously. The reaction was complete after 30 min. After completion, the organic layer was washed with 1% aqueous HCl (15 mL) followed by saturated aqueous NaHCO₃ (5 mL), dried over Na₂SO₄, and concentrated under reduced pressure. This procedure afforded compound 2b in a 95% yield (193 mg, 0.95 mmol).

Sulfonates 2a,2c-2j (Figure 1) and 6 (Figure 2) were synthesized in yields ranging from 77 to 100% using a procedure similar to that described for compound 2b. The spectroscopic data for these compounds are available in the Supplementary Information (SI) section.

1-(Azidomethyl)-4-fluorobenzene (3b)

Mesylated compound 2b (157 mg, 0.77 mmol) was added to a 50 mL round bottom flask containing 5 mL of DMSO (dimethyl sulfoxide) and 200 mg (3.1 mmol) of sodium azide. The reaction mixture was stirred at room temperature for 15 h. After this period of time, 15 mL of dichloromethane were added to the flask. The resulting organic layer was washed with 15 mL of saturated aqueous NaCl solution. After separation, the organic layer was dried over Na₂SO₄, filtered and concentrated under reduced pressure to afford compound 3b in a 91% yield (106 mg).

Azides 3a,3c-3j (Figure 1) and 7 (Figure 2) were synthesized in yields ranging from 72 to 99% using a procedure similar to that described for compound 3b. The spectroscopic data of the azides are available in the SI section.

3-[1'-(4''-Fluorobenzyl)-1',2',3'-triazol-4'-yl]propan-1-ol (4b)

A total of 93 mg (0.62 mmol) of the azide derivative 3b was added to a 10 mL round bottom flask containing 1 mL of dichloromethane, 1 mL of water, 30.7 mg (0.12 mmol, 20 mol%) of CuSO₄.5H₂O, 48.8 mg (0.24 mmol, 40 mol%) of sodium ascorbate and 55 mL of pent-4-yn-1-ol. The resulting reaction mixture was vigorously stirred at room temperature for 24 h. Subsequently, the mixture was extracted with 15 mL of CH₂Cl₂, and the resulting material obtained after solvent removal was purified by silica gel column chromatography. The product was eluted with 100 mL of dichloromethane, 100 mL of dichloromethane/ethyl acetate (1:1 v/v), 100 mL of ethyl acetate and 100 mL of ethyl acetate/methanol (9:1 v/v). The procedure described afforded compound 4b with a 71% yield (103 mg, 0.44 mmol).

Compounds 4a, 4c-4j (yield ranging from 44 to 90%), 5a (87% yield), 5b (49% yield) and 8 (87% yield) were prepared from the corresponding azides using a procedure similar to that described for compound 4b. The compounds were purified by column chromatography using a typical eluotropic sequence (dichloromethane, ethyl acetate and methanol), and the yields are presented in Figures 1 and 2. The structures of the triazoles are supported by the spectroscopic and spectrometric data available in the SI section.

Biological assays

The evaluation of the phytotoxic activities of compounds 4a-4j, 5a, 5b and 8 was performed using an adaptation of the methodology described by Macías et al.²⁸ The bioassays used Petri dishes (90 mm diameter) with one sheet of Whatman No. 1 filter paper as a substrate. The target species were Allium cepa (onion) as the model monocotyledonous plant and Lactuca sativa (lettuce) and Cucumis sativus (cucumber) as the dicotyledonous species. The plants were germinated and grown in aqueous solutions. The compounds to be assayed were dissolved in DMSO at different concentrations, and these solutions were diluted with distilled water, so that, the desired test concentrations (10^-4, 10^-6 and 10^-8 mol L^-1) were obtained. This procedure facilitated the dissolution of the assayed compounds. Twenty five commercial seeds of each target species were placed in each Petri dish. The appropriate treatment, or the negative control (aqueous solution containing DMSO but no test compound), was added (10 mL) to each Petri dish. Three replicates were used for each target species. After the addition of the seeds and the aqueous solutions (10 mL), the Petri dishes were sealed with parafilm to create closed-system models. The seeds were incubated at 25 ºC in a controlled environment growth chamber in the absence of light. The bioassays lasted 5 days for the dicot model species and 7 days for the monocot model species (onion). After the growth period, the plants were frozen at -10 ºC for 24 h to prevent growth during the measurement process. This process facilitated the handling of the plants and allowed the more accurate measurement of radicle elongation. The shoot and root lengths of each radicle were measured manually to the nearest millimeter, using a ruler. The germination rate was obtained by the direct counting of the number of seeds that germinated, but not necessarily developed. Seeds were considered to have germinated if a radicle protruded at least 1 mm. All treatments were replicated three times in a completely randomized design. The percent inhibition or stimulation of radicle growth (root and shoot) was calculated in relation to the radicle growth of the negative control using the following equation:

where S corresponds to the average value of germination or the radicle (root and shoot) length and C corresponds to the average growth of the negative control. When using this equation, stimulatory effects correspond to values above the graphic base line, and inhibitory effects correspond to values below this line (Figures 3-6). Errors were estimated using the derivative method. L. sativa, C. sativus and A. cepa seeds (TOPSEED brand) were purchased from Agristar do Brasil, Petropólis city, Rio de Janeiro State, Brazil. The commercial herbicide 2,4-D (2,4-dichlorophenoxyacetic acid) was used as positive control.

Cluster analysis

The cluster analysis was performed using Statistica^® software (version 5.0), and the clusters were generated on the basis of the activity parameters for the three concentrations. Data were statistically analyzed using Welch's test, with significance set at 0.01 and 0.05. Results are expressed as percentage differences to control. Zero represents control, positive values represent stimulation of the studied parameter, and negative values represent inhibition. Once the germination and growth data were acquired, cluster analysis was used to group compounds with similar phytotoxicity behaviors and associate them with their molecular structure. Complete linkage was used as an amalgamation rule and the distance measurement was based on squared euclidean distances, given by the equation below:

where d(x, y) is the squared euclidean distance (i-dimensional), i represents the number of variables, and x and y the observed values. Regression analyses were performed using the Microsoft Excel 2010 (Microsoft Corporation, USA) and Graph Pad Prism^® (version 4).²⁹

Results and Discussion

Synthesis of triazole derivatives

The compounds 4a-4j were prepared in three steps as outlined in Figure 1.

Commercially available benzylic alcohols were initially converted into their corresponding mesylates 2a-2j by the straightforward reaction with methanesulfonyl chloride.³⁰ Nucleophilic substitution reactions between sodium azide and the mesylates yielded azides 3a-3j.³¹ Cu-catalyzed azide-alkyne cycloaddition (CuAAC), a leading example of the click chemistry approach,³² between compounds 3a-3j and pent-4-yn-1-ol afforded the chemicals 4a-4j in yields ranging from 44 to 90% (Figure 1). The click reaction is a very reliable method to prepare 1,2,3-triazoles. The conditions of the click reactions used to prepare 4a-4j were similar to those reported by Iehl et al.,³³ who have shown that the reaction time should be approximately 2 h. However, in the present study, the product formation was not observed within less than 20 h. The best results were observed for a period of 24 h at room temperature. It is necessary caution when manipulating any kind of azide, organic and inorganic, due to the risk of explosion caused by mechanic shock, electrical spark as well as heating.^34,35

A second set of compounds, 5a,5b and 8, was prepared as depicted in Figure 2.

Chemicals 5a and 5b were obtained, respectively, from azides 3a and 3d and were synthesized to evaluate the influence of shortening the triazole side chain on the compound biological activity (compound 5a compared with 4a and compound 5b compared with 4d). The aim of the preparation of triazole 8 was to evaluate the impact of the presence of two triazole rings on the phytotoxic activity (vide infra).

Evaluation of the phytotoxic activity of triazoles

The strategies employed for the discovery of new chemicals to control weeds (and other agrochemicals) are similar to those used for the discovery of bioactive compounds in the pharmaceutical industry and involve the assessment of the activity of extracts and pure compounds in a given biological system.³⁶ Preliminary laboratory bioassays need to be fast, economical and relevant to the system in question, in addition to being useful to establish the potential activity of a pure compound or an extract. Bioassays should also be followed by studies in greenhouses and fields to determine whether the initial observations are reproducible on a larger scale.^37,38 The most widely used bioassay to evaluate the phytotoxicity of a synthesized compound is one that monitors the germination and growth of plants (root and shoot) of given species such as Lactuca sativa (lettuce), Raphanus sativus L. (radish), Lepidium sativum (cress), Cucumis sativus (cucumber) and Allium cepa (onion), among other species, primarily due to their high sensitivity and fast germination rates. Weeds, which would be the ideal candidates for this initial assessment to identify potential herbicides, are used for testing only after activity is observed against the species mentioned above. This is because weeds generally have low germination rates.^31,39

The effects of 1,2,3-triazoles 4a-4j, 5a, 5b and 8 on the germination and radicle growth (shoot and root) of the species Lactuca sativa (lettuce), Cucumis sativus (cucumber) and Allium cepa (onion) were evaluated at three different concentrations (10^-4, 10^-6 and 10^-8 mol L^-1) and the results are shown in Figures 3-6.

With respect to germination (Figure 3), the most pronounced effects were observed on L. sativa. Compounds 4b-4f, 4j,5b and 8 significantly inhibited the germination relative to the negative control. At 10^-4 mol L^-1, triazoles 4d,4e,4f,4j and 8 exhibited inhibitory activities greater than 90%. Regarding A. cepa, compound 4f was the most efficient in inhibiting germination (ca. -80% at 10^-4 and 10^-6 mol L^-1). Some compounds were more effective inhibiting the germination than the positive control (2,4-D), especially on lettuce and onion.

For C. sativus, fluorinated 4f strongly inhibited the germination at the highest concentration (-85%). The germination of C. sativus was also strongly inhibited by 4b at 10^-6 and 10^-8 mol L^-1, and 8 at 10^-4 mol L^-1 (Figure 3).

The effects of compounds 4a-4j, 5a, 5b and 8 on the radicle growth (shoot and root) of the evaluated species are presented in Figures 4-6. As general trend, the triazole derivatives had inhibitory effects on the tested species. In addition, the halogenated compounds had, in general, superior effects relative to those of their non-halogenated counterparts (compounds 4a and 5a), demonstrating the beneficial effects of the presence of halogen atoms on the biological activity of the evaluated compounds.

The growth of the shoots and roots of L. sativa was strongly inhibited (higher than 60%) by the majority of the test compounds at a concentration of 10^-4 mol L^-1, as shown in Figure 4. Exceptions to this generalization were the derivatives 4a,4g and 5a, which can be considered inactive against this species. Interestingly, although the derivative 4f, which contains a trifluoromethoxyphenyl group, inhibited the radicle growth of L. sativa by 100% at the highest concentration and retained this effect at 10^-6 mol L^-1, the derivative 4g, which contains a trifluoromethylphenyl group, was almost inactive. Only compound 4f was as phytotoxic as the positive control, 2,4-D at the concentration of 10^-4 and 10^-6 mol L^-1.

The effects of the triazole derivatives on A. cepa are outlined in Figure 5. The growth of the shoots and roots of this monocotyledonous species was strongly inhibited by compounds 4b-4f and 8 at 10^-4 mol L^-1, with compound 8 being the most potent (-89%). With the exception of compounds 4a, 5a and 5b, all of the synthesized compounds presented superior phytotoxic effects in the development of the onion root and shoot than the positive control, 2,4-D. This is not a surprisingly result since this commercial agrochemical is used as a selective herbicide controlling dicot species and preserving monocot ones.

The effects of compounds 4a-4j, 5a, 5b and 8 on C. sativus were also evaluated (Figure 6). In this experiment, the most phytotoxic compound was the triazole 4b, which caused almost 90% inhibition of the shoot development and 84% inhibition of the root development at a concentration of 10^-6 mol L^-1. None of the synthesized compound was as phytotoxic as the positive control on the cucumber root and shoot development.

According to Macías et al.,^28,29 the compounds tested in standard bioassays tend to have inhibitory effect only at the highest concentrations; at lower concentrations, their inhibitory effect usually decreases and may sometimes reach a stimulatory effect. This growth stimulation at lower concentrations was observed for compounds 4d, 4e, 4h, 4j, 5a and 8, on the lettuce (Figure 4); for compounds 4b, 4c, 4i, 5a and 5b, on the onion (Figure 5); and 4g, 4h, 5a and 5b, against cucumber (Figure 6).

Cluster analysis

To obtain a better understanding of the relationship between biological activity and the structural features of the evaluated compounds, cluster analysis was performed on the basis of biological activity values (root and shoot growth) for all of the concentrations tested (Figure 7).

Based on activity, the assayed compounds can be divided into three groups (G1, G2 and G3). The composition of each group was as follows: G1 is composed of chemicals with low activity, 4a, 5a and 4g; G2 contains compounds 4h, 4i, and 5b; and G3 contains compounds 4b-4f, 4j and 8.

The compounds belonging to G1 do not possess a halogen directly attached to the benzyl ring. The compounds in G2 contain triazoles with two halogen atoms attached to the aromatic ring and one triazole with a shorter aliphatic side chain (5b). With the exception of 4f, the compounds in G3 are derivatives containing one halogen attached to position 4", or to position 4"", in case of 8 (see Figures 1 and 2 for numbering), with compound 4b, which has a fluorine substituent, being the most potent among all of the synthesized triazoles. Compound 8, which has a bromine attached to position 4'''', was found to be one of the most active compounds. This result points to the fact that the presence of a second triazole ring possibly has a favorable impact in terms of biological activity. Further studies will be carried out to prove such hypothesis.

Conclusions

In summary, thirteen 1,2,3-triazoles were synthesized, purified and fully characterized. The conditions employed were satisfactory, and thus, the formation of side products was not observed. The phytotoxicity of these compounds was investigated against two dicotyledonous species, Lactuca sativa and Cucumis sativus, and one monocotyledonous species, Allium cepa. The products exhibited predominantly inhibitory activity on the target species, and they were more selective for the dicotyledonous species, in particular for lettuce. Some of them were more phytotoxic than the positive control (2,4-D), especially on lettuce and onion. The comparison of the activities of products 4b-4j with the activities of compound 4a, which was synthesized to evaluate the influence of the absence of a halogen substituent on phytotoxic activity, revealed that the halogenated products were much more active. The same relationship was observed when comparing the phytotoxicity of derivatives 5a and 5b. These products were synthesized to evaluate the influence of shortening the chain length on phytotoxic activity. Again, it was confirmed that the presence of a halogen substituent on product 5b was necessary for appreciable phytotoxic activity. It was also noted an appreciable similarity between the phytotoxic activities of compounds 4b and 5b, and thus, confirming that the length of the triazole side chain does not have an important effect on phytotoxic activity.

The most significant results were observed for the fluorinated derivatives (4b and 4f), and the chlorinated derivatives (4c and 4j), and at some concentrations, these compounds inhibited lettuce germination by up to 100%. These compounds also inhibited the growth of shoots and roots by approximately 60% for all target species at a concentration of 10^-4 mol L^-1.

Considering these results, these compounds seem worthy of further investigation and could be exploited for the design of new ones endowed with herbicidal activity. Synthetic efforts are under way in our laboratories to prepare and biologically evaluate new derivatives. The results will be submitted soon.

Supplementary Information

A list of all spectroscopic data is available free of charge at http://jbcs.sbq.org.br as a PDF file.

Acknowledgments

We are grateful to CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), FAPEMIG (Fundação de Amparo à Pesquisa do Estado de Minas Gerais) and PRPq (Pró-Reitoria de Pesquisa da Universidade Federal de Minas Gerais) for their financial support.

Submitted:January 14, 2013

Published online: May 21, 2013

Supplementary Information

The supplementary material is available in pdf: [Supplementary material]

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Publication Dates

• Publication in this collection
  28 June 2013
• Date of issue
  June 2013

History

• Received
  14 Jan 2013
• Accepted
  21 May 2013