Novel 1, 2, 4-Triazoles as Antifungal Agents

The development of innovative antifungal agents is essential. Some fungicidal agents are no longer effective due to resistance development, various side effects, and high toxicity. Therefore, the synthesis and development of some new antifungal agents are necessary. 1,2,4-Triazole is one of the most essential pharmacophore systems between five-membered heterocycles. The structure-activity relationship (SAR) of this nitrogen-containing heterocyclic compound showed potential antifungal activity. The 1,2,4-triazole core is present as the nucleus in a variety of antifungal drug categories. The most potent and broad activity of triazoles have confirmed them as pharmacologically significant moieties. The goal of this review is to highlight recent developments in the synthesis and SAR study of 1,2,4-triazole as a potential fungicidal compound. In this study, we provide the results of a biological activity evaluation using various structures and figures. Literature investigation showed that 1, 2, 4-triazole derivatives reveal the extensive span of antifungal activity. This review will assist researchers in the development of new potential antifungal drug candidates with high effectiveness and selectivity.

2.1. General Pathways. A simple method for preparing 1,2,4triazoles has been introduced from the reaction of formamide and hydrazines without a catalyst using microwave irradiation (Scheme 2) [33].
The synthesis of 1,2,4-triazoles from aliphatic amines and hydrazones has been developed using a cascade C-H functionalization, oxidative aromatization sequence, and double C-N bond formation under iodine as the catalyst (Scheme 6) [37].
3.2. Structure-Activity Relationship of Fluconazole. Fluconazole is well known as one of the most potent antifungal drugs with remarkable interest in medicinal chemistry. Due to the importance of fluconazole as a reference drug, the structure-activity relationship is shown in Figure 3.

1,2,4 Triazole Scaffold for the Development of Antifungal Agents
This part of the review is classified into two parts based on the structural similarities of 1,2,4-triazole derivatives. First, we explained novel analogues of commercial 1,2,4-triazole drugs and then discussed 1,2,4-triazole-based scaffolds with various functional groups such as indole, benzimidazole, quinolone, quinazoline, amine, hydrazone, amide, sulfur, and oxime ether that showed remarkable antifungal properties, as well as SARs of all synthesized compounds.
Montoir et al. [89] reported a novel class of azole antifungal compounds based on a pyrrolotriazinone scaffold. As a result, these compounds demonstrated fungicidal activity against pathogenic Candida species in vitro (fluconazole susceptible and fluconazole resistant) and were more active than voriconazole against two Candida albicans candidates. Compound 4e also showed promising in vitro activity against several filamentous fungi, including Aspergillus fumigatus ( Figure 6).
Xie et al. [90] demonstrated that the entire series of triazole containing isoxazole compounds (5a-f) were antifungal against eight human pathogenic fungi. Compound 5a showed a strong inhibitory activity toward Candida parasilosis and Candida albicans with MIC80 values of 0.0313 μg/ mL. According to the SARs study, mono-fluorine on the phenyl ring possesses antifungal activity. On the other hand, enhancing the number of fluorine atoms (5c-d) may result in a reduction in antifungal activity (Figure 7).
In comparison to the reference drugs, voriconazole, fluconazole, and ravuconazole, an alkyne linked in the side  BioMed Research International chain of the triazole derivatives demonstrated good fungicidal activity against eight human pathogenic fungi, with particularly noticeable activity against Cryptococcus species and Candida. Compounds 6b and 6c shown in vitro antifungal activity against all the investigated fungi with (MIC80 = 0:0156 -0:5 mg/mL), which is greater than fluconazole and ravuconazole. SAR study shows that para fluoro (6e, 6h), para chloro (6b), and para cyano (6c) substituted phenylalkynyl, or pyridinyl alkynyl (6m, 6n) side chains improve activity. Compounds with R groups in the para position are shown to be more active than meta or ortho compounds (Figure 8) [91].
The antifungal efficacy of triazole alcohol derivatives toward 16 Candida isolates from five different species, including fluconazole-susceptible and fluconazole-resistant isolates, was investigated. All of these derivatives with MIC values of 0.063-1 mg/mL showed higher activity than fluconazole (MICs = 0:5 -4 mg/mL) against fluconazolesusceptible isolates; significantly, compounds 7b and 7e were also active against fluconazole-resistant species. However, the effect of chloro substitution depends on the type of species. For example, the 2,4-dichloro substituent 7d was shown to be more effective against C. albicans than 3-Cl (7b) or 4-Cl (7c). In the case of C. krusei, however, the 3-chloro group was better than the 4-chloro or 2,4-dichloro substituents. The addition of fluoro or bromo groups to the benzyl residue, however, had no beneficial impact. Among the fluorobenzyl regioisomers (7f-h), the 3-fluoro 7g analog was more active against Candida species (Figure 9) [92].
Chandrika et al. [93] investigated novel fluconazole (FLC) compounds for antifungal activity against the clinical strains of C. parapsilosis, C. glabrata, and Candida with aryl, alkyl, cycloalkyl, and dialkyl-amino substituents for neoformans using MIC determination. The activity of the alkylamino FLC derivatives was shown to be directionally related to the length of the alkyl chains ( Figure 10).
Tekale et al. [94] investigated the antifungal activities of triazole compounds, including imidazole. The impact of the imidazole side chain on the in vitro fungicidal activity of novel synthesized compounds toward various microorganisms such as aspergillus, niger, aspergillus fumigates, and Candida albicans was demonstrated. Compound 9e had the lowest activity against C. albicans, and compounds 9b & 9d had higher activity against A. niger than the other compounds ( Figure 11).
The MIC 80 values of new triazole compounds containing 1,2,3-triazoles or substituted amines as side chain 10a-o derivatives demonstrated better antifungal properties than those of fluconazole on three significant fungal infections except for 10i. Furthermore, the considerable compounds 10d, 10 k, 10n, 10 m, and 10o were reported on the Aspergillus fumigatus strain (MIC 80 range: 0.125-1 μg/mL). In addition, 10k can be applied to almost all fungi tested, especially Aspergillus spp. In vitro biological assessments of the compounds 10d and 10k showed potent antifungal properties ( Figure 12) [95].
Sadeghpour et al. [96] reported two classes of novel fluconazole-derivatives containing nitrotriazole or 2-(piperazin-1-yl) ethanol moieties, which were evaluated for antifungal activity against standards and clinically isolated yeasts, and their MIC structures were compared with those of fluconazole. Nitrotriazole derivatives 11a-d and compounds 12g and 11b containing two chlorine atoms exhibited good activity against the tested fungus, notably some fluconazole-resistant species. Compounds 11a, 11b, and 12g with 2,4-difluorophenyl or 2,4-dichlorophenyl groups had more excellent antifungal activity ( Figure 13).
Compared to fluconazole and 5-flucytosine, the new fungicidal hybrids of 5-flucytosine and fluconazole showed modest antifungal activity. Surprisingly, a hybrid of 3,4dichlorobenzene can inhibit clinical-resistant strain C. albicans and the growth of C. albicans ATCC 90023 with MIC values of 0.02 and 0.008 mM, respectively. Compound 16e   BioMed Research International inhibited C. albicans rapidly, whereas compound 16a lacked fungicidal activity due to the lack of substituents on the phenyl ring (ure 16) [99]. Xu et al. [100] described a series of novel triazole derivatives having γ-lactam that were screened for antifungal activity against six pathogenic fungi in vitro. Furthermore, the pyridyl-and phenyl-substituted compounds 17d and 17e showed moderate antimicrobial activity against Cryptococcus neoformans and Candida spp. (Figure 17).
Ciprofloxacin and itraconazole were employed to screen 1,2,4-triazole derivatives fused with novel benzene-ethanol which were assessed at concentrations ranging from 0.125 to 64 mg/mL. Furthermore, compounds 22a, 22g, and 22i showed much better growth inhibitory activity on C. albicans with MIC of 32 mg/mL (itraconazole was introduced as the standard drug MIC 1 mg/mL). Electronegativity, like substituent groups on the para and ortho positions of a benzene ring, can be effective in antifungal activity ( Figure 21) [104].
A class of 1,2,4-triazole derivatives has been tested toward Magnaporthe oryzae. Aromatic ring structures revealed that the methyl group at position 1,4 of the phenyl ring 23b and the phenyl moiety at the para position of phenyl 23c reduced antifungal activity. When an electronwithdrawing fluorine atom entered this position (23e), the antifungal activity increased slightly. An electronwithdrawing group (trifluoromethyl group) had a positive efficacy on increasing the antifungal activity of this synthetic series (comparison of antifungal activity 23b with 23f). The introduction of two chlorine atoms to the phenyl moiety Significantly improve activity increases solubility in water increases pharmacokinetic properties serum protein binding is reduced Significantly improve activity increases metabolic stability first pass metabolism is reduced Significantly improve activity increases solubility in water increases pharmacokinetic properties serum protein binding is reduced Significantly improve activity and selectivity increases metabolic stability  8   Compound e(R1 = (p) OCH 3 -C 6 H 4 ) also exhibited promising in vitro antifungal activity againts some filamentous fungi such as Aspergillus funmigatus.  Especially, compound k showed strong activity against all tested fungi.      14 BioMed Research International arachidicola Hori than chlorothalonil and carbendazim (commercial fungicides). Compounds 26d and 26f indicated better actions against most of the fourteen plant pathogens ( Figure 24) [107]. According to the study by Ahuja et al. [108], compound 27c has a lower ED50 value than the triazole fungicide propiconazole. Significantly, compound 27c showed the highest activity compared with other experimental fungi, with an ED50 value of 16 to 21 μg/mL, which is higher than the ED50 values of the standard commercial fungicides used (tilt: 20-25 μg/mL and carbendazim: 150-230 μg/mL ( Figure 25).
Microbiological studies revealed that benzimidazole-1,2,4-triazole hybrid compounds 28m, 28n, 28f, and 28g had good fungicidal activity (MIC50 values of 0.78 to1.56 μg/mL) because of the presence of a fluoro or chloro substituent at the C-para position of phenyl, whereas compounds 28c, 28a, and 28b did not. Compounds 28d and 28e demonstrated adequate fungicidal activity (MIC50 values = 1:56-3.12 μg/mL). Compound 28l exhibited comparable antifungal activity with reference drugs fluconazole and ketoconazole. As a result, chloro or fluoro substitution at the C-5 position of benzimidazole is vital and could have had a significant influence on antifungal activity ( Figure 26) [109].
The antifungal efficacy of 1,2,4-triazoles having quinoline moiety against A. fumigatus and Candida albicans was highest owing to methoxy and chloro substituents. As a result, 32e, 32g, and 32m derivatives with methoxy and    D'Souza et al. [114] investigated the fungicidal of new quinoline-triazoles. Compounds 33b and 34b with chlorine substituents on the aromatic ring demonstrated more antifungal activity than (33e, 33f, 34e, and 34f) that included OCH 3 and CH 3 ( Figure 31).
Sompalle et al. [120] investigated the antifungal activity of a class of 1,2,4-triazole-quinazolinethiones (44a-l) against Aspergillus niger (A. niger) and Aspergillus flavus (A. flavus) in combination with the commonly used antifungal drug fluconazole ( Figure 37). All triazole derivatives with N-alkylated groups were tested for fungicidal activity toward Candida albicans and Aspergillus flavus and anthelmintic activity against Pheretima posthuma, and the compound containing group CH 3 at the ortho position of the phenyl ring showed good inhibition with the inhibition zone 24:17 ± 0:32 and 15:02 ± 0:41 mm against A flavus and C albicans in comparison with a standard antifungal drug, Nystatin, while the antifungal activity of the other structures was lower ( Figure 38) [121].
Jin et al. [122] investigated the fungicidal activity of novel compounds containing 1,2,4-triazole with different substituted groups toward Gibberlla nicotiancola, Pythium solani, Gibberlla saubinetii, and Fusarium oxysporum f.sp. niveum in vitro. Compound 46 had good activity against the case fungus, indicating that 1,2,4-triazole-imidazole can contribute to antifungal properties. Methyl at position Q increased the activity, the activity order is 47>46, and compound 47 demonstrated a remarkable antifungal activity. As a result, positions P and Q may have an impact on the activity at the same time ( Figure 39).
1,2,4-Triazole derivatives with a pyrimidine moiety were evaluated for fungicidal activity, with compounds 50c and 50d showing the best antifungal activity against Phompsis sp. that was even better than pyrimethanil (32.1 mg/mL). Compound 50d, on the other hand, had higher activity against B. cinerea and B. dothidea with 55.1 and 40.1 mg/ mL, respectively, when compared with Pyrimethanil (57.6 and 62.8 mg/mL) ( Figure 40) [123].
Antifungal evaluation [1, 2, 4] of triazolo [5,1-b] quinazolin-8(4H) one scaffolds (51a-n) in vitro exhibited that compounds 51e and 51i display higher activity than standard drug griseofulvin (MIC 500 mg/mL) against C. albicans. Surprisingly, the substitution at the C-6 carbon of the final moiety and para-substituted phenyl ring was responsible for variable biological results, while the triazole with nonsubstituted or diversely para-substituted (Cl, OCH 3 , and NO 2 ) phenyl core or heterocyclic nucleus showed the best properties. In addition, the compounds having OCH 3 group substitution (compound 51f) effectively showed poor inhibition toward A. clavatus and A. niger inhibited the S. aeruginosa, P. aeruginosa, and S. pneumonia strains, although the derivative with the electron-withdrawing group such as NO 2 (compound 51i) efficiently inhibited the E. coli bacterial strain as well as was found potent toward the C. albicans strain. Finally, compound bearing heteroaryl substitution (compound 51l) led to the improvement in the activity against the E. coli strain (Figure 41) [124].
Remarkable antifungal activity of a number of new 1,2,4triazole derivatives against different strains of Aspergillus fumigatus, Candida albicans, and Candida crocus has been reported in comparison with those of commercial fungicides ketoconazole and itraconazole. All of the derivatives investigated, the dichloro urea analogue and bromo substituted triazole, stand out as the most favorable compounds. The most potent compounds against A. fumigatus were 64l, 61b, 61a, and 61c, with MIC values ranging from 0.114 to   Compound containing heteroaryl substiturion ( compound l) effectively increased activity against E.coli strain.
Compounds 51e and 51i display better activity than standard drug griseofulvin (MIC 500 mg/mL against C. albicans) Compounds show poor inhibition against A.niger and A.clavatus para-substituted phenyl ring and the substitution at the C-6 carbon of the final moiety are responsible for varying the biological results Compound having OCH 3 group substitution (compound f) effectively e compound with electron withdrawing group like NO 2 (compound 51i) effeiciently inhibited the E.coli bacterial strin and also found active against C.albicans fungi strain. Compounds53d, 54b and 54c showed excellent activity against all the fungal strains at the MIC value of 3.9 g/mL except for C.albicans MTCC 7315, C. parapsilosis MTCC 1744, which is equal to the standard miconazole. the order of activity is found to be 53d<54b = 54c.
e existence of fluoro, trifluoromethyl, bromo and nitro group on phenyl and furyl in pyrido (2, 3-d) pyrimidine was significant to increase antifungal activity.
Compounds containing an electron-donating group at the 2-or 4-position of the benzene ring or an electron-withdrawing group at the position of the benzyl rings, such as 58a and 58b, exhibited higher activity.
Cheng et al. [134] investigated the fungicidal activity of new groups of 1,2,4-triazole benzoyl aryl amines. The findings revealed a clear relationship between the structure and training in these compounds as well. The electronwithdrawing group oi-pr(isopropyl) at the para position has a favorable impact on high activity, and the preferred groups were alkoxy carbonyls. This compound indicated the most effective fungicidal activities with EC50 values of 0.12, 0.19, and 0.01 mg/mL against S. sclerotiorum, F. graminearum, and G. graminis var. tritici, respectively. Alkoxy carbonyl of these ester carbonyls revealed the highest activities (69a-b and 69c-g). In contrast, no significant increase in the activity was observed when more than one electronwithdrawing group was added to aniline. For instance, if the second electron-withdrawing groups such as CF 3 or Cl were added to the meta situation of aniline, the activity against G. graminis var. tritici would be reduced (69e and 69f) ( Figure 51).
The evaluation indicated that all 1,2,4-triazole derivatives had fungicidal activity, with MIC values ranging from 0.02 to 0.52 mM, which was better than bifonazole (MIC values of 0.32-0.64 mM) and ketoconazole (MIC values of 0.28-1.88 mM). Compound 70c, having a MIC value of 0.02-0.04 mM, exhibited the best antifungal activity rather than compound 70a ( Figure 52) [135].
Wu et al. [136] evaluated the fungicidal activity of a novel series of 1,2,4-triazole derivatives containing an amide moiety. Compounds 71a, 71d, 71e, and 71f had the highest antifungal activity against Botrytis cinerea. Meanwhile, compound 71b, when R was CH 3 , exhibited better antifungal property against Phomopsis sp., compared with that of pyrimethanil. SAR studies revealed that 4-pyridine in the R substituent group and the smaller alkyl substituent groups (H or CH 3 ) could have a favorable influence on the activity, such as 71a>71b>71c. Meanwhile, when R = OH is added to the 4-positions of phenyl and substituted phenyl, the action against Phomopsis sp., B. dothidea, and B. cinerea rises in the sequence 71d>71g>71k. Furthermore, when R = 4-pyridine, the antifungal activities of the corresponding compound 71h against Phomopsis sp., B. dothidea, and B.

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BioMed Research International cinerea were higher than those of compound 71e (R = 2 -pyridine) ( Figure 53). Yurttaş and CantŘrk [137] investigated triazoleoxadiazole compounds against C. krusei, C. glabrata, C. albicans, and C. parapsilosis and found that triazole-oxadiazole derivatives 72e and, particularly, 73i had the highest activity against C. glabrata and C. albicans (MIC90 = 62:5 mg/mL). The oxadiazole rings of these derivatives differ due to the benzothiazole and phenyl rings linked to the acetamide molecule. Meanwhile, compounds 72e (R = NO 2 ) and compound 73e (R = F) were found to have the highest activity ( Figure 54).
Li et al. [138] reported a group of N-phenylacetamide containing 1,2,4-triazole derivatives (74a-f) that were screened in vitro for antifungal assessment, and specific compounds, such as 74b-f derivatives, inhibited the growth of the tested fungus. Among all synthesized compounds, 74a exhibited no antifungal activity. Moreover, monosubstituted halogen substituents in the benzene ring, in either the ortho or the para position, displayed antifungal activity ( Figure 55).

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BioMed Research International special effects were detected notably against fungal infections. Fusarium oxysporum and Aspergillus fumigatus were inhibited with all of them. The most excellent antifungal activities indicated triazole 79c that contains R = 4-nitrophenyl has the highest antifungal activity as well as the high-affinity binding to the receptor. Hydrogen bonds between the N-1 azole ring and some amino acid residues in the target enzyme interact predominantly. These findings might aid in the development of antifungal drugs ( Figure 60). Some studies have been conducted on the fungicidal activities of the novel 1,2,4-triazole derivatives. Compounds 80a-d ( Figure 61) in particular showed high antifungal activity. The relationship between biological activity and structure revealed that compounds with the sulfur atom exclusively in the thiol form exhibited activity. Furthermore, compounds 80a and 80c, at a concentration of 1000 mm, inhibit the growth of C. albicans by 35-40%, respectively [144].
Sidhu and Kukreja [145] reported new compounds based on lead hybridization of 1,2,4-triazoles with fluorinated benzothiazol-2-yl that were tested for fungicidal activity against P. striiformis, D. oryzae, and U. hordei in contrast with conventional fungicides. Furthermore, derivatives 81b   Shingare et al. [146] presented a new series of pyrazole bearing triazolo-thiadiazole derivatives (82a-l) which were evaluated to have antifungal activity versus A. Niger, C. 33 BioMed Research International albicans, and A. clavatus along with nystatin and griseofulvin as standard drugs. Amongst them, compounds 82b and 82j revealed good antifungal activity. Compound 82j has shown the most activity ( Figure 63).
Bai et al. [148] assessed several novel 1,2,4-triazole analogues for antifungal activity against eight phytopathogens and found that the majority of them exhibited acceptable to outstanding fungicidal characteristics. Almost all of the compounds demonstrated moderate to excellent fungicidal activity toward the tested phytopathogens. In general, the fungicidal activity of methyl oxime ether group 85 (R 1 = methyl) was significantly greater than benzyl oxime ether group 86 (R 1 = benzyl). It is clear that electrondrawing groups at the 2-position of the phenyl ring, such as 85b, 85d, 86b were more helpful for fungicidal activity. Because of the halogen substituent effect, compounds 85b, 85d, 85g, 85k, and 86b demonstrated significantly higher inhibitory activities against fungal pathogens than other compounds, with the chlorine atom playing a more significant role in improving fungicidal activity in each position of the benzene ring.
Furthermore, the prevention rates of compounds 85d, 86e, and 85f against all of the fungi examined were meager. It is shown that for benzyl oxime ether series 85, a bulky 2-tert-butyl group on the benzene substituent was not best for the activity. Compound 85d (2-Cl-4-Br) containing two mixed halogen atoms showed broad-spectrum fungicidal activity, with EC50 values of 1.59, 0.46, 0.27, and 11.39 mg/ L against four fungal pathogens ( Figure 65).

Conclusion
A privileged structure in medicinal and organic chemistry is 1,2,4-triazole-hybrids having a broad spectrum of antifungal activity. The 1,2,4-triazole nucleus and its derivatives are essential scaffolds in the discovery and development of drugs that have a multitude of biological activities. An acceptable reason for its broad biological profile is a small and stable cyclic ring structure wherein the nitrogen atoms can act both as hydrogen bond donor and as acceptors at the active site of the receptor. The pentacyclic triazole ring processes plasticity for the synthesis of a number of derivatives due to of its multifold binding sites. This potent scaffold will act as a lead molecule in drug synthesis in the future. The various methods for the regioselective synthesis of 1,2,4-triazolescaffold will be a great tool in medicinal chemistry in the future. The most challenging problem in fungal therapy is antifungal resistance, which may be progressed by drug target overexpression. This review is focused to summarizing recent research on 1,2,4-triazole-hybrids as fungicidal agents over the last decade. It will aid researchers and medicinal chemists in the discovery and the synthesis of new antifungal compounds with 1,2,4-triazole-moiety.

Data Availability
The datasets used and analyzed during the current study are available from the corresponding author on reasonable request. We have presented all data in the form of figures.

Conflicts of Interest
The authors declare that they have no competing interests.

Authors' Contributions
The conception, design, writing, and revision of the study were done by E. Z. The first draft of the manuscript was written by E. Z., Z. K., M. M., A. S., M. F., and A.K. Also, E. Z. and M. M. played the role of the first author in this manuscript; all authors approved the final manuscript.