Synthesis and Antiplasmodial Activity of 1,2,3-Triazole-Naphthoquinone Conjugates

A series of 34 1,2,3-triazole-naphthoquinone conjugates were synthesized via copper-catalyzed cycloaddition (CuAAC). They were evaluated for their in vitro antimalarial activity against chloroquine-sensitive strains of Plasmodium falciparum and against three different tumor cell lines (SKBr-3, MCF-7, HEL). The most active antimalarial compounds showed a low antiproliferative activity. Simplified analogues were also obtained and some structure–activity relationships were outlined. The best activity was obtained by compounds 3s and 3j, having IC50 of 0.8 and 1.2 μM, respectively. Molecular dockings were also carried on Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) in order to rationalize the results.


Introduction
Malaria, a disease caused by parasites of the Plasmodium genus and spread through the bites of infected mosquitoes, was responsible for an estimated 219 million clinical cases and 435,000 deaths worldwide in 2017, mostly among children under the age of five [1]. Most of the malarial infections and deaths are due to Plasmodium falciparum and Plasmodium vivax species. Available therapeutic agents are already limited in their efficacy, and drug resistance threatens to diminish the ability to treat and prevent the disease further. Despite a renewed effort to identify antimalarial compounds, the drug discovery lacks target diversity and most malaria drugs are only efficacious during the asexual blood stage of parasite infection [2]. Thus, it is necessary the search for new antimalarial drugs that overcome the resistance and act through new mechanisms.
Natural products have played a pivotal role in the discovery of lead compounds for the treatment of malaria, from quinine and artemisinin to ozonide-based compounds. Many of these natural products have served as a starting point for the development and design of antimalarial drugs currently in the clinic or in the development phase [3].
Nowadays, molecular hybridization has emerged as a promising tool for the drug design process and medicinal chemistry. In this strategy, two or more different pharmacophoric units are covalently linked into a single hybrid molecule with best properties as compared to the parent drugs [20]. Molecular hybridization [21] is beneficial, as different targets are activated by a single molecule, and is particularly interesting where treatment is limited to a few commercial drugs or in cases where the bioactive compounds present pharmacokinetic and pharmacodynamic limitations or high toxicity.
Molecules 2019, 24,3917 3 of 23 against chloroquine-sensitive strains of Plasmodium falciparum and for antiproliferative activity against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.
Molecules 2019, 24,3917 3 of 23 against chloroquine-sensitive strains of Plasmodium falciparum and for antiproliferative activity against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.
Molecules 2019, 24,3917 3 of 23 against chloroquine-sensitive strains of Plasmodium falciparum and for antiproliferative activity against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.
Molecules 2019, 24,3917 3 of 23 against chloroquine-sensitive strains of Plasmodium falciparum and for antiproliferative activity against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.

Results and Discussion
The 1,2,3-triazole-naphthoquinone derivatives (3) were synthesized using a copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction [28] of the corresponding O-propargylated naphthoquinone (1) with alkyl or aryl azides (2) ( Table 1). against chloroquine-sensitive strains of Plasmodium falciparum and for antiproliferative activity against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.

Results and Discussion
The 1,2,3-triazole-naphthoquinone derivatives (3) were synthesized using a copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction [28] of the corresponding O-propargylated naphthoquinone (1) with alkyl or aryl azides (2) ( Table 1). against chloroquine-sensitive strains of Plasmodium falciparum and for antiproliferative activity against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.

Results and Discussion
The 1,2,3-triazole-naphthoquinone derivatives (3) were synthesized using a copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction [28] of the corresponding O-propargylated naphthoquinone (1) with alkyl or aryl azides (2) ( Table 1). against chloroquine-sensitive strains of Plasmodium falciparum and for antiproliferative activity against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.

Results and Discussion
The 1,2,3-triazole-naphthoquinone derivatives (3) were synthesized using a copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction [28] of the corresponding O-propargylated naphthoquinone (1) with alkyl or aryl azides (2) ( Table 1). against chloroquine-sensitive strains of Plasmodium falciparum and for antiproliferative activity against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.

Results and Discussion
The 1,2,3-triazole-naphthoquinone derivatives (3) were synthesized using a copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction [28] of the corresponding O-propargylated naphthoquinone (1) with alkyl or aryl azides (2) ( Table 1). against chloroquine-sensitive strains of Plasmodium falciparum and for antiproliferative activity against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.

Results and Discussion
The 1,2,3-triazole-naphthoquinone derivatives (3) were synthesized using a copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction [28] of the corresponding O-propargylated naphthoquinone (1) with alkyl or aryl azides (2) ( Table 1). against chloroquine-sensitive strains of Plasmodium falciparum and for antiproliferative activity against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.

Results and Discussion
The 1,2,3-triazole-naphthoquinone derivatives (3) were synthesized using a copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction [28] of the corresponding O-propargylated naphthoquinone (1) with alkyl or aryl azides (2) ( Table 1). against chloroquine-sensitive strains of Plasmodium falciparum and for antiproliferative activity against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.

Results and Discussion
The 1,2,3-triazole-naphthoquinone derivatives (3) were synthesized using a copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction [28] of the corresponding O-propargylated naphthoquinone (1) with alkyl or aryl azides (2) ( Table 1). against chloroquine-sensitive strains of Plasmodium falciparum and for antiproliferative activity against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.

Results and Discussion
The 1,2,3-triazole-naphthoquinone derivatives (3) were synthesized using a copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction [28] of the corresponding O-propargylated naphthoquinone (1) with alkyl or aryl azides (2) ( Table 1). against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.

Results and Discussion
The 1,2,3-triazole-naphthoquinone derivatives (3) were synthesized using a copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction [28] of the corresponding O-propargylated naphthoquinone (1) with alkyl or aryl azides (2) ( Table 1). against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.

Results and Discussion
The 1,2,3-triazole-naphthoquinone derivatives (3) were synthesized using a copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction [28] of the corresponding O-propargylated naphthoquinone (1) with alkyl or aryl azides (2) ( Table 1). against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.

Results and Discussion
The 1,2,3-triazole-naphthoquinone derivatives (3) were synthesized using a copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction [28] of the corresponding O-propargylated naphthoquinone (1) with alkyl or aryl azides (2) ( Table 1). against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.

Results and Discussion
The 1,2,3-triazole-naphthoquinone derivatives (3) were synthesized using a copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction [28] of the corresponding O-propargylated naphthoquinone (1) with alkyl or aryl azides (2) ( Table 1). against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.

Results and Discussion
The 1,2,3-triazole-naphthoquinone derivatives (3) were synthesized using a copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction [28] of the corresponding O-propargylated naphthoquinone (1) with alkyl or aryl azides (2) ( Table 1). against some tumor cell lines. From the obtained results some structure-activity relationships were outlined. Furthermore, docking studies on the Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) suggested that these compounds could act as inhibitors of this enzyme.
In order to deepen the structural determinants responsible of the antiplasmodial activity, we decided to prepare several derivatives. Thus, we wanted to evaluate the influence in the activity of the -CH 2 O-linker, and two compounds (4a and 4j) were synthesized with the triazol ring attached at carbon C-2 of the naphthoquinone nucleus (Scheme 1).

3o
2.2 ± 0.1 >10 >10 >10 3p 9.7 ± 0.2 >10 6.5 ± 0.9 3.9 ± 0.8 3q In order to deepen the structural determinants responsible of the antiplasmodial activity, we decided to prepare several derivatives. Thus, we wanted to evaluate the influence in the activity of the -CH2Olinker, and two compounds (4a and 4j) were synthesized with the triazol ring attached at carbon C-2 of the naphthoquinone nucleus (Scheme 1). Scheme 1. Synthesis of compounds 4a and 4j.
When 4a and 4j were assayed, a loss of activity (IC50 > 10 μg/mL) with respect to the active compounds 3a and 3j, having the -OCH2-linker, was detected. Next, we analysed the antiplasmodial activity of the 2-hydroxy-1,4-naphthoquinone (lawsone) and some (4-aryl-1H-1,2,3-triazol-1-yl) methanol derivatives ( Figure 2 and Supplementary Material) as simplified fragments of the active structures. All of them were inactive (IC50 > 10 μg/mL), which indicated the importance for the biological activity of both the quinone ring and the substituted triazole. Next, we evaluated the influence on the activity of an isosteric modification by replacing the -OCH2linker by -NHCH2-. Thus, the following conjugates were prepared from N-propargylated-1,4-naphthoquinone and alkyl and aryl azides in higher yields than those from O-propargylated-1,4-naphthoquinone ( Table 3). None of the N-derivatives resulted active (IC50 > 10 Scheme 1. Synthesis of compounds 4a and 4j. When 4a and 4j were assayed, a loss of activity (IC 50 > 10 µg/mL) with respect to the active compounds 3a and 3j, having the -OCH 2 -linker, was detected. Next, we analysed the antiplasmodial activity of the 2-hydroxy-1,4-naphthoquinone (lawsone) and some (4-aryl-1H-1,2,3-triazol-1-yl) methanol derivatives ( Figure 2 and Supplementary Material) as simplified fragments of the active structures. All of them were inactive (IC 50 > 10 µg/mL), which indicated the importance for the biological activity of both the quinone ring and the substituted triazole.
9.7 ± 0.2 >10 6.5 ± 0.9 3.9 ± 0.8 3q In order to deepen the structural determinants responsible of the antiplasmodial activity, we decided to prepare several derivatives. Thus, we wanted to evaluate the influence in the activity of the -CH2Olinker, and two compounds (4a and 4j) were synthesized with the triazol ring attached at carbon C-2 of the naphthoquinone nucleus (Scheme 1). Scheme 1. Synthesis of compounds 4a and 4j.
When 4a and 4j were assayed, a loss of activity (IC50 > 10 μg/mL) with respect to the active compounds 3a and 3j, having the -OCH2-linker, was detected. Next, we analysed the antiplasmodial activity of the 2-hydroxy-1,4-naphthoquinone (lawsone) and some (4-aryl-1H-1,2,3-triazol-1-yl) methanol derivatives ( Figure 2 and Supplementary Material) as simplified fragments of the active structures. All of them were inactive (IC50 > 10 μg/mL), which indicated the importance for the biological activity of both the quinone ring and the substituted triazole. Next, we evaluated the influence on the activity of an isosteric modification by replacing the -OCH2linker by -NHCH2-. Thus, the following conjugates were prepared from N-propargylated-1,4-naphthoquinone and alkyl and aryl azides in higher yields than those from O-propargylated-1,4-naphthoquinone ( Table 3). None of the N-derivatives resulted active (IC50 > 10 Next, we evaluated the influence on the activity of an isosteric modification by replacing the -OCH 2 -linker by -NHCH 2 -. Thus, the following conjugates were prepared from N-propargylated-1,4-naphthoquinone and alkyl and aryl azides in higher yields than those from O-propargylated-1,4-naphthoquinone ( Table 3). None of the N-derivatives resulted active (IC 50 > 10 µg/mL), which indicated that the replacing of an acceptor of hydrogen bonds by donor hydrogen bonds in this part of the molecule led to a drastic loss of activity.
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31]. Table 3. Preparation of isosteric analogues of 1,2,3-triazole-naphthoquinones (7b-7d, 7h, 7j-7o, 7r-7t).
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31].
Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of PfDHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding PfDHODH inhibitors [34,35]. On the basis of this information and in order to understand the Molecules 2019, 24, 3917 6 of 23 μg/mL), which indicated that the replacing of an acceptor of hydrogen bonds by donor hydrogen bonds in this part of the molecule led to a drastic loss of activity. Table 3. Preparation of isosteric analogues of 1,2,3-triazole-naphthoquinones (7b-7d, 7h, 7j-7o, 7r-7t).
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31].
Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of PfDHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding PfDHODH inhibitors [34,35]. On the basis of this information and in order to understand the Molecules 2019, 24, 3917 6 of 23 μg/mL), which indicated that the replacing of an acceptor of hydrogen bonds by donor hydrogen bonds in this part of the molecule led to a drastic loss of activity. Table 3. Preparation of isosteric analogues of 1,2,3-triazole-naphthoquinones (7b-7d, 7h, 7j-7o, 7r-7t).
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31].
Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of PfDHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding PfDHODH inhibitors [34,35]. On the basis of this information and in order to understand the Molecules 2019, 24, 3917 6 of 23 μg/mL), which indicated that the replacing of an acceptor of hydrogen bonds by donor hydrogen bonds in this part of the molecule led to a drastic loss of activity. Table 3. Preparation of isosteric analogues of 1,2,3-triazole-naphthoquinones (7b-7d, 7h, 7j-7o, 7r-7t).
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31].
Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of PfDHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding PfDHODH inhibitors [34,35]. On the basis of this information and in order to understand the Molecules 2019, 24, 3917 6 of 23 μg/mL), which indicated that the replacing of an acceptor of hydrogen bonds by donor hydrogen bonds in this part of the molecule led to a drastic loss of activity. Table 3. Preparation of isosteric analogues of 1,2,3-triazole-naphthoquinones (7b-7d, 7h, 7j-7o, 7r-7t).
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31].
Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of PfDHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding PfDHODH inhibitors [34,35]. On the basis of this information and in order to understand the Molecules 2019, 24, 3917 6 of 23 μg/mL), which indicated that the replacing of an acceptor of hydrogen bonds by donor hydrogen bonds in this part of the molecule led to a drastic loss of activity. Table 3. Preparation of isosteric analogues of 1,2,3-triazole-naphthoquinones (7b-7d, 7h, 7j-7o, 7r-7t).
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31].
Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of PfDHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding PfDHODH inhibitors [34,35]. On the basis of this information and in order to understand the Molecules 2019, 24, 3917 6 of 23 μg/mL), which indicated that the replacing of an acceptor of hydrogen bonds by donor hydrogen bonds in this part of the molecule led to a drastic loss of activity. Table 3. Preparation of isosteric analogues of 1,2,3-triazole-naphthoquinones (7b-7d, 7h, 7j-7o, 7r-7t).
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31].
Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of PfDHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding PfDHODH inhibitors [34,35]. On the basis of this information and in order to understand the Molecules 2019, 24, 3917 6 of 23 μg/mL), which indicated that the replacing of an acceptor of hydrogen bonds by donor hydrogen bonds in this part of the molecule led to a drastic loss of activity. Table 3. Preparation of isosteric analogues of 1,2,3-triazole-naphthoquinones (7b-7d, 7h, 7j-7o, 7r-7t).
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31].
Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of PfDHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding PfDHODH inhibitors [34,35]. On the basis of this information and in order to understand the Molecules 2019, 24, 3917 6 of 23 μg/mL), which indicated that the replacing of an acceptor of hydrogen bonds by donor hydrogen bonds in this part of the molecule led to a drastic loss of activity. Table 3. Preparation of isosteric analogues of 1,2,3-triazole-naphthoquinones (7b-7d, 7h, 7j-7o, 7r-7t).
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31].
Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of PfDHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding PfDHODH inhibitors [34,35]. On the basis of this information and in order to understand the Molecules 2019, 24, 3917 6 of 23 μg/mL), which indicated that the replacing of an acceptor of hydrogen bonds by donor hydrogen bonds in this part of the molecule led to a drastic loss of activity. Table 3. Preparation of isosteric analogues of 1,2,3-triazole-naphthoquinones (7b-7d, 7h, 7j-7o, 7r-7t).
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31].
Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of PfDHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding PfDHODH inhibitors [34,35]. On the basis of this information and in order to understand the Molecules 2019, 24, 3917 6 of 23 μg/mL), which indicated that the replacing of an acceptor of hydrogen bonds by donor hydrogen bonds in this part of the molecule led to a drastic loss of activity. Table 3. Preparation of isosteric analogues of 1,2,3-triazole-naphthoquinones (7b-7d, 7h, 7j-7o, 7r-7t).
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31].
Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of PfDHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding PfDHODH inhibitors [34,35]. On the basis of this information and in order to understand the Molecules 2019, 24, 3917 6 of 23 μg/mL), which indicated that the replacing of an acceptor of hydrogen bonds by donor hydrogen bonds in this part of the molecule led to a drastic loss of activity. Table 3. Preparation of isosteric analogues of 1,2,3-triazole-naphthoquinones (7b-7d, 7h, 7j-7o, 7r-7t).
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31].
Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of PfDHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding PfDHODH inhibitors [34,35]. On the basis of this information and in order to understand the Molecules 2019, 24, 3917 6 of 23 μg/mL), which indicated that the replacing of an acceptor of hydrogen bonds by donor hydrogen bonds in this part of the molecule led to a drastic loss of activity. Table 3. Preparation of isosteric analogues of 1,2,3-triazole-naphthoquinones (7b-7d, 7h, 7j-7o, 7r-7t).
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31].
Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of PfDHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding PfDHODH inhibitors [34,35]. On the basis of this information and in order to understand the Molecules 2019, 24, 3917 6 of 23 μg/mL), which indicated that the replacing of an acceptor of hydrogen bonds by donor hydrogen bonds in this part of the molecule led to a drastic loss of activity. Table 3. Preparation of isosteric analogues of 1,2,3-triazole-naphthoquinones (7b-7d, 7h, 7j-7o, 7r-7t).
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31].
Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of PfDHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding PfDHODH inhibitors [34,35]. On the basis of this information and in order to understand the Molecules 2019, 24, 3917 6 of 23 μg/mL), which indicated that the replacing of an acceptor of hydrogen bonds by donor hydrogen bonds in this part of the molecule led to a drastic loss of activity. Table 3. Preparation of isosteric analogues of 1,2,3-triazole-naphthoquinones (7b-7d, 7h, 7j-7o, 7r-7t).
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31].
Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of PfDHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding PfDHODH inhibitors [34,35]. On the basis of this information and in order to understand the μg/mL), which indicated that the replacing of an acceptor of hydrogen bonds by donor hydrogen bonds in this part of the molecule led to a drastic loss of activity. Table 3. Preparation of isosteric analogues of 1,2,3-triazole-naphthoquinones (7b-7d, 7h, 7j-7o, 7r-7t).
In order to rationalize all these results, we carried out molecular dockings on the enzyme dihydroorotate dehydrogenase (DHODH). Dihydroorotate dehydrogenase (DHODH) is an enzyme essential to the fourth and rate limiting step in de novo pyrimidine biosynthesis, and it catalyzes the conversion of dihydroorotate (DHO) to orotate (ORO) with the reduction of ubiquinone. The significance of pyrimidine bases for cell proliferation and metabolism determines human DHODH as an attractive target for the development of new drug candidates in different clinical applications for arthritis, malaria, and cancer [30,31].
Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of PfDHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding PfDHODH inhibitors [34,35]. On the basis of this information and in order to understand the Plasmodium falciparum, the major human malarial parasite, is particularly susceptible to DHODH inhibition because the P. falciparum is dependent on de novo pyrimidine biosynthesis, at least during the parasite's intraerytrocytic stage [32]. The inhibitor-binding site of Pf DHODH, located in proximity to the cofactor-binding site, is characterized by the presence of two regions-the H-bond pocket, comprising His 185, Tyr 528, and Arg 265, and the hydrophobic pocket. The size of the hydrophobic pocket is variable, depending on the conformations of the side chains of Phe 171 and Phe 188. In addition, it is observed that Met 536 and Tyr 168 are two additional residues with a high degree of conformational flexibility in the same hydrophobic pocket [33]. Several computational studies have been done using various approaches with the goal of finding Pf DHODH inhibitors [34,35]. On the basis of this information and in order to understand the probable binding mode and to propose a mode of action of the antiplasmodial activity of synthesized 1,2,3-triazole-naphthoquinone conjugates, a molecular docking study was performed on reported crystal structure of Plasmodium falciparum enzyme dihydroorotate dehydrogenase using the Glide software [36]. The X-ray crystal structure of Pf DHODH (Protein Data Bank (PDB) 1TV5) [37] was retrieved from the Protein Data Bank having a resolution of 2.4 Å, which has been reported to be used frequently for docking studies [38]. Hence, we docked all the 1,2,3-triazole-naphthoquinone conjugates into the putative quinone-binding tunnel formed by the N-terminal domain to see if these compounds could interact with this target and, consequently, to understand the possible key active site interactions.
An analysis of the docking results clearly indicated that one of the most active compounds in this study, 3j, showed substantial binding affinities, a good steric and electronic complementarity, and fits well into the Pf DHODH binding site. In addition, these docking results strongly suggested that the compound 3j and the inhibitor bounded in the putative quinone-binding tunnel shared a common binding mode. The best docking score was −6.70 kcal mol −1 . According to the predicted binding modes, the compound 3j showed a π-π stacking interaction between the triazol ring present in these compounds with the amino acid residue Phe 188 (Figure 3).
probable binding mode and to propose a mode of action of the antiplasmodial activity of synthesized 1,2,3-triazole-naphthoquinone conjugates, a molecular docking study was performed on reported crystal structure of Plasmodium falciparum enzyme dihydroorotate dehydrogenase using the Glide software [36]. The X-ray crystal structure of PfDHODH (Protein Data Bank (PDB) 1TV5) [37] was retrieved from the Protein Data Bank having a resolution of 2.4 Å, which has been reported to be used frequently for docking studies [38]. Hence, we docked all the 1,2,3-triazole-naphthoquinone conjugates into the putative quinone-binding tunnel formed by the N-terminal domain to see if these compounds could interact with this target and, consequently, to understand the possible key active site interactions. An analysis of the docking results clearly indicated that one of the most active compounds in this study, 3j, showed substantial binding affinities, a good steric and electronic complementarity, and fits well into the PfDHODH binding site. In addition, these docking results strongly suggested that the compound 3j and the inhibitor bounded in the putative quinone-binding tunnel shared a common binding mode. The best docking score was −6.70 kcal mol −1 . According to the predicted In addition, three hydrogen bonds were detected, two of them between Arg 265 and Tyr 528 with the carbonyl groups belonging to the quinone moiety and another hydrogen bond between His 185 and the ether group that link the quinone moity and the triazole ring. Moreover, in the favored docking conformation, many hydrophobic side chain residues of the putative quinone-binding tunnel were in close proximity to naphthoquinone-triazole conjugates. In the predicted pose of the compound 3j, the potential hydrophobic interactions involve residues such as Cys 175, Cys 184, Phe 227, Leu 172, Met 536, and Phe 171. The presence of polar contacts with a considerable number of residues, as well as hydrophobic interactions, seem to play a fundamental role in the binding of these compounds, which could explain their antiplasmodial activity.
Finally, some physicochemical descriptors (MW, LogP, H-bond donors, H-bond acceptors, rotable bonds, and TPSA) for the best active compounds were calculated using Molinspiration Cheminformatics software (2019) and the corresponding values are included in Table 4. As we can observe, all of them showed values within the accepted ranges for drug-like molecules.  descriptors of 3d, 3f, 3j, 3l, 3m, and 3s

General Methods
The reactions under microwave irradiation were performed in a Biotage Initiator 2.5 using standard sealed microwave glass vials (2-5 mL) and a normal absorption level. Solvents were dried immediately prior to use by distillation from a drying agent: Tetrahydrofuran (THF) from Na/benzophenone and CH 3 CN from CaH 2 [39]. Commercial reagents were purchased from Sigma-Aldrich Chemical Co. and Alfa Aesar and were used without further purification. Analytical thin-layer chromatography was performed on Polygram SIL G/UV254 silica gel plates and chromatograms were visualized under UV light (254 and 360 nm). Purification on column chromatography was carried out on Merck silica gel 60 (0.063-0.2 mm) with the indicated solvent mixtures. Pre-coated TLC plates SIL G-100 UV254 (Macherey-Nagel) and SILICA GEL GF plates (1000 µm, Analtech) were used for preparative TLC purification. 1 H and 13 C-NMR spectra were acquired in CDCl 3 (0.03% v/v TMS), DMSO-d 6 or CD 3 CN at room temperature using Bruker Avance instruments (500 or 600 MHz for 1 H-NMR and 125 or 150 MHz for 13 C-NMR). Chemical shifts were reported in parts per million (ppm) from tetramethylsilane and referenced to the residual solvent peak (CDCl 3 : δ 7.26 for 1 H-NMR, δ 77.00/77.16 for 13 C-NMR; DMSO-d 6 : δ 2.50 for 1 H-NMR, δ 39.52 for 13 C-NMR; CD 3 CN: δ 1.93 for 1 H-NMR, δ 1.32, 118.26 for 13 C-NMR). For 1 H-NMR, data were reported in the following manner: chemical shift (integration, multiplicity, coupling constant where applicable). The following abbreviations were used: s (singlet), br (broad), d (doublet), t (triplet), dd (double doublet), td (triplet of doublets), m (multiplet). Coupling constants (J) were given in Hertz (Hz). 13 C-NMR were obtained with complete proton decoupling. MS and HRMS data were recorded in a VG Micromass ZAB-2F spectrometer and an ESI instrument LCT Premier XE Micromass (ESI-TOF). IR spectra were recorded on a Bruker IFS 28/55 spectrophotometer. All compounds were named using the ACD40 Name-Pro program, which is based on IUPAC rules. 0.024 mmol) in 0.06 mL of 20% of NH 3 and 0.12 mL of H 2 O. The reaction mixture was stirred for 16 h at room temperature under an oxygen atmosphere. Then, 30 mg (0.14 mmol) of N-propargylated naphthoquinone (6), 8.11 mg (0.041 mmol) of sodium ascorbate, 1.5 mL of H 2 O, and 3 mL of acetone were added. After stirring for 48 h, the reaction mixture was extracted with ethyl acetate (EtOAc). Subsequently, the aqueous phase was acidified with 5% HCl until pH 2 and it was extracted with EtOAc (3 × 15 mL). The combined organic phases were dried over anhydrous MgSO 4 and, after the elimination of the solvent, the corresponding residue was purified by silica gel column cromatography (CC) or TLC-preparative with DCM or 5% DCM/MeOH.   Following the general procedure described in method B, 44.8 mg (0.21 mmol) of the alkyne, 31.4 mg (0.21 mmol) of 4-methoxybenzylazide, and 25% mol of CuI were stirred at room temperature for 72 h. Then, the solvent was removed at reduced pressure and the solid was washed with n-hexane,

Antiproliferative Activity
The human cancer cell lines HL60 (promyelocytic leukemia), HEL (human erythroleukemia), and SK-Br3 (breast adeno carcinoma) were purchased from ATCC and cultured in RPMI medium 10% FBS. The MTT assay, MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide], was used to test the cytotoxicity of 1,2,3-triazolquinones and cell viability [41]. Briefly, cells were plated in 96-well plates at 10,000 cells/well. Sixteen hours after plating, vehicle (0.1% DMSO, final concentration) or compound was added to cells at indicated concentrations. Forty-eight hours following compound addition, MTT (Sigma-Aldrich, St. Louis, MO, USA) was added to each well (0.3 mg/mL, final concentration) and plates were incubated for an additional 2 h at 37 • C. Medium was then aspirated and the formazan product was solubilized in SDS-HCl (20% SDS; HCl 0.02 M). The absorbance of each well was measured at 595 nm using an iMark Microplate Reader (BioRad, Hercules, CA, USA). Nonlinear regression analysis was performed to calculate IC 50 according to the GraphPad Prism 5 program (GraphPad Software, San Diego, CA, USA). The data were expressed by mean ± SEM (n = 3).

Protein Preparation and Docking
The X-ray coordinates of Plasmodium falciparum enzyme dihydroorotate dehydrogenase (Pf DHODH) was extracted from the Protein Data Bank (PDB code 1TV5). The PDB structures were prepared for docking using the Protein Preparation Workflow (Schrodinger, LLC, New York, NY, USA, 2018) accessible from within the Maestro program (Maestro, version 11.5; Schrodinger, LLC: New York, NY, USA, 2018). The substrate and water molecules were removed beyond 5 Å, bond corrections were applied to the cocrystallized ligands, and an exhaustive sampling of the orientations of groups was performed. Finally, the receptors were optimized in Maestro 11.5 by using OPLS3 force field before docking study. In the final stage, the optimization and minimization on the ligand-protein complexes were carried out with the OPLS3 force field and the default value for RMSD of 0.30 Å for non-hydrogen atoms was used. The receptor grids were generated using the prepared proteins, with the docking grids centered on the center of the bound ligand for each receptor. A receptor grid was generated using a 1.00 van der Waals (vdW) radius scaling factor and 0.25 partial charge cutoff. The binding sites were enclosed in a grid box of 20 Å 3 with default parameters and without constrains. The three-dimensional structures of the ligands to be docked were generated and prepared using LigPrep as implemented in Maestro 11.5 (LigPrep, Schrodinger, LLC: New York, NY, USA, 2018) to generate the most probable ionization states at pH 7 ± 1 (retain original ionization state). These conformations were used as the initial input structures for the docking. In this stage, a series of treatments are applied to the structures. Finally, the geometries are optimized using OPLS3 force field. These conformations were used as the initial input structures for the docking. The ligands were docked using the extra precision mode (XP) [42] without using any constraints and a 0.80 van der Waals (vdW) radius scaling factor and 0.15 partial charge cutoff. The dockings were carried out with flexibility of the residues of the pocket near to the ligand. The generated ligand poses were evaluated with empirical scoring function, GlideScore a modified version of ChemScore [43]; GlideScore implemented in Glide was used to estimate binding affinity and rank ligands [44]. The XP Pose Rank was used to select the best-docked pose for each ligand.

Conclusions
In summay, a library of 34 1,2,3-triazolyl naphthoquinone derivatives was prepared using a copper (I) catalyzed Huisgen 1,3-dipolar cycloaddition reaction of O-propargylated naphthoquinone (1) or N-propargylated naphthoquinone (6) and different azides. Some simplified analogues were also synthesized in order to deepen the structural determinants responsible of the antiplasmodial activity. The compounds were evaluated against strains of Plasmodium falciparum F-32 Tanzania (chloroquine sensitive). The results showed that the best antiplasmodial activities were achieved with the derivatives having the ether bridge and aromatic substituents attached at the nitrogen of the triazole ring. The nature of the substituents on the aromatic ring seemed to modulate the antiplasmodial activity. The results obtained from docking studies supported the hypothesis that the enzyme Pf DHODH might be the target of these compounds in the parasite. The most active compounds (3d, 3f, 3j, 3l,  3m, and 3s) showed values for physicochemical descriptors within the accepted ranges for drug-like molecules. All these results suggest that these compounds could serve as promising lead compounds for further research.