New Potential Antimalarial Agents: Design, Synthesis and Biological Evaluation of Some Novel Quinoline Derivatives as Antimalarial Agents

A novel series of dihydropyrimidines (DHPMs) 4a–j; 2-oxopyran-3-carboxylate 7a,b; 1-amino-1,2-dihydropyridine-3-carboxylate 8; and 1,3,4-oxadiazole derivatives 12 with quinolinyl residues have been synthesized in fairly good yields. The structure of the newly synthesized compounds was elucidated on the basis of analytical and spectral analyses. In vitro antimalarial evaluation of the synthesized quinoline derivatives against Plasmodium falciparum revealed them to possess moderate to high antimalarial activities, with IC50 values ranging from 0.014–5.87 μg/mL. Compounds 4b,g,i and 12 showed excellent antimalarial activity against to Plasmodium falciparum compared with the antimalarial agent chloroquine (CQ).


Introduction
Malaria is one of the principal diseases of the developing countries, particularly in Africa, Asia and South America. According to a World Health Organization (WHO) report, there are between 300 million and 500 million cases of malaria worldwide annually and more than one million people die from that disease, most of them are children under the age of five years [1,2]. Among five typically recognized Plasmodium species causing this disease in humans, Plasmodium falciparum is responsible for about 95% of worldwide malaria and has a mortality rate of 1%-3%, and Plasmodium vivax for most morbidity, additionally representing a reservoir of latent infection that hampers current control and future elimination efforts [3][4][5][6]. Due to the toxic side effects and the risk of developing resistance after prolonged treatment with aminoquinolines and their derivatives ( Figure 1) which are nowadays used as antimalarial agents, the growth of and increasing resistance [7,8] of the malaria parasite Plasmodium falciparum to known antimalarial agents demands a continuous effort to develop new antimalarial agents especially, as an effective vaccine for malaria is not available. several quinolines compounds and screened for their antimalarial activities. Hopefully, these compounds will be active on the CQ-resistant strain FcB1 and could lead to the availability of better drugs to treat malaria.
The structure of products 4a-j has been confirmed by both analytical and spectral analyses. The presence of a single proton at a range of δ = 5.41-5.91 ppm corresponding to H-4 of DHPMs in addition to the two NH groups at δ = 7.13-9.24 ppm and 9.29-11.79 ppm supported the suggested DHPMs structures. Also, molecular weight determination (MS) confirmed their structures. (cf. Scheme 1 and Experimental Section).
The synthetic strategies adopted to obtain the target 8 and 12 are somewhat long and linear with few common intermediates. To this aim, the chalcones derivatives 6a,b, which were prepared by reaction of 2-(piperidin-1-yl)quinoline-3-carbaldehyde (1d) [27] with methyl ketones 5a,b, were reacted with ethyl cyanoacetate in ethanol at room temperature to give pyran-3-carboxylate derivatives 7a,b in fairly good yield (Scheme 2). The structures of compounds 7 were established by both analytical and spectral analyses. The IR spectra show two absorption bands at 1690-1682 cm −1 and 1743-1736 cm −1 for the ester and lactone carbonyl groups, respectively. In addition, the 1 H-NMR shows the pyran H-5 at δ 7.33-7.39 ppm and other protons in their expected locations. N-Nucleophilic addition reaction of hydrazine at the lactonic carbonyl group of 7a, gave 1,2-dihydropyridine-3-carboxylate derivative 8. The IR spectra showed absence of the lactonic carbonyl group perilously appeared in the parent 7 and the appearance of new bands at ν 3383, 3182 cm −1 due to NH2 function and 1 H-NMR showed a singlet signal at δ 5.41 ppm attributed to amino group. In addition, quinoline-based fused heterocyclic systems are found to possess potential antimicrobial [9,10], antimalarial [11,12], anti-inflammatory [13,14], antitumor [15], and anti-parasitic activity [16].
Currently there are only limited safe drugs for the treatment of the disease, however, the design of new chemical agents, specifically affecting these targets, could lead to the availability of better drugs to treat malaria. Based on the above information and in continuation with our previous work [11,15,17,18], quinoline-based antimalarials that would not induce resistance, we have designed and prepared several quinolines compounds and screened for their antimalarial activities. Hopefully, these compounds will be active on the CQ-resistant strain FcB1 and could lead to the availability of better drugs to treat malaria.
The structure of products 4a-j has been confirmed by both analytical and spectral analyses. The presence of a single proton at a range of δ = 5.41-5.91 ppm corresponding to H-4 of DHPMs in addition to the two NH groups at δ = 7.13-9.24 ppm and 9.29-11.79 ppm supported the suggested DHPMs structures. Also, molecular weight determination (M S ) confirmed their structures. (cf. Scheme 1 and Experimental Section).
The synthetic strategies adopted to obtain the target 8 and 12 are somewhat long and linear with few common intermediates. To this aim, the chalcones derivatives 6a,b, which were prepared by reaction of 2-(piperidin-1-yl)quinoline-3-carbaldehyde (1d) [27] with methyl ketones 5a,b, were reacted with ethyl cyanoacetate in ethanol at room temperature to give pyran-3-carboxylate derivatives 7a,b in fairly good yield (Scheme 2). The structures of compounds 7 were established by both analytical and spectral analyses. The IR spectra show two absorption bands at 1690-1682 cm´1 and 1743-1736 cm´1 for the ester and lactone carbonyl groups, respectively. In addition, the 1 H-NMR shows the pyran H-5 at δ 7.33-7.39 ppm and other protons in their expected locations. N-Nucleophilic addition reaction of hydrazine at the lactonic carbonyl group of 7a, gave 1,2-dihydropyridine-3-carboxylate derivative 8. The IR spectra showed absence of the lactonic carbonyl group perilously appeared in the parent 7 and the appearance of new bands at ν 3383, 3182 cm´1 due to NH 2 function and 1 H-NMR showed a singlet signal at δ 5.41 ppm attributed to amino group. 2-Chloroquinoline-3-carboxylic acid was prepared by oxidation of 1e using silver nitrate in the presence of sodium hydroxide [28]. Esterification of the carboxylic acid derivative 9 using absolute 2-Chloroquinoline-3-carboxylic acid was prepared by oxidation of 1e using silver nitrate in the presence of sodium hydroxide [28]. Esterification of the carboxylic acid derivative 9 using absolute ethanol and sulfuric acid afforded the ester derivative 10, in a good yield, followed by subsequent hydrazinolysis in boiling ethanol to afford 2-chloroquinoline-3-carbohydrazide 11. The later compound Molecules 2016, 21, 909 3 of 11 2-Chloroquinoline-3-carboxylic acid was prepared by oxidation of 1e using silver nitrate in the presence of sodium hydroxide [28]. Esterification of the carboxylic acid derivative 9 using absolute ethanol and sulfuric acid afforded the ester derivative 10, in a good yield, followed by subsequent hydrazinolysis in boiling ethanol to afford 2-chloroquinoline-3-carbohydrazide 11. The later compound Scheme 2. Synthesis of quinolinyl 1,2-dihydropyridine 8.
2-Chloroquinoline-3-carboxylic acid was prepared by oxidation of 1e using silver nitrate in the presence of sodium hydroxide [28]. Esterification of the carboxylic acid derivative 9 using absolute ethanol and sulfuric acid afforded the ester derivative 10, in a good yield, followed by subsequent hydrazinolysis in boiling ethanol to afford 2-chloroquinoline-3-carbohydrazide 11. The later compound Molecules 2016, 21, 909 3 of 11 2-Chloroquinoline-3-carboxylic acid was prepared by oxidation of 1e using silver nitrate in the presence of sodium hydroxide [28]. Esterification of the carboxylic acid derivative 9 using absolute ethanol and sulfuric acid afforded the ester derivative 10, in a good yield, followed by subsequent hydrazinolysis in boiling ethanol to afford 2-chloroquinoline-3-carbohydrazide 11. The later compound Ph 5a, Scheme 2. Synthesis of quinolinyl 1,2-dihydropyridine 8.
2-Chloroquinoline-3-carboxylic acid was prepared by oxidation of 1e using silver nitrate in the presence of sodium hydroxide [28]. Esterification of the carboxylic acid derivative 9 using absolute ethanol and sulfuric acid afforded the ester derivative 10, in a good yield, followed by subsequent hydrazinolysis in boiling ethanol to afford 2-chloroquinoline-3-carbohydrazide 11. The later compound 2-Chloroquinoline-3-carboxylic acid was prepared by oxidation of 1e using silver nitrate in the presence of sodium hydroxide [28]. Esterification of the carboxylic acid derivative 9 using absolute ethanol and sulfuric acid afforded the ester derivative 10, in a good yield, followed by subsequent hydrazinolysis in boiling ethanol to afford 2-chloroquinoline-3-carbohydrazide 11. The later compound 2-Chloroquinoline-3-carboxylic acid was prepared by oxidation of 1e using silver nitrate in the presence of sodium hydroxide [28]. Esterification of the carboxylic acid derivative 9 using absolute ethanol and sulfuric acid afforded the ester derivative 10, in a good yield, followed by subsequent hydrazinolysis in boiling ethanol to afford 2-chloroquinoline-3-carbohydrazide 11. The later compound 2-Chloroquinoline-3-carboxylic acid was prepared by oxidation of 1e using silver nitrate in the presence of sodium hydroxide [28]. Esterification of the carboxylic acid derivative 9 using absolute ethanol and sulfuric acid afforded the ester derivative 10, in a good yield, followed by subsequent hydrazinolysis in boiling ethanol to afford 2-chloroquinoline-3-carbohydrazide 11. The later compound 2-Chloroquinoline-3-carboxylic acid was prepared by oxidation of 1e using silver nitrate in the presence of sodium hydroxide [28]. Esterification of the carboxylic acid derivative 9 using absolute ethanol and sulfuric acid afforded the ester derivative 10, in a good yield, followed by subsequent hydrazinolysis in boiling ethanol to afford 2-chloroquinoline-3-carbohydrazide 11. The later compound 2-Chloroquinoline-3-carboxylic acid was prepared by oxidation of 1e using silver nitrate in the presence of sodium hydroxide [28]. Esterification of the carboxylic acid derivative 9 using absolute ethanol and sulfuric acid afforded the ester derivative 10, in a good yield, followed by subsequent hydrazinolysis in boiling ethanol to afford 2-chloroquinoline-3-carbohydrazide 11. The later compound 2-Chloroquinoline-3-carboxylic acid was prepared by oxidation of 1e using silver nitrate in the presence of sodium hydroxide [28]. Esterification of the carboxylic acid derivative 9 using absolute ethanol and sulfuric acid afforded the ester derivative 10, in a good yield, followed by subsequent hydrazinolysis in boiling ethanol to afford 2-chloroquinoline-3-carbohydrazide 11. The later compound 11 was subjected to react with carbon disulfide in ethanol in the presence of KOH under reflux followed by acidification by using diluted hydrochloric acid to give 5-(2-chloro-quinolin-3-yl)-1,3,4oxadiazole-2-thiol (12). The IR spectrum showed the presence of the absorption band at 2500 cm −1 due Scheme 2. Synthesis of quinolinyl 1,2-dihydropyridine 8.
2-Chloroquinoline-3-carboxylic acid was prepared by oxidation of 1e using silver nitrate in the presence of sodium hydroxide [28]. Esterification of the carboxylic acid derivative 9 using absolute ethanol and sulfuric acid afforded the ester derivative 10, in a good yield, followed by subsequent hydrazinolysis in boiling ethanol to afford 2-chloroquinoline-3-carbohydrazide 11. The later compound 11 was subjected to react with carbon disulfide in ethanol in the presence of KOH under reflux followed by acidification by using diluted hydrochloric acid to give 5-(2-chloro-quinolin-3-yl)-1,3,4-oxadiazole-2-thiol (12). The IR spectrum showed the presence of the absorption band at 2500 cm´1 due to S-H function, in addition 13 C-NMR revealed signal at δ C 164.54 (C2-1,3,4-oxadiazole) ppm indicates that 12 exists in the thiol form (cf. Scheme 3 and Experimental Section).

Antimalarial Evaluation
Seventeen quinoline derivatives were evaluated in vitro against P. falciparum. The results of the antimalarial screening are presented in Tables 1-3. The basic measurement of antimalarial activity used in this study was the reduction in number of parasitized cells in the test cultures compared to control at 36-48 h of incubation. Compounds exhibiting IC50 P. falciparum >5 μg/mL was considered inactive. If the IC50 is between 0.5 and 5 μg/mL, the compound is classified as moderately active. If the IC50 is <0.5 μg/mL, the compound is classified as active.

General Information
Melting points were determined on digital MFB-595 instrument (Gallenkamp London, UK) using open capillary tubes and are uncorrected. IR spectra were recorded on a FTIR 440 spectrometer (Shimadzu, Tokyo, Japan) using KBr pellets. Mass spectra were obtained on a Qp-2010 plus mass spectrometer (Shimadzu) at 70 eV. 1 H-NMR and 13 C-NMR spectra were recorded on a model Ultra Shield-NMR spectrometer (500 MHz or 400 MHz, Bruker, Coventry, UK) in DMSO-d 6 using tetramethylsilane (TMS) as an internal standard at the College of Science, King Khalid University, Saudi Arabia; chemical shifts are reported as δ ppm units. The elemental analyses (% C, H, N) were done at the Microanalytical Center, Cairo University, Cairo, Egypt. Solvents were dried by standard techniques. The monitoring of the progress of all reactions and homogeneity of the synthesized compounds was carried out and was run using thin layer chromatography (TLC) aluminum sheets silica gel 60 F 254 (Merck, Darmstadt, Germany).