Synthesis and Antimalarial Activity of 1,4-Disubstituted Piperidine Derivatives

In order to prepare, at low cost, new compounds active against Plasmodium falciparum, and with a less side-effects, we have designed and synthesized a library of 1,4-disubstituted piperidine derivatives from 4-aminopiperidine derivatives 6. The resulting compound library has been evaluated against chloroquine-sensitive (3D7) and chloroquine-resistant (W2) strains of P. falciparum. The most active molecules—compounds 12d (13.64 nM (3D7)), 13b (4.19 nM (3D7) and 13.30 nM (W2)), and 12a (11.6 nM (W2))—were comparable to chloroquine (22.38 nM (3D7) and 134.12 nM (W2)).


Introduction
This year's World Health Organization (WHO) report shows that after an unprecedented period of success in global malaria control, progress has stalled [1]. In 2016, there were an estimated 216 million cases of malaria, an increase of about 5 million cases over 2015. Deaths reached 445,000, a similar number to the previous year.
Malaria-related mortality followed the same trend, i.e., a decline from 2010 to 2014, and then an increase in 2015 and 2016. According to this report, it is in the WHO African region that the increase in cases of malaria and associated deaths was the most significant. The African region still accounts for some 90% of worldwide malaria cases and related deaths. Fifteen countries, all but one in sub-Saharan Africa, account for 80% of the global burden of malaria.
One of the biggest challenges facing malaria chemotherapy is the rapid emergence of resistance to existing antimalarial drugs [2]. Chloroquine was replaced as first line therapy by the sulfonamide antimalarials and, later on, artemisinin combination therapy (ACT), following the development of widespread resistance against the drug by Plasmodium falciparum [3]. This challenge underscores the need for the continued search for new antimalarials.
Research is being pursued for the discovery of new antimalarials with less side effects, a faster onset of action and a better rate of response [4,8,[21][22][23][24][25][26]. In the process of searching for new small molecules interacting with the P. falciparum strain, we have identified the target compounds A with various R1, R2 and R3 substituents ( Figure 1). In this paper, we describe the synthesis of some new derivatives with potential antimalarial properties.
Molecules 2019, 24, x 2 of 16 various R1, R2 and R3 substituents ( Figure 1). In this paper, we describe the synthesis of some new derivatives with potential antimalarial properties.
Scheme 2. Synthesis of compound 6c and 6d. Condensation of phenoxyacetyl chloride with compound 3a in the presence of triethylamine at room temperature in acetone gave compounds 5a (55%) and 10 (30%) (Scheme 3). In this reaction we used excess phenoxyacetyl chloride, and we think that this excess probably made the reaction medium acidic which cause the cleavage of the N-Boc protective group. To avoid this side reaction, we used NaH in CH2Cl2 which furnished compound 5a (Scheme 1). Scheme 3. Synthesis of compounds 5a and 10.
A pharmaco-modulation has been achieved on the parent molecule 6 taking advantage of the nucleophilicity of the piperidine nitrogen leading to compounds 17-34 in good yield. Thus, reductive amination [30][31][32][33] of 6 with benzaldehyde derivatives in 1,2-dichloroethane (ClCH2CH2Cl), gave compounds A (Scheme 4) ( Table 2).  Condensation of phenoxyacetyl chloride with compound 3a in the presence of triethylamine at room temperature in acetone gave compounds 5a (55%) and 10 (30%) (Scheme 3). In this reaction we used excess phenoxyacetyl chloride, and we think that this excess probably made the reaction medium acidic which cause the cleavage of the N-Boc protective group. To avoid this side reaction, we used NaH in CH 2 Cl 2 which furnished compound 5a (Scheme 1).
Scheme 2. Synthesis of compound 6c and 6d. Condensation of phenoxyacetyl chloride with compound 3a in the presence of triethylamine at room temperature in acetone gave compounds 5a (55%) and 10 (30%) (Scheme 3). In this reaction we used excess phenoxyacetyl chloride, and we think that this excess probably made the reaction medium acidic which cause the cleavage of the N-Boc protective group. To avoid this side reaction, we used NaH in CH2Cl2 which furnished compound 5a (Scheme 1).
Interestingly, compounds 6c and 6d are inactive against both strains. However, after pharmacomodulation on the nitrogen atom, their derivatives 12d, 14d, 17c, 13c and 14c showed good activity against both strains.
Interestingly, compounds 6c and 6d are inactive against both strains. However, after pharmacomodulation on the nitrogen atom, their derivatives 12d, 14d, 17c, 13c and 14c showed good activity against both strains.

Apparatus, Materials, and Analytical Reagents
All chemical reagents and anhydrous solvents were obtained from commercial sources and used without further purification. The 1 H-and 13 C-NMR spectra were recorded in CDCl 3 at ambient temperature on an AMX 500 spectrometer (Bruker, Palaiseau, France). Some product structures were confirmed by DEPT 135, HMQC and HMBC experiments. Chemical shifts are given in δ (ppm) and coupling constants J (Hz) relative to TMS used as internal standard; multiplicities were recorded as s (singlet), d (doublet), dd (double doublet), t (triplet), dt (double triple), q (quartet) or m (multiplet). Reactions involving anhydrous conditions were conducted in dry glassware under a nitrogen atmosphere. The infrared spectra have been recorded on a model 842 spectrometer (Perkin-Elmer, 842) using polystyrene as reference. The melting points have been measured on a Tottoli S Bucchi device (Buchi, Rungis, France). Microanalysis have been done on a Perkin-Elmer 2400-CMN apparatus (Perkin ElmerVillebon-sur-Yvette, France). GC/MS conditions: Analyses were performed using a 5890 gas chromatogram connected to a G 1019 A mass spectrometer (both from Hewlett Packard, Alpharetta, GA, USA) operating in the electrospray ionization mode (ESI).

General Procedure for the Coupling with Chloroacetyl chloride: Synthesis of Compounds 7a-b
One equiv of compound 3 was dissolved in 25 mL of CH 2 Cl 2 , 2 equiv. of potassium carbonate were added and the mixture was cooled to 0 • C. Two equiv. of chloroacetyl chloride were added to 0 • C, and the mixture was stirred overnight. The reaction was quenched by addition of a saturated solution of NaHCO 3 (25 mL), the aqueous phase was decanted and extracted twice with 15 mL of CH 2 Cl 2 . Combined organic phases were washed with water and brine, dried over MgSO 4 and concentrated in vacuum. The crude product was purified by crystallisation (ether petroleum/ethyl acetate (8:2)).

General Procedure for Synthesis of Compounds 9a-b
To a mixed solution of acetonitrile/acetone (50/50) were added 1 equiv of compound 7; 1 equiv of 2-chlorophenol and 2 equiv of potassium carbonate. After 12 h of stirring under reflux the mixture was concentrated in vacuum. The residual was dissolved in 15 mL of ethyl acetate and (1N) of NaOH (15 mL), the aqueous phase was decanted and extracted twice with 15 mL of ethyl acetate. Combined organic phases were washed with water and brine, dried over MgSO 4 and concentrated in vacuum. The crude product was purified by crystallisation (ether petroleum/ethyl acetate (8:2)).

General Procedure for Deprotection: Synthesis of 6a-b
One equiv of compound 5 was dissolved in 15 mL of CH 2 Cl 2 , and 13 equiv of trifluoroacetic acid were added. After 2 h of stirring at room temperature, the reaction mixture was concentrated under vacuum. The residue was dissolved in 5 mL of ethyl acetate then neutralized with NaHCO 3 (5%). The aqueous layer was extracted with ethyl acetate (4x5mL). The combined organic phases were dried over MgSO 4 , filtered and concentrated under reduced pressure. The crude product was purified by crystallisation (ether petroleum/ethyl acetate (8:2)).

Conclusions
In this study, we have prepared a small library of new nitrogen heterocycles displaying piperidine scaffolds using a flexible synthetic approach. Eighteen new derivatives were prepared in good yield. The antimalarial activity of these compounds has been described. The compounds were tested against P. falciparum 3D7 strains and W2. The best result is observed with the compounds 13b against the 3D7 strain and 12a against the W2 one with a selectivity index greater than chloroquine. We observed that modification with different R groups, for example compound 12a (R 1 = R 2 = R 3 = H) in 13b (R 1 = F, R 2 = H, R 3 = Br) significantly modulated the activity of the tested molecules. These molecules could be further optimized to provide good malaria drug candidates.
Author Contributions: R.S. performed the synthetic, drew the molecules and searched the literatures, A.G. and C.C. designed the target compounds, provided guidance to optimization the synthesis process and wrote paper, S.C. conceived and performed the biological assay. All authors have read and agreed to the published version of the manuscript.
Funding: This work was supported by the French cooperation.