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Seven novel dispiro-1,2,4,5-tetraoxane derivatives were synthesized and characterized by a number of analytical and spectroscopic techniques. The molecules were subsequently screened for in vitro antimalarial activity against chloroquine resistant strain of Plasmodium falciparum (RKL-9). At antimalarial activity screening, two compounds, namely 5d (MIC = 15.6 µg/mL or 64.5 µM) and 5f (MIC = 15.6 µg/mL or 54.6 µM) were found to be about 1.5 times more potent against chloroquine resistant strain-RKL-9 compared to chloroquine (MIC = 25.0 µg/mL or 78.3 µM). Molecular docking studies of potent ligands were also performed in cysteine protease binding pocket residues of falcipain-2 as a target protein.

Introduction

Human malaria is a life-threatening disease transmitted by female Anopheles mosquitoes. It is caused by four parasite species of the genus Plasmodium; P. vivax, P. malariae, P. ovale and P. falciparum. P. falciparum is the most pernicious, causing the majority of malaria related morbidity and mortality (Kumawat et al., 2011). WHO reported that 207 million cases of malaria and 627,000 deaths occurred globally in 2012. Most cases (80%) and deaths (90%) occurred in Africa, and most deaths (77%) were in children under 5 years of age.

The growing drug resistance towards P. falciparum and the lack of an effective antimalarial vaccine emphasize the need to develop a novel, safe, affordable antimalarial drug effective against multi drug-resistant malaria (Casteel, 1997; O’Neill et al., 2010). Tetraoxanes are believed to have a similar mode of activity as the naturally occurring endoperoxides such as artemisinin ((O’Neill et al., 2008; Vennerstrom et al., 1992, 2000). 1,2,4,5-tetraoxanes have been proven to be superior to other synthetic endoperoxides such as 1,2,4- trioxolanes in terms of stability and to trioxane analogues in terms of both stability and activity (Dong et al., 1999; Amewu et al., 2013).

In the present study, seven molecules of dispiro-1,2,4,5-tetraoxanes were synthesized and subsequently screened for their in vitro antimalarial activity against laboratory cultured P. falciparum. These molecules were also tested for their inhibitory potency against falcipain-2 (FP-2).

Materials and Methods

All the chemicals used in the work were procured either from Sigma–Aldrich Corporation, USA or Merck Specialties Pvt. Ltd., Mumbai and were used without further purification. The melting points of the synthe-sized compounds, including intermediate were determined by using Veego-MPI melting point apparatus. The progress of the reactions was monitored on silica gel-G TLC plate using various solvent combinations. The spots were detected with iodine vapors and by observing under UV-light. The UV–visible spectra of the synthesized compounds were recorded on UV–visible spectrophotometer (Shimadzu UV-1800). Infrared spectra were recorded on an FT-IR Perkin-Elmer spectrometer. The ¹H and ¹³C NMR spectra were recorded at 400 MHz and 100 MHz, respectively, on a Bruker Avance-II 400 NMR spectrometer using either DMSO-d₆ or CDCl₃ as solvent with tetramethylsilane (TMS) as an internal standard. Mass spectra were obtained on a Waters Q-TOF MICROMA SS LC mass spectrometer. Elemental analyses (CHN and O) were carried out on Eager Xperience Elemental analyzer (Coates, 2000; Pasto et al., 1992; Mathieson, 1965; Silverstein and Webster, 1963).

Cyclic aldehyde/ketone (1 mL, 10 mmol) was dissolved at room temperature in a CH₂Cl₂/CH₃CN mixture (20 mL, 1:3 v/v) followed by 30% H₂O₂ (10.4 mL, 0.1 mol) and 0.5 mL of concentrated HCl. The reaction mixture was stirred for 2 hours at room temperature and quenched with saturated NaHCO₃ and CH₂Cl₂. The organic layer was separated, and the water layer was filtered and dried (O’Neill et al., 2008; Opsenica et al., 2008; Terent’ev et al., 2012).

Cyclic ketone/aldehyde (0.36 g, 2.3 mmol) was added to a cooled solution (ice bath) of dihydroperoxide (0.34 g, 2.3 mmol) in CH₂Cl₂ (20 mL). The mixture was stirred for 30 min at the same temperature, and then a cooled H₂SO₄/CH₃CN mixture (1.66 mL, 1:10, v/v) was added drop wise. After an additional 50 min of stirring, the reaction was quenched with saturated NaHCO₃ and CH₂Cl₂. The organic layer was separated, and the water layer was filtered and dried (O’Neill et al., 2008; Opsenica et al., 2008; Terent’ev et al., 2012).

Brownish semisolid with a characteristic odor; soluble in dichloromethane, DMSO, chloroform; %yield 43.98; R_f value 0.81 (petroleum ether: acetone: 1:4); spectroscopic analysis: λ_max (in CHCl₃) 242.72 nm; FTIR (ν_max, in cm^-1, film) 2934.34-2863.53 (C-H stretching, cycloalkyl), 1453.03-1392.73 (C-H bending, cycloalkyl), 1096.82 (C-C-O stretching), 736.74 (peroxide, C-O-O- stretching); ¹H NMR (400 MHz, DMSO-d₆, δ in ppm) 1.22-1.43 (m, 6H, 3x>CH₂, cyclohexyl), 1.46-1.78 (m, 2H, 2x>CH₂, cyclo-pentyl), 1.81-1.84 (t, 4H, J=12Hz, 2x>CH₂, cyclohexyl), 2.50-2.51 (t, 4H, J=4Hz, 2x>CH₂, cyclopentyl); ¹³C NMR (100 MHz, DMSO-d₆, δ in ppm) 24.96, 25.29 (2xC, cyclohexyl), 27.52, 27.61, 27.79, 27.85, 28.46, 29.26 (2xC, cyclopentyl), 30.38, 30.84, 32.08, 33.01, 33.11, 33.68, 33.49 (2xC, cyclohexyl), 38.92 (1xC, cyclohexyl), 39.12, 39.33, 39.54, 39.75, 39.96, 40.17 (2xC, cyclopentyl), 53.93, 60.59, 63.16, 63.38, 63.43, 78.42, 78.75, 79.08 (1xC, tetraoxane), 172.41, 172.54, 174.09, 174.24 (1xC, tetraoxane); mass (m/z) calculated 214.26; observed 246.2 (100%), 260.2 (94.14%), 165.2 (79.03%), 196.2 (73.92%).

Reddish brown semisolid with a characteristic odor; soluble in dichloromethane, DMSO, chloroform; %yield 15.28; R_f value (petroleum ether: acetone: 1:3) 0.79; spectroscopic analysis: λ_max (in CHCl₃) 242.41 nm; FTIR (ν_max, in cm^-1, film) 2933.37-2862.09 (C-H stretching, cycloalkyl), 1455.48-1349.89 (C-H bending, cycloalkyl), 1044.19 (C-C-O stretching), 855.70-735.23 (peroxide, C-O-O- stretching); ¹H NMR (400 MHz, CDCl₃, δ in ppm) 1.18-1.40 (m, 8H, 4x>CH₂, cycloheptyl), 1.48-1.88 (m, 6H, 3x>CH₂, cyclohexyl), 1.97-1.99 (dd, 4H, J=8Hz, 2x>CH₂, cycloheptyl), 2.00-2.02 (t, 4H, J=8Hz, 2x>CH₂, cyclohexyl); ¹³C NMR (100 MHz, CDCl₃, δ in ppm) 24.35, 24.58 (2xC, cyclohexyl), 25.53, 28.31(2xC, cyclo-heptyl), 30.44, 34.14, 43.90 (3xC, cyclohexyl), 64.09, 64.18 (2xC, cycloheptyl), 76.70 (2xC, cycloheptyl), 77.02 (1xC, tetraoxane), 77.34 (1xC, tetraoxane).

Brownish yellow solid with characteristic odor; soluble in DMSO, dichloromethane, chloroform; melting range 96-97ºC; %yield 09.96; R_f value (petroleum ether: acetone: 1:2) 0.65; spectroscopic analysis: λ_max (in CHCl₃) 269.30 nm; FTIR (ν_max, in cm^-1, film) 2924.12-2855.05 (C-H stretching, cycloalkyl), 1491.98-1372.76 (C-H bending, cycloalkyl), 1027.81 (C-C-O stretching), 906.12-749.87 (peroxide, C-O-O- stretching); ¹H NMR (400 MHz, CDCl₃, δ in ppm) 1.18-1.49 (m, 8H, 4x>CH₂, cycloheptyl), 1.67-1.74 (m, 4H, 2x>CH₂, cyclopentyl), 2.32-2.46 (m, 4H, 2x>CH₂, cycloheptyl), 2.57-2.60 (t, 4H, J=12Hz, 2x>CH₂, cyclopentyl); ¹³C NMR (100 MHz, CDCl₃, δ in ppm) 25.62 (2xC, cyclopentyl), 28.01, 28.10 (2xC, cycloheptyl), 29.71, 40.40 (2xC, cyclopentyl), 76.72, 77.04, 77.36 (2xC, cycloheptyl), 125.50, 125.64 (2xC, cycloheptyl), 127.65 (1xC, tetraoxane), 127.97 (1xC, tetraoxane).

Creamy semisolid with a characteristic odor; soluble in dichloromethane, chloroform; %yield 52.12; R_f value (isopropyl alcohol: benzene: 1:1) 0.78; spectroscopic analysis: λ_max (in CHCl₃) 253.48 nm; FTIR (ν_max, in cm^-1, film) 2935.02-2861.33 (C-H stretching, methyl and cycloalkyl), 1447.56-1344.27 (C-H bending, methyl and cycloalkyl), 1064.69-952.83 (C-C-O stretching), 846.91-758.74 (peroxide, C-O-O- stretching); ¹H NMR (400 MHz, CDCl₃, δ in ppm) 0.90-0.93 (dd, 3H, J=12Hz, -CH₃, methylcyclohexyl), 1.02-1.10 (m, 6H, 3x>CH₂, cyclohexyl), 1.20-1.32 (m, 6H, 3x>CH₂, methylcyclohexyl), 1.38-1.48(m, 4H, 2x>CH₂, cyclohexyl), 1.52-1.94 (m, 2H, >CH₂, methylcyclohexyl), 2.27-2.40 (m, 2H, >CH₂, methylcyclohexyl); ¹³C NMR (100 MHz, CDCl₃, δ in ppm) 13.66, 14.88 (1xC, -CH₃, methylcyclohexyl), 20.43, 22.59, 22.63 (2xC, cyclohexyl), 22.73, 22.76 (1xC, methylcyclohexyl), 22.82, 23.15 (1xC, methylcyclohexyl), 24.57, 25.26, 25.39 (1xC, methylcyclohexyl), 25.54, 25.57, 25.62 (1xC, methylcyclohexyl), 28.31, 29.48, 29.84, 30.17, 30.40, 30.48, 30.65, 30.89 (3xC, cyclohexyl), 31.09, 31.92, 32.88, 34.15 (1xC, methylcyclohexyl), 76.70, 77.02, 77.34 (1xC, tetraoxane), 107.53, 107.60, 107.66, 108.04, 108.17, 109.26, 109.72 (1xC, tetraoxane); mass (m/z) calculated 242.31; observed 129.1 (100%), 411.2 (77.20%), 525.2 (64.55%), 115.1 (60.78%), 539.3 (51.53%), 503.3 (47.94%), 243.1 (47.09%) [M+H]⁺; elemental analysis calculated C, 64.44; H, 9.15; O, 26.41; Observed C, 64.654; H, 9.462; O, 26.620.

Brown semisolid with a characteristic odor; soluble in dichloromethane, chloroform; %yield 10.67; R_f value (isopropyl alcohol: benzene: 1:1) 0.74; spectroscopic analysis: λ_max (in CHCl₃) 240.51 nm; FTIR (ν_max, in cm^-1, Film) 2927.77-2860.43 (C-H stretching, methyl and cycloalkyl), 1457.38-1375.93 (C-H bending, methyl and cycloalkyl), 1039.11-933.44 (C-C-O stretching), 843.84-736.33 (peroxide, C-O-O- stretching).

Orange liquid with a characteristic odor; soluble in dichloromethane, chloroform; boiling range 104-105ºC; %yield 40.35; R_f value (benzene: carbon tetrachloride: 2:1) 0.51; spectroscopic analysis: λ_max (in CHCl₃) 253.48 nm; FTIR (ν_max, in cm^-1, Film) 2940.15-2869.23 (C-H stretching, cycloalkyl), 1721.64 (C=O stretching, C₂H₅COO-), 1455.05-1337.93 (C-H bending, cycloalkyl), 1234.61-1107.91 (-C-O stretching, C₂H₅COO-), 1025.47 (C-C-O stretching), 855.46-735.67 (peroxide, C-O-O- stretching); ¹H NMR (400 MHz, CDCl₃, δ in ppm) 1.20-1.22 (t, 3H, J=8Hz, -CH₃, ethylcarboxylate), 1.25-1.33 (m, 6H, 3x>CH₂, cyclohexyl), 1.35-1.47 (m, 2H, >CH₂, cyclopentyl), 1.57-1.76 (m, 4H, 2x>CH₂, cyclohexyl), 1.84-1.92 (m, 2H, >CH₂, cyclopentyl), 1.96-1.99 (m, 2H, >CH₂, cyclopentyl), 2.28-2.40 (m, 2H, >CH₂, cyclopentyl), 4.09-4.38 (m, 2H, >CH₂, ethylcarboxylate); ¹³C NMR (100 MHz, CDCl₃, δ in ppm) 14.24 (1xC, -CH₃, ethylcarboxylate), 20.96 (1xC, cyclopentyl), 23.23 (2xC, cyclohexyl), 24.58 (1xC, cyclopentyl), 25.51, 27.39, 28.33 (3xC, cyclohexyl), 34.18 (1xC, cyclopentyl), 38.08 (1xC, cyclopentyl), 38.36 (1xC, -C₂H₅, ethylcarboxylate), 54.80 (1xC, tetraoxane), 76.71 (1xC, tetraoxane), 77.02, 77.34 (1xC, -C=O, ethylcarboxylate); mass (m/z) calculated 286.32; Observed 143.1 (100%), 115.1 (78.27%), 111.0 (52.78%), 157.1 (47.48%), 407.2 (32.10%), 129.1 (26.54%); elemental analysis calculated C, 58.73; H, 7.74; O, 33.53; observed C, 58.784; H, 7.746; O, 33.581.

Yellowish pink semisolid with a characteristic; soluble in methanol, dichloromethane, chloroform; %yield 09.92; R_f value 0.82 (petroleum ether: n-butanol: 2:1); spectroscopic analysis: λ_max(in CHCl₃) 242.72 nm; FTIR (ν_max, in cm^-1, film) 2940.10-2866.00 (C-H stretching, cycloalkyl), 1435.76-1363.54 (C-H bending, cycloalkyl), 1062.74-961.13 (C-C-O stretching), 809.83-738.60 (per-oxide, C-O-O- stretching), 706.26 (-C-Cl stretching, aliphatic-Cl).

All the synthesized compounds were evaluated for in vitro antimalarial activity against chloroquine resistant strain-RKL-9 of P. falciparum (Pf) using 96 well-microtitre plates at the Department of Pharmaceutical Sciences, Dibrugarh University, Dibrugarh, Assam, India. The laboratory adapted strain of Pf was routinely cultured at 37ºC temperature and 5% CO₂ environment in RPMI 1640 medium supplemented with 25 mM HEPES, 1% D-glucose, 0.23% sodium bicarbonate and 10% heat inactivated human serum. For antimalarial testing, the asynchronous parasites of Pf were synchronized to obtain only the ring stage parasitized cells by 5% D-sorbitol treatment. For carrying out the assay, the initial ring stage parasitemia of 0.8–1.5% at 3% hematocrit in a total volume of 100 mL of medium RPMI-1640 was uniformly maintained. A stock solution (1 mg/mL) of sample was prepared by dissolving the test compounds in DMSO and subsequent dilutions were made with the culture medium. Hundred microlitres of the test compounds at 100 µg/mL concentrations in triplicate was incubated with parasitized cell preparation at 37ºC and 5% CO₂ in a CO₂ incubator. After an incubation period of 36-40 hours, blood smears were prepared from each well and stained with 3% Giemsa stain. The slides were microscopically observed and the percent dead rings and schizonts were scored against 200 asexual parasites with respect to the control group. Chloroquine was used as the standard reference drug (Trager and Jensen, 1976).

The three dimensional (3D) crystal structure of falcipain-2 (PDB code 3BPF) was retrieved from the protein data bank (PDB) (Source:www.rcsb.org/pdb). The native autoinducer and all water molecules were removed. The CHARMm force field (FF) was used to add atom types and hydrogens in the proteins. 3D structures of all synthesized compounds were constructed and energy minimized using the Discovery Studio 2.5/Builder module. Docking studies were performed using the CDOCKER module of Discovery Studio 2.5. CDOCKER is a grid-based molecular docking method where the receptor is held rigid while the ligands are allowed to flex during the refinement. The CHARMm force field was used as an energy grid force field for docking and scoring function calculations. Random ligand conformations were generated from the initial structure through high temperature molecular dynamics, followed by random rotations which were further refined by grid-based (GRID 1) simulated annealing and a final grid-based minimization. Of the 10 best poses, one (conformation) having a highest docking score (-CDOCKER energy) was used for the binding energy calculations and further analysis. The higher negative value of CDOCKER energy represents more favorable binding of the complex. This means that ligands with high docking scores are able to fit snugly in the active site pocket with the minimal steric clashes. CDOCKER score (-CDOCKER Energy) includes inter-nal ligand strain energy and receptor-ligand interaction energy, and is used to sort the different conformations of each input ligand (Oliveira et al., 2013; Liu et al., 2012).

Results

The targeted compounds were prepared as outlined in Scheme 1 (Steps 1 and 2). The step 1 involved the preparation of intermediates dihydroperoxide (3) by the acid-catalyzed addition of hydrogen peroxide (2) to cyclic carbonyl compounds (1). The step 2 involved the preparation of targeted dispiro-1,2,4,5-tetraoxanes (5) via cyclization between dihydroperoxide (3) and cyclic carbonyl compounds (4) in the presence of conc. sulfuric acid. The reaction yielded desired pure products though the yields were low.

Scheme 1:Synthesis of targeted compounds 5 (5a to 5g): Reagents and condition (a,b): Step I: Synthesis of dihydroperoxide (3): a; CH₃CN, CH₂CI₂, Concentrated HCI, stirring at room temperature; Step II: Synthesis of dispiro–1,2,4,5-tetraoxanes (5): b; CH₃CN, CH₂CI₂, Concentrated H₂SO₄, stirring at 0-10ºC

FT-IR spectra showed the stretching frequency range between region 2850–2950 cm^-1 due to aliphatic cycloalkyl –C–H stretching, 1250-1000 cm^-1 due to C-C-O stretching and 900-750 cm^-1 due to peroxide, C-O-O- stretching. ¹H NMR spectra of the compounds showed a triplet or multiplet at δ (ppm) 1.00–2.50 due to cycloalkyl –C-H which further confirmed the formation of the desired compounds. The analytical and spectral data of the compounds were in conformity with the structure of the synthesized compounds.

Among the seven compounds, four compounds 5b, 5c, 5d and 5f showed good activity against chloroquine resistant Pf strain RKL-9 with MIC 62.5 µg/mL, 62.5 µg/mL, 15.6 µg/mL, 15.6 µg/mL, respectively and IC₅₀ 3.9 µg/mL compared to chloroquine (MIC 25.0 µg/mL and IC₅₀ 0.4 µg/mL). Compounds 5d (MIC = 15.6 µg/mL or 64.5 µM) and 5f (MIC = 15.6 µg/mL or 54.6 µM) were observed to be about 1.5 times more potent than CQ (MIC = 25.0 µg/mL or 78.3 µM) (Table I).

Compound code	MIC (μM)	MIC (μg/mL)	IC₅₀ (μg/mL)	IC₉₀ (μg/mL)
5a	1166.6	250.0	3.9	9.1
5b	257.9	62.5	3.9	6.5
5c	273.8	62.5	3.9	38.5
5d	64.5	15.6	3.9	12.0
5e	547.6	125.0	3.9	88.2
5f	54.6	15.6	3.9	3.9
5g	958.7	250.0	4.9	146.8
Chloroquine	78.3	25.0	0.4	1.2

A molecular docking study was undertaken to gain insight into the key structural requirements and the basis of the distinct activity profile of the test compounds in P. falciparum parasite. The docking studies of the target compounds were performed into the binding pocket of falcipain-2 (PDB code 3BPF). The results and docked conformations of the ligands in the active site are illustrated in Table II and Figure 1, respectively.

Compound code	CDOCKER energy (kcal/mol)	CDOCKER interaction energy (kcal/moL)
5a	–1.69	–28.65
5b	–14.75	–28.15
5c	–23.13	–28.73
5d	–11.06	–28.10
5e	-2.75	-34.81
5f	–12.96	–26.82
5g	–8.56	–37.87

Discussion

Seven novel dispiro-1,2,4,5-tetraoxane derivatives were synthesized and characterized by a number of analytical and spectroscopic techniques. Two compounds, namely 5d (MIC = 15.6 µg/mL or 64.5 µM) and 5f (MIC = 15.6 µg/mL or 54.6 µM) were found to be about 1.5 times more potent against chloroquine resistant strain-RKL-9 compared to chloroquine (MIC = 25.0 µg/mL or 78.3 µM) on antimalarial activity screening. Molecular docking study results showed that the targeted molecules were snugly fitted into the active site with considerable and diverse CDOCKER energy (-1.6870 to -23.1300) with FP-2 along with the formation of numerous hydrogen bonds and hydrophobic interactions. steric hindrance close to the peroxide ring is unfavorable for activity in dispirotetraoxanes. Amewu and co-workers (Amewu et al., 2013) reported that dispirotetraoxane compounds was found to be equally potent as artemisinin.

Antimalarial activity results reflect that the dispirotetraoxanes are found to be potent compounds against chloroquine resistant Pf strain RKL-9. Substitution on dispirocycloalkane-tetraoxane with methyl and ethyl carboxylic acid groups make more active tetraoxanes than CQ against RKL-9 as observed with 5d and 5f, due to desirable lipophilicity to tetraoxane. Attachment of higher cycloalkanes e.g. six and seven member dispirocycloalkane ring (cyclohexane or cycloheptane) to tetraoxane ring enhanced effectiveness of tetraoxanes than that of lower cycloalkanes e.g. five member dispirocycloalkane ring (cyclopentane) towards their antimalarial activity. Chloro group substituted dispirotetraoxane become less active than corresponding unsubstituted dispirotetraoxanes.

Conclusion

A novel series of compounds with potent antimalarial activity has been developed. Designed molecules have the possibility to introduce chemical diversity around the core skeleton to generate newer and potent molecules.

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| Research | Article |

Synthesis, antimalarial activity evaluation and molecular docking studies of some novel dispiro-1,2,4,5-tetraoxanes