Unsymmetrical Bisquinolines with High Potency against P. falciparum Malaria

Quinoline-based scaffolds have been the mainstay of antimalarial drugs, including many artemisinin combination therapies (ACTs), over the history of modern drug development. Although much progress has been made in the search for novel antimalarial scaffolds, it may be that quinolines will remain useful, especially if very potent compounds from this class are discovered. We report here the results of a structure-activity relationship (SAR) study assessing potential unsymmetrical bisquinoline antiplasmodial drug candidates using in vitro activity against intact parasites in cell culture. Many unsymmetrical bisquinolines were found to be highly potent against both chloroquine-sensitive and chloroquine-resistant Plasmodium falciparum parasites. Further work to develop such compounds could focus on minimizing toxicities in order to find suitable candidates for clinical evaluation.


Compound 11: 7-chloro-N-(3-(4-(quinolin-4-ylamino)piperidin-1-yl)propyl)quinolin-4-amine
HPLC (method A) tR = 6.35 min (93% pure). (An additional peak elutes at 6.94 minutes, 6%.)  3-(Quinolin-4-ylamino)propyl methanesulfonate (0.76 g, 0.00271 mol), N-(piperidin-4yl)quinolin-4-amine (0.65 g, 0.00285 mol), K2CO3 (0.56 g, 0.00407 mol), and a catalytic amount of potassium iodide were heated for 24 hours in refluxing acetonitrile (40 mL), at which point TLC indicated that reaction was complete. After dilution with 10 mL water, the acetonitrile was removed by evaporation under reduced pressure with warming, and the residue was combined with dichloromethane (30 mL) and water (30 mL). An undissolved tan solid was removed by filtration, rinsing with dichloromethane and water. The dichloromethane and water layers were separated, and the aqueous layer was extracted with dichloromethane (3 x 10 mL), followed by drying of the pooled organic layers (MgSO4) and evaporation under reduced pressure with warming to yield a yellow solid. NMR indicated that this was primarily starting material, whereas the solid from filtration was primarily the desired product. The latter was dissolved in excess boiling 95% ethanol, and allowed to evaporate gradually (at room temperature) to approximately 10 mL. Filtration afforded a tan solid (0.80 g). This material was further purified by automated flash chromatography on alumina, eluting with a gradient of 100% hexanes to 100% ethyl acetate. The desired product was obtained as a white solid (0.29 g, 26%, mp =190.2-191.8°C, Rf =0.14 (alumina, 10/90 MeOH/EA v/v)). 3-(8-(Trifluoromethyl)quinolin-4-ylamino)propyl methanesulfonate (0.50 g, 0.00144 mol), 7-chloro-N-(piperidin-4-yl)quinolin-4-amine (0.39 g, 0.00151 mol), K2CO3 (0.20 g, 0.00145 mol), and a catalytic amount of potassium iodide were heated for 5 days in refluxing acetonitrile (40 mL), at which point TLC indicated that reaction was complete. After dilution with 10 mL water, the acetonitrile was removed by evaporation under reduced pressure with warming, and the residue was combined with chloroform (30 mL) and water (30 mL). A beige solid remained undissolved and was removed by filtration, rinsing with chloroform and water. After separation of the chloroform and water layers, the aqueous layer was extracted with chloroform (3 x 7 mL), and the pooled organic layers were dried (MgSO4) and evaporated under reduced pressure with warming to yield a pale yellow, waxy solid, 0.09g. NMR indicated that the solid from filtration was primarily the desired product. This was dissolved in excess boiling 95% ethanol, and allowed to evaporate gradually (at room temperature) to approximately 10 mL. Filtration afforded the desired product as sparkling, beige crystals (0.52 g, 70%, mp = 225-233°C (dec)).  3-(8-(Trifluoromethyl)quinolin-4-ylamino)propyl methanesulfonate (0.66 g, 1.9 mmol), N-(piperidin-4-yl)-8-(trifluoromethyl)quinolin-4-amine (0.59 g, 2.0 mmol), K2CO3 (0.39 g, 2.8 mmol), and a catalytic amount of potassium iodide were heated for 27 hours in refluxing acetonitrile (50 mL), at which point TLC indicated that reaction was complete; however, heating at reflux was continued for an additional 18 hours. The solvent was removed by evaporation under reduced pressure with warming, and the resulting golden brown solid was combined with chloroform (30 mL) and water (30 mL). A large amount of solid material remained undissolved and was removed by filtration, rinsing with chloroform and water (a white solid). This material was dissolved in excess boiling ethyl acetate, followed by gradual evaporation at room temperature to induce crystallization. The fine, off-white crystals thus obtained proved to be the desired product (0.20 g, 19%).  3-(7-Chloroquinolin-4-ylamino)propyl methanesulfonate (0.59 g, 1.9 mmol), N-(piperidin-4-yl)-8-(trifluoromethyl)quinolin-4-amine (0.58 g, 2.0 mmol), K2CO3 (0.39 g, 2.8 mmol), and a catalytic amount of potassium iodide were heated for 24 hours in refluxing acetonitrile (50 mL), at which point TLC indicated that reaction was complete. The solvent was removed by evaporation under reduced pressure with warming, and the resulting ochre solid was combined with chloroform (30 mL) and water (30 mL). A large amount of solid material remained undissolved and was removed by filtration, rinsing with chloroform and water. The chloroform and water layers of the filtrate were separated, and the aqueous layer was extracted with chloroform (3 x 10 mL), dried (MgSO4), and evaporated under reduced pressure with warming. This material was combined with the insoluble material from extraction (above), and taken up in excess boiling 95% ethanol. This was allowed to cool and evaporate gradually at room temperature. The crystals that formed were recovered by filtration and further purified by automated flash chromatography on basic alumina, eluting with a gradient of 100% hexanes to 100% ethyl acetate. The desired product was obtained as a beige solid (0.38 g, 40%, mp = 205-207°C, Rf = 0.2 (alumina, EA)).
Note: Q1 and Q2 denote respectively the quinoline ring system on the left and that on the right of the structure as shown above. Spectra are provided below (Example spectra: Compound 6). Original method used to synthesize compound 6 (for preferred method, see above)

N-(3-(4-aminopiperidin-1-yl)propyl)-7-chloroquinolin-4-amine:
Methyl isobutyl ketone, 125 mL, was allowed to reflux in a round bottom flask equipped with a Dean and Stark water separator. After 30 minutes, sodium carbonate (0.74 g, 7.0 mmol) and 4-aminopiperidine (0.580 mL, 5.5 mmol) were added and reflux was continued for 1 hour, at the end of which time TLC indicated that no more of the amine starting material was present. The reaction was allowed to cool to 85°C, and 3-(7-chloroquinolin-4-ylamino)propyl methanesulfonate (1.45 g, 4.6 mmol) was added. After 16 hours had elapsed, TLC indicated that no more 3-(7-chloroquinolin-4ylamino)propyl methanesulfonate was present. The reaction mixture was washed with water (50 mL) to remove sodium carbonate, and the solvent was then evaporated under reduced pressure. The resulting amber oil was stirred for 1.5 hours in a mixture of 50 mL isopropanol and 5 mL water, and the solvent was then evaporated under reduced pressure. The resulting liquid was partitioned between ethyl acetate (20 mL) and saturated sodium bicarbonate (25 mL), and the aqueous layer was extracted with additional ethyl acetate (3 x 10 mL). During this procedure, a yellow oil remained insoluble; NMR indicated that this was pure product. After removal of water therefrom by heating under reduced pressure, this material was used without further purification.
Note: Q1 and Q2 denote the quinoline ring system on the left and right of the figure, respectively.
List of spectra: Figure C1: 1 H NMR spectrum of compound 6 in CDCl3, full view. Figure C2: 1 H NMR spectrum of compound 6 in CDCl3, expansion 1 of 3 (aromatic region, part 1). Figure C3: 1 H NMR spectrum of compound 6 in CDCl3, expansion 2 of 3 (aromatic region, part 2). Figure C4: 1 H NMR spectrum of compound 6 in CDCl3, expansion 3 of 3 (aliphatic region, part 1). Figure C5: 13 C NMR spectrum of compound 6 in CDCl3, full view. Figure C6: 13 C NMR spectrum of compound 6 in CDCl3, aliphatic region only. Figure C7: COSY spectrum of compound 6 in CDCl3. Figure C8: HSQC spectrum of compound 6 in CDCl3. Figure C9: HMBC spectrum of compound 6 in CDCl3. Figure C10: NOESY spectrum of compound 6 in CDCl3. Figure C11: 1 H NMR spectrum of compound 6 in DMSO-d6. Figure C12: 1 H NMR spectrum of compound 6 in DMSO-d6, aliphatic region only.           figure C12 for expanded aliphatic region). This spectrum was used to confirm the suspected integration of a peak overlapped by the water signal in the CDCl3 spectrum (here at 1.68 ppm).