Article pubs.acs.org/Organometallics
1,1′-Disubstituted Ferrocenyl Carbohydrate Chloroquine Conjugates as Potential Antimalarials Christoph Herrmann,†,‡ Paloma F. Salas,† Jacqueline F. Cawthray,†,§ Carmen de Kock,∥,⊥ Brian O. Patrick,† Peter J. Smith,∥,⊥ Michael J. Adam,*,§ and Chris Orvig*,† †
Medicinal Inorganic Chemistry Group, Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada ‡ Advanced Applied Physics Solutions, 4004 Wesbrook Mall, Vancouver, British Columbia V6T 2A3, Canada § TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia V6T 2A3, Canada ∥ Department of Medicine, University of Cape Town Medical School, Observatory 7925, South Africa ⊥ Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa S Supporting Information *
ABSTRACT: This work presents a new class of organometallic antimalarials, based on a ferrocene scaffold, bearing a chloroquine derivative and a 1,2;3,5-(diisopropylidene)-α-D-glucofuranose moiety in a 1,1′-heteroannular substitution pattern. Synthesis proceeds via orthogonal functionalization of ferrocene, giving 1-acetoxy-1′-(1,3-dioxan-2-yl)ferrocene (15) as the precursor for modular introduction of the carbohydrate (16, 17) followed by subsequent reductive amination with chloroquine building blocks 8−10, yielding the 1-[3-(7-chloroquinolin-4-ylamino)alkylamino]-1′-[6-(1,2;3,5-diisopropylidene)-α-Dglucofuranosidyl]ferrocenes 18−20. After complete characterization of these new, trifunctional conjugates, they were examined for their antiplasmodial activity in a chloroquine-susceptible strain of Plasmodium falciparum (D10) and in two chloroquineresistant strains (Dd2 and K1). Their activity was compared to that of the monosubstituted reference conjugates 1−3 and the 1,2-disubstituted regioisomers 4−6. Compounds 19 and 20 exhibited consistently high activity in in vitro antiplasmodial activity assays performed in Dd2 and K1 strains, performing better than the reference compounds chloroquine and the monosubstituted and 1,2-disubstituted compounds 1−6.
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oquine (CQ; Chart 1, A) has been the first line of treatment against malaria since 1942, but resistance to CQ and other inexpensive antimalarial agents has become a severe problem.4 In Southeast Asia, resistance to all of the widely used antimalarial agents is common, the exception being the recently introduced artemisinins (Chart 1, B),5 yet artemisinin resistance in the border region of Thailand and Cambodia6 and further evidence hinting at a developing artemisinin resistance in Myanmar and Vietnam1 have recently been
INTRODUCTION Malaria is a parasitic disease, endemic in 106 countries in the tropical and subtropical regions of Africa, Asia, and Latin America. In 2010 alone, an estimated 3.3 billion people were at risk of contracting malaria. In the same year, 216 million cases of malaria led to an estimated number of 655 000 casualties,1 while other studies put the number of casualties in 2010 as high as 1 238 000.2 Approximately 81% of all cases and 91% of all casualties occur on the African continent. The disease is transmitted by the female Anopheles mosquito through biting and transfer of Plasmodium parasites via its saliva. Of the five different types of Plasmodium that can infect humans, severe malaria, which can lead to coma and death if untreated, is almost exclusively caused by Plasmodium falciparum.3 Chlor© 2012 American Chemical Society
Special Issue: Organometallics in Biology and Medicine Received: April 27, 2012 Published: June 12, 2012 5736
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Chart 1. The Commonly Used Antimalarials Chloroquine (A) and Artemisinin (B) and the Novel Drug Ferroquine (C)
CQ resistance.17 The most potent metallo antimalarial agents are chloroquine-based metallocenes of Fe(II)13c,g,h,18 and Ru(II).19a The most prominent of these derivatives is ferroquine13h (Chart 1, C), the first organometallic antimalarial to proceed to phase IIb clinical trials.20 Other examples include complexes containing mefloquine, quinine, artemisinin, and chloroquine with metal ions such as Ga(III),21 Au(I),22 Pd(II), and Pt(II).23 Varying the structure of the antimalarial drug by incorporation of a lipophilic, redox-active metal center should increase membrane permeability and aid in the accumulation of drug in the resistant parasite’s food vacuoles, thereby increasing efficacy.24 The combination of both of these approaches led to the synthesis and antiplasmodial activity studies of 1,2-disubstituted carbohydrate chloroquine ferrocene conjugates (4−6, Chart 2) in comparison to their literature-known25 monosubstituted chloroquine−ferrocene counterparts 1−3. We were able to improve the antiplasmodial activity of the conjugates from the micromolar domain of the ferrocenoyl carbohydrates11 into the low nanomolar region for the novel conjugates 4−6. The current paper seeks to expand the 1,2-substitution pattern of the ferrocene backbone of 4−6 from homoannular substitution to heteroannular 1,1′-substitution at opposing cyclopentadienyl rings. We hope to gain insights into the influence of the substitution pattern on antiplasmodial activity in vitro. On the basis of previous syntheses, the synthesis of new, 1,1′-disubstituted conjugates was planned accordingly to yield the regioisomeric analogues of 4−6, with retention of the earlier employed glucofuranosidyl substituent.
reported. In addition to parasitic resistance against commonly used antimalarials, a further problem is a developing resistance of the Anopheles vectors to the pyrethroids used in insecticidetreated mosquito nets.1 Because of the recurring resistance to antimalarial drugs and insecticides, there is a high demand to develop new and inexpensive antimalarials that are able to overcome the parasites’ resistance. Organometallic antimalarials are, in our eyes, a very promising target in this regard. In a previous publication7 we described the synthesis of 1,2-disubstituted ferrocenes bearing a chloroquine substituent and a carbohydrate (4−6, Chart 2), fusing the two main fields of research on Chart 2. Previously Synthesized7,23 Mono- and 1,2Disubstituted Ferrocenyl Chloroquine Glucofuranose Conjugates 1−3 and 4−6
organometallic antimalarials, which are either organometallic derivatives of known antimalarial drugs, leading to a change in the properties of these compounds, or completely novel compounds,8 acting by a mechanism that most likely will differ from the mode of action of the commonly employed drugs. Our rationale is based on glucose consumption being a target in antimalarial research,9 as glucose uptake and metabolism are elevated at all stages of the parasite’s life cycle;10 our group11 and that of Itoh12 previously described ferrocenoyl carbohydrate derivatives with antiplasmodial activity in vitro. Also, incorporation of ferrocene into the structure of chloroquine has been well studied, 13 leading to potent organometallic derivatives. The motivation for research on metal conjugates of known antimalarial drugs13 lies in the well-studied mechanism of action of 4-aminoquinoline-type antimalarial agents such as chloroquine (CQ). CQ forms a complex with ferric heme (ferriprotoporphyrin IX, FP IX) in the parasite's food vacuole. FP IX is a metabolite of the parasite’s sequestering and consumption of hemoglobin.14,15 Ferric heme is toxic and disposed of by the parasite through formation of hemozoin. The drug−ferric heme complex inhibits this detoxification, resulting in the parasite’s death.16 It has been suggested that CQ-resistant parasites accumulate less of the drug in the food vacuoles,14 via an increased efflux from the vacuole, thus diminishing the toxic effect of the drug, leading to
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RESULTS AND DISCUSSION Synthesis of the Conjugates. With the 1,2-disubstituted conjugates already developed, the synthesis needs to be modified to accommodate heteroannular substitution at both cyclopentadienyl rings of ferrocene, leading to a precursor with the ability to form an ether connectivity with a carbohydrate and a carboxaldehyde moiety available for reductive amination. On the basis of the previously described precursor ferrocene (2S,4S,Sp )-1-acetoxy-2-(4-methoxymethyl-1,3-dioxan-2-yl)ferrocene (D)7,26 of the 1,2-disubstituted systems, the precursor would ideally be the corresponding 1-acetoxy-1′(1,3-dioxan-2-yl)ferrocene (15), giving us the possibility to perform similar transformations for the 1,1′-systems (Chart 3). Using 15 as a precursor enabled us to reutilize synthetic manipulations from the previously synthesized 1,2-conjugates. Orthogonal substitution of two equivalent cyclopentadienyl rings at a ferrocene scaffold started from pure ferrocene. In a double deprotonation step, using tmeda as the auxiliary base, the intermediate 1,1′-dilithium salt was generated. Without isolation, transmetalation using tri-n-butyltin chloride yielded 1,1′-bis(tri-n-butylstannyl)ferrocene (11) in approximately 90% yield. In contrast to procedures described in the literature,27 5737
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forms the carboxaldehyde 17, required as an intermediate for reductive amination. The reaction in the microwave proceeds within minutes, yielding 66% of 17 (a reference reaction under reflux yielded only 54% after 3 h). In NMR experiments two AA′BB′ systems are observed for the ferrocene scaffold of 16, leading to resonances in the 1H and 13C NMR spectra of δ(1H/13C) (ppm) 4.35/67.3, 4.18/ 68.7, 4.12/63.1, and 3.85/56.5 for the CH groups and 126.8 (C11) and 86.5 (C1), respectively, for the ipso carbon atoms of both cyclopentadienyl rings. For both the 1,3-dioxanyl moiety and the 1,2;3;5-diisopropylidene moiety attached to the ferrocene scaffold, the expected resonances in the 1H and 13C NMR experiments are observed. The heteroannular substitution could be confirmed in the solid-state crystal structure. The dioxanyl moiety is attached to one of the cyclopentadienyl rings, adopting a chair geometry, as expected for a sixmembered ring. The second cyclopentadienyl ring of the ferrocene scaffold is substituted by 1,2;3,5-(diisopropylidene)α-D-glucofuranose linked at its 6′-position. The core heterocycle and both of the five-membered rings formed via isopropylidene protection adopt an envelope conformation. Formation of carboxaldehyde 17 was confirmed by NMR spectroscopy and IR spectroscopy. NMR signals of the 1,3dioxanyl moiety of 16 (δ(1H/13C) (ppm) 4.19, 3.92/67.0, 67.1 (OCH2) and 2.15, 1.38/26.0 (CH2)) vanish and give rise to new signals in the 1H and 13C NMR spectra, respectively. The signals of the aldehyde group are at 9.98 (1H) and 193.7 ppm (13C), respectively. This change influences the resonances of the corresponding cyclopentadienyl ring as well. Signals shift from 86.5 ppm (ipso-C) to 80.2 ppm. The methylene group signals can be located in the 1H and 13C NMR spectra of 17 at 4.87/74.1 and 4.67/70.9 ppm (compared to 4.18/68.7 and 4.35/67.3 ppm in 16). Most indicative nonetheless is the carbonyl IR stretch of 17 (ν̅ 1681 cm−1) of the newly formed carboxaldehyde. Reductive amination, as the final step, employing the chloroquine derivatives N-(7-chloroquinolin-4-yl)ethane-1,3diamine (8),32 N-(7-chloroquinolin-4-yl)propane-1,3-diamine (9),32 and N-(7-chloroquinolin-4-yl)-2,2-dimethylpropane-1,3diamine (10),32 respectively, yielded the 1,1′-substituted conjugates 18−20 as analogues of the previously discussed 1,2-substituted conjugates (Scheme 3). Synthesis of 18 was performed using a microwave-assisted procedure, substituting for stirring overnight at room temperature, to form the
Chart 3. Comparison of Ferrocene Precursors for 1,2Disubstituted (D) and 1,1′-Disubstituted (15) Chloroquine Glucofuranose Ferrocenes
ferrocene was sublimed prior to lithiation, and the experiments were carried out using dried solvents. Each of the stannyl groups could be independently substituted using 1 equiv of n-butyllithium. Quenching with DMF after addition of 1 equiv of n-butyllithium yielded 1′-(trin-butylstannyl)ferrocene-1-carboxaldehyde (12).28 A procedure reported by Iftime et al.,29 starting from ferrocenecarboxaldehyde to yield 12 in one step,29 proved unsuccessful in our hands. The labile carboxaldehyde 12 was acetal-protected, using 1,3-propanediol, yielding 1-(1,3-dioxan-2-yl)-1′-(tri-nbutylstannyl)ferrocene (13).30 This protecting step was carried out either by using a Dean−Stark trap, refluxing a solution of carboxaldehyde 12 and diol in toluene overnight, or by heating the reaction mixture in the microwave in the presence of freshly activated molecular sieves, both procedures giving comparable yields. The second tri-n-butylstannyl group was removed by reaction with another 1 equiv of n-butyllithium, and salt metathesis with 1,2-diiodoethane yielded 1-(1,3-dioxan-2-yl)-1′iodoferrocene 14. Single crystals of 14 could be obtained and were analyzed by X-ray structure analysis (see Figure 1). In the final step to yield the precursor compound 15, the iodo group of 14 was transformed catalytically using copper(I) oxide. A solid-state structure for 15 was also obtained by X-ray crystallography. As described previously, the introduction of the carbohydrate proceeds through reaction of the acetoxy moiety of 15 with sodium methoxide and reaction with 6-deoxy-6-bromo-1,2;3,5(diisopropylidene)-α-D-glucofuranose (7),31 giving 16 in 45% yield after chromatographic workup (Scheme 2). Compound 16 formed single crystals suitable for X-ray crystallography in pentane overnight (see Figure 2). Cleavage of the cyclic acetal in the second to last step, using excess p-toluenesulfonic acid, Scheme 1. Synthetic Outline for the Ferrocene Precursor 15
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Figure 1. Solid-state structures of 14 and 15 (thermal ellipsoids drawn at the 50% probability level; hydrogen atoms omitted for 15). Selected bond lengths (in Å): for 14, C(1)−C(2) = 1.422(3), C(1)−C(6) = 1.489(3), C(6)−O(1) = 1.407(3), C(6)−O(2) = 1.416(3), C(10)−I(1) = 2.080(2); for 15, C(1)−C(6) = 1.4972(17), C(6)−O(1) = 1.4127(16), C(6)−O(2) = 1.4098(16), C(11)−O(3) = 1.4001(15), (17)-O(4) = 1.193(2), C(17)−O(3) = 1.3674(17), C(17)−C(18) = 1.503(2).
Scheme 2. Introduction of the Carbohydrate and Microwave-Assisted Cleavage of the Cyclic Acetal
transient imine by heating at 80 °C for 12.5 min. Addition of the reducing agent (NaBH4) and subsequent heating in the microwave at 80 °C for 5 min gave 18 in 59% yield. This is
comparable to the yields achieved for 19 and 20 (49% and 57%, respectively), which were synthesized at room temperature over the course of 16 h. 5739
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“handle” is introduced. This can be seen from the 13C NMR signals of the CH cyclopentadienyl groups, in the α-position (C12/C15) to the carbohydrate substituent. They split into two signals, whereas beforehand (and for the opposing cyclopentadienyl ligand) only one signal is observed for each pair of CH groups in α- (C2,5) and β-positions (C13,14 and C3,4) to the substituted ipso carbon (C11 and C1) (see Scheme 3 for numbering). This allowed for easy identification of the different cyclopentadienyl rings in NMR analysis. As was already observed for the 1,2-disubstituted conjugates, there is evidence for hydrogen bonding between the NH at the quinoline moiety and the NH in a β-position to the cyclopentadienyl ring. The substitution and lengths of the alkyl spacer between the two secondary amines have a great influence. Spacer length influences the ring size for the intramolecular hydrogen bond and has the biggest influence. From 18 to 19, the ring size expands from five- to sixmembered, leading to a 1H NMR shift in NHAr of 2.08 ppm (Table 2). Steric encumbrance of the side chain favors the formation of intramolecular hydrogen bonds, with δ(1H) for 19 and 20 being 0.61 ppm, showing a less prominent effect in comparison to ring expansion. For ferroquine, the 1H NMR resonance of the NH group is observed at 7.66 ppm.33 A direct comparison of H shifts and H bonding is not possible, since ferroquine forms a seven-membered ring through H bonding to a tertiary amine, whereas the systems described here form fiveor six-membered rings through H bonding between two secondary amines (see Figure 3). Molecular Electrostatic Surface Potentials. Molecular electrostatic surface potentials for the ferrocene CQ conjugates 1−3 (monosubstituted), 4−6 (1,2-disubstituted), and 18−20 (1,1′-disubstituted) were calculated (Table 3) using the optimized structures of the uncharged compounds (Figure 4). In general, changing the chain length of the chloroquine substituent does not significantly alter the electrostatic potential of the molecular surface, nor does the introduction of a carbohydrate substituent or a change of substitution pattern from the homoannularly disubstituted conjugates 4−6 to the heteroannular conjugates 18−20. The topological polar surface areas of 1−6 and 18−20 are significantly influenced by the presence of the carbohydrate
Figure 2. Solid-state structure of 16 (thermal ellipsoids drawn at the 50% probability level, hydrogen atoms of the non-carbohydrate fragment omitted for clarity). Selected bond lengths for 16 (in Å): C(1)−C(6) = 1.489(3), C(6)−O(1) = 1.423(2), C(6)−O(2) = 1.409(2), C(11)−O(3) = 1.377(2), C(6′)-O(3) = 1.427(2).
NMR analysis confirms formation of the 1,1′-disubstituted conjugates. 1H and 13C NMR signals for the aldehyde group of 17 disappear and give rise to signal sets corresponding to the ethyl-, propyl-, or 2,2-dimethylpropyl-spacered chloroquine substituents. With the introduction of the carbohydrate substituent at the ferrocene scaffold of 15 to generate 16, a stereochemical
Scheme 3. Synthesis of the 1,1′-Disubstituted Conjugates 18−20 from 17a
a
The numbering scheme for Table 1 is given. 5740
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Table 1. Characteristic 1H and 13C NMR and IR Resonances of Compounds 16−20a C1 C11 1′-H C1′ 6-H C6 6′-H C6′ C13,14 C12,C15 ν(CO/N−H) ̅
a
16
17
18
19
20
86.5 126.8 6.04 106.6 5.38 100.5 4.07/3.96 70.9 63.1 56.8/56.5
80.2 127.9 6.04 106.7 9.98 193.7 3.97/3.89 70.9 63.9 57.3/57.0 1681
87.7 126.3 6.02 106.6 3.57 48.2 4.03/3.95 70.9 62.7 56.5/56.3 1580
86.5 126.5 6.01 106.6 3.62 49.3 4.00/3.93 71.0 62.8 56.6/56.4 1582
86.3 126.1 6.01 106.6 3.58 50.1 3.99/3.92 71.1 62.7 56.6/56.3 1581
δ in ppm, 300 or 600 MHz, 300 K, CDCl3, ν̅ in cm−1; for numbering see Scheme 3.
Table 2. 1H NMR Resonances of the NH Protons of the Monosubstituted (1−3), 1,2-Disubstituted (4−6), and 1,1′Disubstituted Conjugates (18−20)a
a
δ in ppm, 300 or 600 MHz, 300 K, CDCl3.
Figure 3. Intramolecular hydrogen bonding in ferroquine (FQ, left) and in 19 and 20 (right).
substituent. PSAs are around 36 Å2 for 1−3, approximately 91 Å2 for 4−6, and 96 Å2 for 18−20, with lower values denoting an increased lipophilicity. Remarkable is the comparison of PSA within the respective triads of monosubstituted and 1,2- and 1,1′-disubstituted conjugates. The PSAs are lower for the 2,2dimethylpropyl spacers of all systems, whereas the change in PSA from ethyl to propyl spacer is negligible in all described systems. This trend has already been observed for the increased 1 H NMR shift of the aromatic, secondary amine groups of the chloroquine substituents of the conjugates. For all conjugates the decrease of PSAs from ethyl- or propyl-spacered systems to the 2,2-dimethylpropyl-spacered systems can be determined to
Figure 4. Molecular electrostatic potentials (MEP) color-coded by constant electron density, ρ, isosurfaces of 1−6 and 18−20. The MEPs represent a maximum potential of 0.05 au and a minimum of −0.05 au, mapped onto electron density isosurfaces of 0.002 e Å−3.
Table 3. Topological Polar Surface Areas (PSA) in Å2 for Conjugates 1−6 and 18−20
PSA (Å2)
1
2
3
4
5
6
18
19
20
38.730
38.476
33.638
92.169
92.973
88.928
98.330
99.003
94.147
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be approximately 5 Å2. This can be attributed to the presence of two additional methyl groups in the system. Biot described a similar trend when comparing ferroquine (FQ) to its methylated counterpart (MeFQ), calculating polar surface areas of 14 Å2 for FQ and only 10 Å2 for MeFQ,33a concluding that an increase in lipophilicity through intramolecular hydrogen bonding (in FQ) does not outweigh the increase in lipophilicity introduced by a methyl substituent but that the increased antiplasmodial activity of FQ over MeFQ stems from a reduced molecule size through hydrogen bonding. Antiplasmodial Activity Studies. Antiplasmodial activity studies were carried out in Plasmodium falciparum parasites, using a chloroquine-susceptible (CQS) strain (D10) and two chloroquine-resistant (CQR) strains (Dd2, K1), and the results are given in Table 4. As a reference compound, chloroquine Table 4. Antiplasmodial Activity Studies in CQ-Susceptible (D10) and CQ-Resistant (Dd2, K1) Strains of P. falciparum and Their Respective Resistance Indices (RI)a IC50 (nM)
a b
Figure 5. IC50 values (in nM) for compounds 17−20 with reference compounds 1−6 and CQ and FQ 13c,25a (dotted lines: CQ susceptibility at 194 nM, CQ-resistance at 1940 nM).13h.
RI
compd
D10
Dd2
K1
Dd2/D10
K1/D10
CQ FQ 17 1 4 18 2 5 19 3 6 20
38.8 1813c >200 60.8 90.2 n.d. 138 147 72.8 368 72.4 97.0
388 1925a >2000 36.2 (29.325a) 157 n.d. 300 (63.025a) 173 104 1150 (50.625a) 167 111
758 1413c n.d.b 1500 119 n.d. 16.1 47.7 135 90.9 242 51.4
10 1.1 10 0.6 1.7 n.d. 2.2 1.2 1.4 3.1 2.3 1.1
20 0.8 n.d. 24.7 1.3 n.d. 0.1 0.32 1.9 0.25 3.3 0.53
conversion into nM units, for comparison with the tested compounds, the margins are defined as 194 and 1940 nM. Both the literature-known monosubstituted compounds 2 and 3 have been tested against D10 and Dd2 strains (Table 4).25a These published results correlate only partially to the results obtained herein; this must be taken into account when discussing the activity of the novel compounds. With the exception of 17 in Dd2, all measured compounds are within the 1940 nM threshold defining chloroquine resistance. The inactivity of 17 is due to a lack of chloroquine substituent at the ferrocene scaffold. The reference compound chloroquine shows an increase in IC50 values from D10 (38.8 nM) by a factor of 10 in Dd2 (388 nM) and by a factor of 20 to K1 (758 nM), resulting in RIs of 10 and 20, respectively. Ferroquine shows activities between 14 and 19 nM in all three P. falciparum strains, with RIs of 1.1 (D10/Dd2) and 0.8 (D10/ K1). Compound 1 has a wide spectrum of RI values: RI = 0.6 shows an activity in CQR Dd2 strains, higher than in the CQS D10 strain, yet unfortunately the compound has RI = 24.7 in the K1/D10 challenge. Both compounds 4 and 19 have RIs between 1 and 2 in both Dd2 and K1 challenges, meaning almost similar activities in both CQS and CQR strains of P. falciparum, with IC50 values ∼100 nM. IC50 values for compound 6 are in the same range as for 4 and 19, yet RI ≈ 3 (D10/K1), signifying a 3-fold reduced activity in the CQR K1 strain. Compounds 2, 3, 5, and 20 show an increased activity in the CQR K1 strain in comparison to the CQR Dd2 strain. RI values between 0.1 and 0.53 are calculated, indicating an increase in activity of 10 for 2 and an increase by 2 for 20. With the exception of 2 (IC50 = 16.1 nM), none of the tested compounds show IC50 values in the proximity of ferroquine. On comparison of results for the 1,2-disubstituted and 1,1′disubstituted compounds (5/19 and 6/20), to probe the effect of the substitution pattern, an increased activity of 19 by 50% (D10) and 40% (Dd2) and a decrease in activity in K1 (200%) in comparison to 5 can be observed. In the D10/Dd2 challenge, the RI drops from 1.2 in 5 to 1.4 in 19, but this is more than accounted for by the significantly higher activity of 19, whereas for D10/K1 RI increases from 0.32 (5) to 1.9 (19). For 20, the IC50 value in D10 increases by a factor of 1.3 compared to 6, but the activities in the CQR strains (Dd2, K1) are increased by 34% (Dd2) and 78% (K1) in comparison to 6. The RI values
Literature-reported25a IC50 values for 1−3 are given in parentheses. n.d. = not determined.
diphosphate salt was tested. The monosubstituted ferrocene− chloroquine conjugates 1−3 were used as a direct comparison to prove or disprove a beneficiary effect of the appended carbohydrate at the ferrocene scaffold. Literature values for ferroquine (FQ), tested in the corresponding strains, are given as well. Compounds are listed according to their side-chain length, linking ferrocene and chloroquine; 1, 4, 18, C2H4; 2, 5, 19, C3H6; 3, 6, 20, CH2C(CH3)2CH2. The resistance index, as a useful tool to compare antiplasmodial activities, is determined as the quotient of IC50 (CQR) and IC50 (CQS). RI > 1 means that the activity in CQR strains is worse than in CQS strains; a value
