1-Aryl-1,2,3,4-tetrahydroisoquinolines as potential antimalarials: Synthesis, in vitro antiplasmodial activity and in silico pharmacokinetics evaluation

Article abstract of DOI:10.1039/c3ra46791k

In the present study, twenty-one 1-aryl-6-hydroxy-1,2,3,4- tetrahydroisoquinoline (THIQ) analogues were synthesized by base-catalyzed Pictet-Spengler reaction, and tested in vitro against P. falciparum using the [³H]hypoxanthine incorporation a

Full text of DOI:10.1039/c3ra46791k

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1-Aryl-1,2,3,4-tetrahydroisoquinolines as potential
antimalarials: synthesis, in vitro antiplasmodial
activity and in silico pharmacokinetics evaluation†
Cite this: RSC Adv., 2014, 4, 22856
Joelle Ngo Hanna,‡a Fidele Ntie-Kang,a Marcel Kaiser,b Reto Brunb
a
*
and Simon M. N. Efange
In the present study, twenty-one 1-aryl-6-hydroxy-1,2,3,4-tetrahydroisoquinoline (THIQ) analogues were
synthesized by base-catalyzed Pictet–Spengler reaction, and tested in vitro against P. falciparum using
the [3H]hypoxanthine incorporation assay. Two compounds were found to be inactive while seventeen
compounds displayed moderate antiplasmodial activity and two compounds were found to be highly
active (IC50 < 0.2 mg mlꢀ1). The two highly active compounds, 1-(4-chlorophenyl)-6-hydroxyl-1,2,3,4-
Received 18th November 2013
Accepted 1st April 2014
tetrahydroisoquinoline
and
6-hydroxyspiro[1,2,3,4-tetrahydroisoquinoline-1:10-cyclohexane],
also
displayed low cytotoxicity, against rat skeletal myoblast cells, with CC50 values of 257.6 and 174.2 mM
respectively. These results justify further investigation of simple 1-aryl-1,2,3,4-tetrahydroisoquinolines as
potential anti-malarial agents.
DOI: 10.1039/c3ra46791k
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drugs.6 Attempts to address the problem of resistance have resul-
ted in the current shi to artemisinin based combination drug
Introduction
regimens. New anti-malarial drugs, especially those with new
structures and novel modes of action against malaria are also
needed. This work describes the synthesis of structurally simple
tetrahydroisoquinolines (THIQs) and their preliminary evaluation
as potential anti-malarial agents. THIQs abound in nature and
have attracted signicant attention due to their diverse biological
activities. The THIQ substructure (Fig. 1) is found in many drugs
and alkaloids that exhibit antitumor, cardiovascular, antibacterial,
antimicrobial, antitubercular, antifungal, antileishmanial, anti-
trypanosomal and antiplasmodial activities.7–20 These compounds
include the naphthylisoquinolines, benzylisoquinolines and
bisbenzylisoquinolines.
Malaria is a life threatening infectious parasitic disease that inicts
both health and economic losses.1 The disease is transmitted by
mosquitoes of the genus Anopheles. The causative agent for
malaria is the Plasmodium species, and there are four types of
human malaria: P. vivax: P. malaria, P. ovale and P. falciparum. P.
vivax and P. falciparum are the most common, and P. falciparum is
responsible for the most deadly type of malaria infection.2 Statis-
tics indicate that P. falciparum causes more than 300 to 500 million
clinical cases of malaria infection, resulting in 1.7 to 2.5 million
deaths annually.3 Malaria is a particularly important disease in
Sub-Saharan Africa where about 90% of cases and death occur, but
is also a serious public health burden in South, East and Central
Asia, South and Central America, Caribbean, Oceania, and Middle
East regions.4 Many attempts have been made to control the
disease by using vector control measures and/or chemoprophy-
laxis, but they have had limited success.5 The control of malaria is,
among other factors, hampered by the emergence of resistance of
the vector (Anopheles sp.) to dichlorodiphenyltrichloroethane
(DDT) and other insecticides, and the ongoing spread of strains of
P. falciparum that are resistant to currently used anti-malarial
The broad spectrum of biological activities of tetrahy-
droisoquinolines, especially reports on the anti-human immu-
nodeciency virus (HIV) activity of michellamine B (1) and
antiplasmodial activity of dioncophylline
C (3), naph-
thylisoquinolines (Fig. 2) isolated from Ancistrocladus kor-
upensis,11 prompted us to investigate 1-aryl-1,2,3,4-tetra-
hydroisoquinolines, as abbreviated analogues of the naturally
occurring compounds, for antiplasmodial activity. Naph-
thylisoquinoline alkaloids are known to exhibit curative
aDepartment of Chemistry, Faculty of Science, University of Buea, P.O. Box 63, Buea,
South West Region, Cameroon. E-mail: smbuangalefange@gmail.com; Tel: +237
99915610
bDepartment of Medical Parasitology and Infection Biology Parasite Chemotherapy,
Swiss Tropical Institute, Basel, Switzerland
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra46791k
‡ Current address: Department of Chemistry, Faculty of Science, University of
Douala, P. O. Box 24157, Douala, Littoral Region, Cameroon.
Fig. 1 1,2,3,4-Tetrahydroisoquinoline.
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Fig. 2 Michellamine B (1) and dioncophyllines B (2), C(3), D(4) and E(5) with the tetrahydroisoquinoline scaffold shown in red.
potentials against malaria.12–14 Dioncophylline
E (5) has 1-aryl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinolines and 1-aryl-
exhibited good antimalarial activity against both chloroquine- 6,7-dimethoxy-1,2,3,4-tetrahydroisquinolines, display anti-
sensitive and -resistant strains of P. falciparum in vitro,13 while HIV15 and bronchodilator16 properties. The current study, which
dioncophylline C was active against the chloroquine-resistant P. forms part of a larger investigation, was undertaken to investi-
berghei Anka CRS parasites12 with dioncophylline B (2), dio- gate the antiplasmodial activity of 1-aryl-1,2,3,4-tetrahy-
ncophylline D (4) and synthetic naphthylisoquinolines also droisoquinolines. The THIQ skeleton presents eight positions
found to display good in vitro antiplasmodial activities.14
A
that can accommodate substituents. The choice of positions 1
noteworthy example of a structurally simple THIQ is the spi- and 6 for this study was based on the ease of synthesis of 1-
rofused compound IVa, which was previously described by substituted-THIQs (which is offered by the Pictet Spengler) and
Kametani et al.,21 as an abbreviated analogue of erysodine, one the presence of alkoxy groups on the aromatic portion of
member of the Erythrina class of alkaloids. Plants of the genus complex THIQs like the naphthoisoquinolines, such as 2–5,
Erythrina have been used extensively in ethnomedicine for a which display antiplasmodial activity.
number of applications including the treatment of liver disor-
The target 6-hydroxy-1,2,3,4-tetrahydroisoquinoline deriva-
ders,20 as antiplasmodial agents, analgesics, and anti-inam- tives (IIIa to IIIu and IVa) were synthesized via base-catalyzed
matory agents,21 as well as in the treatment of diseases such as Pictet–Spengler cyclization as previously reported.19 Overall
female infertility, stomach pain and gonorrhea.22 Moreover, the yields over two steps ranged between 4 and 99%. The
alkaloids of Erythrina type are characterized by their unique compounds were characterized by NMR and mass spectrometry.
skeleton of a tetracyclic spiroamine.23 They are classied in two The corresponding hydrochlorides were subsequently tested in
main groups: alkaloids possessing a skeleton of a 6,5,6,6 vitro for antiplasmodial activity against P. falciparum. An in silico
indoloisoquinoline called erythrinanes and those with a skel- assessment of drug metabolism and pharmacokinetics (DMPK)
eton of 6,5,7-6 indolobenzazepine called schelhammerans or proles was also carried out by the use of computed molecular
homoerythrinane alkaloids (Fig. 3). Erythrina alkaloids are descriptors related to the absorption, distribution, metabolism,
known to display antiplasmodial activity in vitro.24–26 The ratio- excretion and toxicity (ADMET) of compounds.
nale behind the design of THIQ analogues as potential anti-
malarials is that they seem to be the common denominator of
the naturally occurring naphthoisoquinolines (1 to 5) which
display anti-malarial activities (marked in red on Fig. 2). It could
Results and discussion
Antiplasmodial activity versus cytotoxicity
therefore be easily inferred that the THIQ scaffold could contain
the pharmacophore responsible for the anti-malarial activity.
Previous studies have revealed that even simple THIQs such as
All compounds were tested in vitro against P. falciparum as
described in the Experimental section. The biological data were
plotted to obtain IC50 values which represent the concentration
Fig. 3 Homoerythrinane alkaloids.
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required to cause 50% inhibition of parasite growth. Activity concentrations that were two orders of magnitude higher than
criteria were as follows: inactive, IC50 > 5 mg mLꢀ1; moderately the concentrations at which they displayed antiplasmodial
active, 0.5 mg mLꢀ1 < IC50 < 5 mg mLꢀ1; highly active, IC50
<
activity. Therefore, structurally simplied 1,2,3,4-tetrahy-
0.5 mg mLꢀ1. The antiplasmodial activity, presented as IC50 droisoquinolines may be potentially useful as antiplasmodial
values, is provided in Table 1. The P. falciparum strain K1, agents.
known to be resistant to chloroquine and pyrimethamine, was
employed for this assay. Chloroquine was used as the reference values 0.70, 0.76 and 0.89 mM, respectively, and their activity
drug. was in the same range as that of chloroquine. In addition, the
Compounds IIIb, IIIk and IVa were the most active with IC50
Of twenty-one compounds tested, three could be described cytotoxicity of these compounds against myoblast cells could be
as inactive following the criteria established. The rest were considered as an index of drug effectiveness. Compounds IIIb,
either moderately active (twelve) or active (seven). Among the IIIk and IVa had selectivity index values 369.6, 229.1 and >451.5
active compounds at least three (IIIb, IIIk and IVa) displayed respectively. These values are up to tenfold lower than the
antiplasmodial activity comparable to that of chloroquine, selectivity index value reported for chloroquine but signicantly
suggesting the need for further evaluation of these compounds higher than the value of 100 which is required to qualify a
in vivo. In general, the more active compounds tended to compound as a “hit” in the screening of compounds for anti-
possess electron withdrawing substituents on the pendant plasmodial activity.
phenyl group (IIIb-e,j,k,q). However, a number of moderately
At 2.30 mM, IIIa was moderately active, but the presence of
active compounds contained electron donating substituents on chlorine at C-4 resulted in a signicant increase of activity,
the same phenyl group, thereby obscuring the role of electronic maybe due to the electron withdrawing property by inductive
substituent parameters as determinants of antiplasmodial effect and ortho and para directive effect. This activity is not
activity in this series. The present demonstration of anti- maintained in IIIc, IIId, IIIe, IIIj, IIIm, IIIn, IIIo, IIIp and IIIq.
plasmodial activity in the compound would therefore suggest When a methoxyl group (electron donating by resonance) is
that the key pharmacophoric elements for antiplasmodial introduced in compound IIIa to get IIIf, only a slight drop in
activity in this class of alkaloids reside in the skeleton as activity is observed. When additional methoxyl groups are
described by IVa.19
introduced, (IIIg, IIIh, IIIi) the activity drops even further. In
When tested for cytotoxicity, several of the compounds were general, the effects of both mono- and di-substitution on the
found to be considerably less toxic against rat skeletal myco- pendant phenyl group appeared to be largely dependent on the
blast cells than the reference compound podophyllotoxin. In nature of the substituent(s), but C4-substitution, especially with
addition, the tested compounds displayed toxicity at electron withdrawing groups of moderate size, appeared to be
Table 1 In vitro cytotoxicity and antiplasmodial activities of the synthesized 1-aryl-1,2,3,4-tetrahydroisoquinolines, along with selectivity indicesa
IC50 P. falciparum
IC50 P. falciparum
K1 (mM)
Cytotoxicity
Cytotoxicity
L6 (mM)
Selectivity
index
Compounds
Substituent (R)
K1 (mg mLꢀ1
)
L6 (mg mLꢀ1
)
IIIa
IIIb
IIIc
IIId
IIIe
IIIf
IIIg
IIIh
IIIi
IVa
IIIj
IIIk
IIIl
IIIm
H
4-Cl
3-Cl
3,4-Cl2
3-NO2
3-OCH3
2,3-(OCH3)2
2,5-(OCH3)2
2,6-(OCH3)2
Spiro
0.519
0.181
0.347
0.395
0.347
0.646
0.892
1.22
2.304
0.697
1.336
1.343
1.284
2.530
3.126
4.276
>20
0.885
1.581
0.760
3.006
NA
>90
>400
257.6
188.3
98.9
>400
332.5
>400
>400
>400
>400
155.9
174.2
10.9
>200
369.6
140.9
73.7
>300
131.4
>100
>100
>200
>400
98.6
229.1
3.6
66.9
48.9
29.1
>90
84.9
>90
>90
>90
>90
47.44
51.1
3.29
NA
>5
0.193
0.481
0.223
0.906
NA
4-Br
4-CF3
4-Phenyl
2-Br
NA
NA
IIIn
IIIo
IIIp
IIIq
IIIr
IIIs
IIIt
IIIu
2-F
3-Br
3-F
4-F
4-Cl-3-NO2
5-Br-2-OCH3
2-OH-5-NO2
4-CH3
2.31
1.08
1.59
0.595
2.05
0.744
10.3
0.891
0.061
9.495
3.550
6.787
2.446
6.727
2.226
35.978
3.955
0.192
28.3
9.06
33.1
36.5
24.9
9.18
19.5
21.6
NA
116.3
29.8
141.3
150.0
81.7
27.5
68.1
95.9
NA
12.3
8.4
20.8
61.3
12.1
12.3
1.9
24.2
3000
Chloroquine
Podophyllotoxin
0.004
a
NA: not available.
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the most favored substitution pattern (compare IIIq vs IIIp vs appears to be the molecular target responsible for the cardiac
IIIn; IIIb vs IIIc and IIIj vs IIIo). The favorable antiplasmodial toxicity of a wide range of therapeutic drugs.32 HERG has also
activity of spirocylohexyl analogue IVa may be attributed to the been associated with modulating the functions of some cells
increased rigidity of this analogue.
of the nervous system and with establishing and maintaining
cancer-like features in leukemic cells.32 Thus, HERG K+
channel blockers are potentially toxic and the predicted IC50
values oen provide reasonable predictions for cardiac toxicity
In silico pharmacokinetics proles
Molecular modelling was carried out on all synthesised of drugs in the early stages of drug discovery.33
molecules as described in the Experimental section. The The results for een (15) relevant descriptors have been
computed molecular descriptors are those related to the shown in Table 2, while 2D scatter plots of selected molecular
prediction of DMPK/ADMET properties like the n-octanol– descriptors are shown in Fig. 4. It was observed that only the
water partition coefficient, aqueous solubility, brain–blood points of subgure C fell outside the recommended range for
partition coefficient, Caco-2 cell permeability, serum protein the dipole moment parameter. Subgures A and B showed that
binding, number of likely metabolic reactions, solvent acces- all the compounds complied with Lipinski's ro5, since the
sible surface area, blockage of human-ether-a-go-go potassium molecular weights (MW) < 500 Da, logarithm of n-octanol–water
ion (HERG K+) channels and the total volume of the molecule partition coefficient (log P) < 5, number of hydrogen bond
enclosed by the solvent accessible surface area. The bioavail- acceptors (accHB) < 10 and number of hydrogen bond donors
ability of a compound depends on the processes of absorption (donHB) < 5. It was observed that the other computed molecular
and liver rst-pass metabolism.27 Absorption in turn depends descriptors of all the THIQ analogues fell within the recom-
on the solubility and permeability of the compound, as well as mended range for 95% of known drugs (recommended
interactions with transporters and metabolizing enzymes in parameters are shown in the ESI†), except for two compounds
the gut wall. The computed parameters used to assess oral (IIIo and IIIq), as can be seen in subgures 4D-L, corresponding
absorption are the predicted aqueous solubility, log Swat, the respectively to the scatter plots for MW against the number of
conformation-independent predicted aqueous solubility, rotatable bonds (#rotor), cohension index in solids, blood–brain
CI log Swat, the predicted qualitative human oral absorption, barrier partitioning, HERG K+ ion channel blockage, binding
the predicted % human oral absorption and compliance to affinities to human serum albumin, skin permeability, aqueous
Lipinski's famous “Rule of Five” (ro5).28 The size of a molecule, solubilities, as well Caco-2 and MDCK permeabilities. As for
as well as its capacity to make hydrogen bonds, its overall compound IIIo, the dipole moment was computed to be 0.524
lipophilicity and its shape and exibility are important prop- Debye, below the recommended range (1.0 to 12.5 Debye), while
erties to consider when determining permeability. Molecular for compound IIIq, the computed value was 0.890 Debye, only
exibility has been seen as a parameter which is dependent on slightly below the recommended minimum for 95% of known
the number of rotatable bonds (NRB), a property which drugs. The weak dipole moments could be explained by the
inuences bioavailability in rats.29 The blood–brain partition absence of polar groups in the pendant phenyl ring of the two
coefficients (log B/B) were computed and used as a predictor aforementioned compounds. Dipole moments as high as >6
for access to the central nervous system (CNS). Madin–Darby Debye were observed for compounds IIIe, IIIr and IIIt contain-
canine kidney (MDCK) monolayers, are widely used to make ing the polarising nitro group in the pendant phenyl ring. It is
oral absorption estimates, the reason being that these cells however important to mention that the most active compounds
also express transporter proteins, but only express very low (IIIb, IIIk and IVa) had interesting predicted pharmacokinetic
levels of metabolizing enzymes.29 They are also used as an proles (with all computed parameters falling within the rec-
additional criterion to predict BBB penetration. Thus, our ommended range for 95% of known drugs, i.e., #stars ¼ 0). We
calculated apparent MDCK cell permeability could be consid- can therefore conclude that these three compounds could be
ered to be a good mimic for the BBB (for non-active transport). subjected to further investigation as potential anti-malarial
The efficiency of a drug may be affected by the degree to which drug leads.
it binds to the proteins within blood plasma. Binding of drugs
to plasma proteins (like human serum albumin, lipoprotein,
glycoprotein, a, b' and g globulins) greatly reduces the quan-
tity of the drug in general blood circulation and hence the less
Experimental
General experimental procedures
bound a drug is, the more efficiently it can traverse cell The synthesis of organic compounds was carried out with
membranes or diffuse. The predicted plasma-protein binding commercially available chemicals that were used as received
has been estimated by the prediction of binding to human (Scheme 1). All target compounds were tested in the form of the
serum albumin; the log KHSA parameter (recommended range hydrochlorides which were obtained by dissolution of the cor-
is ꢀ1.5 to 1.5 for 95% of known drugs). Human ether-a-go-go responding free base in ethanolic HCl. Nuclear Magnetic
related gene (HERG) encodes a potassium ion (K+) channel Resonance spectroscopy data were recorded using UNITYplus –
that is implicated in the fatal arrhythmia known as torsade de 500 spectrometer (1H NMR at 500 MHz). Chemical shis are
pointes or the long QT syndrome.30 The HERG K+ channel, denoted in unit parts per million (ppm) relative to the solvent
which is best known for its contribution to the electrical (1H NMR peak: 3.30 for CD3OD). The following abbreviations
activity of the heart that coordinates the heart's beating, are used to indicate peak multiplicities and characteristics: s
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Fig. 4 2D pairwise scatter plots of selected ADMET properties: (A) molecular weight against lipophilicity, (B) molecular weight against number of
hydrogen bond acceptors, (C) molecular weight against dipole moments, (D) molecular weight against number of rotatable bonds, (E) molecular
weight against cohesion indices, (F) molecular weight against blood–brain barrier penetration coefficient, (G) molecular weight against predicted
logIC50 of HERG channel blockage, (H) molecular weight against predicted binding coefficient to human serum albumin, (I) molecular weight
against skin penetration parameter, (J) molecular weight against aqueous solubility, (K) molecular weight against Caco-2 penetration, (L)
molecular weight against Madin–Darby canine kidney parameter.
(singlet), d (doublet), dd (doublet of doublets), t (triplet), q aluminum plates (Merck) precoated with silica gel 60 F254 (0.2
(quartet), m (multiplet). Mass spectra were recorded on a BIOTO mm thickness). Visualization of spots was performed with UV
FIIEST mass spectrometer. TLC analyses were carried out on light or by treatment with iodine.
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and 3-chlorobenzaldehyde (2.04 g, 14.5 mmol)], 1.6 g, (47%);
mp, 210 ꢂC 1H NMR (CD3OD) 3.08–3.53 (m, 4, –(CH2)2), 4.91 (s.1,
OH), 5.73 (s, 1, –CH–N), 6.64–7.51 (m, 7, aryl).
1-(3,4-Dichlorophenyl)-6-hydroxy-1,2,3,4-tetrahy-
droisoquinoline (IIId). Yield [from 3-methoxyphenethylamine
(2.0 g, 13 mmol) and 3,4-dichlorobenzaldehyde (2.54 g, 14.5
1
ꢂ
mmol)], 2.4 g (62%); mp, 240 C. H NMR (CD3OD) 3.08–3.53
(M, 4 –(CH2)2), 4.92 (s, 1, OH), 5.77 (s, 1, –CH–N), 6.65–7.65(m, 6,
aryl).
6-Hydroxy-1-(3-nitropheny)-1,2,3,4-tetrahydroisoquinoline
(IIIe). Yield [from 3-methoxyphenethylamine (2.0 g, 13 mmol)
and 3-nitrobenzaldehyde (2194 g, 14.5 mmol)] 2.90 g (84%). 1H
NMR (CD3OD) 3.12–3.56 (m, 4, –(CH2)2), 4.98 (s, 1, OH), 5.93 (s,
ꢂ
1, –CH–N), 6.65–8.37 (m, 7, aryl), mp, 248–250 C.
6-Hydroxy-1-(3-methoxyphenyl)-1,2,3,4-tetrahydroisoquino-
line (IIIf). Yield [from 3-methoxyphenethylamine (2.0 g, 13
mmol) and 3-methoxybenzaldehyde (1.98 g, 14.5 mmol)], 120
1
ꢂ
mg (4%); mp, 264–266 C. H NMR (CD3OD) 3.07–3.55 (m, 4,
(–CH2)2), 3.80 (s, 3, methoxyl), 4.89 (s, 1, OH), 5.65 (s, 1, –CH–N),
6.36–7.40 (m, 7, aryl). MS [M + H]+ 256.13 corresponding to
molecular formula (C16H18NO2).
1-(2,3-Dimethoxyphenyl)-6-hydroxy-1,2,3,4-tetrahydroisoquino-
line (IIIg). Yield [from 3-methoxyphenethylamine (2.0 g, 13 mmol)
and 2,3-dimethoxybenzaldehyde (2.41 g, 14.5 mmol)], 3.6 g (96%);
mp, 240 ꢂC. 1H NMR (CD3OD) 3.05–3.54 (m, 4, –(CH2)2), 3.52–3.89
(s, 6, methoxyl), 4.89 (s, 1, OH), 5.71 (s, 1 –CH–N), 6.63–7.19 (m, 6,
aryl). MS [M + H]+ 286.146 corresponding to (C17H20NO3).
1-(2,5-Dimethoxyphenyl)-6-hydroxy-1,2,3,4-tetrahydroisoquino-
line (IIIh). Yield [from 3-methoxyphenethylamine (2.0 g, 13 mmol)
and 2,5-dimethoxy benzaldehyde (2.4 g, 14.5 mmol)] (3.6 g, 98%);
mp, 260 ꢂC. 1H NMR (CD3OD) 306–3.46 (m, 4, –(CH2)2), 3.69–3.83
(s, 6, methoxyls), 4.89 (s, 1, OH), 5.82 (s, 1, –CH–N), 6.58–7.09 (m,
6, aryl). MS [M + H]+ 286.146 corresponding to (C17H20NO3).
1-(2,6-Dimethoxyphenyl)-6-hydroxy-1,2,3,4-tetrahydroisoquino-
Scheme
analogues.
1 Synthesis of 6-hydroxyl-1,2,3,4-tetrahydroisoquinoline
General procedure for the synthesis of 1-aryl-6-hydroxy-
1,2,3,4-tetrahydroisoquinolines
6-Hydroxy-1-phenyl-1,2,3,4-tetrahydro-isoquinoline (IIIa).
The synthesis followed the procedure earlier described by Kame- line (IIIi). Yield [from 3-methoxyphenethylamine (2.0 g, 13 mmol)
tani et al.21 A mixture of 3-methoxyphenethylamine (2.0 g, 13 and 2,6-dimethoxybenzaldehyde (2.81 g, 14.5 mmol)], (3.70 g,
mmol), hydrobromic acid (10 mL) and acetic acid (10 mL) was 99%); mp, 260–266 ꢂC. 1H NMR (CD3OD) d 2.99–3.69 (m, 4,
stirred and heated under reux for six (6) hours. The cooled –(CH2)2), 3.79 (s, 6, methoxyl), 4.89 (s, 1, OH), 6.05 (s, 1, –CH–N),
mixture was concentrated under reduced pressure to obtain a 6.42–7.46 (m, 6, aryl). MS [M + H]+ 286.146 corresponding to
residue. To this residue, benzaldehyde (1.96 g, 14.5 mmol), tri- C17H20NO3.
ethylamine (1 mL), and ethanol (10 mL) were added. The resulting
6-Hydroxyspiro[1,2,3,4-tetrahydroisoquinoline-1:10-cyclo-
mixture was stirred, heated under reux for 10 hours and hexane] (IVa). Yield [from 3-methoxyphenethylamine (2.0 g, 13
concentrated to remove the solvent. The residue obtained aer mmol) and cyclohexanone (4 g, 14.5 mmol)], 250 mg (10%); mp,
evaporation was diluted with methylene chloride (100 mL) and 240 ꢂC for the hydrochloride analogue. 1H NMR (CD3OD) 1.49–
water (100 mL) with resulting precipitation of solid. The solid was 2.08 (M, 10, cyclohexyl), 3.05–3.47 (t, 4, –(CH2)2), 4.89 (s, 1, OH),
collected by ltration, washed with acetone (3 ꢁ 10 mL) and air 6.60–7.26 (m, 3, aromatic protons). MS [M + H]+ 218.154 cor-
dried to afford IIIa as a white solid (2.80 mg, 10%); mp 260 ꢂC 1H responding to (C14H20NO).
NMR (CD3OD) 3.08–3.58 (m, 4, –(CH2)2), 4.923 (s, 1, OH), 5.07 (s, 1,
1-(4-Bromophenyl)-6-hydroxy-1,2,3,4-tetrahydroisoquinoline
–CH–N), 6.64–7.48 (m, 8, aryl), melting point. In subsequent runs, (IIIj). Yield [from 3-methoxyphenethylamine (2.0 g, 13 mmol)
the Pictet–Spengler reaction was allowed to run for up to 24 hours. and 4-bromobenzaldehyde (0.875 g, 4.73 mmol)], (0.68 g, 52%).
1-(4-Chlorophenyl)-6-hydroxy-1,2,3,4-tetrahydroisoquinoline 1H NMR (CD3OD) d 3.09–3.52 (m, 4, –(CH2)2), 4.89 (s, 1, OH),
(IIIb). Yield [from 3-methoxyphenethylamine (2.0 g, 13 mmol) 5.71 (s, 1, –CH–N), 6.20–7.59 (m, 7, aryl). MS [M + H]+ 304.0344
and 3-chlorobenzaldehyde (2.04 g, 14.5 mmol)], 2.70 g (79%); corresponding to C15H15NOBr.
mp, 270–280 ꢂC. 1H NMR (CD3OD) 3.08–3.53 (m, 4, –(CH2)2),
4.91 (s. 1, OH), 5.73 (s, 1, –CH–N), 6.64–7.51 (m, 7, aryl).
1-(a,a,a-triuoromethylphenyl)-6-hydroxy-1,2,3,4-tetrahy-
droisoquinoline (IIIk). Yield [from 3-methoxyphenethylamine
1-(3-Chlorophenyl)-6-hydroxy-1,2,3,4-tetrahydroisoquinoline (2.0 g, 13 mmol) and a,a,a-triuorotolualdehyde (0.63 g, 3.63
(IIIc). Yield [from 3-methoxyphenethylamine (2.0 g, 13 mmol) mmol)], 0.12 g (16%). 1H NMR (CD3OD) d 3.11–3.52 (m, 4,
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–(CH2)2), 4.90 (s, 1, OH), 5.80 (s, 1, –CH–N), 6.62–7.81 (m, 7, (76%). H NMR (CD3OD) d 2.37 (s, 3, methyl), 3.06–3.53 (m, 4,
aryl). MS [M + H]+ 295.1108 corresponding to C16H15NOF3.
–(CH2)2), 4.92 (s, 1, OH), 5.63 (s, 1, –CH–N), 6.63–7.31 (m, 7,
1-(4-Biphenyl)-6-hydroxy-1,2,3,4-tetrahydroisoquinoline (IIIl). aryl).
Yield [from 3-methoxyphenethylamine (2.0 g, 13 mmol) and 4-
biphenylcarboxaldehyde (1.33 g, 7.27 mmol)], (1.40 g, 70%); mp, In vitro sensitivity assays of P. falciparum ([3H]-hypoxanthine
260–266 ꢂC. 1H NMR (CD3OD) d 2.99–3.69 (m, 4, –(CH2)2), 3.79 (s, incorporation)
6, methoxyl), 4.89 (s, 1, OH), 6.05 (s, 1, –CH–N), 6.42–7.46 (m, 6,
The test samples were assayed on a P. falciparum clone desig-
nated K1 which is chloroquine/pyrimethamine resistant. Test
samples of 10 mg were prepared in dimethyl sulfoxide (DMSO).
The testing procedure was as follows: a serial dilution factor of
aryl). MS [M + H]+ 286.146 corresponding to C17H20NO3.
1-(2-Bromophenyl)-6-hydroxy-1,2,3,4-tetrahydroisoquinoline
(IIIm). Yield [from 3-methoxyphenethylamine (2.0 g, 13 mmol)
and 2-bromobenzaldehyde (1.35 g, 7.27 mmol)], 1.1 g (55%). 1H
1 : 2 was prepared with 100 mL of medium RPMI 1640 without
NMR (CD3OD) d 3.11–3.58 (m, 4, –(CH2)2), 4.91 (s, 1, OH), 6.10
hypoxanthine 10.44 g Lꢀ1; HEPES 5.94 g Lꢀ1; Albumax® 5 g Lꢀ1
;
(s, 1, –CH–N), 6.57–7.81 (m, 7, aryl). MS [M + H]+ 304.0593
corresponding to C15H15NOBr.
neomycin 10 mL Lꢀ1 (100 U mLꢀ1); NaHCO3 50 g Lꢀ1 stock 42
mL Lꢀ1 (2.1 g Lꢀ1), the nal concentration was distributed into
96 wells of the Costar TM 96 – wells microtitre plates. To these
96 wells, 100 mL of medium, washed human red blood cells A+
(RBC) and P. falciparum mix, were added. The plates were
incubated for 48 hours at 37 ꢂC in a chamber and gassed with a
4% CO2, 3% O2, 93% N2. Aer 50 mL of medium and [3H]-
hypoxanthine (0.5 mCi) were added to each well. The plates were
then put back into the chamber and gassed with a 4% CO2, 3%
O2, and 93% N2 mix. The chamber was then placed in the
1-(2-Fluorophenyl)-6-hydroxy-1,2,3,4-tetrahydroisoquinoline
(IIIn). Yield [from 3-methoxyphenethylamine (2.0 g, 13 mmol)
and 2-uorobenzaldehyde (1 g, 7.27 mmol)], 1 g(63%). 1H NMR
(CD3OD) d 3.10–3.53 (m, 4, –(CH2)2), 4.90 (s, 1, OH), 5.97 (s, 1,
–CH–N), 6.66–7.56 (m, 7, aryl). MS [M + H]+ 244.1361 corre-
sponding to C15H15NOF.
1-(3-Bromophenyl)-6-hydroxy-1,2,3,4-tetrahydroisoquinoline
(IIIo). Yield [from 3-methoxyphenethylamine (2.0 g, 13 mmol)
and 3-bromobenzaldehyde (1.35 g, 7.27 mmol)], 1.22 g (55%).
1H NMR (CD3OD) d 3.08–3.53 (m, 4, –(CH2)2), 4.89 (s, 1, OH),
5.72 (s, 1, –CH–N), 6.64–7.66 (m, 7, aryl). MS [M + H]+ 304.0626
corresponding to C15H15NOBr.
1-(3-Fluorophenyl)-6-hydroxy-1,2,3,4-tetrahydroisoquinoline
(IIIp). Yield [from 3-methoxyphenethylamine (2.0 g, 13 mmol)
and 3-uorobenzaldehyde (1 g, 7.27 mmol)], 1.1 g (69). 1H NMR
(CD3OD) d 3.09–3.54 (m, 4, –(CH2)2), 4.89 (s, 1, OH), 5.75 (s, 1,
–CH–N), 6.64–7.54 (m, 7, aryl). MS [M + H]+ 244.1375 corre-
sponding to C15H15NOF.
ꢂ
incubator for 24 hours at 37 C. The assay was terminated by
harvesting the content of each microtiter plate. Data were
transferred into a graphic program (Excel) and analyzed to
determine the IC50. The test scores were grouped as follows;
inactive (no repeat) IC50 > 5 mg mLꢀ1; moderate activity (repeat)
0.5 mg mLꢀ1 < IC50 < 5 mg mLꢀ1; high activity (repeat) IC50 < 0.5
mg mLꢀ1
.
34,35
In vitro sensitivity assays (cytotoxicity)
The test samples were submitted to L-6 (rat skeletal myoblast
cells). Compounds were prepared in DMSO the testing proce-
dure was as follows. A sample of 100 mL medium (RPMI + 10%
FCS + 1.7 mML – glutamine (850 mL 200 mM for 100 mL)), was
added to wells of a microtiter plate. Then 100 mL of a cell
suspension of 4 ꢁ 104 cells per mL was added. Aer a serial
dilution factor of 1 : 3 was prepared into the wells. The plates
1-(4-Fluorophenyl)-6-hydroxy-1,2,3,4-tetrahydroisoquinoline
(IIIq). Yield [from 3-methoxyphenethylamine (2.0 g, 13 mmol)
and 4-uorobenzaldehyde (1 g, 7.27 mmol)], (1.2 g, 75%). 1H
NMR (CD3OD) d 3.08–3.52 (m, 4, –(CH2)2), 4.92 (s, 1, OH), 5.73
(s, 1, –CH–N), 6.65–7.44 (m, 7, aryl). MS [M + H]+ 244.1385
corresponding to C15H15NOF.
1-(4-Chloro-3-nitrophenyl)-6-hydroxy-1,2,3,4-tetrahydroisoquino-
line (IIIr). Yield [from 3-methoxyphenethylamine (2.0 g, 13 mmol)
and 4-chloro-3-nitrobenzaldehyde (1.35 g, 7.27 mmol)], 0.7 g (35%).
1H NMR (CD3OD) d 3.07–3.53 (m, 4, –(CH2)2), 4.91 (s, 1, OH), 5.88 (s,
1, –CH–N), 6.69–8.03 (m, 6, aryl).
ꢂ
were then incubated for 70 hours at 37 C/5%CO2.
In silico modelling
Each compound was sketched using the GaussView soware
1-(5-Bromo-2-methoxyphenyl)-6-hydroxy-1,2,3,4-tetrahydro- and geometry optimisation was carried out using the Gaussian
isoquinoline (IIIs). Yield [from 3-methoxyphenethylamine (2.0 09W package36 with the density functional theory (DFT) using
g, 13 mmol) and 5-bromo-o-anisaldehyde (1.6 g, 7.27 mmol)], the B3LYP/6-31+G(d,p) approach until convergence was
(1.0 g, 46%). 1H NMR (CD3OD) d 3.10–3.45 (m, 4, –(CH2)2), 3.90 reached. This was necessary in order to generate good starting
(s, 3, methoxyl), 4.90 (s, 1, OH), 5.88 (s, 1, –CH–N), 6.69–7.62 (m, geometries for the 3D models. The low-energy 3D chemical
6, aryl).
structures were saved in mol2 format and initially treated with
1-(2-Hydroxy-5-nitrophenyl)-6-hydroxy-1,2,3,4-tetrahydro- LigPrep,37 distributed by Schrodinger, Inc. This implementation
isoquinoline (IIIt). Yield [from 3-methoxyphenethylamine (2.0 was carried out with the graphical user interface (GUI) of the
g, 13 mmol) and 2-hydroxy-5-nitrobenzaldehyde (1.22 g, 7.27 Maestro soware package,38 using the OPLS force eld.39–41 The
1
mmol)], 0.32 g (17.78%). H NMR (CD3OD) d 3.07–3.52 (m, 4, treatment with LigPrep was necessary in order to correct
–(CH2)2), 4.90 (s, 2, OH), 5.96 (s, 1, –CH–N), 6.68–8.27 (m, 6, aryl). protonation states and set the right parameters necessary for
Synthesis of 6-hydroxy-1-(4-methylphenyl)-1,2,3,4-tetrahy- running QikProp.42 Protonation states at biologically relevant
droisoquinoline (IIIu). Yield [from 3-methoxyphenethylamine pH were correctly assigned (group I metals in simple salts were
(2.0 g, 13 mmol) and p-tolualdehyde (0.9 g, 7.27 mmol)], 1.2 g disconnected, strong acids were deprotonated and strong bases
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protonated, while explicit hydrogens were added). A set of the
ADMET-related properties (a total of 46 molecular descriptors)
9 M. Strolin-Bendetti, P. Doistert and P. J. Cannitiati, J. Neural
Transm., 1989, 78, 43.
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property descriptors computed by QikProp, which fall outside
and G. Bringmann, Antimicrob. Agents Chemother., 1997,
41, 2533.
¨
¨
the optimum range of values for 95% of known drugs. The 13 G. Bringmann, K. Messer, K. Wolf, J. Muhlbacher, M. Grune,
methods implemented were developed by Jorgensen et al.43–45
R. Brun and A. M. Louis, Phytochemistry, 2002, 60, 389.
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¨
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Conclusions
A
series of 1-aryl-6-hydroxy-1,2,3,4-tetrahydroisoquinoline
analogues was synthesized using simple methodology. To the
best of our knowledge this is the rst time the antiplasmodial of
these analogues are reported. Following the criteria established
by the WHO/TDR screening program,46 a compound is consid-
ered active against P. falciparum when in vitro IC50 is <1 mM (or
0.2 mg mLꢀ1) and SI > 100. In the present study, three
compounds (IIIb, IIIk and IVb) fell within this category and
could be described as antimalarial hits from which more potent
THIQ analogues could be obtained by further investigation.
Additionally, in vivo and mechanistic studies of these
compounds are underway. The study clearly demonstrates that
1-aryl-6-hydroxy-1,2,3,4-tetrahydroisoquinolines display suffi-
cient antiplasmodial activity and favourable predicted phar-
macokinetic properties to warrant further investigation of their
antiplasmodial activity.
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