Synthesis and trypanocidal action of new adamantane substituted imidazolines

Article abstract of DOI:10.1039/c3md00081h

Introduction of the hydrophobic substituents cyclopentyl, cyclohexyl and phenyl into adamantane imidazolines was accomplished with the synthesis of compounds 4. Members of this series were found to display a range of activities against bloodstream forms of Trypanosoma brucei. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3md00081h

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Synthesis and trypanocidal action of new adamantane
substituted imidazolines†
Cite this: Med. Chem. Commun.,
2013, 4, 856
a
a
a
Ana Koperniku, Ioannis Papanastasiou, George B. Foscolos, Andrew Tsotinis,a
*
Martin C. Taylorb and John M. Kellyb
Received 13th March 2013
Accepted 31st March 2013
Introduction of the hydrophobic substituents cyclopentyl, cyclohexyl and phenyl into adamantane
imidazolines was accomplished with the synthesis of compounds 4. Members of this series were found
to display a range of activities against bloodstream forms of Trypanosoma brucei.
DOI: 10.1039/c3md00081h
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extent amantadine, are effective against the bloodstream forms
of T. brucei. These adamantane amino derivatives readily cross
Introduction
the BBB and are well absorbed by the gastrointestinal tract.15
Furthermore, other adamantane derivatives with hydrophobic
substituents on the adamantane ring show increased trypano-
cidal activity.16
We have previously reported17 the synthesis of compounds 1
and 2 (Fig. 1). The non-spiro analogues 2 were found to be
potent against bloodstream form T. brucei.
Human African trypanosomiasis (HAT) is caused by tsetse y
transmitted protozoan parasites of the Trypanosoma brucei
species complex. HAT is endemic in several areas of Sub-
Saharan Africa and subject to epidemic outbreaks. Drugs used
to treat1 HAT are thought to target a range of biological mole-
cules. Elfornithine2,3 is a specic irreversible inhibitor of the
rst step in polyamine biosynthesis, which is the formation of
putrescine from ornithine by ornithine decarboxylase. Eorni-
thine is relatively expensive and requires administration over 14
days, under medical supervision. Suramin4,5 is a large polyanion
which exerts inhibitory activity on a variety of enzymes involved
in several protozoan metabolic pathways. Melarsoprol6,7 and its
metabolite melarsen oxide inhibit many enzymes and
substrates that contain vicinal thiol groups, e.g. trypanothione,
and disrupt the intracellular thiol-redox balance in T. brucei.
Melarsoprol is extremely toxic, with reactive encephalopathy
killing up to 5% of patients. Diamidines8–10 are nucleic acid
binding drugs that target the kinetoplast. The trypanocidal
activity of pentamidine is the result of selective accumulation
leading to multiple lethal effects, rather than a single effect on a
specic “diamidine target”. More recently, nifurtimox–eorni-
thine combination therapy (NECT) has been introduced and
has led to some improvements in treatment.1 Other promising
drug targets are N-myristoyltransferase,11 protozoan caspases12
and kinases.13
Fig. 1 Amantadine, rimantadine, adamantane analogues 1, 2 and 3.
It has been reported that various anti-Infuenza A virus ami-
noadamantane derivatives,14 e.g. rimantadine and to a lesser
aFaculty of Pharmacy, Department of Pharmaceutical Chemistry, University of Athens,
Panepistimioupoli-Zografou, 157 71 Athens, Greece. E-mail: gfosc@pharm.uoa.gr;
Fax: +30 210 7274747; Tel: +30 210 7274527
bDepartment of Pathogen Molecular Biology, London School of Hygiene and Tropical
Medicine, London WC1E 7HT, UK. E-mail: john.kelly@lshtm.ac.uk; Fax: +44 (0)207
6368739; Tel: +44 (0)207 9272330
† Electronic supplementary information (ESI) available: Experimental data for the
compounds, materials and methods for pharmacological assay. See DOI:
10.1039/c3md00081h
Fig. 2 5-(3-Substituted-1-adamantyl)-2-imidazolines 4a–s.
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In this communication, we extend our previous work to 5-(3-
substituted-1-adamantyl)-2-imidazolines of the general type 4
(Fig. 2), which, besides the imidazoline ring, also bear hydro-
phobic substituents at C3 of the adamantane skeleton. This
increase in lipophilicity is expected to lead to more active try-
panocidals, as in the case of previously reported analogues 3.16
Results and discussion
The synthesis of 2-imidazolines 4a–s was accomplished by the
reaction shown in Scheme 1.
The imidazolines 4a, 4b, 4g, 4j, 4k, 4p and 4r and the 2-
methyl-imidazolines 4c, 4d, 4l and 4m were prepared by
condensing 1-(1-adamantyl)ethylenediamines 5a–c with either
formamidine acetate or acetamidine hydrochloride in ethanol.
The free bases that were obtained were then transformed to
either the corresponding hydrochloride or fumarate.
Treatment of diamines 5a–h with cyanogen bromide led to 2-
amino-2-imidazolines (R2 ¼ NH2) (4e, 4f, 4h, 4i, 4n, 4o, 4q and 3s)
in the form of hydrobromides. The preparation of ethylenedi-
amines 5 was realized by reduction of the corresponding a-
aminoacetonitriles 6a–f, under the appropriate conditions, as
illustrated in Scheme 2. Catalytic hydrogenation of a-amino-
nitriles 6a and 6b under heat and pressure in the presence of
PtO2 in ethanol gave the diamines 5a and 5b, respectively.
Conversely, prolonged hydrogenation of the phenyl-substituted
nitriles 6c and 6d, under the same conditions, led to cyclo-
hexyldiamines 5c and 4d. Under less harsh conditions, however,
the catalytic hydrogenation of nitriles 6c and 6d led to the
formation of the phenyl-substituted diamines 5e and 5f. Last,
reduction of the anilinonitrile 6e, depending on the conditions
used, gave three different products (5g, 5h and 7). Thus, N-
cyclohexyldiamine 5g was formed from 6e upon complete
hydrogenation of its benzene ring and the concomitant reduc-
tion of the CN group, when the reaction was performed at room
temperature and under pressure. The synthesis of the phenyl-
substituted diamine 5h was effected by reducing aminonitrile 6e
with BH3$THF complex. Previous attempts to prepare diamine
5h by reducing aminonitrile 5c with LiAlH4 in THF, led to the
removal of the cyano group and the formation of aniline 7.
The preparation of a-aminoacetonitriles 6 involved the
Strecker reaction18 on the 3-substituted 1-adamantanecarbox-
aldehydes 8a and 8b (Scheme 3).
Scheme 2 Reagents and conditions: (a) H2/PtO2, gas HCl in EtOH/MeOH,
50–60 psi, 60 ꢀC, 4 h; (b) H2/PtO2, gas HCl in EtOH/MeOH, 50–60 psi, 60 ꢀC, 8 h;
(c) H2/PtO2, gas HCl in EtOH, 40–50 psi, rt, 2–3 h; (d) (i) BH3 in THF, 0 ꢀC, (ii)
reflux, 2 h, (iii) H2O, 0 ꢀC, (iv) HCl 18%, reflux, 4 h; (e) (i) LiAlH4, THF, 0 ꢀC, 1 h, (ii)
H2O/NaOH, 0 ꢀC.
3-Cyclopentyl-1-adamantanecarboxaldehyde (8a) was prepared
from 3-cyclopentyl-1-adamantanecarboxylic acid (9), as shown in
Scheme 4.
The synthesis of 3-phenyl-1-adamantanecarboxaldehyde (8b)
(Scheme 5) was accomplished by the same reaction sequence
followed for the preparation of aldehyde 8a.
Scheme 3 Reagents and conditions: (a) (i) Me3SiCN, CHCl3, ZnI2, (ii) NH3/MeOH,
70–75 ꢀC, autoclave, 20 h (6a, 6c); (b) NaCN, MeNH2$HCl, DMSO/H2O, rt for 48 h
and then 60 ꢀC for 7 h (6b, 6d); (c) Me3SiCN, aniline, ACN, rt (6e).
Scheme 1 Reagents and conditions: (a) formamidine acetate (R2 ¼ H) or acet-
amidine hydrochloride (R2 ¼ CH3) in EtOH; (b) cyanogen bromide (R2 ¼ NH2).
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adamantane and a cyclohexyl substituent at N1 imidazoline. In
previous studies with other classes of adamantane derivatives,
we have found that anti-parasite activity is enhanced by these
chemical features.14,16 Interestingly, there was no general
correlation between trypanocidal activity and cytotoxicity. For
example, the addition of the amino group at the C2 position of
the imidazoline ring of the 3-cyclopentyl or 3-cyclohexyl deriv-
atives 4e, 4f and 4h signicantly reduced cytotoxicity, while
having little effect on trypanocidal activity. In contrast, with
derivative 4q, the N1 cyclohexyl substitution was associated with
enhancement of both trypanocidal activity and cytotoxicity.
These insights should provide the basis for the design of
compounds with greater selectivity indices. Calculated log P and
PSA values (Table 2 in ESI†), imply that compounds 4 should be
able to cross the BBB, as various other trypanocidals, with
analogous physicochemical properties, do.19
Scheme 4 Reagents and conditions: (a) (i) LiAlH4, THF, rt, 3 h, (ii) H2O/NaOH,
0
ꢀC; (b) PCC, DCM, 2 h.
Scheme 5 Reagents and conditions: (a) (i) LiAlH4, THF, rt, 3 h, (ii) H2O/NaOH,
0
ꢀC; (b) PCC, DCM, 2 h.
Conclusion
The results of anti-T. brucei screening are summarized in
It is clear from our results that introduction of the cyclopentyl or
cyclohexyl lipophilic substituents into the scaffold of 5-(1-ada-
mantyl)-2-imidazolines 2 increases efficacy against cultured
bloodstream forms of the African trypanosome. Both the anti-
viral and the CNS activity of aminoadamantane derivatives is
explainable in terms of their channel blocking properties. In
T. brucei however, the precise target(s) of these compounds
remain to be determined.
Table 1. In general, the introduction of a cyclopentyl or cyclo-
hexyl group in C3 of adamantane seems to increase the trypa-
nocidal activity in comparison to the previously reported
unsubstituted adamantane analogues 2. The introduction of
phenyl substitution either in C3 of adamantane or in N1 of
imidazoline causes, in all cases, a drastic reduction of trypa-
nocidal action. Cyclopentyl or cyclohexyl substituted derivatives
present similar activity with the exception of compound 4q, the
most active of this series, which bears a cyclopentyl group at C3
Experimental section
General method of preparation of compound 4a–d, 4g, 4j, 4k–m,
4p and 4r: 5-(3-substituted-1-tricyclo[3.3.1.13,7]decyl)-4,5-dihydro-
1H-imidazoles
Table 1 Activitya and cytotoxicityb of 5-(3-substituted-1-adamantyl)-2-imida-
zolines 4a–s
T. brucei
IC50 (mM)
T. brucei
IC90 (mM)
L6 cells
IC50 (mM)
Selectivity
index
The requisite diamine 5 (1.3 mmol) was dissolved in absolute
ethanol (10 mL) and formamidine acetate or acetamidine
hydrochloride (1.7 mmol) was added. The mixture was stirred
under an argon atmosphere at ambient temperature for 24 h
and at 50 ꢀC for another 24 h. The solvent was then evaporated
under vacuum and the residue obtained was treated with HCl
4% (30 mL). The resulting mixture was washed with ethyl
acetate, the aqueous phase was made alkaline with NaOH 4%
and extracted with ethyl acetate. The organic phase was washed
with brine, dried over Na2SO4 and concentrated in vacuo to give
the respective imidazoline, which was then converted to its salt,
on treatment with the appropriate acid.
Cmpd
4ac
4bc
4cd
4dd
4ee
4fe
1.57 ꢁ 0.18
1.50 ꢁ 0.03
2.16 ꢁ 0.05
2.97 ꢁ 0.24
2.17 ꢁ 0.19
1.65 ꢁ 0.10
2.89 ꢁ 0.09
1.86 ꢁ 0.08
1.72 ꢁ 0.03
7.09 ꢁ 0.32
2.42 ꢁ 0.03
1.83 ꢁ 0.03
2.77 ꢁ 0.12
4.51 ꢁ 0.15
3.70 ꢁ 0.15
5.77 ꢁ 0.72
3.56 ꢁ 0.56
3.77 ꢁ 0.33
2.4
3.9
1.6
1.3
7.9
7.1
2.5
2.63 ꢁ 0.03 17.22 ꢁ 1.63
2.23 ꢁ 0.29 11.74 ꢁ 1.96
4gd
4he
4id
4.20 ꢁ 0.09
7.22 ꢁ 1.74
2.52 ꢁ 0.03 19.00 ꢁ 2.57 10.2
2.35 ꢁ 0.03 7.39 ꢁ 0.33 4.3
9.02 ꢁ 0.10 nd
4jd
—
4kd
4ld
7.91 ꢁ 0.24 13.05 ꢁ 0.18 nd
—
—
—
—
—
1.9
5.0
—
4.88 ꢁ 0.11
8.11 ꢁ 0.06 nd
4md
4ne
4oe
4pd
4qe
4rd
4se
7.16 ꢁ 0.12 10.32 ꢁ 0.23 nd
7.39 ꢁ 0.19 11.81 ꢁ 0.19 nd
General method of preparation of compound 4e, 4f, 4h, 4i, 4n,
4o, 4q and 4s: 5-(3-substituted-1-tricyclo[3.3.1.13,7]decyl)-4,5-
dihydro-1H-2-imidazolamines
4.53 ꢁ 0.12
1.82 ꢁ 0.03
0.65 ꢁ 0.01
16.4 ꢁ 3.8
11.1 ꢁ 1.7
5.58 ꢁ 0.49 nd
2.41 ꢁ 0.06
0.84 ꢁ 0.01
24.2 ꢁ 2.5
15.5 ꢁ 0.7
3.46 ꢁ 0.18
3.22 ꢁ 0.13
nd
nd
To a stirred solution of the respective diamine 5 (2.9 mmol) in
dry dichloromethane (10 mL) a solution of cyanogen bromide
(375 mg, 3.52 mmol) in dry dichloromethane (5 mL) was added,
under an argon atmosphere, dropwise and under cooling.
—
Rimantadine 7.04 ꢁ 0.27 13.97 ꢁ 1.67
Amantadine
>132
>132
a
Derivatives 4a–s were tested against in vitro bloodstream form T. brucei Stirring was continued at ambient temperature for 48 h and the
(pH 7.4) (ESI). IC50 and IC90 data are the mean of triplicate experiments
solvent evaporated under vacuum to leave a residue, which
b
ꢁ SEM. Cytotoxicity tests were carried out using rat L6 cells (ESI).
crystallized as hydrobromide salt on treatment with anhydrous
ether.
Selectivity indices were calculated as the ratio of the IC50 values for L6
cells and T. brucei. c Fumarate. d Hydrochloride. Hydrobromide.
e
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