Direct, Late-Stage Mono-N-arylation of Pentamidine: Method Development, Mechanistic Insight, and Expedient Access to Novel Antiparastitics against Diamidine-Resistant Parasites

Article abstract of DOI:10.1002/cmdc.202100509

A selective mono-N-arylation strategy of amidines under Chan-Lam conditions is described. During the reaction optimization phase, the isolation of a mononuclear Cu(II) complex provided unique mechanistic insight into the operation of Chan-Lam mono-N-arylation. The scope of the process is demonstrated, and then applied to access the first mono-N-arylated analogues of pentamidine. Sub-micromolar activity against kinetoplastid parasites was observed for several analogues with no cross-resistance in pentamidine and diminazene-resistant trypanosome strains and against Leishmania mexicana. A fluorescent mono-N-arylated pentamidine analogue revealed rapid cellular uptake, accumulating in parasite nuclei and the kinetoplasts. The DNA binding capability of the mono-N-arylated pentamidine series was confirmed by UV-melt measurements using AT-rich DNA. This work highlights the potential to use Chan-Lam mono-N-arylation to develop therapeutic leads against diamidine-resistant trypanosomiasis and leishmaniasis.

Full text of DOI:10.1002/cmdc.202100509

Accepted Article
Title: Direct, late-stage mono-N-arylation of pentamidine: method
development, mechanistic insights, and expedient access to
novel antiparastitics against diamidine-resistant parasites
Authors: Glenn Burley, Allan John Bell Watson, Harry de Koning,
Katherine Jones, Jack Robertson, Bela Bode, Fergus
McWhinnie, Marzuq Ungogo, Mustafa Aldfer, and Leandro
Lemgruber
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemMedChem 10.1002/cmdc.202100509
Link to VoR: https://doi.org/10.1002/cmdc.202100509
01/2020

10.1002/cmdc.202100509
ChemMedChem
COMMUNICATION
Direct, late-stage mono-N-arylation of pentamidine: method
development, mechanistic insights, and expedient access to
novel antiparastitics against diamidine-resistant parasites
Jack Robertson,[a] Marzuq A. Ungogo,[b] Mustafa M. Aldfer,[b] Leandro Lemgruber,[c] Fergus S.
McWhinnie,[a] Bela E. Bode,[d] Katherine L. Jones,[e]* Allan J. B. Watson,[d]* Harry P. de Koning,[b]*
Glenn A. Burley*[a]
Abstract: A selective mono-N-arylation strategy of amidines under
Chan–Lam conditions is described. During the reaction optimization
phase, the isolation of a mononuclear Cu(II) complex provided unique
mechanistic insight into the operation of Chan–Lam mono-N-arylation.
The scope of the process is demonstrated, and then applied to access
the first mono-N-arylated analogues of pentamidine. Sub-micromolar
activity against kinetoplastid parasites was observed for several
analogues with no cross-resistance in pentamidine and diminazene-
resistant trypanosome strains and against Leishmania mexicana. A
fluorescent mono-N-arylated pentamidine analogue revealed rapid
cellular uptake, accumulating in parasite nuclei and the kinetoplasts.
The DNA binding capability of the mono-N-arylated pentamidine
series was confirmed by UV-melt measurements using AT-rich DNA.
This work highlights the potential to use Chan-Lam mono-N-arylation
to develop therapeutic leads against diamidine-resistant
trypanosomiasis and leishmaniasis.
Amidines are essential functional groups used throughout
medicinal chemistry.[1] The versatility of this motif arises from their
ability to form strong, bifurcated hydrogen bonds and electrostatic
interactions with a range of hydrogen bond acceptors and
conjugate bases.[2] In addition, modulating the basicity of the
Scheme 1. (a) Amidines in antiparasitic agents. (b) Selective amidine mono-N-
arylation and application to development of pentamidine antiparasitics. MGB =
minor groove binder.
amidine group (pKa ~13-14) alters the overall physicochemical
properties, potency, and selectivity of amidine-containing
H
therapeutics.[3] A prominent exemplar of this approach is the
development of the DNA-binding antiparasitic pentamidine for the
treatment of early-stage Trypanosoma brucei gambiense-related
human African trypanosomiasis (HAT) or sleeping sickness,[4]
AIDS-related pneumocystis pneumonia, and leishmaniasis
(Scheme 1a).[5] The broader importance of diamidine
antiparasitics is further exemplified by their use as the main
treatment for animal African trypanosomiasis (AAT), caused by
the related parasite Trypanosoma congolense, which has a
billion-dollar adverse impact on emerging economies.[6]
Poor oral bioavailability and the emergence of pentamidine-
resistant strains has spurred the exploration of new DNA-binding
analogues (Scheme 1a).[7] Modification of the essential amidine
moiety is a nascent strategy to by-pass resistance mechanisms
[a]
[b]
[c]
[d]
[e]
Dr J. Robertson, Dr F. S. McWhinnie, Prof. G. A. Burley
Department of Pure and Applied Chemistry
University of Strathclyde
295 Cathedral Street, Glasgow, G1 1XL (UK)
E-mail: glenn.burley@strath.ac.uk
Mr M. A. Ungogo, Mr M. M. Aldfer, Prof. H. P. de Koning
Institute of Infection, Immunity, and Inflammation
College of Medical, Veterinary, and Life Sciences
University of Glasgow, Glasgow, G12 8TA (UK)
Email: harry.de-koning@glasgow.ac.uk
Dr L. Lemgruber
Glasgow Imaging Facility
Institute of Infection, Immunity, and Inflammation
College of Medical Veterinary and Life Sciences
University of Glasgow, Glasgow, G12 8TA (UK)
Dr B. E. Bode, Dr A. J. B. Watson
EaStCHEM, School of Chemistry
University of St Andrews
North Haugh, St Andrews, Fife, KY16 9ST (UK)
Email: aw260@st-andrews.ac.uk
Dr K. L. Jones
GlaxoSmithKline
Medicines Research Centre
in these parasites by modulating the affinity of diamidines for key
[8]
drug transporters in T. brucei
[e.g., TbAT1/P2 aminopurine
Affinity Pentamidine Transporter
transporters,[9]
High
(HAPT)[10]].[11] However, expedient synthetic access to mono-N-
arylated amidines has been hampered by the lack of robust
synthetic methodology suitable for structure-activity relationship
profiling:[12] existing Pinner-style methodology[13] is unsuited to
this challenge due to forcing reaction conditions limiting functional
group tolerance, and the lack of chemoselective control leading to
competing di/tri-N-arylation.[14] By virtue of readily-available
starting materials, Pd-[15] and Cu-catalyzed cross couplings (e.g.,
Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY (UK)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cmdc.202100509
ChemMedChem
COMMUNICATION
Ullmann,[3c, 16] Chan-Lam[17]) provide the opportunity to selectively
access mono-N-functionalised amidine analogues.
In contrast to our experience with other Chan–Lam
processes,[22-23] reactions using 1 and K2CO3 produced a purple
precipitate within the first 5 minutes of the reaction (Figure S3). X-
Here we report a mild and chemoselective approach to
exemplify amidine mono-N-arylation via the optimization of a
Chan–Lam strategy (Scheme 1b). We expand the substrate
scope and demonstrate its application to access, for the first time,
mono-N-arylated pentamidines as anti-parasitic leads for
treatment of HAT, AAT, and leishmaniasis.[18]
A prominent challenge of amidine functionalization is
controlling mono- vs. di/tri-N-arylation.[17b, 19] Although selective
amidine N-arylation strategies have been reported, these are
ray crystallographic analysis revealed
a
C2-symmetric
mononuclear Cu(II) complex 4 (Scheme 2a) consisting of a
bidentate carbonate ligand and two monodentate benzamidines.
This unique complex suggests that K2CO3 plays a dual role in this
system, acting both as a ligand and base. A dual ligand/base role
has been speculated in some Cu(OAc)2-based Chan–Lam
reactions;[24] however, structural information has been limited to
the observation of a single tetranuclear Cu(II) complex.[22]
embedded within cascade processes alongside C–H activation
20]
events.[17b,
As such, the identification of reaction variables
(a) Identification and reaction-relevance of mononuclear Cu(II) complex 4
which influence mono- vs. di/tri-N-arylation was first established.
Under standard Chan–Lam conditions,[21] a model system
using benzamidine (1) and PhB(OH)2 gave 73% overall yield and
ca. 4:1 selectivity in favor of the mono- (2) vs. N,N’-diarylated
product 3 (Entry 1, Table 1). Using an excess of PhB(OH)2 led to
82% total yield with ca. 9:1 selectivity for 3 (Entry 2), which
suggested selective mono-N-arylation vs. N,N’-diarylation is
possible by controlling reagent stoichiometry. No tri-N-
functionalization products were observed. Vantourout’s B(OH)3-
based conditions were less effective (Entry 3).[22] Li’s NaOPiv-
based conditions provided a high conversion and ca. 40:1
selectivity for 2 (Entry 4);[17b] however, further exploration of these
conditions did not improve the conversion to 2 (Table S5). The
observation of a pronounced base effect led to a focused screen
and identification of K2CO3 as the optimal additive (Entries 5-9).
This screen ultimately provided selective mono-N-arylation set of
conditions using 20 mol% Cu(OAc)2 giving 2 in 81% isolated yield
and without any observable generation of 3 (Entry 10).
O
NH2
O
O
PhB(OH)2 (2 equiv)
Ph
Cu
H2N
Ph
NH2
i-PrOH
Ph
N
4 Å MS, 50 ºC
N
N
2, 61%
Ph
H
H
4
(b) Proposed origin of Cu(II) complex 4
H2N
Cu
Ph
S
OAc
NH
Cu
1
1
NH
NH
K2CO3
AcO
HO
HO
4
[Cu(OAc)2]●2H2O
H2N
Ph
5
6 H2N
Ph
Scheme 2. (a) Structure of Cu(II) complex 4. (b) Chan–Lam bond formation
using complex 4. (c) Proposed mechanistic origin of complex 4. S = solvent.
Exposing 4 to PhB(OH)2 in i-PrOH afforded 2 in 61% yield,
suggesting 4 is a reaction-relevant intermediate (Scheme 2a).
The role of 4 was further investigated by EPR analysis of reaction
mixtures. No EPR signals were observed when [Cu(OAc)2]2H2O
was treated with K2CO3 or PhB(OH)2. In contrast, a paramagnetic
species was observed upon addition of 1 (Figure S2). This
suggests that 1 is responsible for paddlewheel denucleation,
Table 1. Reaction development.[a]
Ph
NH2
HN
NH2
NH
consistent with mechanistic proposals for the Chan–Lam
Cu(OAc)2 (x mol%)
+
Ph B(OH)2
amination.[22,
Complex 4 could form after paddlewheel
25]
Ph
Ph
+
4 Å MS, conditions
Ph
N
Ph
N
Ph
denucleation via ligand exchange processes at, for example,
putative Cu(II) complexes 5 and 6 (Scheme 2b).
1
2
3
Entry
Conditions
2/3 (%)[b]
Supplementary to our main aim of using a Chan-Lam
strategy for the late-stage mono-N-arylation of pentamidine, the
generality of the approach was assessed using a range of
amidines and arylboronic acids, all of which exclusively formed
mono-N-arylated products (2-29, Scheme 3a, Table S6). The use
of 20 mol% Cu(OAc)2 was comparable to stoichiometric
Cu(OAc)2; however, for expediency, the latter was used across
the series based on the faster reaction time (see Table 1, Entries
7 and 9) with comparable yields afforded for both processes (see
yields for catalytic reactions in parentheses).
Both mono- and bisamidines are known Cu-chelators,[26]
which could hamper the wider utility of this approach to mono-N-
arylate pentamidine for structure-activity-relationship (SAR)
profiling. Indeed, standard Chan–Lam conditions failed to deliver
any N-arylation of pentamidine. Gratifyingly, our reaction
conditions were leveraged to directly access a series of
exclusively mono-N-arylated pentamidines (30-36, Scheme 3b) in
a single step from pentamidine.[5b, 27]
1[c]
2[c,d]
3
Cu(OAc)2 (100 mol%), Et3N, CH2Cl2, RT, 16 h
Cu(OAc)2 (100 mol%), Et3N, CH2Cl2, RT, 16 h
Cu(OAc)2 (100 mol%), B(OH)3, CH2Cl2, RT, 16 h
Cu(OAc)2 (100 mol%), NaOPiv, DMF, 50 ºC, 16 h
Cu(OAc)2 (100 mol%), K2CO3, CH2Cl2, RT, 16 h
Cu(OAc)2 (100 mol%), K2CO3, MeOH, RT, 16 h
Cu(OAc)2 (100 mol%), K2CO3, i-PrOH, RT, 2 h
Cu(OAc)2 (50 mol%), K2CO3, i-PrOH, 50 ºC, 24 h
59/14
8/74
45/11
81/2
40/8
59/2
83/5
76/2
81/0
4[d]
5
6
7[c]
8
9[c]
Cu(OAc)2 (20 mol%), K2CO3, i-PrOH, 50 ºC, 24 h
[a] Using 2:1 1:PhB(OH)2 and 2 equiv of additive unless noted. [b] Determined by
HPLC analysis using standard concentration curves of 2 and 3. [c] Isolated yield
on 1 mmol scale. [d] 1:PhB(OH)2 = 1:2. [e] 1:PhB(OH)2 = 1:1.2.
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10.1002/cmdc.202100509
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COMMUNICATION
(a) Scope of the amidine mono-N-functionalization
NH2
NH2
NH
Cu(OAc)2
+
B(OH)2
SMe
K2CO3, 4 Å MS
i-PrOH, RT, 2 h
N
F
t-Bu
CF3
NH2
NH2
NH2
NH2
NH2
Ph
N
Ph
Ph
N
Ph
N
Ph
N
Ph
N
2, 83% (81%)
7, 63% (72%)
8, 71% (67%)
9, 82%
10, 64%
NH2
N
NH2
Ph
OMe
F
NH2
NH2
NH2
NH2
Ph
Ph
N
N
Ph
N
Me
Ph
Ph
N
N
Ph
N
Me
OMe
11, 61%
12, 69%
Me
13, 40% (48%)
14, 42%
15, 39%[a]
16, 68%
F
NH2
NH2
NH2
NH2
N
OMe
Me
F
F
NH2
NH2
Ph
R
Ph
N
Ph
N
Ph
N
OMe
Ph
Ph
N
Ph
N
R = c-Hex: 22, trace
R = c-Prop: 23, trace
21, 41% (33%)
17, 62%
NH2
18, 83%
19, 57%
20, 35%
NH2
NH2
NH2
NH2
NH2
Ph
Ph
MeO
Ph
Br
Ph
NC
Ph
N
N
N
N
N
N
Cl
F
Me
24, 61% (69%)
25, 53%
26, 52%
27, 49%
28, 31%
29, 11%
(b) Library of monoarylated pentamidine derivatives
N
O
NH2
O
O
O
S
=
OMe
N
H
NMe2
CF3
NMe2
Ph
N
F
pentamidine derivatives
30
31
32
33
34
35
36
• Mono-N-functionalization
H2N
• Direct, late-stage derivativization
NH
Scheme 3. (a) Scope of mono-N-arylation. Yields in parentheses for reactions at 50 °C for 24 h using 20 mol% Cu(OAc)2. (b) Library of monoarylated pentamidine
derivatives. [a] Isolated as the corresponding TFA salt.
Table 2. Activity of mono-N-aryl pentamidine analogues 30-36 against kinetoplastid parasites.
Entry
Compound
T. b. brucei
EC50 (M; n = 3)
T. congolense
EC50 (M; n = 3)
L. mexicana
RF
t-test
RF
t-test
EC50
(M; n = 4)
WT
B48
WT
DA-Res
3.260.01
n.d.
1
2
3
4
5
6
7
8
9
30
31
1.100.07
0.910.02
0.650.03
0.190.003
0.510.004
1.090.01
3.040.02
0.00240.0002
0.0820.006
1.310.02
0.860.02
0.370.006
0.280.006
0.510.02
0.820.01
2.980.04
0.280.002
0.890.05
1.19
0.94
0.57
1.47
1.01
0.75
0.98
116
0.042
0.10
2.940.10
n.d.
1.11
---
0.037
---
12.70.7
2.120.07
9.710.56
0.660.04
>20
32
0.0007
0.0002
0.82
1.260.03
0.640.02
n.d.
1.560.06
0.750.003
n.d.
1.23
1.23
---
0.012
0.0008
---
33
34
35
0.0001
0.23
n.d.
n.d.
---
---
>20
36
6.690.11
1.310.02
0.270.008
9.290.17
1.420.01
1.710.02
1.39
1.08
6.4
0.0002
0.013
2x10–7
7.540.59
2.040.04
n.d.
PMD
DA
3.5x10–8
0.0001
10.8
WT = wild-type, PMD = pentamidine, DA = diminazene aceturate, DA-Res = diminazene-resistant cell line, RF = resistance factor being the ratio of EC50 (resistant
line) over EC50(WT). Statistical difference between WT and resistant pairs of cell lines was established using a Student unpaired two-tailed t-test.
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COMMUNICATION
C
B
30000
A
35000
10 μM 36
10 μM 36
+10 μM PMD
10 μM 36
10 μM 36
+10 μM PMD
10 μM 36
10 μM 36
+10 μM PMD
35000
30000
25000
20000
15000
25000
15000
20000
10000
7000
7000
6000
5000
4000
3000
900
3.33 μM 36
3.33 μM 36
+10 μM PMD
3.33 μM 36
3.33 μM 36
+10 μM PMD
3.33 μM 36
3.33 μM 36
+10 μM PMD
10000
8000
6000
4000
6000
5000
4000
1 μM 36
1 μM 36
+10 μM PMD
1 μM 36
1 μM 36
+10 μM PMD
0 μM 36
0 μM 36
+10 μM PMD
1 μM 36
1 μM 36
+10 μM PMD
1200
1000
800
1000
500
0
700
500
300
0 μM 36
0 μM 36
+10 μM PMD
0 μM 36
600
0 μM 36
+10 μM PMD
0
30
60
90
120
150
180
0
30
60
90
120
150
180
0
30
60
90
120
150
180
Time (min)
Time (min)
Time (min)
Figure 1. Real time fluorescence development of 36 with bloodstream forms of WT T. b. brucei (A), bloodstream forms of WT T. congolense, (B) and promastigotes
of L. mexicana (C). Cells were incubated for 3 h in the presence of 36 at 0, 1, 3.3 or 10 µM, in the presence or absence of 10 µM pentamidine. Measurements were
taken at 2-minute intervals. A.U., artificial units of fluorescence intensity.
36
Merge
Hoechst
Mitotracker
DIC
Figure 2. Selected immunofluorescence images of T. brucei labelled with a nuclear marker (Hoechst 33342), a dye for the mitochondrion (MitoTracker Green), and
with 36 (30 min exposure, [10 µM]) or not (control). Charts show the intensity levels of the DAPI and red filter channels measured in the nuclei and kinetoplast.
Scale bars are 5 µm.
This focused series covered analogues ranging from
unsubstituted (30), mono-substituted (31-33) and disubstituted
aromatics (34), a quinolone (35), and the bulky fluorescent
analogue (36) suitable for cellular uptake studies.
Compounds (30-36) were then tested for activity against
three kinetoplastid parasite species that are commonly treated
with pentamidine (T. brucei, L. mexicana) or diminazene
aceturate (T. congolense) (Table 2).[28] EC50 values ranged from
0.19 µM to 1.1 µM, with 33 exhibiting the greatest potency
followed by 34. Indeed, for T. brucei, the SAR was quite flat, with
quinoline (35) and biphenyl (31) analogues displaying similar
EC50s to the phenyl analogue (30).
The compounds were also tested against the related
kinetoplastid parasites L. mexicana and T. congolense. Here, 33
displayed >2-fold and >3-fold higher activity compared to
pentamidine against T. congolense and L. mexicana, respectively.
The SAR profile was considerably less flat than for T. brucei, with
33 being 4.6-fold and 19.2-fold more active than 30 against T.
congolense and L. mexicana, respectively.
The potential for cross-resistance with diamidines and
melaminophenyl arsenicals (MPAs; including melarsoprol,
cymelarsan) is particularly problematic in the development of
next-generation amidine antiparasitics.[4] Resistance likely
emerges from active drug transport [e.g., TbAT1/P2 aminopurine
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10.1002/cmdc.202100509
ChemMedChem
COMMUNICATION
transporter;[29] High Affinity Pentamidine Transporter (TbAQP2)]
into the parasite’s interior.[30] Having studied the SAR of the
transporter-diamidine interactions, we hypothesized that mono-N-
arylation of the pentamidine scaffold would abolish their ability to
act as substrates for the T. brucei drug transporters, and therefore
would not display cross-resistance with pentamidine and
MPAs.[31] To test this hypothesis, 30-36 were also tested against
a T. b. brucei clonal line, B48,[32] lacking both the TbAT1/P2 and
HAPT1/AQP2 transporters, and against a diminazene-resistant T.
congolense[11c] clone. Although minor differences in drug
sensitivity were observed (±50% of WT EC50), these variations
were trivial compared to the resistance levels to pentamidine
(116-fold) and diminazene (6.4-fold), respectively. We therefore
conclude that mono-N-arylated pentamidine analogues are not
cross-resistant with un-substituted diamidines, arising from
differences in their mechanism of uptake.
directly prepare mono-N-arylated analogues of pentamidine,
which displayed promising in vitro activity against three species of
kinetoplast parasites of clinical and veterinary importance. Most
importantly, the mono-N-arylated analogues were not cross-
resistant with pentamidine and diminazene, bypassing known
drug transporters. In addition, 36 accumulated rapidly in all three
kinetoplastid species and localized to the parasite nucleus and
kinetoplast, consistent with a putative mechanism of action being
a DNA minor groove binder. These findings highlight that the
potential utility of mono-N-arylation of diamidines, and potentially
their expansion to guanidine analogues,[35] as an emerging class
of therapeutic agents against neglected parasitic diseases.
Acknowledgements
To gain further insights into the uptake of the mono-N-
arylated pentamidines, the fluorescent analogue (36) was used to
monitor uptake in each of the three parasite species in real-time
(Figure 1). In all three species, 36 was taken up rapidly (3.3 µM
and 10 µM), consistent with an EC50  3 - 7 µM against the
kinetoplastid species (Table 2). The rate of uptake of 36 was
dose-dependent and showed an approximately 2.3 - 10-fold
higher rate at 10 µM [36] than for 3.3 µM after 30 min (Figure 1).
This shows that the uptake mechanism was not saturable in the
lower µM range, unlike TbAT1/P2 and HAPT/TbAQP2, which
display Km values of 0.26 ± 0.03 and 0.036 ± 0.06 µM, respectively,
for pentamidine.[30] Moreover, the rate of uptake of 36 was not
substantially different in the presence of 10 µM pentamidine
(within 10% in all cases), clearly indicating that uptake did not
involve a high affinity pentamidine transporter. Taken collectively,
these data show that mono-N-arylated analogues such as 36
evade known pentamidine transporters TbAT1 and TbAQP2 in T.
brucei.
One of the putative mechanisms of action of pentamidine is
the inhibition of replication via binding to A/T-rich sequences of
duplex DNA.[33] Indeed, selective accumulation of 36 to the
nucleus and kinetoplast of T. brucei was observed, as confirmed
by co-localization with Hoechst 33342 (Figure 2), consistent with
DNA binding. UV melt analyses were undertaken to explore the
ability of 33 and 36 to stabilize DNA duplexes relative to
pentamidine (Table S7, Figure S4). A 7.0 °C stabilization of an A-
tract duplex was observed for 33, relative to a 6.0 °C stabilization
for pentamidine. Although a reduced level of duplex stabilization
was observed using 36 for the same sequence (Tm 4.6 °C), both
analogues exhibited a similar binding bias for an A-tract duplex
relative to a duplex containing an alternating A-T sequence.[34]
These data indicate that the likely mechanism of action of these
analogues is via binding to DNA duplexes with a selectivity profile
similar to that observed for pentamidine. The stronger DNA
binding of 33 correlated with its stronger anti-kinetoplastid activity
compared to 36.
J.R. and G.A.B. thank GlaxoSmithKline (GSK) and the
Engineering and Physical Sciences Research Council (EPSRC)
for an industrial CASE studentship. MMA is supported by a
studentship from the government of Libya, and MAU by a
studentship from the Petroleum Technology Development Fund
(PTDF), Abuja, Nigeria. We thank Dr Alan Kennedy (University of
Strathclyde) and Dr Nicola Bell (University of Glasgow) for
assistance in the analysis of crystal structures.
Keywords: antiparasitics • amidine • arylation • copper •
medicinal chemistry
[1] N. A. Meanwell, J. Med. Chem. 2011, 54, 2529-2591.
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ChemMedChem
COMMUNICATION
COMMUNICATION
Jack Robertson,[a] Marzuq A. Ungogo,[b]
Mustafa M. Aldfer,[b] Leandro
NH2
HN
NH
Lemgruber,[c] Fergus S. McWhinnie,[a]
Bela E. Bode,[d] Katherine L. Jones,[e]*
Allan J. B. Watson,[d]* Harry P. de
Koning,[b]* Glenn A. Burley*[a
O
O
NH
Cu(OAc)2
O
O
B(OH)2
mono-selective
N-arylation
pentamidine
pentamidine derivatives
→ late stage diversification
→ modulation of physchem
→ improved potency
H2N
H2
N
Nucleus and kinetoplast accumulation
NH
NH
Direct, late-stage mono-N-arylation of
pentamidine: method development,
mechanistic insights, and expedient
access to novel antiparastitics against
diamidine-resistant parasites
A selective mono-N-arylation strategy of amidines under Chan–Lam conditions is described. During the reaction optimization phase,
the isolation of a mononuclear Cu(II) complex provided unique mechanistic insight into the operation of Chan–Lam mono-N-arylation.
The scope of the process is demonstrated, and then applied to access the first mono-N-arylated analogues of pentamidine. Sub-
micromolar activity against kinetoplastid parasites was observed for several analogues with no cross-resistance in pentamidine and
diminazene-resistant trypanosome strains and against Leishmania mexicana. A fluorescent mono-N-arylated pentamidine analogue
revealed rapid cellular uptake, accumulating in parasite nuclei and the kinetoplasts. The DNA binding capability of the mono-N-arylated
pentamidine series was confirmed by UV-melt measurements using AT-rich DNA. This work highlights the potential to use Chan-Lam
mono-N-arylation to develop therapeutic leads against diamidine-resistant trypanosomiasis and leishmaniasis.
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