Synthesis and evaluation of phenoxyoxazaphospholidine, phenoxyoxazaphosphinane, and benzodioxaphosphininamine sulfides and related compounds as potential anti-malarial agents

Article abstract of DOI:10.1016/j.bmcl.2013.04.026

A series of phenoxyoxazaphospholidine, phenoxyoxazaphosphinane and benzodioxaphosphininamine sulfides and related cyclic organophosphorus compounds based on the lead anti-tubulin herbicides amiprophos methyl and butamifos were synthesised and evaluated fo

Full text of DOI:10.1016/j.bmcl.2013.04.026

Bioorganic & Medicinal Chemistry Letters 23 (2013) 3580–3583
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Bioorganic & Medicinal Chemistry Letters
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Synthesis and evaluation of phenoxyoxazaphospholidine,
phenoxyoxazaphosphinane, and benzodioxaphosphininamine
sulfides and related compounds as potential anti-malarial agents
Christine Mara a, Enda Dempsey b, Angus Bell b, James W. Barlow a,
⇑
a Department of Pharmaceutical & Medicinal Chemistry, Royal College of Surgeons in Ireland, Stephens Green, Dublin 2, Ireland
b Department of Microbiology, School of Genetics & Microbiology, Moyne Institute of Preventive Medicine, Trinity College Dublin, Dublin 2, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of phenoxyoxazaphospholidine, phenoxyoxazaphosphinane and benzodioxaphosphininamine
sulfides and related cyclic organophosphorus compounds based on the lead anti-tubulin herbicides
amiprophos methyl and butamifos were synthesised and evaluated for anti-malarial activity. Of these
compounds, while none of the phenoxyoxazaphospholidines, phenoxyoxazaphosphinanes or ben-
zodioxaphosphininamine sulphides were more potent than APM, phosphorothioamidate 30, a dual com-
pound also bearing an aminoquinoline motif, showed promising activity against Plasmodium falciparum
Received 18 January 2013
Revised 8 April 2013
Accepted 9 April 2013
Available online 17 April 2013
Keywords:
(IC50 0.038 lM) and warrants further study.
Phenoxyoxazaphospholidines
Phenoxyoxazaphosphinanes
Benzodioxaphosphininamines
Anti-malarial
Ó 2013 Elsevier Ltd. All rights reserved.
Tubulin
Malaria continues to pose a significant global health burden.
According to the World Malaria Report 2011, there were 216 mil-
lion cases of malaria and an estimated 655,000 deaths in 2010,1
although recent evidence suggests the true mortality figure to be
twofold higher.2 The need to develop new antimalarial drugs is
therefore as pressing as ever. Not only do we need new drugs to re-
place those falling prey to resistance—that is ‘running just to stand
still’—but new types of therapy are required in order to implement
the new malaria eradication agenda.3 The two general approaches
used are antimalarial screening and target-based discovery/de-
sign.4 The target-based approach takes advantage of our increas-
ingly detailed understanding of parasite biology but there are
relatively few well validated targets, and even fewer for which po-
tent whole-parasite inhibitors are available. One of the few targets
tional anti-leishmanial herbicidal dinitroanilines.7
A related
series of herbicides, typified by the organophosphate compound
amiprophos methyl (APM, Fig. 1) are believed to share a common
mode of action with the dinitroaniline herbicides. Our previous
work8 showed that phosphoramidate and phosphorothioamidate
compounds based on the leads amiprophos methyl and butamifos
possessed antimalarial activity. Phosphorothioamidates were gen-
erally more active than their oxo congeners and the nature of both
aryl and amido substituents influenced the desired activity. The
aim of the work described in this letter was twofold; first to inves-
tigate whether activity was retained if the phosphoryl or thiophos-
phoryl moiety was constrained within a heterocyclic framework
and secondly to determine whether aromatics other than
phenol derivatives would show activity. Towards the first aim,
we synthesised a range of phenoxyoxazaphospholidine, pheno-
xyoxazaphosphinane and benzodioxaphosphininamine analogues
of APM and butamifos and tested these compounds for activity
against the most lethal human malarial parasite, Plasmodium
falciparum.
that falls into the latter category is tubulin, whose
a/b heterodi-
mers are the principal constituent of cell microtubules.5 The main
barrier to anti-tubulin antimalarial drug development is the issue
of selective toxicity, which we are addressing by focussing on her-
bicide-based compounds that inhibit parasite but not host
microtubules.6
Phosphoramidates and thiophosphoramidates of these structural
types have been investigated as pesticides. Both phenoxy9 and anili-
no10 oxazaphospholidines are known to possess pesticidal activities,
including insecticidal, antimicrobial and herbicidal properties. In addi-
tion, related heterocycles such as oxathiaphospholanes and thiaza-
phospholidines share these actions.11 Most recently, a new series of
carbazolyl phenoxyoxazaphospholan-2-ones, along with their thione
Examples of application of this principle include the therapeu-
tically useful anti-helminthic benzimidazoles and the investiga-
Abbreviation: APM, amiprophos methyl.
Corresponding author. Tel.: +353 1 402 8520; fax: +353 1 402 2765.
E-mail address: jambarlow@rcsi.ie (J.W. Barlow).
⇑
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.04.026

C. Mara et al. / Bioorg. Med. Chem. Lett. 23 (2013) 3580–3583
3581
Figure 1. Antitubulin herbicide APM and active derivative.
Scheme 2. Synthesis of substituted benzodioxaphosphininamines and related
compounds 13–18. Reagents and conditions: (a) PSCl3/Et3N, CHCl3; (b) R0NH2;
Et3N; (c) Et3N; THF, 0 °C to rt; N2.
and selenone analogues, showed antimicrobial activity against
Staphylococcus aureus and Escherichia coli.12 Benzodioxaphosphinin-
amines such as salithion have been primarily investigated for their
insecticidal and anthelmintic activities, both achieved via selective
inhibition of acetylcholinesterase.13 Likewise, 1,3,2-oxazaphospo-
rinanes are known to possess nematocidal activity with low toxicity
towards mammals.14 As with the phenolic compounds previously
synthesised,8 heterocyclic analogues have also been evaluated as
pesticides, including pyridines15 and quinolines.16 However, to date,
none of these compounds have been evaluated for activity against
Plasmodium.
Synthetic approaches towards phenoxyoxazaphospholidines
and phenoxyoxazaphosphinanes typically involve one of two
processes. Shipov’s approach14 was the reaction of a bidentate
nucleophile such as an aminoalkanol with an appropriate dihalo-
thiophosphoric acid derivative, to prepare a series of 2-aryl-
oxy(arylthio)-2-thio(oxo)-1,3,2-oxazaphosphorinanes. Similarly,
Liu et al. prepared a series of chiral 2-thio(oxo)-1,3,2-oxazaphosp-
Scheme 3. Synthesis of substituted phosphoramidates and phosphorothioamidates
19–30. Reagents and conditions: (a) C2H5OPXCl2; Et3N; THF (series A) or PhCH3
(series B); 0 °C; N2; (b) R0NH2; Et3N; THF (series A) or PhCH3 (series B); 0 °C; N2.
Table 1
Anti-malarial activity of phenoxyoxaza-phospholidines and -phosphinanes 1–12 as
assessed by inhibition of pLDH at 72 ha
holidines via the reaction of L-methionol with (thio)phosphoryl
dichlorides.17 Alternatively, these compounds may be prepared
via the reaction of a 2-chloro oxaza-phospholidine or -phosph-
inane with an appropriate phenolate.14 Dihalothiophosphoric acid
derivatives may be prepared either by direct thionation, such as
the reaction of phosphorodichloridous acid esters with PSCl3,18 or
via the reaction of the parent phenols with sulphur or PSCl3.19,20
Using triethylamine as base or phase-transfer catalysis,21 we pre-
pared esters (Scheme 1a). Reaction of these intermediates with
appropriate bidentate nucleophiles afforded 1–12 in 34–60%
yields.
No.
Phosporous heterocycle
R1
IC50
(l
M)
clogPa
1
4-CH3-2-NO2
>128
>128
77
2.56
2
4-CH3-2-NO2
4-CH3-2-NO2
2-CH3-4-NO2
2-CH3-4-NO2
2-CH3-5-NO2
2-CH3-5-NO2
5-CH3-2-NO2
4-CF3
2.62
2.98
2.56
2.62
2.56
2.62
2.62
3.03
3.08
3.44
3
Synthesis of the benzodioxaphosphininamines typically in-
volves the reaction of a saligenin cyclic phosphorochloridiothio-
nate with an appropriate amine,22 as shown in Scheme 2a and b,
or, alternatively, the reaction of saligenin or a substituted deriva-
tive with an appropriate phosphoramidothioic dichloride,13 in
Scheme 2c. In our hands, the phosphorochloridiothionates via
route (a) proved surprisingly stable and were not readily substi-
tuted, whereas route (c) gave the desired product. Non-commer-
cially available saligenin derivatives (e.g., towards 16) were
prepared via reaction of appropriate phenols with HBr and parafor-
maldehyde in acetic acid-acetic anhydride followed by
hydrolysis.23
4
51
5
128
24
6
7
108
>128
108
62
8
Synthesis of phosphoramidate and thiophosphoramidates 19–
30 was accomplished via sequential displacement of the chlorine
atoms in the commercially available ethyl phosphorodichloridate
and ethyl phosphorothiodichloridate, respectively, using
9
10
11
4-CF3
4-CF3
>128
12
4-CF3
64
4
3.03
3.50
Scheme 1. Synthesis of substituted phenoxyoxazaphospholidines and phenoxyox-
azaphosphinanes 1–12. Reagents and conditions: (a) PSCl3/Et3N, 0 °C to rt; (b)
HX(CH2)nYH/Et3N, rt.
APM
a
clogP values calculated using MarvinSketch 5.1.4 from ChemAxon.

3582
C. Mara et al. / Bioorg. Med. Chem. Lett. 23 (2013) 3580–3583
Table 2
phur for nitrogen in the heterocycle improved activity less than
twofold, as seen in a comparison of 12 and 9. However, none of
the compounds tested approached the activity of APM.
The results obtained for compounds 13–18 show the impact on
activity of incorporating the thiophosphoryl group within a ben-
zodioxaphosphinine heterocycle. Although the saligenin derivative
13, with an IC50 of half that of its acyclic analogue8 showed prom-
ise, none of the nitro-substituted saligenin compounds tested
afforded an improvement in activity. For example, compound 16
lacked activity, also seen with its acyclic congener.8
Table 3 depicts the result of varying the aromatic component
within APM-like molecules, including a series of heterocyclic
derivatives. Compounds 19 and 20, containing bulky naphthyl
moieties, showed poor anti-parasitic activity. Within the pyridines
21–27, the most active compounds tested were 23–25, with a CF3
substituent para to the oxygen, and 26. When compared to their
non-pyridine phenol analogues,8 compound 23 was twice as po-
Anti-malarial activity of benzodioxaphosphininamine sulfides 13–18 as assessed by
inhibition of pLDH at 72 ha
No.
Phosporous heterocycle
R0
IC50
(lM)
clogPa
13
i-Pr
62
2.81
14
15
n-Pr
i-Bu
95
58
2.86
3.26
16
17
i-Pr
>128
108
3.23
2.76
i-Pr
tent (25
lM vs 50
lM) while compound 25 was equipotent
(23 M vs 26
l
lM). In addition, the pronounced preference for a
phosphorothioamidate rather than a phosphoroamidate function-
ality as seen in the acyclic compounds8 was not as obvious in the
heterocyclic compounds of Table 3, as illustrated by a comparison
18
i-Bu
118
4
3.22
3.50
APM
of 23 and 24, with IC50s of 25
lines 28 and 29 were quite active, with the more effective being the
6-hydroxy compound 29, with an IC50 of 19 M. 7-Chloro-4-
lM and 29 lM, respectively. Quino-
a
clogP values calculated using MarvinSketch 5.1.4 from ChemAxon.
l
aminoquinolines such as chloroquine are established agents. Many
derivatives including this pharmacophore have been prepared,
including molecules showing dual antimalarial and antitrypanoso-
mal activities,24 or even antitubercular activity.25 We incorporated
this unit as the heterocyclic amine moiety within our organophos-
phorus derivatives, to examine whether this molecular hybridiza-
tion approach would augment activity. The results of this
approach, as demonstrated with compound 30, show a dramatic
improvement in activity. In addition, it is important to know if
compound 30 shows cross resistance with chloroquine. The po-
tency of 30, when tested against the chloroquine-resistant strain
K1/Thailand, was identical to the 3D7 strain, indicating no cross-
resistance with chloroquine in this strain at least (data not shown).
equimolar quantities of phenol (or other aromatic) and amine syn-
thons, as shown in Scheme 3, and the desired compounds were ob-
tained in 35–75% yields.
Tables 1–3 show the anti-malarial activity of the APM ana-
logues, both cyclic and acyclic. In our experiments, APM had an
IC50 value of 4 lM. Firstly, for the phenoxyoxazaphospholidines
and phenoxyoxazaphosphinanes 1–12, the most active compound
tested was 6, the 2-CH3-5-NO2 substituted analogue. Among cer-
tain of the nitro-substituted phenoxyoxazaphospholidines and
phenoxyoxazaphosphinanes, the size of the heterocyclic ring did
have some impact on activity, as exemplified by a comparison of
4 and 5, and also 6 and 7. More important was nature of the phe-
nolic substituent, as can be seen by comparing 1, 4 and 6, where
most activity was exhibited by 6, with an IC50 of 24 lM. A 4-CH3
group seemed not to improve activity significantly, as evidenced
by a comparison of 3 and 1, or 11 and 9. Also, substitution of sul-
In addition, the IC50 of amine 31 was 0.64 lM, 17 times less active
than 30, supporting the molecular hybridization approach and the
contribution of the phosphorothioamidate moiety to the activity of
30.
Table 3
Anti-malarial activity of 19–31 as assessed by inhibition of pLDH at 72 h
No.
R
R0
X
IC50
(l
M)
clogPa
19
20
21
22
23
24
25
26
27
28
29
Naphtal-2-ol
i-Propyl
i-Propyl
i-Propyl
i-Propyl
i-Propyl
i-Propyl
Cyclopentyl
i-Propyl
i-Propyl
O
O
O
O
O
S
O
O
O
S
87
98
128
128
25
29
23
25
128
25
3.69
3.64
2.54
2.49
2.95
3.69
3.39
2.49
1.37
3.94
3.94
1-NO2 naphtal-2-ol
2-OH-4-CH3 pyridine
2-OH-4-CH3-3-NO2 pyridine
2-OH-5-CF3 pyridine
2-OH-5-CF3 pyridine
2-OH-5-CF3 pyridine
2-OH-4-CH3-5-NO2 pyridine
4-OH pyridine
8-OH quinoline
6-OH quinoline
Cyclopentyl
Cyclopentyl
S
19
30
4-CF3 phenyl
S
0.038
6.10
31
0.64
4
1.99
3.50
APM
4-CH3-2-NO2
i-Propyl
S
a
clogP values calculated using MarvinSketch 5.1.4 from ChemAxon.

C. Mara et al. / Bioorg. Med. Chem. Lett. 23 (2013) 3580–3583
3583
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2013.
04.026.
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