Regioselective convergent synthesis of 2-arylidene thiazolo[3,2-: A] pyrimidines as potential anti-chikungunya agents

Article abstract of DOI:10.1039/d0ra00257g

Convergent and convenient regioselective synthesis of novel thiazolo[2,3-a]pyrimidine derivatives was accomplished using the one-pot reaction of 6-ethylthiouracil, bromoacetic acid, anhydrous sodium acetate, acetic anhydride, acetic acid and suitable aldehyde. X-ray crystallographic study reveals the presence of the Z configuration of only one regioisomer confirmed by computational studies as being the most likely isomer present.

Full text of DOI:10.1039/d0ra00257g

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Regioselective convergent synthesis of 2-arylidene
thiazolo[3,2-a]pyrimidines as potential anti-
chikungunya agents†
Cite this: RSC Adv., 2020, 10, 5191
Mohamed Fares, e *ab Patrick M. McCosker,
ac
d
Muhammad A. Alsherbiny,
c
f
f
Anthony C. Willis, Timothy Clark,
Johan Neyts,
Dirk Jochmans
a
and Paul A. Keller
*
Convergent and convenient regioselective synthesis of novel thiazolo[2,3-a]pyrimidine derivatives was
accomplished using the one-pot reaction of 6-ethylthiouracil, bromoacetic acid, anhydrous sodium
acetate, acetic anhydride, acetic acid and suitable aldehyde. X-ray crystallographic study reveals the
presence of the Z configuration of only one regioisomer confirmed by computational studies as being
the most likely isomer present.
Received 27th November 2019
Accepted 17th January 2020
DOI: 10.1039/d0ra00257g
rsc.li/rsc-advances
Arboviruses mainly use arthropod vectors for transmission ligand-based drug design, high throughput screening and
9
–17
between hosts and they are considered a growing global health genome-wide loss of function screen.
Non-steroidal anti-
1,2
threat. This arises from the increasing number of travellers, inammatory drugs (NSAID) have been used to alleviate the
from 450 million in 1990 to nearly 1.4 billion in 2018, and symptoms of the CHIKF because of the lack of effective
consequently travel-related diseases incidence has increased prevention or curing of the viral infection. However, so far
18
3
signicantly. Chikungunya virus (CHIKV) (Alphavirus genus, there is no vaccine or approved medication to prevent or treat
family Togaviridae) is one of the prevalent tropical alphaviruses CHIKV infection.
4,5
that is transmitted by Aedes mosquitoes. Aer an incubation
The recent advances and growing knowledge about the
period of 2–4 days, symptoms start with the onset of high fever arboviruses expand our understanding of the CHIKV life cycle,
1–6
and severe persistent joint pain that may last for weeks to pathogenesis and infection. Intensive research in the past 10
6,7
years. Other symptoms of chikungunya fever (CHIKF) include years has led to the identication of new chemical scaffolds that
headache, vomiting, rash, myalgia (muscle pain), photophobia might be potential anti-CHIKV agents. Rhodanine I is a thiazo-
6,8
and maculopapular rash. Unlike other arbovirus infections, lidin-4-one derivative, presenting a scaffold that exhibited
6
around 15% of the patients may encounter “silent infections”.
broad antiviral activity against HCV, HIV-1 and Chikungunya
19
A variety of strategies have been applied to develop anti- (Fig. 1). A series of rhodanine derivatives was tested for their
CHIKV treatments, including vaccines, and for small molecule anti-CHIKV activity by a cytopathic effect (CPE) reduction assay
inhibitors, random screening, computer aided drug design, (Fig. 1). The 2-methyl analogue II emerged as the best
compound and displayed excellent activity with submicromolar
a
School of Chemistry & Molecular Bioscience, Molecular Horizons, University of
Wollongong, Illawarra Health & Medical Research Institute, Wollongong, NSW
2
522, Australia. E-mail: Ph.fares@yahoo.com; keller@uow.edu.au
b
School of Chemistry, The University of Sydney, NSW 2006, Australia
c
Department of Chemistry and Pharmacy, Computer-Chemistry-Center (CCC),
Friedrich-Alexander-Universit a¨ t Erlangen-N u¨ rnberg (FAU), N a¨ gelbachstrasse 25,
9
1052 Erlangen, Germany
d
NICM Health Research Institute, Western Sydney University, Westmead, NSW 2145,
Australia
e
Research School of Chemistry, The Australian National University, Canberra, ACT
2
601, Australia
f
KU Leuven (University of Leuven), Department of Microbiology and Immunology, Rega
Institute for Medical Research, Laboratory of Virology and Chemotherapy, B-3000,
Leuven, Belgium
†
Electronic supplementary information (ESI) available. CCDC 1968317. For ESI
and crystallographic data in CIF or other electronic format see DOI:
0.1039/d0ra00257g
Fig. 1 Examples of heterocyclic classes showing anti-CHIKV activity,
including thiazolidin-4-one I and II, pyrimidine III and pyrimidine fused
rings IV and IVa–c.
1
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EC50 (EC50 ¼ 0.42 mM) against CHIKV. A docking study also
showed good interaction between the ligand and CHIKV nsp2
19
protease.
Among the main heterocycles that form the basis of small
molecule inhibitors of CHIKV are the pyrimidine and fused
15,20,21
pyrimidine rings, e.g. III, IV and Va–c (Fig. 1).
Recent
reports indicated that hybridization between uracil–coumarin– Scheme 1 Reagents and conditions: (i) NaOEt, C H OH, 6 h, reflux; (ii)
2
5
arenes III resulted in the discovery of a novel anti-CHIKV scaf- ClCH
2
COOH, anhydrous CH
chlorobenzaldehyde, reflux, 4 h.
3
COONa, (CH
3
2 3
CO) , CH COOH, 4-
20
fold that was found to impede CHIKV replication. In an effort
to nd new leads as CHIKV inhibitors, a random screening
identied the [1,2,3]triazolo[4,5-d]pyrimidin-7(6H)ones as
21
acid, and lowering the reaction temperature. This aimed to
favour the formation of the kinetic regioisomer (Z)-A, assumed
to be the desired product, over the thermodynamic regioisomer
potential candidates. Iterative optimization cycles led to the
most potent compound IV with EC50 ¼ 3 mM and a selectivity
21
index greater than 600 (Fig. 1). In 2012, purine-b-lactams Va
and Vb and purine-amino propanol Vc (Fig. 1) conjugates were
screened against nine different viruses including CHIKV. The
amino propanol derivative Vc (EC50 ¼ 11.51–17.11 mM) showed
relatively higher activity other than the purine-b-lactam hybrids.
(Z)-B (Scheme 2), and resulted in the crystallization of only one
regioisomer from the reaction mix (acetic acid) in each case.
This may be due to the low reactivity and the less regioselectivity
of the chloroacetic acid compared to bromoacetic acid.
15
From the convergent reaction conditions, four possible
isomers can be formed: the Z or E congurations of structural
isomers A or B – (Z)-A, (E)-A, (Z)-B and (E)-B (Fig. 4). Analysis of
Therefore, the b-lactam was not essential for activity.
Recently, hybridization of two or more bioactive fragments
has emerged as a concept for the exploration of novel multi-
target acting inhibitors as well as novel anti CHIKV agents
1
the H NMR spectra showed a quartet at 2.48–2.57 ppm
15,20,22,23
assigned to methylene hydrogens of the ethyl substituent –
these were assigned to structural isomer A as the corresponding
resonances of isomer B are expected to resonate more downeld
(ꢀ3 ppm) as a result of a deshielding anisotropic effect of the
adjacent carbonyl group of the thiazole ring.
(Fig. 2).
Therefore, in our ongoing efforts to discover new
scaffolds for CHIKV infection, we investigated a molecular
hybridization approach with the fusion of the uracil and the
22
rhodanine pharmacophoric moieties (Fig. 2).
The synthetic strategy started with the condensation of ethyl
The Z absolute conguration of this class of molecules was
conrmed by X-ray crystallographic analysis of derivative 12
propionylacetate 1 and thiourea 2 to afford the known 6-ethyl
24,25
thiouracil 3 with 69% yield.
Although the synthesis of the
(Fig. 5). This illustrated the planarity of the thiazolopyrimidine
thiazolopyrimidine derivatives from the 6-ethylthiouracil
substrate has not previously been reported, this multicompo-
nent reaction (MCR) was achieved using an analogous proce-
rings with the aryl group, which may be attributed to two
intramolecular hydrogen bonds. The Z-conguration is
presumably stabilized by the two intramolecular hydrogen
bonds between the arylidene H and the oxygen atom of the
thiazole carbonyl group (HB1) and between the sulfur atom and
the aryl hydrogen (HB2). The distance of both hydrogen bonds
26–29
dure
by reacting 3 with the aldehyde, chloroacetic acid,
sodium acetate anhydrous, acetic anhydride and acetic acid in
one pot and at reux for 4 h (Scheme 1).
These conditions consistently gave low yields and two
regioisomers (e.g. chlorobenzaldehyde 8 A : B 16% : 84%), the
˚
is 2.49 and 2.50 A, for HB1 and HB2, respectively, and the angles
ꢁ
ꢁ
1
are 102.81 and 132.94 for HB1 and HB2 respectively.
latter determined by analysis of the H NMR spectrum (Fig. 3) –
attempts to separate the isomers by column chromatography
failed.
Optimization of the reaction involved varying the mode of
addition, replacement of the chloroacetic acid with bromoacetic
1
Fig. 3 H NMR spectrum of the MCR product of the two regioisomers
Fig. 2 Design of the targeted uracil–rhodanine conjugates.
of 8 in a ratio of 1 : 5.
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gave no indication of hydrogen bond stabilisation for the
alcohol, despite attempts at manually inputting the conforma-
tion. This indicated steric and geometric arrangement of the
alcohol intermediate is more signicant for the energy term.
A stacked ring conformer (C3) was also observed with
0
ꢂ1
a favourable D
r
G {298 K} of ꢂ1.30 kcal mol . To better account
for this interaction, Grimme's dispersion (D3) and geometric
counterpoise (gCP) corrections were applied to the energy
30
term, and the stacked C3 was observed to increase by
ꢂ1
ꢂ1
+
1.29 kcal mol , whereas C1/C2 increased by +0.87 and
Scheme 2 Reactions and conditions: (i) BrCH
2
COOH, anhydrous +0.85 kcal mol respectively. The similar increase to the energy
ꢁ
CH
anhydrous CH
anhydrous CH
3
COONa, (CH
COONa, CH
COONa, (CH
3
CO)
2
, CH
3
COOH, 60 C, 3 h; (ii) benzaldehyde,
term for C1/C2 suggested there are no differences in dispersion
effects to account for in these conformers. However, the greater
energy change observed with C3 was indicative the ring stacking
is not a favourable intramolecular force. Based on the subtle
differences in energies of the conformers, C1 is the apparent
favoured alcohol intermediate conformer, which is arranged to
dehydrate to the Z isomer.
ꢁ
3
3
COOH, 60 C, 2 h. (iii) BrCH
2
ꢁ
COOH,
3
3
CO)
2
3
, CH COOH, aldehyde, 60 C, 4 h.
The transition states of the proposed dehydration reactions
0
were calculated and the saddle point D G {298 K} were found to
r
ꢂ1
be +32.58 and +30.05 kcal mol for the E T1 and Z T2 arranged
transition states respectively (Fig. S3a†). With the D3-gCP
0
ꢂ1
r
corrections D G {298 K} were +33.51 and +30.90 kcal mol
for T1 and T2 respectively. The similar increases in energy
ꢂ1
(
+0.93 and +0.85 kcal mol ) indicated negligible differences in
dispersion force correction required when comparing T1 and
T2. What was particularly interesting to note, in contrast to the
alcohol intermediate, is the preference of the water leaving
hydrogen bound to the carbonyl. Despite attempts in manually
positioning the water to leave adjacent to the sulfur, and in turn
have H–S interaction, the transition state only converged to
allow hydrogen bonding of the water with the thiazole carbonyl.
Subsequently, it was proposed the principal factor dictating the
favoured transition state is the arrangement of the aryl group, as
the water has apparent preference for leaving trans to the sulfur,
in favour of Z isomer formation.
Fig. 4 The four possible isomers that can be produced from the MCR
reaction.
Subsequent calculation of the product (E)-6 and (Z)-6
0
ꢂ1
isomers indicated D
r
H {298 K} of ꢂ2.40 and ꢂ6.78 kcal mol
Fig. 5 (A) ORTEP diagram of compound 12, (B) representation of
stabilizing hydrogen bonds of the regioisomer (Z)-A.
respectively, which indicts the Z isomer to be thermodynami-
cally favoured, and D3-gCP corrections were in good agreement
(Fig. 6 and S3b†). Comparison of the calculated (Z)-6 structure
with the X-ray crystal structure of compound 12 indicated
To investigate the observed preference for the Z isomer,
density functional theory (DFT) calculations were performed
using the widely applied M06-2X functional with a Dunnings
˚
˚
similar bond distances of HB1 ¼ 2.43 A and HB2 ¼ 2.50 A for
the proposed intramolecular hydrogen bonds (Fig. 5).
Likewise, the bond angles were in good agreement for HB1
aug-cc-pVDZ basis set. Compound 6 (Ar ¼ C
6 6
H ) was selected
ꢁ
ꢁ
and HB2 calculated at 101.34 and 127.70 , compared with
02.81 and 132.94 from the crystal structure, respectively. In
for the theoretical study and all structures were optimised with
modelled acetic acid solvation (SMD). Analysis of the proposed
mechanism indicated a possible alcohol intermediate and
dehydration step preceded the nal Z/E isomer formation, and
to keep the model simple, the calculations have focussed on
these molecules only, maintaining chirality where applicable
ꢁ ꢁ
1
conclusion, although the energy differences are small between
the investigated reaction pathways, there is a consistent pref-
erence for the Z forming pathway at each step investigated and
this result was reciprocated by D3-gCP energy corrections
(Fig. 6).
(
Fig. S1†). Conformer analysis of the alcohol intermediate
0
The compounds were screened for their antiviral activities in
a viral-cell based assay against Chikungunya virus (Indian
Ocean strain 899) (Table S1†).
Based on the biological results, compounds 6–20 showed
different percentages of viral replication inhibition at
indicated a Gibbs free energy of reaction (D
+
lowest and the E arranged C2 the highest energy conformers
respectively (Fig. S2†). Analysis of the optimised geometries
r
G {298 K}) range of
ꢂ1
0.35 to ꢂ2.30 kcal mol with the Z arranged C1 being the
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Acknowledgements
MF thanks UOW for the University Postgraduate Award and
International Postgraduate Tuition Award scholarships.
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0 0
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component reaction conditions by replacing chloroacetic acid
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