Synthesis and biological evaluation of (E)-cinnamic acid, (E)-2-styrylthiazole and (E)-2-[2-(naphthalen-1-yl)vinyl]thiazole derivatives

Article abstract of DOI:10.24820/ark.5550190.p009.905

Cinnamyl- and thiazole-based compounds have been shown to exhibit diverse medicinal properties and a series of twelve (E)-2-styrylthiazole and (E)-2-[(naphthalen-1-yl)vinyl]thiazole derivatives, which are conjugates of both systems and which satisfy the “Lipinski rule of 5”, have been synthesised and subjected to in vitro biological screening. While insignificant inhibition (60-98% viability at 10 μM) of HeLa (cervical cancer) cells was noted, all five of the (E)-2-[naphthalen-1-yl)vinyl]thiazole derivatives proved remarkably active against SH-SY5Y (neuroblastoma) cells with IC50 values ranging from 2.09 to 8.64 μM. Two of the seven (E)-2-styrylthiazoles were found to be moderately active (with IC50 values of 10.8 and 11.7 mM), whereas the remaining five analogues exhibit significant proliferation of SH-SY5Y cells (with IC50 values of 180-1000 mM). The results warrant further studies on the effects of styrylthiazoles on the differentiation and extension of SH-SY5Y cells in order to assess their activity in neurological degenerative diseases.

Full text of DOI:10.24820/ark.5550190.p009.905

General Papers
ARKIVOC 2016 (vi) 284-296
Synthesis and biological evaluation of (E)-cinnamic acid,
(E)-2-styrylthiazole and (E)-2-[2-(naphthalen-1-yl)vinyl]thiazole
derivatives
Emmanuel O. Olawode,a* Roman Tandlich,a Earl Prinsloo,b Michelle Isaacs,c Heinrich
Hoppe,c,d Ronnett Seldon,f Digby F. Warner,g Vanessa Steenkamph and Perry T. Kayec,e*
aDivision of Pharmaceutical Chemistry, Faculty of Pharmacy; bDepartment of Biotechnology;
cCentre for Chemico- and Biomedicinal Research; dDepartment of Biochemistry and
Microbiology; and eDepartment of Chemistry, Rhodes University, Grahamstown, South Africa.
g
fDrug Discovery and Development Centre (H3-D), Department of Chemistry and Molecular
Mycobacteriology Research Unit, Department of Pathology and Institute of Infectious Disease
and Molecular Medicine, University of Cape Town, Cape Town, 7700, South Africa.
hPhytomedicine Unit, Department of Pharmacology, University of Pretoria, Pretoria, 0028,
South Africa
E-mail: E.Olawode@ru.ac.za; P.Kaye@ru.ac.za
DOI: https://doi.org/10.24820/ark.5550190.p009.905
Abstract
Cinnamyl- and thiazole-based compounds have been shown to exhibit diverse medicinal properties
and a series of twelve (E)-2-styrylthiazole and (E)-2-[(naphthalen-1-yl)vinyl]thiazole derivatives,
which are conjugates of both systems and which satisfy the “Lipinski rule of 5”, have been
synthesised and subjected to in vitro biological screening. While insignificant inhibition (60-98%
viability at 10 µM) of HeLa (cervical cancer) cells was noted, all five of the (E)-2-[naphthalen-1-
yl)vinyl]thiazole derivatives proved remarkably active against SH-SY5Y (neuroblastoma) cells
with IC50 values ranging from 2.09 to 8.64 µM. Two of the seven (E)-2-styrylthiazoles were found
to be moderately active (with IC50 values of 10.8 and 11.7 mM), whereas the remaining five
analogues exhibit significant proliferation of SH-SY5Y cells (with IC50 values of 180-1000 mM).
The results warrant further studies on the effects of styrylthiazoles on the differentiation and
extension of SH-SY5Y cells in order to assess their activity in neurological degenerative diseases.
Keywords: Synthesis, cinnamic acids, styrylthiazoles, 2-[2-(naphthalen-1-yl)vinyl]thiazoles,
biological activity
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Introduction
The naturally occurring cinnamic acid derivatives, p-coumaric acid 1, caffeic acid 2 and ferulic
acid 3, are small phenolic compounds found in fruits, vegetables and flowers (Figure 1)1 or, as
their esters, in essential oils, resins and balsams.2 Cinnamic acid, an important intermediate in the
biochemical shikimic and phenylpropanoic acid pathways, 3 belongs to the class of plant hormones
(viz., auxins) which regulate cell growth and differentiation.4 Cinnamic acid analogues act as
precursors of many commercially important synthetic cinnamic esters5 and as reactants in the
preparation of chalcones and stilbenes.6 Cinnamic acid derivatives have also been reported to
possess antidiabetic,7 hepato-protective,8 antioxidant,9 antimicrobial,10 anti-tuberculosis11 and
anti-cancer properties.12 On the other hand, compounds containing the thiazole nucleus have also
been reported to exhibit various biological activities, the specificity of action often being dictated
by the attached functionalities.13 Thiazole-containing heterocyclic peptides, such as nosiheptide,
GE2270 A and nocathiacin,14-17 isolated from marine organisms, exhibit potent biological profiles
─ observations which support the inclusion of the thiazole nucleus in the design of lead compounds
in the development of new active pharmaceutical ingredients (APIs).18 Attention has thus been
given to the development of novel compounds which contain both the styryl and thiazole moieties
and, in this communication, we report the preparation and biological screening of a series of
styrylthiazoles 4, (E)-2-[(naphthalen-1-yl)vinyl]thiazoles and their cinnamic acid precursors.
R2
OH
O
HO
HO
HO
S
OH
OH
HO
N
MeO
R1
O
O
1 p-Coumaric acid
2 Caffeic acid
3 Ferulic acid
4 Styrythiazoles
Figure 1. Structures of cinnamic acid derivatives: p-coumaric acid 1, caffeic acid 2 and ferulic
acid 3 and the proposed styrylthiazoles 4.
Results and Discussion
Access to the cinnamic acids 7a-k and the targeted (E)-styrylthiazoles 4a-g and the (E)-2-[2-
(naphthalen-1-yl)viny]thiazoles 4h-l, all of which satisfy the “Lipinski rule of 5”,19 is outlined in
Scheme 1. Various synthetic methods have been used for the preparation of cinnamic acid and its
derivatives,6,20,21 including compound 7k, as reported by Master et al.16 whose approach has been
adopted in the current study. Thus, the commercially available aldehydes 6a-k were reacted with
malonic acid in pyridine, in the presence of a catalytic quantity of piperidine, at 90 ⁰C for 1 hour
(Scheme 1). [The lower temperature (90 ⁰C) gave yields comparable with those obtained at 120-
130 ⁰C.16] This procedure permitted the diastereoselective synthesis of the desired (E)-cinnamic
acid analogues 7a-f in good yields (68-98%). Confirmation of the E-configuration is provided by
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1
the large H NMR vicinal coupling constant (ca. 16 Hz) between the vinylic protons which
typically resonate at ca. 7.0 and 9.0 ppm. All of the cinnamic acids 7a-k were subjected to
biological screening, while cinnamic acids 7a and 7k served as precursors for the synthesis of the
styrylthiazoles 4a-g and the (E)-2-[2-(naphthalen-1-yl)vinyl]thiazoles 4h-l.
The synthesis of the (E)-2-styrylthiazoles 4a-g involved three steps, viz., i) conversion of (E)-
cinnamic acid 7a to cinnamamide 8a; ii) thionation to obtain the thioamide 9a; and iii)
condensation with α-halo carbonyl derivatives to give the corresponding thiazoles 4a-f (Scheme
1). Following the method developed by Pozdnev et al.,22,23 (E)-cinnamic acid 7a was reacted with
di-tert-butyl dicarbonate [(Boc)2O], ammonium hydrogen carbonate and pyridine in
tetrahydrofuran to afford cinnamamide 8a in good yield (78.5%). Thionation was achieved by
stirring cinnamamide 8a with Lawesson’s reagent in dry THF at ambient temperature for 8 h,24 the
progress of the reaction being monitored by thin layer chromatography (TLC). Work-up and
column chromatography gave (E)-3-phenylprop-2-enethioamide 9a22-27 in 52% yield, with
1
retention of the (E)-configuration about the double bond being confirmed by the large H NMR
vinylic coupling constant (J = 15.6 Hz) and conversion of the carbonyl group to thiocarbonyl by
the significant downfield shift (= 32 ppm)of the thiocarbonyl (C=S) signal to 198.1 ppm.
Application of the conventional Hantzsch method,13 involving reaction of (E)-3-phenylprop-2-
enethioamide 9a with each of the α-halo carbonyl compounds 10a-f (X=Br or Cl) afforded the (E)-
2-styrylthiazoles 4a-f. Hydrolysis of the styrythiazole-5-carboxylate ester 4f afforded the
corresponding acid 4g as a white solid (65%, method 1).
Similar methods were used to obtain the (E)-2-[2-(naphthalen-1-yl)vinyl]thiazoles 4h-l,
starting from (E)-3-(naphthalen-1-yl)-2-propenoic acid 7k and proceeding via the two
intermediates, (E)-3-(naphthalen-1-yl)-2-propenamide 8k (98%) and (E)-3-(naphthalen-1-
yl)prop-2-enethioamide 9k (77%).
The synthetic derivatives 7a-k and 4a-l were screened for anti-malarial (Plasmodium
falciparum), anti-tuberculosis (Mycobacterium tuberculosis) and anti-bacterial (Pseudomonas
aeruginosa) activity as well as for cytotoxicity, in terms of their capacity to inhibit HeLa and SH-
SY5Y cells. All of these compounds {with the exception of the (E)-2-[2-(naphthalen-1-
yl)vinyl]thiazoles (4h-l), and the para-chlorophenyl (4b) and para-bromophenyl (4c) (E)-
styrythiazoles which have predicted Log P values of 5.83-6.99 (i.e. slightly > 5)} satisfy the
requirements of the “Lipinski rule of five” for in vivo transport and the capacity to traverse
biological membranes.19
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Ar
Phenyl
R
% Yield
99
88
94
98
93
68
90
73
90
73
68
83
94
60
85
79
77
78
69
58
63
94
65
7a
7b
7c
7d
7e
7f
-
N,N-Dimethylanilin-4-yl
p-Chlorophenyl
p-Bromophenyl
p-Fluorophenyl
Quinolin-2-yl
Thiophen-2-yl
Furan-2-yl
-
-
-
-
-
7g
7h
7i
-
-
Pyrrol-2-yl
-
7j 3,4-Methylenedioxyphenyl
-
7k
4a
4b
4c
4d
4e
4f
Naphthalen-1-yl
Phenyl
-
Phenyl
Phenyl
p-Chlorophenyl
p-Bromophenyl
p-Fluorophenyl
p-Nitrophenyl
Carbethoxy
Carboxy
Phenyl
Phenyl
Phenyl
Phenyl
Phenyl
4g
4h
4i
Phenyl
Naphthalen-1-yl
Naphthalen-1-yl
Naphthalen-1-yl
Naphthalen-1-yl
Naphthalen-1-yl
p-Chlorophenyl
p-Bromophenyl
p-Fluorophenyl
p-Nitrophenyl
4j
4k
4l
Scheme 1. Synthesis of (E)-cinnamic acid derivatives, (E)-styrylthiazoles and (E)-2-[2-
(naphthalen-1-yl)vinyl]thiazoles: Reagents and conditions: (i) Piperidine, pyridine, 90 ºC, 1 h; (ii)
(Boc)2O, THF, pyridine, NH4HCO3, rt, 6 h; (iii) Lawesson’s reagent, THF, rt, 8 h; (iv) EtOH, 70
ºC, 1 h; v) KOH, MeOH-H2O.
Preliminary in vitro cytotoxic screening of the synthesised compounds 7a-k and 4a-l was
conducted using HeLa cells, while further cytotoxicity studies were conducted on the human
neuroblastoma SH-SY5Y cells using an xCELLigence Real-Time Cell Analyzer (RTCA), the
output of which is illustrated in Figure 2. The real-time monitoring permits label-free analysis of
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cell viability providing insight into the mode of action of the test compounds.28,29 From the HeLa
cell inhibition data (Table 1) it was apparent that with the exception of the p-chlorophenyl- (7c)
and 2-thiophenyl- (7g) cinnamic acid analogues, which exhibited 35-40% inhibition at 10 M, the
remaining cinnamic acid analogues exhibited low levels of inhibition (< 20% at 10 M). The
styrylthiazoles 4a-g and the (E)-2-[2-(naphthalen-1-yl)vinyl]thiazoles 4h-l showed 2-40%
inhibition of HeLa cells at 10 μM, but remarkably variable activity against SH-SY5Y cells (Table
1, Figure 2). Thus, while the RTCA data revealed that of all the (E)-2-[(2-(naphthalen-1-
yl)vinyl]thiazoles 4h-l inhibited the SH-SY5Y cells remarkably with IC50 values in the range of
2.09 to 8.64 µM. Compounds 4a and 4b inhibited the SH-SY5Y cells moderately with IC50 values
of ca. 11 mM, the remaining compounds exhibited IC50 values ranging from 180 to 1000 mM,
with the carboxy analogue 4g exhibiting the lowest toxicity on SH-SY5Y cells with a predicted
IC50 value > 1000 mM. The proliferative effects may be due to the extended -delocalisation in
the styryl- and naphthalenylthiazole scaffolds ─ a structural feature of all-trans-retinoic acid which
has been reported to: i) activate survival signalling in SH-SY5Y cells; ii) promote cell survival;
and iii) reduce cell susceptibility to neurotoxins.30,31 Further studies are required to compare the
effects of compounds 4a-l and all-trans-retinoic acid on the morphology and differentiation of SH-
SY5Y cells,32 and explore their potential activity against neurodegenerative diseases (e.g.,
Parkinson’s and Alzheimer’s diseases).
Figure 2: RTCA dose-response curves for compound 4l (1-100 μM) on SH-SY5Y cells Cell.
Certain substituted cinnamic acids have been shown to exhibit anti-malarial potential,3,33 and
compounds 7a-k, the styrylthazole derivatives 4a-g and the (E)-2-[(naphthalen-1-
yl)vinyl)]thiazoles 4h-l were subjected to in vitro whole cell PfLDH-based (Plasmodium
falciparum parasite lactate dehydrogenase) bioassay,34 the results of which are summarised in
Table 1. Gravina et al.35 found that, while α-cyano- and α-fluorocinnamate exhibited promising
anti-malarial activity, these compounds were unfortunately toxic to human cells ─ a pattern
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mirrored in the cytotoxicity levels observed for the cinnamic acid analogues 7a-k. The PfLDH
inhibition levels exhibited by the cinnamic acid derivatives 7b-k at a concentration of 20 µM lie
in the range 10-30% (chloroquine exhibits 98% inhibition at 2 nM), the 2-furanyl 7g and 2-pyrrolyl
7h derivatives being the least active and the para-fluorophenyl derivative 7f the most active. The
(E)-2-styrylthiazoles 4a-g and the (E)-2-[(naphthalen-1-yl)vinyl)]thiazoles 4h-l also typically
exhibited low inhibition levels against PfLDH (< 20% inhibition at 20 μM).
Cinnamic acid analogues only appear to feature in anti-tuberculosis agents when present as
components of more complex scaffolds3 and, consequently, the in vitro assays for activity against
M. tuberculosis H37Rv were limited to the (E)-2-styrylthiazoles 4a-g and (E)-2-[2-(naphthalen-1-
yl)vinyl]thiazoles 4h-l. While exhibiting some activity, the MIC90 and MIC99 values for these
compounds were found to exceed 20 µM. Thiazole derivatives have been reported to exhibit
antimicrobial activity36 and compounds 4a-l were also subjected to the antibacterial disc diffusion
susceptibility assay against P. aeruginosa at concentrations of 10–2000 µM. The para-
bromophenylstyrythiazole derivative 4c and (E)-4-(4-fluorophenyl)-2-[2-(naphthalen-1-
yl)vinyl]thiazole 4k exhibited low-level zones of inhibition of ca. 7 and 9 mm at 1000 and 2000
µM, respectively (cf- ampicillin, 24.7 mm at 0.0715 µM and streptomycin, 20 mm at 0.0172 µM).
Table 1. Bioassay data showing: the effects of the cinnamic acid analogues 7a-k, the
styrylthiazoles 4a-g and the (E)-2-[2-(naphthalen-1-yl)vinyl]thiazoles 4h-l on the viability of
HeLa cells; IC50 values for the inhibition of SH-SY5Y cells by compounds 4a-l; the effects of
compounds 7a-k and 4a-l on the viability of PfLDH at 20 µM; and the effects of compounds 4a-
g and 4h-l against Mycobacterium tuberculosis H37Rv
Compd.
R
% HeLa SH-SY5Y % PfLDH
viability IC50 (µM)b viability
MIC90
(µM)d
MIC99
(µM)d
at 10
µMa
at 20 µMc
7a
7b
7c
7d
7e
7f
Phenyl
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
N,N-dimethylanilin-4-yl
p-Chlorophenyl
p-Bromophenyl
p-Fluorophenyl
Quinolin-2-yl
85
65
90
85
90
60
80
75
80
85
80
75
75
70
75
90
90
80
80
80
7g
7h
7i
Thiophen-2-yl
Furan-2-yl
Pyrrol-2-yl
7j
3,4-Methylenedioxyphenyl
Naphthalen-1-yl
7k
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Table 1 (continued)
Compd.
R
% HeLa
viability IC50 (µM)b viability
SH-SY5Y % PfLDH
MIC90 MIC99
(µM)d
(µM)d
at 10 µMa
at 20 µMc
S
4a-g: Ar = Phenyl
4h-l: Ar = Naphthalen-1-yl
R
N
Ar
4a
4b
4c
4d
4e
4f
Phenyl
60
62
60
70
90
82
98
1.08x104
1.17 x104
1.80 x105
5.73 x105
4.03 x105
3.51 x105
> 1x106
95
102
95
> 20
> 20
> 20
> 20
> 20
> 20
> 20
> 20
p-Chlorophenyl
p-Bromophenyl
p-Fluorophenyl
p-Nitrophenyl
Carbethoxy
> 20
> 20
> 20
> 20
> 20
> 20
92
82
98
4g
Carboxy
90
4h
4i
Phenyl
60
65
70
75
80
2.09
5.19
8.64
4.87
2.23
82
90
> 20
> 20
> 20
> 20
> 20
> 20
> 20
> 20
> 20
> 20
p-Chlorophenyl
p-Bromophenyl
p-Fluorophenyl
p-Nitrophenyl
a
4j
4k
4l
95
105
112
b
Control compounds: Untreated HeLa cells: 100% viability; Untreated SH-SY5Y cells: 100%
viability; Chloroquine: 4% viability at 2 nM; Rifampicin: 0.0015µM (MIC90) & 0.00167 µM
c
d
(MIC99).
Conclusions
Various cinnamic acid analogues 7a-k have been prepared and used as precursors for the synthesis
of 2-styrylthiazole derivatives 4a-g and (E)-2-[(naphthalen-1-yl)vinyl)]thiazoles 4h-l. None of
these compounds exhibited significant inhibition of HeLa cells nor significant antimalarial, anti-
tuberculosis or antibacterial activity. However, the (E)-2-[2-[(naphthalen-1-yl)vinyl]thiazoles 4h-
l exhibited remarkable activities against SH-SY5Y cells (with IC50 values ranging from 2.09 to
8.64 µM), while the 2-styrylthiazole derivatives 4a-g showed moderate activities against SH-
SY5Y cells ranging from inhibition in two cases (with IC50 values of 10.8 and 11.7 mM) to
proliferation, with IC50 values ranging from 180 to > 1000 mM. The results indicate that studies
are warranted on the effects of styrylthiazoles on the differentiation and extension of SH-SY5Y
cells in order to assess their therapeutic potential in the treatment of neurological degenerative
diseases.
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Experimental Section
All reagents were obtained from Sigma-Aldrich (South Africa) and used without further
purification. Tetrahydrofuran (THF) and methylene chloride were stored over 4 Å molecular
sieves. Reaction progress and purity of the compounds were checked by thin layer chromatography
(TLC) on pre-coated silica gel G60 F254 plates (Merck®), and viewed under UV light (Syngiene
LF-206.LS lamp, South Africa) at 254 and 365 nm. Melting points were recorded, uncorrected,
using Reichert(R) slide warmer hot plate microscopy. Nuclear magnetic resonance (NMR) spectra
were recorded on Bruker AvanceTM II 600 MHz, Bruker AvanceTM III HD 400 MHz and Bruker
FourierTM 300 MHz spectrometers. The NMR chemical shifts are reported in ppm downfield from
tetramethylsilane (TMS), and the coupling constants are given in Herz (Hz). The NMR analyses
were carried out in deuterated solvents, such as DMSO-d6, CDCl3, acetone-d6 and methanol-d4,
and the spectra calibrated using solvent signals [δH: 7.26 ppm for residual CHCl3, 2.50 ppm for
residual DMSO, 2.05 ppm for residual acetone and 3.31 ppm for residual MeOH; δC: 77.2 ppm
(CDCl3), 39.5 ppm (DMSO-d6), 29.8 ppm (acetone-d6) and 49.0 ppm (MeOH-d4)]. Infrared (IR)
spectra were obtained using a Perkin Elmer Spectrum 400 Frontier / FT-IR spectrometer;
compounds were analysed neat. High resolution mass spectra (HRMS) were recorded on a Waters
API Q-TOF Ultima spectrometer (University of Stellenbosch, Stellenbosch, South Africa).
Compounds 7a-k,37, 8a,16,37 8b,38 9a,16,24 9b,39 4a,39-41 4e-g,42 4h39 and 4k43,44 are known. The
preparation and characterisation of the known compound 4a and the new compounds are
summarised below. NMR data for new compounds and bioassay procedures are provided in the
supplementary data.
Formation of the (E)-styrylthiazoles 4a-g and the (E)-2-[2-(naphthalen-1-yl)vinyl]thiazoles
4h-l.
General procedure, exemplified by the preparation of 4-phenyl-2-styrylthiazole (4a). A
mixture of (E)-3-phenylprop-2-enethioamide (9) (0.082 g, 0.5 mmol) and 2-bromoacetophenone
(0.099 g, 0.5 mmol) in ethanol (1.5 mL) was stirred at 70 0C for 1 h. The solvent was removed in
vacuo and the crude product was extracted with EtOAc. The resulting solution was washed
successively with satd. aq. NaHCO3 and water, and dried (anhydr. Na2SO4), and the solvent was
removed in vacuo. The crude product was purified using column chromatography on silica gel G60,
eluting with hexane-EtOAc (3:2) to give, as a yellowish, fluffy solid, 4-phenyl-2-styrylthiazole
(4a) (0.109 g, 82.6%), mp 130-132 ºC (Lit.39-41 131.0-131.5 ºC) [HRMS: m/z calculated for
C17H14NS (MH+) 264.0847. Found M+1, 264.0841]; δH/ppm (400 MHz; DMSO-d6) 8.07 (1H, s,
thiazolyl-H), 8.00 (2H, d, J = 7.6 Hz, ArH), 7.72 (2H, d, J = 7.3 Hz, ArH), 7.55 (2H, s, HC=CH ),
7.50–7.40 (4H, overlapping m, ArH) and 7.39–7.33 (2H, overlapping m, ArH); δC/ppm (100 MHz;
DMSO-d6) 165.8, 154.8, 135.3, 133.9, 133.7, 128.7, 128.6, 128.5, 127.9, 127.0, 125.9, 121.1 and
113.9 (ArC and HC=CH).
(E)-4-(4-Chlorophenyl)-2-styrylthiazole (4b) as a yellow solid (0.140 g, 94%), mp 150-153 ºC
[HRMS: m/z calculated for C17H13NS35Cl (MH+), 298.0457. Found M+1, 298.0448]; δH/ppm (400
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MHz; DMSO-d6) 8.15 (1H, s, thiazolyl-H), 8.04 (2H, d, J = 7.6 Hz, ArH), 7.73 (2H, d, J = 7.3 Hz,
ArH), 7.56 (2H, s, HC=CH), 7.53 (2H, d, J = 8.3 Hz, ArH), 7.42 (2H, t, J = 7.3 Hz, ArH) and 7.37
(1H, t, J = 7.1 Hz, ArH); δC/ppm (100 MHz; DMSO-d6) 166.2, 153.7, 135.4, 134.3, 132.8, 132.6,
129.0, 128.8, 128.7, 127.8, 127.3, 121.2 and 114.8 (ArC and HC=CH).
(E)-4-(4-Bromophenyl)-2-styrylthiazole (4c) as a fluffy, yellow solid (0.103 g, 60.1%), mp 169-
170 ºC [HRMS: m/z calculated for C17H13NS79Br (MH+), 341.9952. Found M+1, 341.9945];
δH/ppm (400 MHz; DMSO-d6) 8.15 (1H, s, thiazolyl-H), 7.96 (2H, d, J = 8.0 Hz, ArH), 7.73 (2H,
d, J = 7.0 Hz, ArH), 7.66 (2H, d, J = 8.0 Hz, ArH), 7.55 (2H, s, HC=CH), 7.42 (2H, dd, J = 6.6 Hz
and J = 7.1 Hz, ArH), 7.37 (1H, t, J = 7.2 Hz, ArH); δC/ppm (100 MHz; DMSO-d6) 166.8, 154.2,
135.9, 134.8, 133.7, 132.2, 129.5, 129.4, 128.6, 127.8, 121.8, 121.7 and 115.4 (ArC and HC=CH).
(E)-4-(4-Fluorophenyl)-2-styrylthiazole (4d) as a yellow solid (0.091 g, 64.9%), mp 126-128 ºC
[HRMS: m/z calculated for C17H13NSF (MH+), 282.0753. Found M+1, 282.0742]; δH/ppm (400
MHz; DMSO-d6) 8.06 (1H, s, thiazolyl-H), 8.04 (2H, m, ArH ), 7.73 (2H, d, J = 7.1 Hz, ArH),
7.55 (2H, s, HC=CH), 7.42 (2H, t, J* = 6 Hz, ArH), 7.37 (1H, m, ArH) and 7.30 (2H, t, J* = 10
Hz, ArH); δC/ppm (100 MHz; DMSO-d6) 166.5, 163.5 (1JF,C = 246 Hz), 155.3, 136.0, 134.6, 131.0
(4JF,C = 3.6 Hz), 129.5, 129.4, 128.7 (3JF,C = 9.8 Hz), 127.8, 121.8, 116.1 (2JF,C = 21.9 Hz) and
114.4 (ArC, HC=CH and thiazolyl-C).
* Overlapping doublets (JH,H and JF,H).
(E)-4-(4-Chlorophenyl)-2-[2-(1-naphthalenyl)vinyl]thiazole (4i) as a yellow solid, mp 118-120
ºC ; [HRMS: m/z calculated for C21H15NS35Cl (MH+), 348.0614. Found M+1, 348.0606]; /cm-1
1560 (C=C) and 3143 (C=CH, ArH); δH/ppm (400 MHz; CDCl3) 8.30 (1H, d, J = 16.1 Hz,
CH=CHa), 8.26 (1H, d, J = 8.4 Hz, ArH), 7.93-7.86 (4H, m, ArH), 7.83 (1H, d, J = 7.3 Hz, ArH),
7.62-7.57 (1H, m, ArH), 7.56-7.50 (2H, m, ArH), 7.44-7.39 (4H, overlapping m, 2 x ArH,
CH=CHb, and thiazolyl-H); δC/ppm (100 MHz; CDCl3) 167.2, 155.3, 134.2, 133.9, 133.2, 133.0,
131.8, 131.5, 129.5, 129.1, 128.9, 127.9, 126.7, 126.3, 125.8, 124.4, 124.1, 123.7 and 112.7 (ArC,
HC=CH and thiazolyl-C).
(E)-4-(4-Bromophenyl)-2-[2-(1-naphthalenyl)vinyl]thiazole (4j) as a yellow solid, mp 120-122
ºC; [HRMS: m/z calculated for C21H15NS79Br (MH+) 392.0109. Found M+1, 392.0090]; /cm-1
1513 (C=C) and 3028 (C=CH, ArH); δH/ppm (400 MHz; CDCl3) 8.30 (1H, d, J = 15.9 Hz,
CH=CHa), 8.26 (1H, d, J = 8.3 Hz, ArH), 7.90-7.84 (4H, overlapping m, ArH), 7.83 (1H, d, J =
7.3 Hz, ArH) 7.62-7.49 (5H, overlapping m, ArH), 7.45 (1H, s, thiazolyl-H) and 7.41 (1H, d, J =
15.9 Hz, CH=CHb); δC/ppm (100 MHz; CDCl3) 167.0, 155.2, 133.8, 133.3, 133.1, 131.9, 131.7,
131.3, 129.4, 128.8, 128.0, 126.6, 126.1, 125.7, 124.2, 124.0, 123.5, 122.3 and 112.7 (ArC,
HC=CH and thiazolyl-C).
(E)-2-[2-(1-Naphthalenyl)vinyl)]-4-(4-nitrophenyl]thiazole (4l) as a bright yellow solid, mp
148-149 ºC; [HRMS: m/z calculated for C21H15N2O2S (MH+) 359.0854. Found M+1, 359.0839];
/cm-1 1598 (C=C) and 3101 (C=CH, ArH); δH/ppm (400 MHz; DMSO-d6) 8.52 (1H, s, thiazolyl-
H), 8.41 (1H, d, J = 15.8 Hz, CH=CHa), 8.39-8.32 (5H, overlapping m, ArH), 8.05 (1H, d, J = 7.3
Hz, ArH), 8.02-7.98 (2H, overlapping m, ArH), 7.68-7.64 (2H, overlapping m, ArH) and 7.63-
7.58 (2H, overlapping m, ArH and CH=CHb); δC/ppm (100 MHz; DMSO-d6) 166.8, 152.8, 146.8,
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139.9, 133.4, 132.3, 131.1, 130.7, 129.5, 128.7, 127.1, 126.9, 126.3, 125.9, 124.4, 124.3, 123.5,
123.4 and 118.8 (ArC, HC=CH and thiazolyl-C).
Hydrolysis of ethyl (E)-2-styrylthiazole-4-carboxylate (4f).
This was achieved using two different methods.
Method 1.45 A solution of KOH (0.093 g, 0.5 mmol) in MeOH-H2O [(2:1), 300 µL] was added to
ethyl (E)-2-styrylthiazole-4-carboxylate (4f) (0.088 g, 0.25 mmol) in MeOH (250 µL). The
resulting solution was stirred at room temperature for 2 h, and the reaction was monitored by TLC
[hexane-EtOAc (1:1)]. Addition of HCl (20%; 0.5 mL) to the reaction mixture precipitated (E)-2-
styrylthiazole-4-carboxylic acid (4g) (0.038 g, 65.4%) as a yellow solid, mp 178-180 ºC (this
compound has been cited in the literature44 without a mp) [HRMS: m/z calculated for C12H10NO2S
(MH+) 232.0422. Found M+1, 232.0431]; ν/cm-1 1730 (C=O) and 2598-3140 (COOH); δH/ppm
(400 MHz; DMSO-d6) 8.39 (1H, s, thiazolyl-H), 7.71 (2H, J = 8.2 Hz, ArH), 7.53 (2H, apparent
d,  17.6 Hz, CH=CH), 7.42 (2H, t, J = 7.2 Hz, ArH) and 7.36 (1H, m, ArH); δC/ppm (100
MHz; DMSO-d6) 166.4 (C=O), 162.0, 147.8, 135.2, 128.9, 128.1, 128.1, 128.0, 127.4 and 121.0
(ArC and HC=CH).
Method 2.43 The procedure for the synthesis of compound 4a was employed, using (E)-3-
phenylprop-2-enethioamide 9 (0.163 g, 1 mmol) and bromopyruvic acid (0.167 g, 1 mmol) to
obtain the desired product 4g as a yellow solid (0.175 g, 75.5%).
Acknowledgements
The authors thank Rhodes University for a bursary (E.O.O) and Rhodes University and the South
African Medical Research Council (MRC) for generous financial support. This research project
was supported by the South African Medical Research Council (MRC) with funds from National
Treasury under its Economic Competitiveness and Support Package. The xCELLigence Real-
Time Cell Analyzer SP Station was supported through the National Research Foundation (NRF)
Department of Science & Technology (DST) Research Infrastructure Support Programme
(National Nanotechnology Equipment Programme) grant.
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