5-Ethoxymethylidene-4-thioxo-2-thiazolidinone as versatile building block for novel biorelevant small molecules with thiopyrano[2,3-D][1,3]thiazole Core

Article abstract of DOI:10.1080/00397911.2013.800552

5-Ethoxymethylidene-4-thioxo-2-thiazolidinone was studied as versatile core building block in the synthesis of new thiopyrano[2,3-d]thiazole derivatives relevant for medicinal chemistry purposes under hetero-Diels-Alder reaction conditions. Promising compounds (6, 10) were identified among synthesized series with high antitumor and moderate antiviral activity. [Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications for the following free supplemental resource(s): Full experimental and spectral details.]

Full text of DOI:10.1080/00397911.2013.800552

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5-Ethoxymethylidene-4-thioxo-2-
thiazolidinone as Versatile Building Block
for Novel Biorelevant Small Molecules
with Thiopyrano[2,3-d][1,3]thiazole
Core
Dmytro Atamanyuk a , Borys Zimenkovsky a , Vasyl Atamanyuk a
Roman Lesyk a
a Department of Pharmaceutical, Organic, and Bioorganic
&
Chemistry , Danylo Halytsky Lviv National Medical University , Lviv ,
Ukraine
Accepted author version posted online: 27 Aug 2013.
To cite this article: Dmytro Atamanyuk , Borys Zimenkovsky , Vasyl Atamanyuk & Roman Lesyk
(2014) 5-Ethoxymethylidene-4-thioxo-2-thiazolidinone as Versatile Building Block for Novel
Biorelevant Small Molecules with Thiopyrano[2,3-d][1,3]thiazole Core, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 44:2, 237-244, DOI:
10.1080/00397911.2013.800552
To link to this article: http://dx.doi.org/10.1080/00397911.2013.800552
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Synthetic Communications1, 44: 237–244, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2013.800552
5-ETHOXYMETHYLIDENE-4-THIOXO-
2-THIAZOLIDINONE AS VERSATILE BUILDING BLOCK
FOR NOVEL BIORELEVANT SMALL MOLECULES
WITH THIOPYRANO[2,3-d][1,3]THIAZOLE CORE
Dmytro Atamanyuk, Borys Zimenkovsky, Vasyl Atamanyuk,
and Roman Lesyk
Department of Pharmaceutical, Organic, and Bioorganic Chemistry,
Danylo Halytsky Lviv National Medical University, Lviv, Ukraine
GRAPHICAL ABSTRACT
Abstract 5-Ethoxymethylidene-4-thioxo-2-thiazolidinone was studied as versatile core
building block in the synthesis of new thiopyrano[2,3-d]thiazole derivatives relevant for
medicinal chemistry purposes under hetero-Diels–Alder reaction conditions. Promising
compounds (6, 10) were identified among synthesized series with high antitumor and
moderate antiviral activity.
[Supplementary materials are available for this article. Go to the publisher’s online
edition of Synthetic Communications1 for the following free supplemental resource(s):
Full experimental and spectral details.]
Keywords Biological activity; 5-ethoxymethylidene-4-thioxo-2-thiazolidinone; hetero-
Diels–Alder reaction; thiopyrano[2,3-d]thiazole
INTRODUCTION
At present 5-aryl=heteryl=idene-4-thiazolidinone derivatives constitute an
important series of biologically active compounds and important intermediates in
modern medicinal and synthetic organic chemistry by possessing synthetic multi-
functionality[1–3] and bioactivity in diverse pharmacological areas.[4–10] Substituted
ylidene fragment conjugation with the carbonyl group at position 4 of the
Received April 10, 2013.
Address correspondence to Roman Lesyk, Department of Pharmaceutical, Organic, and
Bioorganic Chemistry, Danylo Halytsky Lviv National Medical University, Pekarska 69, 79010, Lviv,
Ukraine. E-mail: dr_r_lesyk@org.lviv.net
237

238
D. ATAMANYUK ET AL.
thiazolidine moiety rendered compounds potentially reactive because of possible
Michael addition of the nucleophilic protein residues to the exocyclic double bond.
This property characterizes 5-arylidene-4-thiazolidinones as frequent hitters or pan
assay interference compounds (PAINS) that are useless in the modern drug
discovery process because of their insufficient selectivity.[11–14] Following our pre-
vious findings we have proposed the possibility of avoiding the mentioned features.
Thus, it was established that fixation of highly active 5-aryl=heteryl=idene-4-thiazo-
lidinone moiety in structure-related 7-aryl=heteryl=thiopyrano[2,3-d][1,3]thiazole
system usually allows conservation of the biological activity vector, and the latter
systems could be considered as cyclic isosteric mimics of their synthetic precursors,
5-arylidene-4-thiazolidinones, without Michael accepting functionalities.[15–19]
Experimental development of this concept gave us an opportunity to synthesize
and select hit and lead compounds possessing antitumor,[15,17] antituberculosis,[19]
and antitrypanosomal[20] activities. Modern approaches in drug discovery and
development processes are oriented on low-molecular-weight compounds. Thus,
we decided to pursue our efforts in simplification of thiopyrano[2,3-d][1,3]thiazoles
while synthesizing new derivatives without aryl- or heteryl substituents in position
7 of the core heterocycle.[21]
Our synthetic strategy was based on 5-ethoxymethylidene-4-thioxo-2-thiazolidi-
none (5-ethoxymethylideneisorhodanine) as heterodiene in hetero-Diels–Alder
reaction. The latter contains a C¼C5-C4 ¼ S group and is an active heterodiene.
According to literature data[22] hetero-Diels–Alder reaction of 5-ethoxymethylidene-4-
thioxo-2-thiazolidinone and dienophiles leads to the formation of [4 þ 2]-adducts that
Scheme 1. Background for applicability of simplified scaffolds in medicinal chemistry optimization
studies. (Figure is provided in color online.)

BIORELEVANT THIOPYRANO[2,3-d] THIAZOLES
239
undergo spontaneous ethanol elimination, forming endocyclic double bonds
(Scheme 1).
This approach allowed us to access low-molecular-weight thiopyrano[2,3-
d][1,3]thiazoles as novel scaffolds for the drug discovery process. Nevertheless, this
type of hetero-Diels–Alder reaction is rarely used. Only a limited number of dieno-
philes, such as acrylic and maleic acid derivatives and b-nitrostyrene, were studied in
the reaction with 5-ethoxymethylideneisorhodanine.[22]
RESULTS AND DISCUSSION
Hence, a series of new 3,5-dihydro-2H-thiopyrano[2,3-d][1,3]thiazoles were
synthesized using hetero-Diels–Alder reaction based on 5-ethoxymethylidene-4-
thioxo-2-thiazolidinone 1.[22] We studied 1 in hetero-Diels–Alder reaction with
several dienophiles, in particular, acrolein, crotonic aldehyde, 3-phenyl-2-propenal,
acetylenedicarboxylic acid and its methyl ester, aroylacrylic acids, 2-norbonene,
imides of 5-norbornene-2,3-dicarboxylic acid (endic acid), and naphthoquinone.
Thus, in the reactions of 1 with acrolein, crotonic aldehyde, and 3-phenyl-2-
propenal, appropriate 5-R-2-oxo-3,5-dihydro-2H-thiopyrano[2,3-d][1,3]thiazole-6-
carbaldehydes were synthesized (2a–c). Using as dienophiles 2-norbornene and
several derivatives of endic acid imide (derivatives of 4-azatricyclo[5.2.1.02,6]dec-8-
ene-3,5-dione) allowed us to obtain the expected tetra- (3) and pentacyclic structures
(4, 5), containing 3,5-dihydrothiopyrano[2,3-d]thiazole-2-one fragment (Scheme 2).
We established that hetero-Diels–Alder reaction of acetylene derivatives as die-
nophiles in [4 þ 2]-cyclocondensation with 1 passed with ethanol elimination and
simultaneous conjugated double-bond rearrangement. Thus, reaction of 1 with pro-
pynoic acid and its ethyl ester as well as acetylenedicarboxylic acid dimethyl ester
afforded corresponding 2-oxo-2H-thiopyrano[2,3-d][1,3]thiazole derivatives 6–8.
Acetylenedicarboxylic acid Diels–Alder adduct underwent unexpected decarboxy-
lation, affording acid 6.
A similar decarboxylation was observed during
Scheme 2. Synthesis of new 5-R-2-oxo-3,5-dihydro-2H-thiopyrano[2,3-d][1,3]thiazole-6-carbaldehydes (2a–c)
and polycyclic fused 3,5-dihydro-2H-thiopyrano[2,3-d][1,3]thiazoles containing norbornane moiety (3–5).

240
D. ATAMANYUK ET AL.
Scheme 3. Synthesis of new 2-oxo-2H-thiopyrano[2,3-d][1,3]thiazole derivatives 6–8 and 6-aryl-2-
oxo-3,5-dihydro-2H-thiopyrano[2,3-d][1,3]thiazol-6-yl methanones 9a and b.
[4 þ 2]-cyclocondensation of 1 with aroylacrylic acids, yielding regioselectively
6-aryl-2-oxo-3,5-dihydro-2H-thiopyrano[2,3-d][1,3]thiazol-6-yl methanones 9a and
b (Scheme 3).
Elimination, double-bond rearrangement, and decarboxylation phenomena
1
were confirmed by H and 13C NMR spectra and mass spectrometry (MS) (Sup-
plementary Information). In the NMR spectra of 6–9 a shift of the proton in the
7 position in weak magnetic field (r ꢀ 5.90–9.09) was observed, the latter being
included into the conjugated double-bond system of 3,5-dihydrothiopyrano
[2,3-d]thiazole. For 9a and b singlets corresponding to two protons in position 5
and one in position 7 were characteristic as well.
Hetero-Diels–Alder reaction of 1 with 1,4-naphthoquinone (Scheme 4)
afforded 10 and was followed by (i) the intermediate dehydrogenation because of
hydroquinone excess, (ii) the formation of an additional endocyclic double bond,
and (iii) EtOH elimination. Consequently, a rearrangement of conjugated double
1
bonds took place, confirmed by the absence of a singlet NH group in H NMR
spectrum and MS of 10.
Scheme 4. Synthesis of 5,10-dihydro-2H-benzo[6,7]thiochromeno[2,3-d][1,3]thiazole-2,5,10-trione 10.

BIORELEVANT THIOPYRANO[2,3-d] THIAZOLES
241
Table 1. Anticancer activity results (GI50) of compound 10 for the most sensitive cancer cell lines (mM)
Type of cancer
Non-small
cell lung cancer
Prostate Breast
cancer cancer
Leukemia
Melanoma
LOX
HOP-62 NCI-H23 IMVI MALME-3M SK-MEL-5 DU-145 MCF7
HL-60(TB)
0.186
SR
0.269
0.295
0.257
0.214
0.251
0.138
0.302
0.309
Synthesized compounds 6, 9a, 9b, and 10 were tested by the National Cancer
Institute Developmental Therapeutics Program (NCI DTP) (www.dtp.nci.nih.gov)
for the in vitro anticancer activity[23–27] on 60 human tumor cell lines at concen-
tration of 10 mM. Growth percent (GP) values were identified, and results are sum-
marized in Table S1 (Supporting Information).
Derivatives 9a and 9b, besides low mean levels of tumor cells growth inhibition,
have shown selectivity for leukemia cells (Table S1). The compound 10 was character-
ized by high levels of growth inhibition of cancer cell lines SK-MEL-5 (melanoma),
OVCAR-3 (ovarian cancer), and MDA-MB-435 (breast cancer) and was tested at
5- to 10-fold dilutions, characterized with the following values of mean dose response
parameters: GI50 ¼ 1.95 mM, TGI ¼ 20.42 mM, and LC50 ¼ 79.43 mM. The results of
compound 10 activity at the most sensitive tumor cell lines are presented in Table 1.
Promising in vitro activity of compound 10 allowed its evaluation in the hollow
fiber assay. This assay provides quantitative indications of the compound’s drug
efficacy on tumor cell death (http://dtp.nci.nih.gov/branches/btb/hfa.html).[28]
However, 10 did not show significant activity in the hollow fiber assay but instead
showed low cytotoxic activity.
Also compounds 6 and 10 were tested for antiviral activity according to the
antimicrobial acquisition and coordinating facility (AACF) screening program.[29]
Studied compounds did not show significant activity against viruses flu A (H1N1),
flu A (H3N2), flu A (H5N1), flu B, measles, PIV (parainfluenza virus), RSV A (res-
piratory syncytial virus A) and SARS, but compound 10 did possess moderate
activity against coronavirus SARS using both systems of results assesment: visual
(EC50vis ¼ 1.7; IC50vis ¼ 23; SIvis ¼ 14) and with neutral red dye (EC50-NR ¼ 6;
IC50-NR ¼ 14; SI-NR ¼ 5.4).
EXPERIMENTAL
All starting materials were purchased from Merck, Sigma-Aldrich, or
Lancaster and used without purification. Melting points are uncorrected and
were measured in open capillary tubes on a Buchi B-545 melting-point apparatus.
1
The H NMR spectra were recorded on a Varian Gemini 400-MHz instrument
and 13C NMR spectra on Varian Mercury-400 100-MHz instrument in dimethyl-
sulfoxide (DMSO-d6) or CF3COOD (compound 10) with tetramethylsilane
(TMS) as an internal standard [chemical shift values are reported in parts per
million (ppm) units, coupling constants (J) are in hertz]. The elemental analyses

242
D. ATAMANYUK ET AL.
(C, H, N) performed on the Perkin-Elmer 2400 CHN analyzer were within ꢁ0.4%
of the theoretical values. Mass spectra were obtained on the Varian 1200 L
instrument.
General Procedure of Thiopyrano[2,3-d][1,3]thiazol-2-one Derivative
(2–10) Preparation
The starting 5-ethoxymethylidene-4-thioxo-2-thiazolidinone[22] and imides of
5-norbornene-2,3-dicarboxylic acid[17] were obtained according to the methods
described previously. To a mixture of 5-ethoxymethylidene-4-thioxo-2-thiazolidi-
none (1) (10 mmol) and appropriative dienophile (10 mmol) for compound 10
[1,4-naphthoquinone (20 mmol)] were added glacial acetic acid (10 mL) and hydro-
quinone (2–3 mg), and the reaction mixture was heated with stirring at 100 ꢂC for
1 h and then allowed to cool overnight to room temperature. The precipitated crys-
tals were filtered off, washed with ethanol, and recrystallized from an appropriate
solvent. Synthetic protocol of 10 involved 2 eq. of 1,4-naphthoquinone to ensure
intermediate redox process.
Spectral and analytical data for compounds 2a, 6, 9a, 10 are as follows. Data
on compounds 2b, 2c, 3–5, 7, 8, and 9b as well as data of primary antitumor activity
assay of all synthesized substances are in the Supporting Information, available
online.
2-Oxo-3,5-dihydro-2H-thiopyrano[2,3-d][1,3]thiazole-6-
carbaldehyde (2a)
Yield 68%; mp 196–198 ꢂC (EtOH). 1H NMR (400 MHz, DMSO-d6): 3.84
(s, 2H, CH2-S), 7.24 (s, 1H, 7-H), 9.40 (s, 1H, CHO), 12.40 (br. s, 1H, NH).
13C NMR (100 MHz, DMSO-d6): 23.38, 106.88, 121.38, 137.23, 140.85, 172.03,
190.40. ESI-MS m=z 200 (MþH)þ.
2-Oxo-2H-thiopyrano[2,3-d][1,3]thiazole-6-carboxylic Acid (6)
Yield 78%, mp 228–230 ꢂC (DMF). 1H NMR (400 MHz, DMSO-d6): 8.57
(d, 1H, J ¼ 1.5 Hz, 5-H), 9.09 (d, 1H, J ¼ 1.5 Hz, 7-H), 13.97 (br. s, 1H, COOH).
13C NMR (100 MHz, DMSO-d6): 126.10, 131.78, 138.74, 139.17, 164.34, 177.72,
178.87. ESI-MS m=z 214 (MþH)þ.
6-(4-Fluorobenzoyl)-3,5-dihydro-2H-thiopyrano[2,3-d]
[1,3]thiazol-2-one (9a)
1
Yield 76%, mp 220–222 ꢂC (AcOH). H NMR (400 MHz, DMSO-d6): 4.66 (s,
2H, CH2), 6.93 (s, 1H, 7-H), 7.37 (t, 2H, J ¼ 8.8 Hz, arom.), 8.12 (dd, 2H, J ¼ 8.6,
5.6 Hz, arom.), 12.10 (br. s, 1H, NH). 13C NMR (100 MHz, DMSO-d6): 114.28,
116.40, 116.62, 120.46, 129.27, 132.04, 132.13, 133.15, 165.89 (d, J ¼ 250.0 Hz,
C-F), 174.18, 195.62. ESI-MS m=z 293 (MþH)þ.

BIORELEVANT THIOPYRANO[2,3-d] THIAZOLES
243
5,10-Dihydro-2H-benzo[6,7]thiochromeno[2,3-d]
[1,3]thiazole-2,5,10-trione (10)
Yield 81%, mp > 250 ꢂC (DMF). 1H NMR (400 MHz, CF3COOD): 7.80 (br. s,
2H, arom.), 7.92 (br. s, 2H, arom.), 9.11 (s, 1H, ¼CH). ESI-MS m=z 300 (MþH)þ.
ACKNOWLEDGMENTS
We are grateful to V. L. Narayanan from Drug Synthesis and Chemistry
Branch, National Cancer Institute, Bethesda, MD, USA, for in vitro evaluation of
anticancer activity and to Chris Tseng, from Division of Microbiology and Infec-
tious Diseases, NIAID=NIH, Bethesda, MD, USA, for in vitro evaluation of anti-
viral activity.
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