Synthesis and characterization of sulfide, sulfoxide and sulfone derivatives of thiopyran: antimicrobial evaluation

Article abstract of DOI:10.1007/s11164-017-3075-4

Abstract: A series of thiopyran derivatives and their oxidized analogous forms were synthesized and characterized by FT-IR, ¹H, ¹³C, ³¹P NMR and mass spectroscopy techniques. The antibacterial and antifungal activities of these synthesized materials were evaluated against Staphylococcus aureus and Bacillus subtilis, as Gram-positive bacteria, and Escherichia coli and Pseudomonas aeruginosa, as Gram-negative bacteria, as well as the fungus Candida albicans. The results revealed that thiopyran S,S-dioxides are the most effective against all the bacteria studied in this work. Furthermore, thiopyran S-oxides showed excellent antifungal activity against Candida albicans.

Full text of DOI:10.1007/s11164-017-3075-4

Res Chem Intermed
DOI 10.1007/s11164-017-3075-4
Synthesis and characterization of sulfide, sulfoxide
and sulfone derivatives of thiopyran: antimicrobial
evaluation
Ghasem Firouzzade Pasha1 Sakineh Asghari1,2
•
•
Mahmoud Tajbakhsh1 Mojtaba Mohseni2,3
•
Received: 22 November 2016 / Accepted: 12 July 2017
Ó Springer Science+Business Media B.V. 2017
Abstract A series of thiopyran derivatives and their oxidized analogous forms were
synthesized and characterized by FT-IR, 1H, 13C, 31P NMR and mass spectroscopy
techniques. The antibacterial and antifungal activities of these synthesized materials
were evaluated against Staphylococcus aureus and Bacillus subtilis, as Gram-positive
bacteria, and Escherichia coli and Pseudomonas aeruginosa, as Gram-negative bacteria,
as well as the fungus Candida albicans. The results revealed that thiopyran S,S-dioxides
are the most effective against all the bacteria studied in this work. Furthermore,
thiopyran S-oxides showed excellent antifungal activity against Candida albicans.
Electronic supplementary material The online version of this article (doi:10.1007/s11164-017-3075-4)
contains supplementary material, which is available to authorized users.
&
Sakineh Asghari
s.asghari@umz.ac.ir
1
2
3
Department of Organic Chemistry, Faculty of Chemistry, University of Mazandaran,
Babolsar 47416-95447, Iran
Nano and Biotechnology Research Group, University of Mazandaran, Babolsar 47416-95447,
Iran
Department of Microbiology, Faculty of Science, University of Mazandaran,
Babolsar 47416-95447, Iran
123

G. F. Pasha et al.
Graphical Abstract
Keywords Antibacterial Á Antifungal Á Thiopyran S-oxides Á
Thiopyran S Á S-dioxides Á Thiopyrans
Introduction
Blocking the growth and multiplication of bacteria is an important action for
antimicrobial agents. Many infectious diseases, caused by a variety of pathogens,
can be controlled by antimicrobials [1]. Resistance to antimicrobial agents is
increasing rapidly, and, therefore, designing new and potential antimicrobial agents
with low risk of toxicity is a crucial issue [2]. The compounds possessing
heterocyclic moieties have attracted much interest due to their biological and
medicinal activities [3, 4]. Thiopyrans, six-membered heterocyclic compounds,
have occupied an important place in the realm of natural and synthetic organic
chemistry. They are key units in the medicinal chemistry and are widely used in the
organic synthesis as versatile building blocks of biological active compounds [5, 6].
Thiopyran derivatives have shown efficient antimicrobial activity in a wide range of
pharmaceutical and medicinal chemistry [7–13]. For instance, compounds A, B and
C (Fig. 1) have been reported as antibacterial [14], anti-inflammatory [15], and
anticancer [16] reagents, respectively. Thiopyran derivatives are also used in the
synthesis of various bioactive compounds with potential biological activities, such
as serricornin [17], tetrahydrodicranenone B [18], cyclopentanoids [19] and
thromoboxanes [20]. Furthermore, it has been reported that oxidation of thiopyran
sulfides to S-oxides and S,S-dioxides can significantly increase the biological
activities of thiopyrans [15, 21–27]. For example, the compounds D, E and
F (Fig. 1) have shown higher biological activities than their sulfide analogous
forms.
In the continuation of our research interest in the synthesis of potentially
bioactive heterocyclic compounds [28–30], we report here on the synthesis of
thiopyrans and their derivatives for the evaluation of their antibacterial and
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Synthesis and characterization of sulfide, sulfoxide and…
Fig. 1 Some biologically active thiopyran derivatives
antifungal activities. Firstly, thiopyrans 1a–5a and their derivatives of S-oxides 1b–
5b and S,S-dioxides 1c–5c were prepared. Then, their antibacterial and antifungal
activities against Staphylococcus aureus and Bacillus subtilis, as Gram-positive
bacteria, as well as Escherichia coli and Pseudomonas aeruginosa, as Gram-
negative bacteria, and the fungus Candida albicans were evaluated.
Experimental
All solvents, meta-chloroperoxybenzoic acid (m-CPBA), methyl-chloroformate,
lithium diisopropylamide (LDA), triphenylphosphine (PPh3), dimethyl acetylenedi-
carboxylate, 3,30-thiodipropionic acid and other inorganic chemicals including
H2SO4, Na2SO4 and Na2CO3, were purchased from Merck and were used without
further purification. The compounds 1a–5a were prepared according to the reported
methods [31–33]. Sulfoxides 1b–5b and sulfone derivatives 1c–5c were prepared
from their corresponding sulfides by m-CPBA according to the reported methods
[27]. The uncorrected melting points were measured by an Electrothermal 9100
apparatus (UK). The FT-IR spectra were recorded on a FT-IR Bruker Vector 22
spectrometer. Mass spectra were recorded on a Finnigan-Matt 8430 mass
1
spectrometer operating in electron impact mode. H, 13C and 31P NMR spectra
were recorded on a Bruker 400 MHz Ultrashield Advance DRX spectrometer in
CDCl3 at 400.1, 100.6 and 169.1 MHz, respectively. Elemental analyses were
performed using a Heraeus CHN-O rapid analyzer. The spectral data for the newly
synthesized compounds are presented in the supplementary data.
123

G. F. Pasha et al.
General procedure for the synthesis of thiopyran-S-oxides 1b–5b
A solution of m-CPBA (1.0 mmol) in AcOEt (2 mL) was added to the solution of
1a–5a (1.0 mmol) in dichloromethane (DCM) (10 mL) dropwise at 5 °C and stirred
for 30 min at this temperature. The resulting solution was diluted with 20 mL of
DCM, washed with 3 9 10 mL of saturated solution of Na2CO3, dried over
anhydrous Na2SO4, and concentrated under vacuum to give S-oxides 1b–5b in
excellent yields as white solids (Table 1).
The spectral data for the newly synthesized sulfoxides 3b–5b are as follows.
Dimethyl 4-oxotetrahydro-2H-thiopyran-3,5-dicarboxylate 1-oxide (3b, C9H12O6S)
White powder, m.p: 121–122 °C, yield 95%,; IR (KBr) (tmax, cm-1): 3436 (OH),
1733 and 1702 (C=O), 1242 (Cs2p-O), 1115 (Cs3p-O), 1107 (SO); MS, m/z (%): 248
(M?) (3), 232 (20), 216 (8), 200 (26), 173 (100), 155 (5), 141 (43), 113 (100), 86
(41), 55 (100); Anal. Calcd for C9H12O6S (248.04): C, 43.54; H, 4.87, Found: C,
43.48; H, 4.86; 1H NMR (400.1 MHz, CDCl3): dH (for enol tautomer) 2.94 (1H, dd,
2
3JHH = 14.0, JHH = 4.8 Hz, CH2), 3.01–3.11 (1H, m, CH2), 3.28–3.50 (2H, m,
CH2), 3.62 (0.67H, dd, 3JHH = 6.0 and 4.8 Hz, CH), 3.77, 3.80, 3.82 and 3.82 (12H,
3
s, 4OCH3), 4.15 (0.32H, dd, JHH = 10.0 and 4.8 Hz, CH), 12.59 (0.67H, s, OH),
3
12.79 (0.32H, s, OH); dH (for keto tautomer) 2.94 (1H, dd, JHH = 14.0,
2JHH = 4.8 Hz, CH2), 3.01–3.11 (1H, m, CH2), 3.28–3.50 (2H, m, CH2), 3.62
3
(0.67H, dd, JHH = 6.0 and 4.8 Hz, CH), 3.78 and 3.81 (6H, s, 2OCH3), 4.15
(0.32H, dd, 3JHH = 10.0 and 4.8 Hz, CH); 13C NMR (100.6 MHz, CDCl3): dC (for
enol tautomer) 23.5, 27.9, 32.6 and 32.7 (4CH2), 44.2 and 45.2 (2CH), 52.4, 52.5
and 53.2 (4OCH3), 90.1 and 98.7 (2Cq), 166.7, 168.3, 170.1, and 171.7 (4C=O,
ester), 171.8 (2Cq); dC (for keto tautomer), 39.8 and 46.9 (2CH2), 52.1 and 52.7
(2OCH3), 57.6 and 59.5 (2CH), 167.9 and 170.3 (2C=O, ester), 199.4 (C=O,
ketone).
Dimethyl 2-[3-(methoxycarbonyl)-1-oxido-4-oxotetrahydro-2H-thiopyran-3-yl]-3-
(triphenylphosphoranylidene)succinate (4b, C31H31O8PS)
White powder, m.p. 149–150 °C, yield 95%; IR (KBr) (tmax, cm-1): 3059 (Csp2-H),
1723 (C=O), 1628 and 1483 (C=C), 1198 (C2sp-O), 1122 (Cs3p-O), 1107 (SO); MS, m/
z (%): 594 (M?) (2), 405 (8), 347 (3), 294 (32), 277 (100), 236 (11), 201 (19), 183
(61), 152 (18), 77 (20); Anal. Calcd for C31H31O7PS (594.14): C, 62.62; H, 5.25,
Found: C, 62.48; H, 5.23.
Diastereomer I-Z-isomer, 1H NMR (400.1 MHz, CDCl3): dH 2.56–2.63 (1H, m,
CH2), 2.98 (3H, s, OCH3), 3.34 (3H, s, OCH3), 3.16–3.26 (2H, m, CH2), 3.72 (3H, s,
OCH3), 3.52–3.54 (1H, m, CH2), 3.81–3.83 (1H, m, CH2), 3.97 (1H, d,
3JHP = 16.4 Hz, CH), 4.30–4.35 (1H, m, CH2), 7.51–7.74 (15H, m, aromatic
protons); 13C NMR (100.6 MHz, CDCl3): dC 34.1 (CH2), 38.5 (d, 1JPC = 124.8 Hz,
P = C), 42.6 (CH2), 45.6 (CH2), 48.9 (d, 2JPC = 14.6 Hz, CH), 48.9, 52.2, and 52.5
1
2
(3OCH3), 64.7 (Cq), 127.5 (d, JPC = 91.8 Hz, Cipso), 128.6 (d, JPC = 12.0 Hz,
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Synthesis and characterization of sulfide, sulfoxide and…
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G. F. Pasha et al.
Cortho), 132.0 (d, 4JPC = 2.5 Hz, Cpara), 133.8 (d, 3JPC = 10.0 Hz, Cmeta), 169.1 (d,
3
2JPC = 7.1 Hz, C=O, ester), 170.3 (C=O, ester), 174.0 (d, JPC = 5.0 Hz, C=O,
ester), 199.2 (C=O, ketone); 31P NMR (161.9 MHz, CDCl3): dP 24.5.
Diastereomer I-E-isomer, 1H NMR (400.1 MHz, CDCl3): dH 2.56–2.63 (1H, m,
CH2), 2.95 (3H, s, OCH3), 3.16–3.26 (2H, m, CH2), 3.30 (3H, s, OCH3), 3.52–3.54
(1H, m, CH2), 3.74 (3H, s, OCH3), 3.81–3.83 (1H, m, CH2), 3.93 (1H, d,
3JHP = 14.0 Hz, CH), 4.30–4.35 (1H, m, CH2), 7.51–7.74 (15H, m, aromatic
protons); 13C NMR (100.6 MHz, CDCl3): dC 34.9 (CH2), 38.5 (d, 1JPC = 124.8 Hz,
P = C), 44.3 (CH2), 45.6 (CH2), 48.9 (d, 2JPC = 14.6 Hz, CH), 50.8, 52.3 and 53.1
1
2
(3OCH3), 65.8 (Cq), 127.5 (d, JPC = 91.8 Hz, Cipso), 128.6 (d, JPC = 12.0 Hz,
C
4
3
ortho), 132.0 (d, JPC = 2.5 Hz, Cpara), 133.8 (d, JPC = 10.0 Hz, Cmeta), 170.4
(C=O, ester), 174.5 (d, 3JPC = 4.0 Hz, C=O, ester), 175.1 (d, 2JPC = 7.0 Hz, C=O,
ester), 201.1 (C=O, ketone); 31P NMR (161.9 MHz, CDCl3): dP 24.70.
1
Diastereomer II-Z-isomer, H NMR (400.1 MHz, CDCl3): dH 2.56–2.63 (1H,
m, CH2), 3.00 (3H, s, OCH3), 3.16–3.26 (2H, m, CH2), 3.31 (3H, s, OCH3), 3.52–
3.54 (1H, m, CH2), 3.67 (3H, s, OCH3), 3.77–3.81 (1H, m, CH2), 3.97 (1H, d,
3JHP = 16.4 Hz, CH), 4.36–4.39 (1H, m, CH2), 7.51–7.74 (15H, m, aromatic
protons);13C NMR (100.6 MHz, CDCl3): dC 33.6 (CH2), 37.6 (d, 1JPC = 124.3 Hz,
P = C), 42.6 (CH2), 45.1 (CH2), 48.5 (d, 2JPC = 14.5 Hz, CH), 49.1, 50.8 and 54.6
1
2
(3OCH3), 64.8 (Cq), 127.5 (d, JPC = 91.8 Hz, Cipso), 128.5 (d, JPC = 12.2 Hz,
ortho), 131.9 (d, 4JPC = 2.8 Hz, Cpara), 133.7 (d, 3JPC = 10.5 Hz, Cmeta), 169.1 (d,
C
3
2JPC = 7.1 Hz, C=O, ester), 169.7 (C=O, ester), 170.4 (d, JPC = 5.0 Hz, C=O,
ester), 199.3 (C=O, ketone); dP 24.3.
1
Diastereomer II-E-isomer, H NMR (400.1 MHz, CDCl3): dH 2.56–2.63 (1H,
m, CH2), 3.00 (3H, s, OCH3), 3.16–3.26 (2H, m, CH2), 3.21 (3H, s, OCH3), 3.52–
3.54 (1H, m, CH2), 3.74 (3H, s, OCH3), 3.77–3.81 (1H, m, CH2), 3.93 (1H, d,
3JHP = 14.0 Hz, CH), 4.36–4.39 (1H, m, CH2), 7.51–7.74 (15H, m, aromatic
protons);13C NMR (100.6 MHz, CDCl3): dC 34.2 (CH2), 37.6 (d, 1JPC = 124.3 Hz,
P = C), 44.4 (CH2), 45.1 (CH2), 48.5 (d, 2JPC = 14.5 Hz, CH), 50.7, 51.9, and 54.8
1
2
(3OCH3), 66.8 (Cq), 127.5 (d, JPC = 91.8 Hz, Cipso), 128.5 (d, JPC = 12.2 Hz,
C
4
3
ortho), 131.9 (d, JPC = 2.8 Hz, Cpara), 133.7 (d, JPC = 10.5 Hz, Cmeta), 169.8
(C=O, ester), 170.1 (d, 2JPC = 7.2 Hz, C=O, ester), 170.9 (d, 3JPC = 4.0 Hz, C=O,
ester), 203.1 (C=O, ketone); dP 24.55.
Dimethyl (2E)-2-[5-(methoxycarbonyl)-1-oxido-3,6-dihydro-2H-thiopyran-4-yl]but-
2-enedioate (5b, C13H16O7S)
Yellow powder, m.p. 98–99 °C, yield 90%; IR (KBr) (tmax, cm-1): 1723 and 1650
(C=O), 1610 (C=C), 1251 (Cs2p-O), 1186 (SO), 1115 (Cs3p-O); MS, m/z (%): 316
(M?)(3), 277 (100), 257 (2), 199 (31), 183 (27), 152 (18), 77 (30), 51 (18); Anal.
1
Calcd for C13H16O7S (332.06): C, 49.36; H, 5.10, Found: C, 49.42; H, 5.12; H
NMR (400.1 MHz, CDCl3): dH 2.52–2.64 (1H, m, CH2), 2.97–3.02 (1H, m, CH2),
3.11–3.14 (2H, m, CH2), 3.69 (3H, s, OCH3), 3.73 (3H, s, OCH3), 3.75 (2H, m,
CH2), 3.82 (3H, s, OCH3), 6.77 (1H, s, olefinic proton);13C NMR (100.6 MHz,
CDCl3): dC 24.8, 43.2 and 46.3 (3CH2), 52.0, 52.2 and 53.1 (3OCH3), 124.3
123

Synthesis and characterization of sulfide, sulfoxide and…
(quaternary olefinic carbon), 124.7 (olefinic carbon, CH), 145.4 and 147.3
(quaternary olefinic carbon), 164.7, 164.9 and 165.4 (3C=O, ester).
General procedure for the synthesis of sulfones 1c–5c
A solution of m-CPBA (3.0 mmol) in AcOEt (5 mL) was added to the solution of
1a–5a (1.0 mmol) in dichloromethane (DCM) (20 mL) dropwise at 5 °C and stirred
for 30 min at this temperature. The resulting solution was diluted with 30 mL of
DCM, washed with 3 9 10 mL of saturated solution of Na2CO3, dried over
anhydrous Na2SO4, and concentrated under vacuum to give S,S-dioxides 1c–5c in
excellent yields as white solids (Table 2).
The spectral data for the newly synthesized sulfoxides 3c–5c are as follows.
Dimethyl 4-oxotetrahydro-2H-thiopyran-3,5-dicarboxylate 1,1-dioxide (3c,
C9H12O7S)
White powder, m.p. 115 °C, yield 95%; IR (KBr) (tmax, cm-1): 3481 (OH), 1738
and 1710 (C=O), 1379 and 1142 (SO2), 1226 (C2sp-O), 1122 (Cs3p-O); MS, m/z (%):
265 (3), 233 (5), 200 (3), 168 (16), 140 (9), 114 (21), 87 (56), 55 (100); Anal. Calcd
1
for C9H12O7S (248.04): C, 40.91; H, 4.58, Found: C, 40.82; H, 4.59; H NMR
(400.1 MHz, CDCl3): (for enol tautomer): dH 3.22–3.28 (1H, m, CH2), 3.46–3.52
(1H, m, CH2), 3.60–3.65 (1H, m, CH), 3.84 and 3.85 (6H, s, 2OCH3), 4.03–4.08
(1H, m, CH), 12.59 (1H, s, OH): (for keto tautomer): dH 3.22–3.28 (1H, m, CH2),
3.46–3.52 (1H, m, CH2), 3.60–3.65 (1H, m, CH), 3.85 (6H, s, OCH3), 4.03–4.08
(1H, m, CH), 12.59 (1H, s, OH); 13C NMR (100.6 MHz, CDCl3): dC (for enol
tautomer) 47.6 (CH), 48.0 and 49.8(2CH2), 52.8 and 53.4 (2OCH3), 93.2 (Cq), 165.7
and 167.9 (2C=O, ester), 170.0 (Cq), (for keto tautomer), 46.7 (2CH), 49.0 (2CH2),
53.3 (2OCH3), 170.7 (C=O, ester), 202.8 (C=O, ketone).
Table 2 Preparation of the
sulfoxides 1b–5b by m-CPBA
Compounds
m-CPBA
Products
m.p
References
1a
2a
3a
4a
5a
1 equiv
1 equiv
1 equiv
1 equiv
1 equiv
1b
2b
3b
4b
5b
110
[39]
84
[40]
121–122
149–150
98–99
This work
This work
This work
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G. F. Pasha et al.
Dimethyl 2-[3-(methoxycarbonyl)-1,1-dioxido-4-oxotetrahydro-2H-thiopyran-3-yl]-
3-(triphenylphosphoranylidene)succinate (4c, C31H31O9PS)
White powder, m.p. 160–161 °C, yield 95%; IR (KBr) (tmax, cm-1): 3058 (Csp2-H),
1720 (C=O), 1625 and 1481 (C=C), 1435 and 1109 (SO2), 1244 (C2sp-O), 1061 (Cs3p-
O); MS, m/z (%): 609 (M?-1) (1), 337 (12), 316 (4), 294 (47), 277 (100), 236 (4),
201 (16), 183 (66), 152 (18), 113 (18), 77 (17); Anal. Calcd for C31H31O7PS
(610.14): C, 60.98; H, 5.12, Found: C, 60.78; H, 5.10.
1
4c-Z-isomer, H NMR (400.1 MHz, CDCl3): dH 2.55–2.59 (1H, m, CH2), 2.99
(3H, s, OCH3), 3.10–3.26 (2H, m, CH2), 3.31 (3H, s, OCH3), 3.36–3.45 (1H, m,
3
CH2), 3.74 (3H, s, OCH3), 3.81–3.82 (1H, m, CH2), 3.96 (1H, d, JHP = 15.6 Hz,
CH), 4.30–4.39 (1H, m, CH2), 7.51–7.74 (15H, m, aromatic protons); 13C NMR
1
(100.6 MHz, CDCl3): dC 33.9 (CH2), 38.5 (d, JPC = 123.7 Hz, P = C), 42.5
(CH2), 44.3 (CH2), 48.9 (OCH3), 49.0 (d, JPC = 14.8 Hz, CH), 52.2 and 53.1
2
1
(2OCH3), 65.8 (Cq), 127.5 (d, JPC = 94.8 Hz, Cipso), 128.5 (d, JPC = 12.0 Hz,
2
C
ortho), 132.0 (d, 4JPC = 2.2 Hz, Cpara), 133.9 (m, Cmeta), 170.2 (d, 2JPC = 12.8 Hz,
C=O, ester), 170.9 (C=O, ester), 170.0 (d, 3JPC = 7.0 Hz, C=O, ester), 201.1 (C=O,
ketone); 31P NMR (161.9 MHz, CDCl3): dP 24.5.
1
4c-E-isomer, H NMR (400.1 MHz, CDCl3): dH 2.55–2.59 (1H, m, CH2), 2.94
(3H, s, OCH3), 3.01 (3H, s, OCH3), 3.10–3.26 (2H, m, CH2), 3.29 (3H, s, OCH3),
3
3.36–3.45 (1H, m, CH2), 3.81–3.82 (1H, m, CH2), 3.92 (1H, d, JHP = 16.0 Hz,
CH), 4.30–4.39 (1H, m, CH2), 7.51–7.74 (15H, m, aromatic protons);13C NMR
1
(100.6 MHz, CDCl3): dC 33.6 (CH2), 37.5 (d, JPC = 116.0 Hz, P = C), 44.4 and
44.6 (2CH2), 48.9 (OCH3), 50.6 (d, JPC = 15.0 Hz, CH), 50.8 and 52.3 (2OCH3),
64.8 (Cq), 127.5 (d, 1JPC = 94.8 Hz, Cipso), 128.6 (d, 2JPC = 11.9 Hz, Cortho), 132.1
(d, 4JPC = 2.2 Hz, Cpara), 133.9 (m, Cmeta), 169.0 (d, 2JPC = 12.8 Hz, C=O, ester),
2
3
171.8 (C=O, ester), 174.0 (d, JPC = 5.0 Hz, C=O, ester), 199.2 (C=O, ketone); dP
24.55.
Dimethyl (2E)-2-[5-(methoxycarbonyl)-1,1-dioxido-3,6-dihydro-2H-thiopyran-4-
yl]but-2-enedioate (5c, C13H16O8S)
Yellow powder, m.p. 83–85 °C, yield 95%; IR (KBr) (tmax, cm-1): 1724 and 1648
(C=O), 1617 (C=C), 1449 and 1118 (SO2), 1252 (C2sp-O), 1050 (Cs3p-O); MS, m/z
(%): 331 (M?-H) (2), 301 (6), 273 (53), 240 (67), 225 (10), 209 (84), 193 (23), 176
(15), 149 (34), 121 (18), 84 (100), 59 (37); Anal. Calcd for C13H16O8S (332.06): C,
1
46.98; H, 4.85, Found: C, 46.85; H, 4.86; H NMR (400.1 MHz, CDCl3): dH 2.89–
3.06 (2H, m, CH2), 3.15–3.37 (2H, m, CH2), 3.68 (3H, s, OCH3), 3.73 (3H, s,
2
OCH3), 3.80 (3H, s, OCH3), 3.94 and 4.03 (2H, AB, JHH = 17.6 Hz, CH2), 6.73
(1H, s, olefinic proton);13C NMR (100.6 MHz, CDCl3): dC 32.3, 46.7 and 50.1
(3CH2), 52.3, 52.5 and 53.2 (3OCH3), 120.1 (quaternary olefinic carbon), 124.7
(olefinic carbon, CH), 145.8 and 147.7 (quaternary olefinic carbon), 164.3, 164.4
and 165.1 (3C=O, ester).
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Synthesis and characterization of sulfide, sulfoxide and…
General procedure for the evaluation of antimicrobial activity
The most widespread method for the assessment of antimicrobial activities is the
Kirby–Bauer disc diffusion technique [42]. In vitro antimicrobial activities of the
compounds 1–5 were assayed using the disc diffusion method by determining the
inhibition zones. The microorganisms, used in this study, are as follows:
Escherichia coli PTCC 1330, Pseudomonas aeruginosa PTCC 1074, Staphylococ-
cus aureus ATCC 35923, Bacillus subtilis PTCC 1023 and Candida albicans ATCC
10231. The late exponential phases of the bacteria were standardized with a final
cell density of 108 cfu/mL. Muller–Hinton agar (Merck) were prepared and
inoculated by the standardized cultures of the studied microorganisms, spread as
uniformly as possible throughout the entire media. Sterile paper discs (diameter of
6 mm; Padtan, Iran) were impregnated with 20 mL of the sample solution (20 mg/
mL in DMSO), and then placed on the upper layer of the seeded agar plate and
incubated at 37 °C for 24 h. The antimicrobial activities of the samples were
compared with the known commercial antibiotics of gentamicin (10 lg/disc),
chloramphenicol (30 lg/disc) and nystatin (100 Units/disc). After the incubation
period, the antibacterial and antifungal activities were estimated by calculating the
mean diameter (mm) of the inhibition halo, where the tested microorganism did not
grow, and the results were reported as mean SD after three repeats.
Results and discussion
Chemistry
Syntheses of the compounds 1a–5a are depicted in Scheme 1. Compound 2a was
prepared by the Dieckmann cyclization reaction of dimethyl 3,30-thiodipropanoate
in the mixture of NaOMe/THF according to the reported procedure [31]. Treatment
of compound 2a with H2SO4 (10%) under refluxing conditions afforded 1a in high
yield [31]. The reaction of compound 2a with methyl-chloroformate in the presence
of LDA at -78 °C led to the formation of compound 3a in good yield [32]. The
three-component reaction of thiopyran sulfide 2a with dimethyl acetylenedicar-
boxylate and triphenylphosphine resulted in phosphorous ylide 4a, which converted
to compound 5a in excellent yield when refluxed in toluene by the intramolecular
Wittig reaction [33].
It is known that the oxidation of sulfides to higher oxidation states, i.e., S-oxides
and S,S-dioxides, usually improves their biological activities [21–24] Therefore, all
the synthesized sulfides in this work were oxidized to their corresponding sulfoxides
and sulfones. The oxidation of sulfides 1a–5a to sulfoxides 1b–5b and sulfones 1c–
5c is presented in Scheme 2. As is clear from Scheme 2, when one equivalent of m-
CPBA is used for the oxidation of sulfides 1a–5a, the corresponding sulfoxides 1b–
5b are obtained as sole products in excellent yields without the formation of any
other oxidation products. Similarly, the sulfones 1c–5c are synthesized using three
equivalents of m-CPBA. The structures of all the synthesized compounds were
1
characterized by FT-IR, H, 13C, 31P NMR and mass spectroscopy techniques. The
123

G. F. Pasha et al.
Scheme 1 Preparation of compounds 1a–5a. Reagents and conditions: (i) 10% H2SO4, reflux for 1 h (ii)
LDA, methyl-chloroformate, THF, -78 °C (iii) triphenylphosphine and dimethylacetylenedicarboxylate,
DCM, r.t., 20 min (iv) toluene, reflux for 2 h
Scheme 2 Preparation of S-oxide and S,S-dioxide derivatives of sulfides 1a–5a
123

Synthesis and characterization of sulfide, sulfoxide and…
mass spectra of these compounds exhibited the molecular ion peaks at the
appropriate regions. Other fragmentations involved the loss of the ester or alkoxy
groups from the molecular ions. The FT-IR spectra of sulfoxides 1b–5b and
sulfones 1c–5c displayed strong absorption bands at 1720–1650 cm-1 for the
carbonyl groups. The S=O stretching vibrational peak for the sulfoxides appeared at
1100 cm-1 and the symmetric and asymmetric stretching peaks of SO2 group of
sulfones displayed at 1110 and 1400 cm-1. In the 1H NMR (CDC13), the methylene
(CH2X) protons appear as multiplets at approximate d values of 2.7–2.9 and 3.3–3.7
(X = S), 2.9–3.0 and 3.8–4.3 (X = SO), 2.9–3.1 and 3.9–4.4 (X = SO2) ppm,
which could be due to the X-substituent effect. These methylene groups (CH2X)
displayed d values at 20–30 ppm (X = S), 25–35 (X = SO), and 30–40 (X = SO2)
ppm in the 13C NMR spectra in agreement with the proposed structures.
Biological evaluation
Antibacterial activity
The contamination and microbial infection caused by the microorganisms, the
increasing number of microbial strains resistant to many antibiotics, and faster
growth of some microorganisms than antibiotics are important concerns for the
human beings. Therefore, design and synthesis of new reagents for the treatment of
microbial infections are challenging tasks [34, 35]. One type of antimicrobial agents
are thiopyran derivatives offering potential biological activities [36, 37]. In order to
investigate the biological activities of all the synthesized thiopyran heterocycles,
their antibacterial and antifungal activities against Gram-positive bacteria (Staphy-
lococcus aureus and Bacillus subtilis), Gram-negative bacteria (Escherichia coli
and Pseudomonas aeruginosa) and a fungus (Candida albicans) were evaluated
in vitro using the Kirby–Bauer disk diffusion method. The results are summarized in
Table 3 and compared with the gentamicin, chloramphenicol and nystatin as
standard drugs. In general, all the samples showed a range of modest to highest
in vitro antibacterial activity against all the tested microorganisms. However,
compounds 1a against the studied Gram-negative bacteria and S. aureus, 1b against
P. aeruginosa, 3a against the Gram-negative bacteria, 4a against all tested four
bacteria, and 5a against P. aeruginosa did not show antibacterial activity.
Compound 1a without any substituent at the alpha position, at the C3 and C5
positions of the six-membered heterocyclic ring, did not exhibit in vitro antibacterial
Table 3 Preparation of the sulfones 1c–5c by m-CPBA
Compounds
m-CPBA
Products
m.p
References
1a
2a
3a
4a
5a
3 equiv
3 equiv
3 equiv
3 equiv
3 equiv
1c
2c
3c
4c
5c
174–175
115
[41]
[41]
115
This work
This work
This work
160–161
83–85
123

G. F. Pasha et al.
activity except against B. subtilis. In compound 1a, the introduction of a methyl
carboxylate group (CO2Me) at the C3 position of the thiopyran-4-one moiety
resulted in 2a, slightly improving the antibacterial activity against all four bacteria.
When another methyl carboxylate group was substituted at the C5 position of the
thiopyran-4-one moiety, this resulted in 3a, and the activity against all bacteria did
not significantly change in comparison to the antibacterial activity of compound 2a.
Replacement of hydrogen at the C3 position in 2a by a dimethyl 2-(triphenylphos-
phoranylidene) succinate group resulted in 4a, which suppressed the antimicrobial
activity against all the tested microorganisms. Removing the carbonyl and
phosphine oxide from 4a through the intramolecular Wittig reaction led to the
formation of 5a which significantly enhanced the antibacterial activities against all
the bacteria except P. aeruginosa. In general, the comparison of antibacterial
activities of the compounds 1a–5a against the four tested bacteria, as shown in
Fig. 2(I), revealed that compound 5a had the highest activity against the two studied
Gram-positive bacteria and E. coli, a Gram-negative bacterium, whereas compound
2a showed the highest effect against P. aeruginosa, a Gram-positive bacterium.
The antibacterial activities of the S-oxide derivatives 1b–5b exhibited the same
trend as the thiopyrans 1a–5a with just a slight improvement. Comparison of the
antibacterial activities of the sulfoxide derivatives 1b–5b against all the tested
bacteria, as shown in Fig. 2(II), showed more growth inhibitory effects for
Fig. 2 Comparison of the antibacterial activities of sulfide derivatives 1a–5a (I), S-oxide derivatives 1b–
5b (II), S,S-dioxide derivatives 1c–5c (III) and the growth inhibitory effect of 2c on E. coli using the disk
diffusion test (IV)
123

Synthesis and characterization of sulfide, sulfoxide and…
compound 5b against the two Gram-negative bacteria, for compound 4b against P.
aeruginosa, and for compounds 2b and 5b against B. subtilis. The antibacterial
activities of the S,S-dioxide derivatives 1c–5c released the maximum activity
against all the tested bacteria. In compound 1c, the introduction of an ester group
(CO2Me) at the C3 position of the thiopyran-4-one resulted in compound 2c, which
impressively increased the antibacterial activity. Furthermore, by the introduction of
a second ester group at the C5 position of thiopyran-4-one, this resulted in
compound 3c, and the antibacterial activity decreased against all the studied bacteria
in comparison to that observed for 2c. Here, the antibacterial activities of these
materials were in the order: 2c [ 3c [ 1c. Similar trends were observed from the
comparisons between 2a with 3a, as well as 2b with 3b. In compound 1c, the
replacement of two hydrogens in the alpha-position at the C3 position of thiopyran-
4-one by a dimethyl 2-(triphenylphosphoranylidene) succinate and an ester group
gave 4c, which improved the activity against the Gram-positive bacteria and
decreased the antibacterial activity against the Gram-negative bacteria. Interest-
ingly, comparing the antibacterial activities of 4c with 5c demonstrated an increase
in the activity against the Gram-negative bacteria as well as a decrease against the
Gram-positive bacteria. Figure 2(III) shows the antibacterial activities of the S,S-
dioxide derivatives 1c–5c. Accordingly, compound 2c had the highest growth
inhibitory effect against all the examined microorganisms. The growth inhibitory
effect of 2c on E. coli using the disk diffusion test is shown in Fig. 2(IV).
Fig. 3 Antimicrobial activity against Gram-negative bacteria E. coli (a), and P. aeruginosa (b), Gram-
positive bacteria S. aureus (c), and B. subtilis (d) and the fungus C. albicans (e)
123

G. F. Pasha et al.
The obtained results revealed that the thiopyran S,S-dioxides with sulfur in its
highest oxidation state had potential antimicrobial activities in comparison with
their corresponding derivatives with sulfur in its lower oxidation states. This trend
of antibacterial activity enhancement has also been observed in other sulfur-
containing heterocycles [15, 21–25]. The comparison of the antibacterial activities
of thiopyran sulfides, thiopyran S-oxides and thiopyran S,S-dioxides against the four
bacteria examined in this study are depicted in Fig. 3a–d. Among the studied
compounds, compound 2c showed excellent antibacterial activity against all four
tested bacteria which is comparable with the activity of standard antibiotics
(gentamicin and chloramphenicol). It seems that the presence of the thiopyran-4-
one-S,S-dioxide moiety and one ester functional group in the backbone of
compounds 1c–5c acted as antibacterial agents. It is suggested that the binding
ability to cell wall components increases via hydrogen bonding in the presence of a
sulfone functional group and, as a result, the lysis of the bacterial cells and the
leakage of the cytoplasmic materials increase, which leads to the cell death of
bacteria [15, 21–27].
Antifungal activity
The in vitro antifungal activities of all the compounds 1–5 were investigated against
Candida albicans (Table 3). Nystatin was used as a standard antibiotic whose
growth inhibitory effect value was reported in Table 3. Among thiopyrans 1–4,
thiopyran-4-one-S-oxides 1b–4b showed higher growth inhibitory activity against
C. albicans than against Nystatin. Compounds 1c, 2a and 4c exhibited moderate
antifungal activities while compounds 1a, 2c, 3a, 3c and 4a did not show any
antifungal activities. Among thiopyrans 5a–c, only 5c showed moderate antifungal
activity against C. albicans (Fig. 3e). It is noteworthy that sulfoxide derivatives
seem to be primarily active against C. albicans. Generally, different structural
parameters and mechanisms of action are responsible for achieving potency against
a range of microorganisms [38].
Conclusions
Some derivatives of thiopyran heterocycles can be used as antimicrobial agents
against important human pathogenic microorganisms, such as S. aureus, B. subtilis,
E. coli, P. aeruginosa and C. albicans, which play a significant role in deadly
infectious diseases. Based on our results, a structure–activity correlation can be
provided. The structure containing one ester group (CO2Me) at the C3 position of
the thiopyran-4-one-S,S-dioxide plays an important role in eliciting the biological
response. Also, the oxidation of sulfur to higher states in the six-membered
S-heterocycles, i.e., sulfide to sulfone, improves the antibacterial activity, whereas
the improvement for the antifungal activity is seen with the thiopyran S-oxides.
Acknowledgements The financial support from the University of Mazandaran is gratefully
acknowledged.
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Synthesis and characterization of sulfide, sulfoxide and…
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