Design and synthesis of new 2-aryl-4,5-dihydro-thiazole analogues: In vitro antibacterial activities and preliminary mechanism of action

Article abstract of DOI:10.3390/molecules201119680

Sixty 2-aryl-4,5-dihydrothiazoles were designed and synthesized in yields ranging from 64% to 89% from cysteine and substituted-benzonitriles via a novel metal- and catalyst-free method. The structures of the title compounds were confirmed mainly by NMR spectral data analysis. Antibacterial activity assays showed that the compounds (S)-2-(2′-hydroxyphenyl)-4-hydroxy-methyl-4,5-dihydrothiazole (7h) and (R)-2-(2′-hydroxyphenyl)-4-hydroxymethyl-4,5-dihydro-thiazole (7h′) exhibited significant inhibition against Ralstonia solanacearum, Pseudomonas syringae pv. actinidiae, Bacillus subtilis and Bacillus cereus, with minimum inhibitory concentrations (MICs) ranging from 3.91 to 31.24 μg·mL^-1. The effect of substituents showed that not only electron-withdrawing groups, but also electron-donating groups could abolish the antibacterial activities unless a 2′-hydroxy group was introduced on the 2-aryl substituent of the 4,5-dihydrothiazole analogues. The results of scanning electron microscope (SEM) and fatty acid exposure experiments indicated that these antibacterial compounds influence fatty acid synthesis in the tested bacteria.

Full text of DOI:10.3390/molecules201119680

Article
Design and Synthesis of New 2-Aryl-4,5-dihydro-thiazole
Analogues: In Vitro Antibacterial Activities and
Preliminary Mechanism of Action
Fangfang Tan 1,:, Baojun Shi 2,:, Jian Li 1, Wenjun Wu 2 and Jiwen Zhang 1,2,
*
Received: 9 September 2015 ; Accepted: 2 November 2015 ; Published: 9 November 2015
Academic Editor: Derek J. McPhee
1
College of Science, Northwest Agriculture and Forestry University, Yangling 712100, China;
15877653513@163.com (F.T.); lijian3773@163.com (J.L.)
Institute of Pesticide Science, Northwest Agriculture and Forestry University, Yangling 712100, China;
2
shibaojun@nwsuaf.edu.cn (B.S.); wuwenjun@nwsuaf.edu.cn (W.W.)
*
:
Correspondence: nwzjw@nwsuaf.edu.cn; Tel.: +86-29-8709-2191
These authors contributed equally to this work.
Abstract:
Sixty 2-aryl-4,5-dihydrothiazoles were designed and synthesized in yields
novel
The structures of the title compounds were confirmed
ranging from 64% to 89% from cysteine and substituted-benzonitriles via
metal- and catalyst-free method.
a
mainly by NMR spectral data analysis.
Antibacterial activity assays showed that
1
the compounds (S)-2-(2 -hydroxyphenyl)-4-hydroxy-methyl-4,5-dihydrothiazole (7h) and
1
1
(R)-2-(2 -hydroxyphenyl)-4-hydroxymethyl-4,5-dihydro-thiazole
(7h )
exhibited
significant
inhibition against Ralstonia solanacearum, Pseudomonas syringae pv. actinidiae, Bacillus subtilis
and Bacillus cereus, with minimum inhibitory concentrations (MICs) ranging from 3.91 to
31.24 µg¨ mL´1. The effect of substituents showed that not only electron-withdrawing groups,
1
but also electron-donating groups could abolish the antibacterial activities unless a 2 -hydroxy
group was introduced on the 2-aryl substituent of the 4,5-dihydrothiazole analogues. The results
of scanning electron microscope (SEM) and fatty acid exposure experiments indicated that these
antibacterial compounds influence fatty acid synthesis in the tested bacteria.
Keywords: synthesis; 4,5-dihydrothiazole; antibacterial activity; antibacterial mechanism
1. Introduction
Ralstonia solanacearum and Pseudomonas syringae pv. Actinidiae are the causes of serious
bacterial diseases that pose
a major threat to agricultural production worldwide [1].
Unfortunately, there have been few reports of effective bactericides against these
pathogens. In the process of screening new agricultural bactericides, we found that some
1
2-arylthiazolines such as (S)-2-(2 -hydroxyphenyl)-4-hydroxymethyl-4,5-dihydrothiazole and
1
(R)-2-(2 -hydroxyphenyl)-4-hydroxymethyl-4,5-dihydrothiazole exhibited potent antibacterial
activities [2–5]. Inspired by these results, we wondered whether the 2-arylthiazoline scaffold could
be a lead compound for agricultural bactericides. In order to investigate the bactericidal activities and
clarify the inevitable structural motifs of 2-arylthiazoline analogues, sixty 2-aryl-4,5-dihydrothiazoles
were then designed and synthesized.
A couple of protocols have been developed for the synthesis of 2-arylthiazoline scaffolds, which
mainly include using ethyl benzimidates reacted with cysteine [6], aryl nitriles condensed with
cysteine in buffered media [7], Ru-catalyzed/TBHP oxidation reactions [8], and so on [9–11]. The
broad application of these methods has been hindered by the following drawbacks: long reaction
Molecules 2015, 20, 20118–20130; doi:10.3390/molecules201119680
www.mdpi.com/journal/molecules

Molecules 2015, 20, 20118–20130
˝
time, high reaction temperatures (>100 C), tedious work-ups and the use of expensive noble
metal catalysts.
We present herein the design and synthesis a series of 2-aryl-4,5-dihydrothiazolines under
mild metal- and catalyst-free conditions [12] as well as their in vitro antibacterial activities
against Ralstonia solanacearum (R. solanacearum), Pseudomonas syringae pv. Actinidiae (P. syringae),
Bacillus subtilis (B. subtilis) and Bacillus cereus (B. cereus). Furthermore, we also explore the preliminary
antibacterial mechanism of compound 7h (Figure 1).
OH
OH
N
S
N
S
OH
OH
(S)-2-(2-hydroxyphenyl)-4-
hydroxymethyl-4,5-dihydrothiazole
(7h)
(R)-2-(2-hydroxyphenyl)-4-
hydroxymethyl-4,5-dihydrothiazole
(7h )
′
Figure 1. Chemical structures of (S)-2-(2-hydroxyphenyl)-4-hydroxymethyl-4,5-dihydrothiazole and
(R)-2-(2-hydroxyphenyl)-4-hydroxymethyl-4,5-dihydrothiazole.
2. Results and Discussion
2.1. Preparation of 2-Arylthiazoline Analogues
The synthetic route to the 2-arylthiazoline analogues is depicted in Scheme 1. Aryl nitriles and
methyl cysteine were used as starting materials and reacted in dry methanol in the presence of sodium
carbonate and refluxed for 12–15 h to obtain methyl-2-aryl-4,5-dihydrothiazole-4-carboxylates in
yields ranging from 64% to 89% [12].
Scheme 1. Synthetic route of 2-arylthiazoline analogues.
The results suggested that ortho-substituents did not favor the reaction, in particular larger
groups, such as -Br and -Cl. Meanwhile the electron-withdrawing groups were better in
1
1
this reaction than electron-donating ones. The carboxylates 1e–10e, 1e –10e were reduced by
NaBH4 in EtOH to furnish the corresponding 2-aryl-4-hydroxymethyly-4,5-dihydrothiazoles 1h–10h,
1
1
1h –10h and treated with 10% sodium hydroxide solution in EtOH to get the corresponding
20119

Molecules 2015, 20, 20118–20130
1
1
2-aryl-4,5-dihydrothiazole-4-carboxylic acids 1a–10a, 1a –10a . The structures of all the compounds
were confirmed mainly by analyses of their NMR spectral and mass spectrometry data or reference
to related literature [3,13–15].
2.2. Antibacterial Activity Assays
All compounds were evaluated for in vitro antibacterial activities against two Gram-positive
bacteria—B. cereus and B. subtilis—and two Gram-negative bacteria—R. solanacearum and P. syringae.
The inhibition zone diameters and minimum inhibitory concentrations (MICs), presented in Tables 1
and 2 were determined by the filter paper and double-dilution methods, respectively. However,
1
only six 2 -hydroxyphenyl-4,5-dihydro-thiazoles showed potent antibacterial activities at the tested
1
concentration, in particular, compounds 7h and 7h showed the highest antibacterial activities for all
tested bacteria.
Table 1. Inhibition Zone Diameters of Six Compounds against the Tested Bacteria.
Diameter of Inhibition Zone (mm) in 10 µL/Disk (Mean ˘ S.D.)
Compounds
B. subtilis
B. cereus
R. solanacearum
P. syringae
7a
7a1
7e
-
-
-
-
-
-
-
-
11.0 ˘ 0.23 (+)
10.5 ˘ 0.56 (+)
12.3 ˘ 0.02 (+)
11.2 ˘ 0.45 (+)
20.5 ˘ 0.56 (+++)
20.0 ˘ 0.22 (+++)
9.1 ˘ 0.11 (++)
9.6 ˘ 0.43 (++)
15.5 ˘ 0.13 (++)
14.9 ˘ 0.55 (++)
-
-
7e1
7h
7h1
16.3 ˘ 0.23 (+++)
18.2 ˘ 0.34 (+++)
16.7 ˘ 0.11 (+++)
17.8 ˘ 0.23 (+++)
All values were means of three replicates, “+++” means transparent; “++” means clear; “+” means visible;
“´” means no inhibitory effect.
Table 2. The MICs of Four Compounds against the Tested Bacteria.
Minimum Inhibitory Concentration (µg¨ mL´1
)
Tested Bacteria
7h
7h1
7e
7e1
Ampicillin
R. solanacearum
P. syringae
B. cereus
31.24
7.81
31.24
3.91
31.24
7.81
31.24
3.91
>125
>125
>125
>125
>125
>125
>125
>125
>125
>125
62.5
B. subtilis
>125
2.3. Mechanism of Antibacterial Action
2.3.1. Fatty Acid Exposure Experiment
The results of the fatty acid exposure experiments was similar to those of yaglingmycin, that
is the diameters of inhibition zone decrease significantly with the increase of the concentration of
some fatty acids. It was worth nothing that compound 7h completely lost the antibacterial activity
against B. subtilis in the presence of the long-chain saturated stearic acid, palmitic acid, and long-chain
unsaturated oleic acid at the concentration of 500 ppm, 100 ppm, 1 ppm, respectively. Nevertheless,
compound 7h did not lose the antibacterial activity against B. subtilis in the presence of short-chain
n-hexylic acid and these results can be found in Figure 2.
20120

Molecules 2015, 20, 20118–20130
The results of stearic acid salt added
30
25
20
15
10
5
0
0
0.1
1
10
100 500 1000
The concentration of stearic acid salt added / ppm
The results of hexylic acid added
30
25
20
15
10
5
0
0
0.1
1
10
100 500 1000
The concentration of hexylic acid added / ppm
Figure 2. The fatty acid exposure experiments of compound 7h.
2.3.2. Scanning Electron Microscope (SEM)
After treating B. cereus with 31.24 µg¨ mL´1 7h for 3 h, the morphological changes to the bacterial
cells were observed by Scanning Electron Microscope (SEM). As shown in Figure 3, the surfaces
of untreated bacterial cells were smooth and intact, while the morphology of treated cells changed
dramatically. The treated cells had obvious depressions and deformations, some clear holes in the
damaged cell walls and cell membranes and the cells became irregular, pitted, and shriveled. The
morphological changes on the surface might result in diffusion of the contents from the cells to
the outside.
Figure 3. Scanning Electron Microscope (SEM) photography of B. cereus. (A) Blank control;
(B) Positive control (treated with yanglingmycin); (C) Treated with Compound 7h.
20121

Molecules 2015, 20, 20118–20130
3. Experimental Section
3.1. General Information
Solvents were of analytical reagent (AR) grade unless otherwise mentioned. TLC was performed
on E. Merck 60 F254 silica gel plates. Column chromatography was carried out with 200~300 mesh
silica gel (Qingdao Haiyang Co., Ltd., Qingdao, Shandong, China); compounds were eluted with
the mixture solution of petroleum ether and ethyl acetate in sequence. Melting points (m.p.s,
uncorrected) were measured using a Yanagimoto apparatus (Shanghai-Measuring Equipment Co.,
Ltd., Shanghai, China). 1H-NMR (500 MHz) and 13C-NMR (125 MHz) were obtained on a
Bruker-Avance-500 spectrometer (Bruker Corporation, Switzerland) with DMSO-d6 or CDCl3 as
solvent and SiMe4 as internal standard. Mass spectra were recorded under electrospray ionization
(ESI) conditions by using a Thermo LCQ Fleet instrument (Thermo Fisher, Waltham, MA, USA).
3.2. Chemistry
1
1
3.2.1. Synthesis of 1e–10e and 1e –10e
˝
Acetyl chloride (1.6 mL, 22 mmol) was slowly added to anhydrous MeOH (100 mL) at 0 C. The
solution was then stirred for 5 min, followed by addition of an amino acid (D-cysteine or L-cysteine)
(20 mmol). After refluxing for 6 h, the reaction mixture was cooled to room temperature, and
evaporated under reduced pressure to furnish the methyl D/L-cysteinate as a white solid in yields of
90% or 85%, respectively. To the anhydrous methanol solution (1.0 mL) of the benzonitriles (1.0 mmol)
and anhydrous Na2CO3 (106 mg, 1.0 mmol) the above synthesized methyl cysteinate (5.0 mmol) was
˝
added, then the resulting mixture was stirred at 80 C for 12 h, cooled to room temperature and
concentrated. The crude mixture was purified by column chromatography on silica gel (PE/EA) to
1
1
afford the desired 2-substituted-phenyl-4-methoxycarbonyl-4,5-dihydrothiazoles 1e–10e and 1e –10e
in 76% to 85% yields.
˝
(S)-2-Phenyl-4-methoxycarbonyl-4,5-dihydrothiazole (1e). White powder. m.p. 60–62 C. 1H-NMR
(CDCl3) δ 7.87 (d, J = 7.6 Hz, 2H), 7.50–7.40 (m, 3H), 5.30 (t, J = 9.1 Hz, 1H), 3.84 (s, 3 H), 3.73
(dd, J = 8.8, 11.0 Hz, 1H), 3.67–3.62 (m, 1H). 13C-NMR (CDCl3) δ 171.3, 171.0, 132.6, 131.7, 128.6 ˆ 2,
128.5 ˆ 2, 78.5, 52.7, 35.3. ESI-MS m/z: Calcd for C11H11NO2S: 222.05 [M + H]+; found: 222.35.
20
˝
rαsD +129.0 (c = 0.1, MeOH).
1
˝
(S)-2-(4 -Hydroxyphenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (2e). White powder. m.p. 166–168 C.
1H-NMR (CDCl3) δ 7.69 (d, J = 8.5 Hz, 2H), 6.76 (d, J = 8.5 Hz, 2H), 5.28 (t, J = 8.8 Hz, 1H), 3.79–3.77
(m, 3H), 3.72–3.62 (m, 2H). 13C-NMR (CDCl3) δ 172.2, 171.5, 159.8, 130.6 ˆ 2, 124.4, 115.6 ˆ 2,
20
˝
77.6, 52.8, 35.3. ESI-MS m/z: Calcd for C11H11NO3S: 238.05 [M + H]+; found: 238.26. rαsD +109.6
(c = 0.1, MeOH).
1
(S)-2-(4 -Fluorophenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (3e). Yellow oil. 1H-NMR (CDCl3) δ
7.89–7.86 (m, 2 H), 7.10 (t, J = 8.7 Hz, 2H), 5.28 (t, J = 9.1 Hz, 1H), 3.85 (s, 3H), 3.76–3.64 (m, 2H).
13C-NMR (CDCl3) δ 171.2, 169.7, 164.8 (d, J = 252 Hz), 130.8, 130.7, 129.0, 115.7, 115.5, 78.4, 52.8, 35.7.
20
˝
ESI-MS m/z: Calcd for C11H10FNO2S: 240.04 [M + H]+; found: 240.18. rαsD +57.8 (c = 0.1, MeOH).
(S)-2-(4 -Bromophenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (4e). White solid. m.p. 74–76 C. 1H-NMR
(CDCl3) δ 7.74–7.73 (m, 2H), 7.56–7.54 (m, 2H), 5.27 (t, J = 9.1 Hz, 1H), 3.84 (s, 3H), 3.74 (dd, J = 8.8,
11.0 Hz, 1H), 3.66 (dd, J = 9.5, 11.0 Hz, 1H). 13C-NMR (CDCl3) δ 171.1, 169.9, 131.7 ˆ 2, 131.5, 130.0 ˆ 2,
126.3, 78.5, 52.8, 35.6. ESI-MS m/z: Calcd for C11H10BrNO2S: 299.96:301.96 = 1:1 [M + H]+; found:
1
˝
20
˝
299.69:301.85 = 1:1. rαsD +7.6 (c = 0.1, MeOH).
(S)-2-(3 -Hydroxyphenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (5e). Colourless oil. 1H-NMR (CDCl3)
1
δ 7.37 (s, 1H), 7.32 (d, J = 7.6 Hz, 1H), 7.22 (t, J = 8.0 Hz, 1H), 6.96 (dd, J = 1.9, 8.2 Hz, 1H), 5.29 (t,
J = 9.1 Hz, 1H), 3.80 (s, 3H), 3.72–3.62 (m, 2H). 13C-NMR (CDCl3) δ 172.2, 171.3, 156.2, 133.4, 129.8,
20122

Molecules 2015, 20, 20118–20130
120.9, 119.4, 114.9, 77.9, 52.8, 35.2. ESI-MS m/z: Calcd for C11H11NO3S: 238.05 [M + H]+; found:
20
˝
238.22. rαsD +50.2 (c = 0.1, MeOH).
(S)-2-(3 -Methylphenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (6e). Colourless oil. 1H-NMR (CDCl3) δ
1
7.72 (s, 1H), 7.64 (d, J = 4.1 Hz, 1H), 7.30 (d, J = 4.4 Hz, 2H), 5.29 (t, J = 9.1 Hz, 1H), 3.84 (s, 3H),
3.73–3.62 (m, 2H), 2.39 (s, 3H). 13C-NMR (CDCl3) δ 171.4, 171.2, 138.3, 132.6, 132.5, 129.0, 128.4, 125.9,
20
78.4, 52.8, 35.3, 21.2. ESI-MS m/z: Calcd for C12H13NO2S: 236.07 [M + H]+; found: 236.41. rαsD +71.7
˝
(c = 0.1, MeOH).
20
1
˝
(S)-2-(2 -Hydroxyphenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (7e). Colorless oil. rαsD +13.5 (c = 0.1,
MeOH) [3].
1
˝
(S)-2-(2 -Fluorophenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (8e). White solid. m.p. 70–72 C. 1H-NMR
(CDCl3) δ 7.95–7.92 (m, 1H), 7.47–7.43 (m, 1H), 7.21–7.11 (m, 2H), 5.26 (t, J = 9.5 Hz, 1H), 3.84 (s, 3H),
3.74–3.62 (m, 2H) 13C-NMR (CDCl3) δ 171.2, 166.0 (d, J = 5.04 Hz), 160.6, (d, J = 255.78 Hz), 133.0 (d,
J = 10.89 Hz), 130.7(d, J = 2.52 Hz), 124.1(d, J = 3.78 Hz), 120.7(d, J = 11.43 Hz), 116.3(d, J = 22.68 Hz),
20
52.8, 40.1, 35.4. ESI-MS m/z: Calcd for C11H10FNO2S: 240.04 [M + H]+; found: 240.12. rαsD +14.4
˝
(c = 0.1, MeOH).
1
(S)-2-(2 -Chlorophenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (9e). Yellow oil. 1H-NMR (CDCl3) δ 7.66
(d, J = 7.6 Hz, 1H), 7.44–7.42 (m, 1H), 7.37–7.28 (m, 2H), 5.31 (t, J = 9.5 Hz, 1H), 3.85 (s, 3H), 3.79
(dd, J = 9.5, 11.0 Hz, 1H), 3.69 (t, J = 10.4 Hz, 1H). 13C-NMR (CDCl3) δ 171.0, 169.0, 132.6, 132.3,
131.3, 130.7, 130.4, 126.7, 78.0, 52.8, 36.4. ESI-MS m/z: Calcd for C11H10ClNO2S: 256.01:258.01 = 3:1
20
˝
[M + H]+; found: 256.27:258.19 = 3:1. rαsD +32.4 (c = 0.1, MeOH).
(S)-2-(2 -Bromophenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (10e). Yellow oil. 1H-NMR (CDCl3) δ
1
7.64–7.62 (m, 1H), 7.65–7.55 (m, 1H), 7.37–7.29 (m, 2H), 5.35–5.31 (m, 1 H), 3.86–3.84 (m, 3H), 3.81–3.69
(m, 2H). 13C-NMR (CDCl3) δ 175.5, 170.9, 170.2, 134.5, 133.6, 131.4, 130.6, 127.2, 121.3, 78.3, 52.8, 36.5.
ESI-MS m/z: Calcd for C11H10BrNO2S: 299.96:301.96 = 1:1 [M + H]+; found: 299.88:301.76 = 1:1.
20
˝
rαsD +22.0 (c = 0.1, MeOH).
1
˝
1
(R)-2-Phenyl-4-methoxycarbonyl-4,5-dihydrothiazole (1e ). White powder. m.p. 60–62 C. H-NMR
(CDCl3) δ 7.87 (d, J = 7.3 Hz, 2H), 7.48 (t, J = 7.4 Hz, 1H), 7.41 (t, J = 7.6 Hz, 2H), 5.30 (t, J = 9.1 Hz, 1H),
3.84 (s, 3H), 3.73 (dd, J = 8.8, 11.0 Hz, 1H), 3.66–3.62 (m, 1H). 13C-NMR (CDCl3) δ 171.3, 170.9, 132.6,
131.6, 128.6 ˆ 2, 128.5 ˆ 2, 78.5, 52.7, 35.3. ESI-MS m/z: Calcd for C11H11NO2S: 222.05 [M + H]+;
20
˝
found: 222.24. rαsD ´129.0 (c = 0.1, MeOH).
1
1
˝
(R)-2-(4 -Hydroxyphenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (2e ). White powder. m.p. 166–168 C.
1H-NMR (CDCl3) δ 7.70–7.69 (m, 2H), 6.78–6.76 (m, 2H), 5.28 (t, J = 9.0 Hz, 1H), 3.79 (s, 3H), 3.72–3.62
(m, 2H). 13C-NMR (CDCl3) δ 172.1, 171.5, 159.7, 130.6 ˆ 2, 124.4, 115.5 ˆ 2, 77.6, 52.8, 35.3. ESI-MS
20
˝
m/z: Calcd for C11H11NO3S: 238.05 [M + H]+; found: 238.05. rαsD ´109.6 (c = 0.1, MeOH).
(R)-2-(4 -Fluorophenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (3e ). Yellow oil. 1H-NMR (CDCl3) δ
7.89–7.86 (m, 2H), 7.10 (t, J = 8.5 Hz, 2H), 5.28 (t, J = 9.1 Hz, 1H), 3.84 (s, 3H), 3.75–3.63 (m, 2H)
13C-NMR (CDCl3) δ 171.2, 169.6, 164.8 (d, J = 252 Hz), 130.8, 130.7, 128.9, 115.7, 115.5, 78.4, 52.8, 35.6.
1
1
20
ESI-MS m/z: Calcd for C11H10FNO2S: 240.04 [M + H]+; found: 240.18. rαsD ´57.8 (c = 0.1, MeOH).
˝
1
1
˝
(R)-2-(4 -Bromophenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (4e ). White solid. m.p. 74–76 C.
1H-NMR (CDCl3) δ 7.75–7.73 (m, J = 8.2 Hz, 2H), 7.57–7.55 (m, J = 8.2 Hz, 2H), 5.28 (t, J = 9.1 Hz,
1H), 3.85 (s, 3H), 3.76–3.72 (m, 1H), 3.68–3.64 (m, 1H). 13C-NMR (CDCl3) δ 171.2, 170.0, 131.8 ˆ 2,
131.6, 130.1 ˆ 2, 126.4, 78.5, 52.9, 35.7. ESI-MS m/z: Calcd for C11H10BrNO2S: 299.96:301.96 = 1:1
20
[M + H]+; found: 299.79:301.86 = 1:1. rαsD ´7.6 (c = 0.1, MeOH).
˝
1
1
1
(R)-2-(3 -Hydroxyphenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (5e ). Colourless oil. H-NMR (CDCl3)
δ 7.38 (s, 1H), 7.32 (d, J = 7.6 Hz, 1H), 7.23 (t, J = 7.9 Hz, 1H), 6.98–6.96 (m, 1H), 5.29 (t, J = 9.1 Hz, 1H),
3.81 (s, 3H), 3.72–3.62 (m, 2H). 13C-NMR (CDCl3) δ 172.2, 171.3, 156.2, 133.5, 129.8, 121.0, 119.4, 114.8,
20123

Molecules 2015, 20, 20118–20130
20
77.9, 52.9, 35.2. ESI-MS m/z: Calcd for C11H11NO3S: 238.05 [M + H]+; found: 238.24. rαsD ´50.2
˝
(c = 0.1, MeOH).
1
1
1
(R)-2-(3 -Methylphenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (6e ). Colourless oil. H-NMR (CDCl3) δ
7.72 (s, 1H), 7.65–7.64 (m, 1H), 7.30 (d, J = 4.7 Hz, 2H), 5.29 (t, J = 9.1 Hz, 1H), 3.84 (s, 3H), 3.74–3.61
(m, 2H), 2.39 (s, 3H). 13C-NMR (CDCl3) δ 171.3, 171.2, 138.3, 132.5, 132.5, 128.9, 128.4, 125.9, 78.4,
20
52.7, 35.3, 21.2. ESI-MS m/z: Calcd for C12H13NO2S: 236.07 [M + H]+; found: 236.16. rαsD ´71.7
˝
(c = 0.1, MeOH).
20
1
1
˝
(R)-2-(2 -Hydroxyphenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (7e ). Colorless oil. rαsD ´13.9 (c = 0.1,
MeOH) [3].
1
1
˝
(R)-2-(2 -Fluorophenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (8e ). White solid. m.p. 70–72 C.
1H-NMR (CDCl3) δ 7.95–7.92 (m, 1H), 7.47–7.42 (m, 1H), 7.20–7.11 (m, 2H), 5.25 (t, J = 9.3 Hz, 1H), 3.84
(s, 3H), 3.74–3.62 (m, 2H). 13C-NMR (CDCl3) δ 171.2, 165.9 (d, J = 5.04 Hz), 160.6 (d, J = 255.78 Hz),
133.0 (d, J = 8.82 Hz), 130.7 (d, J = 2.52 Hz), 124.1 (d, J = 2.52 Hz), 120.7 (d, J = 10.08 Hz), 116.3 (d,
J = 22.68 Hz), 77.2, 52.8, 35.3 (d, J = 3.78 Hz). ESI-MS m/z: Calcd for C11H10FNO2S: 240.04 [M + H]+;
20
˝
found: 240.25. rαsD ´14.4 (c = 0.1, MeOH).
1
1
1
(R)-2-(2 -Chlorophenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (9e ). Yellow oil. H-NMR (CDCl3) δ 7.65
(dd, J = 1.6, 7.6 Hz, 1H), 7.44–7.42 (m, 1H), 7.37–7.29 (m, 2H), 5.30 (t, J = 9.3 Hz, 1H), 3.85 (s, 3 H), 3.79
(dd, J = 9.1, 11.0 Hz, 1H), 3.69 (dd, J = 9.6, 11.2 Hz, 1H). 13C-NMR (CDCl3) δ 171.0, 169.0, 132.6, 132.3,
131.3, 130.7, 130.4, 126.7, 78.0, 52.8, 36.3. ESI-MS m/z: Calcd for C11H10ClNO2S: 256.01:258.01 = 3:1
20
˝
[M + H]+; found: 256.12:258.37 = 3:1. rαsD ´32.4 (c = 0.1, MeOH).
1
1
(R)-2-(2 -Bromophenyl)-4-methoxycarbonyl-4,5-dihydrothiazole (10e ). Yellow oil. 1H-NMR (CDCl3) δ
7.64–7.54 (m, 2H), 7.37–7.24 (m, 2H), 5.35–5.29 (m, 1H), 3.85 (s, 3H), 3.83–3.67 (m, 2H). 13C-NMR
(CDCl3) δ 170.9, 170.1, 134.5, 133.6, 131.4, 130.6, 127.2, 121.3, 78.2, 52.8, 36.5. ESI-MS m/z: Calcd for
20
˝
C11H10BrNO2S: 299.96:301.96 = 1:1 [M + H]+; found: 299.92:301.76 = 1:1. rαsD ´22.0 (c = 0.1, MeOH).
1
1
3.2.2. Synthesis of 1h–10h and 1h –10h
NaBH4 (52.8 mg, 1.1 mmol) was added slowly in parts to a stirred solution of 1e–10e (1 mmol) in
EtOH (10 mL). After 1 h, the reaction was complete (checked by TLC). Ethyl acetate was added, and
then the mixture was washed with saturated aq. NaHCO3 (2 ˆ 5 mL), H2O (2 ˆ 5 mL), saturated aq.
NaCl (2 ˆ 5 mL), and dried with anhydrous Na2SO4. The mixture was concentrated under vacuum,
1
1
and the crude product was chromatographed on silica gel to obtain the products 1e–10e and 1e –10e .
The yields of 1h–10h and 1h –10h were from 76% to 89%.
1
1
˝
(S)-2-Phenyl-4-hydroxymethyly-4,5-dihydrothiazole (1h). Yellow solid. m.p. 76–78 C. 1H-NMR (CDCl3)
δ 7.81 (d, J = 7.6 Hz, 2H), 7.48–7.39 (m, 3H), 4.83–4.77 (m, 1H), 4.02 (dd, J = 4.6, 11.2 Hz, 1H), 3.80
(dd, J = 5.7, 11.3 Hz, 1H), 3.46–3.42 (m, 1H), 3.33–3.29 (m, 1H). 13C-NMR (CDCl3) δ 169.7, 132.9, 131.4,
128.5 ˆ 2, 128.4 ˆ 2, 79.3, 64.5, 34.4. ESI-MS m/z: Calcd for C10H11NOS: 194.06 [M + H]+; found:
20
˝
194.22. rαsD +30.7 (c = 0.1, MeOH).
1
˝
(S)-2-(4 -Hydroxyphenyl)-4-hydroxymethyl-4,5-dihydrothiazole (2h). White solid. m.p. 196–198 C.
1H-NMR (DMSO-d6) δ 10.06 (s, 1H), 7.59 (d, J = 8.5 Hz, 2H), 6.81 (d, J = 8.5 Hz, 2H), 4.91 (t, J = 5.4 Hz,
1H), 4.66–4.60 (m, 1H), 3.68–3.64 (m, 1H), 3.50–3.40 (m, 2H), 3.30–3.27 (m, 1H). 13C-NMR (DMSO-d6)
δ 165.3, 160.2, 129.9 ˆ 2, 124.0, 115.3 ˆ 2, 79.3, 62.3, 34.5. ESI-MS m/z: Calcd for C10H11NO2S: 210.05
20
[M + H]+; found: 210.23. rαsD +124.0 (c = 0.1, MeOH).
˝
(S)-2-(4 -Fluorophenyl)-4-hydroxymethyly-4,5-dihydrothiazole (3h). White solid. m.p. 60–62 C. 1H-NMR
(CDCl3) δ 7.79–7.76 (m, 2H), 7.07 (t, J = 10 Hz, 2H), 4.79–4.74 (m, 1H), 4.03 (dd, J = 4.4, 11.3 Hz, 1H),
3.78 (dd, J = 5.5, 11.2 Hz, 1H), 3.43 (dd, J = 9.0, 10.9 Hz, 1H), 3.35–3.31 (m, 1H). 13C-NMR (CDCl3)
δ 168.4, 164.6 (d, J = 252 Hz) 130.5, 130.4, 129.1, 115.6, 115.4, 79.3, 64.3, 34.6. ESI-MS m/z: Calcd for
1
˝
20
C10H10FNOS: 212.05 [M + H]+; found: 212.31. rαsD +10.9 (c = 0.1, MeOH).
˝
20124

Molecules 2015, 20, 20118–20130
1
˝
(S)-2-(4 -Bromophenyl)-4-hydroxymethyl-4,5-dihydrothiazole (4h). White solid. m.p. 96–98 C. 1H-NMR
(CDCl3) δ 7.67–7.65 (m, 2H), 7.54–7.52 (m, 2H), 4.80–4.75 (m, 1H), 4.03 (dd, J = 4.7, 11.3 Hz,
1H), 3.79 (dd, J = 5.7, 11.3 Hz, 1H), 3.46 (dd, J = 8.8, 10.7 Hz, 1H), 3.33 (dd, J = 9.3,
10.9 Hz, 1H). 13C-NMR (CDCl3) δ 168.7, 131.8, 131.7 ˆ 2, 129.8 ˆ 2, 126.0, 79.4, 64.4, 34.6.
ESI-MS m/z: Calcd for C10H10BrNOS: 271.97:273.97 = 1:1 [M + H]+; found: 272.16:274.04 = 1:1.
20
˝
rαsD +26.6 (c = 0.1, MeOH).
1
˝
(S)-2-(3 -Hydroxyphenyl)-4-hydroxymethyl-4,5-dihydrothiazole (5h). White solid. m.p. 118–120 C.
1H-NMR (CD3OD) δ 7.27–7.23 (m, 3H), 6.98–6.96(m, 1H), 4.78–4.73 (m, 1H), 4.04 (s, 2H), 3.85–3.78
(m, 2H). 13C-NMR (CD3OD) δ 171.1, 156.8, 133.6, 129.5, 119.7, 118.7, 114.2, 78.5, 63.1, 34.2. ESI-MS
20
m/z: Calcd for C10H11NO2S: 210.05 [M + H]+; found: 210.27. rαsD +178.2 (c = 0.1, MeOH).
˝
1
(S)-2-(3 -Methylphenyl)-4-hydroxymethyl-4,5-dihydrothiazole (6h). Yellow oil. 1H-NMR (CDCl3) δ
7.62–7.57 (m, 2H), 7.28–7.27 (m, 2H), 4.81–4.75 (m, 1H), 4.02 (dd, J = 4.7, 11.0 Hz, 1H), 3.79 (dd, J = 5.5,
11.2 Hz, 1H), 3.42 (dd, J = 8.8, 10.7 Hz, 1H), 3.31 (dd, J = 9.5, 10.7 Hz, 1H), 2.38 (s, 3H). 13C-NMR
(CDCl3) δ 169.9, 138.2, 132.8, 132.1, 128.8, 128.3, 125.6, 79.3, 64.4, 34.3, 21.2. ESI-MS m/z: Calcd for
20
C11H13NOS: 208.07 [M + H]+; found: 208.32. rαsD +7.6 (c = 0.1, MeOH).
˝
1
˝
(S)-2-(2 -Hydroxyphenyl)-4-hydroxymethyl-4,5-dihydrothiazole (7h). White solid. m.p. 43–45 C.
20
˝
rαsD +16.5 (c = 0.1, MeOH) [3].
(S)-2-(2 -Fluorophenyl)-4-hydroxymethyl-4,5-dihydrothiazole (8h). White solid. m.p. 78–80 C. 1H-NMR
(CDCl3) δ 7.86–7.82 (m, 1H), 7.46–7.41 (m, 1H), 7.20–7.11 (m, 2H), 4.81–4.75 (m, 1H), 4.00 (dd, J = 4.9,
11.2 Hz, 1H), 3.80 (dd, J = 5.7, 11.3 Hz, 1H), 3.44 (dd, J = 9.1, 10.7 Hz, 1H), 3.30 (dd, J = 9.1, 10.7 Hz,
1H). 13C-NMR (CDCl3) δ 164.6 (d, J = 5.04 Hz), 160.3 (d, J = 255.78 Hz), 132.6 (d, J = 8.82 Hz), 130.5 (d,
J = 1.26 Hz), 124.1 (d, J = 3.78 Hz), 121.1 (d, J = 10.1 Hz), 116.4 (d, J = 25.2 Hz), 78.4, 64.4, 34.4. ESI-MS
1
˝
20
˝
m/z: Calcd for C10H10FNOS: 212.05 [M + H]+; found: 212.41. rαsD +25.9 (c = 0.1, MeOH).
(S)-2-(2 -Chlorophenyl)-4-hydroxymethyl-4,5-dihydrothiazole (9h). Yellow oil. 1H-NMR (CDCl3) δ 7.57
1
(dd, J = 1.6, 7.6 Hz, 1H), 7.44–7.42 (m, 1H), 7.36–7.29 (m, 2H), 4.85–4.80 (m, 1H), 3.95 (dd, J = 4.9,
11.2 Hz, 1H), 3.78 (dd, J = 5.5, 11.2 Hz, 1H), 3.48 (dd, J = 9.1, 10.7 Hz, 1H), 3.36 (dd, J = 8.7, 10.9 Hz,
1H). 13C-NMR (CDCl3) δ 167.5, 132.7, 132.1, 131.1, 130.4, 130.3, 126.7, 79.2, 64.2, 35.5. ESI-MS m/z:
20
˝
Calcd for C10H10ClNOS: 228.02:230.02 = 3:1 [M + H]+; found: 228.21:230.37 = 3:1. rαsD +19.4
(c = 0.1, MeOH).
1
˝
(S)-2-(2 -Bromophenyl)-4-hydroxymethyl-4,5-dihydrothiazole (10h). Yellow solid. m.p. 54–56 C.
1H-NMR (CDCl3) δ 7.64 (d, J = 7.9 Hz, 1H), 7.51 (d, J = 7.6 Hz, 1H), 7.37–7.29 (m, 2H), 4.91–4.86
(m, 1H), 4.04–4.01 (m, 1H), 3.81 (dd, J = 5.5, 11.2 Hz, 1H), 3.55–3.51 (m, 1H), 3.42 (dd, J = 8.8, 10.7 Hz,
1H). 13C-NMR (CDCl3) δ 168.7, 134.8, 133.6, 131.2, 130.2, 127.3, 121.0, 79.5, 64.5, 35.7. ESI-MS m/z:
20
Calcd for C10H10BrNOS 271.97:273.97 = 1:1 [M + H]+; found: 271.86:273.91 = 1:1. rαsD +109.1
˝
(c = 0.1, MeOH).
1
˝
(R)-2-Phenyl-4-hydroxymethyl-4,5-dihydrothiazole (1h ). Yellow solid. m.p. 76–78 C. 1H-NMR (CDCl3)
δ 7.79 (d, J = 7.9 Hz, 2H), 7.47–7.37 (m, 4H), 4.80–4.75 (m, 1H), 4.01 (dd, J = 4.7, 11.0 Hz, 1H), 3.79
(dd, J = 5.5, 11.2 Hz, 1H), 3.44–3.40 (m, 1H), 3.33–3.29 (m, 1H), 2.78 (br. s, 1H). 13C-NMR (CDCl3) δ
169.8, 132.8, 131.4, 128.4 ˆ 2, 128.3 ˆ 2, 79.3, 64.3, 34.3. ESI-MS m/z: Calcd for C10H11NO2S: 194.06
20
[M + H]+; found: 194.23. rαsD ´30.6 (c = 0.1, MeOH).
˝
1
1
˝
(R)-2-(4 -Hydroxyphenyl)-4-hydroxymethyl-4,5-dihydrothiazole (2h ). White solid. m.p. 196–198 C.
1H-NMR (DMSO-d6) δ 10.12 (s, 1H), 7.75–7.60 (m, 2H), 6.95–6.80 (m, 2H), 4.97 (t, J = 5.7 Hz, 1H),
4.72–4.66 (m, 1H), 3.75–3.70 (m, 1H), 3.56–3.51 (m, 1H), 3.48 (dd, J = 8.7, 10.9 Hz, 1H), 3.35 (dd, J = 7.4,
10.9 Hz, 1H). 13C-NMR (DMSO-d6) δ 165.3, 160.2, 129.8 ˆ 2, 124.0, 115.3 ˆ 2, 79.3, 62.3, 34.5. ESI-MS
20
m/z: Calcd for C11H11NO2S: 210.05 [M + H]+; found: 210.16. rαsD ´123.9 (c = 0.1, MeOH).
˝
20125

Molecules 2015, 20, 20118–20130
1
1
˝
(R)-2-(4 -Fluorophenyl)-4-hydroxymethyl-4,5-dihydrothiazole (3h ). White solid. m.p. 60–62 C. 1H-NMR
(CDCl3) δ 7.81 (dd, J = 5.4, 8.8 Hz, 2H), 7.08 (t, J = 8.5 Hz, 2H), 4.81–4.75 (m, 1H), 4.02 (dd, J = 4.6,
11.2 Hz, 1H), 3.79 (dd, J = 5.7, 11.0 Hz, 1H), 3.45 (dd, J = 8.8, 10.7 Hz, 1H), 3.34–3.30 (m, 1H). 13C-NMR
(CDCl3) δ 168.3, 164.6 (d, J = 252 Hz), 130.5, 130.5, 129.2, 115.6, 115.5, 79.3, 64.5, 34.7. ESI-MS m/z:
20
Calcd for C10H10FNOS: 212.05 [M + H]+; found: 212.11. rαsD ´10.9 (c = 0.1, MeOH).
˝
1
1
˝
(R)-2-(4 -Bromophenyl)-4-hydroxymethyl-4,5-dihydrothiazole (4h ). White solid. m.p. 96–98 C. 1H-NMR
(CDCl3) δ 7.68–7.66 (m, 2H), 7.55–7.53 (m, 2H), 4.81–4.76 (m, 1H), 4.03 (dd, J = 4.7, 11.0 Hz, 1H),
3.79 (dd, J = 5.7, 11.3 Hz, 1H), 3.46 (dd, J = 8.8, 10.7 Hz, 1H), 3.33 (dd, J = 9.3, 10.9 Hz, 1H).
13C-NMR (CDCl3) δ 168.5, 131.8, 131.7 ˆ 2, 129.8 ˆ 2, 126.0, 79.4, 64.5, 34.6. ESI-MS m/z: Calcd for
20
C10H10BrNOS: 271.97:273.97 = 1:1 [M + H]+; found: 271.82:273.77 = 1:1. rαsD ´26.6 (c = 0.1, MeOH).
˝
1
1
˝
(R)-2-(3 -Hydroxyphenyl)-4-hydroxymethyl-4,5-dihydrothiazole (5h ). White solid. m.p. 118–120 C.
1H-NMR (CDCl3) δ 7.35 (br. s, 1H), 7.27–7.24 (m, 2H), 6.97–6.96 (m, 1H), 4.75–4.74 (m, 1H), 3.82–3.80
(m, 2H), 3.62–3.61 (m, 2H). 13C-NMR (CDCl3) δ 171.3, 156.8, 133.6, 129.6, 120.0, 118.9, 114.0, 78.5,
20
63.3, 34.2. ESI-MS m/z: Calcd for C10H11NO2S: 210.05 [M + H]+; found: 210.23. rαsD ´178.2
˝
(c = 0.1, MeOH).
1
1
(R)-2-(3 -Methylphenyl)-4-hydroxymethyl-4,5-dihydrothiazole (6h ). Yellow oil. 1H-NMR (CDCl3) δ
7.61–7.56 (m, 2H), 7.29–7.26 (m, 2H), 4.79–4.74 (m, 1H), 4.01 (dd, J = 4.7, 11.3 Hz, 1H), 3.78 (dd, J = 5.5,
11.2 Hz, 1H), 3.41 (dd, J = 8.8, 10.7 Hz, 1H), 3.30 (dd, J = 9.1, 10.7 Hz, 1H), 2.37 (s, 3H). 13C-NMR
(CDCl3) δ 169.9, 138.2, 132.7, 132.1, 128.8, 128.3, 125.6, 79.2, 64.3, 34.2, 21.2. ESI-MS m/z: Calcd for
20
˝
C11H13NOS: 208.07 [M + H]+; found: 208.01. rαsD ´7.6 (c = 0.1, MeOH).
1
1
˝
(R)-2-(2 -Hydroxyphenyl)-4-hydroxymethyl-4,5-dihydrothiazole (7h ). White solid. m.p. 44–46 C.
20
˝
rαsD ´16.1 (c = 0.1, MeOH) [3].
1
1
˝
(R)-2-(2 -Fluorophenyl)-4-hydroxymethyl-4,5-dihydrothiazole (8h ). White solid. m.p. 78–80 C. 1H-NMR
(CDCl3) δ 7.94–7.75 (m, 1H), 7.57–7.34 (m, 1H), 7.24–7.02 (m, 2H), 4.86–4.71 (m, 1H), 4.00 (dd, J = 4.7,
11.3 Hz, 1H), 3.80 (dd, J = 5.7, 11.0 Hz, 1H), 3.44 (dd, J = 9.0, 10.9 Hz, 1H), 3.31 (dd, J = 9.5, 10.7 Hz,
1H), 2.62 (br. s, 1H). 13C-NMR (CDCl3) δ 164.6 (d, J = 5.04 Hz), 160.4, (d, J = 255.78 Hz), 132.6 (d,
J = 8.82 Hz), 130.5 (d, J = 2.52 Hz), 124.1 (d, J = 3.78 Hz), 121.1 (d, J = 11.34 Hz), 116.4 (d, J = 22.68 Hz),
78.4, 64.4, 34.4 (d, J = 2.52 Hz). ESI-MS m/z: Calcd for C10H10FNOS: 212.05 [M + H]+; found: 212.29.
20
˝
rαsD ´25.9 (c = 0.1, MeOH).
1
1
(R)-2-(2 -Chlorophenyl)-4-hydroxymethyl-4,5-dihydrothiazole (9h ). Yellow oil. 1H-NMR (CDCl3) δ
7.58 (d, J = 7.6 Hz, 1H), 7.44 (d, J = 7.6 Hz, 1H), 7.36–7.28 (m, 2H), 4.86–4.80 (m, 1H), 3.96
(dd, J = 4.9, 11.2 Hz, 1H), 3.81–3.77 (m, 1H), 3.53–3.47 (m, 1H), 3.41–3.35 (m, 1H). 13C-NMR
(CDCl3) δ 167.5, 132.7, 132.1, 131.1, 130.4, 130.4, 126.7, 79.2, 64.2, 35.5. ESI-MS m/z: Calcd for
20
C10H10ClNOS: 228.02:230.02 = 3:1 [M + H]+; found: 228.16:230.27 = 3:1. rαsD ´19.4 (c = 0.1, MeOH).
˝
1
1
˝
(R)-2-(2 -Bromophenyl)-4-hydroxymethyl-4,5-dihydrothiazole (10h ). Yellow solid. m.p. 54–56 C.
1H-NMR (CDCl3) δ 7.69–7.63 (m, 1H), 7.55–7.50 (m, 1H), 7.36–7.28 (m, 1H), 4.90–4.85 (m, 1H),
4.03–3.95 (m, 1H), 3.83–3.76 (m, 1H), 3.55–3.51 (m, 1H), 3.46–3.39 (m, 1H). 13C-NMR (CDCl3) δ
168.7, 133.6, 131.7, 131.2, 130.2, 129.8, 127.3, 79.4, 64.4, 35.6. ESI-MS m/z: Calcd for C10H10BrNOS:
20
271.97:273.97 = 1:1 [M + H]+; found: 271.82:273.79 = 1:1. rαsD ´109.1 (c = 0.1, MeOH).
˝
1
1
3.2.3. Synthesis of 1a–10a and 1a –10a
10% sodium hydroxide solution was added slowly in parts to a stirred solution of 1e–10e and
1
1
1e –10e (1mmol) in EtOH (10 mL). After 2 h, the reaction was complete (checked by TLC). Purification
of the crude reaction mixture by column chromatography on silica gel (PE/ EA) afforded the desired
products and the yields were from 81% to 90%.
˝
(S)-2-Phenyl-4-carboxy-4,5-dihydrothiazole (1a). Yellow powder. m.p. 116–118 C. 1H-NMR (CDCl3) δ
7.83 (d, J = 7.6 Hz, 2H), 7.48–7.45 (m, 1H), 7.40–7.38 (m, 2H), 5.26 (t, J = 9.1 Hz, 1H), 3.72–3.64 (m, 2H).
20126

Molecules 2015, 20, 20118–20130
13C-NMR (CDCl3) δ 172.6, 171.8, 132.4, 131.7, 128.9 ˆ 2, 128.1 ˆ 2, 78.1, 35.1. ESI-MS m/z: Calcd for
20
C10H9NO2S: 208.04 [M + H]+; found: 208.25. rαsD +10.5 (c = 0.1, MeOH).
˝
20
1
˝
˝
(S)-2-(4 -Hydroxyphenyl)-4-carboxy-4,5-dihydrothiazole (2a). White solid. m.p. 150–152 C. rαsD +12.5
(c = 0.1, MeOH) [13].
1
˝
(S)-2-(4 -Fluorophenyl)-4-carboxy-4,5-dihydrothiazole (3a). White solid. m.p. 120–122 C. 1H-NMR
(CDCl3) δ 9.14 (br. s, 1 H), 7.87 (dd, J = 5.4, 8.5 Hz, 2H), 7.12 (t, J = 8.5 Hz, 2H), 5.38 (t, J = 9.1 Hz, 1H),
3.78–3.70 (m, 2H). 13C-NMR (CDCl3) δ 173.6, 172.0, 165.1 (d, J = 252 Hz), 130.9, 130.9, 128.3, 115.9,
20
115.7, 77.6, 35.3. ESI-MS m/z: Calcd for C10H8FNO2S: 226.03 [M + H]+; found: 226.08. rαsD +25.3
˝
(c = 0.1, MeOH).
1
˝
1
(S)-2-(4 -Bromophenyl)-4-carboxy-4,5-dihydrothiazole (4a). White power. m.p. 194–196 C. H-NMR
(CDCl3) δ 7.74–7.72 (m, 2H), 7.59–7.58 (m, 2H), 5.35 (t, J = 9.6 Hz, 1H), 3.76 (d, J = 9.8 Hz, 2H).
13C-NMR (CDCl3) δ 172.5, 171.9, 132.0 ˆ 2, 130.9, 130.0 ˆ 2, 127.0, 78.0, 35.2. ESI-MS m/z: Calcd for
20
C10H8BrNO2S: 285.95:287.95 = 1:1 [M + H]+; found: 285.91:287.75 = 1:1. rαsD +14.6 (c = 0.1, MeOH).
˝
20
1
˝
(S)-2-(3 -Hydroxyphenyl)-4-carboxy-4,5-dihydrothiazole (5a).
Yellow oil.
rαsD +13.3 (c = 0.1,
MeOH) [14].
1
˝
1
(S)-2-(3 -Methylphenyl)-4-carboxy-4,5-dihydrothiazole (6a). Yellow power. m.p. 118–120 C. H-NMR
(CDCl3) δ 7.66 (br. s, 1H), 7.64–7.49 (m, 1H), 7.48–7.27 (m, 2H), 5.25 (t, J = 9.1 Hz, 1H),
3.70–3.61 (m, 2H), 2.36 (s, 3H). 13C-NMR (CDCl3) δ 172.6, 172.0, 138.2, 132.5, 132.2, 128.8, 128.3,
125.6, 78.0, 35.0, 21.0. ESI-MS m/z: Calcd for C11H11NO2S: 222.05 [M + H]+; found: 222.33.
20
˝
rαsD +30.2 (c = 0.1, MeOH).
1
˝
128–130 C.
(S)-2-(2 -Hydroxyphenyl)-4-carboxy-4,5-dihydrothiazole (7a).
Yellow solid.
m.p.
20
˝
rαsD +159.9 (c = 0.1, MeOH) [15].
1
(S)-2-(2 -Fluorophenyl)-4-carboxy-4,5-dihydrothiazole (8a). Yellow oil. 1H-NMR (DMSO-d6) δ 7.90–7.87
(m, 1H), 7.69–7.57 (m, 2H), 7.31–7.27 (m, 1H), 5.26 (t, J = 9.0 Hz, 1H), 3.73–3.68 (m, 1H), 3.63–3.59
(m, 1H). 13C-NMR (DMSO-d6) δ 171.7, 171.4, 163.1(d, J = 3.78 Hz), 133.6(d, J = 7.56 Hz), 130.3(d,
J = 1.26 Hz), 124.9 (d, J = 2.52 Hz), 120.2(d, J = 7.56 Hz), 116.6(d, J = 8.82 Hz), 77.2, 35.0(d, J = 5.04 Hz).
20
˝
ESI-MS m/z: Calcd for C11H11NO2S: 226.03 [M + H]+; found: 226.03. rαsD +96.8 (c = 0.1, MeOH).
1
˝
1
(S)-2-(2 -Chlorophenyl)-4-carboxy-4,5-dihydrothiazole (9a). Yellow solid. m.p. 120–122 C. H-NMR
(CDCl3) δ 7.63 (d, J = 7.6 Hz, 1H), 7.46–7.43 (m, 1H), 7.39 (d, J = 7.9 Hz, 1H), 7.32–7.28 (m, 1H),
5.38 (t, J = 9.5 Hz, 1H), 3.92–3.68 (m, 2H). 13C-NMR (CDCl3) δ 173.1, 171.4, 132.5, 131.8, 131.5, 130.8,
130.6, 126.9, 77.5, 36.0. ESI-MS m/z: Calcd for C10H8ClNO2S: 242.00:244.00 = 3:1 [M + H]+; found:
20
˝
242.36:244.48 = 3:1. rαsD +62.6 (c = 0.1, MeOH).
1
(S)-2-(2 -Bromophenyl)-4-carboxy-4,5-dihydrothiazole (10a). Yellow oil. 1H-NMR (CDCl3) δ 7.65 (d,
J = 8.2 Hz, 1H), 7.59–7.54 (m, 1H), 7.38–7.22 (m, 2H), 5.41 (t, J = 9.5 Hz, 1H), 3.87–3.76 (m, 2H).
13C-NMR (CDCl3) δ 172.6, 167.5, 133.9, 131.9, 130.6, 129.7, 127.4, 121.2, 77.8, 36.1. ESI-MS m/z:
20
˝
Calcd for C10H8BrNO2S: 285.95:287.95 = 1:1 [M + H]+; found: 285.87:287.92 = 1:1. rαsD +74.5
(c = 0.1, MeOH).
1
˝
(R)-2-Phenyl-4-carboxy-4,5-dihydrothiazole (1a ). Yellow powder. m.p. 116–118 C. 1H-NMR (CDCl3) δ
7.83 (d, J = 7.6 Hz, 2H), 7.48–7.45 (m, 1H), 7.40–7.38 (m, 2H), 5.26 (t, J = 9.1 Hz, 1H), 3.72–3.64 (m, 2H).
13C-NMR (CDCl3) δ 172.6, 171.8, 132.4, 131.7, 128.9 ˆ 2, 128.1 ˆ 2, 78.1, 35.1. ESI-MS m/z: Calcd for
20
C10H9NO2S: 208.04 [M + H]+; found: 208.26. rαsD ´10.5 (c = 0.1, MeOH).
(R)-2-(4 -Hydroxyphenyl)-4-carboxy-4,5-dihydrothiazole (2a ). White solid. m.p. 150–152 C. rαsD ´12.5
(c = 0.1, MeOH) [13].
˝
20
1
1
˝
˝
1
1
˝
1
(R)-2-(4 -Fluorophenyl)-4-carboxy-4,5-dihydrothiazole (3a ). White solid. m.p. 120–122 C. H-NMR
(CDCl3) δ 10.23 (br. s, 1H), 7.86 (dd, J = 5.4, 8.5 Hz, 2H), 7.13–7.09 (m, 2H), 5.41–5.37 (m, 1H),
20127

Molecules 2015, 20, 20118–20130
3.78–3.69 (m, 2H). 13C-NMR (CDCl3) δ 173.7, 172.0, 165.1 (d, J = 252 Hz), 130.9, 130.9, 128.3, 115.9,
20
115.7, 77.5, 35.3. ESI-MS m/z: Calcd for C10H8FNO2S: 226.04 [M + H]+; found: 226.33. rαsD ´25.3
˝
(c = 0.1, MeOH).
1
1
˝
1
(R)-2-(4 -Bromophenyl)-4-carboxy-4,5-dihydrothiazole (4a ). White power. m.p. 194–196 C. H-NMR
(DMSO-d6) δ 7.85–7.66 (m, 4H), 5.31 (t, J = 8.8 Hz, 1H), 3.84–3.70 (m, 1H), 3.70–3.59 (m, 1H). 13C-NMR
(DMSO-d6) δ 171.6, 167.3, 131.9 ˆ 2, 131.4, 130.0 ˆ 2, 125.4, 78.4, 35.2. ESI-MS m/z: Calcd for
20
C10H8BrNO2S: 285.95:287.95 = 1:1 [M + H]+; found: 285.92:287.86 = 1:1. rαsD ´14.6 (c = 0.1, MeOH).
˝
20
1
1
˝
(R)-2-(3 -Hydroxyphenyl)-4-carboxy-4,5-dihydrothiazole (5a ). Yellow oil. rαsD ´13.3 (c = 0.1,
MeOH) [14].
1
1
˝
1
(R)-2-(3 -Methylphenyl)-4-carboxy-4,5-dihydrothiazole (6a ). Yellow power. m.p. 118–120 C. H-NMR
(CDCl3) δ 7.67 (br. s, 1H), 7.64–7.54 (m, 1H), 7.40–7.27 (m, 2H), 5.25 (t, J = 9.1 Hz, 1H), 3.77–3.56 (m,
2H), 2.36 (s, 3H). 13C-NMR (CDCl3) δ 172.6, 172.0, 138.2, 132.5, 132.3, 128.9, 128.3, 125.7, 78.0, 35.0, 21.0.
20
ESI-MS m/z: Calcd for C11H11NO2S: 222.05 [M + H]+; found: 222.27. rαsD ´30.2 (c = 0.1, MeOH).
˝
1
1
˝
128–130 C.
(R)-2-(2 -Hydroxyphenyl)-4-carboxy-4,5-dihydrothiazole (7a ).
Yellow solid.
m.p.
20
˝
rαsD ´160.0 (c = 0.1, MeOH) [15].
1
1
1
(R)-2-(2 -Fluorophenyl)-4-carboxy-4,5-dihydrothiazole (8a ). Yellow oil. H-NMR (DMSO-d6) δ 7.90–7.87
(m, 1H), 7.69–7.57 (m, 2H), 7.31–7.27 (m, 1H), 5.26 (t, J = 9.0 Hz, 1H), 3.73–3.68 (m, 1H), 3.63–3.59
(m, 1H). 13C-NMR (DMSO-d6) δ 171.7, 171.4, 163.1 (d, J = 3.78 Hz), 133.6 (d, J = 7.56 Hz), 130.3(d,
J = 1.26 Hz), 124.9 (d, J = 2.52 Hz), 120.2 (d, J = 7.56 Hz), 116.6 (d, J = 8.82 Hz), 77.2, 35.0 (d, J = 5.04 Hz).
20
˝
ESI-MS m/z: Calcd for C10H8FNO2S: 226.03 [M + H]+; found: 226.18. rαsD ´96.8 (c = 0.1, MeOH).
1
1
˝
1
(R)-2-(2 -Chlorophenyl)-4-carboxy-4,5-dihydrothiazole (9a ). Yellow solid. m.p. 120–122 C. H-NMR
(CDCl3) δ 7.63 (d, J = 7.6 Hz, 1H), 7.46–7.43 (m, 1H), 7.39 (d, J = 7.9 Hz, 1H), 7.32–7.28 (m, 1H),
5.38 (t, J = 9.5 Hz, 1H), 3.92–3.68 (m, 2H). 13C-NMR (CDCl3) δ 173.1, 171.4, 132.5, 131.8, 131.5, 130.8,
130.6, 126.9, 77.5, 36.0. ESI-MS m/z: Calcd for C10H8ClNO2S: 242.00:244.00 = 3:1 [M + H]+; found:
20
˝
242.06:244.12 = 3:1. rαsD ´62.6 (c = 0.1, MeOH).
1
1
(R)-2-(2 -bromophenyl)-4-carboxy-4,5-dihydrothiazole (10a ). Yellow oil. 1H-NMR (CDCl3) δ 7.65 (d,
J = 8.2 Hz, 1H), 7.59–7.54 (m, 1H), 7.38–7.22 (m, 2H), 5.41 (t, J = 9.5 Hz, 1H), 3.87–3.76 (m, 2H).
13C-NMR (CDCl3) δ 172.6, 167.5, 133.9, 131.9, 130.6, 129.7, 127.4, 121.2, 77.8, 36.1. ESI-MS m/z:
20
Calcd for C10H8BrNO2S: 285.95:287.95 = 1:1 [M + H]+; found: 285.87:287.69 = 1:1. rαsD ´74.5
˝
(c = 0.1, MeOH).
3.3. Biology
3.3.1. Filter Paper Assay
The standard bacterial stains were provided by the Institute of Plant Disease, Northwest
Agriculture and Forestry University. Ampicillin (Sigma, Shanghai, China) was preferred as the
positive control. Mueller–Hinton (Hangzhou Microbial Reagent Co., Ltd., Hangzhou, China) agar
˝
was used as an assay medium. The medium at 45 C was mixed with a suspension containing the
bacterial pathogen at approximately 108 colony forming units (CFU)¨ mL´1. Petri dishes (9 cm in
diameter) were then flooded with the mixture. The tested samples were dissolved in acetone at the
concentration of 1000 ppm, the filter papers (6 mm in diameter) were impregnated with 10 µL/disc
of each compound, then were completely dried and placed on the inoculated agar. The inoculated
˝
plates were incubated at 37 C for 10–12 h. Antibacterial activity was evaluated by measuring the
zone of inhibition against the test organism. Experiments were run in triplicate.
20128

Molecules 2015, 20, 20118–20130
3.3.2. Minimal Inhibitory Concentration (MIC)
Antibacterial activities were measured by the micro-broth dilution method in 96-well culture
plates using the Mueller–Hinton broth, according to the National Committee for Clinical Laboratory
˝
Standards. The tested bacteria were incubated in the Mueller–Hinton broth for 12 h at 37 C at
190 rpm, and the spore concentration was diluted to approximately 1 ˆ 105–1 ˆ 106 CFU/mL with
˝
Mueller–Hinton broth. After incubation for 24 h at 37 C, the MICs were examined by observing the
first few holes transparent [3].
3.3.3. The Fatty Acid Exposure Experiment
˝
The medium at 45 C was mixed with a suspension containing the bacterial pathogen at
´1
approximately 108 CFU¨ mL
and fatty acid or fatty acid salt of certain concentration gradient.
Petri dishes (9 cm in diameter) were then flooded with the mixture. Compound 7h was dissolved in
acetone at the concentration of 100 ppm, the filter papers (6 mm in diameter) were impregnated with
˝
10 µL/disc of each compound. The inoculated plates were incubated at 37 C for 10–12 h. The fatty
acid added experiment was evaluated by measuring the zone of inhibition against the test organism.
The experiments were run in triplicate.
3.3.4. Scanning Electron Microscopy
˝
The tested bacteria were incubated in the Mueller–Hinton broth for 12 h at 37 C at 180 rpm and
the spore concentration was diluted to approximately 1 ˆ 105–1 ˆ 106 CFU/mL with Mueller–Hinton
broth. Compound 7h was dissolved in acetone and added to the broth. After incubation for 2 h at
˝
37 C then centrifuged at 3500 rpm for 20 min. All the cells were washed twice with 0.1 M phosphate
˝
buffer saline (PBS, pH 7.4) and fixed with 2.5% glutaraldehyde (v/v) in 0.1 M PBS at 4 C for 24 h.
Next, the cells were dehydrated using 30%, 50%, 70%, 90%, and 100% ethanol, and then the ethanol
was replaced by tertiary butyl alcohol. The cells were dried at “critical point” in liquid CO2, and the
samples were gold-covered by cathodic spraying before examination [16,17].
4. Conclusions
Aryl nitriles and methyl cysteine reacted in the presence of sodium carbonate in dry methanol
to afford methyl-2-aryl-4, 5-dihydrothiazole-4-carboxylates in high yield. Ortho substituents
did not favor the reaction, especially some large groups, such as -Br and -Cl. Meanwhile
electron-withdrawing groups were better to this reaction than electron-donating ones.
Not only electron-withdrawing but also electron-donating groups could diminish the
1
antibacterial activities unless a 2 -hydroxy was present in the 2-aryl substituent of the
4,5-dihydro-thiazole analogues. These results implied that the intramolecular hydrogen bond
1
between the 2 -hydroxy and the nitrogen-atom of 4,5-dihydrothiazole is very important for the
antibacterial activities of these compounds.
According to the fatty acid exposure experiments at high concentrations of fatty acid (1000 ppm),
when short chain acids were added in culture medium, the bacteria grew normally; inhibition zones
become small while long chain acids were added in it. The overall trend is consistent with the results
of yanglingmycin, whereby by decreasing the concentration of fatty acids added, the inhibition zone
gradually increased. This implied that these antibacterial compounds influence fatty acid synthesis of
the tested bacteria. Scanning electron microscopy results showed that the bacterial cell walls showed
depressions, leading to cell content leakage. Compound 7h may damage cell membranes, leading to
altered cell metabolism and the finally cell lysis and death. The antibacterial mechanism is similar to
that of yanglingmycin.
Acknowledgments: This work was supported by the National Natural Science Foundation of China (31371958,
31070344, 21372185) and Agricultural Science and Technology Research Foundation of Shanxi Province
(2014k02-10-02).
20129

Molecules 2015, 20, 20118–20130
Author Contributions: F.T., B.S. and J.Z. designed research; F.T. performed research; B.S., J.L. and W.W.
performed statistical analysis; F.T. and J.Z. wrote the paper; B.S. and W.W. reviewed the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
Mansfield, J.; Genin, S. Top 10 plant pathogenic bacteria in molecular plant pathology. Mol. Plant Pathol.
2012, 13, 614–629. [CrossRef] [PubMed]
Zhang, J.W.; Zhang, W.J. A new dihydrooxazole antibiotic from the fermentation broth of streptomyces
djakartensis. Heterocycles 2014, 89, 1656–1661.
Li, L.B.; Dan, W.J.; Tan, F.F.; Cui, L.H. Synthesis and antibacterial activities of Yanglingmycin analogues.
Chem. Pharm. Bull. 2015, 63, 33–37. [CrossRef] [PubMed]
Wu, W.J.; Zhang, J.W.; Zhang, W.J. A new dihydrooxazole antibiotic: structure, bioactivity and mechanism.
In Proceedings of the International Conference on Biopesticides 7, Side-Antalya, Turkey, 19–25 October
2014; p. 145.
5.
6.
7.
Zhang, W.J.; Wei, S.P.; Zhang, J.W.; Wu, W.J. Antibacterial activity composition of the fermentation broth of
Streptomyces djakartensis NW35. Molecules 2013, 18, 2763–2768. [CrossRef] [PubMed]
Zamri, A.; Abdallah, M.A. An improved stereocontrolled synthesis of pyochelin, siderophore of
Pseudomonas aeruginosa and Burkholderia cepacia. Tetrahedron 2000, 56, 249–256. [CrossRef]
Inahashi, Y.; Iwatsuki, M.; Ishiyama, A.; Namatame, M.; Nishihara-Tsukashima, A.; Matsumoto, A.;
Hirose, T.; Sunazuka, T.; Yamada, H.; Otoguro, K.; et al. Spoxazomicins A–C, novel antitrypanosomal
alkaloids produced by an endophytic actinomycete, Streptosporangium oxazolinicum K07–0460T. J. Antibiot.
2011, 64, 303–307. [CrossRef] [PubMed]
8.
9.
Fernandez, X.; Fellous, R.; Duñach, E. Novel synthesis of 2-thiazolines. Tetrahedron Lett. 2000, 41, 3381–3384.
[CrossRef]
Ino, A.; Hasegawa, Y.; Murabayashi, A. Total synthesis of the antimycoplasma antibiotic micacocidin.
Tetrahedron Lett. 1998, 39, 3509–3512. [CrossRef]
10. Suzuki, N.; Izawa, Y. A convenient synthesis of 2-phenyl-thiazolines. Tetrahedron Lett. 1974, 15, 1863–1864.
[CrossRef]
11. Kline, T.; Fromhold, M.; McKennon, T.E.; Cai, S.; Treiberg, J.; Ihle, N.; Sherman, D.; Schwan, W.;
Hickey, M.J.; Warrener, P.; et al. Antimicrobial Effects of novel siderophores linked to β-lactam antibiotics.
Bioorg. Med. Chem. 2000, 8, 71–93. [CrossRef]
12. Zunnundzhanov, A.; Bessonova, I.A.; Abdullaev, N.D.; Ogai, D.K. Structure of aeruging from
Pseudomonas aeruginosa. Chem. Nat. Compd. 1987, 4, 461–465. [CrossRef]
13. Loughlin, W.A.; Knevitt, S.A. Approaches to the high-throughput synthesis of analogues of
Dihydroaeruginoic. Acid. Aust. J. Chem. 2000, 53, 457–462. [CrossRef]
14. Maltsev, O.V.; Walter, V. Medium buffer effects on the condensation of L-Cysteine and aryl nitriles to
(R)-2-aryl-4,5-dihydrothiazole-4-carboxylic acids. Synthesis 2013, 45, 2763–2767. [CrossRef]
15. Ferm, R.J.; Riebsomer, J.L. The Chemistry of the 2-imidazolines and imidazolidines. Chem. Rev. 1954, 54,
593–613. [CrossRef]
16. Bandyopadhyay, N.; Zhu, M.; Lu, L.; Mitra, D.; Das, M.; Das, P.; Samanta, A.; Naskar, J.P. Synthesis,
structure, spectral characterization, electrochemistry and evaluation of antibacterial potentiality of a novel
oxime-based palladium(II) compound. Eur. J. Med. Chem. 2015, 89, 59–66. [CrossRef] [PubMed]
17. Sun, D.; Zhang, W.; Lv, M.; Yang, E.; Zhao, Q.; Wang, W. Antibacterial activity of ruthenium(II) polypyridyl
complex manipulated by membrane permeability and cell morphology. Bioorg. Med. Chem. Lett. 2015, 25,
2068–2073. [CrossRef] [PubMed]
Sample Availability: Samples of the compounds are available from the authors.
© 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open
access article distributed under the terms and conditions of the Creative Commons by
Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
20130