The synthesis of 2-alkylthio-3-alkyl-5-arylmethylidene-4H-imidazol-4-ones

Article abstract of DOI:10.1080/10426500600614147

2-alkylthio-3-alkyl-5-arylmethylidene-4H-imidazol-4-ones were synthesized by the S-alkylation and N-alkylation of 2-thioxo-5-arylmethylidene-4- imidazolidinones, which were obtained via a tandem aza-Wittig reaction of vinyliminophosphoranes, carbon disulf

Full text of DOI:10.1080/10426500600614147

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The Synthesis of 2-Alkylthio-3-
alkyl-5-arylmethylidene4H-
imidazol-4-ones
Yong Sun a , Li-Ping Gao a , Zhi-Qiang Guo a & Ming-
Wu Ding b
a Department of Chemistry , Yunyang Teachers
College, Danjiangkou Hubei , P. R. China
b College of Chemistry, Central China Normal
University , Wuhan, P. R. China
Published online: 21 Sep 2006.
To cite this article: Yong Sun , Li-Ping Gao , Zhi-Qiang Guo & Ming-Wu Ding
(2006) The Synthesis of 2-Alkylthio-3-alkyl-5-arylmethylidene4H-imidazol-4-ones,
Phosphorus, Sulfur, and Silicon and the Related Elements, 181:9, 2109-2116, DOI:
10.1080/10426500600614147
To link to this article: http://dx.doi.org/10.1080/10426500600614147
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Phosphorus, Sulfur, and Silicon, 181:2109–2116, 2006
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500600614147
The Synthesis of 2-Alkylthio-3-alkyl-5-arylmethylidene-
4H-imidazol-4-ones
Yong Sun
Li-Ping Gao
Zhi-Qiang Guo
Department of Chemistry, Yunyang Teachers College, Danjiangkou
Hubei P. R. China
Ming-Wu Ding
College of Chemistry, Central China Normal University, Wuhan,
P. R. China
2-alkylthio-3-alkyl-5-arylmethylidene-4H-imidazol-4-ones were synthesized by the
S-alkylation and N-alkylation of 2-thioxo-5-arylmethylidene-4-imidazolidinones,
which were obtained via a tandem aza-Wittig reaction of vinyliminophosphoranes,
carbon disulfide, and excess ammonium hydroxide (28% NH in water).
3
Keywords 4H-imidazol-4-ones; alkylation; aza-Wittig reaction; synthesis
INTRODUCTION
4H-imidazol-4-ones are important heterocycles having biological and
pharmaceutical activities,1−10 and some 2-alkylthio-4H-imidazol-4-
ones show significant fungicidal activities.11−13 Until now, many of
the new derivatives of 2-alkylthio-4H-imidazol-4-ones have been syn-
thesized to evaluate their biological and pharmaceutical activities.
However, most of the 2-alkylthio-4H-imidazol-4-ones reported are of the
5,5-disubstituted type and were generally synthesized from the corre-
sponding α-amino acetic acid13,14 (Scheme 1). Regrettably, 2-alkylthio-
5-arylmethylidene-4H-imidazol-4ones cannot be prepared by this
general method, for the corresponding starting material needed would
Received September 14, 2005; accepted December 20, 2005.
We gratefully acknowledge the financial support of this work by the Natural Science
Foundation of Hubei Education Commission of China (Project No. D200560003) and the
National Natural Science Foundation of China (Project No. 20102001).
Address correspondence to Yong Sun, Department of Chemistry, Yunyang Teachers
College, Danjiangkou, Hubei 442700 P. R. China. E-mail: sunyong376111@yahoo.com.cn
2109

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Y. Sun et al.
SCHEME 1
be unstable vinyl amino acetic acids. Recently, we have become inter-
ested in the synthesis of new imidazolone derivatives, especially in
2-alkylthio-5-arylmethylidene-4H-imidazol-4ones, via a tandem aza-
Wittig reaction, and some of them have been shown potential fungi-
cidal activities.15−24 In the present work, we wish to report further
a new efficient synthesis method of some new 2-alkylthio-3-alkyl-
5-arylmethylidene-4H-imidazol-4-ones derivatives 5 from the stable
vinyliminophosphoranes 1.
RESULTS AND DISCUSSION
The easily accessible iminophosphoranes 125,26 reacted with carbon
disulfide to give vinyl isothiocyanates 2,13−18 which were allowed to
react with excess ammonium hydroxide (28% NH3 in water) smoothly
at r.t. to give 2-thioxo-5-arylmethylidene-4-imidazolidinones 427,28 in
86–90% yields (Scheme 2 and Table I). The formation of 4 can be ratio-
nalized in terms of an initial nucleophilic addition of ammonia to give
the intermediates 3, which cyclize to give 4.
SCHEME 2
The S- alkylation13−18 and N-alkylation of 4 with excess alkyl
halides in the presence of solid potassium carbonate provided 2-
alkylthio-3-alkyl-5-arylmethylidene-4H-imidazol-4-ones 5 in 55–86%
yields (Scheme 3). When activated alkylating reagents (RI, BrCH2COR)
were used, the alkylation could be carried out at r.t. When other alky-
lating reagents were applied, the alkylation had to be carried out at
50–70◦C (Table I).

2-Alkylthio-3-alkyl-5-arylmethylidene-4H-imidazol-4-ones
2111
SCHEME 3
The structures of 4 and 5 have been determined through spectro-
scopic characterization. For example, the 1H NMR spectroscopic data in
5a show the signals of CH, NCH3, and SCH3 at 6.88 ppm, 3.17 ppm,
and 2.74 ppm as single peaks, respectively. The chemical shift of the aryl
hydrogens were in the range 8.10–7.26 ppm and appear as a multiplet.
In the IR spectrum data of 5a, the strong stretching peak of imida-
zolone C O appears at 1726 cm−1. The stretching vibration of C C
shows a strong absorption band at about 1642 cm−1 due to a resonance
effect. The MS of 5a shows a molecular ion peak at m/z 266 with 100%
abundance.
EXPERIMENTAL
Melting points were uncorrected. MS were measured on a Finni-
gan Trace spectrometer. IR were recorded on a PE-983 infrared
TABLE I The Preparation of Derivatives of
2-Thioxo-5-Arylmethylidene-4-Imidazolidinones 4 and
2-Alkylthio-3-Alkyl-5-Arylmethylidene-4H-imidazol-4-ones 5
Entry
Ar
RX
Conditions Masses (g) Yield (%)∗ m.p. (◦C)
4aa
4b
5ab
5b
5c
5d
5e
5f
4-Cl C6H4
2-Cl C6H4
4-Cl C6H4 MeI
r.t./2 h
r.t./3 h
r.t./2 h
1.07
1.02
0.82
0.85
0.85
0.77
1.44
1.59
1.10
1.13
0.79
1.36
90
86
77
72
66
55
86
84
72
69
74
81
286–287
256–258
159–161
114–116
102–104
76–78
183–185
197–199
156–158
134–136
181–182
121–123
4-Cl C6H4 EtBr
60◦C/4 h
70◦C/6 h
70◦C/8 h
50◦C/3 h
r.t./3 h
4-Cl C6H4 n-PrBr
4-Cl C6H4 n-BuBr
4-Cl C6H4 PhCH2Cl
4-Cl C6H4 PhCOCH2Br
4-Cl C6H4 BrCH2COOMe
4-Cl C6H4 ClCH2COOEt
2-Cl C6H4 MeI
5g
5h
5i
r.t./3 h
50◦C/4 h
r.t./3 h
5j
2-Cl C6H4 PhCH2Cl
50◦C/4 h
a
b
Isolated yields of 4 based on vinyliminophosphoranes 1.
Purified yields of 5 based on 2-thioxo-5-arylmethylidene-4-imidazolidinones 4.

2112
Y. Sun et al.
spectrometer as KBr pellets with absorption in cm−1. NMR were
recorded in CDCl3 for 5 or DMSO-d6 for 4 on a Varian Mercury 400 spec-
trometer, and resonances are given in ppm (δ) relative to tetramethyl-
silane (TMS). Elemental analyses were recorded on a Vario EL III ele-
mentary analysis instrument. CS2 is poisonous, and a good hood should
be used. Vinyliminophosphoranes 1 were prepared by the literature
report.25,26
The Preparation of 2-Thioxo-5-arylmethylidene-4-
imidazolidinones 4
To a solution of vinyliminophosphoranes 1 (2.43 g, 5 mmol) in dry
methylene chloride (15 mL) was added excess carbon disufide (5 mL). Af-
ter the reaction mixture was refluxed for 28 h, the solvent was removed
under reduced pressure, and ether/petroleum ether (1:2, 20 mL) was
added to precipitate triphenylphosphine sulfide, which was removed
by filtration. The filtrate was evaporated to give vinyl isothiocyanate
2, which was used directly without further purification. To the solu-
tion of crude 2 in CH3CN (15 mL) was added excess ammonium hy-
droxide (28% NH3 in water) (2 mL, 30 mmol). The mixture was al-
lowed to stand for 2–3 h at r.t., and the precipitated solid was collected
and washed with water and ethanol and recrystallized from ethanol to
give 4.
2-Thioxo-5-(4-chlorophenylmethylidene)-4-imidazolidinone
(4a)
Yellow crystals, 1H NMR (DMSO-d6, 400 MHz): δ 12.43 (s, 1H,
O CNH), 12.21 (s, 1H, C CNH), 7.78–7.47 (m, 4H, Ar-H), 6.48 (s, 1H,
CH); IR (cm−1), 3336 (N H), 3310 (N H), 1716 (C O), 1649 (C C);
MS (m/z, %), 240 (M+, 27), 238 (M+, 70), 203 (2), 150 (62), 122 (23),
115 (31), 88 (71), 58 (100). Elemental anal. calcd. for C10H7N2OSCl: C,
50.31; H, 2.94; N, 11.74. Found: C, 50.44; H, 3.10; N, 11.93.
2-Thioxo-5-(2-chlorophenylmethylidene)-4-imidazolidinone
(4b)
Light yellow crystals, 1H NMR (DMSO-d6, 400 MHz): δ 12.49 (s,
1H, O CNH), 12.29 (s, 1H, C CNH), 7.84–7.40 (m, 4H, Ar H), 6.60
(s, 1H, CH); IR (cm−1), 3345 (N H), 3321 (N H), 1719 (C O),
1653 (C C); MS (m/z, %), 240 (M+, 2), 238 (M+, 6), 203 (6), 151
(12), 124 (14), 115 (18), 85 (62), 58 (100). Elemental anal. calcd. for
C10H7N2OSCl: C, 50.31; H, 2.94; N, 11.74. Found: C, 50.57; H, 3.18; N,
12.01.

2-Alkylthio-3-alkyl-5-arylmethylidene-4H-imidazol-4-ones
2113
The Preparation of 2-Alkylthio-3-alkyl-5-arylmethylidene-4H-
imidazol-4-ones 5
A mixture of 4 (4 mmol), excess alkyl halides (16 mmol), and solid potas-
sium carbonate (2.22 g, 16 mmol) in CH3CN (30 mL) was stirred for
2–8 h at r.t. or 50–70◦C and filtered; the filtrate was condensed, and the
residue was recrystallized from methylene chloride/petroleum ether to
give 5.
2-Methylthio-3-methyl-5-(4-chlorophenylmethylidene)-4H-
imidazol-4-one (5a)
1
Light yellow crystals, H NMR (CDCl3, 400 MHz): δ 8.10–7.26 (m,
4H, Ar-H), 6.88 (s, 1H, CH), 3.17 (s, 3H, NCH3), 2.74 (s, 3H, SCH3); IR
(cm−1), 1726 (C O), 1642 (C C); MS (m/z, %), 268 (M+, 35), 266 (M+,
100), 251 (1), 233 (5), 221 (11), 176 (13), 149 (24), 87 (87). Elemental
anal. calcd. for C12H11N2OSCl: C, 54.03; H, 4.13; N, 10.51. Found: C,
53.99; H, 3.98; N, 10.73.
2-Ethylthio-3-ethyl-5-(4-chlorophenylmethylidene)-4H-
imidazol-4-one (5b)
Yellow crystals, 1H NMR (CDCl3, 400 MHz): δ 8.09–7.27 (m, 4H,
Ar H), 6.85 (s, 1H, CH), 3.64 (q, 2H, NCH2), 3.36 (q, 2H, SCH2),
1.52 (t, 3H, NCH2CH3), 1.26 (t, 3H, SCH2CH3); IR (cm−1), 1729 (C O),
1638 (C C); MS (m/z, %), 296 (M+, 29), 294 (M+, 100), 278 (3), 266
(57), 261 (44), 238 (11), 182 (21), 149 (30). Elemental anal. calcd. for
C14H15N2OSCl: C, 57.05; H, 5.09; N, 9.51. Found: C, 57.11; H, 5.18; N,
9.71.
2-(n-Propylthio)-3-(n-propyl)-5-(4-chlorophenylmethylidene)-
4H-imidazol-4-one (5c)
Yellow crystals, 1H NMR (CDCl3, 400 MHz): δ 8.09–7.27 (m, 4H,
Ar H), 6.84 (s, 1H, CH), 3.55 (t, 2H, NCH2), 3.32 (t, 2H, SCH2), 1.93–
1.84 (m, 2H, NCH2CH2CH3), 1.74–1.65 (m, 2H, SCH2CH2CH3), 1.10
(t, 3H, NCH2CH2CH3), 0.94 (t, 3H, SCH2CH2CH3); IR (cm−1), 1728
(C O), 1639 (C C); MS (m/z, %), 324 (M+, 10), 322 (M+, 27), 307 (4),
294 (100), 280 (54), 247 (39), 238 (41), 182 (90). Elemental anal. calcd.
for C16H19N2OSCl: C, 59.53; H, 5.89; N, 8.68. Found: C, 59.69; H, 6.02;
N, 8.88.
2-(n-Butylthio)-3-(n-butyl)-5-(4-chlorophenylmethylidene)-
4H-imidazol-4-one (5d)
Yellow crystals, 1H NMR (CDCl3, 400 MHz): δ 8.10–7.26 (m, 4H,
Ar H), 6.84 (s, 1H, CH), 3.58 (t, 2H, NCH2), 3.33 (t, 2H, SCH2),

2114
Y. Sun et al.
1.86–1.33 (m, 8H, NCH2CH2CH2CH3 and SCH2CH2CH2CH3), 1.03–
0.93 (m, 6H, NCH2CH2CH2CH3 and SCH2CH2CH2CH3); IR (cm−1),
1728 (C O), 1642 (C C); MS (m/z, %), 352 (M+, 9), 350 (M+, 25), 321 (6),
317 (6), 303 (36), 294 (16), 261 (100), 149 (15). Elemental anal. calcd.
for C18H23N2OSCl: C, 61.63; H, 6.56; N, 7.99. Found: C, 61.89; H, 6.77;
N, 8.21.
2-Benzylthio-3-benzyl-5-(4-chlorophenylmethylidene)-4H-
imidazol-4-one (5e)
1
Light yellow crystals, H NMR (CDCl3, 400 MHz): δ 8.11–7.25 (m,
14H, Ar H), 6.94 (s, 1H, CH), 4.76 (s, 2H, NCH2), 4.54 (s, 2H, SCH2);
IR (cm−1), 1722 (C O), 1640 (C C); MS (m/z, %), 420 (M+, 6), 418 (M+,
14), 385 (14), 328 (8), 294 (2), 150 (6), 90 (100), 64 (25). Elemental anal.
calcd. for C24H19N2OSCl: C, 68.82; H, 4.54; N, 6.69. Found: C, 69.03; H,
4.75; N, 6.98.
2-Benzoylmethylthio-3-benzoylmethyl-5-(4-chlorophen-
ylmethylidene)-4H-imidazol-4-one (5f)
1
Yellow crystals, H NMR (CDCl3, 400 MHz): δ 8.14–7.25 (m, 14H,
Ar H), 6.96 (s, 1H, CH), 4.66 (s, 2H, NCH2), 4.40 (s, 2H, SCH2); IR
(cm−1), 1734 (C O), 1696 (COPh), 1690 (COPh), 1647 (C C); MS (m/z,
%), 476 (M+, 18), 474 (M+, 49), 439 (9), 369 (35), 164 (55), 150 (87), 105
(83), 89 (100). Elemental anal. calcd. for C26H19N2O3SCl: C, 65.75; H,
4.00; N, 5.90. Found: C, 66.01; H, 3.96; N, 6.15.
2-Methoxycarbonylmethylthio-3-methoxycarbonylmethyl-5-
(4-chlorophenylmethylidene)-4H-imidazol-4-one (5g)
Light yellow crystals, 1H NMR (CDCl3, 400 MHz): δ 8.07–7.26
(m, 4H, Ar H), 6.94 (s, 1H, CH), 4.38 (s, 2H, NCH2), 4.35 (s, 3H,
NCH2COOCH3), 4.33 (s, 3H, SCH2COOCH3), 4.12 (s, 2H, SCH2); IR
(cm−1), 1741 (COOEt), 1738 (COOEt), 1731 (C O), 1642 (C C); MS
(m/z, %), 384 (M+, 34), 382 (M+, 90), 351 (15), 323 (100), 310 (13), 266
(57), 250 (39), 164 (79). Elemental anal. calcd. for C16H15N2O5SCl: C,
50.20; H, 3.92; N, 7.32. Found: C, 50.44; H, 4.17; N, 7.15.
2-Ethoxycarbonylmethylthio-3-ethoxycarbonylmethyl-5-(4-
chlorophenylmethylidene)-4H-imidazol-4-one (5h)
White crystals, 1H NMR (CDCl3, 400 MHz): δ 8.06–7.25 (m, 4H,
Ar H), 6.94 (s, 1H, CH), 4.37 (s, 2H, NCH2), 4.28–4.22 (m, 4H,
NCH2COOCH2CH3 and SCH2COOCH2CH3), 4.11 (s, 2H, SCH2), 1.32–
1.28 (m, 6H, NCH2COOCH2CH3 and SCH2COOCH2CH3); IR (cm−1),
1742 (COOEt), 1739 (COOEt), 1731 (C O), 1642 (C C); MS (m/z, %),
412 (M+, 38), 410 (M+, 100), 365 (20), 336 (99), 307 (10), 279 (82), 164

2-Alkylthio-3-alkyl-5-arylmethylidene-4H-imidazol-4-ones
2115
(71), 149 (76). Elemental anal. calcd. for C18H19N2O5SCl: C, 52.62; H,
4.63; N, 6.82. Found: C, 52.83; H, 4.76; N, 6.97.
2-Methylthio-3-methyl-5-(2-chlorophenylmethylidene)-4H-
imidazol-4-one (5i)
Yellow crystals, 1H NMR (CDCl3, 400 MHz): δ 8.88–7.25 (m, 5H,
Ar H and CH), 3.17 (s, 3H, NCH3), 2.72 (s, 3H, SCH3); IR (cm−1),
1730 (C O), 1645 (C C); MS (m/z, %), 268 (M+, 7), 266 (M+, 17), 231
(100), 216 (10), 183 (7), 177 (9), 149 (36), 87 (83). Elemental anal. calcd.
for C12H11N2OSCl: C, 54.03; H, 4.13; N, 10.51. Found: C, 54.25; H, 4.38;
N, 10.78.
2-Benzylthio-3-benzyl-5-(2-chlorophenylmethylidene)-4H-
imidazol-4-one (5j)
1
Yellow crystals, H NMR (CDCl3, 400 MHz): δ 8.89–7.25 (m, 15H,
Ar H and CH), 4.77 (s, 2H, NCH2), 4.53 (s, 2H, SCH2); IR (cm−1),
1725 (C O), 1643 (C C); MS (m/z, %), 420 (M+, 1), 418 (M+, 3), 385
(2), 151 (3), 123 (2), 104 (2), 90 (100), 64 (21). Elemental anal. calcd. for
C24H19N2OSCl: C, 68.82; H, 4.54; N, 6.69. Found: C, 69.09; H, 4.77; N,
6.89.
REFERENCES
[1] B. Trivedi and V. H. Shah, J. Indian Chem. Soc., 70, 645 (1993).
[2] M. Bhalla, P. K. Naithani, T. N. Bhalla, A. K. Saxena, and K. Shanker, J. Indian
Chem. Soc., 69, 594 (1992).
[3] A. Kumar, M. Verma, A. K. Saxena, and K. Shanker, Indian J. Chem., Sect. B, 27B,
301 (1998).
[4] T. Okazaki, K. Kikuchi, T. Watanable, A. Suga, M. Shibasaki, A. Fujimori, O. Ina-
gaki, and I. Yanagisawa, Chem. Pharm. Bull., 46, 777 (1998).
[5] K. Kiec-Kononowicz, E. Szymanska, M. Motyl, W. Holzer, A. Bialecka, and A.
Kasprowicz, Pharmazie, 53, 680 (1998).
[6] A. I. Khodair, H. I. El-Subbagh, and A. M. Al-Obaid, Phosphorus, Sulfur, and Sili-
con., 140, 159 (1998).
[7] H. Sato, M. Tsuda, K. Watanabe, and J. Kobayashi, Tetrahedron, 54, 8687 (1998).
[8] T. S. Sulkowski, D. P. Strike, and H. M. Elockdah, U.S. 5599829 (1997); Chem. Abstr.,
126, 195251h (1997).
[9] B. L. Pilkington, S. E. Russell, A. J. Whittle, W. R. Mound, M. D. Turnbull, A.
M. Kozakiewicz, and W. G. Whittingham, G.B. 2329180 (1999); Chem. Abstr., 131,
44817z (1999).
[10] M. Gaillard-Kelly, F. Goubet, D. Philibert, and J. G. Teutsch, U.S. 5411981 (1995);
Chem. Abstr., 123, 55883d (1995).
[11] G. Emeric, J. Hutt, and J. Perez, W.O. 9602538 (1996); Chem. Abstr., 125, 10818m
(1996).
[12] J. P. Bascou, A. Gadras, J. Perez, G. Emeric, G. Lacroix, and C. Veyrat, E.P. 668270
(1995); Chem. Abstr., 123, 340128t (1995).

2116
Y. Sun et al.
[13] G. Lacroix, R. Peignier, R. Pepin, J. P. Bascou, J. Perez, and C. Schmitz, U.S. 6002016
(1999); Chem. Abstr., 132, 35698e (2000).
[14] J. P. Bascou, G. Lacroix, A. Gadras, and J. Perez, E.P. 629616 (1994); Chem. Abstr.,
122, 187580s (1995).
[15] Y. Sun and M. W. Ding, Phosphorus, Sulfur, and Silicon, 179, 2465 (2004).
[16] Y. Sun and M. W. Ding, Phosphorus, Sulfur, and Silicon, 178, 2137 (2003).
[17] Y. Sun and M. W. Ding, Heteroatom Chem., 14, 348 (2003).
[18] Y. Sun, L. P. Gao, and M. W. Ding, Heterocycl. Commun., 11, 69 (2005).
[19] M. W. Ding, G. P. Zeng, and Z. J. Liu, Phosphorus, Sulfur, and Silicon, 177, 1315
(2002).
[20] M. W. Ding, Y. Sun, X. P. Liu, and Z. J. Liu, Heterocycl. Commun., 9, 135 (2003).
[21] M. W. Ding, Y. Sun, S. J. Yang, X. P. Liu, and Z. J. Liu, Synth. Commun., 33, 1651
(2003).
[22] M. W. Ding, Z. F. Xu, Z. J. Liu, and T. J. Wu, Synth. Commun., 31, 1053 (2001).
[23] M. W. Ding, Z. F. Xu, and T. J. Wu, Synth. Commun., 29, 1171 (1999).
[24] M. W. Ding, Y. Sun, X. P. Liu, and Z. J. Liu, Chinese J. Chem., 21, 577 (2003).
[25] P. Molina, P. M. Fresneda, and F. Hurtado, Synthesis, 45 (1987).
[26] P. Molina, A. Pastor, and M. J. Vilaplana, Tetrahedron, 49, 7769 (1993).
[27] M. I. Husain and G. C. Srivastava, J. Indian Chem. Soc., 58, 1183 (1981).
[28] A. M. Khalil, I. I. Abd El-Gawad, and M. Hammouda, Australian J. Chem., 27, 2035
(1974).