A facile method for the synthesis of aroyl pyrazoles from functionalized ketene dithioacetals

Article abstract of DOI:10.1002/jhet.1954

On treating with ammonium acetate in DMSO aroyl formyl ketene dithioacetals were transformed into valuable intermediates 3-amino-2-aroyl-3-(methylsulfanyl) -2-propenals, which could be explored for the synthesis of aroyl aminopyrazoles.

Full text of DOI:10.1002/jhet.1954

562
A Facile Method for the Synthesis of Aroyl Pyrazoles from Functionalized
Ketene Dithioacetals
Vol 51
a
a
a
b
*
*
N. K. Sreedevi, A. Mathews, K. S. Devaky, and E. R. Anabha
aSchool of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala 686560, India
bDepartment of Chemistry, Sree Narayana College, Alathur678 622Kerala, India
*
E-mail: ksdevakyscs@yahoo.com
Received August 25, 2012
DOI 10.1002/jhet.1954
Published online 22 November 2013 in Wiley Online Library (wileyonlinelibrary.com).
The authors dedicate the manuscript before everlasting memory of late Dr C. V. Asokan.
On treating with ammonium acetate in DMSO aroyl formyl ketene dithioacetals were transformed into
valuable intermediates 3-amino-2-aroyl-3-(methylsulfanyl)-2-propenals, which could be explored for the
synthesis of aroyl aminopyrazoles.
J. Heterocyclic Chem., 51, 562 (2014).
INTRODUCTION
secondary amines in the presence of mild base such as potas-
sium carbonate [14]. To synthesize 3-aminopropenals from
a-formyl ketene dithioacetals, we had to overcome compet-
ing deformylation reactions and fragmentation reactions
under normal conditions. Optimization studies were
conducted (Scheme 1, Table 1) and found that 2-benzoyl-
3,3-(bismethylsulfanyl)acrylalde (1a) on reaction with
ammonium acetate in DMSO at 95ꢀC afforded 3-amino-2-
benzoyl-3-(methylsulfanyl)-2-propenal (2a) in 63% yield.
We prejudiced that 3-amino-2-benzoyl-3-(methyl
sulfanyl)-2-propenal (2a) could be easily transformed into
amino-substituted heterocycles. Contrary to our expectations,
compound 2a underwent fragmentation reactions under basic
conditions or found nonreactive in acidic conditions to-
wards binucleophiles such as malononitrile, amidines,
guanidines, ethyl cyano acetate, cyano acetamide, and so
forth. Oxidation of the aldehyde functionality on 2a might
be resulted in the formation of functionalized a-amino
acids; but the amino propenals underwent fragmentation
reactions, yielding benzoyl ketene-N,S-acetal even under
mild reaction conditions. However, it was reactive towards
hydrazine hydrate in acetonitrile at RT to yield 2-benzoyl-
3-aminopyrazole (3a) in 80% yield (Scheme 2).
Pyrazole, which is an important structural motif present
in biologically active molecules [1–3], plays an important
role in medicinal chemistry [4] and agrochemistry [5].
Functionalized pyrazoles show a wide range of biological
properties, such as antihyperglycemic, analgesic, anti-
inflammatory, antipyretic [6], antibacterial [7], hypoglyce-
mic, and sedative–hypnotic activity [8]. In the present article,
we have described the synthetic investigations on the
reactions of a-formyl ketene dithioacetals, leading to the
synthesis of aroyl aminopyrazoles. Generally, the synthesis
of aminopyrazole involves the reaction of hydrazine or
hydrazine derivatives with 1,3-bielectrophiles such as
a,b-unsaturated nitriles, b-ketonitriles, b-ketoamides, and
so forth, whereas aroyl pyrazoles are synthesized by
aroylation reactions of pyrazoles that still result in the for-
mation of 1-aroyl pyrazoles. In the literature, there are
only limited reports on the synthesis of aroyl pyrazoles.
a-Formyl ketene dithioacetals, synthesized by the reaction
of aroyl ketene dithioacetals with chloromethylene iminium
salt (Vilsmeier–Haack reagent), have been proved for their
synthetic potential in heterocyclic synthesis [9]. Our research
group has explored the valuable intermediate for the synthe-
sis of nitrogen heterocycles and furan derivatives [10–13].
As a continuation to these studies, a-formyl ketene
dithioacetals were transformed into 3-amino-2-aroyl-3-
(methylsulfanyl)-2-propenals, which could be easily
converted to 2-aroyl-3-amino pyrazoles and it is the subject
matter of present article.
The 3-amino-2-benzoyl-3-(methylsulfanyl)-2-prope-
nal (2a) and (5-amino-1H-pyrazol-4-yl)(phenyl)metha-
none (3a) were characterized on the basis of common
1
spectroscopic methods. In the H-NMR spectra, 3-amino-
2-benzoyl-3-(methylsulfanyl)-2-propenal (2a) showed
singlets at d 2.55 because of methylsulfanyl group, at d
6.16 and 10.09 because of NH2 group, and at d 9.25
because of the aldehyde functionality. In the 13C-NMR
spectrum, methyl sulfanyl, carbonyl, and aldehyde groups
RESULTS AND DISCUSSION
were appeared at
d 13.35, 190.81, and 197.46,
Functionalized ketene-S,S-acetals were converted to
ketene-N,S-acetals, by treating them with primary or
respectively. (5-Amino-1H-pyrazol-4-yl)(phenyl)metha-
none (3a) showed peaks at d 6.77 because of NH2 and d
© 2013 HeteroCorporation

May 2014
A Facile Method for the Synthesis of Aroyl Pyrazoles
563
Scheme 1
12.01 because of NH group in addition to the aromatic peaks.
In the 13C-NMR spectrum, the peaks corresponding to
(5-amino-1H-pyrazol-4-yl)(phenyl)methanone (3a) were pres-
ent at 128.40, 131.03, 139.99, 153.35, and 187.93 (CO). Mass
spectral values and carbon, hydrogen, and nitrogen (CHN)
analyses were in accordance with the proposed structure.
The mechanism for the formation of aminopyrazoles
(3a) from 3-amino-2-benzoyl-3-(methylsulfanyl)-2-pro-
penal (2a) is explained as follows. 3-Amino-2-benzoyl-
3-(methylsulfanyl)-2-propenal (2a) was formed from
2-benzoyl-3,3-bis(alkylsulfanyl)acrylaldehyde (1a) follow-
ing an addition–elimination reaction, involving ammonia
released from the ammonium acetate. In this reaction,
DMSO was found to be better solvent, by obstructing the
formyl ketene dithioacetal from deformylation reaction.
The hydrazine hydrate was condensed with the aldehyde
moiety on 2a, to form a hydrazone intermediate (5a), which
on cyclo-aromatization with the elimination of methanethiol
resulted in the formation of aminopyrazoles (3a) in good
yield (Scheme 3).
Table 1
Optimization studies for the synthesis of 3-amino-2-benzoyl-3-(methyl-
sulfanyl)-2-propenal (2a).
Sl. no
Reaction condition
Yield (%)
1
2
NH3 in ethanol/DMF/DMSO at RT
NH3 in the presence of K2CO3 in
acetone or DMF, RT
NH3 in the presence of triethylamine
in ethanol, RT
No reaction
Deformylation
3
No reaction
4
5
6
7
NH4OAc/DMF, RT
No reaction
32%
NH4OAc/DMF, 80ꢀC
NH4OAc/AcOH, 95ꢀC
32%
63%
NH4OAc/DMSO, 95ꢀC
The aroyl acrylaldehydes (1b–e) were treated with
ammonium acetate in DMSO at 90ꢀC to obtain 2-aroyl-3-
amino-3-(methyl sulfanyl)-2-propenals (2b–e), which were
transformed into aryl-(5-amino-1H-pyrazol-4-yl)metha-
nones (3b–e) in 79–85% yields (Scheme 4, Table 2).
In conclusion, we have synthesized 3-amino-2-(4-meth-
oxybenzoyl)-3-(methylsulfanyl)-2-propenal (2) from 2-
aroyl-3,3-bis(alkylsulfanyl)acrylaldehyde (1) and explored
their synthetic potential for the synthesis of aryl-(5-amino-
1H-pyrazol-4-yl)methanones (3).
Scheme 2
Scheme 3. Mechanism for the synthesis of (5-amino-1H-pyrazol-4-yl)(phenyl)methanone (3a).
Scheme 4
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

564
N. K Sreedevi, A. Mathews, K. S. Devaky, and E. R. Anabha
Vol 51
ArH) 9.43 (s, 1H, CHO); 13C-NMR (DMSO-d6): d = 13.3
(SCH3), 21.49 (CH3), 108.14 (C═C), 112.54, 128.73, 129.59,
162.3, 176.58 (C═C), 188.26 (CHO), 191.75 (CO); GC–MS:
m/z (%) = 235 (M+, 3), 219 (2), 207 (24), 202 (28), 187 (22),
173 (11), 158 (6), 144 (7), 116 (5), 119 (89), 91 (100).
Table 2
Synthesis of aryl-(5-amino-1H-pyrazol-4-yl)methanones (3).
Yield %
Compounds 2 and 3
Ar
2
3
3-Amino-2-(4-methoxybenzoyl)-3-(methylsulfanyl)-2-propenal
(2c). This compound was obtained as yellow crystalline solid,
mp: 190–192ꢀC, yield: 70%, Anal. Calcd for C12H13NO3S: C,
57.35; H, 5.21; N, 5.57; S, 12.76, Found: C, 57.34; H, 5.24; N,
5.57; S, 12.76; IR (KBr): 3372, 1690, 1625, 1534, 1364, 1326,
a
b
c
d
e
C6H5
63
67
70
68
69
80
79
83
85
82
4-CH3C6H4
4-CH3OC6H4
4-ClC6H4
1
1234, 1130, 1013 cmÀ1; H-NMR (DMSO-d6): d = 2.39 (s, 3H,
4-BrC6H4
SCH3), 3.80 (s, 3H, OCH3), 6.97 (d, 2H, J = 6 Hz, ArH), 7.42 (d,
2H, J = 9 Hz, ArH), 9.27 (s, 1H, CHO), 8.55 (s, 1H, NH), 12.02
(s, 1H, NH); 13C-NMR (DMSO-d6): d = 13.25 (SCH3), 55.49
(OMe), 109.24 (C═C), 113.64, 130.73, 132.59, 161.29, 177.48
(C═C), 187.26 (CHO), 192.74 (CO); GC–MS: m/z (%)= 251
(M+, 8), 235 (3), 234 (5), 219 (4), 201 (3), 187 (3), 144 (3), 135
(100), 117 (2), 107 (9), 92 (15).
EXPERIMENTAL
General. Melting points were determined on a Buchi 530
instrument (Mumbay, India) melting point apparatus and were
uncorrected. The IR spectra were recorded as KBr pellets on a
Schimadzu IR-470 spectrometer (Mumbay, India), and the
3-Amino-2-(4-bromobenzoyl)-3-(methylsulfanyl)-2-propenal
(2d). This compound was obtained as yellow crystalline solid,
mp: 220–222ꢀC, yield: 69%; Anal. Calcd for C11H10BrNO2S: C,
44.01; H, 3.36; N, 4.67; S, 10.68. Found: C, 43.99; H, 3. 38; N,
4.67; S, 10.67; IR (KBr): 3359, 1608, 1569, 1546, 1438,
1
frequencies are reported in reciprocal centimeter. The H-NMR
spectra were recorded on a Brucker WM 300 (300 MHz)
spectrometer (South Kolkatta, India), using TMS as internal
standard and CDCl3, DMSO-d6 as solvent. The 13C-NMR
spectra were recorded on a Brucker WM 300 (75.47 MHz)
spectrometer using CDCl3, DMSO-d6 as solvent. The Electron
Impact Mass spectra were obtained on a GC–MS-Schimadzu
5050 model instrument. The CHN analyses were performed on
an Elementar Vario EL III Carlo Erba 1108 instrument
(Mumbay, India). All reagents were commercially available and
were purified before use. Anhydrous sodium sulfate was used as
drying agent. All purified compounds gave a single spot upon
TLC analyses on silica gel 7GF using ethyl acetate/hexane
mixture as eluent. Iodine vapors or KMnO4 solution in water
was used as developing agent for TLC.
Synthesis of 3-amino-2-aroyl-3-(alkylsulfanyl)-2-propenal
(2): general procedure. To a solution of 2-aroyl-3,3-bis
(alkylsulfanyl)acrylaldehyde (1) (5 mmol) in DMSO (5 mL),
ammonium acetate (5 mmol, 385 mg) was added, and the
solution was heated at 95ꢀC. After 1 h, the reaction mixture was
poured into ice-cold water, the solid separated was filtered,
dissolved in chloroform, dried over anhydrous sodium sulfate,
and the solvent was evaporated off. The crude product obtained
was recrystallized from chloroform.
1373 cmÀ1 1H-NMR (DMSO-d6): d = 2.4 (s, 3H, SCH3), 6.97
;
(d, 2H, J = 8.03 Hz, ArH), 7.42 (d, 2H, J = 8.28 Hz, ArH), 9.2 (s,
1H, CHO), 8.85 (s, 1H, NH), 12 (s, 1H, NH); 13C-NMR
(DMSO-d6): d = 13.33 (SCH3), 109.14 (C═C), 178.06, 123.95,
130.46, 131.41, 139.49, 187.51 (CHO), 192.64 (CO).
3-Amino-2-(4-chlorobenzoyl)-3-(methylsulfanyl)-2-propenal
(2e). This compound was obtained as yellow crystalline solid,
mp: 208–210ꢀC; yield: 68%, Anal. Calcd for C11H10ClNO2S:
C, 51.66; H, 3.94; N, 5.48; S, 12.54. Found: C, 52.66; H, 3. 98;
N, 5.47; S, 12.50; IR (KBr): 3390, 1602, 1573, 1525, 1411,
1
1249 cmÀ1; H-NMR (DMSO-d6): d = 2.43 (s, 3H, SCH3), 7.23
(d, 2H, J = 9 Hz, ArH), 7.45 (d, 2H, J = 9 Hz, ArH), 9.5 (s, 1H,
CHO), 8.7 (s, 1H, NH), 12.4 (s, 1H, NH); 13C-NMR (DMSO-
d6): d = 13.5 (SCH3), 110.12 and 177 (C═C), 121.85, 130.5,
132.43, 138.5, 188.32(CHO), 191.71 (CO).
General procedure for the synthesis of (5-amino-1H-pyrazol-
4-yl)(aryl)methanones (3). 3-Amino-2-aroyl-3-(alkylsulfanyl)-
2-propenal (2) (5mmol) was dissolved in acetonitrile (5mL), and
hydrazine hydrate (5mmol, 250mg) was added to the solution.
The mixture was stirred at RT for an hour. As the TLC
examination showed complete disappearance of the starting
material, the reaction mixture was poured into ice-cold water and
extracted with ethyl acetate (3Â 20 mL). The organic layer was
washed with water and dried with sodium sulfate, and the solvent
was evaporated off. The crude reaction mixture was purified by
column chromatography using hexane/ethyl acetate (50:50)
solvent mixture as the eluent.
3-Amino-2-benzoyl-3-(methylsulfanyl)-2-propenal (2a). This
compound was obtained as yellow crystalline solid, mp: 164–166ꢀC,
yield: 63%, Anal. Calcd for C11H11NO2S: C, 59.71; H, 5.01;
N, 6.33; S, 14.49. Found: C, 59.72; H, 5.04; N, 6.33; S,
14.50; IR (KBr): 3344, 1631, 1596, 1473, 1404.08,
1353.94,1276.79,1157.21 cmÀ1 1H-NMR (DMSO-d6): d = 2.55
;
(s, 3H, SCH3), 6.16 (s, 1H, NH), 7.39–7.47 (m, 5H, ArH), 9.25
(s, 1H, CHO), 10.09 (s, 1H NH); 13C-NMR (DMSO-d6):
d = 13.35 (SCH3), 101.06 (C═C), 128.06, 128.32, 130.16, 140.54,
164.54 (C═C), 190.81 (CHO), 197.46 (CO).
(5-Amino-1H-pyrazol-4-yl)(phenyl)methanone (3a). This
compound was obtained as yellow crystalline solid, mp: 150–
152ꢀC, yield: 80%, Anal. Calcd for C10H9N3O: C, 64.16; H,
4.85; N, 22.45. Found : C, 64.12; H, 4. 88; N, 22.47; IR (KBr):
3-Amino-2-(4-methylbenzoyl)-3-(methylsulfanyl)-2-propenal
(2b). This compound was obtained as yellow crystalline solid,
mp: 188–190ꢀC, yield: 67%, Anal. Calcd for C12H13NO2S: C,
61.25; H, 5.57; N, 5.95; S, 13.63, Found: C, 61.20; H, 5.61;
N, 5.97; S, 13.63; IR (KBr): 3356, 1604, 1570, 1547, 1373,
3380, 3166, 1598, 1562, 1479, 1347, 1261, 1195 cmÀ1 1H-
;
NMR (DMSO-d6): d = 6.77 (s, 2H, NH2), 7.47–7.73 (m, 6H,
ArH, H-5), 12.01 (s, 1H, NH); 13C-NMR (DMSO-d6):
d = 103.62, 127.81, 128.40, 131.03, 139.99, 153.35, 187.93
(CO); GC–MS: m/z (%) = 187 (M+, 54), 186 (97), 170 (2), 158
(2), 130 (9), 110 (100), 105 (14), 77 (50).
1323, 1284.50, 1180 cmÀ1
;
1H-NMR (DMSO-d6): d = 2.3
(s, 3H, CH3), 2.45 (s, 3H, SCH3), 6.40 (s, 1H, NH2), 12.56 (s,
1H, NH2), 7.20 (d, 2H, J = 8 Hz, ArH), 7.42 (d, 2H, J = 6.6 Hz,
(5-Amino-1H-pyrazol-4-yl)(4-methylphenyl)methanone
(3b). This compound was obtained as yellow crystalline solid,
Journal of Heterocyclic Chemistry
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May 2014
A Facile Method for the Synthesis of Aroyl Pyrazoles
565
mp: 160–162ꢀC, yield: 79%, Anal. Calcd for C11H11N3O: C,
Acknowledgments. We thank DST, New Delhi, and STECK, TVM
for financial assistance; ERA thanks CSIR-New Delhi; and SNK
thanks Mahatma Gandhi University for research fellowships.
65.66; H, 5.51; N, 20.88. Found: C, 65.69; H, 5. 52; N, 20.87;
IR (KBr): 3371, 3150, 1627, 1537, 1492, 1220, 1164 cmÀ1
;
1H-NMR (DMSO-d6): d = 2.37 (s, 3H, CH3), 6.73 (s, 2H,
NH2), 7.31 (d, 2H, J = 9 Hz, ArH), 7.64 (d, 2H, J = 9 Hz, ArH ),
8.57 (s, 1H, H-5), 11.98 (s, 1H, NH); 13C-NMR (DMSO-d6):
d = 20.97 (CH3), 127.74, 127.94, 128.92, 129.31, 137.29, 141,
153.37, 187.69 (CO); GC–MS: m/z (%) = 201 (M+, 59), 200
(100), 186 (29), 185 (1), 172 (4), 157 (2), 144 (8), 119 (14),
110 (81), 91 (40), 77 (6).
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C11H11N3O2: C, 60.82; H, 5.10; N, 19.34. Found: C, 60.85;
H, 5. 12; N, 20.83 cmÀ1
;
1H-NMR (DMSO-d6): d = 3.8
(s, 3H, OCH3), 6.78 (s, 2H, NH2), 7.4 (d, 2H, J = 9 Hz,
ArH), 7.72 (d, 2H, J = 9 Hz, ArH), 8.53 (s, 1H, H-5), 12
(s, 1H, NH); 13C-NMR (DMSO-d6): d = 55.17 (OCH3), 126.54,
127.34, 128.32, 129.51, 136.29, 142.57, 155.37, 189.65 (CO);
GC–MS: m/z (%)= 217 (M+, 54), 186 (97), 170 (2), 158 (2), 130
(9), 110 (100), 105 (14), 77 (50).
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C10H8BrN3O: C, 45.14; H, 3.03; N, 15.79. Found: C, 45.12; H,
3.05; N, 15.79; IR (KBr): 3371, 3150, 1627, 1537, 1492, 1365,
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1309, 1220, 1164, 1043, 908, 825, 767 cmÀ1
;
1H-NMR
(DMSO-d6): d = 6.80 (s, 2H, NH2), 7.64–7.71 (m, 5H, ArH, H-
5), 12.03 (s, 1H, NH); 13C-NMR (DMSO-d6): d = 124.68,
121.91, 130.96, 131.36, 131.43, 138.90, 153.77, 186.50 (CO);
GC–MS: m/z (%) = 267 (M+ + 2, 32), 266 (52), 265 (M+, 30),
264 (54), 248 (1), 207 (9), 186 (18), 185 (12), 183 (7), 157
(14), 155 (16), 77 (23), 76 (25).
(5-Amino-1H-pyrazol-4-yl)(4-chlorophenyl)methanone
(3e). This compound was obtained as yellow crystalline
solid, mp: 162–164ꢀC, yield: 82%, Anal. Calcd for
C10H8ClN3O: C, 54.19; H, 3.64; N, 18.96. Found: C, 54.20;
H, 3.65; N, 18.93; IR (KBr): 3423, 3164, 1625, 1577, 1311,
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127 cmÀ1 1H-NMR (DMSO-d6): d = 6.80 (s, 2H, NH2), 7.56
;
(d, 2H, J = 9 Hz, ArH), 7.74 (d, 2H, J = 9 Hz, ArH ), 8.62
(s, 1H, H-5), 12.03 (s, 1H, NH); 13C-NMR (DMSO-d6):
d = 128.49, 128.72, 129.48, 135.76, 138.55, 153.34, 186.19
(CO); GC–MS: m/z (%) = 223 (M+ + 2, 22), 221 (56), 220
(M+, 86), 207 (21), 205 (7), 186 (12), 177 (2), 139 (12),
110 (100), 77 (7).
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet