Pyrazole derivatives from azines of substituted phenacyl aryl/cyclohexyl sulfides and their antimycobacterial activity

Article abstract of DOI:10.1016/j.bmcl.2010.09.137

Azines derived from substituted phenacyl aryl/cyclohexyl sulfide on treatment with excess phosphorous oxychloride in N,N-dimethylformamide have been found to yield two isomeric pyrazoles in each case. A plausible mechanism has been suggested for the forma

Full text of DOI:10.1016/j.bmcl.2010.09.137

Bioorganic & Medicinal Chemistry Letters 20 (2010) 6920–6924
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Pyrazole derivatives from azines of substituted phenacyl aryl/cyclohexyl
sulfides and their antimycobacterial activity
Ramaiyan Manikannan a, Ramaiyan Venkatesan a, Shanmugam Muthusubramanian a,
,
⇑
Perumal Yogeeswari b, Dharmarajan Sriram b
a Department of Organic Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India
b Medicinal Chemistry & Antimycobacterial Research Laboratory, Pharmacy Group, Birla Institute of Technology & Science—Pilani, Hyderabad Campus, Jawahar Nagar,
Hyderabad 500 078, Andhra Pradesh, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2010
Revised 14 September 2010
Accepted 29 September 2010
Available online 7 October 2010
Azines derived from substituted phenacyl aryl/cyclohexyl sulfide on treatment with excess phosphorous
oxychloride in N,N-dimethylformamide have been found to yield two isomeric pyrazoles in each case.
A plausible mechanism has been suggested for the formation of the products. The antimycobacterial
activity of the isomeric compounds has been tested against Mycobacterium tuberculosis (MTB).
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
x
-Substituted acetophenone azines
Vilsmeier’s reagent
Pyrazole derivatives
Microwave irradiation
Antimycobacterial activity
Tuberculosis is a chronic infectious disease that is transmitted
by cough-propelled droplets that carry the etiologic bacterium,
Mycobacterium tuberculosis. Although currently available drugs kill
most isolates of M. tuberculosis, strains resistant to each of these
have emerged, and multiply resistant strains are increasingly
widespread. The growing problem of drug resistance combined
with a global incidence of several thousands of new cases per year
underscore the urgent need for new antituberculosis therapies.1 In
this connection, several pyrazole derivatives have been tested for
their antitubercular activity. For example, a series of 5-tert-butyl-
N-pyrazol-4-yl-4,5,6,7-tetrahydrobenzo[d]isoxazole-3-carboxam-
ide derivatives have been shown to be novel potent M. tuberculosis
PS inhibitors.2 Another series of 5-aryl-1-isonicotinoyl-3-(pyridin-
2-yl)-4,5-dihydro-1H-pyrazole derivatives exhibited an interesting
in vitro antimycobacterial activity against M. tuberculosis, their MIC
relationship studies revealed that the presence of the p-chloroben-
zoyl moiety at the C4 of the pyrazole ring is fundamental for the
antimycobacterial activity of some of the pyrazole compounds.7
1-(3,5-Diaryl-4,5-dihydro-1H-pyrazol-4-yl)-1H-imidazole
were
tested against a strain of M. tuberculosis H37Rv and showed a good
antimycobacterial activity reaching MIC values of 4 lg/mL for six
compounds of the series.8 Substituted 1-(2-hydroxybenzoyl)-3-
methyl-1H-pyrazol-5(4H)-one has shown promising antitubercu-
lar activity.9
Realizing the importance of pyrazole nucleus in the antimyco-
bacterial studies, it is planned to synthesize arylthio/cyclohexyl-
thio substituted pyrazoles and test their activity and the results
are described in this Letter.
Substituted phenacyl aryl/cyclohexyl sulfides 1 have been pre-
pared10–17 by the reaction of substituted phenacyl bromide with
values ranging from 8 to 16
l
g/ml.3 Similarly 1-acetyl-3,5-diphe-
cyclohexane thiol or benzene thiol. Symmetrical x-aryl/cyclo-
nyl-4,5-dihydro-(1H)-pyrazole derivatives were screened against
M. tuberculosis strain H37Rv.4 Hydrazone products, ethyl 2-[(3,5-
dimethylpyrazole-4-yl)hydrazono]-3-oxobutyrate and methyl
2-[(3,5-dimethylpyrazole-4-yl)hydrazono]-4-methoxy-3-oxobuty-
rate showed 29 and 28% inhibition against M. tuberculosis H37Rv,
hexylthio substituted acetophenone azines 2 were prepared, which
are all new to the best our knowledge, in quantitative yield by the
reaction of
presence of sodium acetate in ethanol under reflux. The reaction
of -arylthio substituted acetophenone azines 2 with Vilsmeier’s
x-arylthioacetophenone and hydrazine sulfate in
x
respectively.5 Pyrazolone derivatives bearing
a
4-substituted
reagent (taken in excess, 1:8 ratio of substrate to reagent) took
place smoothly with quantitative conversion of the starting mate-
rial (Scheme 1).18 In all the cases investigated, one product, 3, was
obtained as the major product with more than 80% yield in many
cases. Another compound 4 is obtained in poor yield in many cases,
though from 2h and 2i, this compound is obtained as the major
benzyl moiety at N2 were found to be more active, even if they re-
sulted to be less active than the pyrazole series.6 Structure–activity
⇑
Corresponding author. Tel./fax: +91 452 2459845.
E-mail address: muthumanian2001@yahoo.com (S. Muthusubramanian).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.09.137

R. Manikannan et al. / Bioorg. Med. Chem. Lett. 20 (2010) 6920–6924
6921
R
S
R
S
R
S
R
Ar
H2C
Ar
C
S
Ar
C
DMF/POCl3
N
N
Ar
+
N
+
NH
Ar
N
N
H
Ar
MWI, 30-60 sec.
N
S
Ar
S
CH2
S
H
R
S
Ar
R
R
R
3
4
5
2
Ar
C6H5
R
a
b
c
d
e
f
p-ClC6H4
p-ClC6H4
p-ClC6H4
p-ClC6H4
p-ClC6H4
p-CH3C6H4
C6H5
p-CH3C6H4
p-OCH3C6H4
p-ClC6H4
2-Naphthyl
p-ClC6H4
g
h
i
p-ClC6H4
C6H5
C6H11
p-CH3C6H4
p-OCH3C6H4
p-ClC6H4
C6H11
j
C6H11
k
l
C6H11
p-NO2C6H4
C6H11
Scheme 1. Synthesis of pyrazoles and pyrroles.
product (Table 1). In a few cases (2a, 2b, 2d, 2i, and 2k), apart from
3 and 4, a third product 5 was also obtained in small amount. All
the compounds were column separated and fully characterized
by NMR spectral data.
rings. Out of the 21 carbon signals noticed in the 13C NMR spec-
trum of 3d, eleven of them are quaternary, the remaining being
methine carbons. The spectral data clearly suggest that a pyrazole
ring has been formed at one end of the azine system with the gen-
eration of a vinyl unit at the other. The signal at 7.44 ppm gives a
strong contour with the carbon at 153.0 ppm in the HMBC
spectrum, indicating that this hydrogen to be the ring hydrogen
of the pyrazole. The other olefinic hydrogen gives HMBC contours
The 1H NMR spectrum of 3d19 has two singlets appearing at
7.25 and 7.44 ppm accounting for one hydrogen each. There are
four doublets each accounting for two hydrogens and a multiplet
accounting for eight hydrogens suggesting the presence four aryl
Table 1
Yield, physical constants, and anti-TB activities of pyrazole derivatives
Entry
Ar
R
Yield (%)
Mp (°C)
MIC (lg/ml)
3
4
3
4
3
4
b
b
a
b
c
d
e
f
g
h
i
j
k
l
C6H5
p-ClC6H4
p-ClC6H4
p-ClC6H4
p-ClC6H4
p-ClC6H4
p-CH3C6H4
C6H5
C6H11
C6H11
C6H11
C6H11
86
83
82
86
81
84
82
38
42
73
78
75
9
8
—
10
15
—
7
60
50
25
118–119
—
—
12.5
6.25
6.25
1.56
6.25
12.5
25.0
6.25
3.13
3.13
1.56
0.78
0.05
1.56
6.25
>25
b
p-CH3C6H4
p-OCH3C6H4
p-ClC6H4
2-Naphthyl
p-ClC6H4
p-ClC6H4
C6H5
p-CH3C6H4
p-OCH3C6H4
p-ClC6H4
p-NO2C6H4
—
—
12.5
a
b
c
—
b
b
128–129
133–134
—
—
3.13
12.5
a
b
c
—
—
—
b
b
b
—
—
>25.0
12.5
6.25
12.5
90–91
93–94
107–108
b
b
—
—
a
c
—
20
97–98
129–131
—
b
C6H11
—
1.56
Isoniazid
Ethambutol
Pyrazinamide
a
b
c
Not isolated.
Viscous liquid.
Not tested.

6922
R. Manikannan et al. / Bioorg. Med. Chem. Lett. 20 (2010) 6920–6924
with the carbons at 134.7 and 135.4 ppm, suggesting these carbons
to be the ipso carbons of the p-chlorophenyl ring. From the single
crystal X-ray analyses20,21 of compounds 3a and 3h, it is found that
the pyrazolyl ring and the olefinic hydrogens are cis to each other
(Figs. 1 and 2).
simple NMR spectrum. For 5d,23 four pairs of doublets accounting
for sixteen hydrogens and one hydrogen which is replaceable by
D2O are there in its 1H NMR spectrum. This indicates that the com-
pound 5d has a NH. The 13C NMR and DEPT spectra also account for
two identical pairs of aryl, four aryl in total and signals for two
quaternary carbons. The structure can be easily arrived at to be
2,3,4,5-tetra substituted symmetrical pyrrole.
The tentative mechanism of the reaction for the formation 3 and
4 are given in Scheme 2. It should be noticed that even in the pres-
ence of excess Vilsmeier’s reagent, only monoformylation occurs,
either giving 3 or 4 as there is no scope for further formylation. This
is contrast with the results of a similar reaction carried out on ace-
tophenone azines, where further formylation has occurred giving
The 1H NMR spectrum of 4b22 shows two singlets at 6.49 and
7.70 ppm. This is in difference to 3, where there are two singlets
at 7.25 and 7.44 ppm. All other hydrogens and carbons are having
almost same pattern. This clearly shows that 3 and 4 are geomet-
rical isomers, with 4 having a Z form.
In some cases (2a, 2b, 2d, 2i, and 2k), small amount of another
compound 5 has also been isolated from the reaction mixture. This
compound is found to be a symmetrical system, with well defined
Figure 1. ORTEP diagram of compound 3a.
Figure 2. ORTEP diagram of compound 3h.

R. Manikannan et al. / Bioorg. Med. Chem. Lett. 20 (2010) 6920–6924
6923
H3C
H3C
H
N
H3C
H
CH
R
S
3R
S
Cl
R
S
N
H
C
H2C
-HCl
Ar
HC
N
Ar
CH2
N
N
Ar
H
Ar
N
Ar
N
H
Ar
N
2
CH2
S
S
S
H
R
R
R
R
S
Ar
C
N
H3C
H
- NHMe2
R
S
CH3
Ar
N
N
S
Ar
H
R
3
N
N
C
R
H
Ar
- NHMe2
S
R
S
Ar
C
N
N
H
Ar
4
S
R
Scheme 2. Possible mechanism for the formation of compounds 3 and 4.
N
N
NHHN
HN NH
Ar
Ar
R
Ar
R
Ar
R
Ar
Ar
R
H
H
3,3-sigmatropic
rearrangement
R
S
S
S
S
R
S
S
2
H
NH2
H2N
Ar
N
Ar
Ar
- NH3
Ar
R
S
R
S
S
R
S
R
5
Scheme 3. Mechanism for the formation of compound 5 (Piloty pyrrole synthesis).
different types of products.24 The stability of 3 over 4 may be the
reason for the higher yield of 3 in the reaction mixture. This is also
confirmed by molecular modeling, optimized using PM3 (semi-
empirical method) in Arguslab 4.0.1, where the E conformer was
found to be stable by 11 KJ/mol.
Compound 5 obtained in some cases can be explained by the
popular Piloty pyrrole synthesis. Before the Vilsmeier’s reagent
attacks the azine, the system would have undergone rearrange-
ment by the Lewis acid character of phosphorous oxychloride
and the mechanism for the formation of 5 is given in Scheme 3.
The antimycobacterial activity of the isomeric pyrazoles (3/4)
has been screened for their in vitro activity against M. tuberculosis
H37Rv (MTB) by agar dilution method for the determination of MIC
in triplicates. The MIC values of 3/4 along with the standard drugs
for comparison are provided in Table 1. The isomer 3 seems to be
more active than 4 in all the cases, the activity being doubled in
many cases. In general, the cyclohexylthio substituted pyrazoles
(h–l) are more active than the arylthio substituted systems (a–g)
in the respective series. Excellent activity has been noticed when
a p-nitrophenylthio ring (3l and 4l) is there in the pyrazole ring.
3d and 3k are as active as ethambutol. The antimycobacterial
activity is remarkable when all the aryl rings carry p-chloro group
(3d) or when the substituents are p-chlorophenylthio and cyclo-
hexylthio groups (3k). It has already been reported that a p-chloro-
benzoyl moiety at the C4 of the pyrazole ring has found to have
remarkable antimycobacterial activity in some of the pyrazole
compounds.7As observed earlier,9 in our case also, some of the
1-substituted pyrazoles (3/4) are found to be antitubercular active.
Acknowledgments
The authors thank Department of Science and Technology, New
Delhi for assistance for the NMR spectrometer facility. Financial

6924
R. Manikannan et al. / Bioorg. Med. Chem. Lett. 20 (2010) 6920–6924
18. General procedure for the synthesis of pyrazole derivatives 3 and 4 and pyrrole
derivative 5: To a mixture of 1-aryl-2-(aryl/cyclohexylsulfanyl)-1-ethanone N-
support from Madurai Kamaraj University, Madurai for one of the
authors (R.M.) is gratefully acknowledged.
[(Z)-1-aryl-2-(aryl/cyclohexylsulfanyl)ethylidene]hydrazones
2
(0.003 mol)
and 3 mL of dimethyl formamide kept in ice bath at 0 °C, phosphorous
oxychloride (0.024 mol) was added dropwise for 5–10 min. The reaction
mixture was then irradiated under microwaves for 30–60 s. The process of the
reaction was monitored by TLC. After completion of the reaction, the reaction
mixture was poured into crushed ice and extracted with dichloromethane. The
organic layer was dried over anhydrous sodium sulfate. The different
compounds present in the mixture, 3, 4 and 5 were separated by column
chromatography using petroleum ether and ethyl acetate mixture as eluent.
19. 3-(4-Chlorophenyl)-1-(E)-1-(4-chlorophenyl)-2-[(4-chlorophenyl)sulfanyl]-1-
ethenyl-4-[(4-chlorophenyl)sulfanyl]-1H-pyrazole (Table 1, entry 3d): colourless
crystal (dichloromethane), yield 86%, mp 128–129 °C, time 30 s; 1H NMR
(300 MHz, CDCl3): d 6.97 (d, J = 8.7 Hz, 2H), 7.16 (d, J = 8.7 Hz, 2H), 7.25 (s, 1H),
7.31–7.44 (m, 9H), 7.49 (d, J = 8.7 Hz, 2H), 7.92 (d, J = 8.7 Hz, 2H). 13C NMR
(75 MHz, CDCl3): d 106.3, 117.1, 127.2, 128.6, 128.8, 129.1, 129.4, 129.5, 130.0,
130.7, 130.8, 131.0, 131.4, 133.3, 133.4, 134.7, 135.4, 136.0, 136.2, 136.3, 153.0.
Anal. Calcd for C29H18Cl4N2S2: C, 58.01; H, 3.02; N, 4.67. Found: C, 58.09; H,
3.08; N, 4.73.
20. Ramesh, P.; Subbiahpandi, A.; Manikannan, R.; Muthusubramanian, S.;
Ponnuswamy, M. N. Acta Crystallogr., Sect. E 2008, 64, o2054.
21. Ramesh, P.; Subbiahpandi, A.; Manikannan, R.; Muthusubramanian, S.;
Ponnuswamy, M. N. Acta Crystallogr., Sect. E 2008, 64, o2132.
22. 4-[(4-Chlorophenyl)sulfanyl]-1-[(Z)-2-[(4-chlorophenyl)sulfanyl]-1-(4-methyl
phenyl)-1-ethenyl]-3-(4-methylphenyl)-1H-pyrazole (Table 1, entry 4b): viscous
liquid, yield 8%, time 30 s; 1H NMR (300 MHz, CDCl3): d 2.36 (s, 3H), 2.38 (s,
3H), 6.49 (s, 1H), 7.05 (d, J = 8.4 Hz, 2H), 7.16–7.26 (m, 8H), 7.34 (d, J = 8.4 Hz,
2H), 7.48 (d, J = 8.4 Hz, 2H), 7.70 (s, 1H), 7.96 (d, J = 8.1 Hz, 2H). 13C NMR
(75 MHz, CDCl3): d 21.2, 21.3, 105.5, 114.0, 127.1, 127.2, 127.4, 128.9, 129.0,
129.1, 129.4, 129.6, 131.0, 131.8, 132.6, 133.7, 134.9, 135.4, 137.0, 137.5, 138.4,
139.3, 153.2.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2010.09.137.
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23. 2,5-Bis(4-chlorophenyl)-3,4-bis[(4-chlorophenyl)sulfanyl]-1H-pyrrole
(5d):
colourless crystal (ethyl acetate), yield 3%, mp 112–113 °C, time 30 s; 1H
NMR (300 MHz, CDCl3): d 6.95 (d, J = 8.7 Hz, 4H), 7.06 (d, J = 8.7 Hz, 4H), 7.40
(d, J = 8.1 Hz, 4H), 7.63 (d, J = 8.1 Hz, 4H), 9.00 (s, 1H). 13C NMR (75 MHz,
*
*
CDCl3): d 114.2, 127.4, 128.3, 128.8, 129.2 , 131.0, 134.4, 136.5, 136.8 ( one
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