Synthesis, stereochemistry, antimicrobial evaluation and QSAR studies of 2,6-diaryltetrahydropyran-4-one thiosemicarbazones

Article abstract of DOI:10.1016/j.ejmech.2011.01.029

A series of 2,6-diaryltetrahydropyran-4-one thiosemicarbazones (11-27) were synthesized and characterized for evaluation of potential antibacterial activity against Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Salmonella typhi, Bacillu

Full text of DOI:10.1016/j.ejmech.2011.01.029

European Journal of Medicinal Chemistry 46 (2011) 1415e1424
Contents lists available at ScienceDirect
European Journal of Medicinal Chemistry
journal homepage: http://www.elsevier.com/locate/ejmech
Short communication
Synthesis, stereochemistry, antimicrobial evaluation and QSAR studies
of 2,6-diaryltetrahydropyran-4-one thiosemicarbazones
,
,
S. Umamatheswari a b, B. Balaji b, M. Ramanathan b, S. Kabilan a
*
a Department of Chemistry, Annamalai University, Annamalainagar 608002, India
b Department of Pharmacology, PSG College of Pharmacy, Coimbatore 641004, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 2,6-diaryltetrahydropyran-4-one thiosemicarbazones (11e27) were synthesized and charac-
terized for evaluation of potential antibacterial activity against Staphylococcus aureus, Escherichia coli,
Pseudomonas aeruginosa, Salmonella typhi, Bacillus subtilis and Klebsiella pneumonia and antifungal
activity against Cryptococcus neoformans, Candida albicans, Rhizopus sp., Aspergillus niger and Aspergillus
flavus were evaluated. Compounds 21 and 22 showed maximum inhibition potency at low concentration
Received 18 September 2010
Received in revised form
25 December 2010
Accepted 15 January 2011
Available online 28 January 2011
(6.25 mg/ml) against P. aeruginosa. For antifungal activity, 20 and 21 were effective against C. neoformans
and 22e24 against C. albicans at minimum concentration. Further, the results of QSAR studies of these
synthesized compounds indicated the importance of weakly polar component of surface area, hydro-
phobicity and ionization potential parameters in defining their antimicrobial activity.
Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords:
Tetrahydropyran-4-one
Thiosemicarbazone
Conformation
Antimicrobial activity
QSAR
1. Introduction
naturally occurring carbohydrates and synthetic compounds with
interesting biological and pharmacological properties [15e18].
Since resistance of pathogenic microorganisms towards avail-
able antibiotics is rapidly becoming a worldwide problem, the
design of new compounds to deal with these resistant strains has
Additionally, a careful analysis of literature on antimicrobial agents
concerning our ongoing research program in this field has been
performed [19]. Therefore in the present study, we have synthe-
sized 2,6-diarylpyran-4-one thiosemicarbazones as potential anti-
microbial molecules and performed Quantitative Structure Activity
Relationship (QSAR) studies on them.
become
a major area of research today. Additionally, the
biochemical similarity of the human and fungi cells is a significant
handicap for development of new antifungal agents because of
problems of toxicity and easily gained resistance [1,2]. The present
work is aimed towards developing novel molecules with potent
antimicrobial activity to tackle this problem of resistance.
2. Chemistry
Thiosemicarbazones are a class of small molecules that have
been evaluated against various diseases such as those caused by
Plasmodium falciparum, Trypanosoma brucei and Trypanosoma cruzi.
The effectiveness of thiosemicarbazone analogues in treating these
diseases is reported due to their activity against cysteine protein-
ases including rhodesain [3e6]. Thiosemicarbazones are also
known to possess tranquilizing, muscle relaxing, psychoanaleptic,
hypnotic, ulcerogenic, antidepressant, antibacterial, antifungal,
analgesic and anti-inflammatory properties [7e14].
The general schematic representation describing the routes of
syntheses is furnished in Scheme 1. 3-Alkyl-2,6-diphenyltetrahy-
dropyran-4-ones (1 and 2) were prepared by the condensation
reaction of benzaldehyde with respective ketones (2-butanone for 1
and 2-pentanone for 2) in the presence of potassium hydroxide in
watereethanol medium stirred for 15 days [20]. Thiosemicarbazones
(11, 12, 18 and 19) are synthesized by refluxing 3-alkyl-2,6-diphe-
nyltetrahydropyran-4-ones (1 or 2) with respective thio-
semicarbazides (thiosemicarbazide or 40-phenylthiosemicarbazide)
in the presence of concentrated hydrochloric acid in methanol
medium for 3 h. 3,5-Dimethyl-2,6-diaryltetrahydropyran-4-ones
(3e10) were obtained through condensation reaction of para-
substituted benzaldehyde with 3-pentanone in the presence of
potassium hydroxide in watereethanol medium stirred for 2e96 h
in mechanical stirrer. The ketones (3e10) were further converted
Heterocyclic compounds carrying pyran skeleton are attractive
targets of organic synthesis owing to their presence in numerous
* Corresponding author. Tel.: þ91 9600619142.
E-mail address: drskabilan08@gmail.com (S. Kabilan).
0223-5234/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.ejmech.2011.01.029

1416
S. Umamatheswari et al. / European Journal of Medicinal Chemistry 46 (2011) 1415e1424
O
O
R1
R2
R
R1
R
R2
O
KOH / EtOH-H2
R3
R3
O
O
+
O
R2
R2
2 h-15 day stirring
R3
R3
H
H
1-10
R4
R4
R4
R4
MeOH / H+
H2NNHCSNHR
5
reflux, 3-72h
R5
N
S
N
H
HN
R1
R2
R
R2
R3
R3
O
R4
R4
11-27
Scheme 1. Schematic diagram showing the synthesis of 2,6-diaryltetrahydropyran-4-one thiosemicarbazones.
into their corresponding thiosemicarbazones by refluxing with
respective thiosemicarbazides in the presence of concentrated
hydrochloric acid in methanol medium for 50e72 h. All the
synthesized compounds were characterized by analytical and spec-
tral (Mass, IR, 1H and NMR) data. The physical and analytical data of
the thiosemicarbazones (11e27) are presented in Table 1. The
stereochemistry of all the synthesized thiosemicarbazones is
established using 1H NMR spectral data. The assignment has been
made based on characteristic signal positions of functional groups,
spin multiplicity, and comparison with those of parent ketones. To
determine the structure of the compounds 18e27, the 1H NMR
spectrum of compound 18 (for 3-alkyltetrahydropyran-4-one 40-
phenylthiosemicarbazone 19) and compound 21 (for 3,5-dime-
thyltetrahydropyran-4-one 40-phenylthiosemicarbazones 20 and
21e27) were used as examples. The spectral assignment and
stereochemistry of compounds 11e17 are made by comparing with
representative compounds 11 and 14 whose stereochemistry was
well established [21]. The 1H NMR and 13C NMR chemical shifts of
compounds 11e27 along with their parent ketones 1 and 4 are given
in Tables 2 and 3, respectively.
2.1. Stereochemistry
The coupling constant values and position of the chemical
shifts were used to predict the conformation of the synthesized
compounds. The observed vicinal coupling constant values [10.0
(J32a,3a), 11.5 Hz (J53a,5a) and 2.0 Hz (J63a,5e)] of H-6a and H-2a of
compound 18 indicate that the six-member heterocyclic ring of
compound 18 adopts normal chair conformation with equatorial
orientation of phenyl groups at C-2 and C-6, and equatorial
Table 1
Physical data of 2,6-diaryltetrahydropyran-4-one thiosemicarbazones 11e27.
Compound
R
R1
R2
R3
R4
R5
Reaction time (h)
Yield (%)
m.p ꢀ
C
Mass
Elemental analysisa
C
H
N
S
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
H
H
CH3
CH2CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH2CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Cl
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Cl
OCH3
H
H
F
Cl
Br
CH3
OCH3
H
H
F
H
H
H
H
H
H
H
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
3
3
90
90
76
80
78
76
74
89
86
74
68
79
71
65
68
65
63
110e112
136e137
140e141
212e214
184e186
158e161
172e173
122e124
128e130
153e154
202e204
196e198
178e180
188e189
223e224
196e198
164e165
340.0281b
354
e
e
e
e
67.98
61.65
e
6.54
5.40
e
11.92
10.77
e
9.11
8.22
e
CH3
CH3
CH3
CH3
CH3
H
50
50
50
50
50
4
390
423.3702b
e
46.95
69.24
63.91
72.22
72.61
67.12
53.11
62.61
73. 42
68.60
e
4.18
7.09
6.47
6.12
6.39
5.38
4.25
5.10
6.79
6.32
e
8.19
11.16
10.20
10.98
9.82
9.07
7.11
8.41
9.14
8.51
e
6.29
8.39
7.72
7.77
7.49
6.91
5.47
6.45
7.08
6.53
e
382
414
416
430
466
499
H
4
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
72
72
72
72
72
72
72
72
Cl
Br
CH3
OCH3
H
H
H
e
458
490
499
499
e
e
e
e
490
e
e
e
e
a
The observed elemental analysis values for C, H, N and S are within ꢁ0.4% from their theoretical values.
M þ H value obtained from High Resolution Mass Spectrum.
b

S. Umamatheswari et al. / European Journal of Medicinal Chemistry 46 (2011) 1415e1424
1417
Table 2
Proton chemical shift values of pyranone thiosemicatbazones 11e27 and parent ketones 1 and 4 [
d
(ppm)].
Compound
H-2a
H-3a
H-5a
H-5e
H-6a
3-CH3/CH2CH3
0.93
1.60, 1.16 (CH2); 0.76 (CH3)
1.01
0.97
0.97
0.98
0.96
0.94
5-CH3
NH
NH2/NHPh
Aromatic protons
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
1
4.26
4.36
4.22
4.21
4.21
4.18
4.09
4.21
4.34
4.20
4.26
4.23
4.17
4.11
4.43
4.19
4.14
4.33
4.29
2.67
2.47
2.76
2.76
2.81
2.88
2.83
2.67
2.52
2.73
2.79
2.79
2.91
2.81
3.19
2.84
2.85
2.60e2.82
2.80
2.35
2.12
e
3.01
3.40
3.16
3.10
3.02
3.09
3.08
2.99
3.44
3.21
3.14
3.05
3.12
3.12
3.58
3.16
3.16
4.62
4.57
4.73
4.75
4.74
4.73
4.71
4.60
4.51
4.71
4.72
4.70
4.76
4.73
4.92
4.76
4.75
4.81
4.29
e
8.76
8.62
8.89
8.93
8.79
8.94
8.92
9.91
9.62
9.72
9.96
9.79
9.89
9.96
8.76
9.88
9.25
e
6.29
6.29
6.32
6.41
6.50
6.98
6.88
8.01
8.12
8.01
8.71
8.64
8.79
8.89
7.98
8.91
8.76
e
7.30e7.42
7.22e7.45
7.29e7.57
7.09e7.41
7.09e7.41
7.01e7.83
6.97e7.59
7.29e7.48
7.12e7.51
7.21e7.55
7.11e7.48
7.11e7.45
7.00e7.86
6.98e7.86
7.01e7.59
7.01e7.59
6.83e7.52
7.27e7.36
7.11e7.39
e
0.94
0.92
0.91
0.90
0.92
e
e
e
e
e
2.32
2.16
e
1.62, 1.18 (CH2); 0.78 (CH3)
e
0.99
0.99
0.98
0.97
0.98
1.01
0.96
0.99
0.86
0.86
0.93
0.91
0.90
0.89
0.91
0.95
0.89
0.93
0.86
0.86
e
e
e
e
e
e
e
4
2.80
e
e
e
Table 3
13C NMR chemical shift values of 2,6-diarylpyran-4-one thiosemicarbazones 11e27 and parent ketones 1 and 4 [
d
(ppm)].
Compound C-2 C-3 C-4 C-5 C-6 C ¼ S Alkyl and ipso carbons
86.81 44.76 153.55 35.69 78.47 179.70 11.44 (3-CH3); 140.68 (C-60), 139.98 (C-20)
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
1
84.81 50.48 158.12 36.12 77.92 175.00 18.66 (3-CH2eCH3); 12.11 (3-CH2eCH3); 140.68(C-60); 139.98 (C-20)
86.22 40.37 156.90 36.07 79.21 179.97 11.28 (3-CH3); 11.19 (5-CH3); 140.01 (C-20); 138.64 (C-60)
86.20 40.32 157.03 35.96 79.27 180.07 11.24 (3-CH3); 11.14 (5-CH3) 138.54 (C-20); 136.84 (C-60)
86.15 40.31 156.27 36.09 79.27 176.82 11.51 (3-CH3); 11.39 (5-CH3); 139.07(C-20); 137.74 (C-60)
86.96 40.37 158.49 36.25 79.68 179.86 21.07 (CH3 at C-20000 and C-60000); 11.20 (3-CH3); 11.37 (5-CH3); 138.79 (C-20); 137.96 (C-20)
86.31 40.56 156.35 35.95 79.24 175.71 11.71 (3-CH3); 11.63 (5-CH3); 55.02 (CH3 at C-20000 and C-60000); 138.04 (C-20); 136.02 (C-60)
86.01 44.12 156.98 36.09 79.06 176.32 11.12 (3-CH3); 141.68 (C-20); 140.98 (C-60); 139.21 (C-40)
84.93 50.97 159.92 35.25 78.24 172.34 18.47 (3-CH2eCH3); 12.21 (3-CH2eCH3); 140.68 (C-60); 139.72 (C-20); 138.57(C-40)
87.01 40.41 158.97 35.97 79.23 176.12 11.24 (3-CH3); 11.16 (5-CH3); 140.94 (C-20); 139.97(C-60); 137.01(C-40)
86.91 41.02 157.03 36.21 80.10 175.23 11.25 (3-CH3); 11.17 (5-CH3); 139.23 (C-20); 138.81C-60), 138.01 (C-40)
86.89 41.03 158.98 35.95 79.88 172.78 11.54 (3-CH3); 11.41 (5-CH3); 140.01 (C-20); 139.90 (C-60), 139.12 (C-40)
87.12 40.65 159.99 35.42 78.13 174.18 11.39 (3-CH3); 11.25 (5-CH3); 21.02 (CH3 at C-20000 and C-60000); 139.98 (C-20); 139.10 (C-60), 138.57 (C-40)
86.89 40.69 158.98 36.24 79.92 171.65 11.77 (3-CH3); 11.68 (5-CH3); 139.10 (C-20); 138.73 (C-60), 138.14 (C-40)
87.02 42.97 157.18 38.92 80.87 171.23 12.06 (3-CH3); 11.91 (5-CH3); 139.42(C-20); 139.03 (C-60), 137.02 (C-40)
87.74 41.73 159.08 36.78 79.02 171.99 11.68 (3-CH3); 11.59 (5-CH3); 140.03 (C-20); 139.97 (C-60), 138.11 (C-40)
87.13 41.12 158.01 36.45 80.05 171.98 11.72 (3-CH3); 11.82 (5-CH3); 140.72 (C-20); 139.53 (C-60), 137.55 (C-40)
85.97 51.65 207.09 50.16 79.39
85.64 51.73 208.13 51.73 85.64
e
9.48 (3,5-CH3); 138.54 (C-20); 136.84 (C-60)
9.69 (3,5-CH3); 140.64 (C-20); 139.54 (C-20)
4
e
R5
H
N
H
H
N
H
S
R3
R2
N
H
H
H
3
+
S
N
N
H
H
6
R4
-
H
4
5
N
1
R
H
O
2
6
3
R2
R2
4
5
1
R
H
O
2
H
H
R4
R3
Fig. 1. Configuration of NeNH/ bond syn to C-5 carbon.
Fig. 2. Conformation of 3-alkyl-2,6-diphenyllpyran-4-one thiosemicarbazones.

1418
S. Umamatheswari et al. / European Journal of Medicinal Chemistry 46 (2011) 1415e1424
Table 4
R5
Antibacterial activity of compounds 11e27 against selected bacterial strains (MIC in
H
mg/ml).
N
Compounds S. aureus E. coli
P. aeruginosa S. typhi B. subtilis K. pneumonia
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
100
50
50
25
25
50
12.5
50
50
50
25
25
100
0.000 200
50
50
25
25
25
50
50
100
50
25
25
0
50
25
25
50
50
12.5
50
100
50
25
25
50
12.5
50
25
12.5
50
100
100
50
25
25
100
50
100
100
50
12.5
25
0.000
H
H
3
25
50
50
100
100
50
S
R3
R2
N
H
50
12.5
25
25
100
50
N
H
6
R4
50
50
4
100
100
100
100
50
25
5
1
CH3
12.5
6.25
6.25
100
O
2
H3C
12.5
50
100
25
12.5
50
H
12.5
50
50
12.5
25
25
25
50
25
12.5
12.5
100
50
R2
100
50
25
200
Ciprofloxacin 25
50
12.5
12.5
R4
R3
C-6, alkyl group at C-3 carbon and NeNHeCSeNHPh bond syn to
C-5 carbon (E-form) akin to compound 11 as shown in Fig. 2.
In the case compound 21 which has methyl substituent at C-3
and C-5 carbon, the observed strong NOE between 4.72 ppm (H-6)
and 4.21 ppm (H-2) confirms that the pyran ring of compound 21
exists in chair conformation with equatorial orientation of aryl
groups at C-2 and C-6 which is also substantiated from their
observed coupling constant values. Moreover the NOE between H-5
proton and NeH proton of thiosemicarbazones moiety reveals that
the NeNHeCSeNHPh moiety is syn to C-5 carbon (E-form).
Furthermore, a remarkable point to be noted that is the methyl
group at C-5 adopts axial orientation in order to avoid the 1,3 spatial
interaction between the methyl and NeNHeCSeNHPh and to retain
the chair conformation. The equatorial orientation of the H-5e
proton is substantiated from the vicinal coupling constant
(J36a,5e ¼ 2.0 Hz). The axial orientation of H-3a was confirmed from
the diaxial coupling constant value (J32a,3a ¼10.0 Hz). These obser-
vations suggest that the six-member heterocyclic ring for compound
21 may take a chair conformation with equatorial orientation of both
the phenyl groups (C-2 and C-6), methyl group at C-3 in equatorial
and a methyl group at C-5 in axial. Hence, the six-member hetero-
cyclic ring in 3,5-dimethyl-2,6-diaryltetrahydropyran-4-one
Fig. 3. Conformation of 3,5-dimethyl-2,6-diarylpyran-4-one thiosemicarbazones
13e17 and 20e27.
orientation of methyl group at C-3. Moreover, in the process of
formation of thiosemicarbazones there are two geometrical
isomers are expected with respect to the orientation of
NeNHeCSeNHPh bond towards either C-3 (Z-form) or C-5
(E-form) carbon. In the NMR spectrum of the compound 18 shows
a single set of signals which reveal that the compound 18 is
formed exclusively in one form. Furthermore, the chemical shift
value of H-5e proton is deshielded about 0.3 ppm than the parent
ketone (see Table 2) which indicates that the NeNHeCSeNH2
bond is syn to C-5. The deshielding of H-5e proton is due to the
non-bonding interaction between H-5e proton and NeH proton of
thiosemicarbazones moiety as shown in Fig. 1. These observations
suggest that the 3-alkyl-2,6-diphenyltetrahydropyran-4-one
40-phenylthiosemicarbazones (18 and 19) are exists in chair
conformation with equatorial orientation of phenyl groups at C-2,
Table 5
Antifungal activity of compounds 11e27 against selected fungal strains (MIC in mg/ml).
Compound
C. neoformans
Rhizopus sp.
C. albicans
A. niger
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
50
50
25
25
50
100
200
12.5
12.5
6.25
6.25
12.5
50
200
200
100
100
200
25
25
100
50
200
200
100
25
0.000
200
100
100
200
50
50
100
50
100
50
25
25
25
50
100
12.5
100
100
100
12.5
6.25
6.25
6.25
50
50
100
50
25
200
25
25
100
25
100
200
100
50
200
100
100
50
25
25
25
27
Amphotericin-B
25
Fig. 4. ORTEP diagram of the compound 14.

S. Umamatheswari et al. / European Journal of Medicinal Chemistry 46 (2011) 1415e1424
1419
Table 6
Observed and predicted antibacterial activities (ꢂlog MIC) of the compounds 11e27 using the best QSAR models.
Entry
S. aureus
E. coli
P. aeruginosa
S. typhi
B. subtilis
K. pneumonia
Observed
Predicted
Observed
Predicted
b
Observed
Predicted
Observed
Predicted
Observed
Predicted
b
Observed
Predicted
3.952
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
4.000
4.301
4.301
4.602
4.602
4.301
4.903
4.301
4.301
4.301
4.602
4.602
4.000
4.903
4.301
4.301
4.903
4.602
4.162
4.163
4.390
4.512
4.540a
4.159
4.888a
4.163
4.153a
4.391
4.512
4.541
4.160
4.891
4.452
4.538a
4.914
0.000
4.602
4.301
4.301
4.301
4.602
4.602
4.301
4.000
4.000
4.000
4.000
4.301
4.602
4.602
4.000
4.301
4.301
3.698
4.000
4.000
4.301
4.903
4.000
4.301
4.301
4.602
4.903
5.204
5.204
4.000
4.602
4.301
4.602
3.698
4.903
3.658
3.735
4.168
4.472
4.552a
3.911
3.801a
4.244
4.327a
4.751
5.058
5.138
4.494
4.409
4.335
4.505a
3.834
4.301
4.301
4.602
4.602
4.602
4.301
4.301
4.000
4.301
4.602
4.602
4.903
4.301
4.000
4.602
4.903
4.301
4.903
4.214
4.215
4.539
4.712
4.752a
4.209
4.209a
4.215
4.200a
4.541
4.713
4.754
4.211
4.214
4.627
4.749a
4.247
0
4.000
4.000
4.301
4.602
4.602
4.000
4.301
4.000
4.000
4.301
4.903
4.602
0.000
4.602
4.903
4.903
4.000
4.301
4.524
4.362
4.362
4.299a
4.525
4.491a
4.282
4.207a
4.032
4.029
3.972
4.189
4.644
4.536
4.322a
4.452
4.301
4.602
4.602
4.301
4.301
4.903
4.301
4.000
4.301
4.602
4.602
4.301
4.903
4.301
4.602
4.903
4.301
4.292
4.454
4.539
4.560a
4.289
4.892a
4.292
4.285a
4.454
4.540
4.560
4.290
4.894
4.497
4.558a
4.911
3.927
4.348
4.729
4.760a
4.088
4.100a
3.921
3.895a
4.326
4.705
4.741
b
4.603
4.739
4.795a
4.128
27
Ciprofloxacin
a
Compounds used in the test set.
Outliers.
b
Table 7
Observed and predicted antifungal activities (ꢂlog MIC) of the compounds 11e27 using the best QSAR models.
Entry
C. neoformans
Rhizopus sp.
C. albicans
A. niger
Observed
Predicted
Observed
Predicted
Observed
Predicted
Observed
Predicted
b
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
4.301
4.301
4.602
4.602
4.301
4.000
3.698
4.903
4.903
5.204
5.204
4.903
4.301
3.698
4.602
4.602
4.000
4.602
4.492
4.437
4.516
4.604
4.631a
3.841
3.739a
4.878
4.821a
4.936
5.028
5.053
4.261
4.150
4.653
4.624a
3.758
3.698
3.698
4.000
4.000
3.698
4.602
4.602
4.000
4.301
4.301
4.000
4.301
4.602
4.000
3.698
4.000
4.301
4.602
3.713
3.793
3.887
3.887
3.887a
4.459
4.488a
4.085
4.213a
4.176
4.175
4.176
4.749
3.813
3.768
3.817a
4.417
3.698
3.698
4.000
4.602
4.301
4.000
4.903
4.000
4.000
4.000
4.903
5.204
5.204
5.204
4.301
4.602
4.602
4.602
3.522
3.674
4.033
4.337
4.422a
4.524
4.520a
4.060
4.219a
4.571
4.877
4.961
5.058
5.068
4.184
4.365a
4.542
0.000
3.698
4.000
4.000
3.698
4.301
4.301
4.000
4.301
4.000
4.301
4.602
4.602
4.602
3.698
4.000
4.000
4.301
3.699
3.813
3.929
3.963a
4.203
4.136a
4.026
4.161a
4.230
4.349
4.383
4.621
4.582
3.837
3.919a
4.125
27
Amphotericin-B
a
Compounds used in the test set.
Outlier.
b
Table 8
QSAR statistics of significant equations.
Organism
n
r2
R
S.D.
Q
q2
F value
P value
Randomized r2
r2pred
S. aureus
E. coli
13
12
13
13
12
12
13
13
13
12
0.83
0.90
0.85
0.78
0.84
0.90
0.82
0.81
0.79
0.84
0.91
0.95
0.92
0.88
0.92
0.95
0.91
0.90
0.89
0.92
0.13
0.08
0.22
0.13
0.10
0.13
0.22
0.13
0.29
0.15
7.00
0.75
0.81
0.71
0.69
0.79
0.84
0.66
0.73
0.68
0.73
24.6 (2,10)
39.6 (2,9)
27.3 (2,10)
38.8 (1,11)
24.4 (2,9)
40.3 (2,9)
22.2 (2,10)
45.6 (2,10)
18.7 (2,10)
23.6 (2,9)
1.375eꢂ04
3.465eꢂ05
8.911eꢂ05
6.474eꢂ05
2.330eꢂ04
3.223eꢂ05
2.109eꢂ04
9.407eꢂ06
4.199eꢂ04
2.649eꢂ04
0.15
0.18
0.19
0.08
0.17
0.18
0.16
0.16
0.15
0.19
0.71
0.74
0.79
0.82
0.89
0.90
0.88
0.84
0.75
0.69
11.88
4.18
6.77
9.20
7.31
4.14
6.92
3.07
6.13
P. aeruginosa
S. typhi
B. subtilis
K. pneumonia
C. neoformans
Rhizopus
C. albicans
A. niger

1420
S. Umamatheswari et al. / European Journal of Medicinal Chemistry 46 (2011) 1415e1424
Table 9
3. Results and discussion
Best multiple linear regression model and t-values.
3.1. In vitro antimicrobial activity
Variable
Coefficient
S.E.
t-Value
Eq. (1) for S. aureus
Intercept
WPSA
1.461
0.423
3.448
4.029
6.801
All the synthesized novel 2,6-diaryltetrahydropyran-4-one thi-
osemicarbazones (11e27) were assessed for their antibacterial
activity in vitro against Staphylococcus aureus (ATCC-25825),
Escherichia coli (ATCC-25835), Pseudomonas aeruginosa (ATCC-
10145), Salmonella typhi (ATCC-24915), Bacillus subtilis (ATCC-451)
and Klebsiella pneumonia (ATCC-15490). The in vitro activities of the
compounds were tested in Sabourauds dextrose broth (SDB, Hi-
media, Mumbai) for fungi and in Nutrient broth (NB, Hi-media,
Mumbai) for bacteria by the twofold serial dilution method [22].
The antibacterial potency of the compounds was compared with
standard drug ciprofloxacin, a broad spectrum antibiotic and their
minimum inhibitory concentration (MIC) values are summarized in
Table 4. In vitro antifungal activity of the synthesized compounds
11e27 was examined against four fungal strains viz. Cryptococcus
neoformans, Candida albicans, Rhizopus sp., and Aspergillus niger.
Amphotericin-B was used as the standard drug. The MIC values of
the test compounds and the standard are furnished in Table 5.
A close investigation of the in vitro antibacterial and antifungal
activity profile of the 2,6-diaryltrtrahydropyran-4-one thio-
semicarbazones gives a clear picture of the structure activity
correlations among the compounds 11e27 under study. The
compounds 13e20 and 23e27 with halogens or methoxy functions
of aryl groups present at C-2 and C-6 positions of pyran moiety
along with alkyl substituent at C-3 and C-5 positions and phenyl
group at N-40 of thiosemicarbazones moiety exert varied range of
antimicrobial activity while the compounds 11 and 12 with no
substituents on the phenyl groups of pyran ring did not exhibit
significant antibacterial activity.
2.461eꢂ03
0.486
6.106eꢂ04
0.071
HB acceptors
Eq. (2) for E. coli
Intercept
Volume
20.576
2.573
10.008
6.667
7.942
ꢂ1.482eꢂ03
ꢂ2.312
2.222eꢂ04
0.291
IP
Eq. (3) for P. aeruginosa
Intercept
WPSA
2.291
0.303
7.549
2.987
5.031
3.154eꢂ03
0.293
1.055eꢂ03
0.058
Log P
Eq. (4) for S. typhi
Intercept
WPSA
3.965
0.080
49.035
6.226
3.507eꢂ03
5.632eꢂ04
Eq. (5) for B. subtilis
Intercept
WPSA
2.962
0.226
13.100
3.433
6.949
1.745eꢂ03
0.301
5.083eꢂ04
4.335eꢂ02
HB acceptors
Eq. (6) for K. pneumonia
Intercept
WPSA
IP
20.373
3.948
6.010
8.966
5.053
5.998eꢂ03
ꢂ2.399
6.689eꢂ04
0.474
Eq. (7) for C. neoformans
Intercept
FOSA
4.245
0.326
12.991
5.700
3.320
ꢂ3.959eꢂ03
0.175
6.944eꢂ04
0.052
Log P
Eq. (8) for Rhizopus
Intercept
FOSA
4.025
0.119
33.684
7.767
5.330
3.241eꢂ03
ꢂ7.374eꢂ03
4.172eꢂ04
1.383eꢂ03
FISA
Eq. (9) for C. albicans
Intercept
PISA
0.0741
ꢂ3.489eꢂ03
0.1150
0.760
0.097
2.772
6.050
1.258eꢂ03
0.019
3.2. QSAR studies
Polarizability
LigPrep [23], QikProp [24] and Strike [25] programme from
Schrödinger were used for the QSAR model development. All the
synthesized molecules were sketched in Maestro [26]. LigPrep was
used to convert the 2D molecular structures to 3D molecular
structures with minimum molecular energy. Molecular descriptors
and physicochemical properties were generated by QikProp for
each of the compound, based on the accurate 3D molecular files
generated by LigPrep, reflecting Monte Carlo simulation studies
[27]. QikProp properties were used as input to Strike, which is
a collection of chemically-aware statistical tools for examining
correlations within data.
In the present study, we constructed models for the synthesized
compounds using forward stepwise multiple regression. This
method starts with an equation containing only one descriptor and
adds additional ones in turn. If the added descriptor increases the
statistical significance of the equation, it remains in the equation;
Eq. (10) for A. niger
Intercept
FOSA
2.471
0.252
9.800
4.268
5.889
2.036eꢂ03
0.229
4.771eꢂ04
0.038
Log P
thiosemicarbazones (20e27) adopts a chair conformation with the
orientation of NeNHeCSeNHPh moiety syn to the C-5 carbon
(Fig. 3). The chair conformation and the axial orientation methyl
group at C-5 carbon of pyran ring of 3,5-dimethyl-2,6-diary-
ltetrahydropyran-4-one thiosemicarbazones (13e17 and 20e27) are
supported by single crystal XRD studies of compound 14 [21] which
also confirm the orientation of NeNHeCSeNHPh moiety is syn to
the C-5 carbon (E-form). The ORTEP diagram of compound 14 is
shown in Fig. 4.
Table 10
Intercorrelation matrix of independent variables obtained in multiple linear regression equation.
FOSA
1.000
FISA
PISA
WPSA
Volume
HB acceptor
Polarizability
Log P
IP
FOSA
FISA
PISA
WPSA
Volume
HB acceptor
Polarizability
Log P
0.051
1.000
ꢂ0.359
ꢂ0.819
1.000
ꢂ0.654
ꢂ0.078
ꢂ0.067
1.000
0.292
ꢂ0.894
0.561
0.008
1.000
0.651
0.002
ꢂ0.150
ꢂ0.406
0.301
0.175
ꢂ0.946
0.658
0.046
0.988
0.175
1.000
ꢂ0.131
ꢂ0.949
0.697
0.324
0.891
ꢂ0.722
0.103
0.033
0.557
ꢂ0.370
ꢂ0.730
ꢂ0.279
0.006
1.000
ꢂ0.106
0.940
1.000
IP
1.000

S. Umamatheswari et al. / European Journal of Medicinal Chemistry 46 (2011) 1415e1424
1421
Fig. 5. Plot between observed and predicted activities of the compounds against different bacterial strains in the training set and test set. (a) S. aureus (b) E. coli (c) P. aeruginosa
(d) S. typhi (e) B. subtilis (f) K. pneumonia.

1422
S. Umamatheswari et al. / European Journal of Medicinal Chemistry 46 (2011) 1415e1424
otherwise it is removed. This procedure continues until a statisti-
cally significant equation is obtained [28]. Using the similar
method, we obtained all the models. The observed and predicted
values using QSAR Eqs. (1)e(10) are tabulated in Tables 6 and 7. The
statistical values of all the Eqs. (1)e(10) are tabulated in Table 8 and
the QSAR models are tabulated in Table 9. Intercorrelation matrix of
independent variables obtained in the multiple linear regression
equations were tabulated in Table 10.
Statistically important QSAR model was identified on the basis
of squared correlation coefficient (r2), standard deviation (S.D.),
Fisher’s value (F), Q value and t-value for the training set. Robust-
ness of the derived models were verified by using three different
types of validation viz. (i) Leave one out (LOO) internal validation or
cross-validation (q2), (ii) external validation by predicting the
activity of test set and (iii) randomization test. In Table 8, Q indi-
cates quality factor or quality ratio which is calculated by r/S.D.
chance correlation, due to the excessive number of parameter
(which increases also the r and s values) is detected by the exam-
ination of Q value [29]. The high values of Q in the present study
indicate the high predictive power of the QSAR models and also no
over fitting. All the constructed models show a very good linear
relationship having a squared correlation coefficient r2 > 0.78. In
Table 9, intercept indicates the point where the equation cuts either
x-axis or y-axis.
Fig. 6. Plot between observed and predicted activities of the compounds against different fungal strains in the training set and test set. (a) C. neoformans (b) Rhizopus (c) C. albicans
(d) A. niger.

S. Umamatheswari et al. / European Journal of Medicinal Chemistry 46 (2011) 1415e1424
1423
The graphs of predicted ꢂlog MIC versus observed ꢂlog MIC for
the training and test sets are depicted in Figs. 5 and 6.
intercorrelation coefficients between polarizability and volume
(0.988) and between polarizability and hydrophobicity (0.940). The
QSAR Eq. (10) for A. niger indicates that increase in FOSA and
hydrophobicity increases the antifungal activity of the synthesized
compounds because of the increased facilitation of the perme-
ability of the molecules through the fungal cell [34]. Plot between
observed and predicted activities of the synthesized compounds
11e27 against different bacterial and fungal strains was shown in
Figs. 5 and 6.
Eq. (1) depicts that increase in Weakly Polar component of
Solvent Accessible surface area (WPSA) and Hydrogen Bond (HB)
acceptor of the compounds favours the antibacterial activity against
S. aureus. The WPSA descriptor pertains to the surface area for all
halogens, sulfur, and phosphorous atoms [28]. Experimental results
also depicts that the presence of halogens in the compounds
increases the activity against S. aureus.
Eq. (2) represents that decrease in volume and ionization
potential (IP) favours the antibacterial activity of the synthesized
compounds against E. coli. There is a high mutual correlation
between polarizability and molecular volume (0.988). The equation
with molecular volume or polarizability gave exactly the same
statistics with that of ionization potential. Hence, it is hard to
predict, the most important polarizability or molecular volume
effect. We prefer the latter because of the variation in R2 with
phenyl substitution. In the case of P. aeruginosa, positive correlation
was observed between antibacterial activity and the descriptors
WPSA and hydrophobicity (Eq. (3)). Singh et al. also observed
positive contribution of the hydrophobicity in the QSAR study with
other bacterial species [30].
In the case of S. typhi, the QSAR model developed resulted in
a monoparametric model with good r2 and rp2red and the model
depicted that WPSA positively correlates with the antibacterial
activity against S. typhi (Eq. (4)). Even though the sample size
allowed us to go for development of multiparametric model, the
high colinearity among the parameters restricted us to go for
monoparametric model only. When B. subtilis inhibition activity
was correlated with the descriptors, the descriptors like WPSA and
HB acceptors showed a statistically significant correlation (Eq. (5)).
The model described by Eq. (6) demonstrated the importance of
WPSA and IP in describing the antibacterial activity against K.
pneumonia. The negative correlation of molecular descriptor IP
with antibacterial activity reveals that decrease in value of IP will
lead to increase in antibacterial activity against K. pneumonia.
Descriptor WPSA had a positive correlation with the antibacterial
activity of the synthesized compounds against K. pneumonia as
seen in other Eqs. (1), (3)e(5).
Eq. (7) represents that decrease in hydrophobic surface area
(FOSA) and increase in lipophilicity will increase the antifungal
activity of the synthesized compounds against C. neoformans. The
hydrophobic surface area (FOSA) represents the aliphatic portion of
the solvent accessible surface area that is composed of saturated
carbons and their attached hydrogen [31]. An increase in hydro-
phobic substituents like fluoro, bromo and chloro increases the
antifungal activity of the synthesized compounds against C. neo-
formans which is in accordance with the report of other study [32].
Eq. (8) indicates that FOSA and FISA correlate in a positive and
negative way respectively with the antifungal activity of the
synthesized compounds against Rhizopus sp. FOSA is also a measure
of the hydrophobic property of a molecule and as it increases, the
polarity of the molecule will decrease [31]. FISA denotes the
hydrophilic component of surface area, and its correlation with
lipophilicity is 0.949. Thus an increase in hydrophobicity increases
the antifungal activity of the synthesized compounds against
Rhizopus sp.
4. Conclusion
In summary, a series of thiosemicarbazone derivatives have
been synthesized successfully in appreciable yields and screened
for their in vitro antimicrobial activity against bacterial strains S.
aureus, E. coli, P. aeruginosa, S. typhi, B. subtilis and K. pneumonia and
fungal strains C. neoformans, C. albicans, Rhizopus sp., and A. niger.
The results showed that some azole derivatives exhibited signifi-
cant antibacterial and antifungal activities, even comparable or
superior to reference drugs (Ciprofloxacin and Amphotericin-B). Of
the synthesized thiosemicarbazones, 17, 25, 27 against S. aureus 12,
16, 17, 24, 25 against E. coli, 21, 22 against P. aeruginosa, 13, 14, 17, 21,
24, 26, 27 against B. subtilis, 14, 15, 21, 24e26 against K. pneumonia
presented better antibacterial activities than reference drug
Ciprofloxacin. Thiosemicarbazones 18e22 against C. neoformans,
16, 23 against Rhizopus sp., 17, 22e24 against C. albicans, 22e24
against A. niger show better antifungal activities than Amphoter-
icin-B. QSAR analysis indicated that the compounds should possess
high WPSA (halogen substitutions) to have high antibacterial
activity against S. aureus, P. aeruginosa, S. typhi, B. subtilis and K.
pneumonia. To have better activity against E. coli and K. pneumonia
compounds should have decreased ionization potential. Increase in
hydrophobic substituents in the compounds will favour activity
against P. aeruginosa, C. neoformans and A. niger. Further the
obtained regression models show a good capacity to explain the
observed values of antimicrobial activity, high statistical signifi-
cance and predictive capacity. These observations may promote
further development of our research in this field. Further devel-
opment of this group of thiosemicarbazones may lead to chemical
entities with better pharmacological profile than standard drugs.
Thus, in the future this class of thiosemicarbazones may be used as
templates for the construction of better drugs to combat bacterial
and fungal infections. Further the mechanistic studies of the
synthesized compounds will be explored as continuation of this
work.
Acknowledgements
This research work supported by Council of Scientific and
Industrial Research, New Delhi. The authors would like to
acknowledge NMR research centre, IISc-Bangalore and Department
of Chemistry, IIT-Madras, respectively for recording NMR and single
crystal XRD. The authors also acknowledges the Department of
Science and Technology, SERC division, New Delhi for the financial
assistance towards QSAR programme.
Appendix. Supplementary data
Decrease in PISA and increase in polarizability indicates high
antifungal activity against C. albicans as mentioned in Eq. (9).
Supplementary data related to this article can be found online at
doi:10.1016/j.ejmech.2011.01.029.
Descriptor PISA quantifies the
p component of solvent accessible
surface area i.e. it reflects hydrophobicity of the molecule due to
aromatic region. Polarizability is related to size and hydrophobicity
of the compound. Sharma et al. also observed that increase in
polarizability by introducing bulkier substitution, results in active
antifungal agents [33]. The present study showed greater
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