Antimicrobial evaluation of a set of heterobicyclic methylthiadiazole hydrazones: Synthesis, characterization, and SAR studies

Article abstract of DOI:10.1021/jf404537d

To exploit the potential antimicrobial activities of azabicyclic skeleton based compounds, a set of 2r,4c-diaryl-3-azabicyclo[3.3.1]nonan-9-one-4-methyl- 1,2,3-thiadazole-5-carbonyl hydrazones were synthesized. Unambiguous structural elucidation has been

Full text of DOI:10.1021/jf404537d

Article
pubs.acs.org/JAFC
Antimicrobial Evaluation of a Set of Heterobicyclic Methylthiadiazole
Hydrazones: Synthesis, Characterization, and SAR Studies
Paulrasu Kodisundaram,† Shanmugasundaram Amirthaganesan,§ and Thirunavukkarasu Balasankar*,#
†Department of Chemistry, Annamalai University, Annamalai nagar 608002, Tamilnadu, India
§Department of Chemistry, Saveetha School of Engineering, Saveetha University, Thandalam, Chennai 602105, Tamilnadu, India
#Chemistry Section, FEAT, Annamalai University, Annamalai nagar 608002, Tamilnadu, India
S
* Supporting Information
ABSTRACT: To exploit the potential antimicrobial activities of azabicyclic skeleton based compounds, a set of 2r,4c-diaryl-3-
azabicyclo[3.3.1]nonan-9-one-4-methyl-1,2,3-thiadazole-5-carbonyl hydrazones were synthesized. Unambiguous structural
elucidation has been carried out by investigating IR, H1, C13 NMR, and elemental analysis. 2D NMR spectra (1H−1H
COSY, HSQC, HMBC, and NOESY) were recorded for a representative compound, 12, to confirm the proposed structure for
9−15. Antimicrobial activity assessment of synthesized hydrazones 9−15 has been evaluated by screening against selective
strains. Both bacteria and fungi of various forms along with standard drug have been taken for the analysis. Difference in the
potency of activity against the strains has been evaluated on the basis of SAR, and it has been revealed that substitution of
electron-withdrawing halogens (chloro, fluoro, and bromo) at para positions of the phenyl (10, 12, and 13) enhanced the
antifungal and antibacterial activities against tested organisms compared to other hydrazone derivatives.
KEYWORDS: antibacterial, antifungal, hydrazone derivatives, hydrazides
INTRODUCTION
EXPERIMENTAL METHODS
■
■
Chemicals and Instrumentation Techniques for Structural
Characterization. 4-Methyl-1,2,3-thiadiazole-5-carboxylic acid hydra-
zide was purchased from Sigma-Aldrich. Other chemicals used for the
synthesis of azabicyclic ketones were of analytical grade. Solvents were
used after distillation. Reaction progress and purity were monitored by
TLC. Melting points were determined in an Electrothermal 9100
instrument in open capillaries and are uncorrected. FT-IR analysis has
been done in an AVATAR 330 FT-IR Thermo Nicolet spectrometer
by making pellet of compound with KBr. Both one- and two-
dimensional NMR spectra were recorded in a Bruker AMX 400 NMR
spectrometer. Sample was prepared in a 5 mm diameter tube using
DMSO-d6 solvent (10 mg in 0.5 mL). 1H NMR were data collected at
400.13 MHz operating frequency, and 13C NMR was at 100.62 MHz.
Chemical shifts (δ) are expressed in parts per million with respect to
TMS. CHN analysis was provided from Heraeus Carlo Erba 1108.
Synthetic Procedure. Typical Procedure for Synthesis of 2,4-
Diaryl-3-azabicyclo[3.3.1]nonan-9-ones (1−7). An often used typical
procedure reported by Baliah and Jeyaraman24 was adopted for the
synthesis of 2,4-diaryl-3-azabicyclo[3.3.1]nonan-9-ones with conven-
ient modification. A reaction mixture of cyclohexanone (1 equiv),
respective aryl aldehyde (2 equiv), and ammonium acetate (1 equiv)
dissolved in ethanol was kept in the water bath by maintaining the
bath temperature at 60−75 °C with continuous stirring. The crude
product thrown out from the solution was filtered off. It was washed
through etheric ethanol solution. An ethanol/chloroform/acetone
mixture was used for recrystallization of 1−7.
3-Azabicyclonones (3-ABN) have attracted attention owing to
their extensive scale of microbial potencies. Antifungal and
antibacterial studies proved that the 3-ABN skeleton seems to
be an effective pathogen killer.1,2 Further reports revealed the
analgesic,3 anti-inflammatory,4 antipyretic,5 and antiphologistic6
capabilities of azabicyclic ketones. Narcotic antagonism and
antitussive and sedative properties of these heterobicycles were
also described.7 Calcium antagonist, hypotensive, and local
anesthetic activities are measured.8 A wide variety of naturally
abundant diterpenoid/norditerpinoid alkaloids have been
found to possess various pharmacological actions due to the
presence of the 3-azabicyclonone pharmocopore. Of late,
hydrazide−hydrazone derivatives have received the attention
of various medicinal chemists as a result of their effectual
biological potencies, namely, antimicrobial, antitubercular, and
also anticonvulsant actions.9−11 Research in recent decades has
provided evidence that compounds with the thiadiazole ring
possess an essential structure with broad-spectrum biological
activity. Hence, these compounds are of great interest for
medicinal chemists and have been investigated for their
insecticidal,12 herbicidal,13 antimicrobial,14 antimycobacterial,15
antimicotic,16 antiviral,17 antimalarial,18 antidepressive,19 anti-
convulsant,20 cardiotonic,21 anti-inflammatory,22 and antileuke-
mic and anticancer activities.23 The above observations provide
impetus to synthesize a system that combines 3-azabicyclonone
and thiadiazole moieties together to produce the corresponding
hydrazones 9−15 with the anticipation of creating several
promising antimicrobial agents.
General Procedure for the Synthesis of 2r,4c-Diaryl-3-
azabicyclo[3.3.1]nonan-9-one-4-methyl-1,2,3-thiadazole-5-carbon-
yl Hydrazones (9−15). A mixture of 1 mmol of 2,4-diaryl-3-
azabicyclo[3.3.1]nonan-9-ones (1 equiv) and 1.5 mmol of 4-methyl-
Received: October 9, 2013
Accepted: November 19, 2013
Published: November 19, 2013
© 2013 American Chemical Society
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1,2,3-thiadiazole-5-carboxylic acid hydrazide (1.5 equiv) was dissolved
in the solvent mixture of chloroform and methanol (1:1 v/v). Acetic
acid (1−2 mL) was added as catalyst. The reaction mixture was
refluxed for about 3−4 h. After the completion of reaction, the solvent
mixture was removed under vacuum. The separated solid was
subjected to cold water/ethanol mixture washing. Compounds were
recrystallized from ethanol.
CH3), 2.77 (s, 3H, CH3 at thiadiazole ring), 7.12−8.24 (m, 8H
aromatic hydrogens); 13C (400 MHz, DMSO-d6) δ 45.80 (C-1), 64.20
(C-2), 62.00 (C-4), 40.20 (C-5), 28.00 (C-6), 20.70 (C-7), 28.00 (C-
8), 164.20 (C-9), 162.20 (NHCO), 20.70 (CH3 at phenyl ring), 15.20
(CH3 at thiadiazole ring), 161.40 (C-4″), 135.94 (C-5″), 127.2−138.2
(aromatic carbons and ipso carbons).
Data for 15: white solid; yield 55%; mp 196 °C; IR (KBr, νmax
cm−1) 3342 (N−H st), 3050, 2920 and 2847 (C−H st), 1620 (CN
st); 1H NMR (400 MHz, DMSO-d6) δ 2.72 (m, 2H, H-1e, H-7a), 4.31
(d, 2H, H-2a, H-4a), 1.84 (s, 1H, N−H), 3.85 (s, 1H, H-5e), 1.44 (m,
1H, H-7e), 2.01 (m, 2H, H-6e, H-8e), 1.77 (m, 2H, H-6a,H-8a), 10.52
(s, 1H, amide NH), 2.78 (s, 1H, CH3 at thiadiazole ring), 6.88−7.76
(m, 8H aromatic hydrogens); 13C (400 MHz, DMSO-d6) δ 46.00 (C-
1), 63.60 (C-2), 61.19 (C-4), 39.70 (C-5), 27.40 (C-6), 21.50 (C-7),
28.50 (C-8), 164.26 (C-9), 162.71 (NHCO), 15.40 (CH3 at
thiadiazole ring), 164.11 (C-4″), 135.20 (C-5″), 128.02−136.59
(aromatic carbons and ipso carbons).
Biological Screening. Bacterial strains such as Bacillus subtilis,
Klebsiella pneumonia, Escherichia coli, Pseudomonas aeruginosa, and
Staphylococcus aureus and fungal strains such as Aspergillus flavus,
Aspergillus niger, Candida albicans, and Candida6 obtained from the
Faculty of Medicine, Annamalai University, Annamalainagar, Tamil
Nadu, India, were used to screen the antimicrobial activity of the newly
synthesized compounds 9−15. The bacterial and fungal strains were
cultured in Sabourauds dextrose broth (SDB) at a pH 7.4 0.2 (Hi-
media, Mumbai, India) and nutrient broth (NB) (Hi-media) at pH 5.6,
respectively.
In Vitro Antibacterial and Antifungal Activity by 2-Fold
Serial Dilution Method. The in vitro potency of compounds 9−15
was examined by a 2-fold serial dilution method.25 Stock solutions of
9−15 were made in DMSO (1 mg/mL). Compounds were tested in
the concentrations of 200, 100, 50, 25, 12.5, 6.25, and 3.12 μg/mL (2-
fold serial dilution) with SDB and NB. Then SDB and NB were
suspended with 100 μL of bacterial spores from 24-h-old bacterial
cultures on NB at 37 1 °C and 100 μL fungal spores from 1−7-day-
old SDB slant cultures at 28 1 °C, respectively. Plating techniques
were used to determine the colony-forming units (cfu) of the seeded
broth in the adjusted range of 104−105 cfu/mL. Final inoculum sizes
of 105 and (1.1−1.5) × 102 cfu/mL were used for antibacterial and
antifungal assays, respectively. Microbial spore supplemented broth
with DMSO at highest concentrations used in our experiments was
used as the negative control. The growth of the microbes in the test
medium was measured on the basis of the turbidity of the culture after
24 h of bacterial incubation and 72−96 h of fungal incubation. The
lowest concentration of the test compound with the clear solution of
test medium was considered as the minimum inhibitory concentration
(MIC). Drug standards were streptomycin for antibacterial activity and
fluconazole for fungal studies.
Data for 9: white solid; yield 58%; mp 211 °C; IR (KBr, νmax cm−1)
1
3330 (N−H st), 3058, 2920, 2844 (C−H st), 1624 (CN st); H
NMR (400 MHz, DMSO-d6) δ 2.82 (m, 1H, H-1e), 4.47 (s, 1H, H-
2a), 2.00 (s, 1H, N−H), 4.34 (s, 1H, H-4a), 3.29 (s, 1H, H-5e), 1.49
(m, 2H, H-6e, H-6a), 3.01 (m, 1H, H-7a), 1.27 (s, 1H, H-7e), 1.61 (m,
2H, H-8a, H-8e), 10.33 (s, 1H, amide NH), 2.92 (s, 3H, CH3 at
thiadiazole), 7.37−7.65 (m, 10H aromatic hydrogens); 13C (400 MHz,
DMSO-d6) δ 45.90 (C-1), 65.70 (C-2), 64.10 (C-4), 39.5 (C-5), 27.29
(C-6), 21.40 (C-7), 28.67 (C-8), 164.29 (C-9), 162.37 (NHCO), 15.3
(CH3 at thiadiazole ring), 164.17 (C-4″), 135.31 (C-5″), 126.92−
141.68 (aromatic carbons and ipso carbons).
Data for 10: white solid; yield 54%; mp 204 °C; IR (KBr, νmax
cm−1) 3054, 2926, 2850(C−H st), 1630, 1602 (CC st), 3332 (N−
H st); 1H NMR (400 MHz, DMSO-d6) δ 2.69 (d, 1H, H-1e), 4.32 (s,
1H, H-2a), 1.91 (s, 1H, N−H), 4.23 (s, 1H, H-4a), 2.96 (s, 1H, H-5e),
1.51 (m, 1H, H-6a), 2.73 (m, 1H, H-7a), 1.25 (m, 1H, H-7e), 1.38 (m,
2H, H-8a, H-6e), 1.67 (m, 1H, H-8e), 10.78 (s, 1H, amide NH), 2.99
(s, 3H, CH3 at thiadiazole), 7.31−7.70 (m, 8H aromatic hydrogens);
13C (400 MHz, DMSO-d6) δ 45.00 (C-1), 64.53 (C-2), 62.35 (C-4),
39.43 (C-5), 27.99 (C-6), 20.63 (C-7), 26.83 (C-8), 164.02 (C-9),
162.90 (NHCO), 15.18 (CH3 at thiadiazole ring), 161.24 (C-4″),
135.84 (C-5″), 126.83−142.48 (aromatic carbons and ipso carbons).
Data for 11: white solid; yield 52%; mp 200 °C; IR (KBr, νmax
cm−1) 3335 (N−H st), 3032 and 2925 (C−H st), 1631 (CN st); 1H
NMR (400 MHz, DMSO-d6) δ 2.77 (d, 1H, H-1e), 4.38 (s, 1H, H-2a),
1.62 (s, 1H, N−H), 4.24 (s, 1H, H-4a), 3.26 (s, 1H, H-5e), 1.62 (m,
1H, H-6a), 1.80 (s, 1H, H-6e), 2.89 (m, 1H, H-7a), 1.85 (s, 1H, H-7e),
1.97 (m, 2H, H-8a, H-8e), 10.62 (s, 1H, amide NH), 2.77 (s, 3H, CH3
at thiadiazole ring), 3.63 (s, 6H, methoxy hydrogen at aromatic ring),
7.20−7.76 (m, 8H aromatic hydrogens); 13C (400 MHz, DMSO-d6) δ
46.00 (C-1), 65.01 (C-2), 63.60 (C-4), 39.80 (C-5), 27.40 (C-6),
21.52(C-7), 28.50 (C-8), 164.94 (C-9), 162.71 (NHCO), 15.40 (CH3
at thiadiazole ring), 55.30 (OCH3), 164.26 (C-4″), 135.35 (C-5″),
113.94−136.59 (aromatic carbons and ipso carbons).
Data for 12: white solid; yield 67%; mp 204 °C; IR (KBr, νmax
cm−1) 3030, 2944 and 2850 (C−H st), 1634 (CN st), 1607 (CC
1
ring st), 3334 (N−H st); H NMR (400 MHz, DMSO-d6) δ 2.50 (d,
1H, H-1e), 4.30 (s, 1H, H-2a), 1.61 (s, 1H, N−H), 4.21 (s, H, H-4a),
3.08 (s, 1H, H-5e), 1.49 (m, 2H, H-6a, H-6e), 1.40 (m, 1H, H-7e),
2.67 (m, 1H, H-7a), 1.61 (m, 1H, H-8a), 1.63 (m, 1H, H-8e), 11.78 (s,
1H, amide NH), 3.09 (s, 3H, CH3 at thiadiazole), 7.23−7.73 (m, 8H
aromatic hydrogens); 13C (400 MHz, DMSO-d6) δ 44.83 (C-1), 63.78
(C-2), 61.59 (C-4), 39.19 (C-5), 28.09 (C-6), 20.59 (C-7), 30.65 (C-
8), 163.53 (C-9), 162.87 (NHCO), 15.18 (CH3 at thiadiazole ring),
161.25 (C-4″), 135.85 (C-5″), 114.60−138.56 (aromatic carbons and
ipso carbons).
RESULTS AND DISCUSSION
■
Chemistry. Synthesis of diversely substituted diaryl 3-
azabicyclononanones (1−7)24 and their methylthiadiazole
hydrazones (9−15) was carried out according to the steps
shown in Scheme 1. Compounds 9−15 were achieved by the
Data for 13: white solid; yield 61%; mp 201 °C; IR (KBr, νmax
cm−1) 3339 (N−H st), 3054, 2920 and 2850 (C−H st), 1620 (CN
1
st); H NMR (400 MHz, DMSO-d6) δ 2.52 (m, 1H, H-1e), 4.44 (d,
1H, H-2a), 1.86 (s, 1H, N−H), 4.24 (s, 1H, H-4a), 3.26 (s, 1H, H-5e),
1.80 (m, 2H, H-6a, H-6e), 1.12 (m, 1H, H-7e), 2.92 (m,1H, H-7a),
1.96 (m, 2H, H-8a, H-8e), 10.11 (s, 1H amide NH), 2.71 (s, 3H, CH3
at thiadiazole ring), 6.88−7.76 (m, 8H aromatic hydrogens); 13C (400
MHz, DMSO-d6) δ 46.00 (C-1), 65.29 (C-2), 63.67 (C-4), 40.01 (C-
5), 26.70 (C-6), 20.81 (C-7), 28.50 (C-8), 164.90 (C-9), 164.20
(NHCO), 15.43 (CH3 at thiadiazole ring), 162.71 (C-4″), 135.12 (C-
5″), 128.02−136.59 (aromatic carbons and ipso carbons).
Scheme 1. Synthesis of 3-Azabicyclo[3.3.1]nonane and
Hydrazones
Data for 14: white solid; yield 62%; mp 214 °C; IR (KBr, νmax
cm−1) 3328 (N−H st), 3048, 2924 and 2845 (C−H st), 1624 (CN
1
st); H NMR (400 MHz, DMSO-d6) δ 2.63 (d, 1H, H-1e), 4.15 (s,
1H, H-2a), 1.66 (s, 1H, N−H), 4.25 (d, 1H, H-4a), 2.98 (s, 1H, H-5e),
1.54 (m, 3H, H-6a, H-6e, H-8a), 2.84 (m, 1H, H-7a), 1.24 (m, 1H, H-
7e), 1.62 (m, 1H, H-8e), 11.02 (s, 1H, amide NH), 2.32 (d, 3H, p-
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reaction of the compounds 1−7 with 4-methyl-1,2,3-
thiadiazole-5-carboxylic acid hydrazide (8) (Scheme 2). Acetic
Scheme 2
Figure 1. Numbering of the azabicyclo[3.3.1]nonane.
1
In H NMR, characteristic hydrazide NH proton (N−
NHCO−) is expected in the most downfield region. A broad
peak resonating at 10.33 ppm was assigned for NH proton of
the hydrazone part. Signal broadening is due to the faster
exchange of NH proton with solvent moisture than the
resonance time scale. The imine NH proton of the bicyclic ring
was observed at 1.96 ppm. Methylene protons (both Hax and
Heq) of C-6 and C-8 resonated as multiplets at 1.49 and 1.61
ppm, respectively. In contrast, C-7 protons are magnetically
nonequivalent and observed distinctly at 2.94 and 1.27 ppm,
respectively, for H-7a and H-7e. Benzylic protons H-2 and H-4
were observed at 4.48 and 4.34 ppm, respectively. In addition,
signals appearing at 3.28 and 2.94 ppm were designated
correspondingly for H-5 and H-1 bridgehead protons. The
methyl substituent of the thiadiazole ring was observed at 2.92
ppm. Protons resonating with closer chemical shift values were
distinct by two-dimensional NMR correlations.
acid is preferred as suitable catalyst, and it was added in a small
amount (∼1−2 mL). Reaction progress, completion, and purity
were checked by TLC (chloroform/ethyl acetate/hexane
1
mixture was used as eluent). Detailed investigations of IR, H
NMR, and C13 NMR spectral data with CHN analysis (Table
1) were made to identify and establish the newly synthesized
compounds 9−15. To substantiate the proposed structure,
1H−1H COSY, NOESY, HSQC, and HMBC were recorded for
compound 12. Figure 1 shows the numbering patterns of the
compound. C-2′ and C-4′ are the ipso carbons of aryl groups at
C-2 and C-4, respectively. Besides, the carbons of the
thiadiazole part are designated using C-4″ and C-5″. Structural
elucidation of 9 (aryl groups without substituent) has been
described, and it was confirmed from the two-dimensional
NMR reports of 12.
In 13C NMR of compound 9, two downfield resonances at
164.29 and 162.37 ppm were consigned for CN (C-9) and
CO (NNHCO−) carbons, respectively. The carbon
resonances at 65.70 and 64.10 ppm are respectively due to C-2
and C-4 carbons. Furthermore, the carbons involved in the
fusion of bicyclic part C-5 and C-1 were consigned from the
resonances at 39.50 and 45.90 ppm, respectively.
The assignment given for the respective carbons and protons
of compound 9 is further confirmed from the 2D NMR spectra
recorded for compound 12. The benzylic protons showed
correlation with N(3)−H in the HOMOCOSY spectrum. In
addition, H-2 and H-4 were found to be positively correlated
with H-1 and H-5, respectively. From this, benzylic protons and
bridgehead proton assignment is confirmed. Methylene protons
Structural Elucidation of Compound 9. Hydrazone
formation was supported by CN stretching frequency at
1641 and 1646 cm−1 in the IR spectrum. We have observed the
piperidine N−H stretching frequency around 3325−3375
cm−1. Respective aliphatic and aryl C−H stretchings were
found in the region of 3070−2800 cm−1.
a
Table 1. Analytical Data of Compounds 9−15
elemental analysis found (calculated) (%)
compound
molecular formula
molecular weight
C
H
N
9
C24H25N5OS
431.55
500.44
491.61
467.16
587.00
441.25
500.44
66.75 (66.80)
57.52 (57.60)
63.49 (63.52)
61.59 (61.65)
48.88 (48.91)
67.91 (67.94)
57.57 (57.60)
5.79 (5.84)
4.57 (4.63)
5.85 (5.95)
4.85 (4.96)
3.87 (3.93)
6.29 (6.36)
4.57 (4.63)
16.19 (16.23)
13.94 (13.99)
14.16 (14.25)
14.87 (14.98)
11.83 (11.88)
15.20 (15.24)
13.89 (13.99)
10
11
12
13
14
15
C24H23Cl2N5OS
C26H29N5O3S
C24H23F2N5OS
C24H23Br2N5OS
C26H29N5OS
C24H23Cl2N5OS
a
The observed microanalysis values for C, H, and N were within 0.4% of the theoretical values.
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a
Table 2. In Vitro Antibacterial and Antifungal Activities (MIC , μg/mL) of Compounds 9−15 by 2-fold Serial Dilution Method
bacterial strain
fungal strain
A. niger C. albicans
100 100
12.5 6.25
50
6.25
6.25
25
6.25
compound
B. subtilis
K. pneumoniae
E. coli
P. aeruginosa
S. aureus
A. flavus
Candida6
9
100
6.25
100
6.25
6.25
100
25
100
6.25
200
6.25
6.25
100
25
100
12.5
200
12.5
12.5
50
100
50
50
50
50
100
6.25
100
6.25
6.25
50
10
12.5
25
11
100
25
100
25
25
12
12.5
12.5
50
12.5
12.5
50
13
12.5
50
50
14
100
100
12.5
15
50
25
12.5
12.5
25
streptomycin
fluconazole
12.5
12.5
25
12.5
25
25
12.5
25
a
MIC, minimum inhibitory concentration.
at the 6-, 7-, and 8-positions of the bicyclic ring including the
downfield H-7a proton were confirmed from their mutual
correlations. Benzylic carbons, ipso carbons of aryl group, and
methylene carbons (C-6, C-7, and C-8) are assigned with
respect to their HSQC and HMBC correlations (refer to the
Supporting Information). Assignments for the other com-
pounds, 10−15, were made by comparison with compound 9.
X-ray crystallographic study confirmed that 2r,4c-diphenyl-9t-
ethynyl-3-azabicyclo[3.3.1]nonan-9t-ol exists in twin-chair
conformation with equatorial orientations of the phenyl
groups.26 Taken together, all of the above observations
substantiate the proposed structure and twin-chair (CC)
conformation of 2r,4c-diaryl-3-azabicyclo[3.3.1]nonan-9-one-
4-methyl-1,2,3-thiadazole-5-carbonyl hydrazones 9−15 (Table
2).
Biological Activity. Antibacterial Activity. Synthesized
compounds 9−15 were examined for their antibacterial
potencies. In vitro studies by 2-fold serial dilution method
were adopted. Streptomycin was used as a positive control.27
Table 1 shows the MICs of test compounds 9−15. Analysis of
in vitro antimicrobial effects of all the 2r,4c-diaryl-3-
azabicyclo[3.3.1]nonan-9-one-4-methyl-1,2,3-thiadazole-5-car-
bonyl hydrazones 9−15 revealed a diverse range (6.25−200
μg/mL) against the pathogens. The compounds deprived of
any substituents at the aryl rings in 9 hinder the growth of S.
aureus at a MIC value of 50 μg/mL and that of B. subtilis, K.
pneumoniae, E. coli, and P. aeruginosa at a MIC value of 100 μg/
mL. However, compounds 10, 12, and 13 possessing para halo
(electron-withdrawing substituents chloro, fluoro, and bromo)
substituted aryl groups in azabicyclononane moiety account for
the enhanced inhibitory effects against B. subtilis and K.
pneumonia at MIC values of 6.25 μg/mL and against E. coli at a
MIC value of 12.5 μg/mL when compared to the standard
antibiotic streptomycin. John Francis Xaiver et al. have also
documented that electron-withdrawing group (fluoro, bromo,
and chloro) substituted azabicyclononan-9-one derivatives
exhibited outstanding antibacterial and antifungal activities.28
Compound 15 with an ortho chloro substituent in the phenyl
moiety displays good antibacterial activity against B. subtilis, K.
pneumoniae, and P. aeruginosa (MIC = 25 μg/mL), E. coli (MIC
= 50 μg/mL), and S. aureus (MIC = 100 μg/mL). Among the
tested compounds 9−15, compounds 10, 12, and 13 exhibit
remarkable potencies against B. subtilis, K. pneumonia, and E.
coli, whereas these compounds showed lesser activity against P.
aeruginosa and S. aureus than standard streptomycin. Other
compounds displayed reduced inhibitory effects against various
bacterial strains compared to the standard streptomycin.
Antifungal Activity. The results of the present study also
provide evidence for the antifungal effects of an array of 2r,4c-
diaryl-3-azabicyclo[3.3.1]nonan-9-one-4-methyl-1,2,3-thiada-
zole-5-carbonyl hydrazones 9−15 with MIC values ranging
from 6.25 to 200 μg/mL (Table 1). Compound 9, which lacks
any substituents at the aryl groups, showed mild and
comparatively less antimicrobial activity against A. flavus (50
μg/mL), A. niger (100 μg/mL), C. albicans (100 μg/mL), and
Candida6 (100 μg/mL), compared to fluconazole, a known
antifungal agent used as positive control. Compounds 10, 12,
and 13 exerted 2-fold increased antifungal activity against A.
flavus, A. niger, and C. albicans at MIC values of 6.25−12.5 μg/
mL and show 4-fold increased activity against Candida6 at a
MIC value of 6.25 μg/mL when compared to the standard,
fluconazole. In addition, compound 15 exerted 2-fold increases
in antifungal activity against A. flavus, A. niger, C. albicans, and
Candida6 at MIC values of 6.25−25.0 μg/mL, whereas the
enhanced antifungal activity of compounds of 10, 12, and 13
may be due to the chloro, fluoro, and bromo substituents at the
para position of aryl groups. Furthermore, phenyl rings with
electron-donating methoxy and methyl groups at the para
position of compounds 11 and 14 hinder the growth of all
fungal strains at MIC values ranging from 25 to 100 μg/mL.
The results of the present study demonstrate that electron-
withdrawing groups at the para position of the aromatic ring in
the azabicyclononan-9-one moiety exert superior inhibitory
effect against various tested microbes compared to the other
test compounds and standard drug. Taken together, our
findings provide evidence that the type of functional groups and
the patterns of substitution on the azabicyclononan-9-one
moiety could be responsible for the substantial influence on the
antimicrobial activity of the hydrazone derivatives. The
antifungal and antibacterial activities of compounds 9−15
create promising leads for the development of potent
antimicrobial agents.
ASSOCIATED CONTENT
■
S
* Supporting Information
Spectral data (1H NMR, C13 NMR, H−H COSY, NOESY,
HSQC, and HMBC) for compound 12. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*(T.B.) E-mail: drtbalasankar@rediffmail.com. Fax: +91-4144
239141.
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dx.doi.org/10.1021/jf404537d | J. Agric. Food Chem. 2013, 61, 11952−11956

Journal of Agricultural and Food Chemistry
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank NMR Research Centre, IISC Bangalore, and
Department of Chemistry, Annamalai University, for recording
the NMR spectrum. We are grateful to CECRI, Karaikudi, for
providing analytical data. We extend our gratitude to the
RMMCH, Annamalai University, for the antimicrobial studies.
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