Microwave-assisted synthesis, characterization, and antimicrobial activity of some odorant Schiff bases derived from naturally occurring carbonyl compounds and anthranilic acid

Article abstract of DOI:10.1080/00397911.2016.1245749

Nine odorant Schiff bases, namely 2-(4-methoxybenzylideneamino) benzoic acid, 2-(benzylideneamino) benzoic acid, 2-(3-phenylallylidene amino) benzoic acid, 2-(3,7-dimethyloct-2,6-enylideneamino) benzoic acid, 2-(3,7-dimethyloct-6-enylideneamino) benzoic a

Full text of DOI:10.1080/00397911.2016.1245749

Microwave Assisted Synthesis, Characterization and Antimicrobial Activity of Some
Odorant Schiff Bases Derived from Naturally Occurring Carbonyl Compounds and
Anthranilic Acid
Prasoon Kumar Kaushik1, V. K. Varshney1, Pawan Kumar2, Pallavi Bhatia3, S. V.
Shukla4
1Chemistry Division, Forest Research Institute, Dehra Dun, India,
2Chemical Sciences Division, CSIR-Indian Institute of Petroleum, Dehradun, India,
3Forest Pathology Division, Forest Research Institute, Dehra Dun, India,
4Fragrance & Flavor Development Center, Makrandnagar, Kannauj, India
Corresponding author V. K. Varshney, E-mail: vinay.varshney5@gmail.com
Abstract
Nine odorant Schiff bases namely 2-(4-methoxybenzylideneamino) benzoic acid, 2-
(benzylideneamino) benzoic acid, 2-(3-phenylallylidene amino) benzoic acid, 2-(3,7-
dimethyloct-2,6-enylideneamino) benzoic acid, 2-(3,7-dimethyloct-6-enylidene)
aminobenzoic acid, 2-(4-isopropylbenzylideneamino)benzoic acid, 2-(3,4-
dimethoxybenzylideneamino) benzoic acid, 2-(1-phenylethylideneamino) benzoic acid,
2-[(4-(2,6,6-trimethylcyclohex-2-enyl)-but-2-enylidene amino)benzoic acid were
prepared by condensation of anthranilic acid with corresponding naturally occurring
carbonyl compounds (anisaldehyde, benzaldehyde, cinnamaldehyde, citral, citronellal,
cuminaldehyde, veratraldehyde, acetophenone and α-ionone) employing conventional
and microwave irradiation methods. These compounds were characterized with the aid of
elemental and spectral (FT-IR, 1H NMR and 13C NMR) analysis. Microwave irradiation
method was efficient in terms of reduced reaction time, solvent use, and increased yields
of these compounds without affecting their olfactory characteristics. These Schiff bases
also exhibited olfactory characteristics for various fragrance compositions and varied
1

antimicrobial activity against Aspergillus niger, Penicillium chrysogenum,
Staphylococcus aureus and Escherichia coli.
Graphical Abstract
KEYWORDS: Schiff bases, anthranilic acid, olfactory property, antimicrobial activity,
microwave irradiation
INTRODUCTION
Nature has served as a guide towards the design of novel aroma molecules for their use in
various fragrance and flavour (F&F) applications [1]. In spite of the use of more than 3000
odoriferous compounds of diverse chemical structures in F&F industry, creation of new
odorant molecules in augmenting the aroma of compositions for perfumery, flavor or new
extended applications is intensive [2, 3]. Schiff bases (azomethines / anils / imines) having
azomethine group (-CH=N-) belong to a group of compounds which possess varied
biological activities including antimicrobial, anticancer, antineoplastic, antiviral,
antitumour, anti-HIV, antitubercular, anticonvulsant, anthelmintic, antiplatelet,
antimalarial, antibiotics, anti-inflammatory, diuretic, and analgesic [5-15]. Besides, Schiff
bases are also well known in flavoring and perfumery due to their odor characteristics [16]
.
2

Several Schiff bases are reported to exhibit floral, fruity odors, namely of the citrus type
and, more particularly, reminiscent of the odor of the orange-flower [17]. Schiff bases are
also known to be useful as intermediates in producing other fragrance materials [18, 19].
Primary amines and carbonyl compounds (aldehydes or ketones) are widely used as
substrates for the preparation of Schiff bases with biological and olfactory properties.
These carbonyl compounds occur widely in nature and constitute, due to their interesting
odoriferous properties, a novel group of raw materials for F&F industry. Microwave
(MW) assisted synthesis of organic compounds has acquired significant importance over
conventional organic synthesis in terms of reduced solvent consumption and reaction
time, improved yields and easier workup matching with green chemistry approach [20]
Several Schiff bases have been synthesized using microwave irradiation and
.
characterized [21-25]
.
In the present paper, we report synthesis of nine odorant Schiff bases of naturally
occurring carbonyl compounds (five aromatic aldehydes - anisaldehyde, benzaldehyde
cinnamaldehyde, cuminaldehyde, vertraldehyde; two acyclic terpene aldehydes - citral,
citronellal; an aromatic acetophenone, and a cyclic terpenoid ketone- α-Ionone) (Figure
1) with anthranilic acid by conventional and MW assisted methods, and their
characterization through elemental analysis and spectral (FT-IR, 1H NMR and 13C NMR)
data. Their olfactory and antimicrobial properties were also examined. Anisaldehyde (4-
methoxy benzaldehyde), veratraldehyde (3,4-dimethoxybenzaldehyde), acetophenone
(methyl phenyl ketone) and α-Ionone (4-(2,6,6-triethylcyclohex-2-enyl) but-3-ene-2-one)
3

occur in many essential oils. Benzaldehyde is the main, characteristic component of
bitter almond oil. Cinnamaldehyde (3-phenyl-2-propenal) is the chief component of
cassia oil (ca. 90 %) and Sri Lanka cinnamon bark oil (ca. 75%). Cuminaldehyde (4-
isopropylbenzaldehyde) is a constituent of the essential oils of eucalyptus, myrrh, cassia,
cumin and others. Citral (3, 7-dimethyl-2,6-octadien-1-al) occurs in lemon-grass oil (up
to 85 %) and in Litsea cubeba oil (up to 75 %) while citonellal (3,7-dimethyl-6-octen-1-
al) is a constituent of citronella oil (upto 45%), Backhousia citriodora oil (upto 80%) and
Eucalyptus citriodora oil (upto 85%) [26]. Anthranilic acid (2-aminobenzoic acid) occurs
in oats and being a component of the major biochemical pathway of bacteria, plant and
animals is also the starting material for several other types of naturally occurring
compounds.
Though, Schiff bases of acetophenone, anisaldehyde, benzaldehyde, cinnamaldehyde,
citral, and citronellal with anthranilic acid have been prepared by conventional
condensation method for their use as ligands for synthesis of metal complexes [27-30], to
the best of our knowledge this is the first report on the MW assisted synthesis of the
Schiff bases derived from acetophenone, anisaldehyde, benzaldehyde cinnamaldehyde,
citral, citronellal, cuminaldehyde, α-Ionone, and vertraldehyde with anthranilic acid.
Their olfactory properties and antimicrobial activity are also reported for the first time.
RESULTS AND DISCUSSION
Nine odorant Schiff bases 2-(4-methoxybenzylideneamino) benzoic acid (Sb1), 2-
(benzylideneamino) benzoic acid (Sb2), 2-(3-phenylallylidene amino) benzoic acid (Sb3),
4

2-(3,7-dimethyloct-2,6-enylidene)amino benzoic acid (Sb4), 2-(3,7-dimethyloct-6-
enylidene) aminobenzoic acid (Sb5), 2-(4-isopropylbenzylideneamino)benzoic acid
(Sb6), 2-(3,4-dimethoxybenzylideneamino) benzoic acid (Sb7), 2-(1-
phenylethylideneamino) benzoic acid (Sb8), 2-[(4-(2,6,6-trimethylcyclohex-2-enyl)-but-
2-enylidene amino)benzoic acid (Sb9) (Figure 2) were prepared from naturally occurring
seven aldehydes (anisaldehyde, benzaldehyde, cinnamaldehyde, citral, citronellal,
cuminaldehyde, veratraldehyde) and two ketones (acetophenone and α-Ionone) with
anthranilic acid by conventional and MW assisted methods. A large number of
experiments were performed to optimize the irradiation power and its duration and the
optimized conditions are reported. MW irradiation resulted in no solvent consumption,
reduced reaction time from 4 to 14 minutes, and improved yields from 46% to 89% over
conventional method.
The Schiff bases were characterized by analysis of the elements (C, H and N), FT-
IR, 1H NMR and 13C NMR spectral data. Elemental analysis of the Schiff bases was in
agreement to their molecular formula assigned. The FT-IR spectra of the free aldehydes
(anisaldehyde, cinnamaldehyde, cuminaldehyde, benzaldehyde, citronellal, citral,
vertraldehyde), and ketones (acetophenone and α-Ionone) have a strong band at 1690-
1700 cm-1 (due to carbonyl stretch), two bands of moderate intensity in the region 2700-
2800 cm-1 (due to H-C=O stretch in aldehydes only), while two moderate bands at 3500
and 3580 cm-1 corresponding to N-H stretch were observed in the IR spectrum of
anthranilic acid. In the IR spectra of the Schiff bases, these bands disappeared and a new
band appeared in the range of 1613-1658 cm-1 attributed to the v(C=N) mode of
5

azomethine linkage showing the condensation between the -CHO / >C=O group of the
carbonyl compounds and amino group of anthranilic acid resulting in the formation of the
respective Schiff base. The characteristic phenolic v(O-H) mode due to presence of a
hydroxyl group in these Schiff bases were observed around 3478-3331 cm-1
The 1H NMR spectra of the Schiff bases Sb1-Sb8 and Sb9 exhibit singlet of integration
intensity equivalent to one hydrogen at 8.37-8.78 ppm and three hydrogens at 2.99 ppm
attributed to to (-CH=N-) and to (-CH3 -C=N-) protons, respectively. A singlet at 9.78-
9.98 ppm due to aromatic hydroxyl proton was observed in the spectra of all the Schiff
bases. A broad signal at 8.3-8.4 ppm due to free NH2 protons in the spectra of anthranilic
acid was absent in the observed spectra of the Schiff bases which indicated the formation
of the azomethine linkage. The multiplets within the 6.62-7.93 ppm range were assigned
to protons belonging to aromatic rings. The spectra of Sb1 and Sb7 showed a singlet with
an integration equivalent to three hydrogens at 3.90 and 3.98 ppm, respectively
corresponding to the methoxyl protons. In 13C NMR spectra, azomethine and carboxyl
carbons of the Schiff bases appeared between 160.1-165.2ppm and 168.2-169.7 ppm,
respectively. Methoxy carbons of Sb1 and Sb7 were observed at 55.6 and 56.2 ppm,
respectively. Methyl carbons of the isopropoyl moiety in Sb4, Sb5 and Sb6 were found
at 19.6, 25.6; 19.6, 25.6 and 23.4 ppm, respectively. Vinylic carbons of Sb3 emerged at
119.7 and 138.5 ppm. The NMR spectral assignments of the Schiff bases support the
inferences drawn from the elemental and IR studies.
OLFACTORY EVALUATION
6

Schiff bases were evaluated for their olfactory properties in terms of their odor profile
and odor strength and data are shown in Table 1.
A smaller odor diversity was observed between the Schiff bases of the aldehydes and
ketones. The Schiff bases possessed an interesting profile, described as sweet floral
differed in the notes and the strength, except Sb2 and Sb6 which had bitter almond and
herbaceous sweet very tenacious profile, respectively. Schiff bases Sb8 and Sb9 derived
from the ketones exhibited similar odor profile differed, however, in the strength. Though
Sb1 of benzaldehyde and Sb7 of veratraldehyde had similar odor pofile but differed in the
notes (Sb1 with fruity undertone) and the strength (Sb1 of medium and Sb7 of low
strength) which could be linked to the presence of another methoxy group in Sb7.
Difference in the notes of Sb4 of citral (orange blossom to fresh orange peel) and Sb5 of
citronellal (fruity) could be explained on the basis of an additional double bond in citral
than that in the citronellal.
Odor notes of the synthesized Schiff bases were natural and relative to those of the
previously reported Schiff bases. The olfactory properties of these Schiff bases indicated
their use in fragrance compositions of the varied nature.
Antimicrobial Activity
In search of new and effective antimicrobial agents of natural origin, natural product
derived Schiff bases have shown promise [15, 32]. Madurahydroxy lactones, occurring in
Actinomadura rubra, derived Schiff bases were effectively inhibited Bacillus subtilis,
7

Micrococcus flavus, Sarcina lutea, and S. aureus with MIC values ranged from 0.2 to 3.1
µg/mL [33]. Chitosan, isatin and nystatin-dextran-derived Schiff bases reported to exhibit
antifungal activity against Botrytis cinerea andColletotrichum lagenarium [34]
,
Microsporum audouinii, and Microsporum gypseum [35], and Candida albicans and C.
neoformans [36], respectively.
Antimicrobial activity of the synthesized Schiff bases was, therefore, examined against
two bacteria - Staphylococcus aureus (gram +ve) and Escherichia coli (gram –ve), and
two fungi- Aspergillus niger and Penicillium chrysogenum. All the Schiff bases (Sb1-
Sb9) showed moderate to good activity against the pathogenic organisms tested. The
solvent used for the preparation of compound solutions (DMSO) did not show inhibition
against the tested organisms (negative control). Compared to the positive control,
compounds showing the zones of inhibition between 9-16 mm and > 17 mm were
considered to be moderately active and highly active. Percent of activity index of tested
compounds against the microorganisms are also reported (Table 2).
The MIC values (Table II) of the highly active compounds ranged between 20-30 μg/ml
with 59.25-83.33% activity index while those for moderately active compounds were
40μg/ml with 39.58-77.08% activity index. Compounds Sb5, Sb6, Sb7, and Sb9 found
highly active while compound Sb1, Sb2, and Sb4 found moderate active against all four
microorganisms tested. Compound Sb3 was highly active against the fungi with 70.83%
activity index at 30μg/ml and moderately active against bacteria with 70.37-80% activity
index at 40μg/ml, however compound Sb8 was moderately active against the fungi with
8

75-77.08% activity index at 40μg/ml and highly active against bacteria with 74.07-82%
activity index at 30μg/ml.
CONCLUSION
Microwave irradiation proved to be advantageous for preparation of nine odorant Schiff
bases of naturally occurring seven aldehydes (anisaldehyde, benzaldehyde, citral,
citronellal, cinnamaldehyde, cuminaldehyde, veratraldehyde) and two ketones
(acetophenone and α-Ionone) with anthranilic acid in terms of reduced reaction duration
(4-14 minutes), no solvent consumption and increased yield (13% - 20%) over
conventional method. Elemental analysis, FT-IR 1H NMR and 13C NMR data supported
the structures of these compounds. Olfactory attributes of these Schiff bases were not
influenced by the microwave irradiation and indicative of their use in various fragrance
compositions. These compounds showed moderate to good antimicrobial activity against
bacteria - Staphylococcus aureus and Escherichia coli, and fungi- Aspergillus niger and
Penicillium chrysogenum.
EXPERIMENTAL
General
All chemicals obtained from commercial suppliers were of highest purity grade and used
without further purification. These include acetophenone (Himedia), anisaldehyde (Loba
chemie), anthranilic acid (Sigma aldrich), benzaldehyde (Merck), cinnamaldehyde
(Himedia), citral (Himedia), citronellal (Merck), cuminaldehyde (Sigma aldrich), α-
ionone (Sigma Aldrich), vertraldehyde (Himedia), Nutrient Agar Media (Himedia) and
9

Potato Dextrose Agar (Himedia). Melting points were measured on open glass capillary
method on Buchi Melting Point (b-540) and are uncorrected. Elemental analysis of
carbon, nitrogen and hydrogen were carried out on CHN elemental analyser (Thermo
Scientific). Infrared (IR) spectra were recorded at room temperature from 4000 cm-1 to
500 cm-1 with KBr pellets at a resolution of 4cm-1, using Thermo-Nicolet 8700, Research
spectrometer (Thermo Scientific). 1H NMR (500 MHz) and 13C NMR (125 MHz) spectra
were recorded on Bruker Avance-II instrument using CDCl3 as solvent and
tetramethylsilane (TMS) as an internal slandered. Chemical shifts (δ) are reported in
ppm. The microwave assisted synthesis was performed in scientific microwave oven,
(SRL-Milestone Start S Labstation for synthesis, (Operating between 140-1600W). All
the reactions were carried out at power 300-500W.
Olfactory Evaluation
Olfactory properties of the Schiff bases were studied employing standard olfactory
method [31]. Sample solutions (10 wt. % in ethyl alcohol) were used to evaluate their
odour properties.
Antimicrobial Activity
Antimicrobial activity of the Schiff bases was evaluated according to the guidelines of
the National Committee for Clinical Laboratory Standards (NCCLS, 1997) using the agar
disc diffusion method. The bacterial cultures for Staphylococcus aureus (ATCC 259235)
and Escherichia coli (ATCC 27853) were obtained from Microbial Type Culture
Collection (MTCC), Chandigarh and fungal cultures of Aspergillus niger and Penicillium
10

chrysogenum were obtained from National Type Culture Collection (NTCC), Forest
Research Institute, Dehradun.
DETERMINATION OF MIC
MIC values were determined by testing performed according to the guidelines of NCCLS
(Tuite, J., 1969). Solutions of the test compounds, ciprofloxacin and griseofulvin were
prepared in DMSO at different concentrations of 10, 20, 30, 40, 50, 60 μg/ml to
determine the MIC. All determinations were done in triplicate and found the same result.
ACKNOWLEDGEMENT
Authors are thankful to the Director, Forest Research Institute, Dehradun, India and Dr.
Suman L. Jain, Scientist, Chemical Sciences Division, CSIR-Indian Institute of
Petroleum, Dehradun for providing necessary facilities to carry out this work.
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Table 1. Olfactory properties of the Schiff bases
Sl. No. Schiff base Odor profile
Odor strength
A.
1.
2.
3.
4.
Aldehyde and Anthranilic acid
Sb1
Sb2
Sb3
Sb4
Sweet, floral with fruity undertones
Medium
Medium
Medium
Intense
Bitter almond
Sweet, floral, honeysuckle
Sweet floral, orange blossom to fresh orange
peel
5.
6.
7.
B.
8.
9.
Sb5
SB6
Sb7
Very tenacious, sweet floral, fruity
Herbaceous sweet very tenacious
Sweet-floral
Intense
Medium
Low
Ketone and Anthranilic acid
Sb8
Sb9
Heavy, sweet-floral
Medium
Intense
Heavy and very sweet-floral
17

Table 2. Zone of inhibition, MIC and % activity index of odorant Schiff bases
Schiff base
Bacteria
Zone of
inhibition
(MIC)
Fungi Zone of
*% Actevity % Actevity index
inhibition (MIC)
index
(fungi)
(bacteria)
E.
S.
A.
P.
E.
S.
A.
P.
coli aureu nige chrysogenu
col aureu niger chrysogenu
s
r
m
i
s
m
Sb1
Sb2
Sb3
Sb4
Sb5
14. 15.0
12.0 12.0 (40)
(40)
56 55.55 50.0 50.00
0
0
(40)
(40)
11. 11.0
9.5 10.0 (40)
(40)
44 40.74 39.5 41.66
8
0
(40)
(40)
20. 19.0
17.0 17.0 (30)
(30)
80 70.37 70.8 70.83
3
0
(40)
(40)
19. 18.5
18.5 18.0 (40)
(40)
76 68.51 77.0 75.00
8
0
(40)
(40)
19. 20.0
19.0 20.0 (20)
(20)
76 74.07 79.1 83.33
6
0
(20)
(20)
18

Sb6
Sb7
Sb8
15. 16.0
18.5 18.0 (30)
(30)
60 59.25 77.0 75.00
8
0
(20)
(20)
19. 20.0
16.0 16.5 (20)
(20)
78 74.07 66.6 68.75
6
5
(30)
(30)
20.0
18.0 18.5 (40)
(40)
82 74.07 75.0 77.08
0
20. (30)
5
(30)
Sb9
19. 19.0
19.0 20.0 (30)
(30)
76 70.37 79.1 83.33
6
0
(20)
(20)
Ciprofloxaci 25. 27.0
-
-
-
-
-
-
n
0
(20)
(20)
-
Griseofulvin
-
24.0 24.0 (20)
(20)
-
-
-
-
-
-
-
-
DMSO
-
-
-
-
1 - 8 mm = less active; 9 - 16 mm = moderate active; >17 mm = highly active
*% Activity index = Zone of inhibition by test compound (diameter) / Zone of inhibition
by standard (diameter) x 100
19

Figure 1. Naturally occurring carbonyl compounds
20

Figure 2. Structures of the synthesized odorant Schiff bases
21