Synthesis, characterization, and biological activity of organophosphates derived from substituted benzoxazole

Article abstract of DOI:10.1002/hc.20551

The reaction of phosphorus oxychloride/phosphorus thiochloride with 2-(2'-mercaptophenyl)benzoxazole in 1:1, 1:2, and 1:3 molar ratio in the presence of stoichiometric amounts of triethylamine has afforded a series of organophosphates. These organophospha

Full text of DOI:10.1002/hc.20551

Heteroatom Chemistry
Volume 20, Number 5, 2009
Synthesis, Characterization, and Biological
Activity of Organophosphates Derived from
Substituted Benzoxazole
Mahesh Kumar Samota∗, Priyanka Jhajharia, and Gita Seth
Department of Chemistry, University of Rajasthan, Jaipur 302 004, India
Received 11 August 2008; revised 11 August 2008
erocyclic compound, benzoxazole is structurally re-
ABSTRACT: The reaction of phosphorus oxychlo-
lated to biologically important bases and are used
ride/phosphorus thiochloride with 2-(2ꢀ-mercapto-
in research as a starting material for the synthesis
phenyl)benzoxazole in 1:1, 1:2, and 1:3 molar ratio
of bioactive compounds [1,2]. Benzoxazole has been
in the presence of stoichiometric amounts of triethyl-
used in household as cosmetics and medicine. It is
amine has afforded a series of organophosphates.
found within the chemical structures of pharmaceu-
These organophosphates have been characterized by
tical drugs such as “flunoxaprofen.” The aromaticity
elemental analyses, infrared, 1H NMR, 31P NMR,
of benzoxazole makes it stable. Although as a hete-
and mass spectral studies. The antifungal activ-
rocycle, it has reactive sites that allow for functional-
ity of these organophosphates has been evalu-
ization. Benzoxazole derivatives are used primarily
ated against pathogenic fungi Aspergillus niger
as antibacterial and antifungal agents [3,4].
and Fusarium oxysporium. The antifungal screen-
Organophosphates are used in agriculture, in
ing data reveal that these compounds are more
the home, in gardens, and in veterinary purposes
fungitoxic than 2-(2ꢀ-mercaptophenyl) benzoxazole.
[5–8]. Organophosphates exert their biological ac-
These organophosphates were also found to be in-
tion on insects and mammals by attacking the neu-
secticidal when tested against Periplenata ameri-
ral transmission system and inhibiting the function
ꢁ
C
cana.
2009 Wiley Periodicals, Inc. Heteroatom Chem
of acetylcholinesterase enzyme [9]. Organophos-
phorus compounds containing a heterocyclic moi-
ety increase the protonation at the site of pesti-
cides and enhance their biological activity [10]. The
numerous applications of organophosphates pro-
moted us to undertake the synthesis of phospho-
rylated/thiophosorylated derivatives of 2-substituted
benzazoles [11,12]. In continuation of our previous
work on phosphorylated/thiophosorylated deriva-
tives of 2-substituted benzazoles [13–16], we re-
port herein the synthesis, characterization, and bi-
ological activity of organophosphates derived from
2-substituted benzoxazole.
20:309–315, 2009; Published online in Wiley InterScience
(www.interscience.wiley.com). DOI 10.1002/hc.20551
INTRODUCTION
Heterocyclic compounds were involved at the very
beginning of life in the genesis of DNA and have
played an essential role in many living systems. The
nucleic acid bases are derivatives of the aromatic
N-heterocycles pyrimidine and purine. Being a het-
Correspondence to: Mahesh Kumar Samota; e-mail: samota27@
yahoo.co.in.
Contract grant sponsor: Council of Scientific and Industrial
Research (CSIR), New Delhi, India.
RESULTS AND DISCUSSION
The
reactions
of
phosphorus
oxychloride
Contract grant number: 9/149 (325)/2K3-EMR-I.
2009 Wiley Periodicals, Inc.
c
ꢀ
(POCl3)/phosphorus thiochloride (PSCl3) with
309

310 Samota, Jhajharia, and Seth
FIGURE 1
2-(2ꢀ-mercaptophenyl)benzoxazole in 1:1, 1:2, and
1:3 molar ratio in the presence of stoichiometric
amounts of triethylamine in dry tetrahydrofurane
(THF)/CH2Cl2 yielded the phosphorylated and
thiophosphorylated benzoxazole derivatives 2a–b,
3a–b, and 4a–b (Table 1). A schematic presentation
of these reactions is given in Fig. 1.
All these reactions are quite facile and completed
in 12–14 h of refluxing. The resulted compounds are
hygroscopic that are soluble in dimethylformamide
and dimethylsulfoxide.
3a–b, 4a–b), suggesting the deprotonation of the
SH group and the formation of P S C bonds.
These gets support by the appearance of new bands
in the region 585–542 cm−1 due to νP S C, 1265–
1250 cm−1 due to νP O, 815–812 cm−1 and 700–
690 cm−1 due to νP S (I), and νP S(II) bonds,
respectively [17]. The organophosphates (2a–b)
exhibit absorption bands at ∼530–510 cm−1 and
∼610–600 cm−1 due to νs (P Cl) and νas (P Cl) vi-
brations, respectively [18].
1H NMR and 13P NMR Spectra
IR Spectra
1
The characteristic signals in H NMR and 31P NMR
The tentative assignments of some important bands
are summarized in Table 2. The absorption band
of medium intensity at 2570 cm−1, due to the SH
group of 2-(2ꢀ-mercaptophenyl)benzoxazole, disap-
peared in the spectra of organophosphates (2a–b,
spectra of 2-(2ꢀ-mercaptophenyl) benzoxazole (1a)
and organophosphates (2a–b, 3a–b, 4a–b) are sum-
1
marized in Table 3. The H NMR spectrum of 1a
shows SH (thiophenolic) proton signal appeared
at δ4.54 ppm and aromatic protons appeared at
Heteroatom Chemistry DOI 10.1002/hc

Synthesis, Characterization, and Biological Activity of Organophosphates Derived from Substituted Benzoxazole 311
δ6.8–7.8 ppm. The absence of SH (thiophenolic)
proton signal in these organophosphates indicates
deprotonation of the thiophenolic group of 1a
and formation of the P S C bonds. All these
organophosphates show multiplets in the region
δ6.9–8.0 ppm attributable to the aromatic protons.
The 31P NMR spectra of these organophosphates ex-
hibit signals at δ62.5–76.5 ppm, indicating the pres-
ence of a tetracoordinated phosphorus atom [19,20].
Mass Spectra
Mass spectra were recorded for determining the
molecular mass of the organophosphates. The split-
ting pattern of the mass spectrum of compound
(C13H8NOS)P(O)Cl2 (2a) is represented in Fig. 2. The
molecular ion peak appears at m/z 342.16, thus con-
firming the formation of compound 2a. This peak
after removal of two chlorine radicals generates the
ion at m/z 272.02. It may further lose the radical PO,
and a peak appears at m/z 225.1. After removal of a
sulfur radical, a peak appears at m/z 192.18, which
splits off a phenyl moiety producing an ion at m/z
115.14. It may further lose the ion at m/z 102. This
peak after removal of a cyanide molecule generates
the ion at m/z 77. Relative abundance of the ions is
represented in Table 4.
onud
Clacd)/F
(
)
(
Aanlyis
Antifungal Activity
The results of antifungal activity of 1a and
organophosphates (2a–b, 3a–b, and 4a–b) are pre-
sented in Table 5. The antifungal screening data
lro
reveals that these organophosphates are more fun-
gitoxic than 1a. The enhanced activity of these
organophosphates may be ascribed to the increased
lipophilic nature of these compounds. It is also
noted that sulfur-containing compounds are more
active than their oxygen-containing counterparts.
The fungicidal activity increased as the concentra-
tion was increased.
C
)
(
edl
i
Y
C
(
.
Insecticidal Activity
mp.
The results of the insecticidal activity of 1a and
organophosphates (2a–b, 3a–b, and 4a–b) have been
compared with conventional insecticide Malathion,
taken as standards (Table 6). The results reveal that
these organophosphates are more toxic than 1a, and
their insecticidal activity increases with the increase
in the concentration. Organophosphates (2b, 3b,
and 4b) having P S bond resulted in higher toxi-
city than 2a, 3a, and 4a having P O bond, with the
same substituents attached to phosphorus.
Clacd)
(
onud
F
W.t
M.
Cpmound
Heteroatom Chemistry DOI 10.1002/hc

312 Samota, Jhajharia, and Seth
TABLE 2 IR Spectral Data (cm−1) of (1a) and Organophosphates (2a–b, 3a–b, and 4a–b)
Compound
ν(S H)
ν(P S C)
ν(P O)
ν(P S)
ν(P Cl)
1a
2a
2570
585
1265
590 (asymmetric)
518 (symmetric)
600 (asymmetric)
510 (symmetric)
510
2b
3a
3b
565
545
575
545
542
690 (II)
1260
1250
495
680 (II)
700 (II)
4a
4b
565
560
EXPERIMENTAL
metrically by the Volhard method. Phosphorous was
estimated as ammonium phosphomolybdate. The
molecular weights were determined by the Rast cam-
phor method.
All the chemicals used were of reagent grade. Phos-
phorus oxychloride (POCl3)/phosphorus thiochlo-
ride (PSCl3) were purchased from Fluka. All the sol-
vents were dried and distilled before use. Melting
points were determined by the capillary method and
are uncorrected. All operations involving phospho-
rus compounds were carried out in dry equipment
under nitrogen atmosphere. The IR spectra of all
the compounds were recorded as KBr disks on a
Shimadzu 8400 S FTIR spectrophotometer in the
Synthesis of 2- Substituted Benzoxazole
2-(2ꢀ-Mercaptophenyl)benzoxazole (1a) was pre-
pared by reacting the equimolar amounts of
o-aminophenol and thiosalicylic acid in freshly
prepared polyphosphoric acid. The reaction was
carried out according to the procedure reported in
the literature [21,22].
range 4000–425 cm−1. The H NMR spectra were
1
recorded on a JEOL FX 90Q/JEOL AL 300 MHz FT
NMR spectrometer in DMSO-d6/CDCl3 using TMS
as an internal reference. The 31P NMR spectra were
recorded on a JEOL AL 300 MHz FT NMR spec-
trometer at 121.49 MHz in DMSO-d6/CDCl3 using
TMS and 85% H3PO4 as internal and external refer-
ence, respectively, at room temperature. FAB mass
spectra were recorded on a JEOL SX 102/ DA-6000
mass spectrometer. Nitrogen was estimated by the
Kjeldahl method, and sulfur was estimated by the
Messenger method. Chlorine was estimated volu-
Synthesis of (C13H8NOS)P(O)Cl2 (2a) and
(C13H8NOS)P(S)Cl2 (2b)
2-(2ꢀ-Mercaptophenyl)benzoxazole (1a) (0.001 mol)
in dry THF (30 mL) and triethylamine (0.001 mol)
in dry THF (20 mL) were taken in a flame-dried
three-necked round-bottom flask, and temperature
was kept at 0◦C by keeping the flask in ice bath. To
this mixture, a solution of POCl3/PSCl3 (0.001 mol)
in dry THF (30 mL) was added dropwise at 0◦C.
After mixing the reactants, stirring was continued for
4 h at 0◦C. Furthermore, the reaction was removed
from the ice bath and refluxed under nitrogen atmo-
sphere for 14–16 h with continuous stirring. Then
it was cooled and filtered through a closed sintered
TABLE 3 1H NMR and 31P NMR Spectral Data (δ, ppm) of
1a and Organophosphates (2a–b, 3a–b, and 4a–b)
1H NMR
Aromatic Proton
31P NMR
Compound
SH
1a
2a
2b
3a
3b
4a
4b
4.54 (bs)
6.8–7.6 (m)
7.0–7.9 (m)
6.9–7.2 (m)
7.1–7.6 (m)
6.9–7.9(m)
7.3–7.8 (m)
6.9–8.0 (m)
TABLE 4 Mass Spectral Data of 2a
62.5
65.8
64.6
68.5
65.8
76.5
Compound
m/z(%)
2a
342.16 (10.53%), 306.66 (9.24%), 272.02
(9.40%), 225.1 (5.6%), 192.18 (14.6%), 115.14
(23.5%), 102 (89%), 77 (45%)
bs = broad singlet; m = multiplets.
Relative abundance is given in parentheses.
Heteroatom Chemistry DOI 10.1002/hc

Synthesis, Characterization, and Biological Activity of Organophosphates Derived from Substituted Benzoxazole 313
FIGURE 2
funnel to separate Et3N·HCl formed during the re-
action. The filtrate was concentrated under reduced
pressure and was kept for crystallization in vacuum
desiccators for 2 days. The product was recrystal-
lized in ethanol and dried in vacuo.
three-necked round-bottom flask, and temperature
was kept at 0◦C by keeping the flask in ice bath. To
this mixture, a solution of POCl3/PSCl3 (0.001 mol)
in dry THF (30 mL) was added dropwise at 0◦C. The
reaction was carried out in a similar manner as de-
scribed above.
(C13H8NOS)2P(O)Cl (3a) and
(C13H8NOS)2P(S)Cl (3b)
2-(2ꢀ-Mercaptophenyl)benzoxazole (1a) (0.002 mol)
in dry THF (30 mL) and triethylamine (0.002 mol)
in dry THF (20 mL) were taken in a flame-dried
(C13H8NOS)3P(O) (4a) and
(C13H8NOS)3P(S) (4b)
2-(2ꢀ-Mercaptophenyl)benzoxazole (1a) (0.003 mol)
in dry THF (30 mL) and triethylamine (0.003 mol)
Heteroatom Chemistry DOI 10.1002/hc

314 Samota, Jhajharia, and Seth
TABLE 5 Antifungal Screening Data of 1a and Organophosphates (2a–b, 3a–b, and 4a–b)
Inhibition after 72 h (%)
Aspergillus niger
Fusarium oxysporium
Compound
50 (ppm)
100 (ppm)
200 (ppm)
50 (ppm)
100 (ppm)
200 (ppm)
1a
2a
2b
3a
3b
4a
4b
Bavistin
38.0
58.2
47
62
78
81.32
80.06
82.36
78.35
83
40
48
63.33
64
69.01
73.39
78
82.78
93
64
74.26
76
78.39
84.64
92.06
97.24
100
64.66
68.02
64.98
71
69
78.06
89
60.12
64.01
66
72.80
66.73
73
63.46
60.14
64.22
64.78
68.42
77
100
75
in dry THF (20 mL) were taken in a flame-dried
three-necked round-bottom flask, and temperature
was kept at 0◦C by keeping the flask in ice bath. To
this mixture, a solution of POCl3/PSCl3 (0.001 mol)
in dry THF (30 mL) was added dropwise at 0◦C. Then
the reaction was carried out in a similar manner as
describe above. The results of analytical and physical
studies are summarized in Table 1.
ium oxysporium. The medium was then poured into
the Petri dish/Petri plates and a small disc (0.7 cm) of
the fungus culture was cut with a sterile cork borer
and transferred aseptically to the center of a Petri
dish, containing the medium with a certain amount
of the compound (1a, 2a–b, 3a–b, and 4a–b). Con-
trol setup without compounds was also maintained,
and a standard fungicide (Bavistin) was also main-
tained. These Petri dishes were wrapped in poly-
thene bags containing a few drops of dimethylfor-
mamide and were placed in an incubator at 25
2◦C. Three replicates were used in each case. The
colony diameter, after 72 h, compared with control
was taken as a measure of fungi toxicity. The per-
BIOLOGICAL ACTIVITY
Antifungal Activity
The antifungal activity of 1a and organophosphates
(2a–b, 3a–b, and 4a–b) was evaluated by the radial
growth method using potato dextrose agar medium
having the composition glucose 20 g, starch 20 agar–
agar 20 g- and distilled water 1000 mL. To this
medium, the requisite amount of the compound, af-
ter being dissolved in dimethylformamide (DMF) so
as to obtain certain final concentrations (50, 100 and
200 ppm), was added. The organisms used in these
investigations included Aspergillus niger and Fusar-
centage inhibition was calculated as 100(C–T)C−1
,
where C and T are the diameters of the fungus colony
in the control and test plates, respectively (Table 5).
Insecticidal Activity
The insecticidal effect of 1a and organophosphates
(2a–b, 3a–b, and 4a–b) has been evaluated on
the mortality rate of Periplenata americana (adult
TABLE 6 Insecticidal Activity of 1a and Organophosphates (2a–b, 3a–b, 4a–b)
Mortality (%)
24 (h)
48 (h)
72 (h)
20
40
60
20
40
60
20
40
60
Compound
(μg/cm2)
(μg/cm2)
(μg/cm2)
(μg/cm2)
(μg/cm2)
(μg/cm2)
(μg/cm2)
(μg/cm2)
(μg/cm2)
1a
2a
2b
3a
3b
4a
4b
20
55
62
58
63
60
65
80
28
78
84
80
90
82
92
95
35
85
89
86
94
88
96
99
25
76
84
78
80
77
82
88
30
87
92
92
93
91
95
99
38
92
95
93
95
95
98
100
30
78
89
80
86
82
88
90
35
88
92
92
96
90
97
99
40
92
96
94
98
95
99
100
Malathion
(standard)
Heteroatom Chemistry DOI 10.1002/hc

Synthesis, Characterization, and Biological Activity of Organophosphates Derived from Substituted Benzoxazole 315
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and topical method. The effect of different con-
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ACKNOWLEDGMENTS
[13] Pareek, S.; Vyas, Sushma,; Seth, G.; Vyas, P. C.
Heteroatom Chem 2008, 19, 154.
[14] Samota, M. K.; Kumar, A.; Kaur, J.; Mittal, S.; Seth,
G. Phosphorus Sulfur Silicon 2006, 181, 1919.
[15] Mittal, S.; Samota, M. K.; Kaur, J.; Seth, G. Phos-
phorus Sulfur Silicon 2006, 181, 2843.
The authors are thankful to the Head, Department
of, Chemistry, University of Rajasthan, Jaipur, India,
for providing necessary facilities to carry out this
research work.
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