Novel coumarin isoxazoline derivatives: Synthesis and study of antibacterial activities

Article abstract of DOI:10.1080/00397911.2016.1242748

A highly efficient and mild protocol for the syntheses of ethyl-3-[7-benzyloxy-4-methyl-2-oxo-2H-8-chromenyl]-5-aryl-4,5-dihydro-4-isoxazole carboxylates and ethyl-3-[7-benzyloxy-3-chloro-4-methyl-2-oxo-2H-8-chromenyl]-5-aryl-4,5-dihydro-4-isoxazole carboxylates in good yields via [3 + 2] cycloaddition of in situ–generated nitrile oxides from 7-benzyloxy-4-methyl-coumarin hydroxymoylchlorides and 7-benzyloxy-3-chloro-4-methyl-coumarin hydroxymoylchlorides respectively with ethyl-3-aryl prop-2-enoate has been developed. The new compounds are screened for antibacterial activity.

Full text of DOI:10.1080/00397911.2016.1242748

Novel Coumarin Isoxazoline Derivatives: Synthesis and Study of Antibaterial
Activities
1
2
1
3
Garbapu Suresh , Ratnakaram Venkata Nadh , Navuluri Srinivasu Kishore Kaushal
1
Division of Chemistry, Department of Science and Humanities, Vignan’s Foundation for
Science Technology and Research University, Guntur, India
2
GITAM University – Bengaluru Campus, Karnataka, India
3
API Process Research & Development, Dr. Roddy’s Laboratories Ltd, Hyderabad, India
Address Correspondence to: G. Suresh, Division of Chemistry, Department of Science
and Humanities, Vignan’s Foundation for Science Technology and Research University,
Guntur-522213, India Email: garbapusuresh@gmail.com
Abstract
A highly efficient and milder protocol for the syntheses of ethyl-3-[7-benzyloxy-4-
methyl-2-oxo-2H-8-chromenyl]-5-aryl-4,5-dihydro-4-isoxazole carboxylates and ethyl-3-
[
7-benzyloxy-3-chloro-4-methyl-2-oxo-2H-8-chromenyl]-5-aryl-4,5-dihydro-4-isoxazole
carboxylates in good yields via [3+2] cycloaddition of in situ generated nitrile oxides
from 7-Benzyloxy-4-methyl-coumarin hydroxymoylchlorides and 7-Benzyloxy-3-chloro-
4
-methyl-coumarin hydroxymoylchlorides respectively with ethyl-3-aryl prop-2-enoate
has been developed. The new compounds are screened against their antibacterial activity.
GRAPHICAL ABSTRACT
KEYWORDS: [3+2] cycloaddition, 1,3-dipolar cycloaddition, hydroximoyl chloride,
nitrile oxide, coumarin isoxazoline, antibacterial activity.

INTRODUCTION
Coumarins (2H-1-benzopyran-2-ones) are most important plant derived metabolites
[
1]
which exhibits an interesting biological and pharmacological activities . Coumarin
moiety is used as an important synthetic intermediate and plays a vital role in the
synthesis of numerous heterocyclic compounds. Most importantly it is well documented
that, large number of pharmaceutical drug products like Novobuocin (Antibiotic),
Ensaculin (Anti Dementia)) and Warfarin (Antithrombotic) are marketed widely in USA,
Canada and rest of the world which contains 7-hydroxy-4-methyl-2-coumarin as an
[
2]
important structural element . Coumarins containing heterocyclic moieties have
significant medicinal value due to their potential pharmacological activities such as
[3]
antibacterial activities, antifungal and anti-tuberculosis .
According to the prior art literature, compounds containing 2-isoxazoline rings
showed enormous biological activities such as antinociceptive, anticonvulsant,
[
4]
antipsychotic, anti-stress and analgesic effects . Moreover, isoxazoline derivatives have
played an important role, as valuable intermediates in most of the synthetic heterocyclic
pharmacological drug products. The traditional way for the synthesis of isoxazolines
involve the base catalyzed cyclocondensation of hydroxyl amine hydrochlorides and
chalcones. Hemalata Kour et.al teaches the production of isoxazoline coumarin
derivatives, which comprises reaction of various substituted chalcone derivatives with
[
5]
hydroxyl amines (free base) in the presence of refluxing methanolic NaOH solution .
Desai and his co-workers reported the synthesis of isoxazoline coumarin derivatives by
cyclocondensation of hydroxyl amine hydrochlorides and substituted chalcones in

[
6]
presence of refluxing methanolic NaOH solution for 7.5hours . Recently Imen Zghab
and his co-workers speculated the coumarin isoxazoline derivatives by cycloaddition of
nitrile oxides with suitable substituted ethyl cinnamates in the presence of triethylamine
[
7]
and refluxing toluene . All these synthesis methods are toilsome; it requires relatively
harsh reaction conditions to make coumarin isoxazoline derivatives.
1
,3-dipolar cycloaddition was the most effective method for the synthesis of five
[
8]
membered heterocycles which are difficult to be prepared by other means . The most
widely used method for the synthesis of isoxazolines is the 1,3-dipolar cycloaddition
[
9]
(
[3+2] cycloaddition) of nitrile oxides to activated dipolarophiles . Aryl nitrile oxides
are highly reactive and are predominantly generated in situ by either, dehydration of
[
10]
primary nitroalkane derivatives, Mukiyama reaction
followed by an in situ dehydrohalogenation using a base . The main advantage of the
,3-dipolar cycloaddition of aryl nitrile oxides to activated double bond is high
and halogenation of aldoxime
[
11]
1
regioselective with good yields. On the basis of this prior art evidence, we set out to
prepare a new series of biological active coumarin appended isoxazoline pharmacophores
1
1a-d and 12a-d via [3+2] cycloaddition.
As a part of our research for coumarin isoxazoline derivatives, we proposed to investigate
the behavior of coumarin derived aryl nitrile oxides 8 and 9, which are generated in situ
from hydroximoylchlorides (aldehyde chlorooximes) 6 and 7 under basic medium,
toward different substituted cinnamates (dipolarophiles) 10. This led to new series of
ethyl-3-[7-benzyloxy-4-methyl-2-oxo-2H-8-chromenyl]-5-aryl-4,5-dihydro-4-isoxazole

carboxylates 11a-d and ethyl-3-[7-benzyloxy-3-chloro-4-methyl-2-oxo-2H-8-
chromenyl]-5-aryl-4,5-dihydro-4-isoxazole carboxylates 12a-d in good yields. The
present study describes the synthesis, characterization and antibacterial activities of novel
coumarin isoxazoline derivatives.
RESULTS AND DISCUSSION
The required key intermediates for this study 7-benzyloxy-4-methyl-coumarin
aldehyde chlorooximes 6a-d and 7-Benzyloxy-3-chloro-4-methyl-coumarin aldehyde
chlorooximes 7a-d were prepared as shown in Scheme-1. The first step of the synthesis
involves the preparation of 7-hydroxy-4-methyl coumarins 2a-d via Pechmann
condensation of commercially available Resorcinols 1a-d and ethyl acetoacetate in the
[12]
presence of H SO
. Further treatment of 7-hydroxy coumarin derivatives 2a-d with
2
4
[
13]
hexamine in acetic acid undergo Duff formylation
methyl-coumarins 3a-d which were protected as benzyloxy derivatives 4a-d with benzyl
to give 8-formyl-7-hydroxy-4-
[14]
bromide in the presence of K CO in DMF . Compounds 4a-d was treated with
2
3
hydroxylamine hydrochloride in the presence of methanol and sodium acetate gets
[15]
converted in to oximes 5a-d . Finally, the key intermediates, coumarin derived
hydroximoyl chlorides 6a-d and 7a-d were synthesized by the chlorination of oximes 5a-
[
16]
d with equimolar quantities of N-chlorosuccinimide in DMF at 20-25°C . The obtained
syrupy mass on purification by column chromatography gave 7-benzyloxy-4-methyl-
coumarin aldehyde chloroximes 6a-d and 3-chlorinated products 7a-d as off white solids.
1
The absence of oxime proton at δ 11.23 in H NMR indicates the formation of
hydroxymoylchloride derivatives.

Based on our experimental results and in conjunction with known biological
properties for heterocycles as an integral part of the coumarin skeleton we chosen useful
and stable coumarin derived hydroximoyl chlorides to synthesize novel coumarin
appended isoxazoline derivatives as shown in Scheme-2. Treatment of coumarin derived
hydroximoyl chlorides 6a-d and 7a-d with triethylamine in dichloromethane at room
temperature undergo dehydrohalogenation and results in situ generation of coumarin aryl
[
17]
nitrile oxides 8a-d and 9a-d
0 (dipolarophiles) via [3+2] cycloaddition, yielded the corresponding coumarin
isoxazoline derivatives 11a-d and 12a-d. All these newly synthesized compounds were
reacted instantaneously with different ethyl cinnamates
1
1
13
purified by column chromatography and characterized by Mass, H NMR and C NMR.
To demonstrate the structure elucidation of the first series of coumarin isoxazoline
derivatives 11a-d we selected compound 11a which was obtained by the reaction of
equimolar quantities of 7-benzyloxy-4-methyl-coumarin hydroximoyl chloride 6a and
ethyl-3-phenylprop-2-enoate 10 in the presence of triethylamine in dichloromethane at
room temperature for 24 h. Its positive quasi molecular ion peak was observed at m/z
1
4
83.9 (M+H), compatible with the molecular formulae C H NO In H NMR of 11a
2
9
25
6.
the newly formed isoxazoline ring H-4′′ appeared at δ 4.68 (d, J = 8.8 Hz) and H-5′′ at δ
1
3
6
.07 (d, J = 8.8 Hz). In C NMR of 11a isoxazoline ring carbons appeared at δ 60.5 (C-
4
′′), 88.2 (C-5′′) and 158.7 (C-3′′).

To demonstrate the structure elucidation of the second series of coumarin isoxazoline
derivatives 12a-d we selected the compound 12a which was obtained by the reaction of
equimolar quantities of 7-benzyloxy-3-chloro-4-methyl-coumarin hydroximoyl chloride
7
a and ethyl-3-phenylprop-2-enoate 10. The compound 12a showed its quasi molecular
ion peaks at m/z 518.1 (M+H) and m/z 520.3 (M+H+2), with 3:1 isotopic peak intensities
ratio in mass spectrum confirmed the molecular formulae C H ClNO with one chlorine
2
9
24
6
atom. The newly formed isoxazoline ring proton H-4′′ appeared at δ 4.64 (d, J = 8.6 Hz)
1
3
and H-5′′ at δ 6.09 (d, J = 8.6 Hz). In C NMR of 12a isoxazoline ring carbons appeared
at δ 62.3 (C-4′′), 88.7 (C-5′′) and 159.7 (C-3′′).
Antibacterial Activity
[
18]
It is well known that coumarin isoxazoline derivatives possess antibacterial activity
.
Furthermore, halogen substituents were introduced into the basic structure anticipating an
improved biological activity because their incorporation was proved to be influencing the
[
19]
[20]
biological activity in various heterocycles
as well as coumarins . Electron
withdrawing groups like halogens will increase bactericidal potential as they alter the
[
21]
nature of the compound in such a way as to promote binding to the target(s)
.
According to Rajendra Prasad et al in designing the compounds bearing electron

withdrawing substituents (with high degree of binding linearity) results in high molecular
[
22]
weights to exhibit an improved antibacterial activity . Similarly, the significant
inhibition shown by substituted isoxazolines. Similarly, the significant inhibition shown
by substituted isoxazolines was attributed to substituents like hydroxymethyl / hydroxy /
[23]
methoxy / ethyl ester groups . Tejaskumar Shah and Vikas Desai also reported that the
presence of methoxy-, chloro-, and fluoro groups enhanced the antibacterial activity in
[
24]
isoxazoline derivatives
.
All these newly synthesized compounds 11a-d and 12a-d were evaluated for in vitro
antimicrobial activity against Gram-positive bacteria (Staphylococcus aureus –MTCC
9
6) and Gram-negative bacteria (Escherichia coli– MTCC 40) by agar disc diffusion
[
25]
method . The antibacterial screening data showed that most of the compounds 11a-d
and 12a-d showed moderate to excellent activities against the used microorganisms
(Table-2) compared to the reference drug (Chloramphenicol). These results suggested
that the introduction of halogen substituent increased the hydrophobicity of the
[
26]
synthesized compounds and lead to the increase of the antibacterial activity . In the
present study, it is observed that good activity was shown by the prepared derivatives
against the studied Gram-positive bacteria and very poor activity against Gram-negative
bacteria. The cell outer layers of Gram-positive and Gram-negative bacteria may explain
[
27]
the decent antibacterial activities of these prepared derivatives . The permeability of
drug constituents in Gram-positive bacteria is due to an ineffective and penetrable outer
barrier made of peptidoglycan layer. On the other hand, outer cell wall of Gram-negative
[
28]
bacteria consists multiple impermeable peptidoglycan and phospholipidic layers
.
Among the compounds screened, 12c showed high activity. The observed antibacterial

activity profile suggested that the presence of halogen functional group --Bromine had
enhanced the activity.
CONCLUSION
In conclusion, we have successfully achieved two important aspects in this work. One is a
highly practical method for the synthesis of novel coumarin appended isoxazoline
derivatives 11a-d and 12a-d in a good yield via [3+2] cycloaddition (1,3-diploar
cycloaddition) under mild reaction conditions. Second one is the antibacterial activities of
the synthesized compounds. Interestingly all these new compounds are active and
showing moderate to good antibacterial activity against Gram-positive bacteria and
Gram-negative. The observed antibacterial activity profile suggested that the presence of
Bromine had enhanced the activity.
EXPERIMENTAL
Melting points were determined in open glass capillaries on a Fisher–Johns melting point
1
13
apparatus and are uncorrected. NMR ( H 400 MHz; C 125 MHz) were recorded at room
temperature in CDCl as solvent and TMS as an internal standard (δ = 0 ppm), and the
3
values were reported in the following order: chemical shift (δ in ppm), multiplicity (s =
singlet, d = doublet, t = triplet, q = quartet, m = multiplet, qq = quartet of quartet),
coupling constants (J in Hz), and integration. All the reactions were monitored by thin
layer chromatography on precoated silica gel 60 F254 (mesh); spots were visualized
under UV light at 254 nm.

Typical Experimental Procedure For The Synthesis Of Hydroximoyl Chlorides (6a-
D) And (7a-D):
To a stirred solution of 7-benzyloxy-4-methyl-coumarin aldehyde oxime 5a (8.0g,
2
0
5.8 mmol) in DMF (200.0 mL) was added N-chlorosuccinimide (4.1g, 31.0 mmol) at
°C, raised to RT over 2 hours and quenched the reaction with ice cold water (600.0 mL),
extracted with ethyl acetate (3×200.0 mL). The combined organic layer was washed with
saturated NaCl solution (200.0 mL), dried over Na SO , concentrated and purified by
2
4
1
column chromatography to give 6a, 5.6 g (63%). H NMR spectrum (400 MHz, CDCl ),
3
δ, ppm (J, Hz): 2.41 (3H, s, 4-CH ); 5.37 (s, 2H, OCH C H ); 6.13 (s, 1H, H-3); 7.24-
3
2
6
5
7
1
.35 (5H, m, OCH C H ); 7.41 (1H, d, J = 8.8 Hz, H-6); 7.8 (1H, d, J = 8.8 Hz, H-6);
2 6 5
1
2.4 (1H, s, N-OH) and 7a, 2.6 g (27%); H NMR spectrum (400 MHz, CDCl ), δ, ppm
3
(J, Hz): 2.45 (3H, s, 4-CH ); 5.3 (s, 2H, OCH C H ); 7.34-7.46 (5H, m, OCH C H ); 7.8
3 2 6 5 2 6 5
(1H, d, J = 8.8 Hz, H-6); 7.9 (1H, d, J = 8.8 Hz, H-6); 12.4 (1H, s, N-OH).
Following the same procedure as depicted in a 6a and 7a, the other hydroximoyl
chlorides 6b-d and 7b-d were prepared from the corresponding aldehyde oximes.
Hydroximoyl chlorides are highly unstable and are readily prepared whenever required.
Typical Experimental Procedure For The Synthesis Of Coumarin Isoxazoline
Compounds (11a-D):
To a stirred solution of hydroximoyl chloride 6a (0.5g, 1.45 mmol) in
Dichloromethane (12.5 mL) and Et N (0.19g, 1.89 mmol) was added the corresponding
3
ethyl-3-phenyl-prop-2-enoate 10 (0.29g, 1.66 mmol) dissolved in Dichloromethane (12.5
9

mL) and the reaction mixture was stirred for 24 h at room temperature. Water (30.0 mL)
was added and extracted in to Dichloromethane (2x30.0 mL). The combined organic
layer was washed with saturated NaCl solution (60.0 mL), dried over Na SO , and
2
4
concentrated to give the crude isoxazoline which was purified by column
chromatography using silica gel in ethyl acetate: pet ether (6:4), obtained 11a, 0.46 g,
5.7 %, yield.
6
Following the same procedure as illustrated in 11a, the other coumarin isoxazoline
derivatives 11b-d were prepared from the corresponding hydroximoyl chlorides. The
physical, spectral and analytical data for these compounds are mentioned as follows.
Ethyl-3-[7-(Benzyloxy)-4-Methyl-2-Oxo-2H-8-Chromenyl]-5-Phenyl-4,5-Dihydro-4-
Isoxazole Carboxylate (11a)
1
Off white solid; Yield 0.46 g (66%); MR: 130-132°C; H NMR spectrum (400 MHz,
CDCl ), δ, ppm (J, Hz): 0.95 (3H, t, J = 7.2,OCH CH ); 2.42 (3H, s, 4-CH ); 3.95 (2H, q,
3
2
3
3
J = 7.2,OCH CH ); 4.68 (1H, d, J = 8.8 Hz, H-4′′); 5.21 (2H, s, -OCH C H ); 6.07 (1H,
2
3
2
6
5
d, J = 8.8 Hz, H-5′′); 6.29 (s, 1H, H-3); 6.91 (1H, d, J = 9.2 Hz, H-6); 7.23-7.38 (5H, m,
1
3
OCH C H ); 7.51-7.62 (5H, m, Ar); 7.72 (1H, d, J = 9.2 Hz, H-5); C NMR spectrum
2
6
5
(
125 MHz, CDCl ), δ, ppm: 13.7 (OCH CH ); 18.7 (CH ); 60.5 (C-4′′); 63.1 (OCH CH );
3 2 3 3 2 3
7
1
1
0.1 (OCH C H ); 88.2 (C-5′′); 103.9 (C-6); 104.2 (C-3); 112.6 (C-4a); 113.9 (C-8);
2 6 5
27.1 (C-2′,6′,4′,2′′′,6′′′,4′′′ Ar); 129.5 (C-3′,5′,3′′′,5′′′ Ar); 142.1 (C-5); 150.3 (C-1′′′, 1′);
53.5 (C-8a); 158.7 (C-3′′, 4); 166.9 (C-2,7); 167.8 (COOEt); DIPMS: m/z= 483.9
1
0

(
M+H); Elemental analysis: Found, %: C 71.92; H 5.18; N-2.81. C H NO . Calculated,
29 25 6
%: C 72.04; H 5.21; N 2.90.
Typical Experimental Procedure For The Synthesis Of 3-Chlorinated Coumarin
Isoxazoline Compounds (12a-D)
To a stirred solution of hydroximoyl chloride 7a (0.5 g, 1.32 mmol) in
Dichloromethane (10.0 mL), triethylamine (0.17 g, 1.71 mmol) was added the
corresponding ethyl-3-phenyl-prop-2-enoate 10 (0.26 g, 1.51 mmol) dissolved in
Dichloromethane (10.0 mL) and reaction mixture was stirred for 24 h at room
temperature. Added water (50.0 mL) and extracted in to Dichloromethane (2x50.0 mL).
The combined organic layer was washed with saturated NaCl solution (60.0 mL), dried
over Na SO , and concentrated to give the crude isoxazoline which was purified by
2
4
column chromatography using silica gel in ethyl acetate: pet ether (8: 2), obtained 12a,
.51 g, 74.5 % yield.
0
Following the same procedure as illustrated in 12a coumarin isoxazoline derivatives 12b-
d were prepared from the corresponding hydroximoyl chlorides. The physical, spectral
and analytical data for these compounds are mentioned as follows.
Ethyl-3-[7-(Benzyloxy)-3-Chloro-4-Methyl-2-Oxo-2H-8-Chromenyl]-5-Phenyl-4,5-
Dihydro-4-Isoxazole Carboxylate (12a)
1
Off white solid; Yield 0.51 g (75%); MR: 118-120°C; H NMR spectrum (400 MHz,
CDCl ), δ, ppm (J, Hz): 1.03 (3H, t, J = 7.3,OCH CH ); 2.45 (3H, s, 4-CH ); 3.97 (2H,
3
2
3
3
1
1

q, J = 7.3,OCH CH ); 4.64 (1H, d, J = 8.6 Hz, H-4′′); 5.24 (2H, s, OCH C H ); 6.09 (1H,
2
3
2
6
5
d, J = 8.6 Hz, H-5′′); 6.38 (1H, d, J = 9.1 Hz, H-6); 7.28-7.37 (5H, m, OCH C H ); 7.51-
2
6
5
1
3
7
.59 (5H, m, Ar); 7.81 (1H, d, J = 9.1 Hz, H-5); C NMR spectrum (125 MHz, CDCl ),
3
δ, ppm: 14.1 (OCH CH ); 18.9 (CH ); 62.3 (C-4′′); 64.7 (OCH CH ); 72.3 (OCH C H );
2
3
3
2
3
2
6
5
8
2
8
5
8.7 (C-5′′); 105.3 (C-6); 113.7 (C-4a); 115.9 (C-8); 119.3 (C-3); 122.5 (C-
′,6′,4′,2′′′,6′′′,4′′′ Ar); 123.5 (C-3′,5′,3′′′,5′′′ Ar); 142.6 (C-5); 148.2 (C-1′′′, 1′); 151.8 (C-
a); 154.2 (C-3′′); 159.7 (C-4); 163.8 (C-7); 166.9 (C-2,); 169.8 (COOEt); DIPMS: m/z=
18.1 (M+H), 520.3 (M+H+2); Elemental analysis: Found, %: C 67.02; H 4.67; N 2.69.
C H ClNO . Calculated, %: C 67.25; H 4.67; N 2.70.
29
24
6
Screening Of Antibacterial Activity
All these newly synthesized compounds 11a-d and 12a-d were evaluated for in vitro
antimicrobial activity against Gram-positive bacteria (Staphylococcus aureus –MTCC
9
6) and Gram-negative bacteria (Escherichia coli– MTCC 40) by agar disc diffusion
method. All the derivatives at the concentration of 50 μg/ml, 100 μg/ml and 200 μg/ml
were tested against Gram-positive bacteria (Staphylococcus aureus –MTCC 96) and
Gram-negative bacteria (Escherichia coli – MTCC 40). The molten nutrient agar was
inoculated with 100 μl of the inoculum (1 x 108 cfu/ml) and poured into the Petri plate,
the disc (5mm) (Hi-Media), was saturated with 100 μl of the test compound, allowed to
dry and was introduced on the upper layer of the seeded agar plate. The plates were
incubated at about 37ºC and microbial progress was determined by measuring the
diameter of zone of inhibition after 24 hours. Pure solvents were used as control and
inhibitory zones were nearly negligible compared to the inhibition zones of the samples.
1
2

To clarify any solvent effect on the biological screening, separate studies were carried out
using Dimethyl sulfoxide (DMSO) as a control and it showed no activity against any
bacterial strains. Chloramphenicol was used as standard drug for the purpose of
comparison of antibacterial activities of the derivatives. The antibacterial activities were
carried out in triplicate and average values were compiled.
SUPPLEMENTAL MATERIAL
1
13
Elemental analysis, Mass, H NMR, C NMR Spectra and characterization data for all
newly synthesized coumarin isoxazoline derivatives can be accessed on the publisher’s
website.
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6

Table-1. Yields of coumarin isoxazoline derivatives
Entry
R
R1
Product
11a
Yield* (%)
1
2
3
4
5
6
7
8
H
H
66
74
77
69
75
74
69
72
Cl
CF3
11b
11c
Br
CH3
H
CF3
3,5-difluoro
11d
12a
H
Cl
CF3
12b
12c
Br
CH3
CF3
3,5-difluoro
12d
*
yields are after column purification
1
7

Table-2. Antibacterial activity data of the synthesized coumarin isoxazoline derivatives
Comp.
Zone of inhibition (mm)*
Escherichia coli (MTCC 40)
Staphylococcus aureus (MTCC
96)
(Gram-negative)
(Conc. μg/ml)
(Gram-positive) (Conc. μg/ml)
2
00
100
11
12
20
12
11
13
24
22
30
50
5
200
17
18
29
21
22
24
31
24
33
100
12
13
21
19
18
18
29
22
30
50
5
1
1
1
1
1
1
1
1
1a
1b
1c
1d
2a
2b
2c
2d
15
18
27
15
22
11
28
22
8
10
19
4
19
11
6
7
14
20
16
21
26
24
20
23
Chloramphenicol 31
indicates average of triplicate
*
1
8

Scheme-1. Synthesis of coumarin derived hydroximoyl chlorides 6a-d and 7a-d.
1
9

Scheme-2. Synthesis of coumarin isoxazoline derivatives
2
0