Synthesis and biological evaluation of novel angularly fused polycyclic coumarins

Article abstract of DOI:10.1016/j.bmcl.2005.05.063

In a three-step sequence, an array of angularly fused polycyclic heterocycles with coumarin, benzofuran and pyridine rings were synthesized from 4-bromomethylcoumarins and salicylonitrile. All the final compounds were fully characterized and screened for anti-microbial, anti-inflammatory and analgesic activities. Several compounds exhibited promising inflammation inhibiting and anti-microbial properties.

Full text of DOI:10.1016/j.bmcl.2005.05.063

Bioorganic & Medicinal Chemistry Letters 15 (2005) 3584–3587
Synthesis and biological evaluation of novel angularly
fused polycyclic coumarins
Imtyaz A. Khan,a Manohar V. Kulkarni,a,* M. Gopal,b
M. S. Shahabuddinb and Chung-Ming Sunc,*
aDepartment of Chemistry, Karnatak University, Dharwad-580003, India
bDepartment of Biochemistry, Kuvempu University, PG Centre, Davangere-577441, India
cDepartment of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan
Received 23 March 2005; revised 12 May 2005; accepted 14 May 2005
Available online 20 June 2005
Abstract—In a three-step sequence, an array of angularly fused polycyclic heterocycles with coumarin, benzofuran and pyridine
rings were synthesized from 4-bromomethylcoumarins and salicylonitrile. All the final compounds were fully characterized and
screened for anti-microbial, anti-inflammatory and analgesic activities. Several compounds exhibited promising inflammation inhib-
iting and anti-microbial properties.
Ó 2005 Elsevier Ltd. All rights reserved.
Coumarins have been found to exhibit a wide range of
biological and controlled therapeutic activities in view
of their extensive occurrence in nature and low toxicity.1
Their potential for anti-inflammatory activity and relat-
furano[3,2-b]pyridines have been reported as potential
anti-allergic agents17 and endothelin receptor antago-
nists18 and the tetrahydropyridines have been identified
as potential anti-depressants19 (Fig. 1).
ed metabolic processes has been recently reviewed.2–4
A
variety of coumarin derivatives with diverse substituents
at C4 have been found to exhibit anti-coagulant,5
cytochrome P450 inhibiting,6 anti-microbial7 and anti-
tumor8 activities. Introduction of an azomethine linkage
at the allylic position with respect to the biogenetic
C3–C4 double bond in the form of oximes, amidines,
oxadiazoles, isoxazolines, etc., has resulted in com-
pounds with promising anti-proteolytic, anti-oxidant
and anti-inflammatory properties.9–11 To this end,
we have found that bi-heterocycles, such as 4-(2-
benzo[b]furanyl)coumarins and 4-aryloxymethylcouma-
rins from vanillin, exhibit good inflammation inhibiting
properties and high levels of molecular tolerance in ani-
mal models.12,13 Studies on the mode of the anti-bacte-
rial action of benzopyranopyridine esters14 have
revealed that they act via the DNA gyrase inhibition
pathway. The other properties of this skeleton, such as
DNA adduct formation15 and energy transfer in photo-
physical processes16, make them promising molecular
skeletons for further biological explorations. The benzo-
It was envisaged that the two tricyclic templates could be
modified into a pentacyclic system with a common pyri-
dine ring, which might have interesting biological func-
tions in animal model studies (Fig. 2). The present paper
reports the synthesis and preliminary biological evalua-
tion of a series of pyridine-fused benzofuranocoumarins
from various 4-bromomethylcoumarins.
The present synthetic sequence was initiated by an allylic
substitution of various 4-bromomethylcoumarins 1 with
salicylonitrile 2 giving rise to the o-cyano-4-pheno-
xymethylcoumarins 3 at ambient temperature (Scheme
1). An intramolecular carbanion addition across the
nitrile was brought about by refluxing ethers 3 in
ethanol in the presence of anhydrous potassium
carbonate, resulting in the formation of 3-amino-
benzofuranylcoumarins 4. Aromatisation is the driving
force for such an intramolecular condensation. The last
step in the sequence was the construction of the fused
pyridine ring by a C–C bond formation. An attempt
was first made to achieve this ring closure via an intra-
molecular approach using the N-acetyl and the N-ben-
zoyl derivatives 4a. Conversion to 4a was achieved
by the reaction of the amines 4 with acetyl or benzoyl
*
Corresponding author. Tel.: +886 35131511; fax: +886 38630110;
e-mail: drmuk274@yahoo.co.in
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.05.063

I. A. Khan et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3584–3587
3585
Figure 1. Structures of biologically active pyridine-fused coumarins and benzofurans.
detected. Alternatively, the [5 + 1] approach was then
attempted with orthoesters as one-carbon electrophiles.
The amines 4 were refluxed with excess orthoesters,
which resulted in a high yielding ring closure at the C3
position of coumarin. The driving force for such a facile
cyclisation is due, in part, to the built-in leaving group
abilities of the incipient alkoxy imines 4b. Another plau-
sible reason for the ring closure is the aromatic stability
of the resulting pentacyclic ring system. The formation
of the intermediates in the synthetic sequence and the
spectral properties of all the fused coumarins 5 were
consistent with their structures.20 The isolated yields of
the intermediates 3 and 4 were in the range of 70–80%
after purification by crystallization. For the ring
Figure 2. Novel angularly fused pentacyclic heterocycles.
chlorides. The high temperature (120–150 °C) reactions
of anilides 4a in polyphosphoric acid (PPA) led to the
hydrolysis of anilides and no cyclised product was
Scheme 1. Synthesis of new angularly fused polycyclic coumarins.

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I. A. Khan et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3584–3587
Table 1. Biological activity of benzofuropyridinocoumarins 5
the mean number of constrictions in control group
and the mean number of constrictions in the treated
group.
Compound
R
R1
Dose
% Inhibition
% MPE
(mg/kg) of inflammation
(3 h)
Among the compounds tested, we found that compound
5b showed significant inhibition of inflammation at 200
and 300 mg/kg doses, respectively. In the analgesic activ-
ity results, four compounds 5a, 5b, 5f and 5g were found
to give about one-third of the protection compared to
that of acetylsalicylic acid. The most promising com-
pound in the series was the 7-methoxy derivative 5f,
which showed 78% inhibition of inflammation at
300 mg/kg dose level and provided considerable protec-
tion against the acetic acid induced abdominal constric-
tions (Table 1). In the anti-microbial screening by the
cup-plate method,24 all the compounds had a MIC of
25 lg/ml against Pseudomonas chinchori, whereas at
the same concentration they were inactive against
Micrococcus aureus. In the anti-fungal screening, all
the compounds inhibited the growth of Aspergillus
fumigatus at 25 lg/ml, whereas against Penicillium wort-
manni they were active at a concentration of 100 lg/ml.
5a
5b
5c
5d
5e
5f
7-CH3 –H
200
300
400
24.6
31.4
18.3
1.3
22.9
10.8
7-CH3 –CH3 200
80.7
88.7
97
22.9
24.9
18.3
300
400
7-CH3 –C2H5 200
35.4
41.7
46.5
2.4
4.9
7.1
300
400
7-Cl
7-Cl
–H
200
300
400
19.2
26.3
29.4
1.8
3.4
6.8
–CH3 200
300
400
21.6
25.9
27.4
4.7
7.3
9.2
7-OCH3 –C2H5 200
35.7
78.1
67.2
24.3
24.4
39.5
300
400
In summary, we have described a three-step synthetic se-
quence for the synthesis of polycyclic-fused coumarins
by N–C–C3 annealation, a method which has a poten-
tial for wider applicability to create new fused hetero-
cyclic systems for their preliminary pharmacological
evaluation.
5g
6-CH3 –H
200
300
400
89.0
96.3
67.2
20.9
32.9
20.9
Dicyclofenac sodium
Acetyl salicylic acid
100
100
94.5
NA
NA
74.9
closure triethyl orthoformate, acetate and propionate
were employed to generate a library of 14 compounds.
The isolated yield in the last step was around 70–80%
after recrystallization from dioxane. Out of these, seven
compounds (5a–5g) containing the representative
groups R and R1 have been selected for the biological
activity screening.
Acknowledgments
One of the authors thanks the UGC-New Delhi for a
FIP-fellowship; the Chairman, Department of Chemis-
try, Karnatak University Dharwad, for providing facil-
ities; and NSC of Taiwan for financial support.
These were tested for their anti-inflammatory, analgesic
and anti-microbial activities. In the in vivo acute toxicity
experiments, all the compounds showed LD50 values
>800 mg/kg body weight. No tremors and convulsions
were observed upon inspection and a post mortem
examination revealed no haemorrhagic spots. To evalu-
ate the in vivo anti-inflammatory activity of the polycy-
clic coumarins, the carrageenan induced rat paw oedema
method described by Winter et al.21 was used employing
dicyclofenacsodium as the standard drug. The com-
pounds were administered as suspensions in 2% Tween
80. Albino rats (Wister strain) obtained from NIMH-
ANS-Bangalore were used in groups of six animals.
The paw volumes were measured using a plethysmome-
ter. The percentage of inhibition for inflammation was
calculated according to literature methods.22 The anal-
gesic activity was evaluated by acetic acid induced
abdominal constriction method using Swiss albino mice
of either sex (20–30 gm) and a group of six animals was
used. The analgesic response was assessed by counting
the number of abdominal constrictions for 20 min start-
ing 3 min after the injection of the acetic acid solution.
Analgesic activity was calculated23 as the percentage
maximum possible effect (% MPE) from the ratio of
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