Synthesis and biological evaluation of novel angularly
fused polycyclic coumarins
Imtyaz A. Khan,
aManohar V. Kulkarni,
a,*M. Gopal,
bM. S. Shahabuddin
band Chung-Ming Sun
c,*a
Department of Chemistry, Karnatak University, Dharwad-580003, India
b
Department 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-ed metabolic processes has been recently reviewrelat-ed.2–4A
variety of coumarin derivatives with diverse substituents at C4 have been found to exhibit anti-coagulant,5
cytochrome P450 inhibiting,6anti-microbial7 and
anti-tumor8activities. 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 formation15and energy transfer in
photo-physical processes16, make them promising molecular
skeletons for further biological explorations. The
benzo-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).
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
Bioorganic & Medicinal Chemistry Letters 15 (2005) 3584–3587
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2005.05.063
* Corresponding author. Tel.: +886 35131511; fax: +886 38630110; e-mail:[email protected]
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
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.20The isolated yields of
the intermediates 3 and 4 were in the range of 70–80% after purification by crystallization. For the ring
Scheme 1. Synthesis of new angularly fused polycyclic coumarins.
Figure 1. Structures of biologically active pyridine-fused coumarins and benzofurans.
Figure 2. Novel angularly fused pentacyclic heterocycles.
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.
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.21was 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
the mean number of constrictions in control group and the mean number of constrictions in the treated group.
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. 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.
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.
References and notes
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Table 1. Biological activity of benzofuropyridinocoumarins 5
Compound R R1 Dose (mg/kg) % Inhibition of inflammation (3 h) % MPE 5a 7-CH3 –H 200 24.6 1.3 300 31.4 22.9 400 18.3 10.8 5b 7-CH3 –CH3 200 80.7 22.9 300 88.7 24.9 400 97 18.3 5c 7-CH3 –C2H5 200 35.4 2.4 300 41.7 4.9 400 46.5 7.1 5d 7-Cl –H 200 19.2 1.8 300 26.3 3.4 400 29.4 6.8 5e 7-Cl –CH3 200 21.6 4.7 300 25.9 7.3 400 27.4 9.2 5f 7-OCH3 –C2H5 200 35.7 24.3 300 78.1 24.4 400 67.2 39.5 5g 6-CH3 –H 200 89.0 20.9 300 96.3 32.9 400 67.2 20.9 Dicyclofenac sodium 100 94.5 NA
Acetyl salicylic acid 100 NA 74.9
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