2.1 Combination of thiazole ring and coumarin nucleus

Thiazoles have been extensively used in various chemical reactions as a parent, substituent as well as an intermediate (Siddiqui et al., 2009). This heterocycle has also been reported for the synthesis of various thiazolyl coumarin derivatives (Arshad et al., 2011). The present review highlights the synthesis of various carboxamides and Schiff’s bases of thiazolyl coumarin. A series of carboxamide derivatives of 2′-amino-4′-(6-bromo-3-coumarinyl) thiazole 1 showing analgesic and anti-inflammatory activities were reported (Venugopala and Jayashree, 2003), Fig. A.2, Table B.1. Title compounds 2–3 were obtained by treating the parent amine 1 with different aromatic acid chlorides in pyridine medium and the acetyl derivative of 1 was prepared by treating the same with acetyl chloride. Parent amine 1 was prepared by the reaction between 3-bromoacetyl-6-bromocoumarin and thiourea. The former was prepared by bromination of 3-acetyl-6-bromocoumarin in alcohol free chloroform. 3-acetyl-6-bromocoumarin was prepared by mixing appropriate molar quantities of 5-bromosalicyaldehyde and ethylacetoacetate in the presence of piperidine as a catalyst. Further, three series of substituted Schiff’s bases of aminothiazolyl coumarins were prepared and screened for their analgesic and anti-inflammatory activities, Figs. A.3–5, Table B.1. Schiff’s bases of aminothiazolyl coumarin 7–9 (Jayashree et al., 2004), aminothiazolyl bromocoumarin 10a–m (Venugopala and Jayashree, 2004) and aminothiazolyl chlorocoumarin 12–14 (Jayashree et al., 2005a,b), were synthesised by refluxing 2′-amino-4′-(3-coumarinyl) thiazole 6, 2′-amino-4′-(6-bromo-3-coumarinyl) thiazole 1 and 2′-amino-4′-(6-chloro-3-coumarinyl) thiazole 11 with different aldehydes in absolute ethanol as medium, respectively. Parent amines, such as compounds 6, 1 and 11 were prepared by cyclising 3-acetylbromo-6H-coumarins 5, 3-bromoacetyl-6-bromocoumarin and 3-bromoacetyl-6-chlorocoumarin with thiourea, respectively. Bromination of 3-acetylcoumarin 4 (Munshi et al., 2004) in alcohol free chloroform yielded the compound 5. 3-acetylcoumarin 4 was prepared by mixing appropriate molar quantities of salicylaldehyde and ethylacetoacetate in the presence of piperidine as catalyst. Further, the variation in reaction time with respect to different substituents for formation of compounds belonging to the series 10a–m was also reported (Venugopala and Jayashree, 2004). It was observed that, the order of reactivity of reactants when correlated with the reaction time required for product formation is given as; 10i at lowest (60 min) < 10b, 10f and 10k (90 min) < 10a, 10c, 10d, 10g, 10h, 10j, 10l and 10m (120 min) < 10e (150 min).

In addition to this, the synthesis of some of the Schiff’s bases of aminothiazolyl bromocoumarin 10a–m was also attempted by microwave irradiation on 2′-amino-4′-(6-bromo-3-coumarinyl) thiazole 1 with substituted aromatic aldehydes in absolute ethanol at different time intervals (Venugopala and Jayashree, 2008). This method served as an alternative method for enhancing the rate and efficiency of the reaction. Further, comparison of the rate of product formation for all three methods namely, the conventional refluxing method (method A), conventional heating method (method B) and microwave-induced organic reaction enhancement method (MORE, method C) was done. It was found that, products were formed within 65–113 s at 260 W in case of method C when compared with method A and method B, illustrating the efficiency of microwave technique over conventional techniques.

Moreover, synthesis of substituted 2′-arylamino-4′-(3-coumarinyl) thiazoles and 2′-arylamino-4′-(6-bromo-3-coumarinyl) thiazoles, 17a–u was also performed by condensation of 3-bromoacetylcoumarin with thiourea and substituted phenylthioureas (Venugopala et al., 2004), respectively, Fig. A.6, Table B.1. Initially, substituted salicylaldehydes were reacted with ethylacetoacetate in the presence of piperidine, to give 3-acetyl-6-H/bromocoumarins 5, 15, respectively. These on reaction with bromine in alcohol free chloroform yielded the respective 3-bromoacetyl-6-bromocoumarin 16.

2.2 Combination of triazole-thiadizine ring and coumarin nucleus

Triazoles such as 1, 2, 4-triazole have been reported for various pharmacological activities like anti-bacterial (Bhat et al., 2001), anti-viral and anti-fungal (Al-Masoudi et al., 2006). A few triazolyl coumarins have been reported so far along with their biological activities (Shi and Zhou, 2011). Further, some thiaziadinyl coumarins possessing biological activity were also reported (Rao and Reddy, 2009). It was thought worthwhile to bring linkers on the existing coumarin nucleus with other heterocyclic system and in our studies we have tried using triazole-thiadiazinyl ring system, Fig. A.7, Table B.1. A series of novel triazolo-thiadiazinyl bromocoumarin derivatives 18a–k (Jayashree et al., 2005a,b), triazolo-thiadiazinyl chlorocoumarins 19a–k (Jayashree et al., 2006) and triazolo-thiadiazinyl coumarins 20a–l (Jayashree et al., 2007) were prepared and reported. Here, substituted triazole and 3-bromoacetyl-6-bromocoumarin 16, 3-bromoacetyl-6-chlorocoumarin and 3-acetylbromo-6H-coumarins 5 were refluxed with dimethyl formamide in absolute ethanol medium and catalytic pyridine for about 120 min, respectively, to yield title compounds. For the synthesis of 1, 2, 4-triazoles, substituted aromatic acids were converted to their corresponding methyl esters, acid hydrazides, potassium dithiocarbamate salts as intermediates.

2.3 Preparation of coumarinyl chalcones

Chalcones or 1, 3-diaryl-2-propen-1-ones consist of open-chain flavonoids in which two aromatic rings are joined by a three-carbon α, β-unsaturated carbonyl system. Various substituted natural and synthetic chalcones have shown to exhibit significant pharmacological potential (Sahu et al., 2012). In our research, coumarinyl chalcones were prepared and screened for anti-bacterial activity, as shown in Fig. A.8, Table B.1. They were also used as intermediates for further synthesis of other test compounds. Synthesis of coumarinyl chalcones was undertaken by means of both conventional as well as microwave method (Jayashree et al., 2008). Title compounds 21–22 were obtained by refluxing as well as by microwave irradiation of substituted 6-(H/chloro/bromo) acetylcoumarins and different aromatic aldehydes with ethanol in the presence of piperidine. Different 6-(substituted) acetylcoumarins (Chopra et al., 2006) were synthesised by means of Pechmann reaction, involving the reaction of 5-(substituted) salicylaldehydes with ethylacetoacetate in the presence of piperidine, at freezing temperature for 120–180 min. The application of microwave energy to achieve coumarinyl chalcones significantly reduced the reaction time from 420 min of reflux to 2–3 min of microwave heating along with improved product yield.

On the contrary, preparation of some novel coumarinyl chalcones 23a–f was achieved by reaction between 3-(substituted) acetylcoumarins with different aromatic aldehydes in the presence of 30% ethanolic sodium hydroxide (Jayashree et al., 2009).

2.4 Combination of pyrazoline ring and coumarin nucleus

Substituted pyrazolines have been extensively reviewed for a range of biological activities (Shaaban et al., 2012). Pyrazolinyl coumarins have also been synthesised and reported for their biological activities (Khode et al., 2009). Synthesis of some of the 5-(substituted phenyl)-1-phenyl (2-pyrozoline-3″-yl) substituted coumarins was attempted (Jayashree et al., 2008), as shown in Fig. A.9, Table B.1. The title compounds 24–25 were achieved by refluxing different substituted coumarinyl chalcones 21–22 with phenyl hydrazine in the presence of piperidine as catalyst in ethanol for about 600 min approximately equivalent to 10 h of conventional heating.

2.5 Combination of pyridine ring and quinoline with coumarin nucleus

Pyridine fused coumarins have been synthesised and tested for biological activities such as anti-cholinesterase and anti-microbial (Alipour et al., 2012). We reported the synthesis of some of the useful heterocyclic substitutions on coumarin with pyridine and quinoline, Fig. A.10, Table B.1. 3-coumarinoyl pyridinium bromides 26a–l and 3-coumarinoyl quinolinium bromides 27a–c were prepared by 2 h of refluxing followed by reaction at room temperature for 4–5 h in the presence of dry toluene between 3-bromoacetylcoumarins and methyl and ethyl esters of nicotinic acid and isonicotinic acid to give 26a–l and with quinoline to give 27a–c (Porwal et al., 2010). Also, preparation of a series of substituted pyridinyl coumarins 28a–g was reported (Jayashree et al., 2010). Few of the compounds from the synthesised 26a–l series were taken in glacial acetic acid and ammonium acetate and refluxed at 130 °C for 6 h with different chalcones to obtain 7 derivatives of pyridinyl coumarins 28a–g.

2.6 Other substituted coumarin analogues

Synthesis of novel coumarin analogues such as 29–32 was also carried out (Jayashree et al., 2011), as shown in Fig. A.11, Table B.1. Analogues such as 29a–b, 31a–b and 32a–b were obtained by benzoylation of coumarins using different acid chlorides in dry pyridine as catalyst. The parent coumarins were obtained by reaction of different substituted resorcinols with redistilled ethylacetoacetate in the presence of concentrated sulphuric acid, initially for 2 h at low temperatures but later maintained at room temperature for 18 h. Carboxamide analogues 30a–b were obtained by reacting amino coumarin with 5-amino salicylic acid. Amino coumarin was prepared by conversion of 6-nitro-3-acetylcoumarin in aqueous ammonia solution by aqueous sodium hydrosulfite, at boiling temperature for 15 min. The former was prepared by treating 5-nitrosalisaldehyde and ethylacetoacetate in absolute alcohol in the presence of catalytic piperidine.

Alternatively, an attempt was made to generate functionally diverse coumarins with the help of microbiology technique, namely biotransformation (Das and Rosazza, 2006). Here, instead of using the conventional synthetic route for obtaining coumarin, it was thought to undertake certain functional group modifications/bioconversions of simple coumarin substrates using micro-organisms. As a result, experiments on the microbial biotransformation of some of the hydroxy and ethoxy substituted coumarins were performed using different bacterial and fungal strains. The microbial biotransformation of hydroxy and ethoxy substituted coumarin substrates under suitable fermentation conditions, led to their successful bioconversion into methoxy and hydroxy substituted coumarin derivatives, respectively (Nigam et al., 2013).

3.1 Analgesic and anti-inflammatory

The analgesic and anti-inflammatory activities of synthesised test compounds were evaluated in in vivo models using the acetic acid-induced abdominal constriction method (Collier et al., 1968) and the carrageenan induced rat hind paw oedema method (Kulkarni et al., 1986). Series of carboxamides of 2′-amino-4′-(6-bromo-3-coumarinyl) thiazole 1–3, were screened for analgesic and anti-inflammatory activities (Venugopala and Jayashree, 2003). Among all carboxamide derivatives tested, it was found that compounds bearing substitutions at meta, ortho and para positions such as 2c, 2e and 2h showed analgesic activity at 41.66% better than that of the standard acetylsalicylic acid at 37.45%. The effect of the nitro group on the analgesic activity was more pronounced at meta position among all derivatives screened. Whereas, compounds bearing chloro and bromo substitutions were found to be more effective as analgesics, when substituted at ortho and para positions in 2′-amino-4′-(6-bromo-3-coumarinyl) thiazole carboxamides. However, overall effect of substitutions clearly suggested the order of increase in analgesic activity with meta> para> ortho position when compared to that of the parent carboxamide 2a. Further, test compounds such as 1 and 3 showed analgesic activity at 6.95% and 26.53%, respectively when compared to their carboxamide analogues 2a–j at 32.36–41.66%. From this study, it was clearly understood that, the substituted aryl carboxamide derivatives of 2′-amino-4′-(6-bromo-3-coumarinyl) thiazole were more potent as analgesics than their counterparts. Alternatively, the anti-inflammatory activity of carboxamide derivatives 2a–j was found to be feeble when compared with the parent compound 1 and carboxamide 3, showing anti-inflammatory potential at 46.96% and 53.59% respectively against the standard phenylbutazone at 45.30%. However, among the carboxamides of 2′-amino-4′-(6-bromo-3-coumarinyl) thiazole the anti-inflammatory activity was found to be relatively better for meta> para> ortho substituents. Further, compounds with bromo and chloro substitutions on the meta position such as 2g and 2j showed activity better than the meta-nitro derivative 2c and remaining test compounds. It was also found that, the N-acetyl derivative of carboxamide 3 showed relatively lower analgesic and anti-inflammatory activities when compared with that of other substituted aryl carboxamide derivatives of 2′-amino-4′-(6-bromo-3-coumarinyl) thiazole.

Further, when three series Schiff’s bases of 2′-amino-4′-(3-coumarinyl) thiazole 6 were screened for analgesic and anti-inflammatory activities (Jayashree et al., 2004; Jayashree et al., 2005a,b and Venugopala and Jayashree, 2004), it was found that chloro and bromo substitutions at the 6th position on the parent aminothiazolyl coumarin 6 enhanced the analgesic potential of their Schiff’s bases when compared with Schiff’s bases of the parent compound 6. The analgesic activity of Schiff’s bases of 2′-amino-4′-(6-H/chloro/bromo-3-coumarinyl) thiazoles was found to be in the order with 6-chloro substituted 12a–m–14 >6-bromo substituted 10a–m >6-H/unsubstituted 7a–j–9. Further, findings also revealed for the improvement in the analgesic activity of Schiff’s bases of 2′-amino-4′-(6-substituted-3-coumarinyl) thiazole presumably because of the retention of different substituents at ortho, meta and para positions as compared to the 2′-amino-4′-(6-H/unsubstituted-3-coumarinyl) thiazole series. Among the para and meta di-substituted Schiff’s bases of aminothiazolyl coumarin, the test compound 12f, a di-methoxy derivative of the parent 2′-amino-4′-(6-chloro-3-coumarinyl) thiazole 11 showed analgesic activity better than test compounds 10l and 7f, which were also di-methoxy derivatives, but from 2′-amino-4′-(6-bromo-3-coumarinyl) thiazole 1 and 2′-amino-4′-(3-coumarinyl) thiazole 6, respectively. Further, the test compounds such as 8–9 and 13–14 were found to show feeble analgesic activity compared to that of the substituted aryl Schiff’s bases of aminothiazolyl coumarin. The test compound 12e, showing more than two-fold activity than that of the standard analgesic, exhibited maximum analgesic activity among the Schiff’s bases of aminothiazolyl coumarin, thereby highlighting the effect of tri-methoxy substitution at ortho, para and meta positions of the aryl group of 2′-amino-4′-(6-chloro-3-coumarinyl) thiazole nucleus. However, the trend observed for the anti-inflammatory activity of the Schiff’s bases of substituted aminothiazolyl coumarin was paradoxical. Here, the Schiff’s bases of 2′-amino-4′-(6-bromo-3-coumarinyl) thiazole 10a–m were found to be more active than 6-chloro substituted 12a–m–14 and 6-H/unsubstituted 7a–j–9 analogues. Further, all the substituted aryl derivatives of 2′-amino-4′-(6-bromo-3-coumarinyl) thiazole except, the test compound 10k, an unsubstituted aryl derivative of the parent, showed anti-inflammatory activity more than that of the standard phenylbutazone. However, the most active anti-inflammatory derivative among all Schiff’s bases of aminothiazolyl coumarin was found to be the compound 10b, an ortho, para, meta tri-methoxy aryl derivative of 2′-amino-4′-(6-bromo-3-coumarinyl) thiazole. The screening of anti-inflammatory and analgesic activities of different series of synthesised Schiff’s bases of aminothiazolyl coumarin, clearly illustrated that, the substitution made with the tri-methoxy group at ortho, meta and para positions of the aryl Schiff’s bases of 2′-amino-4′-(6-substituted-3-coumarinyl) thiazoles as in case of test compounds 10b and 12e, greatly increased the analgesic as well as anti-inflammatory potential of the respective scaffolds. Further, chloro and bromo substitutions at the 6th position of the 2′-amino-4′-(3-coumarinyl) thiazole, displayed improved analgesic and anti-inflammatory activities over their unsubstituted counterparts. Thus, from variation in response to the analgesic and anti-inflammatory activities of these compounds, it was understood that, the Schiff’s bases of aminothiazolyl coumarin would interact better with different molecular targets involved in understanding analgesic and anti-inflammatory pathways.

In the quest to develop potential NSAIDs, evaluation of series of substituted 2′-arylamino-4′-(3-coumarinyl) thiazoles and 2′-arylamino-4′-(6-bromo-3-coumarinyl) thiazoles, 17a–u in terms of analgesic and anti-inflammatory activities was done (Venugopala et al., 2004). It was observed that the substitution of the bromo group at the 6th position of the 2′-arylamino-4′-(3-coumarinyl) thiazoles did not bring any significant change in their analgesic as well as anti-inflammatory activities as compared to the standard ibuprofen at 74% activity and diclofenac sodium at 72.98% activity. Therefore, it was understood that the substituted aryl amino coumarinyl thiazoles were not effective ligands for these activities. However, from other studies it was found that, the series of their Schiff’s bases 12a–m–14, 10a–m and 7a–j–9 had proved to be much better ligands for analgesic and anti-inflammatory activities. Few of the coumarinyl chalcones were also tested for analgesic and anti-inflammatory activities (Jayashree et al., 2009). These compounds were also randomly subjected to anti-oxidant activity using the DPPH radical scavenging method but, none of the compounds exhibited anti-oxidant potential. In contrast, among the different 6-(chloro/bromo/H)-aryl substituted coumarinyl chalcones, test compounds 21k and 21g bearing chloro and bromo groups on the 6th position of benzopyrone nucleus were not just equally active as analgesics but were also highly active at 92.30% activity as compared to both the standard diclofenac at 74% and 6-H/unsubstituted derivatives 21b and 22a at 76–78%. However, randomly selected chalcones screened for anti-inflammatory activity did show activity but, none of the coumarinyl derivatives were nearly active as anti-inflammatory standard ibuprofen at 64.70%. It could be suggested from the screening of coumarinyl chalcones that the halogen substitution at the 6th position of benzopyrone nucleus had a greater influence on the analgesic activity than unsubstituted analogues. It can be understood that these chalcones may be acting through mutually exclusive molecular pathways involved for the three activities.

Screening of few of the 5-((substituted phenyl)-1-phenyl-2-pyrozoline-3″-yl)-6-halogen substituted coumarins for analgesic and anti-inflammatory activities (Jayashree et al., 2008), such as 24c and 25a both at the 6th position unsubstituted derivatives, has shown anti-inflammatory activity comparable to the standard aspirin. Moreover, test compounds such as 24c, 24e, 24l and 25a–b exhibited potent analgesic activity as compared to the standard aspirin. This scaffold of substituted coumarin consisting of 5-(substituted phenyl)-1-phenyl-2-pyrozoline-3″-yl)-6-halogen could be thereof undertaken for further modifications to arrive at potentially active analgesics and anti-inflammatory agents.

3.2 Anti-bacterial activity

The evaluation of anti-bacterial activity of substituted coumarins synthesised was performed in vitro by measuring the zone of inhibition using the agar diffusion method (Di Cuollo et al., 1965) and also by measurement of minimum inhibitory concentration (MIC) (Ericsson et al., 1960), using two fold dilutions in 96-well plate, against some of the gram positive strains like Bacillus subtilis and Staphylococcus aureus and gram negative strains like Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa.

Schiff’s bases of amino thiazolyl bromocoumarins were tested for anti-bacterial activity against B. subtilis and E. coli (Venugopala and Jayashree, 2008). The MIC of all test compounds showed promising activity and the compound 10a, equally active as the standard ampicillin exhibited MIC in the range of 141–147 μg and the compound 10g exhibited weak activity at 280 μg when compared to that of the standard ampicillin at 145 μg and 135 μg against B.subtilis and E. coli, respectively. Schiff’s bases of such 2′-amino-4′-(6-bromothiazolyl bromo-3-coumarinyl) thiazole may not be suitable candidates as potent anti-bacterial agents however, there need to be extensive studies on their structural modifications.

On evaluation of anti-bacterial activity of series of substituted triazolo-thiadiazinyl coumarin derivatives when studied against B. subtilis, S. aureus, E. coli, K. pneumoniae and P. aeruginosa (Jayashree et al., 2005a,b, 2006, 2007), it was found that, compounds 18i, 19i and 20j bearing the pyridinyl group at para position of the substituted phenyl ring on the 5th position, possessed better anti-bacterial activity among all the derivatives of 6-bromo/chloro/H substituted series of compounds 18a–k, 19a–k and 20a–l, against all bacterial strains as compared to the standard amoxicillin and gentamycin. Alternatively, 2-chlorophenyl derivatives of 6-bromo/chloro/H substituted 3-(5-(substituted phenyl)-1, 2, 4-triazolo-[3, 4-b] [1, 3, 4]-thiadiazin-5-yl)-2H-1-benzopyran-2-ones were among the least active anti-bacterial. However, a clear distinction of the effect of 6-bromo/chloro/H substituted 3-(5-(substituted phenyl)-1, 2, 4-triazolo-[3, 4-b] [1, 3, 4]-thiadiazin-5-yl)-2H-1-benzopyran-2-ones on the anti-bacterial activity could not be established owing to a mixed activity profile for all derivatives.

Further, when anti-bacterial activity of some of the coumarinyl chalcones against B. subtilis, S. aureus, E. coli and K. pneumoniae was performed (Jayashree et al., 2009), they were found to be weak to moderately active as anti-bacterial, however, the test compound 23d, a di-methoxy substituted 6-chlorocoumarinyl chalcone showed activity as compared to the standard streptomycin against all bacterial strains. This scaffold from the coumarinyl chalcone series could be further exploited to enhance and develop potential anti-bacterial agents.

The preliminary screening of certain 3-coumarinoyl pyridinium bromide 26a–l and 3-coumarinoyl quinolium bromide derivatives 27a–c against B. subtilis, S. aureus, E. coli and P. aeruginosa (Jayashree et al., 2010; Porwal et al., 2010), showed that these compounds were either inactive or moderately active as anti-bacterial on a few strains. The anti-bacterial sensitivity of series of derivatives of 3-coumarinoyl pyridinium bromide and 3-coumarinoyl quinolium bromide 26–27 against various strains was found in order of; S. aureus> B. subtilis> E. coli> P. aeruginosa as compared to the standard amoxicillin and gentamycin, which were active against all the strains. Thus, the preliminary screening revealed that such compounds were better anti-bacterial agents against gram positive strains as compared to gram negative organisms. Test compounds such as 26a, 26c, 26d and 26i, bearing 6-chloro/H on the benzopyrone ring showed better activity against all bacterial strains as compared to the 6-bromo analogues and were further selected for determination of MIC at 100 percent growth inhibition. However, test compounds 26a and 26d were found to be equally active as the standard amoxicillin against E. coli and gentamycin against B. subtilis, respectively. Therefore, it could be inferred that compounds 26a, 26c, 26d and 26i were not potential enough against all bacterial strains when compared to the standard gentamycin. Also, the quinolium bromide derivatives of coumarin were less active as anti-bacterial when compared to pyridinium bromide derivatives. However, the same compounds did show greater anti-bacterial activity against E. coli with MIC better than standard amoxicillin.

Further, some of the substituted pyridinyl coumarins such as 28b, 28d, 28f and 28g were also evaluated for their anti-bacterial activity. All compounds showed maximum activity against B. subtilis, moderate against E. coli and minimum against S. aureus as compared to standard gentamycin. Test compounds such as 28b, 28f and 28g exhibited 100 percent growth inhibition against B. subtilis at MIC value same as that of the gentamycin. It was found that, all the four test compounds were highly active against E. coli with MIC value better than standard amoxicillin.

3.3 Miscellaneous

Evaluation of some of the salicylate derivatives 29–32 of coumarins for their anti-diabetic potential was attempted (Jayashree et al., 2011) based on the rationale (Hogale et al., 1987). When test compounds were screened for this activity by in vitro non-enzymatic glycosylation of haemoglobin method and by advanced glycation end product inhibition method (Jedsadayanmata, 2005) using quercetin as standard, the results showed that they were poor candidates for anti-diabetic activity.

4.1 Hydrophobicity

Hydrophobicity is generally parameterised by partition coefficient (log p), determined by partitioning of a drug between organic and aqueous phases. For our experiments, this physical constant was determined on some of the synthesised coumarin derivatives by the classical shake flask method (Sangster, 1989), using n-octanol/n-heptanol and phosphate buffer (pH 7.4). Determination of log p for the series of compounds such as 18–20 was carried out (Jayashree et al., 2005a,b, 2006, 2007). Substitution of halogens at 6th position on the parent compound 20a resulted in an increase in the partition coefficient, which was clearly evident as the corresponding compounds such as 18a and 19a showed log p value in the range of 1.27–1.41 as compared to that of the parent compound at 1.18. The results also showed that triazolo thiadiazinyl 6-bromo substituted coumarins were more hydrophobic than their 6-chloro and 6-H counterparts. Substitution of the hydroxy group irrespective of its positions in the ring led to a decrease in the lipophilic character of compounds in the case of 6-chloro/bromo/H series. Interestingly, para-methyl substituted compounds such as 18h and 20i with the highest partition coefficient among all did not possess any promising anti-bacterial activity. However, compounds bearing the pyridinyl group at para position, such as 18i, 19i and 20j, having optimum lipophilicity with log p in the range of 1.04–1.22, showed potent anti-bacterial activity as compared to that of the standard amoxicillin and gentamycin. Also, compounds bearing ortho, para-dichlorophenoxy group such as 18k, 19k and 20l possessed partition coefficient in the range of 1.34–1.47 and were moderately active as anti-bacterial. The compounds with minimum log p such as 18d–e, 19d–e and 20e–f were least active against bacterial strains. These findings helped in understanding the effect of lipophilicity in terms of log p on the anti-bacterial activity of triazolo thiadiazinyl coumarins. Further, compounds with log p values above and below the optimum range however, did not show any significant biological activity. From our study it was understood that, to develop a potentially useful anti-bacterial agent using this scaffold, compounds should bear at least an optimum log p value as seen in case of pyridinyl substituted compounds.

To understand the pattern of lipophilic character of coumarinyl pyrazolines, few of the test compounds such as 24a, 24e, 24j, 25a and 25b were randomly selected (Jayashree et al., 2008). Presence of an electronegative atom at the 6th position of coumarin moiety in such studies greatly influenced log p value. However, the average range of log p for bromo/chloro and unsubstituted derivatives was found to be 0.08, 0.02 and 0.03, respectively. The increase in the lipophilicity of coumarinyl pyrazolines was found to be in the order of: bromo>chloro⩾ unsubstituted compounds. A similar pattern was also observed for compounds such as 25a and 25b. Further, to clearly understand the effect of various substituted coumarinyl pyrazolines on the partition coefficient, some more related studies may be required in future. Further, the partition coefficient of substituted pyridinyl and quinolium coumarin derivatives was also studied (Jayashree et al., 2010). The log p value of substituted pyridinyl coumarins varied with the position of substituents on the pyridine ring. Compounds such as 26a–c, bearing the para-ethoxy carbonyl group were less lipophilic than their corresponding meta analogues 26d–f. Alternatively, other derivatives bearing the para-methoxy group, for example 26g–i possessed higher partition coefficient as compared to their corresponding meta derivatives 26j–l. Further, in case of quinolium bromide salts of coumarin, compounds 27b–c bearing halogen in form of the chloro/bromo group at the 6th position of the coumarin nucleus exhibited higher log p as compared to the unsubstituted derivative 27a. The presence of the chloro substituted phenyl group at the 4th position in the pyridine ring of the derivatives like 28d–f, led to an increase in the log p value of the corresponding 4th position unsubstituted derivatives 28a–c. However, when the biological activity was correlated with partition coefficient of these derivatives, it was found that the derivative 26e with highest log p was inactive against all bacterial strains tested. On the other hand, it was noted that most of derivatives with quinolium bromide salts of coumarins with optimum log p range of 1.10–1.41, were moderately active as anti-bacterial.

From the results, it could be understood that substitution of electronegative atom in form of halogens such as the chloro/bromo group at the 6th position of the coumarin nucleus leads to an increase in the partition coefficient of the parent/unsubstituted compound. It was also realised that, compounds possessing an optimum balance of lipophilic and hydrophilic characters were biologically active when compared to the standards.

4.2 Other parameters

We wanted to understand the effect of substituents on few other important physicochemical properties such as determination of dissociation constant in terms of pKa value. The dissociation constant of both acidic and basic drug is governed by pKa. The lower the pKa of an acidic drug, the stronger will be the acid. Whereas, the higher the pKa of a basic drug, the stronger will be the base. To have better understanding, one such study was performed (Jayashree et al., 2010) where, estimation of pKa of the substituted pyridinyl coumarin derivatives was done. It was observed that compounds, including the bromide salts 26–28, were in the range of 6–6.5, showing moderate to weak basic character. Such compounds would get ionised in the acidic pH as in case of gastric pH, but they remain relatively unionised at intestinal pH, thereby showing better absorption from the intestine. It was also observed that substitution of different groups on the coumarin nucleus did not bring any noticeable change in their dissociation constant value. However, there is a need for further investigation to determine pKa of other substituted coumarins.