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498 Letters in Organic Chemistry, 2014, 11, 498-508

Recent Advances on the Synthesis of Chalcones with Antimicrobial Activities: A Brief Review

Marina Ritter1, Rosiane Mastelari Martins1, Daiane Dias2* and Claudio M.P. Pereira1,3*

1Post-graduate Program in Biochemistry and Bioprospection, Federal University of Pelotas, Pelotas, RS, Brazil; 2Chemistry and Food School, Federal University of Rio Grande, Rio Grande, RS, Brazil and 3Laboratory of Bioactive Heterocyclics and Bioprospection, Federal University of Pelotas, Pelotas, RS, Brazil

Received November 22, 2013: Revised January 18, 2014: Accepted January 28, 2014

Abstract: The increasing resistance of bacteria and fungi to antimicrobial drugs has been a matter of concern in public health in the last decades. The continuing need for novel therapeutic compounds is still urgent regarding the number of new diseases and resistant strains of microorganisms. Chalcones are ketones �,�-unsaturated with one aromatic ring bonded to carbonyl and another aromatic ring bonded to an olefin function. Literature has already reported the antimicro-bial potential of several chalcones against a wide range of fungal and bacterial strains, including resistant ones; clearly in-dicating that they are attractive target compounds for new antimicrobial drug development. Several studies have shown that the introduction of different functional groups is a strategy used to improve the biological activity of these com-pounds, moreover, the structure of chalcones can also be employed as an intermediate reaction, enabling different reac-tions and giving rise to new molecules. In this review, we describe recent advances on the synthesis of chalcones with an-timicrobial activities.

Keywords: Antimicrobial, chalcones, heterocyclics, isoxazoles, pyrazoles, thiazoles.

INTRODUCTION Fungal and bacterial infections represent a major public

health problem currently. Despite the huge range of antimicrobial drugs available and their medicinal-chemical evolution, the indiscriminate use of these drugs has resulted in the emergence of resistant pathogens. The microbial resistance imposes serious limitations on options for treatment and has caused great harm to public health, since it is the primary cause of death in intensive care units in hospitals worldwide [1]. It is estimated that more than 70% of pathogenic bacteria develop resistance to at least one antibiotic available for clinical use [2]. Thus, the continuing need for novel therapeutic compounds is still urgent as re-gards the number of new diseases and resistant strains of microorganisms [3, 4].

The introduction of biological models performed in vitro and in large scale has been especially important in this context because they can evaluate a large number of samples in a short period of time and provide a breakthrough in the research for new drugs. The technological advances that have contributed to the search for new compounds refer to the discovery of new molecular tools and the development of analytical techniques of purification and organic synthesis.

*Address correspondence these authors at the Chemistry and Food School, Federal University of Rio Grande (FURG) - Avenida Itália, Km 8, Zip code: 96203-900, Rio Grande RS, Brazil; Tel/Fax: + 00 55 53 3233-6960; E-mail: daianezd@gmail.com; and Laboratory of Bioactive Heterocyclics and Bioprospection, Federal University of Pelotas (UFPel) - Campus Universitário Capão do Leão s/n, Zip code: 96900-010, Capão do Leão RS, Brazil; Tel/Fax: +55 53 3275 7358; E-mail: claudio.martin@pq.cnpq.br

As a result, more effective, active and/or less toxic substances can be used as prototypes of drugs with pharmacological activity similar to the original ones or even larger [5].

The literature has already reported the antimicrobial po-tential of several chalcones against a wide range of fungal and bacterial strains, including resistant ones [16-20, 23]. Moreover, chalcones are simple and versatile compounds with a main structure which may contain different functional groups. Thus, due to their synthetic and biological versatil-ity, they are attractive target compounds for new drug devel-opment.

Chalcones are ketones �,�-unsaturated with one aromatic ring bonded at carbonyl and another aromatic ring bonded an olefin function [6]. These molecules are found in natural products, belonging to the flavonoid family or obtained by synthesis process [7]. The most widely used method for the synthesis of chalcones is the Claisen-Schmidt reaction, oc-curring by condensation of an aromatic ketone with an aro-matic aldehyde, and catalyst. In this respect many catalysts are used to prepare these following enones NaOH, Ba(OH)2,KOH, AlCl3 and HCl [8].

Due to the importance of the chacones, much work on improving the yields and reaction conditions has been ac-tively pursued. For example, modification and improvements include the use of microwave irradiation in solvent free con-ditions [9-11]. In particular, microwave (MW) irradiation has been widely exploited in the last decades to carry out a striking number of organic syntheses, benefiting from the dielectric heating in terms of reduced reaction times and in-

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Antimicrobial Activity of Chalcones Letters in Organic Chemistry, 2014, Vol. 11, No. 7 499

creased yields, especially when coupled with solvent-free techniques. In the last decades, developments have been made to prove how glycerol is not only a good replacement for other solvents but can feasibly be considered a benign solvent, especially used for organic synthesis. Very recently was reported the use of glycerol as a solvent for the synthesis of chalcones [12].

Chalcones are important in organic synthesis and they are precursors to synthetic heterocyclics compounds, such as isoxazoles [13], pyrazoles [14, 15] and thiazoles [16].

Isoxasoles containing trihalomethyl groups are useful in-termediates in organic synthesis used for the synthesis of fungicides and enaminones such as versatile synthetic inter-mediates, mainly in the preparation of antimalarial trifluoromethyl quinolines [13].

Another important class of compounds is pyrazol, which are heterocycles with five membered ring, containing two adjacent nitrogen centers. Pyrazol derivatives are key sub-structures in a large variety of compounds with important biological activities and pharmacological properties [14, 15].

Similar to pyrazoles, thiazoles are heterocycles with five membered ring that contains both sulfur and nitrogen in po-sitions 1 and 3. These compounds have pharmacological applications and biological activities, such as anti-inflammatory, analgesic, antimicrobial, anti-HIV, antihyper-tensive and herbicidal activity [16].

RECENT ADVANCES ON THE SYNTHESIS OF CHALCONES

Sivakumar et al. in 2009 obtained a series of chalcones according to the methodology published previously [17], (Scheme 1). The chalcones were synthesized by the reaction of equimolar (10 mmol) mixture of substituted appropriated aryl aldehydes 1 and acetophenone 2 in methanol (10 mL) and stirred at room temperature in basic media (NaOH). The solid precipitate was washed with ethanol and water, recrys-tallized and then dried, resulting in the compounds 3a-x’with good yields (about 80%), which are shown in (Table 1)[18].

In this context, the antimicrobial property of 3-Hydroxy-4-methoxychalcone coated on polymeric biomaterials against Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa was reported. The decrease in the bacterial adhe-sion and slime production on coated polymer was seen from SEM photomicrographs. Its results showed that bacterial adhesion decreased considerably on all the coated surfaces. Chalcone showed considerable slimicidal activity (45 and 48% reduction) against E. coli and S. aureus respectively. Furthermore, the majority of the adhered bacterial cells

showed damaged cell wall indicating the mechanism of ac-tion of chalcone [18].

A novel chalcone based on 6-carbethoxy-2-cyclohexen-1-one (6a-o) was synthesized by Shakil et al. in 2013 pre-pared from substituted acetophenones (4a-h) and benzalde-hydes (5a-i) in equimolar quantity, using NaOH in aqueous media. The mixture was placed in the centre of microwave oven beside a beaker with ice and irradiated for successive periods of 10 s, for a total of 5 to 7 min (Scheme 2). After irradiation, the product (6a-p) was neutralized with cold HCl. The precipitate formed was filtered and recrystallized with ethanol. In this same work, chalcones (6a-p) were used as intermediary in further reactions to obtain cyclohexenones and indazoles and their antibacterial activity was also tested [19].

The synthesized compounds were tested for their fungi-cidal activity against two phyto-pathogenic fungi Rhizocto-nia solani and Sclerotium rolfsii. The inhibitory effects of synthesized compounds on the growth of test fungi were expressed in terms of 50% lethal concentration (LC50). Cy-clohexenone derivatives, in general, showed better antifungal and antibacterial activity than parent chalcones. Chalcones 6n and 6o were highly effective as fungicide against R. so-lani with LC50 values 2.36 and 2.49 mg L-1 respectively. Likewise, the commercial fungicide Hexaconazole showed LC50 values of 1.27 and 1.12 mg L-1 against S. rolfsii. and R. solani, respectively [19].

Very recently, a series of chalcone analogues was synthe-sized via Claisen Schmidt condensation by Tran et al. in 2012. Aryl aldehydes 7a-d were reacted with substituted acetophenones 8a-i using KOH as base catalyst in methanol at room temperature for several hours, affording the desired chalcones 9a-d’ with yields between 45 and 79%. The reac-tions are shown in (Scheme 3) [20].

This study has shown that combinations of chalcones with conventional antibiotics could be an effective alterna-tive in the treatment of infection caused by resistant strains of microorganisms. The synthesized chalcones analogues with different substituents in their two rings were screened for their in vitro antibacterial activity against Methicillin-sensitive Staphylococcus aureus (MSSA) and Methicillin-resistant Staphylococcus aureus (MRSA) alone or in combi-nation with several conventional antibiotics. Results showed that of the synthesized chalcone analogues, four compounds exhibited positive interaction with antibiotics such as gen-tamicin, doxycycline and ciprofloxacin, demonstrating sig-nificant synergism against MRSA with very low MICs. The combination of doxycycline with 9d had the most synergistic effect against both MSSA and MRSA, in which the rates in the increase in the susceptibility of bacteria with doxycycline

OO

CH3

O

H

R1

R2

R3

R1

R2

R3

R4

R5

R6

R7

R4

R5

R6

R7

+

MeOH NaOH

r. t. A B

1 23a-x'

Scheme 1. Synthesis of chalcones 3a-x’.

500 Letters in Organic Chemistry, 2014, Vol. 11, No. 7 Ritter et al.

Table 1. Chalcones 3a-x’.

Substitution at A-ring Substitution at B-ring

R1 R2 R3 R4 R5 R6 R7

3a NMe2

3b SMe

3c NO2 SMe

3d

3e OH SMe

3f OMe SMe

3g OH SMe

3h Me SMe

3i Cl SMe

3j NO2 SMe

3k OMe

3l Cl Cl SMe

3m OMe

3n Br SMe

3o OMe OMe OMe

3p OH SMe

3q Cl Cl OMe OMe

3r NO2 OMe OMe

3s NO2 OMe OMe

3t NO2 NMe2

3v NO2 OMe

3w Cl Cl NMe2

3x Me OMe

3y OH OMe

3z NO2 OMe

3a’ O-CH2-O NO2

3b’ NO2 Cl

3c’ Br NO2

3d’ OMe Cl

3e’ OEt Cl

3f’ SO2CH3

3g’ NO2 SO2CH3

3h’ Cl SO2CH3

3i’ Cl Cl SO2CH3

3j’ Cl Cl Cl

3k’ NO2 Cl

Antimicrobial Activity of Chalcones Letters in Organic Chemistry, 2014, Vol. 11, No. 7 501

Table 1. Contd….

Substitution at A-ring Substitution at B-ring

R1 R2 R3 R4 R5 R6 R7

3l’ OH Cl

3m’ O-CH2-O SMe

3n’ NH2 SMe

3o’ OCH3 OCH3 SMe

3p’ OCH3 SMe

3q’ OEt SMe

3r’ Br SMe

3s’ F SMe

3t’ O-CH2-O Cl

3v’ Cl Cl OMe

3w’ O-CH2-O OMe

3x’ O-CH2-O OH

O

CH3

R1

+

O

H

R2

MW5-7 min

O

R1 R2

(a) R1: H, R2: 3,4-(OCH3)2 (b) R1: H; R2: 4-CH3,; (c) R1: H, R2: 4-OCH3; (d) R1: H, R2: 3,4,5-(OCH3)3;(e) 4-CH3, R2: 3,4-(OCH3)2; (f) 4-CH3, R2: 4-CH3; (g) R1: 4-CH3, R2: 3,4,5-(OCH3)3; (h) R1: 4-OCH3, R2: 3,4-(OCH3)2; (i) R1: 4-NO2, R2: -CH2O2-; (j) R1: -CH2O2-, R2: 4-OCH3; (k) R1: 4-Cl, R2: 4-OCH3; (l) R1: 3,4-(Cl)2, R2: 4-OCH3;(m) R1: 3,4-(OCH3)2, R2: 4-OCH3; (n) R1: 4-F, R2: 3-NO2; (o) R1: 4-F, R2: H; (p) R1: H, R2: H;

4a-i 5a-g 6a-p

NaOHH2O

Scheme 2. Synthesis of compounds 6a-p.

CH3

O

R1

+H

O

R2

KOHMethanol

r. t.

O

R1R2

R1: (a) 2-OH; (b) 2-OH, 4-OCH3; (c) 4-OCH3; (d) H; (e) 4-BrR2: (a) H; (b) 4-Cl; (c) 3,4-(Cl)2; (d) 2-OH; (e) 4-OCH3; (f) 2-OCH3 (g) 2,4-(OCH3)2; (h) 3,4-(OCH3)2; (i) 3,4,5-(OCH3)3

7a-d 8a-i 9a-d'

Scheme 3. Synthesis of compound 9a-d’.

were eight to sixteen fold, respectively. In such combina-tions, the MICs of doxycycline against MSSA and MRSA were very low of about 0.125 and 0.25 �g mL-1, respectively. The combinations of ciprofloxacin with 9d and 9u demon-strated a significant synergism against MRSA with very low MICs (0.0625 �g mL-1) for ciprofloxacin in both combina-tions. The rates of increase in the susceptibility of MRSA with ciprofloxacin were eight-fold. From the results of bio-activities, some preliminary remarks on the structure–activity relationship could be drawn as follows: (i) a free hydroxyl group in position(s) 2 and/or 4 of B ring appears to be very important to anti-MRSA activity alone or in combi-nation with antibiotics; (ii) in contrast, a free hydroxyl group

in position 20 of A ring, like 2’- hydroxychalcone analogues (9a-d) seems to be unnecessary; (iii) methylation of the hy-droxyl group may also be responsible for the abolishment in the anti-SA activity; and (iv) chlorine substitution seems to be unnecessary for the anti SA activity [20].

Jin et al. in 2012 synthesized a series of chalcones (12a-q), L-phenylalanine-derived C5-substituted rhodanine (38a-q) and derivatives containing thiobarbituric acid (39a-e) or 2-thioxo-4-thiazolidinone (40a-e). Chalcones (12a-q) were synthesized by Claisen-Schmidt condensation from the reac-tion of substituted acetophenones (10a-q) with phthalalde-hyde (11) and catalyzed by KOH in ethanol (12a-q) (Scheme4) [21].

502 Letters in Organic Chemistry, 2014, Vol. 11, No. 7 Ritter et al.

The compounds L-phenylalanine-derived C5-substituted rhodanine (14a-q) were obtained via the Knoevenagel condensation reactions of the appropriate intermediates (12a-q) with (S)-2-(4-oxo-2-thioxothiazolidin-3-yl)-3-phenylprop-anoic acid (13) at reflux in ethanol and acetic acid with piperidine for 12 h, as shown in (Scheme 4).

Finally, chalcones (12a-q) were reacted with thiobarbi-turic acid (13) or 2-thioxo-4-thiazolidinone (14), using piperidine in ethanol and acetic acid, for 12 h in reflux, re-sulting in the compounds 15a-e and 16a-e, respectively. The reactions are shown in (Scheme 5).

The in vitro antimicrobial activities of all synthesized compounds (14a-q, 17a-e and 18a-e) were evaluated against selected Gram-positive and Gram-negative bacteria and multidrug-resistant bacteria. Their antibacterial activities were compared with the known antibacterial agents, oxacil-lin and norfloxacin, which were used as references. Although the rhodanine derivatives (14a-q,) exhibited significant lev-els of antibacterial activity with MICs of 2-16 �g mL-1, they

were less active than the reference drug oxacillin. In contrast, several derivatives (14c-e, 14g, 14i, 14j, 14q) showed anti-bacterial activities that were very similar to that of the refer-ence drug norfloxacin. The chalcone derivatives containing thiobarbituric acid or 2-thioxo-4-thiazolidinone (17a-e, 18a-e) were the least active of the compounds tested with MIC values of 64 �g mL-1. None of the compounds inhibited the growth of E. coli (MIC > 64 �g mL-1).

The compounds 14a-q exhibited significant activity against multidrug-resistant S. aureus (MRSA CCARM 3167 and MRSA CCARM 3506) and quinolone-resistant S. aureus(QRSA CCARM 3505 and QRSA CCARM 3519) with MIC values of 2-8 �g mL-1. These values represented a 2-4-fold increase in relation to the standard drug norfloxacin, with compound 14q showing the most potent levels of activity against all of the multidrug-resistant clinical isolates tested. Moreover, compounds 17a-e and 18a-e did not exhibit activ-ity against the multidrug-resistant bacteria (MIC > 64 �gmL-1).

CH3

O

R

+H

ONaOH

Ethanol

O

R

R: (a) 2-F; (b) 4-F; (c) 2-Cl; (d) 3-Cl; (e) 4-Cl; (f) 2,4-(Cl); (g) 2-Br; (h) 3-Br; (i) 4-Br; (j) 4-CH3; (k) 2,4-(Cl)2; (l) 2-OCH3; (m) 3-OCH3; (n) 4-OCH3; (o) 4-NHCOCH3; (p) H; (q) C6H6(3,4-fused)

10a-q 1112a-q

OHH

O

23°C3-4h CHO

O

CHO

O

NS

O

S

COOH+

SN

S

O

PiperidineAcOHEtOH

Reflux, 12 h

14a-q12a-q 13

COOHR R

Scheme 4. Synthesis of compounds 14a-q.

O

CHORR: (a) 2-F; (b) 4-F; (c) 2-Cl; (d) 3-Cl; (e) 4-Cl12a-e

SNH

S

NH

NH

O

O SO

R

O

RS

NH

NH

HN

O

S

O

O

S15

16

17a-e

18a-e

AcOH, EtOHPiperidine12h, reflux

AcOH, EtOHPiperidine12h, reflux

O

Scheme 5. Thiazole-based chalcones 17a-e and 18a-e.

Antimicrobial Activity of Chalcones Letters in Organic Chemistry, 2014, Vol. 11, No. 7 503

No clear structure-activity relationships were observed, indicating that the antibacterial activity was not significantly affected by the position or physicochemical properties of the different substituents on the phenyl ring. Based on all of these antibacterial activity data, the L-phenylalanine-derived C5-substituted rhodanines (14a-q) showed good activity against the Gram-positive bacteria and multidrug-resistant bacteria, whereas the chalcone derivatives containing thiobarbituric acid or 2-thioxo-4- thiazolidinone (17a-e, 18a-e) did not exhibit any antibacterial activity against these bac-teria at 64 �g mL-1. These data support the importance of the presence of a free carboxyl group to potent antibacterial ac-tivity.

The synthesis of thiazole-based chalcones was reported by Liaras et al. in 2011, promoting the reaction of 1-(4-methyl-2-(methylamino)thiazol-5-yl)ethanone 19 with aro-matic aldehydes 20a-j employing solution of NaOH (10% aq) and methanol, as shown in (Scheme 6). The molecules 21a-j were obtained from moderate to good yields (32-84%) and purified by recrystallization in dioxane or ethanol [22].

Regarding the carbonyl derivatives, the possibility reac-tion of aldehydes and heterocycles ketones without interfer-ence of thiazole ring on Claisen-Schmidt were shown.

The synthesized compounds were tested for their in vitroantimicrobial properties against Gram positive and Gram negative bacteria and also against a series of fungi. All the compounds tested had activity against all the bacteria tested roughly comparable to the Ampicillin activity, and rather less than of Streptomycin. The minimal inhibitory concentra-tion (MIC) was at a range of 7.64-45.87 �mol mL-1 x 10-2

and minimal bactericidal concentration (MBC) was at 30.58-68.37 �mol mL-1 x 10-2. Exceptions to this generalization are that compound 21f possesses three fold better MIC against Enterococcus faecalis than does Ampicillin, and the com-pounds 21d and 21g possess three fold better MIC against Micrococcus flavus than does Ampicillin. Among the com-pounds tested, the best antifungal activity was expressed by chalcone 21f (MIC 15.29 �mol mL-1 x10-2) against Aspergil-lus versicolor, N. viride and F. sporotrichoides, as well as compound 21i against N. viride, Penicillium ochrochloron

and F. sporotrichoides, even though these compounds are less potent than reference drugs (ketoconazole and bifona-zole). The lowest activity was observed for chalcone 21a,with MIC 19.38-38.80 �mol mL-1 x10-2. Regarding the rela-tionships between the structure of the heterocyclic scaffold and the detected antibacterial properties, the introduction of the bulky electron-withdrawing nitro and chloro groups clearly enhanced the antibacterial properties [22].

Avupati et al. in 2012 developed the synthesis of novel 2,4-thiazolidinediones having chalcones 24a-x in their struc-tures. Firstly, terephthaldehyde 20 and 1,3-thiazolidine-2,4-dione 21 reacted by Knoevenagel condensation, using piperidine as a catalyst under reflux for 8 h in ethanol to pre-pare the intermediary (Z)-4-((2,4-dioxo-1,3-thiazolidin-5-ylidene)methyl)benzaldehyde 22. Next, the intermediary 22was reacted with appropriate substituted aromatic or het-eroaromatic ketones 23a-x in dimethylformamide, in the presence of pulverized sodium hydroxide (Scheme 7). All the synthesized compounds 24a-x were obtained in good yields and different substituents are listed in (Table 2) [23].

The compounds 26r and 26s were found to be more ac-tive than other compounds with MIC 16-32 �g mL-1 against all tested microorganisms. Compounds 26a, 26e-26h, 26k,26m, 26n, 26w and 26x showed comparatively less antimi-crobial activity with MIC 256-512 �g mL-1 against all tested microorganisms. From the results of antibacterial activity, compound 26s was found to be more active against all Gram-positive bacteria with MIC value 16 �g mL-1. Among all the tested compounds, compound 26r showed significant inhibition against all Gram-negative bacteria with MIC 16 �g mL-1, except against Proteus vulgaris. A broad spectrum of antifungal activity of the compound 23s was obtained against all the fungi with MIC 16 �g mL-1, while other com-pounds displayed less antifungal activity. From the obtained data on antimicrobial activity we can conclude that the com-pounds substituted with halogens on the phenyl ring at meta and para positions enhanced the antibacterial activity (F > Cl) as seen in the case of compounds 26o-26r. It is notewor-thy that compounds 26b-26d, 26k and 26l having electron donating substituents (4-CH3 > 4-OCH3 > 3-OCH3 > 4- NH2

N

S O

CH3

CH3

NH

H3C+

H

O

R

NaOH

MeOH

19 20a-j O

N

SNH

H3C

CH3

R

21a-jR: (a) H; (b) 4-NO2; (c) 3-NO2; (d) 4-Cl,; (e) 3-Cl; (f) 2-Cl;

(g) 4-OMe; (h) 2-OMe; (i) 2,6-(Cl)2; (j) 2,4-(Cl)2

Scheme 6. Thiazole-based chalcones 21a-j.

H

O

H

O

+

S

NH

O

O

S

NH

O

OH

O

S

NH

O

OR

O

22 23 24

Piperidine

EtOH,Reflux, 8h

Ketones25a-x

NaOH,DMF

26a-x

Scheme 7. 2,4-thiazolidinediones having chalcones in their structures. 26a-x.

504 Letters in Organic Chemistry, 2014, Vol. 11, No. 7 Ritter et al.

> 3-NH2) on the phenyl ring at meta and para positions were found to enhance the antibacterial activity [23].

Banday et al. in 2011 synthesized various steroidal chal-cones 29a-j having pregnenolone 27 as precursor, using a series of aldehydes 28a-j. To obtain these compounds 29a-j,a solution of 1 eq. pregnenolone 27 (1mmol in 10 mL of ethanol) was added in a solution of 2 eq. KOH and 1.2 eq. of appropriated aldehydes 28a-j. (Scheme 8) [24].

All the compounds showed significant antimicrobial ac-tivity against all microbial strains used for testing (Bacillus subtilis, Staphylococcus epidermidis, Proteus vulgaris,Pseudomonas aeruginosa, Aspergillus niger and Penicillium chrysogenum). However, these compounds are less potent than reference drugs (kenamycin and fluconazole) [24].

Novel steroidal chalcones were synthesized by Kakati etal. in 2012 having pregnenolone 27 as precursor. To promote the synthesis of chalcones, it was necessary to use pregnenolone as acetate 30. The reaction between 30 and 31a-p compounds by microwave irradiation resulted in products 32a-p (Scheme 9) [25].

These compounds were screened for antimicrobial activ-ity against two bacterial strains Bacillus subtilis and Es-cherichia coli and two fungal strains Aspergillus niger and Candida albicans. The compounds 32a, 32c, 32e, 32h, 32i and 32m exhibited a significant inhibitory activity against the microbial strains. Among all the compounds analysed, compound 32e showed to be the most promising having both bactericidal and fungicidal activities with MIC values 150

Table 2. Substituent from aromatic or heteroaromatic ketones 25a-x.

R R R

24a C6H5 24i (2-OH,5-Me)C6H3 24q (3-F)C6H4

24b (4-Me)C6H4 24j (6-OH,5-Me)C6H3 24r (4-F)C6H4

24c (3-OMe)C6H4 24k (3-NH2)C6H4 24s 3,5-(C7H7O)C6H3

24d (4-OMe)C6H4 24l (4-NH2)C6H4 24t Thyofen-2-yl

24e (2-OH)C6H4 24m (3-NO2)C6H4 24u Pyridin-2-yl

24f (4-OH)C6H4 24n (4-NO2)C6H4 24v Pyridin-3-yl

24g 2,4-(OH)2C6H3 24o (3-Cl)C6H4 24w Naphthalen-2-yl

24h 2,5-(OH)2C6H3 24p (4-Cl)C6H4 24x Fluoren-2-yl

HO

O

+

HO

OR

KOH

EtOH

27

28a-j

29a-j

R H

O

R: (a) C6H5; (b) 2-Me(C6H4); (c) 3-Me(C6H4); (d) 4-Me(C6H4); (e) 3-F(C6H4); (f) 4-F(C6H4); (g) 4-OMe(C6H4); (h) 2-OMe(C6H4); (i) 3-Br(C6H4); (j) Furan-2-yl

Scheme 8. Synthesis of steroidal chalcones 29a-j.

AcO

O

+

AcO

O

I2-Al2O3

MW (5-7min)

30 31a-p 32a-p

H

O

R

R

R: (a) H; (b) 4-OMe; (c) 4-Cl; (d) 2-Br; (e) 4-F; (f) 4-NO2; (g) 4-Br; (h) 2,4-(F)2; (i) 2,3,5-(F)3; (j) 4-OH; (k) 3-OMe, 4-OH; (l) 3,4-(OH)2; (m) 2,4-(Cl)2; (n) 4-CH3; (o) 2,4-(OCH3)2; (p) 2-NO2

Scheme 9. Synthesis of steroidal chalcones in microwave 32a-p.

Antimicrobial Activity of Chalcones Letters in Organic Chemistry, 2014, Vol. 11, No. 7 505

and 300 �g mL-1, respectively. Furthermore, the presence of �, �-unsaturated carbonyl moiety in the synthesized com-pounds was found to be essential for the activity, whereas its manipulation through epoxidation of the double bond dimin-ished the activity [25].

Rizvi et al. in 2010 synthesized novel chalcones from two precursors, 2-chloro-6-methyl-3-formylquinoline 33 and 2-chloro-6-methoxy-3-formylquinoline 34. The interesting compounds 36a-s and 37a-s were prepared based on Claisen-Schmidt condensation with these formyl quinolines and 19 different methyl arylketones (Table 3), in the presence of sodium hydroxide (Scheme 10) [26].

All compounds were screened for antimicrobial activity against bacterial and fungal strains. Amongst the compounds tested for antibacterial study, 33f-i, 33l, 33o-p, 34f-i, 34l,and 34o-p showed remarkable antibacterial activity. Espe-

cially, compounds 34h and 34o-p from substituted heteroaryl derivatives showed antibacterial activities almost equivalent to standard Chloramphenicol (1.00 mmol mL-1) against all the three bacterial strains i.e., Escherichia coli, Micrococcus luteus and Staphylococcus aureus. Among the two series of compounds, unsubstituted thiophenyl derivatives (33a, 34a)exhibited almost equivalent antibacterial activities to that of the standard. The antimicrobial activity decreased considera-bly by the substitution of methyl and halo groups at position 5 of thiophenyl ring (33d, 34d, 33g, 34g, 33j, 34j, 33k, 34k). Incorporation of chlorine at position 5 of thiophenyl ring (33g, 34g) enhanced the activity to a considerable extent; it further increased by the incorporation of another chlorine atom at position 2 (33h, 34h). However, incorporation of same groups at positions 3 and 4 (33b, 33c, 33f, 33i, 34b,34c, 34f, 34i) exhibited no difference in the activity except of bromo derivatives. In case of furanyl derivatives, the in-

Table 3. Methyl arylketones a-s.

Ketones Ar Ketones Ar Ketones Ar

a

S

h

S ClClo O

b

S

H3C

i

S

Br

pO

O

c

S

CH3

jS Br

q

dS CH3 k

S Ir

e

SH3C CH3

lHN

s

f

S

Cl

mO CH3

gS Cl

n

OH3C CH3

N

H

O

R

Cl

Ar

O

CH3(35a - s)

NaOH N

R

Cl

Ar

O

33 R = CH334 R = OCH3

(36a - s) and (37a - s)36: R = CH3

37: R = OCH3

Scheme 10. Synthesis of quinolin chalcones 33a-s and 34a-s.

506 Letters in Organic Chemistry, 2014, Vol. 11, No. 7 Ritter et al.

corporation of a second methyl group at position 2 (33n,34n) considerably decreased the activity in relation to mono substituted one (33m, 34m). In general, the antibacterial activity was enhanced by the substitution of aromatic rings with electron withdrawing groups and was suppressed by the incorporation of electron donating methyl groups. With re-spect to antifungal activity, among the compounds under investigation, un-substituted thiophenyl derivatives (33a,34a) were found almost equivalent in their antifungal activi-ties to the standard Flucanazole (1.00 mmol mL-1). In gen-eral, the activity decreased in the compounds having substi-tution at the positions 2, 3 and 5 by the incorporation of elec-tron donating methyl groups at aromatic rings while it en-hanced by the substitution of electron withdrawing groups. Among the electron withdrawing groups, the activity in-creased with the electronegativity of the substituent. How-ever, such substitutions at position 4 (33c, 34c) displayed no marked difference in the activities [26].

A series of novel chalcones was obtained by Yesuthan-gam et al. in 2011 by condensation of different o-hydroxyacetophenones 38a-f with indole-3-aldehyde 39 (in the ratio of 1.0:1.10) in ethanol and the presence of piperidine. The reaction was refluxed until the formation of interesting compounds 40a-f (Scheme 11) [27].

The in vitro antimicrobial activity of these chalcones was also investigated against E.coli, S. cerevisiae and B.subtilis.All chalcones (40a-f) were active against E.coli, and, chal-cones 40a and 40b were also active against S.cerevisiae. No

activity was found with B.subtilis. Compound 40a showed higher antifungal activity compared to other chalcones. The biological activities can be compared with the enzymatic superoxide anion generation efficiency. The higher antimi-crobial activity of 40a correlated with higher enzymatic su-peroxide anion generation in the dark. Chalcones with higher E1/2 values showed better antimicrobial activity [27].

As shown before, chalcones are versatile compounds with a main structure which may contain different functional groups, including ferrocene. In general, these compounds are widely applied in several areas and have most important ap-plications on the development of more active drugs, espe-cially in cases where there is resistance to current drugs [28].

Prasath et al. in 2012 synthesized quinoline-appended ferrocenyl chalcones (43a-f and 46a-f) using conventional method or ultrasonic radiation. (Schemes 12 and 13) [22]. Posteriorly, the antimicrobial properties of the compounds were assayed. The results revealed that 43a, 43c, 43d, 46aand 46f exhibited high activity against both gram-positive and gram-negative bacteria. Results elucidated the fact that electron-withdrawing carbonyl in 43a-f increases the antimi-crobial activities of the compounds compared to others where the carbonyl group is not directly attached to fer-rocene [29].

Chemically, chalcone nucleous consists of open-chain flavonoids in which the two aromatic rings are joined by a 3-carbon �,�-unsaturated carbonyl system. In connection with

O

CH3

OHR+

HN

O

H PiperidineEtOH

O

OH

HN

R

38a-f 39 40a-fR: (a) H; (b) 4-OCH3; (c) 5-OCH3; (d) 3,4-(OCH3)2; (e) 3,4,6-(OCH3)2; (f) 4,6-(OCH3)2

Scheme 11. Synthesis of benzopyrrole chalcones 40a-f.

FeCH3

O

+ KOHEtOH Fe

O

N

R1

R2

R3

Cl

R: (a) R1=H, R2=H, R3=H; (b) R1=CH3, R2=H, R3=H; (c) R1=H, R2=CH3, R3=H; (d) R1=H, R2=H, R3=CH3; (e) R1=OCH3, R2=H, R3=H; (f) R1=H, R2=OCH3, R3=H;

41 42a-f 43a-f

N

O

H

Cl

R3

R2

R1

Scheme 12. Synthesis of quinolin chalcones ferrocene derivatives 43a-f.

FeH

O

+KOH

EtOH

R: (a) R1=H, R2=H, R3=Br; (b) R1=CH3, R2=H, R3=H; (c) R1=C6H5, R2=CH3, R3=H; (d) R1=C6H5, R2=Cl, R3=CH3; (e) R1=C6H5, R2=NO2, R3=H; (f) R1=(2-Cl)C6H5, R2=Cl, R3=H;

N

O

CH3

R1

R2

R3

N

OR1

R2

R3

Fe

44a-f 45 46a-f

Scheme 13. Synthesis of quinolin chalcones ferrocene derivatives 46a-f.

Antimicrobial Activity of Chalcones Letters in Organic Chemistry, 2014, Vol. 11, No. 7 507

structure, the compounds can be divided into three groups: 3-(aryl, heteryl)-1-phenyl-1- propen-3-one derivatives (1st group), 1-(aryl, heteryl)-3-phenyl-1-propen-3-one derivatives (2nd group) and disubstituted chalcone derivatives (3rd group) [30]. The 1st group chalcones are prepared by the condensation of heteroaryl aldehyde with acetophenone whereas 2nd group chalcones are prepared by the reaction of heteroaryl ketone with benzaldehyde [31]. It is interesting to mention here that in our brief revision is shown the recent advances to synthetize chalcones with antimicrobial activi-ties, and further chemical modification via fragment changes guided by structure can lead to discover better agents as po-tential clinical candidates.

CONCLUSION Chalcones are compounds of great interest in pharmaceu-

tical chemistry. The literature has already reported the antim-icrobial potential of several chalcones against a wide range of fungal and bacterial strains, including resistant ones; clearly indicating that they are attractive target compounds for new antimicrobial drug development. This activity may be related to carbonyl group �,�-unsaturated present between aromatic rings. Moreover, chalcones are very versatile, al-lowing a series of substitutions on the aromatic rings. Sev-eral studies have shown that the introduction of different functional groups is a strategy used to improve the biological activity of these compounds, according to the results of the biological evaluation. The structure of chalcones can also be employed as a reaction intermediate, enabling different reac-tions and giving rise to new molecules.

CONFLICT OF INTEREST The authors confirm that this article content has no con-

flict of interest.

ACKNOWLEDGEMENTS The authors are grateful to FAPESP, CEPEMA-USP

(Centro de Capacitação e Pesquisa em Meio Ambiente), CNPq (310472/2007-5), Program INCT-CNPq 573.667/2008-0, FAPERGS and CAPES for financial sup-port.

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