Design, synthesis and exploring the quantitative structure–activity relationship of some antioxidant flavonoid analogues

Design, synthesis and exploring the quantitative structure–activity relationship of some antioxidant flavonoid analogues

Bioorganic & Medicinal Chemistry Letters xxx (2014) xxx–xxx Contents lists available at ScienceDirect Bioorganic & Medicinal Chemistry Letters journ...

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Bioorganic & Medicinal Chemistry Letters xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Bioorganic & Medicinal Chemistry Letters journal homepage: www.elsevier.com/locate/bmcl

Design, synthesis and exploring the quantitative structure–activity relationship of some antioxidant flavonoid analogues Sreeparna Das a, Indrani Mitra b, Shaikh Batuta a, Md. Niharul Alam a, Kunal Roy b,⇑, Naznin Ara Begum a,⇑ a b

Bio-Organic Chemistry Laboratory, Department of Chemistry, Siksha Bhavana, Visva-Bharati (Central University), Santiniketan 731 235, West Bengal, India Drug Theoretics and Cheminformatics Laboratory, Department of Pharmaceutical Technology, Jadavpur University, Kolkata 700 032, India

a r t i c l e

i n f o

Article history: Received 14 June 2014 Revised 2 September 2014 Accepted 3 September 2014 Available online xxxx Keywords: Flavonoids Antioxidants DPPH assay QSAR In silico method

a b s t r a c t A series of flavonoid analogues were synthesized and screened for the in vitro antioxidant activity through their ability to quench 1,1-diphenyl-2-picryl hydrazyl (DPPH) radical. The activity of these compounds, measured in comparison to the well-known standard antioxidants (29–32), their precursors (38– 42) and other bioactive moieties (38–42) resembling partially the flavone skeleton was analyzed further to develop Quantitative Structure–Activity Relationship (QSAR) models using the Genetic Function Approximation (GFA) technique. Based on the essential structural requirements predicted by the QSAR models, some analogues were designed, synthesized and tested for activity. The predicted and experimental activities of these compounds were well correlated. Flavone analogue 20 was found to be the most potent antioxidant. Ó 2014 Elsevier Ltd. All rights reserved.

Oxidative stress in human arises from an imbalance in the antioxidant status, that is, production of excessive Reactive Oxygen Species (ROS) during cellular metabolism versus living cell’s own defense and repair mechanisms. This oxidative stress plays a crucial role in the age-associated diseases such as cardiovascular and cerebrovascular diseases, some forms of cancer and Parkinson’s and Alzheimer’s diseases.1,2 Antioxidants primarily reduce this oxidative stress. Endogenous antioxidants are produced in the living cells itself and they are the integral part of body’s own defense systems. Various enzymes, for example, superoxide dismutase, catalase and glutathione peroxidase, vitamin E, uric acid and serum albumins represent this class of antioxidants. But consumption of exogenous antioxidants or dietary antioxidants (i.e., antioxidants obtained from dietary supplements) is also important to fight against oxidative stress and associated diseases in human beings.2 Among the dietary antioxidants, flavonoids play substantial role besides ascorbate, tocopherols and carotenoids. Flavonoids represent one of the most diverse and extensively spread groups of plant derived natural products.1,2 Moreover, these compounds can also be synthesized in the laboratory using simple synthetic methods and easy laboratory set-up.3–5 The quest for novel and non-toxic antioxidants with highly specified nutritional and therapeutic properties is an extremely ⇑ Corresponding authors. Fax: +91 3328371078 (K.R.); tel.: +91 9434431810; fax: +91 3463261526 (N.A.B.). E-mail addresses: [email protected] (K. Roy), [email protected] ac.in (N.A. Begum).

important and challenging job. Over the past few years, a plethora of medicinal plants have been put to trial as the source of naturally occurring antioxidants. On the other hand, design and synthesis of new antioxidants is a highly cherished goal for researchers working in the field of ‘Antioxidant Chemistry’. Moreover, it is extremely desirable that these synthetic products must be non-toxic and their synthetic protocols should be energy efficient, economically viable and environment friendly. But synthetic antioxidants like butylated hydroxyl toluene (BHT) and butylated hydroxyl anisole (BHA) which are frequently used as preservatives in processed foods are found to be carcinogenic. Moreover these synthetic antioxidants show adverse effects on the lungs and liver.6,7 Flavones represent a major sub-class of flavonoid type of compounds which have structure consisting of two aromatic rings (A and B) linked by three carbons in an oxygenated heterocycle (Ring C)8 (Fig. S1(a) in Supplementary data). Chalcones (Fig. S1(b) in Supplementary data) represent another sub-class of the flavonoid family.8 These flavonoids have wide spectrum of biological and pharmacological applications as cardioprotective, chemoprotective antimicrobial, antiviral, anti-allergic, hepato-protective and anticancer agents.1,2,8 It is believed that most of these bioactivities of flavonoid type compounds may be originated due to their behavior as antioxidants.1,2 The antioxidant effects of flavonoids are believed to be due to their (a) interactions with several enzymes; (b) ability to scavenge ROS and other free radicals; (c) ability to chelate metal ions and (d) synergistic effects when

http://dx.doi.org/10.1016/j.bmcl.2014.09.028 0960-894X/Ó 2014 Elsevier Ltd. All rights reserved.

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attached/added with other antioxidants.1,2 Moreover, the antioxidant activities of these type of compounds are on an intimate terms with their structural pattern [i.e., nature and position of substituent(s) on Ring-A, B and C], geometry and physico-chemical parameters.1–3,8 Although antioxidant activity of various flavone type of compounds have been well examined but there are few reports on their systematic study on structure–activity relationship and this type of systematic study on their antioxidant activity and molecular structures is of great importance to develop potential drugs which can be used to treat free radical associated diseases. In the present work, we have synthesized a series of chalcone and flavone analogues by varying the substitution patterns on ring A and ring B or by varying the length of the conjugation pattern of the parent flavone moiety [structures are shown in Fig. S2(a and b) in Supplementary data]. Flavonoid analogues (both isolated and synthesized) along with their precursors and standard antioxidants were evaluated for in vitro antioxidant activity by their measuring 1,1-diphenyl-2-picryl hydrazyl radical (DPPH) radical scavenging ability (Table 1). We have also extended this work towards the development of Quantitative Structure–Activity Relationship (QSAR) models9 on

the basis of relevant physico-chemical, spatial and electronic properties of these compounds. Genetic Function Approximation (GFA) method10 with linear and spline options was applied as the chemometric tool to develop the QSAR models which would give us a clear idea about the relationship between the antioxidant activity and the structural patterns of these compounds. This was further explored as our guiding tool for lead optimization. Based on the essential structural requirements for showing potent antioxidant activity as explored from the developed QSAR models, twenty flavonoid analogues were designed. Some of these designed compounds were synthesized and tested for their DPPH radical scavenging potential. The predicted activity of the designed molecules was found to be in accordance with experimental data predicted by in silico methods. A total of forty two compounds (among which twenty three flavonoid analogues were synthesized presently) were tested for DPPH radical scavenging antioxidant activity11,12 and their activity data was used to develop QSAR models.9,10 Table 1 displays structures as well as the IC50 (sample concentration having 50% radical inhibition activity) values of these compounds. Three replicates were performed for each experiment. The results are expressed as mean ± standard deviation (SD).

Table 1 DPPH radical scavenging activity of flavonoid analogues and related compounds used for the development of QSAR models

1 2 a,b c

Compound tested

DPPH Radical scavenging activity [(IC50 ± SD) (mM)]

(E)-1,3-Diphenyl-2-propen-1-one (1)2 (E)-3-(4-Nitrophenyl)-1-phenylprop-2-en-1-one (2)1 (E)-1-Phenyl-3-p-tolylprop-2-en-1-one (3)2 (E)1-(2-Hydroxyphenyl)-3-phenylprop-2-en-1-one (4)1 (E)-1-(2-Hydroxyphenyl)-3-p-tolylprop-2-en-1-one (5)1 (E)-3-(4-Chlorophenyl)-1-(2-hydroxyphenyl)prop-2-en-1-one (6)2 (E)-1-(2-Hydroxy-phenyl)-5-phenyl-penta-2,4-dien-1-one (7)1 (E)-3-Furan-2-yl-1-(2-hydroxy-phenyl)-prop-2-en-1-one (8)1 (E)-1-(2-Hydroxyphenyl)-3-(4-methoxyphenyl)prop-2-en-1-one (9)1 (E)-1,5-Diphenyl-penta-2,4-dien-1-one (10)1 (E)-4-Phenylbut-3-en-2-one (11)1 (Z)-3-Hydroxy-1-(4-methoxybenzofuran-5-yl)-3-phenylprop-2-en-1-one or pongamol (12)2 2-Phenyl-4H-chromen-4-one (13)1 2-(4-Methoxyphenyl)-4H-chromen-4-one (14)1 2-(4-Chlorophenyl)-4H-chromen-4-one (15)1 2-p-Tolyl-4H-chromen-4-one (16)1 3-Hydroxy-2-phenyl-4H-chromen-4-one (17)1 2-(4-Chlorophenyl)-3-hydroxy-4H-chromen-4-one (18)2 3-Hydroxy-2-(4-methoxyphenyl)-4H-chromen-4-one (19)2 3-Hydroxy-2-p-tolyl-4H-chromen-4-one (20)1 2-(3,4-Dihydroxyphenyl)-3,5,7-trihydroxy-4H-chromen-4-one (21)1 2-(4-(Dimethylamino)phenyl)-3-hydroxy-4H-chromen-4-one (22)2 2-(Benzo [d][1,3]dioxol-5-yl)-3-hydroxy-4H-chromen-4-one (23)1 3-Hydroxy-2-styryl-chromone-4-one (24)2 2-Furan-2-yl-3-hydroxy-chromone-4-one (25)1 3-Methoxy-2-phenyl-4H-furo[2,3-h]chromen-4-one or karanjin (26)1 a 3,5,7-Trihydroxy-2-phenyl-4H-chromen-4-one or galangin (27)2 b 5,7-Dihydroxy-2-phenylchroman-4-one or pinocembrin (28)2 b Gallic acid or 3,4,5-trihydroxybenzoic acid (29)1 Ascorbic acid or (R)-5-[(S)-1,2-dihydroxyl]-3,4-dihydroxyfuran-2(5H)-one (30)1 Trolox or 6-hydroxy-2,5,7,8-tetramethyl-chroman-2-carboxylic acid (31)1 Butylated hydroxy anisole (BHA) or 2-tert-butyl-4-methoxyphenol or (32)1 Salicylic acid or 2-hydroxybenzoic Acid (33)1 Cinnamic acid or (E)-3-phenylprop-2-enoic acid (34)1 p-Hydroxy cinnamic acid or (E)-3-(4-hydroxyphenyl)-2-propenoic acid (35)1 Piperine or 1-[5-(1,3-Benzodioxol-5-yl)-1-oxo-2,4-pentadienyl] piperidine (36)1 Carbazol-9-yl-methanol (37)1,c Piperonal or Benzo[d][1,3]dioxole-5-carbaldehyde (38)1 Salicylaldehyde or 2-hydroxybenzaldehyde (39)2 Cinnamaldehyde or (2E)-3-phenylprop-2-enal (40)1 Furfural or furan-2-carbaldehyde (41)2 ortho-Hydroxy acetophenone or 1-(2-hydroxyphenyl) ethanone (42)1

3139.260 ± 8.620 2815.900 ± 5.991 115.160 ± 2.425 77.986 ± 1.145 7.723 ± 0.226 6.103 ± 0.122 7.133 ± 0.371 4.273 ± 0.129 190.000 ± 2.427 113.000 ± 3.807 302.180 ± 3.769 11.640 ± 0.375 9590.000 ± 8.544 681.783 ± 4.923 0.725 ± 0.052 6.338 ± 0.334 0.387 ± 0.004 0.663 ± 0.019 0.114 ± 0.004 0.005 ± 8.5  1005 0.011 ± 9.6  1005 0.022 ± 0.001 0.094 ± 0.004 0.039 ± 0.001 0.034 ± 0.002 4.232 ± 0.051 0.191 ± 0.006 59.300 ± 1.299 0.011 ± 0.0002 0.033 ± 0.0004 0.012 ± 0.0001 0.019 ± 0.0001 278.303 ± 4.640 1727.617 ± 7.142 18.450 ± 0.679 90.693 ± 2.467 3.690X109 ± 3.185 906.943 ± 4.121 96.650 ± 0.976 372.167 ± 2.142 214.777 ± 1.275 154.887 ± 1.798

a

Training set compounds for QSAR modeling. Test set compounds for QSAR modeling. Previously isolated from seed extract of Pongamia glabra Vent. and ethanol extract of Indian propolis.13–15 Synthesized according to the previously reported method.15

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The chalcone analogues (1–6 and 9) were prepared by Claisen– Schmidt reaction between appropriately substituted acetophenone (a) and benzaldehyde (b) derivatives according to the previously reported method3–5 as shown in Scheme S1 (Supplementary data). In case of compounds 7 and 10, benzaldehyde part was replaced by cinnamaldehyde whereas furfuraldehyde was used in case of compound 8. Acetone was used in place of acetophenone part for the preparation of compound (11). In case of the flavone analogues (13–20 and 22–25), chalcone analogues with desired substitution patterns prepared by Claisen–Schmidt reaction3–5 were employed to synthesize the series of flavone derivatives as shown in Scheme 2 (Supplementary data). In case of compound 23, same procedure was followed except in this case 3,4-methylenedioxy benzaldehyde (piperonal) was used as the precursor for the synthesis of corresponding chalcone analogue. The detailed synthetic protocols are discussed in Supplementary data. Structures of all the synthesized compounds were confirmed either on the basis of previously reported data or spectroscopic methods (Supplementary data). The in vitro antioxidant activity of all the compounds was evaluated through their ability to quench the synthetic free radical, DPPH according to the method of Koleva et al.11 with little modification (Supplementary data).12 DPPH free radical scavenging activity was calculated using the following formula11,12:

% of free radical scavenging activity ¼ ½ðAbsControl  AbsSample Þ=AbsControl   100 The sample (compound tested) concentration having 50% radical inhibition activity (IC50) was calculated from the plot of DPPH free radical scavenging activity (%) against the sample concentration. The IC50 data for the tested compounds are shown in Table 1. In addition to the chalcone (1–12) and flavone (13–28) analogues, for better comparison purpose we have also used some standard antioxidants (29–32), other related bioactive moieties (33–37) and the precursors of the flavonoid molecules (38–42). DPPH is a stable free radical which accepts an electron or hydrogen radical to become a stable diamagnetic molecule. In the presence of a hydrogen donor free radical scavenger, DPPH is reduced and a free radical is generated from the scavenger.11 The reaction of DPPH is monitored by measuring the decrease of the absorbance of its radical at 517 nm. Upon reduction of this radical by an antioxidant, the absorbance at 517 nm disappears (Scheme S3 in Supplementary data).

The DPPH radical scavenging activity of the molecules, as measured in comparison to the well known standard antioxidants and their precursors was analyzed further to quantitate the structural requirements of this class of molecules in order to exhibit antioxidant potential. The IC50 values (Table 1) of the compounds were correlated with the structural features through the utilization of appropriate molecular descriptors. A few QSAR models9 were developed using the genetic function technique,10 which can select different combination of descriptors for the development of meaningful models, and the acceptability of the models was determined based on appropriate internal and external validation statistics. The best GFA (genetic function approximation) models10 thus obtained using both linear (Eq. (1)) and spline (Eq. (2)) terms are given below with a description of the different important descriptors appearing in the developed models (Table 2).

pC ¼ 13:5 þ 7:57  Atype O 58  2:443 vmp þ 9:643 vmc þ1:63  HOMO  0:007  JursPNSA2 ntraining ¼ 30; R2 ¼ 0:854; R2a ¼ 0:823; Q 2 ¼ 0:764; r 2m ðLOOÞ ¼ 0:709; Dr2m ðLOOÞ ¼ 0:087; ntest ¼ 12; R2pred

¼ 0:840; r2m ðtestÞ ¼ 0:776; Dr2m ðtestÞ ¼ 0:016; average r2m ðoverallÞ ¼ 0:717; Dr 2m ðoverallÞ ¼ 0:069 ð1Þ

pC ¼ 7:22 þ 6:68  Atype O 58  2:153 vmp þ8:713 vmc þ 0:020  V m þ2:70 < Atype C 19  1 > ntraining ¼ 30; R2 ¼ 0:846; R2a ¼ 0:814; Q 2 ¼ 0:792; r 2m ðLOOÞ ¼ 0:748; Dr 2m ðLOOÞ ¼ 0:027; ntest ¼ 12; R2pred ¼ 0:782; r 2m ðtestÞ ¼ 0:776; Dr 2m ðtestÞ ¼ 0:050; average r 2m ðoverallÞ ¼ 0:748; Dr 2m ðoverallÞ ¼ 0:046 ð2Þ Based on the essential structural requirements of the molecules, as defined by the appearance of the significant descriptors in the developed equation, twenty different molecules (D1–D20) were designed by changing the structural features about the parent moiety [Table 3a and b]. D1–D16 are flavone analogues [Table 3a] and D17–D20 are chalcone analogues [Table 3b]. The activities of the twenty designed compounds thus reported were predicted using the developed models. An average of the calculated values from the developed models was considered as the in silico predicted activity for each of the compounds [Table 3a and b].

Table 2 Interpretations of the descriptors appearing in the equations Statistical tool GFA-linear

Descriptor

Sign

Interpretation

Atype_O_58

+

3

HOMO

 + +

Jurs-PNSA-2



3

 + +

Presence of ketonic fragment favours activity ) in a molecule is disfavoured Presence of three consecutive bonds ( Presence of three bonds with a branching ( ) in a molecule is favoured It refers to the energy of the highest occupied molecular orbital and bears negative value due to the abundance of electrons. Thus decrease in HOMO value i.e., reduced electron richness which in turn reduces the nucleophilicity of the molecules favours activity It refers to the total charge weighted negative surface area and bears negative value due to the abundance of negatively charged surface area. It signifies the electron density over the molecule. More negatively charged surface area implicated by the presence of aromatic ring in the molecular structure improves activity ) in a molecule is disfavoured Presence of three consecutive single bonds ( Presence of three single bonds with a branching ( ) in a molecule is favoured Presence of more than one sp2 carbon bonded to an alkyl substituent and a heteroatom (=CRX) favours the antioxidant activity of the molecules Presence of ketonic fragment favours activity It defines the molecular volume inside the contact surface. Molecules with increased volume i.e., bulky molecules exhibit improved activity

vvp 3 v vc

GFA spline

3

vvp vvc

Atype_O_58 Vm

+ +

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Table 3a Molecular structures of designed flavone analogues along with their predicted and experimental DPPH radical scavenging activity

R4

Designed flavone analogues

D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 D14 D15 D16

O

Structure R

R6

R5

3

1

R2

R

Predicted DPPH Radical scavenging activity [IC50 (mM)]

Experimental DPPH radical scavenging activity [(IC50 ± SD) (mM)]

1.380 0.143 0.106 0.638 0.545 0.156 1.109 0.419 0.058 51.404 1.535 6.653 2.812 1.671 0.406 0.625

44.286 ± 2.054 ND 0.709 ± 0.019 ND 0.452 ± 0.003 ND ND 0.612 ± 0.006 ND ND ND ND 11.015 ± 0.797 29.600 ± 1.749 ND 27.850 ± 1.190

O

R1 = R2 = R3 = R5 = H, R4 = OH, R6 = tBu R1 = R2 = H, R3 = R5 = R4 = OH, R6 = tBu R1 = R2 = R4 = OH, R3 = R5 = H, R6 = tBu R1 = R2 = R4 = OH, R3 = R5 = H, R6 = Me R1 = R2 = R4 = OH, R3 = R5 = H, R6 = OtBu R1 = R2 = H, R3 = R4 = R5 = OH, R6 = OtBu R1 = R3 = R5 = H, R2 = R4 = OH, R6 = NMe2 R1 = R2 = R4 = OH, R3 = R5 = H, R6 = NMe2 R1 = R2 = H, R3 = R4 = R5 = OH, R6 = NMe2 R1 = R2 = H, R3 = R4 = R5 = OH, R6 = NHtBu R1 = R3 = R5 = H, R2 = R4 = OH, R6 = NHtBu R1 = OH, R2 = R3 = R4 = R5 = H, R6 = NHtBu R1 = OH, R2 = R3 = R4 = R5 = H, R6 = CHMe2 R1 = R3 = R5 = H, R2 = R4 = OH, R6 = CHMe2 R1 = OH, R3 = R5 = H, R2 = R4 = R6 = Me R1 = R2 = R4 = OH, R3 = R5 = H, R6 = Cl

ND = not determined.

Table 3b Molecular structures of designed chalcone analogues along with their predicted and experimental DPPH radical scavenging activity

R2 Designed chalcone analogues

R1

Structure

R3 D17 D18 D19 D20

R4

O

Predicted DPPH radical scavenging activity [IC50 (mM)]

Experimental DPPH radical scavenging activity [(IC50 ± SD) (mM)]

0.008 0.114 0.599 0.171

17.110 ± 0.386 ND ND ND

R5

R1 = OH, R2 = R3 = R5 = H, R4 = tBu R1 = R2 = OH, R3 = H, R5 = Me, R4 = tBu R1 = OH, R2 = R3 = Me, R5 = H, R4 = CHMe2 R1 = OH, R2 = R3 = R5 = R4 = Me

ND = not determined.

Some selected designed compounds (D1, D3, D5, D8, D13, D14, D16 and D17) were synthesized and tested experimentally11,12 to make a comparison between the predicted and experimental activity. Method shown in Scheme S2 was followed for the synthesis of designed flavone analogues D13 and D14.3–5 We have synthesized D17 by the reported method3–5 as shown in Scheme S1. In case of designed flavone analogues (D1, D3, D5, D8, D13, D14 and D16), we have developed a synthetic protocol as the above methods failed here. We have at first protected selectively of the ortho-hydroxy groups of polyhydroxy acetophenone moiety by tosylation according to the method of Lai et al.16 but unlike the reported method, in our case we have used N,N-DIPEA in place of Et3N and catalytic amount of DMAP as shown in the Scheme S4 of Supplementary data. The detailed synthetic protocols and spectral data used to confirm their structures are given in Supplementary data. Subsequent experimental determination of DPPH radical scavenging activity of the synthesized designed compounds reveals that predicted values of some of these compounds (D3, D5 and D8) [Table 3a] were actually very near to the corresponding experimental values suggesting the good predictive ability of the models. Thus it may be inferred that these three molecules exhibit potent free radical scavenging activity when compared to available standard antioxidant molecules. In general, large variation between experimental and theoretically predicted IC50 data was observed in the case of designed compounds having an isopropyl group at 4 position of ring B, for example, D13 and D14 [Table 3a].

From the literature survey, it is evident that the presence of ortho-dihydroxyl group (catechol system) on Ring-B and C2–C3 double bond conjugated with 4-oxo group (a,b-unsaturated carbonyl system) along with hydroxyl group at 3-position in Ring-C [Fig. S1(a)] are some important structural features of flavonoids essential for their antioxidant and antiradical activities. Moreover, the number and position of hydroxyl group in flavones moiety also control the antioxidant/radical scavenging properties of these type of compounds.17,18 Therefore, we have incorporated various electron releasing groups at 40 -position of Ring-B of flavonoid moiety. We have also extended the (a,b-unsaturated carbonyl part to study the effect of conjugation on their antioxidant and antiradical activities as in the case of 7, 10 and 24 (Table 1). We have also studied the effect when the hydroxyl group at 3-position is replaced by methoxy group and C7-H and C8-H are replaced by a furan ring as in the case of karanjin (26). It is interesting to note that these modifications of structural features appreciably influence the radical scavenging activity of flavonoids. Replacement of Ring-B (phenyl ring) by furyl or methylenedioxy phenyl moiety also enhanced the activity as in the case of 8, 23 and 25 (Table 1). In case of chalcone type of compounds, a general trend is observed. Presence of hydroxyl group at 20 position on Ring-A is an important structural feature which enhances their antiradical/antioxidant activity as in the case of compounds 4-8 (Table 1). The general trend of experimental IC50 values clearly indicates that chalcone analogues 1–12 (Table 1) and designed chalcone analogue D17 [Table 3b] showed lower DPPH radical scavenging than flavone analogues 13–28 (Table 1).

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Moreover, another general trend of activity was observed in the case of flavone analogues. Flavone analogues having hydroxyl group at 3-position and an electron releasing group at 40 -position, for example, compounds 17–23 (Table 1), D3, D5 and D8 [Table 3a] showed comparable activity to that of the standard antioxidants (29–32) (Table 1). But inspite of the absence of this particular structural feature of flavone moiety, compounds 23–27 showed very high antioxidant activity (comparable to that of the standard antioxidants) as it is evident from their very low IC50 values (Table 1). In summary, most of the flavone analogues showed higher DPPH radical scavenging antioxidant activity than corresponding precursors (38–42) and other bioactive moieties (33–37) (Table 1) which partially resembles to the parent flavone skeleton. However, it was also interesting to note that the flavone analogue 20 was found to be the most potent DPPH radical scavenger having the lowest IC50 value among the all the tested compounds. Moreover, based on the essential structural requirements as predicted by the developed QSAR models, twenty different molecules were designed by changing the structural variants about the parent moiety and some of these compounds were synthesized to screen for the antioxidant activity. The predicted activity of these compounds was in accordance with experimental data predicted by in silico methods. In future, this work can be extended by involving more number of flavones moieties having wide range of substitution pattern to get more accurate correlations among structures, and antioxidant activity of this class of bioactive moieties. This may eventually lead to develop novel and potential drugs which can be used to treat free radical induced oxidative stress related diseases in human beings. Acknowledgments We thank the SERB-DST [sanction order no. SR/SO/BB-0007/ 2011 dated 21.08.2012 to N.A.B.] and CSIR, India [Sanction No. 01 (2504)/11/EMR-II) to N.A.B.] for their financial support. S.D. and S.B. are thankful to CSIR and UGC (MANF) respectively for their fellowships. M.N.A. thanks to SERB-DST [sanction order no. SR/SO/BB-0007/2011 dated 21.08.2012 to N.A.B.] for his fellowship.

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We thank the Dept. of Chemistry, Siksha Bhavana, Visva-Bharati (Central University) and its DST-FIST programme for necessary infrastructural and instrumental facilities. Supplementary data Supplementary data (Figures S1–S2, Schemes S1–S3, detailed experimental procedures for the synthesis of the flavonoid analogues and the designed compounds along with their spectroscopic data used for the confirmation of their structures, Scheme S4, method of assay for DPPH radical scavenging antioxidant activity of the flavonoids) associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.bmcl.2014.09.028. References and notes 1. Piotta, P. G. J. Nat. Prod. 2000, 53, 1035. 2. Silva, M. M.; Santos, M. R.; Caroco, G.; Rocha, R.; Justino, G.; Mira, L. Free Radical Res. 2002, 36, 1219. 3. Desari, N.; Sestili, I.; Stein, M. L.; Tramontano, E.; Corrias, S.; Colla, P. L. Antiviral Chem. Chemother. 1998, 9, 497. 4. Alam, S.; Mostahar, S. J. Appl. Sci. 2005, 5, 327. 5. Begum, N. A.; Roy, N.; Laskar, R. A.; Roy, K. Med. Chem. Res. 2011, 20, 184. 6. Shahidi, F.; Zhong, Y. In Bailey’s Industrial Oil and Fat Products. 6th ed.; Shahidi, F., Ed.; John Wiley & Sons, 2005; Vol. 6, pp 491–512. 7. Kahl, R.; Kappus, H. Z Lebensm Unters Forsch 1993, 196, 329. 8. Harborne, J. B.; Mabry, T. J. The Flavonoids: Advances in Research; Chapman and Hall: London, 1982. 9. Roy, K.; Sengupta, C.; De, A. U. J. Sci. Ind. Res. 2001, 60, 699. 10. Rogers, D.; Hopfinger, A. J. J. Chem. Inf. Comput. Sci. 1994, 34, 854. 11. Koleva, I. I.; van Breek, T. A.; Linssen, J. P. H.; Groot, A. D.; Evstatieva, L. N. Phytochem. Anal. 2002, 13, 8. 12. Roy, N.; Laskar, R. A.; Sk, I.; Kumari, D.; Ghosh, T.; Begum, N. A. Food Chem. 2010, 126, 1115. 13. Begum, N. A.; Choudhury, D. N.; Banerji, J.; Das, B. P. Indian J. Chem. Soc. 2005, 82, 69. 14. Roy, N.; Mondal, S.; Laskar, R. A.; Basu, S.; Mandal, D.; Begum, N. A. Colloids Surf. B 2010, 76, 317. 15. Das, B. P.; Begum, N. A.; Choudhury, D. N.; Banerji, J. Indian J. Chem. Soc. 2005, 82, 62. 16. Lai, G.; Tan, P. Z.; Ghoshal, P. Synth. Commun. 2003, 33, 1727. 17. Rice-Evans, C. A.; Miller, N. J.; Paganga, G. Free Radical Biol. Med. 1996, 20, 933. 18. Amic´, D.; Davidovic´-Amic´, D.; Bešlo, D.; Trinajstic´, N. Croat. Chem. Acta 2003, 76, 55.

Please cite this article in press as: Das, S.; et al. Bioorg. Med. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.bmcl.2014.09.028