Chalcone: A Privileged Structure in Medicinal Chemistry

Privileged structures have been widely used as an effective template in medicinal chemistry for drug discovery. Chalcone is a common simple scaffold found in many naturally occurring compounds. Many chalcone derivatives have also been prepared due to their convenient synthesis. These natural products and synthetic compounds have shown numerous interesting biological activities with clinical potentials against various diseases. This review aims to highlight the recent evidence of chalcone as a privileged scaffold in medicinal chemistry. Multiple aspects of chalcone will be summarized herein, including the isolation of novel chalcone derivatives, the development of new synthetic methodologies, the evaluation of their biological properties, and the exploration of the mechanisms of action as well as target identification. This review is expected to be a comprehensive, authoritative, and critical review of the chalcone template to the chemistry community.

A comprehensive review on tyrosinase inhibitors

Tyrosinase is a multi-copper enzyme which is widely distributed in different organisms and plays an important role in the melanogenesis and enzymatic browning. Therefore, its inhibitors can be attractive in cosmetics and medicinal industries as depigmentation agents and also in food and agriculture industries as antibrowning compounds. For this purpose, many natural, semi-synthetic and synthetic inhibitors have been developed by different screening methods to date. This review has focused on the tyrosinase inhibitors discovered from all sources and biochemically characterised in the last four decades.

Potency of Michael reaction acceptors as inducers of enzymes that protect against carcinogenesis depends on their reactivity with sulfhydryl groups

Induction of phase 2 enzymes and elevations of glutathione are major and sufficient strategies for protecting mammals and their cells against the toxic and carcinogenic effects of electrophiles and reactive forms of oxygen. Inducers belong to nine chemical classes and have few common properties except for their ability to modify sulfhydryl groups by oxidation, reduction, or alkylation. Much evidence suggests that the cellular "sensor" molecule that recognizes the inducers and signals the enhanced transcription of phase 2 genes does so by virtue of unique and highly reactive sulfhydryl functions that recognize and covalently react with the inducers. Benzylidene-alkanones and -cycloalkanones are Michael reaction acceptors whose inducer potency is profoundly increased by the presence of ortho- (but not other) hydroxyl substituent(s) on the aromatic ring(s). This enhancement correlates with more rapid reactivity of the ortho-hydroxylated derivatives with model sulfhydryl compounds. Proton NMR spectroscopy provides no evidence for increased electrophilicity of the beta-vinyl carbons (the presumed site of nucleophilic attack) on the hydroxylated inducers. Surprisingly, these ortho-hydroxyl groups display a propensity for extensive intermolecular hydrogen bond formation, which may raise the reactivity and facilitate addition of mercaptans, thereby raising inducer potencies.

Structure of chalcone synthase and the molecular basis of plant polyketide biosynthesis

Chalcone synthase (CHS) is pivotal for the biosynthesis of flavonoid antimicrobial phytoalexins and anthocyanin pigments in plants. It produces chalcone by condensing one p-coumaroyl- and three malonyl-coenzyme A thioesters into a polyketide reaction intermediate that cyclizes. The crystal structures of CHS alone and complexed with substrate and product analogs reveal the active site architecture that defines the sequence and chemistry of multiple decarboxylation and condensation reactions and provides a molecular understanding of the cyclization reaction leading to chalcone synthesis. The structure of CHS complexed with resveratrol also suggests how stilbene synthase, a related enzyme, uses the same substrates and an alternate cyclization pathway to form resveratrol. By using the three-dimensional structure and the large database of CHS-like sequences, we can identify proteins likely to possess novel substrate and product specificity. The structure elucidates the chemical basis of plant polyketide biosynthesis and provides a framework for engineering CHS-like enzymes to produce new products.

Antioxidant constituents from licorice roots: isolation, structure elucidation and antioxidative capacity toward LDL oxidation

The present study analyzed the antioxidative properties of natural compounds from the root of the plant Glycyrrhiza glabra (licorice) toward LDL oxidation. Seven constituents, with antioxidant capacity were isolated from Glycyrrhiza glabra. The isolated compounds were identified as the isoflavans Hispaglabridin A (1), Hispaglabridin B (4), Glabridin (3), and 4'-O-Methylglabridin (2), the two chalcones, isoprenylchalcone derivative (5) and Isoliquiritigenin (6), and the isoflavone, Formononetin (7). Among these compounds, Glabridin constituted the major amount in the crude extract (11.6%, w/w) as detected by high-performance liquid chromatography (HPLC) analysis. The antioxidative capacities of the isolated compounds (1-7) were tested against beta-carotene destruction and LDL oxidation. The isoflavans (1-4) at a concentration of 50 microM inhibited beta-carotene consumption, following 90 min of incubation at 50 degrees C, similar to the inhibitory effect of the whole licorice crude extract (at 16 mg/1). The chalcones (5 and 6) exhibited moderate inhibition and the isoflavone 7 was almost inactive, whereas vitamin E (50 microM) completely inhibited beta-carotene consumption. The inhibitory effect of the constituents 1-7, at a concentration of 30 microM on 2,2'-azobis (2-amidinopropane) dihydrochloride (AAPH)-induced LDL oxidation was determined by measuring the amount of the thiobarbituric acid reactive substances (TBARS) and the amount of lipid peroxides. While compounds 1-6 exhibited high inhibitory activity, compound 7 and vitamin E were not active. A dose-dependent inhibitory effect of Glabridin, on the formation of cholesteryl linoleate hydroperoxide (CLOOH), in an AAPH-induced LDL oxidation system was also shown. Glabridin, at 5 or 40-60 microM concentration, inhibited the CLOOH formation by 62% and 90%, respectively. These results suggest that constituents 1-6 are very potent antioxidants toward LDL oxidation with Glabridin being the most abundant and potent antioxidant. As LDL oxidation is a key event in the formation of the early atherosclerotic lesion, the use of these natural antioxidants may be proven beneficial to attenuate atherosclerosis.

Antimalarial alkoxylated and hydroxylated chalcones [corrected]: structure-activity relationship analysis

Chalcones with 2',3',4'-trimethoxy, 2',4'-dimethoxy, 4'-methoxy, 4'-ethoxy, 2',4'-dihydroxy, and 4'-hydroxy groups on ring B were synthesized and evaluated in vitro against Plasmodium falciparum (K1) in a [3H] hypoxanthine uptake assay. The other ring A was quinoline, pyridine, naphthalene, or phenyl rings with electron-donating or electron-withdrawing substituents of varying lipophilicities. Trimethoxy 6 and 27, dimethoxy 7, 8, 29, and methoxy 31 analogues had good in vitro activities (IC(50) < 5 microM). 3-Quinolinyl ring A derivatives were well represented among the active compounds. Hydroxylated chalcones were less active than the corresponding alkoxylated analogues. When evaluated in vivo, 8 and 208 were comparable to chloroquine in extending the lifespan of infected mice. Multivariate data analysis showed that in vitro activity was mainly determined by the properties of ring B. Quantitative structure-activity relationship models with satisfactory predictive ability were obtained for various B ring chalcones using projections to latent structures. A model with good predictability was proposed for 19 active chalcones. Size and hydrophobicity were identified as critical parameters.

Cooperative N-heterocyclic carbene/Lewis acid catalysis for highly stereoselective annulation reactions with homoenolates

A new approach that takes advantage of N-heterocyclic carbene/Lewis acid cooperative catalysis provides access to cis-1,3,4-trisubstituted cyclopentenes from enals and chalcone derivatives with high levels of diastereoselectivity and enantioselectivity. The presence of Ti(OiPr)(4) as the Lewis acid allows for efficient substrate preorganization, which translates into high levels of diastereoselectivity. Additionally, we demonstrate the possibility of controlling the absolute stereochemistry of NHC-catalyzed reactions by employing a catalytic amount of a chiral Lewis acid as the unique source of optically active promoter.

Potent antimitotic and cell growth inhibitory properties of substituted chalcones

A series of substituted chalcones was synthesised and screened for cytotoxic activity against the K562 human leukaemia cell line. (E)-3-(3"-Hydroxy-4"-methoxyphenyl)-2-methyl-1-(3',4',5'- trimethoxyphenyl)-prop-2-en-1-one [IC50 (K562) 0.21 nM] was found to be the most active. A relationship between the conformation and cytotoxicity of the chalcones is discussed.

In vitro antimalarial activity of chalcones and their derivatives

A series of chalcones and their derivatives have been synthesized and identified as novel potential antimalarials using both molecular modeling and in vitro testing against the intact parasite. A large number of chalcones and their derivatives were prepared using one-step Claisen-Schmidt condensations of aldehydes with methyl ketones. These condensates were screened in vitro against both chloroquine-sensitive and chloroquine-resistant strains of Plasmodium falciparum and shown to be active at concentrations in the nanomolar range. The most active chalcone derivative, 1-(2,5-dichlorophenyl)-3-(4-quinolinyl)-2-propen-1-one (7), had an IC50 value of 200 nM against both a chloroquine-resistant strain (W2) and a chloroquine-sensitive strain (D6). The resistance indexes for all compounds were substantially lower than for chloroquine, suggesting that this series will be active against chloroquine-resistant malaria. Structure-activity relationships (SAR) of the chalcones in the context of a homology-based model structure of the malaria trophozoite cysteine protease, the most likely target enzyme, are presented.

Antioxidant and prooxidant actions of prenylated and nonprenylated chalcones and flavanones in vitro

Prenylated flavonoids found in hops and beer, i.e., prenylchalcones and prenylflavanones, were examined for their ability to inhibit in vitro oxidation of human low-density lipoprotein (LDL). The oxidation of LDL was assessed by the formation of conjugated dienes and thiobarbituric acid-reactive substances (TBARS) and the loss of tryptophan fluorescence. At concentrations of 5 and 25 microM, all of the prenylchalcones tested inhibited the oxidation of LDL (50 microg protein/ml) induced by 2 microM copper sulfate. The prenylflavanones showed less antioxidant activity than the prenylchalcones, both at 5 and 25 microM. At 25 microM, the nonprenylated chalcone, chalconaringenin (CN), and the nonprenylated flavanone, naringenin (NG), exerted prooxidant effects on LDL oxidation, based on TBARS formation. Xanthohumol (XN), the major prenylchalcone in hops and beer, showed high antioxidant activity in inhibiting LDL oxidation, higher than alpha-tocopherol and the isoflavone genistein but lower than the flavonol quercetin. When combined, XN and alpha-tocopherol completely inhibited copper-mediated LDL oxidation. These findings suggest that prenylchalcones and prenylflavanones found in hops and beer protect human LDL from oxidation and that prenylation antagonizes the prooxidant effects of the chalcone, CN, and the flavanone, NG.

Natural products as antiparasitic drugs

Natural products are not only the basis for traditional or ethnic medicine. Only recently, they have provided highly successful new drugs such as Artemisinin. Furthermore, screening natural products found in all sorts of environments such as the deep sea, rain forests and hot springs, and produced by all sorts of organisms ranging from bacteria, fungi and plants to protozoa, sponges and invertebrates, is a highly competitive field where all of the major pharmaceutical companies are encountered. Already, many new natural product groups have revealed antiparasitic properties of surprising efficacy and selectivity, as will be shown in this review for plant-derived alkaloids, terpenes and phenolics. Many novel lead structures, however, have severe chemico-physical drawbacks such as poor solubility. Here, innovative drug formulations and carrier systems might help, as discussed by the authors in another article of this series.

A common pharmacophore for a diverse set of colchicine site inhibitors using a structure-based approach

Modulating the structure and function of tubulin and microtubules is an important route to anticancer therapeutics, and therefore, small molecules that bind to tubulin and cause mitotic arrest are of immense interest. A large number of synthetic and natural compounds with diverse structures have been shown to bind at the colchicine site, one of the major binding sites on tubulin, and inhibit tubulin assembly. Using the recently determined X-ray structure of the tubulin:colchicinoid complex as the template, we employed docking studies to determine the binding modes of a set of structurally diverse colchicine site inhibitors. These binding models were subsequently used to construct a comprehensive, structure-based pharmacophore that in combination with molecular dynamics simulations confirms and extends our understanding of binding interactions at the colchicine site.

In vitro antifungal evaluation and structure-activity relationships of a new series of chalcone derivatives and synthetic analogues, with inhibitory properties against polymers of the fungal cell wall

Here we report the synthesis, in vitro antifungal evaluation and SAR study of 41 chalcones and analogues. In addition, all active structures were tested for their capacity of inhibiting Saccharomyces cerevisiae beta(1,3)-glucan synthase and chitin synthase, enzymes that catalyze the synthesis of the major polymers of the fungal cell wall.

Chalcone Derivatives: Promising Starting Points for Drug Design

Medicinal chemists continue to be fascinated by chalcone derivatives because of their simple chemistry, ease of hydrogen atom manipulation, straightforward synthesis, and a variety of promising biological activities. However, chalcones have still not garnered deserved attention, especially considering their high potential as chemical sources for designing and developing new effective drugs. In this review, we summarize current methodological developments towards the design and synthesis of new chalcone derivatives and state-of-the-art medicinal chemistry strategies (bioisosterism, molecular hybridization, and pro-drug design). We also highlight the applicability of computer-assisted drug design approaches to chalcones and address how this may contribute to optimizing research outputs and lead to more successful and cost-effective drug discovery endeavors. Lastly, we present successful examples of the use of chalcones and suggest possible solutions to existing limitations.

Study of the collision-induced radical cleavage of flavonoid glycosides using negative electrospray ionization tandem quadrupole mass spectrometry

Negative electrospray ionization tandem quadrupole mass spectrometry was used to study the collision-induced dissociation (CID) of the O-glycosidic bond from different commercially available flavonoid glycosides. Depending on the structure, flavonoid glycosides can undergo both a collision-induced homolytic and heterolytic cleavage of the O-glycosidic bond producing deprotonated radical aglycone ((Y(0) - H)(-*)) and aglycone (Y(0) (-)) product ions. The relative abundance of the radical aglycone to the aglycone fragment from flavonol-3-O-glycosides increased with increasing number of hydroxyl substituents in the B ring and in the order kaempferol - <quercetin - <myricetin-3-O-glycoside. The nature and position of the sugar substitution of the flavonol glycosides also affected the fragmentation to the radical aglycone. Obtained under similar conditions, the product ion spectrum of kaempferol-7-O-neohesperidoside showed only a minor radical aglycone product ion as opposed to kaempferol-3-O-rutinoside. The relative abundance of the radical aglycone to the aglycone fragment from flavone-7-O-glycosides was also dependent on the substitution in the B ring. CID of apigenin-7-O-glucoside produced relatively more of the radical aglycone fragment than luteolin-7-O-glucoside, while only the aglycone fragment was found from diosmetin-7-O-rutinoside. The position of the sugar substitution also affected the fragmentation of the flavone glycosides, such that the product ion spectrum of luteolin-4'-O-glucoside showed only the aglycone fragment as opposed to luteolin-7-O-glucoside. No radical aglycone fragments were found from the flavanone-7-O-glycosides and dihydrochalcone glycoside investigated in the study.

Advance in dietary polyphenols as α-glucosidases inhibitors: a review on structure-activity relationship aspect

The dietary polyphenols as α-glucosidases inhibitors have attracted great interest among researchers. The aim of this review is to give an overview of the research reports on the structure-activity relationship of dietary polyphenols inhibiting α-glucosidases. The molecular structures that influence the inhibition are the following: (1) The hydroxylation and galloylation of flavonoids including catechins improve the inhibitory activity. (2) The glycosylation of hyroxyl group and hydrogenation of the C2=C3 double bond on flavonoids weaken the inhibition. (3) However, cyaniding glycosides show higher inhibition against than cyanidin. Proanthocyanidins oligomers exhibit a stronger inhibitory activity than their polymers. (4) The hydroxylation on B ring and the glycosylation of stilbenes reduce the inhibitory activity. (5) Caffeoylquinic acids display strong inhibition against α-glucosidases. However, hydroxycinnamic acid, ferulic acid, and gallic acid hardly inhibited α-glucosidases. (6) The coupled galloyl structures attached to C-3 and C-6 of the 4C(1) glucose core of ellagitanin gave basic inhibitory activity. (7) The mono-glycosylation of chalcones slightly lowers the inhibition. However, the diglycosylation of chalcones significantly decreased the activity.

Chalcones as potent tyrosinase inhibitors: the effect of hydroxyl positions and numbers

The inhibition of tyrosinase is one of the major strategies to treat hyperpigmentation. Various limitations are associated with many of these inhibitors, such as high cytotoxicity, poor skin penetration and low stability in formulations. In continuation of our previous study [J. Agric. Food Chem. 51 (2003) 1201], showing that isoliquiritigenin chalcone (ILC) is a potent tyrosinase inhibitor, the present study aims to characterize the chalcone family as new tyrosinase inhibitors, and demonstrate their potential whitening potency. Nine mono-, di-, tri- and tetrahydroxychalcones were tested as inhibitors of tyrosinase mono- and diphenolase activities, showing that the most important factor in their efficacy is the location of the hydroxyl groups on both aromatic rings, with a significant preference to a 4-substituted B ring, rather than a substituted A ring. Neither the number of hydroxyls nor the presence of a catechol moiety on ring B correlated with increasing tyrosinase inhibition potency. 4-Hydroxychalcone (4-HC), ILC and Butein inhibited tyrosinase and shortened the lag period of enzyme monophenolase activity from about 490 min (control) to 30 min (ILC). As pigmentation also results from auto-oxidation, the antioxidant activity of 4-HC, ILC and Butein, were tested. Results showed that chalcones are also potent antioxidants, with Butein the most potent. We may conclude that chalcones are potentially potent new depigmentation agents, with their double effect of reduction and antioxidant activity. A deeper understanding of the relation between their structures to their potency will contribute to designing the optimal agents.

The antioxidant activity of phloretin: the disclosure of a new antioxidant pharmacophore in flavonoids

Phloretin is a dihydrochalcone flavonoid that displays a potent antioxidant activity in peroxynitrite scavenging and the inhibition of lipid peroxidation. Comparison with structurally related compounds revealed that the antioxidant pharmacophore of phloretin is 2,6-dihydroxyacetophenone. The potent activity of 2,6-dihydroxyacetophenone is due to stabilisation of its radical via tautomerisation. The antioxidant pharmacophore in the dihydrochalcone phloretin, i.e., the 2,6-dihydroxyacetophenone group, is different from the antioxidant pharmacophores previously reported in flavonoids.

Antileishmanial chalcones: statistical design, synthesis, and three-dimensional quantitative structure-activity relationship analysis

A large number of substituted chalcones have been synthesized and tested for antileishmanial and lymphocyte-suppressing activities. A subset of the chalcones was designed by using statistical methods. 3D-QSAR analyses using 67 (antileishmanial activity) and 63 (lymphocyte-suppressing activity) of the compounds for the training sets and 9 compounds as an external validation set were performed by using the GRID/GOLPE methodology. The Smart Region Definition procedure with subsequent region selection as implemented in GOLPE reduced the number of variables to approximately 1300 yielding 3D-QSAR models of high quality (lymphocyte-suppressing model, R2 = 0. 90, Q2 = 0.80; antileishmanial model, R2 = 0.73, Q2 = 0.63). The coefficient plots indicate that steric interactions between the chalcones and the target are of major importance for the potencies of the compounds. A comparison of the coefficient plots for the antileishmanial effect and the lymphocyte-suppressing activity discloses significant differences which should make it possible to design chalcones having a high antileishmanial activity without suppressing the proliferation of lymphocytes.

Structure-activity relationships of antileishmanial and antimalarial chalcones

A series of oxygenated chalcones which have been evaluated earlier for antimalarial activity (Plasmodium falciparum K1) were tested for antileishmanial activity against Leishmania donovani amastigotes. A comparison of structure-activity relationships reveal that different physicochemical and structural requirements exist for these two activities. Antileishmanial activity is associated with less lipophilic chalcones, in particular those with 4'-hydroxyl-substituted B rings and hetero/polyaromatic A rings. In contrast, chalcones with good antimalarial activity have alkoxylated B rings and electron-deficient A rings. Visualization of the steric and electrostatic fields generated from comparative molecular field analysis (CoMFA) indicate that the ring A of chalcones make a more significant contribution to antileishmanial activity while both rings A and B are important for antimalarial activity. Despite different requirements, two alkoxylated chalcones (8, 19) were identified which combined good antimalarial and antileishmanial activities.

Synthesis and biological evaluation of chalcones and their derived pyrazoles as potential cytotoxic agents

A series of substituted chalcones and their corresponding pyrazoles were synthesized and evaluated for in vitro cytotoxic activity against a panel of human cancer cell lines. Out of 93 compounds screened, 8 compounds, 1s, 3i,j,n, 4i,j,n and 4s, showed marked activity. Compounds 4j,n and 4s were found to be the most promising in this study. SAR is also discussed.

LAP5 and LAP6 encode anther-specific proteins with similarity to chalcone synthase essential for pollen exine development in Arabidopsis

Pollen grains of land plants have evolved remarkably strong outer walls referred to as exine that protect pollen and interact with female stigma cells. Exine is composed of sporopollenin, and while the composition and synthesis of this biopolymer are not well understood, both fatty acids and phenolics are likely components. Here, we describe mutations in the Arabidopsis (Arabidopsis thaliana) LESS ADHESIVE POLLEN (LAP5) and LAP6 that affect exine development. Mutation of either gene results in abnormal exine patterning, whereas pollen of double mutants lacked exine deposition and subsequently collapsed, causing male sterility. LAP5 and LAP6 encode anther-specific proteins with homology to chalcone synthase, a key flavonoid biosynthesis enzyme. lap5 and lap6 mutations reduced the accumulation of flavonoid precursors and flavonoids in developing anthers, suggesting a role in the synthesis of phenolic constituents of sporopollenin. Our in vitro functional analysis of LAP5 and LAP6 using 4-coumaroyl-coenzyme A yielded bis-noryangonin (a commonly reported derailment product of chalcone synthase), while similar in vitro analyses using fatty acyl-coenzyme A as the substrate yielded medium-chain alkyl pyrones. Thus, in vitro assays indicate that LAP5 and LAP6 are multifunctional enzymes and may play a role in both the synthesis of pollen fatty acids and phenolics found in exine. Finally, the genetic interaction between LAP5 and an anther gene involved in fatty acid hydroxylation (CYP703A2) demonstrated that they act synergistically in exine production.

Inhibitory activity of cyclohexenyl chalcone derivatives and flavonoids of fingerroot, Boesenbergia rotunda (L.), towards dengue-2 virus NS3 protease

Boesenbergia rotunda (L.) cyclohexenyl chalcone derivatives, 4-hydroxypanduratin A and panduratin A, showed good competitive inhibitory activities towards dengue 2 virus NS3 protease with the Ki values of 21 and 25 microM, respectively, whilst those of pinostrobin and cardamonin were observed to be non-competitive. NMR and GCMS spectroscopic data formed the basis of assignment of structures of the six compounds isolated.

Analgesic and antiinflammatory effects of chalcones isolated from Myracrodruon urundeuva allemão

The present work showed analgesic and antiinflammatory activities from a fraction containing three dimeric chalcones (chalcone enriched fraction - CEF), isolated from the stem-bark ethyl acetate extract of Myracrodruon urundeuva Allemao (Anacardiaceae). M. urundeuva is a popular medicinal plant used widely in Northeast Brazil, mainly as a topical female genital tract antiinflammatory. We observed that the CEF (5 and 10 mg/kg body wt., i.p. or p.o.) inhibited acetic acid-induced abdominal contractions in mice. In the formalin test, the CEF (5 and 10 mg/kg body wt.) was more effective intraperitoneally and inhibited predominantly the second phase of response. Naloxone reversed this effect, indicating an involvement of the opioid system. The CEF (10 and 20 mg/kg body wt.) also increased the reaction time to thermal stimuli in the hot-plate test in mice, after i.p. but not after p.o. administration. In the carrageenan-induced paw edema test in mice, the CEF (20 and 40 mg/kg body wt.) decreased paw volume significantly, after i.p. administration 2-4 hours after carrageenan injection. The CEF (40 mg/kg body wt.) was also active orally during the same period of time. The present work is the first report on peripheral and central analgesic effects and antiinflammatory activity of natural dimeric chalcones.