Synthesis and Chemical Properties of 3-Phosphono-coumarins and 1,2-Benzoxaphosphorins as Precursors for Bioactive Compounds

Combined coulometric and voltammetric analytical approach provides novel insights into the antioxidant capacity of natural coumarins and furanocoumarins

Despite intense investigation on the subject, the information available on the antioxidant capacity of natural coumarins and furanocoumarins is controversial. In the present study the antioxidant capacity of 19 pure naturally occurring compounds, among which 5 coumarins and 14 furanocoumarins is determined, using both the coulometrically determined antioxidant capacity (CDAC) and linear sweep voltammetry (LSV). None of the coumarins and furanocoumarins assayed exhibited appreciable scavenging activity against the DPPH radical. In contrast, they were differently reactive against Br2. While coumarin did not show detectable CDAC, its 7-hydroxy derivative, namely umbelliferon, had a strikingly high CDAC value (35 mmol e- g-1) that remained unchanged in the pH 5-9 range. The presence of 5-alkoxy substituents appeared to increase critically the CDAC values so that bergapten (5-methoxypsoralen) exhibited the highest CDAC value (12 mmol e- g-1) among the assayed furanocoumarins. For all the compounds investigated, CDAC and LSV values were strictly correlated (R2 = 0.91). These findings suggest that complex mechanisms beyond pure hydrogen atom transfer or electron transfer and not only related to reactivity with Br2 may concur to determine the antioxidant capacity of coumarins. CDAC and LSV of non-volatile fraction of bergamot peel essential oil and its defurocoumarinized counterpart were investigated as real coumarins- and furanocoumarins-rich real samples. Owing to the predominant persistence of bergamottin, removal of bergapten did not significantly reduce the antioxidant capacity, while it minimizes the risks associated with phototoxicity.

Antioxidants: a comprehensive review

Antioxidants had a growing interest owing to their protective roles in food and pharmaceutical products against oxidative deterioration and in the body and against oxidative stress-mediated pathological processes. Screening of antioxidant properties of plants and plant derived compounds requires appropriate methods, which address the mechanism of antioxidant activity and focus on the kinetics of the reactions including the antioxidants. Many studies have been conducted with evaluating antioxidant activity of various samples of research interest using by different methods in food and human health. These methods were classified methods described and discussed in this review. Methods based on inhibited autoxidation are the most suited for termination-enhancing antioxidants and, for chain-breaking antioxidants while different specific studies are needed for preventive antioxidants. For this purpose, the most commonly methods used in vitro determination of antioxidant capacity of food and pharmaceutical constituents are examined and also a selection of chemical testing methods is critically reviewed and highlighting. In addition, their advantages, disadvantages, limitations and usefulness were discussed and investigated for pure molecules and raw plant extracts. The effect and influence of the reaction medium on performance of antioxidants is also addressed. Hence, this overview provides a basis and rationale for developing standardized antioxidant capacity methods for the food, nutraceuticals, and dietary supplement industries. Also, the most important advantages and shortcomings of each method were detected and highlighted. The underlying chemical principles of these methods have been explained and thoroughly analyzed. The chemical principles of methods of 1,1-diphenyl-2-picrylhydrazyl (DPPH•) radical scavenging, 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulphonate) radical (ABTS·+) scavenging, ferric ions (Fe3+) reducing assay, ferric reducing antioxidant power (FRAP) assay, cupric ions (Cu2+) reducing power assay (Cuprac), Folin-Ciocalteu reducing capacity (FCR assay), superoxide radical anion (O2·-), hydroxyl radical (OH·) scavenging, peroxyl radical (ROO·) removing, hydrogen peroxide (H2O2) decomposing, singlet oxygen (1O2) quenching assay, nitric oxide radical (NO·) scavenging assay and chemiluminescence assay are overviewed and critically discussed. Also, the general antioxidant aspects of the main food and pharmaceutical components were discussed through several methods currently used for detecting antioxidant properties of these components. This review consists of two main sections. The first section is devoted to the main components in food and their pharmaceutical applications. The second general section includes definitions of the main antioxidant methods commonly used for determining the antioxidant activity of components. In addition, some chemical, mechanistic, and kinetic properties, as well as technical details of the above mentioned methods, are provided. The general antioxidant aspects of main food components have been discussed through various methods currently used to detect the antioxidant properties of these components.

Elucidating the Mechanism of Coumarin Homodimerization Using 3-Acetylcoumarin Derivatives

The current study is a continuation of our previous investigations into the radical homodimeric reaction mechanism of 3-acetylcoumarin. In the current study, the effects of different substituents on the coumarin ring of 3-acetylcoumarin are investigated both experimentally and theoretically. Several 3-acetylcoumarin derivatives (substituted at C-6, C-7, and C-8) were tested in the optimized reaction conditions under ultrasound irradiation, and biscoumarin species were isolated and characterized. The elucidation of the substituent's effect was further investigated by means of DFT calculations (free-energy calculations, NBO analysis), both in the initial substituted coumarins and in the formed radicals. It was observed that the presence of substituents at the C-6 and C-8 positions in the coumarin moiety would not affect significantly the formation of a radical, while a group at position C-7 could either stabilize or destabilize the formed radical depending on the electronic properties of the substituent.

Evaluation of Antibacterial Activity against Nosocomial Pathogens of an Enzymatically Derived α-Aminophosphonates Possessing Coumarin Scaffold

The purpose of the present study was to evaluate the synergistic effect of two important pharmacophores, coumarin and α-amino dimethyl phosphonate moieties, on antimicrobial activity against selected strains of multidrug-resistant nosocomial pathogenic bacteria. The previously developed enzyme-catalysed Kabachnik-Fields protocol allowed us to obtain the studied compounds with high yields which were free from metal impurities. The structure-activity relationship revealed that inhibitory activity is strongly related to the presence of the trifluoromethyl group (CF3-) in the coumarin scaffold. MIC and MBC studies carried out on six selected pathogenic bacterial strains (Gram-positive pathogenic Staphylococcus aureus (ATCC 23235) strain, as well as on Gram-negative Acinetobacter baumannii (ATCC 17978), Pseudomonas aeruginosa (ATCC 15442), Enterobacter cloacae (ATCC 49141), Porphyromonas gingivalis (ATCC 33277), and Treponema denticola (ATCC 35405)) have shown that tested compounds show a strong bactericidal effect at low concentrations. Among all agents investigated, five exhibit higher antimicrobial activity than those observed for commonly used antibiotics. It should be noted that all the compounds tested showed very high activity against S. aureus, which is the main source of nosocomial infections that cause numerous fatalities. Furthermore, we have shown that the studied coumarin-based α-aminophosphonates, depending on their structural characteristics, are non-selective and act efficiently against various Gram-positive and Gram-negative pathogens, which is of great importance for hospitalised patients.

Enzymatic Synthesis of a Novel Coumarin Aminophosphonates: Antibacterial Effects and Oxidative Stress Modulation on Selected E. coli Strains

The objective of the present study was to evaluate the synergistic effect of two important pharmacophores, coumarin and α-amino dimethyl phosphonate moieties, on antimicrobial activity toward selected LPS-varied E. coli strains. Studied antimicrobial agents were prepared via a Kabachnik-Fields reaction promoted by lipases. The products were provided with an excellent yield (up to 92%) under mild, solvent- and metal-free conditions. A preliminary exploration of coumarin α-amino dimethyl phosphonate analogs as novel antimicrobial agents was carried out to determine the basic features of the structure responsible for the observed biological activity. The structure-activity relationship revealed that an inhibitory activity of the synthesized compounds is strongly related to the type of the substituents located in the phenyl ring. The collected data demonstrated that coumarin-based α-aminophosphonates can be potential antimicrobial drug candidates, which is particularly crucial due to the constantly increasing resistance of bacteria to commonly used antibiotics.

Diastereoselective Synthesis of Novel Spiro-Phosphacoumarins and Evaluation of Their Anti-Cancer Activity

Herein we present the regio- and diastereoselective synthesis of novel pyrrolidine-fused spiro-dihydrophosphacoumarins via intermolecular [3 + 2] cycloaddition reaction. The presented approach is complementary to existing ones and provides an easy entry to the otherwise inaccessible derivatives. Additionally, the unprecedented pathway of the reaction of 4-hydroxycoumarin with azomethine ylides is described. The anti-cancer activity of the obtained compounds was tested in vitro, the most potent compound being 2.6-fold more active against the HuTu 80 cell line than the reference 5-fluorouracil, with a selectivity index > 32.

Efficient Synthesis of Fluorescent Coumarins and Phosphorous-Containing Coumarin-Type Heterocycles via Palladium Catalyzed Cross-Coupling Reactions

Quantum-chemical calculations on the spectral properties of some aryl substituted 3-phosphonocoumarins were performed, and the effect of the substituents in the aryl moiety was evaluated. The structures possessing promising fluorescent properties were successfully synthesized via Suzuki and Sonogashira cross-coupling. The synthetic protocol was also applied for the phosphorous chemoisomer of 3-phosphonocoumarin, 1,2-benzoxaphosphorin, and their carboxylate analogues. The optical properties of the arylated and alkynylated products were experimentally determined. The obtained quantum-chemical and experimental results give the possibility for a fine tuning of the optical properties of phosphorous-containing coumarin systems by altering the substituent at its C-6 position.

Experimental and Theoretical Study on the Homodimerization Mechanism of 3-Acetylcoumarin

In the present study, the reaction conditions for homodimerization process of 3-acetylcoumarin were achieved under sonication using combination of zinc and metallic salt (ZnCl2 or Zn(OAc)2). Appropriate frequency and sound amplitude have been identified as significant variables for the initiation of the reaction. On the base of first principal calculations and experimental results, the mechanism of the reaction was investigated. The relative stability of the possible intermediates has been compared, including evaluation on the ionic and radical reaction pathways for the dimerization process. Theoretical results suggested that the radical mechanism is more favorable. The C-C bond formation between the calculated radical intermediates occurs spontaneously (∆G = −214 kJ/mol for ZnCl2, −163 kJ/mol in the case of Zn(OAc)2), which proves the possibility for the homodimerization of 3-acetylcoumarin via formation of radical species. Both experimental and theoretical data clarified the activation role of the solvent on the reactivity of the Zn-salt. The formation of complexes of solvent molecules with Zn-atom from the ZnCl2 reduces the energy barrier for the dissociation of Zn-Cl bond and facilitate the formation of the dimeric product.

Coumarins Synthesis and Transformation via C–H Bond Activation—A Review

For several decades, coumarins have attracted considerable attention due to the fact of their application in diverse fields such as medical science and biomedical research as well as several industrial branches. Recently, many compounds containing the coumarin moiety have been intensively studied, mainly due to the fact of their biological activities such as antitumor, antioxidative, anti-HIV, vasorelaxant, antimicrobial, and anticancer. They are also widely used as fluorescent dyes and probes because of their great structural flexibility and large fluorescent quantum yields. For this reason, numerous attempts have been made to develop new and more practical methods for the synthesis of these compounds. This review aims at providing a comprehensive overview of coumarin synthesis methods by direct C–H bond activation in order to demonstrate the current state-of-the-art methods as well as the current limitations.

Antioxidants and antioxidant methods: an updated overview

Antioxidants had a growing interest owing to their protective roles in food and pharmaceutical products against oxidative deterioration and in the body and against oxidative stress-mediated pathological processes. Screening of antioxidant properties of plants and plant-derived compounds requires appropriate methods, which address the mechanism of antioxidant activity and focus on the kinetics of the reactions including the antioxidants. Many studies evaluating the antioxidant activity of various samples of research interest using different methods in food and human health have been conducted. These methods are classified, described, and discussed in this review. Methods based on inhibited autoxidation are the most suited for termination-enhancing antioxidants and for chain-breaking antioxidants, while different specific studies are needed for preventive antioxidants. For this purpose, the most common methods used in vitro determination of antioxidant capacity of food constituents were examined. Also, a selection of chemical testing methods was critically reviewed and highlighted. In addition, their advantages, disadvantages, limitations and usefulness were discussed and investigated for pure molecules and raw extracts. The effect and influence of the reaction medium on the performance of antioxidants are also addressed. Hence, this overview provides a basis and rationale for developing standardized antioxidant methods for the food, nutraceuticals, and dietary supplement industries. In addition, the most important advantages and shortcomings of each method were detected and highlighted. The chemical principles of these methods are outlined and critically discussed. The chemical principles of methods of 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulphonate) radical (ABTS^·+) scavenging, 1,1-diphenyl-2-picrylhydrazyl (DPPH·) radical scavenging, Fe³⁺-Fe²⁺ transformation assay, ferric reducing antioxidant power (FRAP) assay, cupric ions (Cu²⁺) reducing power assay (Cuprac), Folin-Ciocalteu reducing capacity (FCR assay), peroxyl radical (ROO·), superoxide radical anion (O₂^·-), hydrogen peroxide (H₂O₂) scavenging assay, hydroxyl radical (OH·) scavenging assay, singlet oxygen (¹O₂) quenching assay, nitric oxide radical (NO·) scavenging assay and chemiluminescence assay are outlined and critically discussed. Also, the general antioxidant aspects of main food components were discussed by a number of methods, which are currently used for the detection of antioxidant properties of food components. This review consists of two main sections. The first section is devoted to the main components in the food and pharmaceutical applications. The second general section comprises some definitions of the main antioxidant methods commonly used for the determination of the antioxidant activity of components. In addition, some chemical, mechanistic and kinetic basis, and technical details of the used methods are given.