Bioactive Coumarins from Marine Sources: Origin, Structural Features and Pharmacological Properties

An Overview on the Synthesis of Lamellarins and Related Compounds with Biological Interest

Lamellarins are natural products with a [3,4]-fused pyrrolocoumarin skeleton possessing interesting biological properties. More than 70 members have been isolated from diverse marine organisms, such as sponges, ascidians, mollusks, and tunicates. There is a continuous interest in the synthesis of these compounds. In this review, the synthetic strategies for the synthesis of the title compounds are presented along with their biological properties. Three routes are followed for the synthesis of lamellarins. Initially, pyrrole derivatives are the starting or intermediate compounds, and then they are fused to isoquinoline or a coumarin moiety. Second, isoquinoline is the starting compound fused to an indole moiety. In the last route, coumarins are the starting compounds, which are fused to a pyrrole moiety and an isoquinoline scaffold. The synthesis of isolamellarins, azacoumestans, isoazacoumestans, and analogues is also described. The above synthesis is achieved via metal-catalyzed cross-coupling, [3 + 2] cycloaddition, substitution, and lactonization reactions. The title compounds exhibit cytotoxic, multidrug resistance (MDR), topoisomerase I-targeted antitumor, anti-HIV, antiproliferative, anti-neurodegenerative disease, and anti-inflammatory activities.

Synthesis of Coumarin-Conjugated Oligonucleotides via Knoevenagel Condensation to Prepare an Oligonucleotide Library

DNA-encoded libraries (DELs) are attracting attention as a screening tool in the early stages of drug discovery. In the development of DELs, drug candidate compounds are chemically synthesized on barcode DNA. Therefore, it is important to perform the synthesis under mild conditions so as to not damage the DNA. On the other hand, coumarins are gaining increasing research focus not only because they possess excellent fluorescence properties, but also because many medicines contain a coumarin skeleton. Among the various reactions developed for the synthesis of coumarins thus far, Knoevenagel condensation followed by intramolecular cyclization under mild conditions can yield coumarins. In this study, we developed a new synthetic method for preparing a coumarin-conjugated oligonucleotide library via Knoevenagel condensation. The results showed that coumarins substituted at the 5-, 6-, 7-, or 8-positions could be constructed on DNA to afford a total of 26 coumarin-conjugated DNAs. Moreover, this method was compatible with enzymatic ligation, demonstrating its utility in DEL synthesis. The developed strategy for the construction of coumarin scaffolds based on Knoevenagel condensation may contribute to the use of DELs in drug discovery and medicinal chemistry.

Cascade Annulation Strategy for Expeditious Assembly of Hydroxybenzo[c]chromen-6-ones and Their Photophysical Property Studies

A 1,8-diazabicyclo[5.4.0]undec-7-ene-promoted cascade double-annulation of ortho-alkynyl quinone methide (in situ generated from modular propargylamine) for constructing of 2-aryl-4-hydroxybenzo[c]chromen-6-ones is developed. This synthetic strategy offers remarkable operational simplicity as it allows the use of benchtop-grade solvents without the need for predrying measures and inert atmosphere protection. Additionally, it demonstrates good functional group compatibility. The photophysical properties of these compounds were also examined, revealing bright fluorescence with high quantum yields.

Recent Advances in Biologically Active Coumarins from Marine Sources: Synthesis and Evaluation

Coumarin and its derivatives have significantly attracted the attention of medicinal chemists and chemical biologists due to their huge range of biological, and in particular, pharmacological properties. Interesting families of coumarins have been found from marine sources, which has accelerated the drug discovery process by inspiring innovation or even by the identification of analogues with remarkable biological properties. The purpose of this review is to showcase the most interesting marine-derived coumarins from a medicinal chemistry point of view, as well as the novel and useful synthetic routes described to date to achieve these chemical structures. The references that compose this overview were collected from PubMed, Mendeley and SciFinder.

Anticancer Activities of Marine-Derived Phenolic Compounds and Their Derivatives

Since the middle of the last century, marine organisms have been identified as producers of chemically and biologically diverse secondary metabolites which have exerted various biological activities including anticancer, anti-inflammatory, antioxidant, antimicrobial, antifouling and others. This review primarily focuses on the marine phenolic compounds and their derivatives with potent anticancer activity, isolated and/or modified in the last decade. Reports on the elucidation of their structures as well as biosynthetic studies and total synthesis are also covered. Presented phenolic compounds inhibited cancer cells proliferation or migration, at sub-micromolar or nanomolar concentrations (lamellarins D (37), M (38), K (39), aspergiolide B (41), fradimycin B (62), makulavamine J (66), mayamycin (69), N-acetyl-N-demethylmayamycin (70) or norhierridin B (75)). In addition, they exhibited anticancer properties by a diverse biological mechanism including induction of apoptosis or inhibition of cell migration and invasive potential. Finally, phlorotannins 1–7 and bromophenols 12–29 represent the most researched phenolic compounds, of which the former are recognized as protective agents against UVB or gamma radiation-induced skin damages. Finally, phenolic metabolites were assorted into six main classes: phlorotannins, bromophenols, flavonoids, coumarins, terpenophenolics, quinones and hydroquinones. The derivatives that could not be attributed to any of the above-mentioned classes were grouped in a separate class named miscellaneous compounds.

Effect of the π-bridge on the light absorption and emission in push-pull coumarins and on their supramolecular organization

A family of eight π-extended push-pull coumarins with cross-conjugated (amide) and directly conjugated (p-phenylene, alkyne, alkene) bridges were synthesized through a convergent strategy. Using an experimentally calibrated computational protocol, their UV-Visible light absorption and emission spectra in solution were investigated. Remarkably, amide-, alkyne- and alkene-bridges undergo comparable vertical excitations. The different nature of these bridges manifests during excited-state relaxation and fluorescence. We predict that these molecules can serve as building blocks for p-type semiconductors with low reorganization energies, below 0.2 eV. Since solid-state self-assembly is crucial for this application, we examined the effect of the π-bridge over the supramolecular organization in this family of compounds to determine if stacking prevails in these π-extended coumarin derivatives. Amide and alkyne spacers allow coplanar conformations which crystallize readily; p-phenylene hinders planarity yet allows facile crystallization; alkene-bridged molecules eluded all crystallization attempts. All the crystals obtained feature dense face-to-face π-stacking with 3.5-3.7 Å interlayer distances, expected to facilitate charge transfer processes in the solid state.

Pd(II)-Catalyzed Fujiwara-Moritani Reactions for the Synthesis and Functionalization of Substituted Coumarins

Highly substituted coumarins, privileged and versatile scaffolds for bioactive natural products and fluorescence imaging, are obtained via a Pd(II)-catalyzed direct C-H alkenylation reaction (Fujiwara-Moritani reaction), which has emerged as a powerful tool for the construction and functionalization of heterocyclic compounds because of its chemical versatility and its environmental advantages. Thus, a selective 6-endo cyclization led to 4-substituted coumarins in moderate yields. Selected examples have been further functionalized in C3 through a second intermolecular C-H alkenylation reaction to give coumarin-acrylate hybrids, whose fluorescence spectra have been measured.

Cascade Lactonization/Benzannulation of Propargylamines with Dimethyl 3-Oxoglutarate for Modular Assembly of Hydroxylated/Arene-Functionalized Benzo[c]chromen-6-ones

A DBU-mediated cascade strategy of propargylamines with dimethyl 3-oxoglutarate for constructing a functionalized benzo[c]chromen-6-one core has been achieved. This cascade process presumably involves a sequence of 1,4-conjugate addition, followed by lactonization, alkyne-allene isomerization, enol-keto tautomerization, 6π-electrocyclization, and aromatization. This protocol features mild reaction conditions, simple operation, rich structural diversity, and good functional group tolerance. A photophysical survey reveals that the benzo[c]chromen-6-one products exhibit fluorescence properties and show potential for exploring fluorescent material applications.

Design, synthesis, and evaluation of novel coumarin-dithiocarbamate derivatives (IDs) as anti-colorectal cancer agents

Colorectal cancer (CRC) is a common malignant tumour of human digestive tract. The high mortality rate of CRC is closely related to the limitations of existing treatments. Thus, there is an urgent need to search for new anti-CRC agents. In this work, twenty novel coumarin-dithiocarbamate derivatives (IDs) were designed, synthesized and evaluated in vitro. The results suggest that the most active compound ID-11 effectively inhibited the proliferation of CRC cell lines while shown little impact on normal colon epithelial cells. Mechanism studies revealed that ID-11 displayed bromodomain-containing protein 4 inhibitory activity, and induced G2/M phase arrest, apoptosis as well as decreased the expression levels of the key genes such as c-Myc and Bcl-2 in CRC cell lines. Moreover, the ADMET properties prediction results shown that ID-11 possess well metabolic characteristics without obvious toxicities. Our data demonstrated that compound ID-11 may be a promising anti-CRC agent and deserved for further development.

Coumarin-1,2,3-triazole Hybrid Molecules: An Emerging Scaffold for Combating Drug Resistance

Undoubtedly, antibiotics have saved billions of lives, but lack of novel antibiotics, development of resistance mechanisms in almost all clinical isolates of bacteria, and recurrent infections caused by persistent bacteria hamper the successful treatment of the infections. Due to the widespread emergence of resistance, even the new families of anti-microbial agents have a short life expectancy. Drugs acting on a single target often lead to drug resistance and are associated with various side effects. For overcoming this problem, either multidrug therapy, or a single drug acting on multiple targets may be used. The latter is called 'hybrid molecules,' which are formed by clubbing two biologically active pharmacophores together, with or without an appropriate linker. In this rapidly evolving era, the development of natural product-based hybrid molecules may be a super-alternative to multidrug therapy, for combating drug resistance caused by various bacterial and fungal strains. Coumarins (benzopyran-2-one) are one of the earliest reported plant secondary metabolites having a clinically proven diverse range of pharmacological properties. On the other hand, 1,2,3-triazole is a common pharmacophore in many drugs responsible for polar interactions, improving the solubility and binding affinity to biomolecular targets. In this review, we discuss recent advances in Coumarin-1,2,3-triazole hybrids as potential anti-bacterial agents, aiming to provide a useful platform for the exploration of new leads with a broader spectrum, more effectiveness and less toxicity with multiple modes of action for the development of cost-effective and safer drugs in the future.

Phytochemical Screening, Antimicrobial and Antioxidant Activities of Selected Mangrove Species

Background:

Mangrove plant extracts are used in folkloric medicine as aphrodisiac, sedative, antioxidant, antimicrobial and antimalarial. Screening for antimicrobial, phytochemical and antioxidant activities of fourteen mangroves plant species (Aegiceras corniculatum, Acanthus ilicifolius, Avicennia alba, Avicennia marina, Avicennia officinalis, Bruguiera cylindrica, Bruguiera gymnorhiza, Ceriops tagel, Excoecaria agallocha, Kandelia candel, Rhizophora apiculata, Rhizophora mucronata, Sonneratia alba, and Sonneratia caseolaris) in various solvents are reported in this work.

Methods:

The antimicrobial screening was carried out using agar well diffusion method. In this study, nine pathogenic strains were used, including three Gram-positive and six Gram-negative bacteria. Phytochemical screening, total flavonoids, total phenolic and antioxidant activity was tested by DPPH radical scavenging assay. Four phenolic compounds (gallic acid, vanillin, tannic acid and quercetin) were quantified by LC-MS/MS in selected mangrove species.

Results:

Antimicrobial screening showed Staphylococcus aureus, Staphylococcus epidermidis, Enterococcus faecalis and Pseudomonas aeruginosa were most susceptible to ethyl acetate extract as compared to ethanol and methanol extract. Ethyl acetate extract of Avicennia marina and Bruguiera gymnorhiza showed strong antibacterial activity against Staphylococcus aureus, Staphylococcus epidermidis, Enterococcus faecalis and Pseudomonas aeruginosa. Phytochemical analysis revealed the presence of saponins, phenolics, flavonoids, alkaloids, tannins, and terpenoids, which was found to be variable as per the solvent used for extraction. In addition, total phenolics and total flavonoids content with different solvents were found in the range of 11.08 to 196.76 mg GAE/g and 12.92 to 110.3 mg QE/g of extract respectively. Moreover, antioxidant capacities expressed in terms of IC50 (mg/mL) showed that methanol extract exhibited higher antioxidant capacity followed by ethanol extract. LC-MS/MS analysis showed gallic acid and tannic acid are present in higher concentration in Aegiceras corniculatum and Sonneratia caseolaris than other species. The vanillin and quercetin were found in the least concentration.

Conclusion:

Mangroves species are rich source of antioxidant, phenolics and antimicrobial compounds.

Marine Pharmacology in 2014-2015: Marine Compounds with Antibacterial, Antidiabetic, Antifungal, Anti-Inflammatory, Antiprotozoal, Antituberculosis, Antiviral, and Anthelmintic Activities; Affecting the Immune and Nervous Systems, and Other Miscellaneous Mechanisms of Action

The systematic review of the marine pharmacology literature from 2014 to 2015 was completed in a manner consistent with the 1998-2013 reviews of this series. Research in marine pharmacology during 2014-2015, which was reported by investigators in 43 countries, described novel findings on the preclinical pharmacology of 301 marine compounds. These observations included antibacterial, antifungal, antiprotozoal, antituberculosis, antiviral, and anthelmintic pharmacological activities for 133 marine natural products, 85 marine compounds with antidiabetic, and anti-inflammatory activities, as well as those that affected the immune and nervous system, and 83 marine compounds that displayed miscellaneous mechanisms of action, and may probably contribute to novel pharmacological classes upon further research. Thus, in 2014-2015, the preclinical marine natural product pharmacology pipeline provided novel pharmacology as well as new lead compounds for the clinical marine pharmaceutical pipeline, and thus continued to contribute to ongoing global research for alternative therapeutic approaches to many disease categories.

Mutation at G103 of MtbFtsZ Altered their Sensitivity to Coumarins

Coumarins are natural polyphenol lactones comprising of fused rings of benzene and α-pyrone. The current study demonstrates the inhibitory effect of coumarins with various substitutions on Mycobacterium smegmatis mc² 155. We also demonstrate the effect of pomegranate (Punica granatum) extract containing ellagic acid, on M. smegmatis as well as their affect on MtbFtsZ (FtsZ from Mycobacterium tuberculosis). The ellagic acid extracts from pomegranate peels inhibit mycobacteria with a MIC of 25 μM and 0.3 to 3.5 mg/mL, respectively, but failed to inhibit the polymerization of MtbFtsZ. However, the coumarins were shown to inhibit the polymerization and GTPase activity of the protein as well as have an inhibitory affect on M. smegmatis mc² 155. Docking of the most active coumarin (7-Dimethyl-4-methyl coumarin with MIC of 38.7 μM) to the GTP binding site suggests that it interacted with the G103 residue. Based on the docking results two mutants of varying activity (G103S and G103A) were constructed to elucidate the interaction of MtbFtsZ and coumarins. Mutation of G103 with Serine (a bulky group) results in an inactive mutant and substitution with alanine produces a variant that retains most of the activity of the wild type. There is a disruption of the protofilament formation of the MtbFtsZ upon interaction with coumarins as demonstrated by TEM. The coumarins increase the length of Mycobacteria five times and MtbFtsZ localization is disturbed. The mutant proteins altered the GTPase and polymerization activity of coumarins as compared to wild type protein. The results here support that coumarins inhibit proliferation of Mycobacteria by targeting the assembly of MtbFtsZ and provide the possible binding site of coumarins on MtbFtsZ. This study may aid in the design of natural products as anti-mycobacterial agents. The currently reported GTP analogs for FtsZ are toxic to the human cell lines; natural coumarins targeting the GTP binding site of MtbFtsZ may hold promise as an important drug lead for tuberculosis treatment.

Synthesis and evaluation of anticancer activity of 6-pyrazolinylcoumarin derivatives

A series of novel 6-pyrazolinylcoumarins has been synthesized via multi-step protocol. The synthetic procedure was based on the acetylation of hydroxycoumarins; Fries rearrangement and Claisen–Schmidt condensation; the target 6-[5-aryl-4,5-dihydropyrazol-3-yl]-5-hydroxy-7-methylcoumarins (33–49) were obtained under reactions of hydrazine and 2-aryl-5-methyl-2,3-dihydropyrano[2,3-f]chromen-4,8-diones as the last phase of the protocol. Anticancer activity screening in NCI60-cell lines assay allowed identification of compound 47 with the highest level of antimitotic activity with mean GI₅₀ value of 10.20 μM and certain sensitivity profile toward the Leukemia cell lines CCRF-CEM and MOLT-4 (GI₅₀/TGI values 1.88/5.06 μM and 1.92/4.04 μM respectively).

Effect of xanthotoxin (8-methoxypsoralen) on the anticonvulsant activity of classical antiepileptic drugs against maximal electroshock-induced seizures in mice

The effects of xanthotoxin (8-methoxypsoralen) on the anticonvulsant activity of four classical antiepileptic drugs (carbamazepine, phenobarbital, phenytoin and valproate) were studied in the mouse maximal electroshock seizure model. Tonic hind limb extension (seizure activity) was evoked in adult male albino Swiss mice by a current (25 mA, 500 V, 50 Hz, 0.2 s stimulus duration) delivered via auricular electrodes. Total brain concentrations of antiepileptic drugs were measured by fluorescence polarization immunoassay to ascertain any pharmacokinetic contribution to the observed anticonvulsant effects. Results indicate that xanthotoxin (50 and 100 mg/kg, i.p.) significantly potentiated the anticonvulsant activity of carbamazepine against maximal electroshock-induced seizures (P<0.05 and P<0.001, respectively). Similarly, xanthotoxin (100 mg/kg, i.p.) markedly enhanced the anticonvulsant action of valproate in the maximal electroshock seizure test (P<0.001). In contrast, xanthotoxin (100 mg/kg, i.p.) did not affect the protective action of phenobarbital and phenytoin against maximal electroshock-induced seizures in mice. Moreover, xanthotoxin (100 mg/kg, i.p.) significantly increased total brain concentrations of carbamazepine (P<0.001) and valproate (P<0.05), but not those of phenytoin and phenobarbital, indicating pharmacokinetic nature of interactions between drugs. In conclusion, the combinations of xanthotoxin with carbamazepine and valproate, despite their beneficial effects in terms of seizure suppression in mice, were probably due to a pharmacokinetic increase in total brain concentrations of these antiepileptic drugs in experimental animals.