Isocoumarins: a new class of selective carbonic anhydrase IX and XII inhibitors.

Isocoumarins, isomeric to comarins which act as effective carbonic anhydrase (CA, EC 4.2.1.1) inhibitors, were investigated for the first time as inhibitors of this enzyme. A series of 3-substituted and 3,4-disubstituted isocoumarins incorporating phenylhydrazone, 1-phenyl-pyrazole and pyrazolo-substituted pyrimidine trione/thioxo-pyrimidine dione moieties were investigated for their interaction with four human (h) CA isoforms, hCA I, II, IX and XII, known to be important drug targets. hCA I and II were not inhibited by these compounds, whereas hCA IX and XII were inhibited in the low micromolar range by the less bulky derivatives. The inhibition constants ranged between 2.7-78.9 µM against hCA IX and of 1.2-66.5 µM against hCA XII. As for the coumarins, we hypothesise that the isocoumarins are hydrolysed by the esterase activity of the enzyme with formation of 2-carboxy-phenylacetic aldehydes which act as CA inhibitors. Isocoumarins represent a new class of CA inhibitors.

PdCl2-catalyzed synthesis of a new class of isocoumarin derivatives containing aminosulfonyl / aminocarboxamide moiety: First identification of a isocoumarin based PDE4 inhibitor.

While anti-inflammatory properties of isocoumarins are known their PDE4 inhibitory potential was not explored previously. In our effort the non-PDE4 inhibitor isocoumarins were transformed into the promising inhibitors via introducing an aminosulfonyl/aminocarboxamide moiety to the C-3 benzene ring attached to the isocoumarin framework. This new class of isocoumarins were synthesized via a PdCl2-catalyzed construction of the 4-allyl substituted 3-aryl isocoumarin ring starting from the appropriate 2-alkynyl benzamide derivative. Several compounds showed good inhibition of PDE4B in vitro and the SAR indicated superiority of aminosulfonamide moiety over aminocarboxamide in terms of PDE4B inhibition. Two compounds 3q and 3u with PDE4B IC50 = 0.43 ± 0.11 and 0.54 ± 0.19 µM and ≥ 2-fold selectivity over PDE4D emerged as initial hits. The participation of aminosulfonamide moiety in PDE4B inhibition and the reason for selectivity though moderate shown by 3q and 3u was revealed by the in silico docking studies. In view of potential usefulness of moderately selective PDE4B inhibitors the compound 3u (that showed PDE4 selectivity over other PDEs) was further evaluated in adjuvant induced arthritic rats. At an intraperitoneal dose of 30 mg/kg the compound showed a significant reduction in paw swelling (in a dose dependent manner), inflammation and pannus formation (in the knee joints) as well as pro-inflammatory gene expression/mRNA levels and increase in body weight. Moreover, besides its TNF-α inhibition and no significant toxicity in an MTT assay the compound did not show any adverse effects in a thorough toxicity studies e.g. teratogenicity, hepatotoxicity, cardiotoxicity and apoptosis in zebrafish. Thus, the isocoumarin 3u emerged as a new, safe and moderately selective PDE4B inhibitor could be useful for inflammatory diseases possibly including COVID-19.

Semisynthesis, Antiplasmodial Activity, and Mechanism of Action Studies of Isocoumarin Derivatives.

In this study, eight natural isocoumarins (1-8) were isolated from a marine-derived Exserohilum sp. fungus. To explore their structure-activity relationship and discover potent antimalarial leads, a small library of 22 new derivatives (1a-1n, 2a, 3a-3c, 4a-4c, and 7a) were semisynthesized by varying the substituents of the aromatic ring and the aliphatic side chains. The natural compound (1) and three semisynthetic derivatives (1d, 1n, and 2a), possessing an all-cis stereochemistry, exhibited strong antiplasmodial activity with IC50 values of 1.1, 0.8, 0.4, and 2.6 µM, respectively. Mechanism studies show that 1n inhibits hemozoin polymerization and decreases the mitochondrial membrane potential but also inhibits P. falciparum DNA gyrase. 1n not only combines different mechanisms of action but also exhibits a high therapeutic index (CC50/IC50 = 675), high selectivity, and a notable drug-like profile.

Synthesis of Novel 1,2,3-Triazole Derivatives of Isocoumarins and 3,4-Dihydroisocoumarin with Potential Antiplasmodial Activity In Vitro.

BACKGROUND: Malaria greatly affects the world health, having caused more than 228 million cases only in 2018. The emergence of drug resistance is one of the main problems in its treatment, demonstrating the need for the development of new antimalarial drugs. OBJECTIVE: Synthesis and in vitro antiplasmodial evaluation of triazole compounds derived from isocoumarins and a 3,4-dihydroisocoumarin. METHODS: The compounds were synthesized in 4 to 6-step reactions with the formation of the triazole ring via the Copper(I)-catalyzed 1,3-dipolar cycloaddition between isocoumarin or 3,4- dihydroisocoumarin azides and terminal alkynes. This key reaction provided compounds with an unprecedented connection of isocoumarin or 3,4-dihydroisocoumarin and the 1,2,3-triazole ring. The products were tested for their antiplasmodial activity against a Plasmodium falciparum chloroquine resistant and sensitive strains (W2 and 3D7, respectively). RESULTS: Thirty-one substances were efficiently obtained by the proposed routes with an overall yield of 25-53%. The active substances in the antiplasmodial test displayed IC50 values ranging from 0.68-2.89 µM and 0.85-2.07 µM against W2 and 3D7 strains, respectively. CONCLUSION: This study demonstrated the great potential of isocoumarin or 3,4-dihydroisocoumarin derivatives because practically all the tested substances were active against Plasmodium falciparum.

An Indirect Strategy for Trifluoromethylation via an Iridium Catalyst: Approach to Generate Isocoumarin Skeletons in Bioactive Molecules.

3-Bromo-1,1,1-trifluoroacetone was first disclosed as an effective indirect trifluoromethylation reagent to construct the important 3-trifluoromethyl isocoumarin skeleton. The reaction proceeds through a ligand-promoted, iridium-catalyzed ortho-selective C-H alkylation of benzoic acid and an intermolecular cyclization reaction promoted by silver acetate. A wide range of 3-trifluoromethyl isocoumarins can be easily obtained in moderate to good yields. Importantly, the isocoumarin skeleton can be easily formed in bioactive compounds, highlighting the importance of this reaction.

Rosuvastatin based novel 3-substituted isocoumarins / 3-alkylidenephthalides: Ultrasound assisted synthesis and identification of new anticancer agents.

A new class of 3-substituted isocoumarin/3-alkylidenephthalide based novel small molecules derived from rosuvastatin were designed and synthesized via the ultrasound assisted Cu-mediated coupling-cyclization in a single pot with remarkable regioselectivity. The phthalides were generally obtained at lower temperature whereas the use of elevated temperature afforded isocoumarins. Two compounds e.g. 3n and 4d showed promising cytotoxic effects when tested against HCT 116, HepG2 and PA-1 cell lines at 10 µM. Indeed, 4d was found to be a potent cytotoxic agent (IC50 ∼ 0.76-4.51 µM). Both 3n and 4d were tested for their effects on PANC-1 cells. Considerable decrease in p-Akt substrates shown by 4d and 3n at 50 µM (western blot analysis) indicated their ability to inhibit p-Akt signal transduction pathway and arresting growth of PANC-1 cells in vitro. This was further supported by the cytotoxic effect of 4d on PANC-1 cells (MTT assay) that was better than rosuvastatin. While none of 3n and 4d showed any significant effect on non-cancerous HEK cell line (indicating their potential selectivity towards cancer cells) these compounds were further evaluated for their toxicities in Zebrafish embryo. The NOAEL (No Observed Adverse Effect Level) for teratogenicity, hepatotoxicity and cardiotoxicity was found to be 100 µM for both compound. Thus, 4d as a novel and potent but safer cytotoxic agent with potential to treat colorectal/ovarian and pancreatic cancer is of further medicinal interest.

Synthesis of 11,12-dihydro benzo[c]phenanthridines via a Pd-catalyzed unusual construction of isocoumarin ring/FeCl3-mediated intramolecular arene-allyl cyclization: First identification of a benzo[c]phenanthridine based PDE4 inhibitor.

In spite of their various pharmacological properties the anti-inflammatory potential of benzo[c]phenanthridines remained underexplored. Thus, for the first time PDE4 inhibitory potential of 11,12-dihydro benzo[c]phenanthridine/benzo[c]phenanthridine was assessed in vitro. Elegant synthesis of these compounds was performed via a multi-step sequence consisting of a Pd-catalyzed unusual construction of 4-allyl isocoumarin ring and FeCl3-mediated intramolecular regio- as well as site-selective arene-allyl cyclization as key steps. The overall strategy involved Sonogashira coupling followed by isocoumarin and isoquinolone synthesis, then chlorination and subsequent cyclization to afford a range of 11,12-dihydro derivatives. One of these dihydro compounds was converted to the corresponding benzo[c]phenanthridine that showed concentration dependent inhibition of PDE4B affording an initial hit molecule. The SAR study suggested that 11,12-dihydro analogs were less potent than the compound having unsaturation at the same part of the ring.

Atomic Resolution Analyses of Isocoumarin Derivatives for Inhibition of Lysyl-tRNA Synthetase.

Aminoacyl-tRNA synthetases, the essential enzyme family for protein translation, are attractive targets for developing antibacterial, antifungal, and antiparasitic agents and for treating other human diseases. The antimalarial natural product cladosporin was discovered recently as a novel lysyl-tRNA synthetase (LysRS) specific inhibitor. Here, we report a thorough analysis of cladosporin derivatives using chemical synthesis, biophysical, and biochemical experiments. A series of isocoumarin derivatives with only one nonhydrogen atom/bond change per compound was synthesized. These changes include replacements of methyltetrahydropyran moiety by methylcyclohexane or cyclohexane, lactone by lactam, hydroxyl groups by methoxyl groups, and dismission of the chiral center at C3 with a Δ3,4 double bond. We evaluated these compounds by thermal shift assays and enzymatic experiments and further studied their molecular recognition by the Plasmodium falciparum LysRS through total five high-resolution crystal structures. Our results showed that the methyltetrahydropyran moiety of cladosporin could be replaced by a more stable methylcyclohexane without reducing binding ability. Removing the methyl group from the methylcyclohexane moiety slightly decreased the interaction with LysRS. Besides, the replacement with a lactam group or a conjugated Δ3,4 double bond within the scaffold could be two more options to optimize the compound. Lastly, the two phenolic hydroxyl groups were critical for the compounds to bind LysRS. The detailed analyses at atomic resolution in this study provide a foundation for the further development of new antibiotics from cladosporin derivatives.

Rh(III)-Catalyzed Annulation of Boc-Protected Benzamides with Diazo Compounds: Approach to Isocoumarins.

A mild rhodium-catalyzed annulation of Boc-protected benzamides with diazo compounds via C-C/C-O bond formation has been explored. In the presence of [Cp*RhCl2]2, AgSbF6 and Cs2CO3, Boc-protected benzamides can be effectively annulated to yield isocoumarins in 0.5⁻2 h.

Ultrasound Assisted Synthesis of 3-(het)aryl Isocoumarin Derivatives and their in vitro Pharmacological Evaluation.

BACKGROUND: In view of numerous biological activities of 3-substituted isocoumarins a number of analogues based on this scaffold were synthesized for their in vitro pharmacological evaluation. METHODS: The syntheses of 3-substituted isocoumarins were carried out via a Pd/C-catalyzed Suzuki- Miyaura coupling of 3-chloroisochromen-1-one with a range of boronic acid derivatives. This C-C bond forming reaction was facilitated by ultrasound irradiation to afford the desired products in good yields. A number of 3-(het)aryl isocoumarin derivatives were prepared by using this methodology and subsequently tested for their TNF-α inhibitory properties in vitro followed by cytotoxicities via the MTT assay. RESULTS: Several compounds showed inhibition of TNF-α with one compound showing an IC50 value of 9.01±1.25 µM. Three compounds also showed promising cytotoxic properties against two cancer cell lines with IC50 ~ 0.9-2.7 µM. CONCLUSION: The isocoumarin framework could be an effective template for the design and discovery of new inhibitor of TNF-α for the potential treatment of related diseases.

3,4-Dihydroisocoumarins, Interesting Natural Products: Isolation, Organic Syntheses and Biological Activities.

BACKGROUND: 3,4-dihydroisocoumarins are an important small group belonging to the class of naturally occurring lactones isolated from different bacterial strains, molds, lichens, and plants. The structures of these natural compounds show various types of substitution in their basic skeleton and this variability influences deeply their biological activities. These lactones are structural subunits of several natural products and serve as useful intermediates in the synthesis of different heterocyclic molecules, which exhibit a wide range of biological activities, such as anti-inflammatory, antiplasmodial, antifungal, antimicrobial, antiangiogenic and antitumoral activities, among others. Their syntheses have attracted attention of many researchers reporting many synthetic strategies to achieve 3,4-dihydroisocoumarins and other related structures. OBJECTIVE: In this context, the isolation of these natural compounds from different sources, their syntheses and biological activities are reviewed, adding the most recent advances and related developments. CONCLUSION: This review aims to encourage further work on the isolation and synthesis of this class of natural products. It would be beneficial for synthetic as well as the medicinal chemists to design selective, optimized dihydroisocoumarin derivatives as potential drug candidates, since dihydroisocoumarin scaffolds have significant utility in the development of therapeutically relevant and biologically active compounds.

Design and Synthesis of Metabolically Stable tRNA Synthetase Inhibitors Derived from Cladosporin.

Selective and specific inhibitors of Plasmodium falciparum lysyl-tRNA synthetase represent promising therapeutic antimalarial avenues. Cladosporin was identified as a potent P. falciparum lysyl-tRNA synthetase inhibitor, with an activity against parasite lysyl-tRNA synthetase >100-fold more potent than that of the activity registered against the human enzyme. Despite its compelling activity, cladosporin exhibits poor oral bioavailability; a critical requirement for antimalarial drugs. Thus, the quest to develop metabolically stable cladosporin-derived analogues, while retaining similar selectivity and potency to that of the natural compound, has begun. Chemogenomic profiling of a designed library allowed an entirely innovative structure-activity relationship study to be initiated; this shed light on structural evidence of a privileged scaffold with a unique activity against tRNA synthetases.

Ultrasound Assisted Synthesis of 3,4-Diyne Substituted Isocoumarin Derivatives: Identification of Potential Cytotoxic Agents.

BACKGROUND: The 3,4-diyne substituted isocoumarins have been designed, synthesized and explored as potential anti-proliferative agents. METHOD: Ultrasound assisted synthesis of these compounds was carried out by using a three-step method involving (i) Pd/C-Cu catalyzed cross-coupling between the methyl 2-iodobenzoate and buta- 1,3-diynylbenzene followed by (ii) I2-mediated electrophilic cyclization of the resultant 2-(alk-1- ynyl)benzoate ester and (iii) subsequent alkynylation of 4-iodo-3-(phenylethynyl)-isocoumarin under Pd/C-Cu catalysis. CONCLUSION: The synthesized compounds showed promising growth inhibition when tested against MDA-MB 231 and K562 cancer cell lines.

Synthesis of benzil-o-carboxylate derivatives and isocoumarins through neighboring ester-participating bromocyclizations of o-alkynylbenzoates.

Bromide mediated neighboring ester-participating bromocyclizations of o-alkynylbenzoates are described here for the synthesis of benzil-o-carboxylates. 4-bromoisocoumarins are also synthesized when phenyl o-alkynylbenzoate is used as the substrate. Mechanistic studies suggest that the whole process is composed of an electrophilic bromocyclization and a dibromohydration-based ring-opening, and the neighboring ester group participates in the bromocyclization. Interestingly, the two oxygen atoms of the keto carbonyls in benzil-o-carboxylates are both derived from water. The electrophilic bromo source is in situ generated from the oxidation of bromide.

P450 BM3-Catalyzed Regio- and Stereoselective Hydroxylation Aiming at the Synthesis of Phthalides and Isocoumarins.

Cytochrome P450 BM3 monooxygenases are able to catalyze the regio- and stereoselective oxygenation of a broad range of substrates, with promising potential for synthetic applications. To study the suitability of P450 BM3 variants for stereoselective benzylic hydroxylation of 2-alkylated benzoic acid esters, the biotransformation of methyl 2-ethylbenzoate, resulting in both enantiomeric forms of 3-methylphthalide, was investigated. In the case of methyl 2-propylbenzoate as a substrate the regioselectivity of the reaction was shifted towards ß-hydroxylation, resulting in the synthesis of enantioenriched R- and S-configured 3-methylisochroman-1-one. The potential of P450 BM3 variants for regio- and stereoselective synthesis of phthalides and isocoumarins offers a new route to a class of compounds that are valuable synthons for a variety of natural compounds.

Palladium-catalyzed carbonylative synthesis of isocoumarins and phthalides by using phenyl formate as a carbon monoxide source.

A simple and efficient palladium-catalyzed intramolecular carbonylative synthesis of isocoumarins and phthalides from the easily available starting materials by employing phenyl formate as a CO surrogate has been achieved. The approach affords target compounds in good to excellent yields with the advantages of lower toxicity, milder conditions, easy operation and wide functional group tolerance.

Synthesis of natural urolithin M6, a galloflavin mimetic, as a potential inhibitor of lactate dehydrogenase A.

Glycolysis is the main route for energy production in tumors. LDH-A is a key enzyme of this process and its inhibition represents an attractive strategy to hamper cancer cell metabolism. Galloflavin is a reliable LDH-A inhibitor as previously identified by us; however, its poor physicochemical properties and chemical tractability render it unsuitable for further development. Therefore, a rational design was undertaken with the aim to reproduce the pharmacophore of galloflavin on simpler, potentially more soluble and synthetic accessible scaffolds. Following a process of structural simplification, natural urolithin M6 (UM6), which is an ellagitannin metabolite produced by gut microbiota, was identified as a putative galloflavin mimetic. In the present study, the synthesis of UM6 is described for the first time. An efficient synthetic pathway has been developed, which involved five steps from readily accessible starting materials. The key reaction steps, a Suzuki coupling and an intramolecular C-H oxygenation, have been optimized to improve the synthetic feasibility and provide the best conditions in terms of reaction time and yield. Moreover, this route would be suitable to obtain other analogs for SAR studies. Preliminary biological tests revealed that UM6 was able to smoothly reproduce the behavior of galloflavin, confirming that our approach was successful in providing a new and accessible structure in the search for new LDH-A inhibitors.

Inhibition of the enzymes in the leukotriene and prostaglandin pathways in inflammation by 3-aryl isocoumarins.

The biosynthesis of leukotrienes in one of the arachidonic acid pathways and PGE2 in the other by 5-LOX and mPGES1 respectively, play pivotal roles in augmenting inflammatory responses. PGE2 is known to participate in cancer pathological processes as well. Isocoumarins are natural compounds with a wide range of biological activities. In this study, 3-aryl isocoumarin derivatives are synthesized and tested against 5-LOX enzyme in vitro and PGE2 production in HeLa cells. Most of the compounds show high activity, and 1c is identified as a dual inhibitor with an IC50 of 4.6 ± 0.26 µM and 6.3 ± 0.13 µM against 5-LOX and PGE2 production respectively. Another compound 7f, exhibits an IC50 of 12.4 ± 0.14 µM against 5-LOX. Further investigations reveal that the mechanism of action of 1c and 7f against 5-LOX is mixed and competitive modes of action respectively. Thunberginol A (7c) exhibits IC50 of 15.8 ± 0.03 µM against PGE2 production. 1c and 7c inhibit the mRNA expression of mPGES1 and COX-2. The study has identified a novel scaffold, 1c with a dual inhibitory activity which can be further optimized to compete against Licofelone which is under clinical trials (with IC50 of 6.0 µM for mPGES1 & 0.2 µM for 5-LOX). To conclude, 3-aryl isocoumarin derivatives appears as promising tools to fight against inflammatory diseases as well as cancer.

Synthesis of Cyclohexane-Fused Isocoumarins via Cationic Palladium(II)-Catalyzed Cascade Cyclization Reaction of Alkyne-Tethered Carbonyl Compounds Initiated by Intramolecular Oxypalladation of Ester-Substituted Aryl Alkynes.

A cationic Pd(II)-catalyzed cascade cyclization reaction of alkyne-tethered carbonyl compounds was developed. This reaction is initiated by intramolecular oxypalladation of alkynes with an ester group followed by 1,2-addition of the formed C-Pd(II) bond to the carbonyl group, providing a highly efficient method for the synthesis of cyclohexane-fused isocoumarins.

Synthesis, antiproliferative activities, and computational evaluation of novel isocoumarin and 3,4-dihydroisocoumarin derivatives.

A series of novel isocoumarin derivatives were synthesized using Castro-Stephens cross-coupling. Moreover, novel 3,4-dihydroisocoumarin derivatives were obtained by catalytic hydrogenation of the corresponding isocoumarin precursors. The antiproliferative activity of all compounds was evaluated in vitro in different tumor cells. Furthermore, docking calculations were performed for the kallikrein 5 (KLK5) active site to predict the possible mechanism of action of this series of compounds. Theoretical findings indicate that the 3,4-dihydroisocoumarin derivative 10a forms hydrogen bonds with Ser190 and Gln192 residues of KLK5. This derivative was the most active compound in the series with potent antiproliferative activity and high selectivity index (SI > 378.79) against breast cancer cells (MCF-7, GI50 = 0.66 µg mL(-1)). This compound represents a promising matrix for developing new antiproliferative agents.