Recent Trends in the Synthesis and Bioactivity of Coumarin, Coumarin–Chalcone, and Coumarin–Triazole Molecular Hybrids

Molecular hybridization represents a new approach in drug discovery in which specific chromophores are strategically combined to create novel drugs with enhanced therapeutic effects. This innovative strategy leverages the strengths of individual chromophores to address complex biological challenges, synergize beneficial properties, optimize pharmacokinetics, and overcome limitations associated with single-agent therapies. Coumarins are documented to possess several bioactivities and have therefore been targeted for combination with other active moieties to create molecular hybrids. This review summarizes recent (2013–2023) trends in the synthesis of coumarins, as well as coumarin–chalcone and coumarin–triazole molecular hybrids. To cover the wide aspects of this area, we have included differently substituted coumarins, chalcones, 1,2,3– and 1,2,4–triazoles in this review and considered the point of fusion/attachment with coumarin to show the diversity of these hybrids. The reported syntheses mainly relied on well-established chemistry without the need for strict reaction conditions and usually produced high yields. Additionally, we discussed the bioactivities of the reported compounds, including antioxidative, antimicrobial, anticancer, antidiabetic, and anti-cholinesterase activities and commented on their IC50 where possible. Promising bioactivity results have been obtained so far. It is noted that mechanistic studies are infrequently found in the published work, which was also mentioned in this review to give the reader a better understanding. This review aims to provide valuable information to enable further developments in this field.


Introduction
Molecular hybridization (MH) is an established drug design method used by medicinal chemists to create and refine new lead compounds [1].It is the rational combination of two or more pharmacophoric units with distinct modes of action into a single molecule [2,3].MH is expected to increase the biological activity of newly created pharmacophores [4] through alterations to their pharmacokinetic profiles, modes of action, potency, selectivity, and biological activity [5].
Coumarin derivatives are popular and attractive in the field of medicinal/organic chemistry [58][59][60][61][62][63][64][65][66][67][68].Hence, several review papers on this topic have been published.For example, Adimule et al. (2022) arranged a recent advance in the one-pot synthesis of coumarin derivatives from different starting materials using metal nanoparticles as heterogeneous catalysts [7].Song et al. (2020) released an update on coumarin derivatives with anticancer activities by underlining interactions between coumarin derivatives and diverse enzymes and receptors in cancer cells [11].Additionally, the recent development of coumarin hybrids as antifungal agents was summarized by Hu et al. [14].Furthermore, an overview of synthetic approaches and the biological activity of coumarin derivatives was provided by Annunziata et al. [67].The present review focuses on coumarin hybrids.However, it starts with recent developments in coumarin chemistry to give the reader the necessary background and transition needed for the chemistry of coumarin hybrids.We attempt to summarize the synthesis strategies and biological activities of coumarin, coumarinchalcone and coumarin-triazole hybrids.This review will be beneficial for synthetic and medicinal chemists aiming to develop these hybrids into lead drug compounds.Elbastawesy et al. (2015) constructed twelve novel coumarin derivates, starting from resorcinol 7 (Scheme 2) [69].After subsequent cyclization, following an SN2 reaction with ethyl chloroacetate and then amidation with hydrazine, compound 10 was obtained which, after further reactions, gave compound 11.From this important intermediate, coumarins 13a-l were obtained in good yields.Scheme 1. Synthesis of coumarins 6a-w from dicarboxylic acid 1 [68].Elbastawesy et al. (2015) constructed twelve novel coumarin derivates, starting from resorcinol 7 (Scheme 2) [69].After subsequent cyclization, following an S N 2 reaction with ethyl chloroacetate and then amidation with hydrazine, compound 10 was obtained which, after further reactions, gave compound 11.From this important intermediate, coumarins 13a-l were obtained in good yields.Scheme 6. Synthesis of coumarins 30a-d from 4-hydroxy coumarin 21 [73].Xu et al. (2020) converted carboxylic acid 31 into cinnamoyl chlorides 32 which, upon reaction with compound 21, gave coumarin derivatives 33a-i in medium yields (Scheme 7) [74].Scheme 7. Synthesis of coumarins 33a-g from substituted carboxylic acid 31 [74].Alshibl et al. (2020) synthesized coumarin derivates via Knoevenagel condensation between 34 and chlorosulfonic acid to produce coumarin sulfonyl chloride 35, which underwent an SN2 reaction with p-substituted aniline 36 to give coumarin-3-sulfonamides 37a-d in medium-to-excellent yields (Scheme 8) [75].
In line with the development of coumarin derivatives, Wang et al. (2020) synthesized 3-substituted coumarin derivatives via a three-step synthetic route from substituted benzyl chlorides/bromides 38 (Scheme 9) [76].The starting materials 38 were treated with mercaptoacetic acid in the presence of sodium hydroxide to give benzylmercaptoacetic acid 39 which, upon treatment with hydrogen peroxide, gave benzylsulfonylacetic acid 40.Finally, the target compounds 42a-d were synthesized via a Knoevenagel reaction between 40 and substituted salicylaldehydes 41 and were obtained in low-to-medium yields.In line with the development of coumarin derivatives, Wang et al. (2020) synthesized 3-substituted coumarin derivatives via a three-step synthetic route from substituted benzyl chlorides/ bromides 38 (Scheme 9) [76].The starting materials 38 were treated with mercaptoacetic acid in the presence of sodium hydroxide to give benzylmercaptoacetic acid 39 which, upon treatment with hydrogen peroxide, gave benzylsulfonylacetic acid 40.Finally, the target compounds 42a-d were synthesized via a Knoevenagel reaction between 40 and substituted salicylaldehydes 41 and were obtained in low-to-medium yields.
With the same purpose of developing coumarin derivatives, Zhou et al. (2022) converted 4-hydroxycoumarin 21 to nitro product 60 (Scheme 14) [81].The subsequent reduction of the nitro group gave the amino compound 61, which underwent substitution with various acid chlorides to form the target compounds 62a-j in good yields.Scheme 13.Synthesis of coumarins 59a-j from 4-hydroxy coumarin 21 [80].
With the same purpose of developing coumarin derivatives, Zhou et al. (2022) converted 4-hydroxycoumarin 21 to nitro product 60 (Scheme 14) [81].The subsequent reduction of the nitro group gave the amino compound 61, which underwent substitution with various acid chlorides to form the target compounds 62a-j in good yields.
In a multi-step route, a series of coumarin-triazole hybrids were successfully synthesized by Basappa et al. (2020) (Scheme 40) [107].The reaction between substituted salicylaldehydes 175 and diethylmalonate gave compound 176 which, upon reaction with hydrazine hydrate, gave hydrazides 177.The reaction of 177 with carbon disulfide resulted in the formation of the intermediate dithiocarbazate salts 178.The in-situ reaction of 178 with hydrazine hydrate produced the target compounds 179a-e in medium-togood yields.
In a multi-step route, a series of coumarin-triazole hybrids were successfully synthesized by Basappa et al. (2020) (Scheme 40) [107].The reaction between substituted salicylaldehydes 175 and diethylmalonate gave compound 176 which, upon reaction with hydrazine hydrate, gave hydrazides 177.The reaction of 177 with carbon disulfide resulted in the formation of the intermediate dithiocarbazate salts 178.The in-situ reaction of 178 with hydrazine hydrate produced the target compounds 179a-e in medium-togood yields.Scheme 40.Synthesis of coumarin-triazole 179a-e from substituted salicylaldehyde 175 [107].

Antioxidant Activity
Alshibl et al. (2020) synthesized coumarin derivates and examined their antioxidation activity using a DPPH assay (Scheme 8, Figure 3) [75].Compound 37c exhibited the highest antioxidant activity, with an IC 50 value of 14.51 ± 1.827 µg/mL.The capacity of hydroxy-3-benzoylcoumarins to scavenge peroxyl radicals was studied using the ORAC method [85].ORAC assesses the ability of antioxidants (or their complex mixtures) to inhibit the bleaching of a target molecule (probe) induced by peroxyl radicals.The results from the ORAC method showed that compound 83e (Scheme 18, Figure 3) had the best values among the synthesized compounds.The ORAC-FL value was 8.51 ± 0.32, the ORAC-PGR value was 1.17 ± 0.10, and the hydroxy radical scavenging value was 90.9 ± 8.2% for compound 83e.
The method of screening the antioxidant activity of coumarin-clubbed chalcone hybrids developed by Konidala et al. (2021) was examined using an in vitro DPPH assay (Scheme 24, Figure 3) [91].The results indicated that compound 101v showed the most potent antioxidant activity, with 77.92% scavenging activity (100 μg/mL).The capacity of hydroxy-3-benzoylcoumarins to scavenge peroxyl radicals was studied using the ORAC method [85].ORAC assesses the ability of antioxidants (or their complex mixtures) to inhibit the bleaching of a target molecule (probe) induced by peroxyl radicals.The results from the ORAC method showed that compound 83e (Scheme 18, Figure 3) had the best values among the synthesized compounds.The ORAC-FL value was 8.51 ± 0.32, the ORAC-PGR value was 1.17 ± 0.10, and the hydroxy radical scavenging value was 90.9 ± 8.2% for compound 83e.
The method of screening the antioxidant activity of coumarin-clubbed chalcone hybrids developed by Konidala et al. (2021) was examined using an in vitro DPPH assay (Scheme 24, Figure 3) [91].The results indicated that compound 101v showed the most potent antioxidant activity, with 77.92% scavenging activity (100 µg/mL).
The coumarin-triazole derivatives developed by Shaikh et al. were also evaluated for antioxidant activity using in vitro DPPH assay.Compound 112a (Scheme 27, Figure 3) was found to be the most potent candidate, with an IC 50 value of 15.20 µg/mL [94].
The potential antioxidant activity of several coumarin hydrazides was also investigated using the cupric ion reducing antioxidant capacity (CUPRAC) method.All studied compounds reduced cupric ions, and the highest activity was observed for compound 158j, with 1.82 ± 0.08 mg TEAC/mg compound (Scheme 35, Figure 3) [102].
Additionally, Alshibl et al. (2020) successfully synthesized and tested coumarin derivates and found compound 37d (Scheme 8, Figure 4) to possess the highest IZ at a concentration of 125 µg/mL.The antimicrobial activity, expressed as the diameter (mm) of inhibition zones, for Gram-positive bacteria was S. aureus = 30 mm, B. subtilis = 29 mm, and B. megaterium = 29 mm; for Gram-negative bacteria, the antimicrobial activity was E. coli = 31 mm and P. aeruginosa = 31 mm, and for fungi it was S. cerevisiae = 32 mm and C. albicans = 30 mm [75].
Abduljabbar and Hadi (2021) constructed several coumarin derivatives and tested their antimicrobial activity using the disc-well-diffusion method [77].The highest activity was demonstrated by compound 45b (Scheme 10, Figure 4), which showed the highest IZs at a concentration of 1000 µg/mL (P.aeruginosa = 22 mm and E. coli = 14 mm).
The antimicrobial activity screening of coumarin-chalcone hybrids developed by Vazquez-Rodriguez et al.The coumarin-clubbed chalcone hybrids developed by Konidala et al. (2021) [91] were also evaluated in vitro using the well-diffusion method.Compound 101a (Scheme 24, Figure 5) was the most potent, with an MIC value of 10 µM against a Gram-positive bacterium (Staphylococcus aureus), an MIC value of 8 µM against a Gram-negative bacterium (Escherichia coli), and an antifungal activity MIC value of 10 µM against Aspergillus niger.
The coumarin derivatives tested by Zhou et al. (2022) [81] showed compound 62i (Scheme 14, Figure 6) to exhibit the best in vitro activity against lung cancer cell motility via an analysis of invasion assay cells treated with 15 µM of the compound.The values of the Relative Invaded Number (RIN) for each lung cancer cell are as follows: A549 = 0.8, H460 = 0.8, H1650 = 0.75, and H1975 = 0.78.
Ghouse et al. (2023) synthesized 3-substituted coumarin derivatives and examined their anticancer activity using their CA inhibitory potential against the isoforms hCA I, II, IX, and XII [82].The results showed that compounds 69d and 71b (Scheme 15, Figure 6) exhibited the best K i values.Compound 69d showed hCA I, K i > 100 µM; hCA II, K i > 100 µM; hCA IX, K i = 5.56 µM; hCA XII, K i = 9.8 µM, and compound 71b showed hCA I, K i > 100 µM; hCA II, K i > 100 µM; hCA IX, K i = 9.2 µM; and hCA XII, K i = 8.0 µM.In silico studies revealed key interaction residues of the molecule 71b and confirmed the binding mode and stability of ligand interactions within the binding pockets of the enzymes hCA IX and hCA XII, which were overexpressed in hypoxic cancer cells.
The anticancer activity of coumarin-chalcone hybrids tested by Amin et al. (2013) [84] against a human colon cancer cell line (HCT-116) showed compound 79o (Scheme 16, Figure 6) to exhibit excellent anticancer activity, with an IC 50 of 0.01 µM.The investigation was extended for the inhibition of PI3K (P110α/p85α), and the IC 50 value was found to be 50.78µM.
Similarly, a series of coumarin-triazole derivatives were synthesized and evaluated as anticancer agents against a human colorectal cancer cell line by Al-Wahaibi et al. ( 2018) [97].The compounds 129c and 133c (Scheme 30, Figure 7) exhibited marked cytotoxic activity, with IC 50 values of 4.363 µM and 2.656 µM, respectively.
The potential antitubercular activity of coumarin-triazole derivatives was investigated against MTB H37Ra (ATCC 25177) by Shaikh et al. (2016) [95].Compound 115f (Scheme 28, Figure 11) exhibited interesting and highly promising antitubercular activity (MIC = 1.80 μg/ mL).Compound 115f was reported to inhibit the DprE1 enzyme of MTB and has the most active binding mode in the active site of the DprE1 enzyme.
N. Yadav et al. (2018) successfully synthesized a series of novel coumarin-triazole derivatives as antiplasmodials [101].The compounds were evaluated for in vitro antiplasmodial activity against a chloroquine-sensitive strain of Plasmodium falciparum (3D7).Compound 151i (Scheme 34, Figure 11) was found to be the most active, with an IC50 value of 0.763 ± 0.0124 μg/mL.

Conclusions and Prospective
Herein, we reviewed recent trends (2013-2023) in the synthesis and bioactivity of coumarin, coumarin-chalcone molecular hybrids, and coumarin-triazole molecular hybrids.The chemistry to synthesize these compounds was enabled by several established reactions including Pechmann condensation, Claisen-Schmidt condensation, and azidealkyne cycloaddition reactions.The synthesized compounds showed diversity in their structures in terms of substituent types, substitution pattern, and the fusion/attachment point to coumarin.The reactions proceeded in a few steps under mild conditions and usually gave good-to-excellent yields.A wide range of bioactivities including antioxidant, antimicrobial, anticancer, antidiabetic, and anti-cholinesterase activities were also reviewed.Although some structure-activity studies (SAR) have been presented in some cases, extensive SARs are lacking in the literature.Additionally, though significant progress was made in synthesis and bioactivity screening, the mechanism of action remains to be investigated in detail.Several questions need to be addressed: do the hybrids interact with the known receptors of one or both chromophores or do they target new receptors?How does the synergistic effect take place?An understanding of this mechanism is critical for optimizing the structures of the hybrids to deliver optimum synergistic effects.The development of molecular hybrids continues to be an active and dynamic field with potential future prospects in medicinal chemistry and health science.It is our hope that these compounds will be useful for the treatment of various diseases.

Figure 1 .
Figure 1.Structures of coumarin, chalcone and triazole and their derivatives.

Figure 1 .
Figure 1.Structures of coumarin, chalcone and triazole and their derivatives.

Figure 7 .
Figure 7.More examples of coumarin-based compounds as anticancer agents.

67. The reaction of 67 with various benzenesulfonylchlorides 68 yielded sulfonamides 69a-e in good yields. On the other hand, 67 underwent a reaction with phenylisothiocyanates 70 to give carbothioamides 71a-c in good yields. Scheme 14. Synthesis of coumarins 62a-j from
[81]l.(2023) synthesized 3-substituted coumarin derivatives, starting from salicylaldehyde 63 (Scheme 15)[82].A reaction between salicylaldehyde 63 and diethyl malonate gave compound 64 which, upon hydrolysis with sodium hydroxide, yielded compound 65.The amidation of 65 with N-Boc piperazine in the presence of EDC•HCl, HOBt, and DIPEA gave 66. Deprotection of the Boc group using TFA led to the formation of compound 4-hydroxy coumarin 21[81].The amidation of 65 with N-Boc piperazine in the presence of EDC•HCl, HOBt, and DIPEA gave 66. Deprotection of the Boc group using TFA led to the formation of compound