Review on Natural Coumarin Lead Compounds for Their Pharmacological Activity

Coumarin (2H-1-benzopyran-2-one) is a plant-derived natural product known for its pharmacological properties such as anti-inflammatory, anticoagulant, antibacterial, antifungal, antiviral, anticancer, antihypertensive, antitubercular, anticonvulsant, antiadipogenic, antihyperglycemic, antioxidant, and neuroprotective properties. Dietary exposure to benzopyrones is significant as these compounds are found in vegetables, fruits, seeds, nuts, coffee, tea, and wine. In view of the established low toxicity, relative cheapness, presence in the diet, and occurrence in various herbal remedies of coumarins, it appears prudent to evaluate their properties and applications further.


Introduction
Coumarins (2H-1-benzopyran-2-one) (1) consist of a large class of phenolic substances found in plants and are made of fused benzene and -pyrone rings [1]. More than 1300 coumarins have been identified as secondary metabolites from plants, bacteria, and fungi [2]. The prototypical compound is known as 1,2-benzopyrone or, less commonly, ashydroxycinnamic acid and lactone, and it has been well studied. Coumarins were initially found in tonka bean (Dipteryx odorata Wild) and are reported in about 150 different species distributed over nearly 30 different families, of which a few important ones are Rutaceae, Umbelliferae, Clusiaceae, Guttiferae, Caprifoliaceae, Oleaceae, Nyctaginaceae, and Apiaceae. (See Scheme 1.) Although distributed throughout all parts of the plant, the coumarins occur at the highest levels in the fruits (Bael fruits (Aegle marmelos) [3], Tetrapleura tetraptera TAUB (Mimosaceae) [4], bilberry, and cloudberry), seeds (tonka beans) (Calophyllum cerasiferum Vesque and Calophyllum inophyllum Linn) [5] followed by the roots (Ferulago campestris) [6], leaves (Murraya paniculata) [7], Phellodendron amurense var. wilsonii [8], and latex of the tropical rainforest tree Calophyllum teysmannii var. inophylloide [9] green tea and other foods such as chicory. They are also found at high levels in some essential oils such as cassia oil [10], cinnamon bark oil [11], and lavender oil [6]. Environmental conditions and seasonal changes could influence the incidence of coumarins in varied parts of the plant. The function of coumarins is far from clear, although suggestions include plant growth regulators, bacteriostats, fungistats, and even waste products [12].
Biosynthesis of coumarin is reviewed by Bourgaud et al. [11]. There are types of coumarins found in nature due to various permutations brought about by substitutions and conjugations; however, most of the pharmacological and biochemical studies have been done on coumarin itself and on its primary metabolite, 7-hydroxycoumarin in man [13]. Some of this earlier pharmacological work on coumarin has been reviewed [14], and other more comprehensive reviews [13,15,16] deal with the occurrence, chemistry, and biochemical properties of both simple and more complex natural coumarins.

Classification of Coumarins
Natural coumarins are mainly classified into six types based on the chemical structure of the compounds (Table 1). The physicochemical properties and therapeutic applications of natural coumarins depend upon the pattern of substitution.
These coagulation factors (factors II, VII, IX, and X) require -carboxylation for their biological activity. Coumarins produce their anticoagulant effect by inhibiting  vitamin K conversion cycle, thereby causing hepatic production of partially carboxylated and decarboxylated proteins with reduced procoagulant activity [51,52]. In addition to their anticoagulant effect, vitamin K antagonists inhibit carboxylation of the regulatory anticoagulant proteins C and S and therefore have the potential to exert a procoagulant effect. In the presence of calcium ions, carboxylation causes a conformational change in coagulation proteins [53][54][55] that promotes binding to cofactors on phospholipid surfaces. The carboxylation reaction requires the reduced form of vitamin K (vitamin KH 2 ), molecular oxygen, and carbon dioxide and is linked to the oxidation of vitamin KH 2 to vitamin K epoxide. Vitamin K epoxide is then recycled to vitamin KH 2 through two reductase steps. The first, which is sensitive to vitamin K antagonist [47,49,50], reduces vitamin K epoxide to vitamin K 1 (the natural food form of vitamin K 1 ), while the second, which is relatively insensitive to vitamin K antagonists, reduces vitamin K 1 to vitamin KH 2 . Treatment with vitamin K antagonists leads to the depletion of vitamin KH 2 , thereby limiting thecarboxylation of vitamin K-dependent coagulant proteins. The effect of coumarins can be counteracted by vitamin K 1 (either ingested in food or administered therapeutically) because the second reductase step is relatively insensitive to vitamin K antagonists ( Figure 1). Patients treated with a large dose of vitamin K 1 can also become warfarin resistant for up to a week because vitamin K 1 accumulates in the liver and is available to the coumarin-insensitive reductase. (1) itself has a very low antibacterial activity, but compounds having long chain hydrocarbon substitutions such as ammoresinol (5) and ostruthin (6) show activity against a wide spectrum of Gram +ve bacteria such as Bacillus megaterium, Micrococcus luteus, Micrococcus lysodeikticus, and Staphylococcus aureus [19]. Another coumarin compound anthogenol (7) from green fruits of Aegle marmelos [3] shows activity against Enterococcus. Imperatorin (2), a furanocoumarin isolated from Angelica dahurica and Angelica archangelica (Umbelliferae) [56], shows activity against Shigella dysenteriae [57]. Grandivittin (8), agasyllin (9), aegelinol benzoate (10) and osthole (11) have been isolated from the roots of Ferulago campestris (Apiaceae) [32]. Felamidin (12) was also isolated from Ferulago campestris [6]. Aegelinol and agasyllin showed significant antibacterial activity against clinically isolated Gram-positive and Gram-negative bacterial strains such as Staphylococcus aureus, Salmonella typhi, Enterobacter cloacae, and Enterobacter aerogenes. Antibacterial activity was also found against Helicobacter pylori where a dosedependent inhibition was shown between 5 and 25 mg/mL. (See Scheme 4.)

Coumarins for Antihypertensive Activity.
Dihydromammea C/OB (32) is a new coumarin that has been isolated from the seeds of the West African tree Mammea africana Sabine (Guttiferae) [68]. The

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Scheme 9 elucidated by single crystal X-ray method [69]. Antihypertensive effects of the methanol and dichloromethane extracts of stem bark from Mammea africana in N -nitro-L-arginine methyl ester induced hypertensive male albino Wistar rats weighing 250-300 g of 12-16-week old rats have been used in the studies [70]. Dichloromethane and methanol extracts of stem bark from Mammea africana exhibited a significant antihyperglycemic activity and improved the metabolic alterations in streptozotocin-induced male albino Wistar diabetic rats (3-month-olds, weighing 200-250 g) [71]. Vasodilatory effects of the coumarin are reported on cultured myocardial cells as well [72]. Scopoletin (33) was isolated form the fruits of Tetrapleura tetraptera TAUB (Mimosaceae) and it produces hypotension in laboratory animals in vitro and in vivo through its smooth muscle relaxant activity [4]. Visnadine (34), an active ingredient extracted from the fruit of Ammi visnaga, exhibited peripheral and coronary vasodilator activities and has been used for the treatment  of angina pectoris [2]. Khellactone (35) was isolated from Phlojodicarpus sibiricus and it exhibited vasodilatory action [73]. (See Scheme 12.)

Coumarins for Multiple Sclerosis.
Osthole (11) could be a potential therapeutic agent for the treatment of multiple sclerosis [75].

Scheme 17
3.15. Coumarins for Neuroprotective Activity. Esculetin (3) also exhibited neuroprotective effects on cerebral ischemia/reperfusion injury in a middle cerebral artery occlusion model in mice at 20 g/mL and was administered intracerebroventricularly at 30 min before ischemia [81].

Analysis of Coumarins by Different Methods
Various methods for the isolation and analysis of coumarins are chromatography (paper chromatography, thin layer chromatography, gas chromatography, and high-performance

Conclusion
This paper covers natural coumarin lead compounds and their broad pharmacological properties and their methods of identification according to their official pharmacopoeias. Natural coumarins are of great interest due to their widespread pharmacological properties, and this attracts many medicinal chemists for further backbone derivatization and screening them as several novel therapeutic agents.