Viral and Human immunodeficiency viral (HIV) infections have no complete and effective remedy, and hence are posing the greatest threats to human health. The classical treatment involves use of drugs that have serious side effects. Owing to the urgent need for new drugs, compounds having multiple functional scaffolds are synthesized 60. Heteronuclei such as imidazopyridine (Fig. 49), purine (Fig. 50), benzoxazole (Fig. 51), benzothiazole (Fig. 52) and benzimidazole (Fig. 53) and its nucleoside have been linked to the coumarin nucleus via methylenethio linker, and evaluated for antiviral activity on HUH 5–2 cells.
Inhibitory activity against hepatitis virus RNA polymerase can be shown by coumarin-linked benzoxazole-5-carboxylic acids. Recently, it has been found that the benzoxazole (Fig. 44) moiety conjugated with a coumarin moiety with a methylene thio linker exhibited potent inhibitory effects on hepatitis virus. The thio methylene (-SCH2-) linker has been used to connect a heterobicycle with various aromatic rings by synthetic methods to form hybrid compounds for antiviral bioassays. The coumarin can be linked with heterobicycles like benzimidazole, imidazopyridine, purine, benzoxazole, and benzothiazole to evaluate the activity 55.
The C4-substituted aryloxymethyl, arylaminomethyl, and dichloroacetamidomethyl coumarins, along with the corresponding 1-azacoumarins, have been demonstrated to be potential anti-microbial and anti-inflammatory agents. To expand the structural diversity of synthetic courmarins for biological functions, attempts have also been made to attach a chloramphenicol side chain at C-3 position of courmarin. In addition, the bi- and tri-heterocyclic coumarins and 1-azacoumarins with benzofuran, furan and thiazole ring systems along with biocompatible fragments like vanillin have shown remarkable potency as anti-inflammatory agents in animal models.
Brahmbhatt et al. synthesized 4-methyl-3-phenyl-6-[4-(3-aryl-1-phenyl-1H-pyrazol-4-yl) - 6 - aryl-pyridin-2-yl] coumarin derivatives 19 (Fig. 9) and were screened for their antibacterial activity against Escherichia coli (Gram negative bacteria), Bacillus subtillis (Gram positive bacteria) and anti-fungal activity against Candida albican by agar cup diffusion method. DMF was used as blank, Streptomycin was used as anti-bacterial standard and Clotrimazole was used as anti-fungal standard drug at concentration of 1000μg/ml. All the synthesized compounds showed activity against both gram positive and gram negative bacteria but lesser activity compared to standard drug.
Some important compounds isolated from coumarin are Warfarin, Acenocoumarol, Armillarisin A, Novobiocin, Clorobiocin, Hymecromone etc. coumarin and its derivatives could be synthesized in laboratory by Pechmann reaction, perkin reaction, Reformatsky reaction and Knovenegal reaction. Coumarins have been under extensive studies for their versatile biodynamic activities, for example coumarins with phenolic hydroxyl group have the ability to scavenge reactive oxygen species and thus prevent the formation of 5- HETE and 5- HHT in arachadonic acid pathway of suppression of inflammation. Recent in vivo studies have revealed the role of coumarins in hepatotoxicity and also in depletion of cytochrome P450. Similarly 1-azacoumarins which is part of quinoline alkaloids are known for their diverse biological activity and recently, a 6- functionalized 1-aza coumarins are undergoing human clinical trials as an orally active antitumor drug in view of its farnesyl protein-inhibiting activity in the nanomolar range. Furthermore, several synthetic coumarins with a variety of pharmacophoric groups at C-3, C-4 and C-7 positions have been intensively screened for anti-microbial, anti-HIV, anti-cancer, lipidlowering, anti-oxidant, and anti-coagulation activities. Specifically, coumarin-3-sulfonamides and carboxamides were reported to exhibit selective cytotoxicity against mammalian cancer cell lines.
Patel et al synthesized some 4-aryl-2,6-di(coumarin-3-yl)pyridines by the reaction of 3-coumarinoyl methyl pyridinium salts with1-[2H-1-benzopyran-2-on-3-yl]-3-aryl-prop-2-ene-1-ones in the presence of ammonium acetate and acetic acid under the Krohnke reaction conditions (Fig. 10). All the synthesized compounds were screened for antimicrobial activity.
Lobato-García , Nancy Romero-Ceronio , Synthesis of 3-carboxylated Coumarins by Knoevenagel Condensation and Exploratory Anti-inflammatory Activity Evaluation by model, , Vol.
Parmar V et al synthesized 4-methyl coumarin derivatives 7,8-dihydroxy-3-ethoxycarbonyl methyl -4-methylcoumarin (DHEMC) and 7,8-diacetoxy-3 - ethoxycarbonylmethyl - 4 - methylcoumarin (DAEMC) (Fig. 19) by using Pechman condensation of pyrogallol with ethyl acetoacetate 26 by introduction of an ethoxycarbonylmethyl group at C-3 position. DAEMC was obtained by acetylation of DHEMC. Both were examined on the inflammatory process induced by lipopolysaccharide (LPS) in activated primary rat microglial cultures by Anna Rita Togna et al 27. LPS induced production of nitric oxide and other pro inflammatory mediators such as prostaglandins PGE2 and thromboxane were inhibited in the presence of 100µM of DHEMC and DAEMC. Their experimentation showed that 4-methyl coumarin derivatives can modulate inflammatory pathways in microglial cells, probably by acting at the protein expression level.
It isreported a very simple, fast and general procedure where the condensation ofsalicylaldehyde or its derivative with various derivatives of ethyl acetate(e.g., R3CH2COEt; R3: CO2Et, COMe, CN, p-C6H4-NO2) in thepresence of piperidine under the solvent-free condition leads to coumarinsynthesis (Fig.1)Figure 1: Synthesis of coumarinssynthesis by the Knoevenagel condensation under microwaveirradiation.
Kullampalayam Krishnasamy Sivakumar et al synthesized 28 some mannich base of 5-methyl-[(2-oxo-2H-chromen-3-yl) carbonyl]-2,4-dihydro-3H-pyrazol-3-one (Fig. 20) by using conventional and non conventional (microwave) techniques. Ethyl 2-oxo-2H-chromene-3-carboxylate was prepared by cyclization of salicylaldehyde with diethylmalonate in presence of catalytic amount of piperidine. Reaction of this ester compound with hydrazine hydrate in ethanol formed 2-oxo-2H-chromene-3-carbohydrazide.
However, previously both the Knoevenagel reaction  and synthesis of coumarin by the Knoevenagelcondensation  have been the subject of microwave induced reactions,in the case of coumarins the only example that has been always given is thesynthesis of 3-ethoxycarbonylcoumarin (i.e., ethyl 2H-1-benzopyran-2-oxo-3-carboxylate).
Singh Om et al developed 32 a facile, convenient and high yielding synthesis of combinatorial library of 3- alkanoyl/aroyl/heteroaroyl-2H-chromene-2-thiones (Fig. 23) by condensation of easily accessible β-oxodithioesters and salicylaldehyde/ substituted 2-hydroxybenzaldehyde ander solvent free conditions. The assessment of radical scavenging activity of the compounds towards the stable free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) was measured and these compounds were found to scavenge DPPH radicals efficiently. The newly synthesized compounds exhibited profound antioxidant activity. Five selected compounds were able to protect curcumin from the attack of suphur free radical generated by radiolysis of glutathione (GSH).