Synthesis of bioactive nitrogen heterocycles and functionalized nanomaterials for biological and catalytic applications
Authors:   , Krishnan Anand 
Published: August 2021

Aromatic heterocycles are highly important structural units found in a large number of biologically active natural compounds, pharmaceuticals and catalytic compounds. They have a crucial role in organic syntheses, which results in the generation of high value products. Among heterocycles, those containing nitrogen are the most indispensable structural motifs and are widely used against dreaded diseases such as Malaria, TB, HIV/AIDS and Cancer. The inclusion of highly electronegative atoms such as fluorine in these organic molecules render them very reactive towards proteins. Furthermore these molecules exhibit strong interactions with surfaces of quantum range particles of elemental gold. Various approaches for the synthesis of novel gold nanoparticles linked to potent bioactive molecules are documented and their application as drug delivery systems are of immense value to human health. Also many chemical and physical methods are available for the synthesis of gold, silver and palladium nanoparticles however these methods are usually laborious and produce toxic by-products. The green approach is to use plant extracts to synthesise various size and shape nanoparticles which could be used in biological and catalytic systems. A simple one-pot two component and three component reaction using formyl quinoline, 2-aminothiophenol, thiosemicarbazone and trifluoromethylbenzaldehyde as a reactant to synthesise quinoline, pyridine and pyran based bioactive small molecules; these products are a quinoline type bearing a benzothiazole moiety, quinoline thio semicarbazone ligand, ﬂuorine substituted dihydro pyridine, ﬂuorine substituted dihydropyran and ﬂuorine substituted pyridine derivatives. In total, fifteen compounds were synthesized eleven of which were novel; all compounds were characterized by spectroscopic techniques. In vitro anti-bacterial activities of the synthesized compounds were investigated against a representative panel of pathogenic strains. Compounds 6, 7, 8, 11 and 13 exhibited excellent anti-bacterial activity compared with ﬁrst line drugs. Potent p53–MDM2 interaction inhibitors 2-thio-1,2-dihydroquinoline-3-carbaldehyde thiosemicarbazone and fluorine substituted new pyridine scaffold were successfully identiﬁed by structure-based design. An efficient one-pot four component route to the synthesis of trifluorinated pyrrolophenanthroline and fluoroquinoline pyrrolophenanthrolines was designed. In this reaction 1-butyl-2,3-dimethylimidazolium tetrafluoroborate ionic liquid (DMTIL) was used as a reaction medium; no catalyst was required. The structure of the pyrrolophenanthrolines was deduced by IR and NMR analysis. These compounds were studied with Bovine Serum Albumin (BSA) through molecular docking. Hydrophopic, electrostatic and hydrogen bonding interaction played a crucial role in the binding to sub domain of BSA. Interaction studies of DMTIL with BSA by emission, absorption, synchronous fluorescence, circular dichroism (CD) and three dimensional emission (3D) spectroscopic techniques were under taken. The results from emission titration experiments revealed the existence of a strong interaction between BSA and DMTIL ionic liquid. It showed that compounds with lesser number of hydrogen bonds are found to be more active which is attributed to hydrophobic interaction and electrostatic interaction which also played a vital role in DMTIL binding to sub domain IB of BSA. A novel copper-loaded boron nitride nanosheet (Cu/BN) catalyst was prepared and fully characterized. It was used as an efﬁcient and chemoselective catalysts for the synthesis of α-aminophosphonates by the Kabachnik-Fields reaction; twenty one α-aminophosphonates were synthesised. The enhanced catalytic activity and product yield was attributed to the increase of surface acidity. Overall, this methodology offered competitive advantages such as recyclability of the catalyst without further puriﬁcation or without using additives or cofactors, low catalyst loading, broad substrate applicability and high yields. The application of this new nanocatalyst in organic synthesis will provide a novel pathway for the synthesis of pharmaceutically important compounds. Gold nanoparticle surfaces were modified with self-assembled monolayers of important thiol and disulfide bioactive molecules since considerable interest is due to their potential application as anti-cancer agents. Herein, a carbazole was conjugated to lipoic acid by using an amide coupling catalyst HBTU and DIEA reaction. The structure of the carbazole thio octanic acid (CTN) was identified by IR and NMR. CTN was attached to the gold nanoparticles surface and the capping behaviour was characterized by UV-vis spectroscopy, TEM, DLS and FTIR. The cytotoxicity of CTNAuNPs on A549 cell lines was determined using the MTT assay. The results suggest CTN and CTNAuNPs possess anti-proliferative properties in the cancerous A549 cells. Furthermore a dual thiol ligand was synthesized by using equimolar 4-aminothiophenol (4-ATP) and amino oxadiazole thiol (AXT). This dual ligand was attached to the gold nanoparticles surface (DTAu) and the capping behaviour was characterized by UV-vis spectroscopy, TEM, DLS and FTIR. The cytotoxicity of DTAu on A549 cell lines was determined using the MTT assay. The results suggest dual ligands (4-ATP, AXT) and DTAu possess anti-proliferative properties in the cancerous A549 cells. South African indigenous plants and agroforestry waste were also used in the synthesis of silver, gold and palladium nanoparticles (NPs). Green protocols such as the use of environmentally benign solvents and non-hazardous reagents were an added advantage to physical and chemical means. Furthermore these reactions were rapid and the size and shape of the NPs could be manipulated by choosing the correct medium. The formulation of natural medicinal compounds capped onto NPs was assessed for their anti-cancer activity, in A549 lung cancer line, and catalytic reduction of dyes and nitrobenzene derivatives were studied. These NPs displayed: Significant cytotoxicity to lung cancer cells with minimal effect on normal healthy cells. Outstanding catalytic reduction of pharmaceutical and textile waste effluents such as dyes and nitro aromatic compounds. In addition, palladium nanoparticles containing capped Moringa olifera compounds were used effectively in the Suzuki coupling reaction of iodobenzene and phenylboronic acid. The reaction was rapid and was conducted in an aqueous medium.