Current Microwave Chemistry - Volume 10, Issue 1, 2023

Limited crude petroleum and growing awareness of fossil fuel depletion have enabled the development of alternative fuels and new energy sources. Biodiesel, also known as fatty acid methyl esters (FAME), has received a lot of attention due to its biodegradability, renewability, cost effective and nontoxicity. The purity of biodiesel production and uniform heating are the major hurdles for large scale biodiesel production. Recent microwave energy-based heating method has proved the potential for cleaner chemical production, short time duration, uniform heating, and purity over conventional heating method. The goal of this review is to discuss the biodiesel production using microwave-assisted heating. The different feedstocks used for biodiesel production, effects of microwave irradiation, factors affecting the rate of microwave-assisted transesterification to produce biodiesel were comprehensively discussed. Microwave irradiation has been compared to other technologies aiming to enhance the efficiency of overall process. The primary knowledge gaps in biodiesel production can be identified based on this research, ensuring the biodiesel industry's longterm sustainability.

In the past few years, using microwave power to heat and wield chemical reactions has become a gradually more popular subject in the scientific community. Microwave-supported organic synthesis is confirmed to be involved in rapidly synthesizing novel compounds with selectivity and enhanced biological activities. Microwave flash heating for chemical synthesis is a spectacular reduction in reaction times, high yield and purity of the products, etc. A catalysis field wherein small organic molecules like L-Proline efficiently and selectively catalyzes organic transformations. Microwave-assisted L-Proline catalyzed reactions are valuable tools for making different acyclic, heterocycles, and carbocyclic scaffolds that signify the main framework of most bioactive compounds. In synthetic organic chemistry, microwave irradiation speedily discarded the conventional heating methods in the world of multicomponent and step-wise synthetic chemistry. This review discusses only L-Proline Catalyzed Organic Reactions under microwave activation using modern organic transformations, including condensation, addition, asymmetric, multi-components, and other modular reactions.

Introduction: Nitrogen containing heterocycles such as azoles have gained popularity in medicinal chemistry research due to their versatile pharmacological activities. Imidazole’s are one such class of adaptable compounds. The aim of the study was to explore pharmacological activities of 2,4,5- trisubstituted imidazole’s and also to develop a novel method of synthesis using microwave chemistry. Methods: In the present study, the in-silico studies of 2,4,5-trisubstituted imidazole’s was carried out to predict their anti-leishmanial as well as COX-2 inhibitory activity. Although, the results are not satisfactory for the anti-leishmanial activity, the molecules showed comparable docking scores with standard celecoxib for the COX-2 inhibitory activity. Later, the microwave-assisted green synthesis of trisubstituted imidazole’s was attempted using green catalyst and solvent, molecular iodine and ethanol respectively. The synthesised derivatives (TG-1-4) were purified and characterised. Results: The derivatives were subjected to in-vitro COX-2 inhibitory assay, which showed good results. The molecules under study showed exemplary results against COX-2 PDB in molecular docking studies. A novel microwave-irradiation method was developed for the synthesis and also the in-vivo studies carried out for testing COX-2 inhibition was fruitful. Conclusion: In conclusion, the selected derivatives can be further studied in-vivo to develop new COX-2 inhibitors.

Background: Microwave synthesis has developed as a powerful tool for the cost-effective and greener synthesis of organic molecules, including quinazolines. Irradiation with microwave leads to the excitation of molecules and equitable distribution of thermal energy in a much shorter time than conventional synthesis. This results in shorter reaction time and, more often than not, higher efficiency. Objective: The primary objective of the work presented in this article was to prepare hydrazine hydrate or thiourea derivative of quinazolines through microwave synthesis as small-molecule scaffolds for further need-based functionalisation, isolation, and characterisation. We, herein, report the synthesis of two quinazolinone derivatives of thiourea and hydrazine, 3-amino-2-phenylquinazolin-4(3H)-one (QH) and 4-oxo-2-phenylquinazoline-3(4H)-carbothioamide (QTh), respectively. Method: A multi-step synthetic strategy starting from anthranilic acid was employed to synthesise the small molecule quinazolinones 3-amino-2-phenylquinazolin-4(3H)-one (QH) and 4-oxo-2- phenylquinazoline-3(4H)-carbothioamide (QTh). The compounds were synthesised by reacting hydrazine and thiourea with 2-benzamidobenzoyl chloride in DMF under microwave irradiation (800 W at 135 °C for 4 min) in the presence of potassium carbonate. The acid chloride was prepared by chlorination of 2-benzamidobenzoic acid, which in turn was synthesised from anthranilic acid by benzoylation. This method is an efficient alternative approach to synthesising quinazolinones from benzoxazin-4-ones. Results: We have successfully synthesised, isolated, and characterised the quinazolinone derivative QH (yield: 81%) and QTh (yield: 85%). The structures of the compounds were established through spectroscopic techniques. Theoretical optimisation of the structures was also achieved using DFT. The HOMOLUMO difference for QH and QTh was calculated to be 4.60 and 4.47 eV, respectively. Conclusion: The reported protocol is advantageous over conventional methods of quinazoline synthesis from benzoxazin-4-ones. The time required for the reaction is much less (4 min) as compared to the usual requirements of reflux (> 4 h); the higher energy gap of QH indicates greater stability than that of QTh.

Background: In recent years, microwave radiation has been widely used in organic synthesis, including solvent-free mode. However, the reaction conditions of phthalic anhydride with amino acids under solvent-free microwave activation have not been studied so far. Objective: In the present work, the effect of microwave activation on the interaction of phthalic anhydride with amino acids in solvent-free conditions has been studied in detail. Methods: The microwave heating dynamics of phthalic anhydride, glycine and their equimolar mixture under microwave conditions have been investigated, and the dependence of the heating rate on the microwave power is defined. Results: The common conditions for the synthesis of phthaloylamino acids have been determined as continuous heating at a power of 200 W at 130 °C for 5-6 min and additional heating for 5-10 min at a temperature close to the melting point of the corresponding amino acid. We have applied the developed two-step solvent-free microwave reaction protocol successfully for the synthesis of phthaloyl derivatives of glycine, alanine, β-alanine, 4-aminobenzoic acid, γ-aminobutyric acid, isoleucine, leucine, phenylalanine. Conclusion: Reaction conditions for synthesizing phthaloylamino acids by microwave activation without solvent have been established. The solvent-free microwave reaction between phthalic anhydride and amino acid has been found to proceed in the melted phthalic anhydride.