Current Green Chemistry - Volume 11, Issue 2, 2024

Organocatalysed reactions are becoming powerful tools in the construction of complex molecular skeletons. It gains extra importance when used as a chemical approach to the chemical fixation of carbon dioxide (CO2). Carbon dioxide is an increasingly dangerous environmental hazard as the global climate temperature rises through the greenhouse effect. Meanwhile, in the past decades, significant advances can be noted in the use of organocatalysis for CO2 capture and its conversion into valuable chemicals. Therefore, herein we review a full set of organocatalysts used in CO2 fixation for the synthesis of N-heterocycles since they are present in several chemical structures with biological relevance.

The development of synthetic methodologies to obtain a diverse range of heterocyclic scaffolds has been a very attractive area of research due to their vast therapeutic importance. Conventional approaches that require the use of organic solvents, which are generally flammable, toxic, and not ecofriendly, are replaced either with greener alternatives or by completely avoiding their use. In literature, several solvent-free methods have already been reported for the synthesis of vast varieties of organic compounds. This review focuses on the solvent-free methods developed for the synthesis of different types of nitrogen and oxygen heterocycles which have exhibited diverse therapeutic applications.

Electro-organic synthesis, an atom-efficient, sustainable, mild process, permits an ecofriendly and elegant green path to synthesize structurally complex, still valuable molecules, avoiding the use of conventional harsh oxidizing and reducing agents and long-route reaction protocols. Being one of the oldest forms of reaction setups in a laboratory, it deals with fundamental redox chemistry through the direct application of electrical potential. Here flow of electrons acts as an oxidizing agent at the anode at the same time reducing agent at the cathode, depending upon the requirement of the reaction. Simultaneously, it minimizes the generation of reagent waste during the reaction. However, electrifying organic synthesis plays more than preventing the waste footprint. This technology provides an alternative roadmap through nonclassical bond disconnections to access desired target molecules by cutting down a number of steps with the formation of apparently looking difficult bonds with excellent regio-, chemo-and stereoselectivity. Hence, it emerges as an alternative and attractive technique for the contemporary synthetic communities. Consequently, in recent years, multiple milestones have been achieved in the electro-organic synthesis of fascinating natural products through oxidative C-C bond formation, C-H/N-H functionalization, very rare oxidative N-N dimerization, RCDA dimerization, etc. Thus, synthesis of extremely complex natural products through finding new electro-synthetic route as a key methodology have become one of the alluring synthetic targets to synthetic chemists because of their versatile utilities in medicine, agriculture, food, and cosmetic industry. This review presents advances in electrochemistry in the total synthesis of 20 complex natural products reported since 2013. Enabling synthetic steps are analyzed alongside innate advantages as well as future prospects are speculated.

The development of more sustainable chemical reactions and processes has been the focus of recent research activities. Advances in the field of organic synthesis have led to the emergence of new methodologies and techniques involving non-conventional energy sources. These include the applications of mechanical energy (mechanochemistry) and microwave radiation (MW) methods. This article reviews the advances in multistep organic synthesis of biologically relevant organic molecules using mechanochemistry and microwave techniques. Among them, various heterocyclic molecules (with nitrogen, oxygen, and sulphur atoms), amides, and peptides have been synthesized by multistep mechanochemical or MW reactions. Performing multiple synthetic steps using more sustainable methods shows cumulative advantages over multistep processes under conventional conditions in terms of reduced solvent use, shorter reaction times, better turnovers, and reaction yields. Simplification of protocols by carrying out two or more reaction steps in the same reaction vessel is another advantage of multistep syntheses.

Introduction: Multicomponent reactions (MCRs) and green chemistry are essential criteria for the development of efficient chemical syntheses for valuable organic compounds. Methods: The design, synthesis, and development of sustainable procedures for the production of novel biological and pharmaceutical molecules have gained high importance. Herein, an environmentally benign synthesis of mono- and bis-2,3-dihydroquinazolin-4(1H)-ones as pharmaceutically active compounds was carried out in good to high yields of 80-99% within 45-120 minutes. Results: The desired products were synthesized via three-component and pseudo five-component condensations of isatoic anhydride, a primary amine (aniline or ammonium acetate), and an aldehyde/dialdehyde using sulfamic acid (20%) as a solid acidic catalyst under the solvent-free condition at 100°C. Conclusion: The easy work-up procedure, metal-free and environmentally benign catalyst, green reaction conditions for performing MCRs, and high yields of pure products are some advantages of the presented protocol.

Introduction: Sonication has been introduced as a green and effective activation method for the selective monoamination of β-dicarbonyl compounds. The simple one-pot process resulted in different substituted β-amino-α,β-unsaturated esters and ketones at room temperature with quantitative yields. Aqueous NH4OH was used as a safe and economical nitrogen source for pressurized NH3gas. The process is considered green, accounting for not using any solvents and catalysts, besides some aqueous NH4OH-involved reactions using nontoxic water. A broad variety of useful synthetic intermediates, β- amino-α,β-unsaturated esters and ketones have been prepared in short reaction time. Methods: Using this developed protocol, we were able to synthesize a series of structurally diverse β- amino-α,β-unsaturated esters and unsaturated amino ketones. Results: The synthesis of target compounds was achieved in a truly green process with high atom economy and excellent yields in a catalyst-free one-pot system in an aqueous medium using simple, commercially available, inexpensive ammonium hydroxide as the source of the nitrogen. The high atom economy has been accompanied by the formation of a small amount of nontoxic waste, water. Conclusion: In conclusion, a simple, convenient, and high-yielding catalyst-free environmentally benign method was developed for the synthesis of unsaturated amino acid esters and unsaturated amino ketones.

An ultrasound-assisted catalyst-free green protocol has been developed for the synthesis of a series of diversely substituted Knoevenagel condensation products from the reaction between functionalized aromatic aldehydes/isatin derivatives and substituted malononitriles as the C-H acids in water at ambient conditions. The method is simple, straightforward, and highly efficient. The major advantages of this newly developed protocol are expedient catalyst-free synthesis, good to excellent yields, energy efficiency, use of water as reaction medium, easy isolation of products, no need for column chromatographic purification, eco-friendliness, and operational simplicity.