Current Organic Chemistry - Volume 27, Issue 12, 2023

Currently, the use of enzymes as efficient tools for the preparation of highly valuable molecules in various industries has proven to be a tremendous development. The preparation of esters via biotechnological processes constitutes an important eco-friendly approach for several industries, especially in the field of flavour and fragrances. This is particularly due to the accrued customer's interest in products labelled as natural ensured by using enzymes as natural catalysts. This mini-review article is dedicated to underline the recent advances (from 2010 to 2022) in the bio-acylation of achiral and chiral alcohols (esterification and transesterification). The preparation conditions, such as enzyme nature, acyl donor, solvents, etc., are highlighted.

Nitrogen ring junction heterocycles play a crucial role in synthetic organic chemistry due to their remarkable activity. The fused nitrogen ring junction compounds are abundant in nature; they have excellent biological activity and are used against various health issues. To make selective nitrogen ring junction products from the nitrogen ring junction heterocycles, expensive chemicals and catalysts, like expensive transition metal complexes and metal composites, are required. To neglect the drawbacks of conventional synthesis methods like long reaction times, by-product formation, lower selectivity, and low yields, an alternative of nonconventional light-mediated techniques can be opted for. The light source uses a radical mechanism that reduces by-product formation, provides a regio-selective product, increases yield, decreases reaction time, is cost-effective, and does not require special catalysts or chemicals. There are a variety of light sources, viz., UV, visible, IR, laser, and X-ray. The UV, visible light, white, green, and blue LED light sources are widely used in the photochemical method. This review emphasizes the light-mediated synthesis of nitrogen-ring junction heterocyclic compounds.

Ultrasound is well explored in the preparation of several heterocycles as it is an eco-friendly, safer and cheaper technique compared to the previous conventional synthetic approach. Ultrasonication offers a great deal of interest for chemists as it not only lowers the reaction time but also markedly reduces the use of hazardous chemicals and enhances the purity of synthesized compounds. Since the application of ultrasound waves acts as a nonpolluting energy source, therefore, it is an area of great importance in the field of green and sustainable chemistry. Also, nitrogen and oxygen-based pharmaceuticals have been found to be a beneficial tool for modifying solubility, lipophilicity, polarity and other important pharmacokinetic parameters which aid in optimizing ADMET characteristics of drug candidates. Further, nitrogen and oxygen-containing compounds have been reported to exhibit antibacterial, antiviral, anti-inflammatory, anticancer, analgesic, antihyperglycemic action etc. Therefore, the review focues to coveron covering the latest applications of ultrasonication in the preparation of oxygen and nitrogen nitrogencontaining heterocycles holding immense importance in therapeutically active molecules, which will aid new researchers in their ongoingnd future research.

Visible light-catalyzed organic chemical transformations with the benefits of green and sustainable, streamlined steps, and mild conditions have drawn increasing interest and attention due to the abundance of visible light, green and non-toxic, and low-cost. These reactions undoubtedly offer new ways, new methods, and new technologies for organic synthetic chemistry, which has developed quickly in the field of organic synthesis in recent years. Cyclization reactions stand out as a significant tool since they allow for the creation of many bonds, even stereocenters, in a single step while maintaining predictable stereochemistry. In this review, we summarize the development of visible-light-induced cyclization reactions.

In this review, we report β-cyclodextrin catalyzed green transformations of biologically active heterocycles. β-Cyclodextrin is a seminatural product, water-soluble, highly efficient, and biodegradable catalyst. β-Cyclodextrin is a versatile catalyst and promotes a variety of multicomponent transformations, biomimetic reactions, C-C bond formation, and synthesis of some biologically active natural products. It has been applicable to attain some name reactions, deprotection of THP/MOM/Ac/Ts ethers, oxidative cleavage of epoxides, oxidative dehydrogenation of alcohol, regioselective cyclization of chalcone epoxides and 2'-aminochalcones. The catalyst is useful to carry out diastereoselective reactions, and it also plays a very important role in phase transfer catalysts.

Pyrazoles are well-known five-membered heterocyclic compounds and are found in a wide variety of synthetic and natural compounds. Compounds carrying pyrazole scaffolds exhibit a wide range of biological activities such as anticancer, antimicrobial, anticonvulsant, antioxidant, analgesic and anti-inflammatory effects. Pharmaceuticals with many different activities in the pyrazole structure are currently on the market (e.g., celecoxib, lonazolac, tepoxalin, rimonabant, pyrazofurin, epirizole). The pyrazole ring contains the N-N double bond, which is thought to have a key role in biological activity, and compounds with this bond are difficult to produce by organisms, so their relative abundance is very low in nature. For this reason, many studies have been carried out on this structure and it has been revealed that the structure has a unique effect spectrum. Microwave-assisted synthesis has opened up some new opportunities compared to conventional synthesis. It is possible to use less solvent and reduce processing time with microwave synthesis. In addition, better selectivity and thermal stability are provided by microwave synthesis. Ultrasound-assisted synthesis is often used to enhance conventional solvent extraction, while microwaves reveal bioactive compounds by heating without any solvent. In the traditional method of pyrazole synthesis; polar solvents, acidic and basic catalysts are needed in large quantities in the synthesis of pyrazole scaffolds. This review aims to summarize the recent advancements in the synthesis methods of a pyrazole ring with nontraditional methods. Therefore this article will provide readers with a new perspective on the synthesis of pyrazole scaffolds as an environmentally friendly alternative.

A vital driving force for chemists to discover novel synthetic protocols is the improvement of more effective synthetic technologies and sustainable methodologies. This is associated with the development of innovative research that stimulates the creative reevaluating of known conceptions. Currently, these robust methodologies, as well as green synthetic procedures, have been designed for the total synthesis of secondary metabolites. Flow chemistry and flow photochemistry have emerged as powerful tools to promote valuable transformations in the total synthesis of natural products as key step(s). Flow chemistry development offers many merits over a traditional batch format, namely a round-bottom flask. The advantages of this green tool comprise waste minimization, simple scale-up, reduction of reaction time, safety betterment as, well as energy and cost efficiency. Flow chemistry comprises a fascinating prospect for the synthesis of promising organic molecules and bioactive complex natural products as it represents a suitable modern synthetic technology for the improvement of sustainable chemistry. Continuous flow chemistry is an assembly of chemical processes carried out in continuous flowing streams. Compared to conventional organic synthesis, it is a process that strengthens technology and is superior in enhancing and scaling up synthesis, accurately controlling reaction rate, and providing the desired products with maximum yields. In the past and likely in the future natural products and their analogue will continue to deliver the stimulation for drug discovery and development programs. Total synthesis of natural products is very useful to synthesize natural products in the laboratory as many secondary metabolites are available in low quantities from their sources of origin. So, this review wishes to cover the brilliant applications of flow chemistry in the total synthesis of natural products in the field of novel technological advances.

Heterogeneous catalysis using continuous flow processing is one of the most demanding subjects from the viewpoint of manufacturing industrial-scale organic compounds. An amalgamation of the two areas of technology, i.e., heterogeneous catalysis and flow chemistry, has opened new avenues for green synthetic chemistry. These processes are particularly convenient in terms of short diffusion paths and improved mixing due to the sensing of high local concentration of catalytic species on solid catalytic surface when the liquid/ gaseous reagents pass through the column, ultimately resulting in quicker and more efficient reaction with increased reaction rates and higher turnover numbers. It imparts several key benefits over conventional batch systems, such as time and energy-saving methodologies, better productivity, reproducibility, economic viability, waste reduction, and ecofriendly nature. Also, it eradicates the need for any intermediate isolation, separation of catalysts, and use of excess reagents. The present review article focuses on heterogeneous catalysis under continuous flow conditions. Various key reactions, for instance, carbon-carbon bond formation, hydrogenation, condensation, and oxidation, are presented well, along with their recent developments in the manufacturing of active pharmaceutical ingredients and platform chemicals. Asymmetric catalysis has also been discussed with its applications in the synthesis of complex organic molecules. It is anticipated that the review article will proliferate significant interest in modernizing chemical syntheses through continuous flow processes.