Current Organic Chemistry - Volume 25, Issue 24, 2021

Bridged polycyclic frameworks represent a unique tool to form curved units; the bicyclo[2.2.1]hepta-2,5-diene system is widely exploited to design and induce concave topologies. In particular, bridged benzocyclotrimers (BCTs) are characterized by a flat aromatic base decorated with bridged polycyclic motifs, which provide the suitable curvature underlying the concave-convex topology. In the 1960s, these molecules attracted interest for their own chemical and physical properties. Later, the improvements in synthetic procedures to produce bridged BCTs have paved the way for their utilization to design and prepare molecular containers, bowls, cages, and baskets that are able to accommodate target molecules, recognize them, and modulate their functions. In this frame, we aim to describe the historical evolution of the concept, from the first bridged BCTs explored to confirm the existence of strained alkynes, and the phenomenon of bond alternation (Mills-Nixon hypothesis), to the most recent gated molecular baskets developed as dynamic synthetic receptors for molecular delivery. The main synthetic approaches which have been used to perform cyclotrimerization of bridged polycyclic alkenes, and related mechanisms, are also examined and discussed, with a specific focus on the syn/anti stereoselectivity issue and its consequences at a mechanistic level. The present review covers literature contributions published until mid 2021.

Isoxazole derivatives are aromatic five-membered ring heterocyclic compounds with one oxygen atom and one nitrogen atom at adjacent positions. These compounds have a broad spectrum of applications in medicinal chemistry, such as antimicrobial, anticancer, antitumor, antitubercular, analgesic, anti-inflammatory, antidepressant, and anticonvulsant activities and some of them are well-known drugs for the treatment of various diseases in the market. The use of this class of compounds in agriculture as herbicides, insecticides have been widely reported. In organic materials, these compounds have been applied as semiconductors, single-walled nanotubes, liquid crystals, chiral ligands, scaffolds for peptidomimetics, dyes, and high-temperature lubricants. Isoxazole derivatives also play an important role in organic synthesis because they can be converted into several important synthetic units such as β-hydroxy ketones, γ -amino alcohols, α , β-unsaturated oximes, β -hydroxy nitriles, α-hydroxy-β-diketones, and β - dicarbonyl compounds. Due to such a wide range of applicability, the synthesis of isoxazole derivatives has attracted intensive research from chemists. This review will focus on considerable studies on the synthesis of isoxazoles which date back from 2012.

In recent years, reductive dehydration of primary nitro compounds has been demonstrated as an attractive and alternative approach for the direct synthesis of nitriles. Although scattered information on the conversion of alkyl nitrites and/or nitroalkanes to the corresponding nitriles has been reported in the literature, a research paper describing comprehensive information at one place is scarce. The present review has attempted to focus on the uncatalyzed as well as catalyzed reductive dehydration of nitro compounds directly into nitriles in the presence of dehydrating reagents. Non-metal and metal-catalyzed dehydration of nitro compounds and the use of radical chemistry in accessing nitriles have been highlighted in this review article.

Quinoxaline is a versatile heterocyclic moiety that possesses a wide range of biological activities. Therefore, many researchers have been performing the synthesis of quinoxaline derivatives on a daily basis. In addition, high demands for their synthesis often result in an increased generation of different waste chemicals. However, to minimize the utilization and generation of toxic organic substances, the present review focuses on the various green synthetic approaches for the synthesis of quinoxaline and its derivatives. Moreover, due to the quick manufacturing of novel medications using a quinoxaline scaffold, multiple study reports are published in a short period of time. Therefore, to fully comprehend the current state of the quinoxaline scaffold in medicinal chemistry, it is necessary to combine recent findings with previous understanding. Besides, compared to conventional methods, these green methods minimize the use and generation of harmful chemicals and improve reaction efficiency in terms of product yields, purity, energy consumption, and post-synthetic procedures. Therefore, in this review, we have attempted to shed light on various green synthetic strategies leading to the synthesis of quinoxaline scaffold and its derivatives, such as ultrasound irradiation, microwave irradiation, grindstone technique, environmentally benign solvents/catalysts based, and reactant immobilized on a solid support, etc.

Maclura tricuspidata, which belongs to the Moraceae family, is widely distributed in Asia, including Korea. All parts of this plant, such as roots, fruits, and leaves, are rich in diverse bioactive compounds, especially xanthones and isoflavonoids. Investigation of the leaves of M. tricuspidata through various chromatographic techniques yielded fourteen compounds, including five new compounds. The new compounds were determined to comprise an isoflavonoid structure with hydroxyethyl moiety and defined as hydroxyethylisoflavones A-E (10-14) on the basis of 1D and 2D NMR and MS data. These compounds efficiently inhibited α-glucosidase activity.