4,6-Dihydroxysalicylic Acid-Catalyzed Oxidative Condensation of Benzylic Amines and Aromatic Ketones for the Preparation of 2,4,6-Trisubstituted Pyridines and Its Application to Metal-Free Synthesis of G-Quadruplex Binding Ligands

Innovative green oxidation of amines to imines under atmospheric oxygen

In recent years, the development of environmentally benign molecular construction methods has been of great importance, and especially, resource recycling, high atomic efficiency, and low environmental impact are in high demand. From this point of view, attention has also been focused on the development of one-pot synthesis of pharmaceuticals and functional molecules. Imines are excellent synthetic intermediates of these useful molecules, and the environmentally friendly oxidative synthesis of imines from amines has been energetically developed using oxygen (or air), which is abundantly available on the Earth, as an oxidant. This review focuses on the latest innovative and green oxidation systems of amines to imines under atmospheric oxygen, and their application to one-pot/eco-friendly and sustainable synthesis of pharmaceuticals and functional molecules. In particular, catalytic systems that activate molecular oxygen are categorized and described in detail as transition metal catalytic systems, photoirradiated catalytic systems, and organocatalytic systems.

Metal-Free Synthesis of 2-Substituted Quinazolines via Green Oxidation of o-Aminobenzylamines: Practical Construction of N-Containing Heterocycles Based on a Salicylic Acid-Catalyzed Oxidation System

Conventional quinazoline synthesis methods involve a highly multistep reaction, and often require excess amounts of substrate to control the product selectivity, leading to significant resource wastage. Hence, in this study, from the viewpoint of green chemistry, we developed a novel metal-free synthetic method for 2-substituted quinazoline derivatives by the 4,6-dihydroxysalicylic acid-catalyzed oxidative condensation of o-aminobenzylamines and benzylamines using atmospheric oxygen. In this system, the use of a catalytic amount of BF₃‧Et₂O (10 mol%) as a Lewis acid successfully led to the efficient oxidative condensation and intramolecular cyclization of these amines, followed by aromatization to afford the corresponding 2-arylquinazolines in up to 81% yield with excellent atom economy and environmental factor. Furthermore, to expand this green oxidation method to gram-scale synthesis, we investigated the development of an oxidation process using salicylic acid itself as an organocatalyst, and established a method for the practical green synthesis of a series of nitrogen-containing heterocycles. We expect that the findings will contribute to the development of practical synthesis methods for pharmaceutical manufacturing and industrial applications, along with further advancements in green chemistry.

Recent advances and future challenges in the synthesis of 2,4,6-triarylpyridines

2,4,6-Triarylpyridines are key building blocks to access functional molecules that are used in the design of advanced materials, metal-organic frameworks, supramolecules, reactive chemical intermediates and drugs. A number of synthetic protocols to construct this heterocyclic scaffold have been developed to date, the most recent of which (2015-present) are included and discussed in the present review. An emphasis has been placed on the utility of each synthetic approach in view of the scope of aryl/hetaryl substituents, limitations and an outlook of each method to be used in applied sciences.

[3 + 2 + 1] Pyridine Skeleton Synthesis Using Acetonitrile as C4N1 Units and Solvent

The first [3 + 2 + 1] methodology for pyridine skeleton synthesis via cascade carbopalladation/cyclization of acetonitrile, arylboronic acids, and aldehydes was developed. This reaction proceeds via six step tandem reaction sequences involving the carbopalladation reaction of acetonitrile, a nucleophilic addition, a condensation, an intramolecular Michael addition, cyclization, and aromatization. Delightfully, both palladium acetate and supported palladium nanoparticles catalyzed this reaction with similar catalytic performance. The characterization results of the fresh and used supported palladium nanoparticle catalysts indicated that the reaction might be performed via a Pd(0)/Pd(II) catalytic cycle that began with Pd(0). Furthermore, the products showed good fluorescence characteristics. The green homogeneous/heterogenous catalytic methodologies pave a new way for constructing the pyridine skeleton.

Oxidative Synthesis of Acid Blue 7 Dye Catalyzed by CuO/Silicotungstic Acid in Water-Phase

A catalytic oxidation reaction for Acid Blue 7 dye synthesis was evaluated in water. Without lead oxide or manganese oxide derivatives as oxidants, polyoxometalate catalysts were investigated to reduce the usage of harmful heavy metal. A catalyst was prepared by mixing silicotungstic acid with copper oxide, and aqueous hydrogen peroxide (30%) was used as an oxidizing agent. This reaction proceeded to produce Acid Blue 7 from the corresponding leuco acid after 45 min at 95 °C and was viable for a 10 g-scale synthesis.

One-Pot Construction of Diverse β-Lactam Scaffolds via the Green Oxidation of Amines and Its Application to the Diastereoselective Synthesis of β-Amino Acids

In this study, a simple one-pot construction of β-lactam scaffolds was successfully achieved via 4,6-dihydroxysalicylic acid-catalyzed organocatalytic oxidation of amines to imines using molecular oxygen. Although some imines are highly unstable and difficult to isolate by conventional methods, the organocatalytic oxidation of amines described herein, followed by their direct reaction with acyl chlorides in the presence of a base, afforded a series of new β-lactam derivatives with excellent cis selectivity, which could not be synthesized and isolated by previously reported methods. Thus, this one-pot protocol will be one of the powerful methods applicable to the synthesis of various potential drug candidates and functional molecules. Furthermore, the subsequent hydrolysis of these β-lactams successfully afforded the corresponding β-amino acids as almost single diastereomers in up to 99% yields.

One-Pot Synthesis of 1,5-Diketones under a Transition-Metal-Free Condition: Application in the Synthesis of 2,4,6-Triaryl Pyridine Derivatives

We developed a facile and green one-pot synthetic method for substituted 1,3,5-triaryl-1,5-diketones by Claisen-Schmidt condensation following Michael addition reaction of aryl ketones and aryl aldehydes under a transition-metal-free condition. This convenient one-pot synthetic strategy has several advantages, including being transition-metal-free, having no extra additives or reagents, having a broad substrate scope, having a high isolated yield, having a minimum amount of base employment, having a shorter reaction time, use of cheap starting materials, cost-effectiveness, and being environment friendly. Some of the chemical structures of 1,5-diketones were confirmed by X-ray single-crystal diffraction analysis. The application of 1,5-diketones was demonstrated in the preparation of 2,4,6-triaryl pyridine derivatives under a catalyst-free system using ammonium acetate as a nitrogen source.

1,6-Addition of vinyl p-quinone methides with cyclic sulfamidate imines: access to 4-hydroxyaryl-2,6-diarylpyridines

A base-promoted 1,6-addition–cyclization reaction of vinyl para-quinone methides with cyclic sulfamidate imines in an open atmosphere is reported. This method delivers good to high yields of 2,4,6-trisubstituted pyridines with a valuable phenolic moiety at the C4-position.