Rapid Access to Benzimidazo[1,2-a]quinoline-Fused Isoxazoles via Pd(II)-Catalyzed Intramolecular Cross Dehydrogenative Coupling: Synthetic Versatility and Photophysical Studies

Imidazopyridine Family: Versatile and Promising Heterocyclic Skeletons for Different Applications

In recent years, there has been increasing attention focused on various products belonging to the imidazopyridine family; this class of heterocyclic compounds shows unique chemical structure, versatile optical properties, and diverse biological attributes. The broad family of imidazopyridines encompasses different heterocycles, each with its own specific properties and distinct characteristics, making all of them promising for various application fields. In general, this useful category of aromatic heterocycles holds significant promise across various research domains, spanning from material science to pharmaceuticals. The various cores belonging to the imidazopyridine family exhibit unique properties, such as serving as emitters in imaging, ligands for transition metals, showing reversible electrochemical properties, and demonstrating biological activity. Recently, numerous noteworthy advancements have emerged in different technological fields, including optoelectronic devices, sensors, energy conversion, medical applications, and shining emitters for imaging and microscopy. This review intends to provide a state-of-the-art overview of this framework from 1955 to the present day, unveiling different aspects of various applications. This extensive literature survey may guide chemists and researchers in the quest for novel imidazopyridine compounds with enhanced properties and efficiency in different uses.

An Aldehyde-Driven, Fe(0)-Mediated, One-Pot Reductive Cyclization: Direct Access to 5,6-Dihydro-quinazolino[4,3-b]quinazolin-8-ones and Photophysical Study

A short, proficient, and regioselective synthesis of biheterocyclic 5,6-dihydro-quinazolino[4,3-b]quinazolin-8-ones has been revealed via an Fe(0)-powder-mediated, one-pot reductive cyclization protocol. Mechanistic investigation proved that water acts as a source of hydrogen for the reduction of the nitro group and the reaction rate was accelerated by an aldehyde. The designed transformation works under aerobic conditions, providing a series of bio-inspired molecular scaffolds. In addition, the photophysical study showed blue fluorescence emission with a good fluorescence quantum yield.

Chemodivergent Synthesis of Indeno[1,2-b]indoles and Isoindolo[2,1-a]indoles via Mn(III)-Mediated or Electrochemical Intramolecular Radical Cross-Dehydrogenative Coupling

Chemodivergent synthesis of indeno[1,2-b]indoles and isoindolo[2,1-a]indoles from the same starting materials involving radical cross-dehydrogenative couplings have been developed. Mn(OAc)₃·2H₂O selectively promoted an intramolecular radical C-H/C-H dehydrogenative coupling reaction to provide indeno[1,2-b]indoles, while an intramolecular radical C-H/N-H dehydrogenative coupling reaction could proceed via electrochemistry to deliver isoindolo[2,1-a]indoles. Plausible mechanisms of the chemodivergent reactions were proposed.

Recent Advances in Palladium-Catalyzed Oxidative Couplings in the Synthesis/Functionalization of Cyclic Scaffolds Using Molecular Oxygen as the Sole Oxidant

Over the past years, Pd(II)-catalyzed oxidative couplings have enabled the construction of molecular scaffolds with high structural diversity via C–C, C–N and C–O bond-forming reactions. In contrast to the use of stoichiometric amounts of more common oxidants, such as metal salts (Cu and Ag) and benzoquinone derivatives, the use of molecular oxygen for the direct or indirect regeneration of Pd(II) species presents itself as a more viable alternative in terms of economy and sustainability. In this review, we describe recent advances on the development of Pd-catalyzed oxidative cyclizations/functionalizations, where molecular oxygen plays a pivotal role as the sole stoichiometric oxidant.

1 Introduction

2 Oxidative C–C and C–Nu Coupling

2.1 Intramolecular Oxidative C–Nu Heterocyclization Reactions

2.1.1 C–H Activation

2.1.2 Wacker/Aza-Wacker-Type Cyclization

2.1.3 Tandem Wacker/Aza-Wacker and Cyclization/Cross-Coupling Reactions

2.2 Intermolecular Oxidative C–Nu Heterocoupling Reactions

2.3 Intramolecular Oxidative (C–C) Carbocyclization Reactions

2.4 Intermolecular Oxidative C–C Coupling Reactions

2.4.1 Cyclization Reactions

2.4.2 Cross-Coupling Reactions

2.4.3 Homo-Coupling Reactions

3 Aerobic Dehydrogenative Coupling/Functionalization

4 Oxidative C–H Functionalization

5 Summary

An Approach to Vinylidenequinazolines from Isoxazoles and Dioxazolones

An effective strategy for the synthesis of vinylidenequinazolines has been efficaciously developed, which involves Rh(III)-assisted C-H amidation followed by ring-opening and intramolecular annulation. This protocol shows a straightforward way to construct diverse quinazoline units with a wide functional group compatibility from readily available isoxazoles and dioxazolones.

Benzo[4,5]imidazo[1,2-a]pyridines and benzo[4,5]imidazo[1,2-a]pyrimidines: recent advancements in synthesis of two diversely important heterocyclic motifs and their derivatives

Nitrogen heterocycles are some of the most important compounds that are found in nature or synthesized otherwise.

Copper-Catalyzed One-Pot Synthesis of Quinazolinones from 2-Nitrobenzaldehydes with Aldehydes: Application toward the Synthesis of Natural Products

A novel, efficient, and atom-economical approach for the construction of quinazolinones from 2-nitrobenzaldehydes has been unveiled via copper-catalyzed nitrile formation, hydrolysis, and reduction in one pot for the first time. In this reaction, urea is used as a source of nitrogen for nitrile formation, hydrazine hydrate is used for both the reduction of the nitro group and the hydrolysis of nitrile, and atmospheric oxygen is used as the sole oxidant. The method portrays a wide substrate scope with good functional group tolerances. Moreover, this method was applied for the synthesis of schizocommunin, tryptanthrin, phaitanthrin-A, phaitanthrin-B, and 8H-quinazolino[4,3-b]quinazolin-8-one.

Cu(II)-Promoted Cascade Synthesis of Fused Imidazo-Pyridine-Carbonitriles

A Cu(II)-promoted synthesis of an aza-fused N-heterocycle having a benz-imidazopyridine scaffold is developed via an addition-cyclization reaction followed by an Ullmann-type C-N coupling between o-iodoanilines and γ-ketodinitriles. This protocol features a broad substrate scope, giving products in 32-84% yields. The compounds show excellent photoluminescence properties having two absorption maxima in the region between 270-280 and 338-350 nm and emission maxima in the range of 502-533 nm. The HOMO-LUMO energy gap of 3.49-3.57 eV was determined using Gaussian 09 at the B3LYP/6-31G (d, p) basis set level. We also demonstrated a few postsynthetic modifications.