Benzo-fused five-membered N/O/S heterocyclic compounds, such as indole, benzofuran, and benzothiophene, possessing a single heteroatom, have important applications in medicinal chemistry, agrochemistry, and material chemistry. Metal-catalyzed reactions are well-established synthetic pathways for the formation of C-X bonds, enabling the direct synthesis of heterocycles. This approach offers advantages over traditional methods, such as fewer steps, increased atom economy, low catalyst loading, regioselectivity, and stereoselectivity. Due to their widespread uses in the pharmaceutical industry, the formation of C-N, C-O, and C-S bonds has gained significant attention. This article focusses on the metal-catalysed synthesis and corresponding mechanistic approaches for N/O/S heterocycles, particularly indole, benzofuran, and benzothiophene, reviewing the progress of the past five years and discussing unexplored future opportunities.

Dibenzofuran and its derivatives are ubiquitous and important medicinal and natural products. Many contain electron-donating substituents. Herein, we report a Pd-catalysed C−H functionalisation protocol that works with electron-rich arenes. We use tetrabutylammonium acetate (NBu₄OAc), which we suspect can act as base and ligand, rendering this protocol a simple and efficient route to dibenzofurans.

Abstract

Dibenzofuran and its derivatives are ubiquitous and important medicinal and natural products. Many contain electron-rich aryl rings. Forming the key intramolecular Ar−Ar bond using traditional cross-coupling is difficult. The C−H functionalisation (C−H activation) approach is, in principle, far more useful. However, we previously found that the well-established conditions, which promote C−H functionalisation through Concerted Metalation-Deprotonation (CMD), proved unsatisfactory. Herein, we report a Pd-catalysed C−H functionalisation protocol that works with electron-rich arenes. We use tetrabutylammonium acetate (NBu₄OAc), which we suspect can act as base, ligand and solvent, rendering this protocol a simple and efficient route to electron-rich dibenzofurans.

Tau be or not tau be. A variety of fluorescent thiophene-vinylene-based ligands was synthesized. Ligands with specific chemical composition displayed selectivity towards distinct protein aggregates in tissue sections with Alzheimer's disease pathology and distinct heterocyclic moieties governed the selectivity of the ligand towards Aβ or tau pathology. We foresee that these findings will aid in designing ligands towards disease-associated protein aggregates.

Abstract

Distinct aggregated proteins are correlated with numerous neurodegenerative diseases and the development of ligands that selectively detect these pathological hallmarks is vital. Recently, the synthesis of thiophene-based optical ligands, denoted bi-thiophene-vinyl-benzothiazoles (bTVBTs), that could be utilized for selective assignment of tau pathology in brain tissue with Alzheimer's disease (AD) pathology, was reported. Herein, we investigate the ability of these ligands to selectively distinguish tau deposits from aggregated amyloid-β (Aβ), the second AD associated pathological hallmark, when replacing the terminal thiophene moiety with other heterocyclic motifs. The selectivity for tau pathology was reduced when introducing specific heterocyclic motifs, verifying that specific molecular interactions between the ligands and the aggregates are necessary for selective detection of tau deposits. In addition, ligands having certain heterocyclic moieties attached to the central thiophene-vinylene building block displayed selectivity to aggregated Aβ pathology. Our findings provide chemical insights for the development of ligands that can distinguish between aggregated proteinaceous species consisting of different proteins and might also aid in creating novel agents for clinical imaging of tau pathology in AD.