Fused 1H-/2H-Indazole as Bioactive Privileged Scaffolds: Synthetic Strategies and Pharmaceutical Applications

Bioheterocycles represent a pharmaceutically privileged class of compounds due to their broad spectrum of medicinal applications. Among them, indazole (benzopyrazole or isoindazole) is a fused nitrogen-containing heterocycle with significant importance in pharmaceuticals, bioactive molecules, and natural products. The indazole system exists in tautomeric forms 1H-, 2H-, and 3H-indazole, with 1H-indazole being the most prevalent and stable. Conventionally, indazoles are synthesized via the reaction of aromatic aldehydes or ketones with substituted phenylhydrazines. Naturally occurring alkaloids such as nigellicine, nigeglanine, and nigellidine feature the indazole nucleus as their core structure. Owing to its versatile biological profile, the indazole scaffold exhibits diverse pharmacological activities, including antibacterial, anticancer, anti-inflammatory, and neuroprotective effects. Moreover, it constitutes the structural backbone of several clinically relevant drugs such as Asitinib, Bozitinib, and Entrectinib. These attributes underscore the prominence of indazole as a key framework in contemporary medicinal chemistry and drug discovery. Overall, the article aims to provide a comprehensive understanding of how innovative C-H functionalization strategies have accelerated the design and development of bioactive indazole derivatives for pharmaceutical applications.