Pyridinium and related azinium salts in organic synthesis and medicinal chemistry.

Pyridinium salts represent a versatile and synthetically valuable class of nitrogen-containing heterocyclic compounds that continue to play a crucial role in modern organic synthesis. Their unique electronic structure, characterized by an electron-deficient aromatic ring and a positively charged nitrogen center, enable diverse reactivity patterns, including nucleophilic addition, hydride transfer, single-electron transfer (SET), ylide formation, and cycloaddition processes. Recent advances in photoredox catalysis, electrochemical activation, and transition metal-catalyzed transformations significantly expanded the synthetic utility of pyridinium salts, particularly through the emergence of redox-active Katritzky salts as efficient radical precursors. This review provides an overview of the classification, synthesis, mechanistic behavior, and synthetic applications of pyridinium salts. Classical and base-mediated transformations are discussed alongside modern photochemical, transition metal-catalyzed, reductive, and electrochemical methodologies, highlighting their roles in constructing structurally diverse heterocyclic frameworks. Particular emphasis is placed on key mechanistic pathways, including SET processes, polar nucleophilic addition, hydride transfer, pyridinium ylide chemistry, and cascade annulation mechanisms that govern pyridinium reactivity and product selectivity. Recent developments in medicinal chemistry are also summarized, with emphasis on pyridinium-derived scaffolds in antiviral, anticancer, cardiovascular, and alkaloid-inspired drug discovery. The integration of sustainable synthetic technologies with pyridinium-mediated transformations significantly expanded the accessible chemical space from simple pyridine precursors while improving efficiency, selectivity, and environmental compatibility. Overall, this review highlights the ongoing importance of pyridinium salts as powerful synthetic platforms and pharmacologically relevant motifs, offering insights for future advances in organic synthesis, chemical biology, and drug discovery.