Cyclization Reactions of N-Aminopyridinium Salts: Synthetic Scope and Mechanistic Insights

N-Aminopyridinium salts are readily available and bench-stable nitrogen-containing precursors that have demonstrated broad synthetic utility in diverse cyclization reactions. As practical 1,3-dipoles, N-aminopyridinium salts can engage with dipolarophiles in cycloaddition reactions to construct nitrogen-containing heterocycles. Moreover, as a class of novel aminating or bifunctional aminating reagents, they are capable of generating nitrogen-centered radicals under visible-light irradiation, which can initiate amination-cyclization processes. In recent years, cyclization strategies involving N-aminopyridinium salts have made remarkable progress and garnered significant attention. This review highlights recent advances in the cyclization chemistry of N-aminopyridinium salts and their derivatives, with a particular focus on transition-metal catalyzed, transition-metal-free systems and visible-light-induced transformations. The scope, limitations, and synthetic applications of various cyclization strategies are critically discussed, aiming to reveal the innovative potential of N-aminopyridinium salts as nitrogen-based precursors in synthetic chemistry. We hope this review provides valuable insights and inspiration for future research in synthetic methodology, drug discovery, and the design of functional materials.