Advancements in the Synthesis of Heterocyclic N-Oxide via Inter- and Intramolecular Cyclization Reactions

Heterocyclic N-oxides have become indispensable scaffolds in energetic chemistry, pharmaceuticals, and synthetic organic chemistry. This core scaffold is also present in numerous therapeutic agents, with activities ranging from antimycobacterial and anthelmintic to anti-inflammatory, antioxidant, and anticancer. Over the years, tremendous effort has been devoted to developing efficient synthetic methods starting from simple precursors. Both intermolecular and intramolecular strategies have been widely explored for the construction of cyclic N-oxide derivatives. This review briefly discusses synthetic approaches to the construction of aromatic and aliphatic heterocyclic N-oxides and their derivatives via cyclization reactions from 2011 to 2025. Various synthetic approaches, including inter- and intramolecular cyclization strategies, have been discussed, along with their scope, mechanisms, and limitations. This review not only provides updates on synthetic methods for heterocyclic N-oxide derivatives but also draws the attention of researchers from diverse research fields.