Recent Advances in [5 + 4] Cycloaddition Reactions: Catalysis, Mechanism, and Reaction Optimization

Medium‐sized rings, especially nine‐membered scaffolds, represent the core framework of several promising biologically active compounds. Among the various synthetic strategies reported, cycloaddition reactions, particularly [5 + 4] cycloaddition reactions, represent a unique and relatively underexplored class of pericyclic reactions that form the nine‐membered ring systems. However, the construction of a nine‐membered ring remains a significant challenge due to thermodynamic barriers, involving unfavorable enthalpic and entropic factors, as well as transannular interactions. Numerous catalytic systems have been developed to overcome these challenges. In this review article, we examined literature from 2000 to 2025 on [5 + 4] cycloaddition reaction, highlighting the role of various catalytic strategies such as metal catalysts, organocatalysts, and photocatalysts.