Cycloaddition-Based Retrosynthetic Logic for the Diversification and Molecular Editing of Polycyclic Aromatic Hydrocarbons.

Expanding the chemical and structural diversity of polycyclic aromatic hydrocarbons (PAHs) represents a significant aspirational goal in organic chemistry, materials science, and chemical biology, as it enables precise modulation of their molecular properties for a wide range of applications. Conventional approaches to PAH diversification typically rely on lengthy multistep sequences involving prefunctionalized aromatic substrates and strong oxidants, which limits efficiency and substrate generality. In contrast, retrosynthetic disconnection strategies based on cycloaddition-cycloreversion (retro-cycloaddition) or cycloaddition-rearomatization processes offer a conceptually distinct and streamlined approach to PAH construction and modification. These methods enable atom- and step-economical transformations directly from simple, unfunctionalized PAH templates, thereby circumventing the need for preinstalled functional groups. Recent advances have demonstrated that such disconnection strategies enable diverse transformations, including regioselective PAH growth via bay-, M- and L-region annulative π-extension (APEX) reactions, K-region aza-APEX reactions, formal C-H amination, ring-expansion through dearomative atom insertion, and π-extension via two-carbon fragment exchange. This review summarizes these developments and highlights the emerging potential of cycloaddition-based retrosynthetic logic as a unifying platform for the diversification and molecular editing of PAHs.