Photochemical Hetero-cycloadditions.

Hetero-cycloaddition reactions are powerful, atom-economical strategies for the rapid construction of heterocyclic frameworks from readily available building blocks. Over the past two decades, advances in photochemistry, photocatalysis, and modern excitation strategies have driven remarkable developments in the field. By accessing electronically excited states, light enables cycloaddition pathways inaccessible under classical thermal conditions, providing access to reactive radical, ionic, or excited-state intermediates under mild conditions. Technological advances in light sources and photocatalyst design have improved reaction efficiency, site-, regio-, and stereoselectivity, while considerably expanding the accessible chemical space. Beyond established hetero-[2 + 2], hetero-(3 + 2), and hetero-[4 + 2] processes, new reaction classes, including π-σ hetero-cycloadditions exploiting strained small-ring systems, have emerged as powerful approaches to structurally complex saturated scaffolds of increasing relevance in medicinal chemistry. In this Review, we critically examine the evolution of photochemical hetero-cycloadditions from 2000 to 2026, discussing fundamental concepts and terminology, cycloaddition classes, heterocyclic targets, and activation mechanisms. We highlight the conceptual advances that have shaped modern photochemical hetero-cycloaddition chemistry and conclude by discussing current limitations and future directions for the field.