4-π-Photocyclization: Scope and Synthetic Applications

4-π-Photocyclizations (also referred to a valence isomerizations) allow the generation of versatile bicyclic cyclobutene building blocks from a range of 1,3-dienes in a totally atom-economical process. Herein, the historical developments and recent innovations in 4-π-photocyclizations are reviewed, illustrated by applications of these intriguing reactions in total synthesis, medicinal chemistry, and materials chemistry.