text: The earliest example of a [4+4] photocycloaddition was the photodimerization of anthracene, discovered in 1936. This reaction has remained one of the most reliable photocycloadditions and is the basis of continuing investigation.
  Other polycyclic aromatics undergo anthracene-like photocycloaddition, such as acridizinium salts. Biologically active natural products include pleuromutilin, kalmanol, crenulide, and longipenol.
  [4+4] photocycloaddition approaches to the creation of cyclooctanoid systems offer several appealing features, including convergent assembly of the eight-membered ring from two relatively simpler four-carbon fragments, formation of two new carbon-carbon bonds, and the potential for introduction of up to four new stereocenters in one step.