text: Tethered intramolecular [2+2] reactions entail the formation of cyclobutane and cyclobutanone via intramolecular 2+2 photocycloadditions. Tethering ensures formation of a multi-cyclic system.
  The length of the tether affects the stereochemical outcome of the [2+2] reaction. Longer tethers tend to generate the "straight" product where the terminal carbon of the alkene is linked to the -carbon of the enone. When the tether consists only two carbons, the “bent” product is generated where the -carbon of the enone is connected to the terminal carbon of the alkene.
  Tethered [2+2] reactions have been used to synthesize organic compounds with interesting ring systems and topologies. For example, [2+2] photocyclization was used to construct the tricyclic core structure in ginkgolide B.