Editing Intramolecular reaction (section)

===<span id="Tethered intramolecular 2+2 reactions">Tethered intramolecular [2+2] reactions</span>===
Tethered intramolecular [2+2] reactions entail the formation of [[cyclobutane]] and [[cyclobutanone]] via intramolecular [[2+2 photocycloaddition]]s. Tethering ensures formation of a multi-cyclic system.
[[File:23 fig. 1.png|center|407 px|Tethered intramolecular [2+2] reactions]]

The length of the tether affects the [[stereochemistry|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 <math>\alpha</math>-carbon of the [[Alpha-beta Unsaturated carbonyl compounds|enone]].<ref>{{cite journal|author1=Coates, R. M. |author2=Senter, P. D. |author3=Baker, W. R. |journal=J. Org. Chem.|year=1982|volume=47|page=3597|title=Annelative Ring Expansion via Intramolecular [2 + 2] Photocycloaddition of α,β-Unsaturated γ-Lactones and Reductive Cleavage: Synthesis of Hydrocyclopentacyclooctene-5-carboxylates|issue=19|doi=10.1021/jo00140a001}}</ref> When the tether consists only two carbons, the “bent” product is generated where the <math>\beta</math>-carbon of the enone is connected to the terminal carbon of the alkene.<ref>{{cite journal |last1=Tamura |first1=Y. |last2=Kita |first2=Y. |last3=Ishibashi |first3=H. |last4=Ikeda |first4=M. |title=Intramolecular photocycloaddition of 3-allyloxy- and 3-allylamino-cyclohex-2-enones: formation of oxa- and aza-bicyclo[2,1,1]hexanes |journal=[[Journal of the Chemical Society|J. Chem. Soc. D]] |year=1971 |volume=19 |issue=19 |pages=1167 |doi=10.1039/C29710001167 }}</ref>
[[File:23 fig. 2.png|center|407 px|Effects of the length of tether on [2+2]  photocyclization  reaction]]

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.<ref>{{cite journal|author1=Corey, E. J. |author2=Kang, M. C. |author3=Desai, M. C. |author4=Ghosh, A. K. |author5=Houpis, I. N. |journal=J. Am. Chem. Soc.|year=1988|volume=110|pages=649–651|title=Total Synthesis of (.+-.)-Ginkgolide B|issue=2|doi=10.1021/ja00210a083|pmc=6746322|pmid=31527923}}</ref>
[[File:23 fig. 3.png|center|407 px|Tethered [2+2] reaction in the total synthesis of (<u>+)</u>  - Ginkgolide B]]