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In organic chemistry, an electrocyclic reaction is a type of pericyclic rearrangement where the net result is one pi bond being converted into one sigma bond or vice versa.〔(IUPAC Gold Book )〕 These reactions are usually categorized by the following criteria: * Reactions can be either photochemical or thermal. * Reactions can be either ring-opening or ring-closing (electrocyclization). * Depending on the type of reaction (photochemical or thermal) and the number of pi electrons, the reaction can happen through either a conrotatory or disrotatory mechanism. * The type of rotation determines whether the cis or trans isomer of the product will be formed. ==Classical Examples== The Nazarov cyclization reaction is a named electrocyclic reaction converting divinylketones to cyclopentenones. A classic example is the thermal ring-opening reaction of 3,4-dimethylcyclobutene. The cis isomer exclusively yields ''cis,trans-2,4-hexadiene'' whereas the trans isomer gives the trans,trans diene:〔''The preparation and isomerization of - and -3,4-dimethylcyclobutene.'' Tetrahedron Letters, Volume 6, Issue 17, 1965, Pages 1207-1212 Rudolph Ernst K. Winter 〕 This reaction course can be explained in a simple analysis through the frontier-orbital method: the sigma bond in the reactant will open in such a way that the resulting p-orbitals will have the same symmetry as the HOMO of the product (a butadiene). The only way to accomplish this is through a conrotatory ring-opening which results in opposite signs for the terminal lobes. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「electrocyclic reaction」の詳細全文を読む スポンサード リンク
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