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Claisen Rearrangement

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When a simple or substituted allyl ether of a phenol is heated at about 200oC an isomerization called claisen rearrangement occurs in which the allyl group gets migrated from the ether oxygen atom to a carbon atom of the aromatic ring. The migrant allyl group goes preferentially to the ortho position or to the para position if both the ortho positions are blocked. Migration to a meta position has not been observed.

The Claisen rearrangement stands as one of the most powerful methods for introducing molecular complexity from easily obtained starting materials. The term Claisen rearangement has subsequently expanded to include any such process involving a 1,5-diene possessig a hetero atom in the 3-position. 

Claisen Rearrangement

Mechanism

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This is an organic reaction where an allyl vinyl ether is converted into an unsaturated carbonyl compound with the input of a heat or Lewis acid.The Claisen rearrangement is a pericyclic reaction. This is an exothermic reaction, according to Woodward-Hoffmann rules exhibit a pathway of suprafacial reaction.
  1. This step is an electro cyclic process. The electrons are pushed around the 6 membered ring starting the electron flow by having the aromatic C=C attacking the allyl C=C and displacing and O leaving group. 
  2. It is a tautomerization reaction, where the ketone, actually a dienone undergo tautomerisation to form the more stable aromatic phenol. 

Claisen Condensation Mechanism

Aromatic Claisen Rearrangement

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The aromatic Claisen rearrangement is a aromatic variation of Claisen rearrangement, it is the [3,3]-sigmatropic rearrangement of an allyl phenyl ether to an intermediate compound which undergoes tautomerizes to an ortho-substituted phenol. 

The aromatic claisen rearrangement is accompanied by a rearomatization. 

Aromatic Claisen Rearrangement

Johnson Claisen Rearrangement

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The Johnson Claisen Rearrangementhas been used in the efficient synthesis of $\beta$, $\beta^{'}$-disubstituted esters containing quatenary centers in the synthesis of both enantiomers of bicyclo nonane-3,8-dione and in the synthesis of $\gamma$-hydroxy \alpha$, $\beta$-unsaturated ketones and nitriles.

At elevated temperature the rearrangement proceeds via the chair like transition state which presents the minimization of 1,3-syn diaxial interactions leading to the substituted E-olefin in a highly diastereocontrolled fashion

Johnson Claisen Rearrangement

The orthoester Johnson-Claisen rearrangement has been extended with great success for many applications in natural product synthesis.