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Claisen Rearrangement Prof. Dr. A. G. Nikalje Pharmaceutical chemistry

Claisen Rearrangement Prof. Dr. A. G. Nikalje Pharmaceutical chemistry Y. B. Chavan College Of Pharmacy, Aurangabad. Contents Introduction Characteristics Mechanism Stereochemistry Applications Johnson-clasien rearrengement. Introduction

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Claisen Rearrangement Prof. Dr. A. G. Nikalje Pharmaceutical chemistry

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  1. Claisen Rearrangement Prof. Dr. A. G. Nikalje Pharmaceutical chemistry Y. B. Chavan College Of Pharmacy, Aurangabad.

  2. Contents • Introduction • Characteristics • Mechanism • Stereochemistry • Applications • Johnson-clasien rearrengement

  3. Introduction • The Claisen rearrangement was the first to be discovered. • When simple or substituted allyl ether of a phenol is heated at about 200ºC, an isomerization takes place called claisen rearrangement. • This occurs in which the allyl group gets migrates from the ether oxygen atom to a carbon atom of the aromatic ring.

  4. The original sigmatotropic rearrangement occurred when an aryl allyl ether was heated without solvent & an o- allyl phenol resulted. This is the Cliasen rearrangement.The thermal (3,3)- sigmatotropic rearrangement of allyl vinyl ether to the corresponding gama and delta –unsaturated carbonyl compounds is Claisen rearrangement. • 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.

  5. Characteristics of Claisen Rearrangement • It is sigmatotropic rearrangement. • This is a one step mechanism without ionic intermediates or any charge, just like a cycloaddition. It is a unimolecular process with constrained trasition state. • Claisen rearrangement does not need a catalyst and it also follows first order kinetics. • No cross products were obtained in a mixture so, the reaction is intramolecular. • Mechanism • This a one step mechanism without ionic intermediates or any charges,just like cyclo addition. The arrow goes round in a ring. The difference between this and cyclo aaddition is that one of the arrows starts on a sigma bond instead a pi bond. The second step in the reaction is the simple ionic proton transfer to regenerate aromaticity.

  6. Mechanism of ortho isomerisation • The ortho rearrangement is concerted process and it follows cyclic mechanism which proceeds through the formation of cyclic 6 membered transition state in which the rupturing of oxygen allyl bond is syncronous with the formation of C-C bond at an ortho position. • Thus there occurs the formation of cyclohexadien which convert to more stable aromatic compound phenol.

  7. Mechanism of para isomerisation If the ortho positions are blocked the very first step still involves the ortho migration . As hydrogen is absent at ortho position , this prevents aromatization , then undergoes further rearrangement involving migration of allyl gr.again through a cyclic 6 membered transition state ultimately gates aromatised to the phenol. Hence the mechanism of Para rearrangement involves a double rearrangement and thus ultimately it is not rearrange during the reaction i.e. there is absence of inversion in the Para claisen rearrangement.

  8. Stereochemistry • Studies reveled that the stereochemical information is transferred from the double bonds to the newly formed sigma-bond. • The actual T.S. depends on the nature of the substituents at various positions of starting allyl vinyl ether.

  9. The stereoselectivity will depend on the energy difference between diastereomeric chair like T.S. in allylic systems , the observed stereoselctivity can usually be rationalized by assuming that unfavorable 1,3-diaxail integration are minimized in the chair like T.S. with large groups adopting an equatorial position when geometry of the ring or other steric effects discover a chair like structure the reaction can proceed through a boat like T.S.

  10. Applications • Claisen rearrangement is also applicable to allyl ethers of enols • Rearrangement of 1,5-hexadienes ( Cope rearrangement) • Rearrangement of allyl amine oxides • 4) Oxygen atom of the allyl ether is replaced by sulphur • 5) Synthesis of o-eugenol (volatile oil as desensitizer & carminative)

  11. SYNTHETIC APPLICATIONS 1)Asymmetric total synthesis of putative structure of cytotonic diterpinoid (-) –sclerophytin A

  12. 2) Enantio total synthesis of (+) and (-) saudin. The rearrangement took place via chair like transition state. 3) In k.k. Nicolaous biomimetic synthesis of 1-o-methyl –forbesione,construction of the 4-oxatricyclo(4,3,1,0)decan –2-one. Double claisen rearrangement involved followed by Dies-Alder reaction

  13. Johnson-Clasian rearrangement • This is a modified method. • This is highly streoselective reaction. • It is well studied for synthesis of trans- disubstituted olifinic bonds. • Temp. required is 100-180oc. • The johnson-clasian rearrangement of (E)- allilic alcohols mainly give syn. products while (Z)- allilic alcohols gives anti product.

  14. Refercences 1) Strategic application of named reactions in organic synthesis pg. no.

  15. THANK YOU

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