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II.Pericyclic Reactions 7 M.Sc. Semester-IV Core Course-9 (CC-9) Synthetic Organic Chemistry II.Pericyclic Reactions 7. Claisen Rearrangement, The Nazarov Reaction Dr. Rajeev Ranjan University Department of Chemistry Dr. Shyama Prasad Mukherjee University, Ranchi II Pericyclic Reactions 20 Hrs Molecular orbital symmetry, Frontier orbitals of ethylene, 1,3-butadiene, 1, 3, 5-hexatriene, allyl system, Classification of pericyclic reactions. FMO approach, Woodward-Hoffman correlation diagram method and PMO approach for pericyclic reaction under thermal and photochemical conditions. Electrocyclic reactions: Conrotatary and disrotatary motion, 4n and (4n+2) systems, Cycloaddition reaction: [2+2] and [4+2] cycloaddition reaction, Cycloaddition of ketones, Secondary effects in [4+2] cycloaddition. Stereochemical effects on rate of cycloaddition reaction, Diels-Alder reaction, 1,3-dipolar cycloaddition, Chelotropic reaction, The Nazarov reaction. Sigmotropic rearrangement: Suprafacial and antarafacial shift involving H and carbon-moieties, Peripatetic cyclopropane bridge, Retention and inversion of configuration, [3,3]-, [1,5]-, [2,3]-, [4,5]-, [5,5]-, and [9,9]- Sigmatropic rearrangements, Claisen rearrangements (including Aza-Claisen, Ireland-Claisen), Cope rearrangements (including Oxy-Cope, Aza-Cope), Sommelet-Hauser rearrangements, Group transfer reaction, Ene reaction, Mislow - Evans rearrangement, Walk rearrangement. Coverage: 1.Claisen Rearrangement 2.The Nazarov Reaction 2 Claisen Rearrangement : [3, 3] Sigmatropic Rearrangement How and Why? Claisen rearrangement: A thermal rearrangement of allyl phenyl ethers to form 2- allylphenols. Rainer Ludwig Claisen O OH 1851-1930 200-250°C Allyl phenyl ether 2-Allylphenol Dr. Rajeev Ranjan 3 Claisen Rearrangement : [3, 3] Sigmatropic Rearrangement Mechanism: O O heat Allyl phenyl Transition ether state O OH keto-enol tautomerism H A cyclohexadienone o-Allylphenol intermediate Dr. Rajeev Ranjan 4 Claisen Rearrangement : [3, 3] Sigmatropic Rearrangement Claisen reactions are generally irreversible and synthetically useful: re- aromatises O O O OH A [3,3]-sigmatropic reaction is pivotal to the Fischer indole synthesis: H H H steps H N N NH N NH NH NH -NH3 H Important: Don’t get confused with the Claisen Reactions of esters. Dr. Rajeev Ranjan 5 Claisen Rearrangement The Ireland-Claisen reaction is a useful method for constructing esters, particularly of difficult medium-ring products, with high stereoselectivity. OMe O O O OMe OMe Some more complex examples: H O O H n n via H O O O O O O n n OSi(tBu)Me2 H OSi(tBu)Me2 O OSi(tBu)Me O O 2 O OSi(tBu)Me 6 2 Application to the synthesis of ascidialactone, a marine natural product. 6 Claisen Rearrangement : Examples 1. 2. 3. 4. Dr. Rajeev Ranjan 7 Claisen Rearrangement: Examples Rearrangement of allyl vinyl ethers 5. 6. 7. 8. Dr. Rajeev Ranjan 8 Claisen Rearrangement: Examples 9. 10. Rearrangement via Ortho Esters 11. 12. 9 The Nazarov Cyclisation Reaction : The Nazarov cyclisation usually carried out under acidic/thermal conditions. The position adjacent to the ketone is a mixture of isomers in each case. Only the relative stereochemistry between the lower hydrogens is controlled. O O O H H irradiation (h) Heat ( and acid H H for catalysis H H (AcOH or H3PO4) cis trans All we need to know is the number of electrons involved (i.e. 4n or 4n+2) and whether the reaction is photochemical or thermal: Mechanism: H H H O O O next slide 10 Dr. Rajeev Ranjan The Nazarov Cyclisation, cont.... H H O O O H H enol -> ketone -H H H H H H H Stereochemistry in the key cyclisation step: O H 4n, thermal O H H H H hence conrotation, H antarafacial Note: although drawn as a localised cation, H the positive charge is spread over five atoms O through a delocalised pi system of p-orbitals. H There are a total of 4 electrons in the pi system (i.e. two in each alkene), hence it is a 4n electron system, and obeys the rules as H H usual. 11 Thank You Dr. Rajeev Ranjan University Department of Chemistry Dr. Shyama Prasad Mukherjee University, Ranchi 12.
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