Sigmatropic rearrangements, Claisen allyl ether

The [3, 3] sigmatropic rearrangement of allyl vinyl ethers is known as Claisen rearrangement. Allylic alcohols can be converted to allyl vinyl ethers by mercuric acetate catalysed exchange with ethyl vinyl ether. The allyl vinyl ether need not be isolated because it undergoes rearrangement to 4-pentenal. 

The thermal [3,3]-sigmatropic rearrangement of allyl vinyl ethers, first reported by Claisen in 1912, allows the preparation of y,6-unsaturated carbonyl compounds. This reaction is called [3,3]-sigmatropic rearrangement because the new a-bond has a 3,3 relationship to the former a-bond. The aromatic variant of the Claisen rearrangement, in which an aryl allyl ether (5) is converted thermally to the corresponding y,6-unsaturated carbonyl compound (6), is followed by a re-aromatization providing an ortho-di y phenol (7). 

Claisen rearrangement. Thermal rearrangement of allyl ethers of enols or phenols to y,A-unsat-urated ketones or omicron-allylphenols, respectively, by a 3,3-sigmatropic shift. 

The ability of charged substituents to accelerate the 3,3-sigmatropic rearrangement of allyl vinyl ethers (the Claisen rearrangement) has also been documented. The effect of oxyanion substituents on the rate and course of aliphatic Claisen rearrangements has been the subject of particular attention. - In 1972, Ireland and Mueller reported that the lithium enolate derivatives of allyl esters undergo rapid and effi-... 

Since the first report of the rearrangement of allyl ethers of phenols and enols to the corresponding C-allyl derivatives by Claisen [1] in 1912, the mechanistic, stereochemical and synthetic aspects of the reaction have been extensively explored [2]. In modern terminology, the reaction, normally referred to as the Qai-sen rearrangement, is referred to as a [3,3]-sigmatropic rearrangement It has been demonstrated [2] that this reaction, espedaUy the aliphatic version, tolerates various substituents. This fact, coupled with accurate predictability of the stereochemical outcome of the reaction, has resulted in this reaction becoming one of the very important and quite frequently employed methods in the synthesis of complex molecules. 

Thermal [3,3]-sigmatropic rearrangements of allyl aryl ethers and allyl vinyl ethers are known as the Claisen rearrangements [68], These reactions are sensitive to solvent polarity and the rates of the reactions are increased by increasing the solvent polarity [69]. The simplest examples are the thermal conversion of allyl phenyl ether to ort/io-allyl phenol and of allyl vinyl ether to 4-pentenal [70]. 

Conjugate addition of vinyllithium or a vinyl Grignard reagent to enones and subsequent oxidation afford the 1.4-diketone 16[25]. 4-Oxopentanals are synthesized from allylic alcohols by [3,3]sigmatropic rearrangement of their vinyl ethers and subsequent oxidation of the terminal double bond. Dihydrojasmone (18) was synthesized from allyl 2-octenyl ether (17) based on Claisen rearrangement and oxidation[25] (page 26). 

This series of rearrangements includes the dithia-Claisen rearrangement mentioned above (Section IV.E.l) as well as the palladium-catalyzed [3,3]-sigmatropic isomeriza-tions of allyl methyl N-aryldithiocarbonimidates 627 (refluxing dioxane, 20 h, 62-90%) (equation 275)376 and a Pd11-catalyzed tandem [2,3]-sigmatropic shift, followed by 1,3-dipolar cycloaddition which takes place at equilibrium between O-allyl ethers of oximes 628 and the corresponding N-allyl nitrones 629 (equation 276)377. 

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