Simple Enolate Claisen Rearrangement

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. 

Recently, the enolate method has emerged as a widely and frequently employed method in the synthesis for reasons like the easy access to the starting materials, simplicity and mildness of the experimental procedures, accurate predictability of the stereochemistry of the products and good to excellent realization of the stereoselection. This method can be further sub-divided into sub-topics like the simple enolate method [11], the Ireland silyl ketene acetal method, the chelated ester enolate method, imidate method, and the N,0- and N,S-acetal method. The present discussion will be restricted to simple enolate method as the remaining sub-topics are covered separately in this book. 

Although it was possible to effect the enolate Claisen rearrangement of allyhc esters of simple carboxylic acids, the reaction conditions were rather harsh and so the methods might not be extendable to more complex and sensitive substrates. Further, low yields of these rearrangements severely limited their usefulness. 

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Simple Enolate Claisen Rearrangement 215 Tab. 5.1.1 Half-life times for rearrangement of silyl ketene acetals at 32°C. 

I 5 Simple and Chelate Enolate Claisen Rearrangement C7H15... 

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