Claisen-type -sigmatropic rearrangement

Stereoselective Claisen-type [3,3] sigmatropic rearrangements of propynyl vinyl ethers, which are intermediates in the reaction of propynyl alcohols with aldehydes or ortho esters leading to a-allenic aldehydes or esters, have been reported15,16. 

The Fischer indole synthesis. The Fischer acid-catalyzed conversion of an 7V-arylhydrazone 42 into an indole is one of the most powerful and versatile methods for the preparation of indoles . The mechanism involves a [3,3] sigmatropic Claisen-type rearrangement of a protonated enehydrazine tautomer 43 to give intermediate 44, which spontaneously cyclizes by loss of ammonia, probably via indoline 45, to an indole 46 (Scheme 27). For unsymmetrical ketones, two isomeric indoles are possible and the general result is that the indole derived from the more stable (usually the more highly substituted) enehydrazine is formed. 

For specific introductions to the Claisen rearrangement see refs 1-16 and 21 -41. Conventionally, the term Claisen rearrangement is used for [3,3] sigmatropic rearrangements of type I -> IV (X = O). For another notation, especially for polyhetero-Claisen systems, see ref 42. 

Several variants of [3,3] sigmatropic Claisen-type rearrangements have been used for the stereoselective synthesis of di- and trisubstituted olefinic systems 11,28,279,28°. 

The original sigmatropic rearrangement occurred when an aryl allyl ether was heated without solvent and an ortho-allyl phenol resulted. This is the Claisen rearrangement. The first step in this reaction is a pericyclic reaction of a type that you will learn to call a [3,3]-sigmatropic rearrangement. 

Some major exceptions, for which equivalent reaction types cannot be found in nature, is the Cope rearrangement - although [3,3]-sigmatropic rearrangements such as the Claisen rearrangement are known [50, 51], On the other hand, some biocatalysts can accomplish reactions impossible to emulate in organic chemistry, e.g., the selective functionalization of ostensibly nonactivated positions in organic molecules, such as the hydroxylation of aliphatics. 

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