Claisen-type -sigmatropic

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. 

Furthermore, a rather bold but chemically more feasible mechanism can account for the proposed migration of the acetoxy group. This is double, symmetry allowed, suprafacial-suprafacial, Claisen-type, 3,3-sigmatropic shift. 

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°. 

A close analog of the Cope is the Claisen rearrangement. Historically, the Claisen, having been known and studied since 1912, is much the older of the two. It is a [3,3]-sigmatropic change (Equation 12.110), with oxygen as one atom of the chain. It is most commonly encountered with unsaturated phenyl ethers of the type illustrated in Equation 12.111, which rearrange to ortho- or para-substi-... 

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