Sigmatropic Claisen variant

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°. 

This section will focus on recent examples of asymmetric [3,3]-sigmatropic rearrangements involving dienes and polyenes. Attention will be given to Cope and Claisen rearrangements, as well as to several of their variants. For more exhaustive reviews of the subject, the reader is referred elsewhere69,70. 

Extremely useful ramifications of the Claisen rearrangement emerged with Johnson s discovery of the orthoester variant of this transformation. His approach (Scheme 2.156) involved the following sequence of steps, which were carried out in one reaction vessel (i) transesterification of the orthoester with an allylic alcohol to give 490 (ii) elimination to form the intermediate ketene acetal 491 and (iii) [3,3] sigmatropic rearrangement to yield the y, -unsaturated ester 492. The Johnson-Claisen procedure is properly considered to be one of the most efficient methods available to prepare y,(5-unsaturated esters such as 492. ° ... 

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). 

The reaction outlined in O Scheme 59 is an example of a variant of the Claisen rearrangement of allyl ketene aminal (so-called Eschenmoser-Claisen rearrangement) [87], The reaction dose not require an acid catalyst glycal was just heated with dimethylacetamide dimethyl acetal to form ketene aminal, which underwent the sigmatropic rearrangement to form the corresponding )/,5-unsaturated amide. 

Most [3,3]-sigmatropic rearrangements take place thermally, and the Cope, oxy-Cope and Claisen rearrangements are among the most important rearrangements in this class. Important variants of the Claisen rearrangement include the Johnson modification via orthoesters, the Eschenmoser modification via ketene N,O-acetals, the Ireland modification via ketene silylacetals and the Corey modification via boron ester enolates [696], The aza-Claisen rearrangement has also seen... 

An accelerated diastereoselective variant of the amide acetal Claisen rearrangement is the reaction between the lithium alkoxide of ( )- or (Z)-2-butenol with salts derived from alkylation of propanamides or fluoroacetamides with methyl triflate or dimethyl sulfate. This reaction yields directly the [3,3] sigmatropic rearrangement products of the corresponding N.O-ketene acetals at room temperature, e.g., formation of 7/8 and 12/13 (Table 8)4fiI-462. 

A review which covers sigmatropic rearrangements in addition to Diels-Alder reactions, 1,3-dipolar cycloadditions, electrocyclic reactions, and ene reactions has appeared. The stereochemistry of [3,3]-sigmatropic reactions of chiral carbon compounds has been reviewed, as have diastereoselective Claisen rearrangements of substrates bearing chiral auxiliary and enantioselective variants of achiral substrates. Examples of [3,3]-sigmatropic rearrangements used in the synthesis of various types of 3-chromene derivatives have been reviewed. ... 

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