Alkenes from Claisen-Ireland

Claisen-Ireland rearrangements frequently are used for the synthesis of alkenes.This works particularly well if the allyl ester is derived from a secondary allyl alcohol. In this case a stereogenic double bond is formed in the rearrangement. The examples in Figure 11.43 show that the alkene is mostly fnms-configured if this C=C bond is 1,2-disubstituted and almost completely -configured if it is trisubstituted. 

A key aspect of stereoselectivity in the Ireland-Claisen rearrangement is the preferential formation of syn (erythro) or anti (threo) pentenoic acids from appropriately substituted allyl silyl ketene acetals. The stereochemical outcome of the reaction is determined by two features (1) the geometry of the silyl ketene acetal and aUyUc alkene, and (2) whether the rearrangement proceeds via a chair-like or boat-Uke transition state. 

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