Chelation Ireland-Claisen rearrangement, ester enolate

Further variations of the Claisen rearrangement protocol were also utilized for the synthesis of allenic amino acid derivatives. Whereas the Ireland-Claisen rearrangement led to unsatisfactory results [133b], a number of variously substituted a-allenic a-amino acids were prepared by Kazmaier [135] by chelate-controlled Claisen rearrangement of ester enolates (Scheme 18.47). For example, deprotonation of the propargylic ester 147 with 2 equiv. of lithium diisopropylamide and transmetallation with zinc chloride furnished the chelate complex 148, which underwent a highly syn-stereoselective rearrangement to the amino acid derivative 149. 

Although the Ireland version of the Claisen rearrangement is by far the most used application for the rearrangement of a-alkoxy-substituted esters, several other metals can be used for chelation. Oh et al. investigated the influence of the chelating metal on the rearrangement with respect to an asymmetric Claisen rearrangement [52]. The chelated enolates were obtained from the esters 100 and... 

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