Asymmetric chelated Claisen rearrangements in the presence of chiral ligands--scope and limitations

Claisen rearrangements of glycine crotyl ester enolates in the presence of chelating metal salts and chiral ligands provide ,-unsaturated amino acids in a highly stereoselective fashion. Best results are obtained with electron withdrawing protecting groups, isopropylates of aluminum and magnesium, and the cinchona alkaloids as chiral ligands. While the use of quinine gives rise to the (2R)-configured amino acids, quinidine provides the opposite enantiomer. The different enantiomers can also be obtained by using only one of the chiral ligands by simply changing the reaction conditions. A mechanistic rational for the stereochemical outcome of the reaction is given, which is supported by several experiments.