J. Ezquerra et al., Stereoselective Michael addition of glycine anions to chiral Fischer alkenylcarbene complexes. Asymmetric synthesis of beta-substituted glutamic acids, J ORG CHEM, 64(18), 1999, pp. 6554-6565
The reaction of lithium enolates of achiral N-protected glycine esters with
chiral alkoxyalkenylcarbene complexes of chromium provided the correspondi
ng Michael adducts with either high anti or syn selectivity depending on th
e nature of the nitrogen protecting group, ana high diastereofacial selecti
vity when carbene complexes containing the (-)-8-phenylmenthyloxy group wer
e employed. Subsequent oxidation of the metal-carbene moiety followed by de
protection of the amine group and hydrolysis of both carboxylic esters affo
rded enantiomerically enriched 3-substituted glutamic acids of natural as w
ell as unnatural stereochemistry. Alternatively, when the deprotection seep
was performed previously to the oxidation, cyclic aminocarbene complexes w
ere formed, which finally led to optically active 3-substituted pyroglutami
c acids.