title: Stereoselective Michael Addition of Glycine Anions to Chiral Fischer Alkenylcarbene Complexes. Asymmetric Synthesis of beta-Substituted Glutamic Acids.
abstract: The reaction of lithium enolates of achiral N-protected glycine esters with chiral alkoxyalkenylcarbene complexes of chromium provided the corresponding Michael adducts with either high anti or syn selectivity depending on the nature of the nitrogen protecting group, and high diastereofacial selectivity when carbene complexes containing the (-)-8-phenylmenthyloxy group were employed. Subsequent oxidation of the metal-carbene moiety followed by deprotection of the amine group and hydrolysis of both carboxylic esters afforded enantiomerically enriched 3-substituted glutamic acids of natural as well as unnatural stereochemistry. Alternatively, when the deprotection step was performed previously to the oxidation, cyclic aminocarbene complexes were formed, which finally led to optically active 3-substituted pyroglutamic acids.
authors.affiliation: Departamento de Química Física y Analítica, Facultad de Química, Universidad de Oviedo, Julián Clavería 8, 33071 Oviedo, Spain.
authors.forename: Jesús
authors.initials: J
authors.lastname: Ezquerra
authors.forename: Concepción
authors.initials: C
authors.lastname: Pedregal
authors.forename: Isabel
authors.initials: I
authors.lastname: Merino
authors.forename: Josefa
authors.initials: J
authors.lastname: Flórez
authors.forename: José
authors.initials: J
authors.lastname: Barluenga
authors.forename: Santiago
authors.initials: S
authors.lastname: García-Granda
authors.forename: María-Amparo
authors.initials: MA
authors.lastname: Llorca