title: Stereoselective formal [3 + 2] cycloaddition of N-alkylidene glycine ester anions to chiral Fischer alkenylcarbene complexes. Asymmetric synthesis of 3,4,5-trisubstituted prolines.
abstract: The reaction between N-alkylidene glycine ester enolates, generated from glycine esters aldimines with LDA in THF at low temperature, and chiral alkoxyalkenylcarbene complexes of chromium provided directly 2,4,5-trisubstituted-3-pyrrolidinylcarbene complexes with total exo selectivity and very high syn and facial diastereoselectivity when carbene complexes bearing the (-)-8-phenylmenthyloxy group were employed. Oxidation of the metal carbene moiety followed by basic hydrolysis of the esters afforded enantiomerically highly enriched syn,exo-3,4,5-trisubstituted prolines, whereas acidic hydrolysis of the same functional groups proceeded with epimerization at the alpha-amino acid center leading to anti,exo-3,4,5-trisubstituted prolines of very high enantiomeric purity as well.
authors.affiliation: Instituto Universitario de Química Organometálica Enrigue Moles-Unidad Asiciada al CSIC, Univerisdad de Oviedo, Spain.
authors.forename: Isabel
authors.initials: I
authors.lastname: Merino
authors.forename: Santosh
authors.initials: S
authors.lastname: Laxmi Y  R
authors.forename: Josefa
authors.initials: J
authors.lastname: Flórez
authors.forename: José
authors.initials: J
authors.lastname: Barluenga
authors.forename: Jesús
authors.initials: J
authors.lastname: Ezquerra
authors.forename: Concepción
authors.initials: C
authors.lastname: Pedregal