title: Construction of Quaternary Stereogenic Centers via [2 + 2] Cycloaddition Reactions. Synthesis of Homochiral 4,4-Disubstituted 2-Azetidinones and Imine Substituent Effects on beta-Lactam Formation.
abstract: A study on the asymmetric construction of quaternary stereogenic centers via [2 + 2] cycloaddition reaction of ketenes with ketimines is described. Reaction of achiral ketenes and chiral alpha-alkoxy ketone-derived imines resulted in formation of new beta-lactams as single diastereomers. The cycloaddition was extended to pyruvate imines, aralkyl ketone-derived imines, and dialkyl ketimines. In these cases the asymmetric induction was satisfactorily achieved using beta-silylalkanoyl ketenes and the Evans-Sjögren ketene. C,C-Bis (trimethylsilyl)methylamine ketimines derived from enolizable dialkyl ketones cleanly led to the corresponding C(4) disubstituted beta-lactams without deprotonation. Therefore, a general methodology for a convergent asymmetric synthesis of beta-lactams in which C(4) exists as a quaternary carbon is provided.
authors.affiliation: Departamento de Química Orgánica, Facultad de Química, Universidad del Pais Vasco, Apdo 1072, 20080 San Sebastián, Spain, Departamento de Química Inorgánica, Universidad del Pais Vasco, Apdo 644, 48080 Bilbao, Spain, and Organisch-Chemisches Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland.
authors.forename: Claudio
authors.initials: C
authors.lastname: Palomo
authors.forename: Jesus M.
authors.initials: JM
authors.lastname: Aizpurua
authors.forename: Jesus M.
authors.initials: JM
authors.lastname: García
authors.forename: Regina
authors.initials: R
authors.lastname: Galarza
authors.forename: Marta
authors.initials: M
authors.lastname: Legido
authors.forename: Raquel
authors.initials: R
authors.lastname: Urchegui
authors.forename: Pascual
authors.initials: P
authors.lastname: Román
authors.forename: Antonio
authors.initials: A
authors.lastname: Luque
authors.forename: Juan
authors.initials: J
authors.lastname: Server-Carrió
authors.forename: Anthony
authors.initials: A
authors.lastname: Linden