title: Generation of quaternary stereocenters by asymmetric Michael reactions: enamine regiochemistry as configuration switch.
abstract: Regioselective enamine formation from cyclic beta-diketones 1 is obtained by the appropriate choice of activating agent: Brønsted acid catalyzed condensation gives endocyclic enamines 3 as the thermodynamically favored products. Activation with Lewis acid BF(3).OEt(2) affords betaines 8 as intermediate products, which can be reacted with L-valine diethylamide (2) to preferentially furnish exocyclic enamines 4 as kinetic products. Derivatives with quaternary stereocenters were accessible from both isomeric enamines by using asymmetric, copper(II)-catalyzed Michael reactions at ambient temperature. Both regioisomers afford the triketones 7 with the same constitution but bearing the opposite absolute configuration at the quaternary stereocenter. Thus, both enantiomers of the product are prepared by using the same chiral auxiliary derived from L-valine.
authors.affiliation: Institut für Organische Chemie der Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany.
authors.forename: Burkard
authors.initials: B
authors.lastname: Kreidler
authors.forename: Angelika
authors.initials: A
authors.lastname: Baro
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Frey
authors.forename: Jens
authors.initials: J
authors.lastname: Christoffers