title: Enantioconvergent synthesis by sequential asymmetric Horner-Wadsworth-Emmons and palladium-catalyzed allylic substitution reactions.
abstract: A new method for enantioconvergent synthesis has been developed. The strategy relies on the combination of an asymmetric Horner-Wadsworth-Emmons (HWE) reaction and a palladium-catalyzed allylic substitution. Different alpha-oxygen-substituted, racemic aldehydes were initially transformed by asymmetric HWE reactions into mixtures of two major alpha,beta-unsaturated esters, possessing opposite configurations at their allylic stereocenters as well as opposite alkene geometry. Subsequently, these isomeric mixtures of alkenes could be subjected to palladium-catalyzed allylic substitution reactions with carbon, nitrogen, and oxygen nucleophiles. In this latter step, the respective (E) and (Z) alkene substrate isomers were observed to react with opposite stereospecificity: the (E) alkene reacted with retention and the (Z) alkene with inversion of stereochemistry with respect to both the allylic stereocenter and the alkene geometry. Thus, a single gamma-substituted ester was obtained as the overall product, in high isomeric purity. The method was applied to a synthesis of a subunit of the iejimalides, a group of cytotoxic macrolides.
authors.affiliation: Department of Chemistry, Technical University of Denmark, Building 201, Kemitorvet, DK-2800 Kgs. Lyngby, Denmark.
authors.forename: T M
authors.initials: TM
authors.lastname: Pedersen
authors.forename: E L
authors.initials: EL
authors.lastname: Hansen
authors.forename: J
authors.initials: J
authors.lastname: Kane
authors.forename: T
authors.initials: T
authors.lastname: Rein
authors.forename: P
authors.initials: P
authors.lastname: Helquist
authors.forename: P O
authors.initials: PO
authors.lastname: Norrby
authors.forename: D
authors.initials: D
authors.lastname: Tanner