title: Catalytic enantioselective [5+2] cycloaddition between oxidopyrylium ylides and enals under dienamine activation.
abstract: Benzopyrylium ylides generated in situ from 1-acetoxyisochroman-4-ones reacted with α,β-unsaturated aldehydes in the presence of a bifunctional secondary-amine/squaramide catalyst to furnish [5+2] cycloaddition products in good yield with high diastereo- and enantioselectivity. The reaction proceeds by dienamine activation and involves β,γ-functionalization of the enal. The dienamine intermediates showed exclusive β,γ-reactivity and provided direct access to compounds with the 8-oxabicyclo[3.2.1]octane framework. The ability of the bifunctional secondary-amine/squaramide catalyst to engage in hydrogen-bonding interactions with the ylide made it particularly effective in terms of both the yield and the stereoselectivity of the transformation.
authors.affiliation: Department of Organic Chemistry II, University of the Basque Country (UPV/EHU), P.O. Box 644, 48080 Bilbao (Spain) http://www.ehu.es/GSA.
authors.forename: Ane
authors.initials: A
authors.lastname: Orue
authors.forename: Uxue
authors.initials: U
authors.lastname: Uria
authors.forename: Efraim
authors.initials: E
authors.lastname: Reyes
authors.forename: Luisa
authors.initials: L
authors.lastname: Carrillo
authors.forename: Jose L
authors.initials: JL
authors.lastname: Vicario
keywords: asymmetric catalysis; cycloaddition; dienamines; organocatalysis; ylides