title: [2+2] Photocycloaddition of 3-alkenyloxy-2-cycloalkenones: enantioselective Lewis acid catalysis and ring expansion.
abstract: By application of substoichiometric amounts (50 mol %) of a chiral Lewis acid, the intramolecular [2+2] photocycloaddition of the title compounds was achieved with high enantioselectivity (up to 94 % ee). Upon cleavage of the cyclobutane ring the resulting tricyclic products underwent ring-expansion reactions under acidic conditions and formed anellated seven- or eight-membered-ring systems without racemization. The ring expansion could be combined with a diastereoselective reduction (triethylsilane) or allylation (allyltrimethylsilane) upon BF3 catalysis (48-87 % yield).
authors.affiliation: Lehrstuhl für Organische Chemie I und Catalysis Research Center (CRC), Technische Universität München, Lichtenbergstrasse 4, 85747 Garching (Germany) http://www.oc1.ch.tum.de/home_en/
authors.forename: Richard
authors.initials: R
authors.lastname: Brimioulle
authors.forename: Thorsten
authors.initials: T
authors.lastname: Bach
keywords: Lewis acids; cycloaddition; enantioselectivity; photochemistry; ring expansion