title: Enantioselective intramolecular [2+2] photocycloaddition reactions of 4-substituted coumarins catalyzed by a chiral Lewis acid.
abstract: Eight coumarins, which carry a terminal alkene tethered by a CH(2)XCH(2) group to their 4-position (X = CH(2), CMe(2), O, S, NBoc, NZ, NTs, NBn), were synthesized in overall yields of 51-80 %. Starting materials for the syntheses were either commercially available 4-hydroxycoumarin or 4-formylcoumarin. The intramolecular [2+2] photocycloaddition of these coumarins gave diastereoselectively products with a tetracyclic 3,3a,4,4a-tetrahydro-1H-cyclopenta[2,3]cyclobuta[1,2-c]chromen-5(2H)-one skeleton. Direct irradiation at λ = 300 nm in dichloromethane (c = 10 mM) led to product formation in good yields for most substrates, presumably via a singlet excited state intermediate. Due to the low coumarin absorption at λ >350 nm the photocycloaddition was slow upon irradiation at λ = 366 nm. Addition of a chiral oxazaborolidine-based Lewis acid (50 mol %) increased the reaction rate at λ = 366 nm and induced a significant enantioselectivity in the [2+2] photocycloaddition. Six out of eight coumarin substrates (X = CH(2), CMe(2), O, NBoc, NZ, NTs) gave the respective products in yields of 72-96 % and with 74-90 % enantiomeric excess (ee) upon irradiation in dichloromethane (c = 20 mM) at -75 °C. The Lewis acid presumably acts by coordination to the coumarin carbonyl oxygen atom, which leads to a bathochromic shift (redshift) of the UV absorption and which increases the singlet state lifetime. A second electrostatic interaction of the hydrogen atom at C3 with the oxygen atom of the oxazaborolidine is likely.
authors.affiliation: Lehrstuhl für Organische Chemie 1 and Catalysis Research Center (CRC), Technische Universität München, Lichtenbergstrasse 4, 85747 Garching, Germany.
authors.forename: Richard
authors.initials: R
authors.lastname: Brimioulle
authors.forename: Hao
authors.initials: H
authors.lastname: Guo
authors.forename: Thorsten
authors.initials: T
authors.lastname: Bach