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 CH₂XCH₂ group to their 4-position (X=CH₂, CMe₂, 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=CH₂, CMe₂, 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.