Enantioselective intramolecular [2+2] photocycloaddition reactions of 4-substituted coumarins catalyzed by a chiral Lewis acid

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.