Enantioselective Intramolecular [2 + 2]-Photocycloaddition Reactions of 4-Substituted Quinolones Catalyzed by a Chiral Sensitizer with a Hydrogen-Bonding Motif

Abstract

Six 2-quinolones, which bear a terminal alkene linked by a three- or four-membered tether to carbon atom C4 of the quinolone, were synthesized and subjected to an intramolecular [2 + 2]-photocycloaddition. The reaction delivered the respective products in high yields (78-99%) and with good regioselectivity in favor of the straight isomer. If conducted in the presence of a chiral hydrogen-bonding template (2.5 equiv) at low temperature in toluene as the solvent, the reaction proceeded enantioselectively (83-94% ee). An organocatalytic reaction was achieved when employing a chiral hydrogen-bonding template with an attached sensitizing unit (benzophenone or xanthone). The xanthone-based organocatalyst proved to be superior as compared to the respective benzophenone. Closer inspection revealed that the reaction of 4-(pent-4-enyloxy)quinolone leading to a six-membered ring, annelated to the cyclobutane, was less enantioselective (up to 41% ee with 30 mol % catalyst) than the reaction of 4-(but-3-enyloxy)quinolone leading to a five-membered ring (90% ee with 5 mol % and 94% ee with 20 mol % catalyst). Photophysical data (emission spectra, laser flash photolysis experiments) proved that the latter photocycloaddition was significantly faster, supporting the idea that the dissociation of the substrate from the catalyst prior to the photocycloaddition is responsible for the decreased enantioselectivity. Under optimized conditions, employing 10 mol % of the xanthone-based organocatalyst at -25 °C in trifluorotoluene as the solvent, three of the other four substrates gave the intramolecular [2 + 2]-photocycloaddition products with high enantioselectivities (72-87% ee). In all catalyzed reactions, the yields based on conversion were moderate to good (40-93%).