Intramolecular [2+2] photocycloaddition of 3- and 4-(but-3-enyl) oxyquinolones: Influence of the alkene substitution pattern, photophysical studies, and enantioselective catalysis by a chiral sensitizer

The intramolecular [2+2] photocycloaddition of four 4-(but-3-enyl) oxyquinolones (substitution pattern at the terminal alkene carbon atom: CH ₂, Z-CHEt, E-CHEt, CMe₂) and two 3-(but-3-enyl) oxyquinolones (substitution pattern: CH₂, CMe₂) was studied. Upon direct irradiation at λ=300 nm, the respective cyclobutane products were formed in high yields (83-95 %) and for symmetrically substituted substrates with complete diastereoselectivity. Substrates with a Z- or E-substituted terminal double bond showed a stereoconvergent reaction course leading to mixtures of regio- and diastereomers with almost identical composition. The mechanistic course of the photocycloaddition was elucidated by transient absorption spectroscopy. A triplet intermediate was detected for the title compounds, which-in contrast to simple alkoxyquinolones such as 3-butyloxyquinolone and 4-methoxyquinolone-decayed rapidly (τ≈1 ns) through cyclization to a triplet 1,4-diradical. The diradical can evolve through two reaction channels, one leading to the photoproduct and the other leading back to the starting material. When the photocycloaddition was performed in the presence of a chiral sensitizer (10 mol %) upon irradiation at λ=366 nm in trifluorotoluene as the solvent, moderate to high enantioselectivities were achieved. The two 3-(but-3-enyl)oxyquinolones gave enantiomeric excesses (ees) of 60 and 64 % at -25 °C, presumably because a significant racemic background reaction occurred. The 4-substituted quinolones showed higher enantioselectivities (92-96 % ee at -25 °C) and, for the terminally Z- and E-substituted substrates, an improved regio- and diastereoselectivity. Upon UV irradiation quinolones A undergo a rapid cyclization at the T₁ level (τ≈1 ns) through the respective 1,4-diradicals (e.g., 1), leading to the [2+2]-photocycloaddition products in high yields. Lifetimes and transient absorption spectra have been carefully measured. The high cyclization rate is one key element in the enantioselective formation of products, such as 3, when sensitized by catalyst (+)-2 (10 mol %).