Enantioselective Visible-Light-Mediated Formation of 3-Cyclopropylquinolones by Triplet-Sensitized Deracemization

3-Allyl-substituted quinolones undergo a triplet-sensitized di-pi-methane rearrangement reaction to the corresponding 3-cyclopropylquinolones upon irradiation with visible light (lambda=420 nm). A chiral hydrogen-bonding sensitizer (10 mol %) was shown to promote the reaction enantioselectively (88-96 % yield, 32-55 % ee). Surprisingly, it was found that the enantiodifferentiation does not occur at the state of initial product formation but that it is the result of a deracemization event. The individual parameters that control the distribution of enantiomers in the photostationary state have been identified.