Asymmetric\nPhotoinduced Excited-State Nazarov Reaction

We\nreport herein the first asymmetric photoinduced excited-state\nNazarov reaction of non-aromatic dicyclic divinyl ketones by using\nhydrogen-bonding catalysis. The enantioselectivity of photoinduced\nelectrocyclization is highly dependent on the structural features\nof the substrate and its interaction with chiral catalysts. For the\nsimple dicyclic divinyl ketone substrates, there is no discernible\nselectivity of the hydrogen bond coordination between the thiourea\nand carbonyl groups of the substrates in the ground state. However,\nwe found that the direction of the electrocyclization was well controlled\nin each coordination model and the <i>N</i>,<i>N</i>′-dimethylamine motif acts as a base in the regioselective\ndeprotonation process, which leads to the formation of two stereoisomers\nwith high enantioselectivity. Photolysis of dicyclic divinyl ketones\nbearing a 1,3-dioxolane motif in the presence of bifunctional hybrid\npeptide–thiourea chiral catalysts gave the tricyclic <i>cis</i>-hydrofluorenones with good enantioselectivity. Mechanistic\nand DFT studies suggested that the amide and thiourea groups in the\nbifunctional chiral catalysts play a key role as H-bond donors, which\ncoordinate with both the carbonyl group and the 1,3-dioxolane motif\nto provide a more favorable chiral species, and control the direction\nof the electrocyclization. Due to the presence of the rigid 1,3-dioxolane\nring, the deprotonation/protonation process occurs regiospecifically\nwith high driving force. This photo-electrocyclization is mild (room\ntemperature and neutral solution), which results a broad reaction\nscope and functional group tolerance and demonstrates its synthetic\npotential in organic synthesis.