| Oxazole is a motif frequently found in natural products,pharamcuticals,fluorecent materials and flavoring agents.Various methods have been developed to construct such units,including acid-promoted dehydration of 2-acylaminoketones,oxidation of oxazolines,metal/halogen-promoted intramolecular cyclization,transition-metal catalyzed bimolecular annulation and so on.Dispite these achievements,the development of efficient,green and substainable methods to access structurally diverse oxazoles is still expected by synthetic chemists.Visible light photoredox catalysis,being renascent in 2008 and featuring the chareacter of clean energy,high efficiency and mild condition,has been applied to heterocycle synthesis.In this thesis,we developed a[3+2]cycloaddition/oxidation sequence of 2H-azirines and aldehydes under visible light photocatalytic conditions.First,we indentified the optimal condition for oxazole synthesis through investigating the parameters such as solvents,photocatalysts,additives,oxidants:2H-azirines,aldehyes,MgSO4 and Li2CO3 were stirred at room temperature under the irradiation of 7 W blue LED using 9-mesityl-10-methylacridinium perchlorate as the photocatalyst,1,2-dicloroethane as the solvent and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone(DDQ)as the oxidant.Next,we probed the substrate scope under the opitimal reaction conditions and the results showed that this reaction was compatile with various 2H-azirines and aldehyes.Moreover,this strategy can be extended to the paparation of bioactive pyrrole and 2,5-dihydroimidazole utilizing acrylonitrile and Ms-protectd imine as reaction partners.A plausible mechanism that elaborated the reaction process to access oxazoles was proposed based on literatures and experimental datas.All the desired products were characterized by 1H NMR,13C NMR,HRMS and two of them were unambiguously determined by X-ray diffraction analysis.This research has been published on Organic Letters(2015,17,4070-4073.). |
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