Visible Light-induced Cyclization To Construct Pyrimido[1,2-b]-indazole Derivatives

As a large proportion of natural light,visible light has the advantages of clean,efficient,green,sustainable and renewable,etc.,and through the mediation of photocatalysts,many reactions that are difficult to complete under thermodynamic conditions can be realized.Therefore,visible light-induced strategy is widely used in chemical synthesis.Pyrimido[1,2-b]-indazoles are a special class of nitrogen-containing fused heterocycles,and they not only have good fluorescence properties,but some derivatives also exhibit biological activities such as anticancer and sedation,which has potential value in biology,materials and pharmaceuticals.The substitution of different functional groups on the ring of pyrimido[1,2-b]-indazoles can significantly change their fluorescence properties and biological activities.Therefore,the exploration and development of novel and efficient synthesis strategies for pyrimido[1,2-b]-indazole derivatives will be beneficial to chemical synthesis,drug discovery and development of fluorescent materials.In this paper,the construction of pyrimido[1,2-b]-indazole derivatives by visible light-induced cyclization of 3-aminoindazoles and ynals is studied.The specific contents include the following two parts:1)Under the irradiation of visible light,the three-component cyclization reaction of 3-aminoindazoles,ynals and chalcogen compounds was realized with FeCl₃ as additive and rose bengal as photocatalyst.Studies have shown that this reaction finally generates a series of selenium/tellurium pyrimido[1,2-b]-indazole derivatives through the formation of an onium ion intermediate and an intramolecular cyclization reaction.This method has the characteristics of mild conditions,green efficiency,high atom-economy,and wide substrate adaptability,which provides a new strategy and idea for the synthesis of pyrimido[1,2-b]-indazole derivatives.2)A new method for the regioselective construction of2-/4-substituted pyrimido[1,2-b]-indazole derivatives induced by different reaction systems was developed.The experimental results show that under the reaction system of metal Ag,the dehydration condensation between3-aminoindazoles and ynals occured and through intramolecular cyclization,the 4-substituted pyrimido[1,2-b]-indazole derivatives were formed;under the visible light-induced system,3-aminoindazoles transformed to free radical intermediate and underwent addition and cyclization with ynals to generate 2-substituted pyrimido[1,2-b]-indazole derivatives.This strategy has the advantages of mild conditions,high regioselectivity,and good functional group tolerance.Meanwhile,the study of optical properties found that the product has strong fluorescence emission in the yellow light region of 514～521 nm.