Study On The Photocatalytic Synthesis Of Thiazole Compounds Starting From Thioamides Compounds

Five-membered heterocycles containing nitrogen and sulfur atoms are important class of heterocycle compounds,which widely existed in natural products and drug molecule s.Over the past decades,great efforts have been devoted to the development of efficient methods for the synthesis of heterocycle compounds with thioamides as synthons.Traditional methods for the synthesis of the heterocycles containing nitrogen and sulfur atoms through the reaction of thioamides with dielectrophiles which usually depend on transition metal catalyst,microwave and unavailable substrates.Therefore,the development of mild,green and effective methods for the synthesis of the different heterocycles containing nitrogen and sulfur atoms has remained an attractive but challenging topic.Recently,radical reaction initiated by photocatalysis as a green method,has been extensively studied by chemical workers.Whereas,the reported reactions mainly focus on the difunctionalization of unsaturated compounds which contain alkene and alkyne units via the addition of C-,S-and Sn-centered radicals,and the report on the synthesis of heterocycles is very limited.Therefore,this thesis mainly focused on the development of photocatalytic cascade reaction of thioamides with unsaturated compounds containing alkene and alkyne units,and synthesized different thiazole compounds.In the first part,we developed a photocatalytic cacade reaction of thioamides and p-quinone methides,and efficiently synthesized 1,2,4-dithiazoles compounds.The effects of different photosensitizers,solvents and additives were studied.The yield of product was increased when the ammonium sulfide was added.The controlled experiments indicated that the final product not only generated from the homocoupling,but also came from the reaction of intermediate with ammonium sulfide.We also explored the reaction of 1,2,4-dithiazole with isocyanides,and synthesized thiazol-5(4H)-imines.In the second part,we investigated the cascade cyclization reaction of thioamides with alkynes promoted by acridinium salt under the irradiation of visible light,and synthesized thiazole compounds.The effects of different photosensitizers,solvents and additives were studied.A possible mechanism was proposed,firstly,thioamide was induced into a sulfur radical cation by the excited-state photosensitizer,then the proton transfer gave a sulfur radical.The addition of sulfur radical with alkyne generated a vinyl radical which tended to undergo intramolecular cyclization and affored thiazoles as final products.