Constructions Of Several N-heterocyclic Compounds Via Tandem Reactions

N-heterocycles, which exist in a wide range of natural products with remarkable biological activities, have attracted continuous interest in organic synthesis. Thus, the development of efficient methods for the construction of N-heterocycles is of highly desirable.Tandem reaction is an effective strategy in diversity-oriented synthesis. Compared with the conventional multi-step synthesis, the tandem reaction simplifies the operation process without the separation and purification of the intermediates, which is particularly prominent for the sensitive, unstable intermediates; Meanwhile, it is environmently friendly owing to avoiding the waste of raw materials and solvents; In addition, it also exhibits high efficiency for bonds formation and easily achieves molecular diversity and complexity. The thesis is mainly focused on the tandem reactions for the synthesis of some N-heterocyclic compounds starting from N’-(2-alkynylbenzylidene)hydrazides, N?[2-(1-alkynyl)phenyl]carbodiimides, o-alkenylphenyl carbodiimides, o-alkynylphenyl isothiocyanates, o-alkenylphenyl isothiocyanates, 2-haloaryl isothiocyanates and 2-isocyanoacetates.Firstly, based on the electrophilicity and [3+2] cyclization characteristics with nucleophiles of isoquinolin-2-ium-2-ylamide intermediates, we have developed a highly efficient route to H-pyrazolo[1,5-a]isoquinolines via a silver-catalyzed tandem reaction of N’-(2-alkynylbenzylidene)hydrazides with 2-phenylethanols in the presence of oxidant.Secondly, a facile synthetic protocol for the construction of indolyl imidazole derivatives via base-promoted tandem reaction of N-[2-(1-alkynyl)phenyl]-carbodiimides with isocyanides is disclosed. In this reaction, the molecular diversity and complexity could be easily introduced via a simple operation. Subsequently, the cascade reaction of o-alkenylphenyl carbodiimides with isocyanides catalyzed by a copper(I) iodide under mild conditions is described, which provides an efficient route for the generation of 4,5-dihydroimidazo[1,5-a]quinazolines. The present method has advantages of mild conditions and simple procedure.Thirdly, a copper(I) chloride-catalyzed tandem reaction of 2-isocyanoacetates with o-alkynylphenyl isothiocyanates is explored, which generates 5H-benzo[d] imidazo[5,1-b][1,3]thiazines via a nucleophilic addition and a [3+2] cyclization. Based on the above results, a new method for the construction of 5H-benzo[d]imidazo [5,1-b][1,3]thiazine scaffold via a a copper(I) chloride-catalyzed tandem reaction of 2-isocyanoacetates with o-alkenylphenyl isothiocyanates has also been developed through the similar reaction mechanism.Finally, a copper(I) chloride-catalyzed tandem reaction of 2-isocyanoacetates with 2-haloaryl isothiocyanates is explored, which generates benzo[d]imidazo [5,1-b]thiazoles in good yields.