Study On Cycloaddition Reaction Of Benzimidazole And Benzothiazole Quaternary Ammonium Salts

Both benzimidazolium salts and benzothiazole salts belong to azaromatic quaternary ammonium salts.Since the heterocyclic quaternary ammonium salt is easily converted the reactive intermediates,nitrogen ylides,in the presence of suitable bases,so they have been widely employed in the synthetic reactions by researchers in various fields.Multicomponent reaction as well as domino cycloaddition reaction is one of the most important procedure for synthesizing versatile monocyclic and polycyclic compounds.The 1,3-dipolar cycloaddition of nitrogen ylides with various diplarophiles is the most powerful method for constructing five-membered heterocycles.In this paper,we carried out some addition reactions of benzimidazole and benzothiazole quaternary ammonium to synthesis a series N,S-containing heterocycles such as pyrrolo[2,1-b]thiazole and its benzo derivatives.1.In the first chapter,we studied the three-component reaction of N-butyl-N’-phenacyl benzimidazolium bromide,aromatic aldehydes and indan-1,3-dione in ethanol in the presence of triethylamine,which resulted in the unusual charge-separated zwitterionic salts in good yields.Furthermore,1H NMR spectra and single crystals reveal that the zwitterionic salts usually exist in two kinds of conformation isomers due to hinder interconvert of the different conformations.All products were characterized by IR,1H NMR,13C NMR,HRMS spectroscopy.The single crystal structures of four products were determined by X-ray diffraction method.2.The triethylamine promoted cycloaddition reactions of N-phenacyl and N-alkoxycarbonyl methylbenzothiazolium bromides with aromatic aldehydes and malononitrile(ethyl cyanoacetate,pivaloylacetonitrile)in ethanol afforded functionalized tetrahydrobenzo[d]pyrrolo[2,1-b]thiazoles in good yields and with various diastereoselectivity.The oxidation reaction of the functionalized tetrahydrobenzo[d]pyrrolo[2,1-b]thiazoles with DDQ in different solvents resulted in diverse benzothi azole derivatives and benzo[d]pyrrolo[2,1-b]thiazoles.The reaction mechanism and the stereochemistry of this tandem[3+2]cycloaddition reaction and sequential oxidation reaction were illustrated on the analysis of the chemical structures of the reactive intermediates and products.All products were characterized by IR,1H NMR,13C NMR,HRMS spectroscopy.The single crystal structures of thirteen products were determined by X-ray diffraction method.3.The triethylamine promoted 1,3-dipolar cycloaddition reactions of N-phenacyl benzothiazolium bromides with nitroalkenes in ethanol resulted in a mixture of two isomeric tetrahydrobenzo[d]pyrrolo[2,1-b]thiazoles with cis/trans/cis-and all-trans-configurations.However,the functionalized dihydrobenzo[d]pyrrolo[2,1-b]thiazoles can be selectively prepared in refluxing ethanol and the benzo[d]pyrrolo[2,1-b]thiazoles can be obtained in satisfactory yields when DDQ was used as oxidizer.On the other hand,the similar reaction with 1-methy-1-nitroalkenes in refluxing ethanol afforded benzo[d]pyrrolo[2,1-b]thiazoles with splitting out of nitro group.The stereochemistry of the spiro compounds was clearly elucidated on the basis of NMR spectra and seven single crystal structures.