A Catalyst-Free Synthetic Route to Thiazolo[3,4-a]benzimidazol Dreivatives through a Three-Component Reaction

Thiazolo[3,4-a]benzimidazole and its derivatives were commonly synthesized through the condensation of o-phenylenediamines, 2-mercaptoacetic acid, and compounds containing a carbonyl group in the presence of various catalysts.1,2 these methods have several shortcomings such as use of expensive or toxic transition metals as catalysts, moderate yields of products, and tedious workup. As a part of our current studies on the synthesis of heterocylic compounds using multicomponent reactions,3,4 we report herein a catalyst-free route for the synthesis of thiazolo[3,4-a][1,3]benzimidazol-3(4H)-yl]acetate 3 using the reaction of ophenylenediamine derivatives 1, aryl isothiocyanate 2, and 4-chloro-3-oxo-butanoate in CH2Cl2-toluene under reflux (Scheme 1).Initially, a mixture of o-phenylenediamine (1a) and phenyl isothiocyanate (2a) in CH2Cl2 wasstirred at room temperature for ten minutes. After this time, 4-chloro-3-oxo-butanoate in toluene was added dropwise to the mixture and reaction was stirred under reflux for one hour, after completion of the reaction (monitored by TLC), compound 3 was separated by column chromatography (silica gel, n-hexane_EtOAC, 10:1).In summary, we have described a convenient process for the synthesis of thiazolo[3,4- a]benzimidazole derivatives. Use of readily available starting materials and good yields of theproducts are the main advantages of this method. The reaction occurs under netrual conditions, and the starting materials can be mixed without any activation.