Studies On The Metal-free Synthesis Of Benzoxazoles Via The Oxidation Of Catechols

Benzoxazoles have a wide range of biological activities,such as antibacterial,antituberculosis,antitumor and antivira,etc.This kind of compounds also play an important role in the field of organic synthesis and functional materials.The traditional methods for the synthesis of benzoxazoles are mainly based on o-aminophenols,which encounter dangerous nitrification process for the synthesis of raw material in industry.Furthermore,most of methods for benzoxazoles require the participation of transition metal catalysts and ligands,which limit their applications in biomedicine because of the problems of metal ion contaimination.In addition,the existing metal-free approaches suffer from harsh conditions such as high temperature,the use of toxic solvents and the requirement for assistance of acids or bases.Therefore,it is of great academic and ind ustrial value to develop simple and green synthetic methods for benzoxazoles by using readily available raw materials.In this paper,from the perspective of directly oxidative construction of benzoxazole compounds without any metal s,we successfully realized highly effective,mild and green synthetic methods for the synthesis of benzoxazoles by selective transformation of catechols compounds,alternative to o-aminophenols.The main research results of this paper are as follows:（1）A metal-free synthesis method is developed for the construction of benzoxazoles from catechols and amines using DDQ as the oxidant.The method is so simple that have no need of any noble metal catalysts,ligands,bases,acids or other additives.42 examples of 2-substituted benzoxazole derivatives are obtained in excellent yields.Compared with the known methods of the synthesis of benzoxazoles,this method avoids the problem of residues of metal impurities.In addition,this approach is featured with easily avaivable starting materials,gentler reaction temperature,avoidance of toxic solvents and auxiliary acid or base reagents.（2）A green method is developed to provide benzoxazoles that promoted by the green oxidant O₂ from catechols and amines.A series of 2-aryl and 2-alkyl substituted benzoxazole compounds are synthesized in satisfied yields from aromatic and aliphatic methylamines.Morever,aliphatic amines show excellent reactivity in the system.This method is featured by using the molecular oxygen as the green oxidant and the widely existed water as th e green solvent.In addition,this reaction has no need of any additives or organic solvents.On the basis of metal-free synthetic chemistry,the reaction system well conforms to the idea of green chemistry and provides a green and gentle method for the synthesis of benzoxazoles.