Niobium-promoted reaction of α-phenylglyoxylic acid with ortho-functionalized anilines: synthesis of 2-arylbenzothiazoles and 3-aryl-2H-benzo[b][1,4]benzoxazin-2-ones

A new and general method to prepare 2-arylbenzothiazoles and 3-aryl-2H-benzo[b][1,4]benzoxazin-2-ones by the reaction of alpha -arylglyoxylic acid with o-aminothiophenol and o-aminophenol respectively is described. The use of ammonium niobium oxalate (ANO) as the catalyst and PEG-400 as the solvent were crucial to afford the title compounds in good yields and selectivity. The reaction time can be reduced from hours to few minutes when ultrasound is used as an alternative energy source.