Synthesis of imidazo[5,1-b]thiazoles or spiro-β-lactams by reaction of imines with mesoionic compounds or ketenes generated from N-acyl-thiazolidine-2-carboxylic acids

New mesoionic compounds (2H, 3H-thiazolo[3,2-c]oxazol-7-ones) (β) or ketenes ((3-acyl-1,3-thiazolidin-2-ylidene)methanone) (β′) were generated from N-acetyl and N-benzoyl-thiazolidine-2-carboxylic acids () using different methods, and their reactivity towards N-(phenylmethylene)benzenesulfonamide () and N-(phenylmethylene)aniline () was tested. When () were treated with () and acetic anhydride in refluxing toluene solution, only imidazo[5,1-b]thiazoles () were obtained from the mesoionic compound intermediates (β). When the ketene intermediates (β′) were generated from () by means of Mukaiyamaʹs reagent, only spiro-β-lactams () were isolated.