Mesoionic isoxazolo[2,3‐ a ]pyrimidinediones and 1,3,4‐oxadiazolo[3,2‐ a ]pyrimidinediones as potential adenosine antagonists

Several derivatives of two novel mesoionic ring systems, i.e. , isoxazolo[2,3‐ a ]pyrimidinedione and 1,3,4‐oxadiazolo[3,2‐ a ]pyrimidinedione, were prepared for evaluation as adenosine antagonists. Whereas both ring systems are relatively stable when the 6‐position ( i.e. , that position corresponding to the purine 1‐position) is substituted by an alkyl group, neither ring system is stable when this position is unsubstituted. An example of a 6‐unsubstituted non‐mesoionic isoxazolopyrimidinedione exhibited similar behavior. As adenosine antagonists, the mesoionic compounds were found to be less potent than their previously evaluated mesoionic thiadiazolo[3,2‐ a ]pyrimidinedione counterparts.