Compounds of the following formula (I): ##STR00001## where A, a, b, R.sup.1, R.sup.2, R.sup.3, R.sup.4 and R.sup.6 are described herein, are useful as inhibitors of platelet adenosine diphosphate. Pharmaceutical compositions containing these compounds, methods of using these compounds as antithrombotic agents and processes for synthesizing these compounds are also described herein.