the invention provides compounds represented by the formula i , each of which compounds may have sphingosine - 1 - phosphate receptor agonist and or antagonist biological activity : and wherein the variables y , r 4 , n , o , a , a 1 , a 2 , x , z , r 1 , r 3 , r 2 , p , q and r are as defined in the specification . these compounds are useful for treating a disease or condition selected from the group consisting of glaucoma , dry eye , angiogenesis , cardiovascular conditions and diseases , and wound healing .