A compound of formula I ##STR00001## wherein W is a bicyclic hetroaryl of the structure ##STR00002## X is --O--, --S--, --SO.sub.2--, --CHR.sup.5--, --CHR.sup.5O--, --CHR.sup.5S--, --CHR.sup.5SO.sub.2--, --CHR.sup.5CO-- or --CH.sub.2CHR.sup.5--; Y is a bond or --CHR.sup.6--; Z is an aryl or heteroaryl group of the following structure: ##STR00003## A is --CH-- or --N--; B is --O-- or --S--; and R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.7 and R.sup.8 are as described herein. Further provided is a method for treating diabetes and related diseases employing a glycogen phosphorylase inhibiting amount of the above compound, either alone or in combination with another therapeutic agent.