There is provided compounds of formula I ##STR00001## wherein R.sup.1, R.sub.x, Y, R.sup.y, n and B have meanings given in the description which are useful as competitive inhibitors of trypsin-like proteases, such as thrombin, and in particular in the treatment of conditions where inhibition of thrombin is required (e.g. thrombosis) or as anticoagulants.