There is provided a compound of formula I ##STR00001## wherein R.sup.a, R.sup.1, R.sup.2, Y and R.sup.3 have meanings given in the description and pharmaceutically-acceptable derivatives (including prodrugs) thereof, which compounds and derivatives are useful as, or are useful as prodrugs of, competitive inhibitors of trypsin-like proteases, such as thrombin, and thus, in particular, in the treatment of conditions where inhibition of thrombin is required (e.g. thrombosis) or as anticoagulants.