There is provided compounds of formula I, 1 wherein Y and R1 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.