Investigations on P2–P3-heterocyclic dipeptide surrogates directed towards identification of an orally bioavailable thrombin inhibitor led us to pursue novel classes of achiral, non- covalent P1-arginine derivatives. The design, synthesis, and biological activity of inhibitors NC1–NC30 that feature three classes of monocyclic P1-arginine surrogates will be disclosed:(1)(hetero) aromatic amidines, amines and hydroxyamidines,(2) 2- ...