Design, Synthesis, and Modeling of Novel Cyclic Thrombin Receptor-Derived Peptide Analogues of the Ser 42 -Phe-Leu-Leu-Arg 46 Motif Sequence with Fixed Conformations of Pharmacophoric Groups:  Importance of a Phe/Arg/NH 2 Cluster for Receptor Activation and Implications in the Design of Nonpeptide Thrombin Receptor Mimetics †