[formula: see text] The new crystalline phosphate reagent 3-(diethoxyphosphoryloxy)-1,2,3-benzotriazin-4(3H)-one (DEPBT) mediates amide bond formation with a remarkable resistance to racemization. Comparative racemization studies were carried out and DEPBT proved to be superior to typical phosphonium and uronium coupling reagents. DEPBT is easily prepared and is exceedingly stable with a shelf life of months at room temperature. The advantageous properties of DEPBT render it a useful and unique addition to the arsenal of coupling reagents.

The development of potent antithrombotic agents from the fibrinogen platelet receptor binding sequences Fg-alpha 572-575 -Arg-Gly-Asp-Ser- and Fg-gamma 400-411 -HHLGGAKQAGDV, believed to be a cryptic RGD-type sequence, is described. The tetrapeptide Ac-RGDS-NH2 itself is capable of inhibiting platelet aggregation in vitro at high concentrations, IC50 91.3 +/- 0.1 microM [in vitro antiaggregatory activity employing dog platelet rich plasma (PRP)/ADP], due to low platelet fibrinogen receptor affinity, Ki 2.9 +/- 1.9 microM (purified, reconstituted human platelet GPIIb/IIIa), relative to fibrinogen, Ki 38.0 +/- 6.0 nM. The peptide is also unstable to plasma, suffering total loss of in vitro activity upon incubation in PRP for 3 h (T1/2 90 min). Only modest improvements in potency were achieved with linear analogues of Ac-RGDS-NH2, while dramatic results were achieved with cyclic analogues, culminating in the cyclic disulfide Ac-cyclo-S,S-[Cys-(N alpha-Me)Arg-Gly-Asp-Pen]-NH2 (SK&F 106760) with improved plasma stability (100% activity after 3 h), affinity (Ki 58 +/- 20 nM purified human receptor), and potency (IC50 0.36 +/- 0.4 microM dog PRP/ADP). The affinity of this peptide is 2 orders of magnitude greater than that of Ac-RGDS-NH2. The affinity of the analogue is also comparable to fibrinogen. This peptide constitutes a first potent small peptide entry into the class of novel antithrombotic agents called fibrinogen receptor antagonists.

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Unusual acid cleavage reactions are reported for derivatives containing acylated N-methyl-α-aminoisobutyryl (NMeAib) residues. The bond linking the NMeAib residue to the following amino acid is cleaved. Through X-ray diffraction studies of the NMeAib containing molecules, we have shown that the carbonyl oxygen atom of the preceding residue is in proximity to the carbonyl carbon of the NMeAib residue. Thus, it can act as an internal nucleophile leading to a cleavage reaction by way of an oxazolinium ion intermediate. Kinetic experiments for the cleavage reaction were carried out on a series of benzoyl dipeptide derivatives (p-X-C6H4C(O)-NMeAib-Phe-OMe) where X is varied from NO2 to Cl. The value of ρ = −1.335 for the Hammett linear free-energy relationship strongly supports the intermolecular oxazolinium intermediate proposed.