Abstract: This invention relates to novel cyclic compounds linked by a heterocyclic ring system, which are useful as antagonists of the platelet glycoprotein IIb/IIIa complex, to pharmaceutical compositions containing such cyclic compounds, and to methods of using these compounds for the inhibition of platelet aggregation. A representative compound of the invention is cyclo-(D-Val-N(Me)Arg-Gly-Asp-?5-aminomethyl!-2-furoate).

Abstract: In this invention, a peptide of the following general formula is subjected to cyclization reaction to produce a cyclic peptide, preferably a synthetic calcitonin derivative (elcatonin) which is a useful medicine. ##STR1## (wherein A and B form a peptide of the formula Ser-Asn-Leu-Ser-Thr (SEQ ID NO: 47); X means a hydroxyl group, a carboxy-protecting group, an amino acid residue or a peptide residue; provided that the side-chain carboxyl group of .alpha.-L-aminosuberic acid is condensed with an amino acid or a peptide).The cyclic peptide can be obtained by subjecting a peptide of the above general formula to (1) cyclization reaction by chemical condensation, (2) cyclization reaction in the presence of an alkali metal salt and (3) reactions using the techniques of liquid phase synthesis and solid phase synthesis in combination.