PURPOSE: To prepare cargtosin useful as an oxytocic, in high yield, with short step, by condensing a novel cyclic peptide with a straight-chain peptide by azide process.

CONSTITUTION: The compound of formulaI(R is H, benzyl or tert-butyl; Tyr, Ile, Gln, Asn are L-amino acid; Asu is L-aminosuberic acid) is condensed with the compound of formula II (Leu is L-leucine) to obtain the compound of formula III. When R is other than H, the protecting group is removed from the product to afford cargtosin (the cmpound of formula III wherein R is H). The compound of formulaI is novel, and obtained by reacting hydrazine with the compound of formula V derived, e.g. from the compound of formula IV (But is tert-butyl; R1 is lower alkyl).