The hexapeptide fragment containing the amino acid residues Nos. 16-21 is prepared as follows. The tripeptide fragment containing residues Nos. 19-21 is prepared by the solid-phase method as described above, cleaved from the resin support with MeOH-TEA and treated with trifluoroacetic acid (TFA) to obtain the corresponding methyl ester as the TFA salt, H-Arg(NO2)-Ile-Gly-OCH3.TFA (9). The amino acids corresponding to residues No. 18 (as Boc-Asp(Bzl)-OH), No. 17 (as Boc-Ile-OH.1/2H2O), and No. 16 (as Boc-Arg(NO2)-OH) are then coupled in succession to the above tripeptide (9) by classical methods. The resulting protected hexapeptide methyl ester is deprotected and treated with hydrazine to obtain the corresponding Boc-hexapeptide hydrazide as the acetate salt, Boc-Arg-Ile-Asp-Arg-Ile-Gly-NHNH2.2HOAc (16).
