Partial retro-inverso analogues of somatostatin: pairwise modifications at residues 7 and 8 and at residues 8 and 9.

Peptide bonds between residues 7 and 8 residues 8 and 9, postulated internal cleavage sites of the peptide hormone somatostatin, were subjected to "pairwise" retro-inverso modification, where atoms of these peptide bonds were interchanged to give the analogues [gPhe7-m-(RS)-Trp8]somatostatin (I) and [gTrp8-m-(RS)-Lys9]somatostatin (II). Key fragments containing the modifications were synthesized by using [bis(trifluoroacetoxy)iodo]benzene for the generation of gem-diaminoalkyl-containing precursors from peptide amides. The versatility of solution synthetic methods was utilized to allow the incorporation of the modified segments. Protecting groups, removable selectively and under mild conditions, included tert-butyl-based groups for the side chains and the tert-butylmercapto group for the cysteine thiols. The excellent results obtained in the syntheses of analogues I and II, and previously of somatostatin on a larger scale [Moroder, L., Gemeiner, M., Goehring, W., Faeger, E., Thamm, P., & Wunsch, E. (1981) Biopolymers 20, 17-31], suggest the general feasibility of this route for the synthesis of centrally modified analogues. The purification of the products by Sephadex LH-20 chromatography afforded the separation of diastereomers of both analogues. The two isomers of I showed significant but different activities while those of analogue II were marginally active.