Ketomethylene and (cyanomethylene)amino pseudopeptide analogues of the C-terminal hexapeptide of neurotensin.

A series of pseudopeptide analogues of the C-terminal hexapeptide of neurotensin (NT8-13), namely [Tyr11 psi[COCH2]Phe12]-, [Ile12 psi[COCH2]Phe13]-, and [Tyr11 psi[CH(CN)NH]Ile12]NT8-13 with different stereochemistries, has been synthesized and evaluated for its potency in displacing labeled NT from rat cortex membranes. Ketomethylene pseudohexapeptides were prepared from the corresponding Boc-protected ketomethylene dipeptide derivatives, previously formed, using different solid phase synthesis (SPS) conditions, while (cyanomethylene)amino analogues were directly prepared by SPS using Fmoc strategy. H-Arg-Arg-Pro-Tyr psi[COCH2]-Phe-Leu-OH was nearly as potent as NT8-13 and [Phe12]NT8-13 in binding to the receptor. Comparison of the affinities for the pseudohexapeptides, here reported, with those of the psi-[CH2NH] analogues indicates the importance of the CO group in the amide or surrogate linkage at 11-12 and 12-13 positions in the receptor binding process.