Synthesis and biological activity of a cyclic pseudohexapeptide analog of somatostatin

A cyclic pseudohexapeptide analog of somatostatin, cyclo(Pro psi[ CH2S ]Phe-D-Trp-Lys-Thr-Phe) was synthesized by solid phase methods and diphenylphosphoryl azide ring closure. The resulting crystalline compound possessed 23% of the growth hormone inhibitory activity of the parent tetradecapeptide and approximately 6% of the activity of the all-amide cyclic hexapeptide analog in spite of the absence of one of two postulated intramolecular hydrogen bonds.