Cyclic ureidopeptides, analogues of deltorphine tetrapeptide (Formula I), were amidinated in hope of preparing new derivatives with high resistance to enzymatic degradation, interesting opioid receptor binding profile and potentially improved blood-brain barrier permeability.

Amidination of corresponding cyclic ureidopeptides was performed using two reagents: N,N’-bis-Boc-S-methyl-isothiourea and 1H-1-pyrazole-1-carboxyamidine (with the latter generally giving better results). The new analogues were purified by reverse-phase HPLC and their purity and structural identity were established by analytical HPLC, HR-MS, and NMR studies.

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