Dehydro‐enkephalins. III. Synthesis and biological activity of [ΔAla2, Leu5]‐enkephalin

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Abstract
[ΔAla2, Leu5]‐enkephalin has been prepared and shown to be more active than the parent saturated enkephalin in a binding assay using rat brain membranes and [3H] dihydromorphine as a tracer. In a comparison of potencies against [3H] dihydromorphine and [3H]‐[d‐Ala2, d‐Leu5]‐enkephalin as tracers, [ΔAla2, Leu5]‐enkephalin showed preference for μ opiate receptors, possibly due to the hydrophobicity of the ΔAla2 residue. A synthetic tetrapeptide enkephalin [ΔAla2]‐desLeu5‐enkephalin had weak activity and high selectivity for the μ receptors. O‐Acylation of a serine residue in the peptide was achieved by coupling between the peptide and a carboxylic acid using DCC and a catalytic amount of 4‐dimethylaminopyridine.