Preparation and opioid activities of N-methylated analogs of [D-Ala2,Leu5]enkephalin

Analogs of opioid pentapeptide [D-Ala2,Leu5]enkephalin were prepared using two kinds of N-methylation reactions, namely quaternization and amide-methylation. Quaternization reaction with CH3I-KHCO3 in methanol was applied to the deprotected N-terminal group of the pentapeptide derivatives affording trimethylammonium group-containing analogs. [Me3+Tyr1,D-Ala2,Leu5]enkephalin and its amide were found to show opioid activity on guinea pig ileium assay only slightly lower than the parent unmethylated peptides. Application of amide-methylation reaction using CH3I-Ag2O in DMF to the protected pentapeptide yielded a pentamethyl derivative in which all of the five N atoms were methylated. Deprotection of the derivative gave pentamethyl analogs of [D-Ala2,Leu5]enkephalin, which showed no significant activity on the guinea pig ileum assay and opiate-receptor binding assay.