[7-Thiazolidine-4-carboxylic acid)]oxytocin and [1-beta-mercaptopropionic acid,7-(thiazolidine-4-carboxylic acid)]oxytocin have been synthesized by a solid-phase method. Alpha-N-tert-Butoxycarbonyl- and S-ethylcarbamoyl-protecting groups were employed. The dipeptide Boc-Cys(Ec)-thiazolidine-4-carboxylic acid as well as individual residues was coupled to a H-Gly-dehydroalanine-resin with dicyclohexylcarbodiimide in the presence of 1-hydroxybenzotriazole. The appropriate protected polypeptide intermediates were cleaved from the resin by acidolysis, deprotected in NH3, and oxidized to the cyclic disulfide analogues with ICH2CH2I. Purification was effected by partition chromatography and gel filtration. Relative to oxytocin and [1-beta-mercaptopropionic acid]oxytocin, these analogues exhibit greatly enhanced oxytocic and avian vasodepressor potencies and unchanged rat pressor potencies.

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