New tetrapeptides having the formula A-Ser(benzyl)-Try(benzyl)-D-Ala-Leu-R.sub.1 wherein A is t-butoxycarbonyl, cyclohexylcarbonyl, benzyloxycarbonyl and p-nitrobenzyloxycarbonyl; Ser(benzyl) is D-Ser(benzyl) or L-Ser(benzyl); Tyr(benzyl) is D-Tyr(benzyl) or L-Tyr(benzyl); Leu is D-Leu or L-Leu, and R.sub.1 is lower alkoxy, hydrazino, amino, lower alkylamino or di(lower alkyl)amino.
新的四肽具有以下公式:A-Ser(苄基)-Try(苄基)-D-Ala-Leu-R.sub.1,其中A为叔丁氧羰基,环己基羰基,苄氧羰基和对硝基苄氧羰基;Ser(苄基)为D-Ser(苄基)或L-Ser(苄基);Tyr(苄基)为D-Tyr(苄基)或L-Tyr(苄基);Leu为D-Leu或L-Leu,R.sub.1为低烷氧基,
肼基,
氨基,低烷基
氨基或双(低烷基)
氨基。